The origins of the mini global warming accompanying El Niño events and the
implications for the role of diabatic processes in El Niño-Southern
Oscillation (ENSO) are explored. The evolution of global mean surface
temperatures, zonal means and fields of sea surface temperatures, land
surface temperatures, precipitation, outgoing longwave radiation, vertically
integrated diabatic heating and divergence of atmospheric energy transports,
and ocean heat content in the Pacific are documented using correlation and
regression analysis. For 1950-98, ENSO linearly accounts for 0.06C of
global warming. Individual warming events peak 3 months after SSTs in the
34 region, somewhat less than found in previous studies. During and
following El Niño events the heat from the ocean is redistributed within
the tropical Pacific. Warming at the surface progressively extends to about
latitude with lags of several months. A build up of ocean heat
content in the equatorial west Pacific progresses eastward and peaks some
months before the surface event, before spreading polewards along the coast
of the Americas and westwards in the off-equatorial tropics. Only in the
equatorial region are the subsurface ocean heat content anomalies linked to
SST anomalies locally. While the progressive development of ocean heat
content anomalies resembles that of the delayed oscillator paradigm, the
damping of anomalies through heat fluxes into the atmosphere introduces a
substantial diabatic component to the discharge and recharge of the ocean
heat content. This contributes to the delayed atmospheric warming in the
tropics and subtropics as El Niño decays, but much of the delayed warming
outside of the tropical Pacific comes from persistent changes in atmospheric
circulation forced from the tropical Pacific. A major part of the ocean heat
loss to the atmosphere is through evaporation and the heat is realized in the
atmosphere as latent heating in precipitation. This diabatic heating drives
large-scale overturning that influences the response throughout the tropics
and subtropics and sets up teleconnections in the extratropics. Reduced
precipitation and increased solar radiation in Australia, southeast Asia,
parts of Africa and northern South America contribute to surface warming that
peaks several months after the El Niño event. Teleconnections contribute
to the extensive warming over Alaska and western Canada through a deeper
Aleutian low and stronger southerly flow into these regions 0 to 12 months
later. Because the temperature response is greater over land than ocean
opposite changes contribute to the overall mean. The 1976/77 climate shift
and the effects of two major volcanic eruptions in the past two decades are
reflected in different evolution of ENSO events. At the surface for 1979-98,
the warming in the central equatorial Pacific develops from the west and
progresses eastward, while for 1950-78, the anomalous warming begins along
the coast of South America and spreads westward. The eastern Pacific south of
the equator warms 4 to 8 months later for 1979-98 but cools from 1950-78.
1. Introduction
It has been suggested that the time scale of El Niño-Southern Oscillation (ENSO) is determined by the time required for an accumulation of warm water in the tropics to essentially recharge the system, plus the time for the El Niño itself to evolve [ Wyrtki, 1985]. However, it has not been clear how much of the exchange of heat in the equatorial Pacific Ocean is merely with other parts of the ocean, and thus is adiabatic, as is the case for the so-called delayed oscillator paradigm for ENSO [ Suarez and Schopf, 1988; Jin, 1997], versus heat exchanges with the atmosphere involving diabatic processes.
During and following an El Niño, the global surface air temperature
typically warms up by perhaps 0.1C with a lag of about 6 months [
Newell and Weare, 1976; Pan and Oort, 1983; Jones, 1989;
Wigley and Santer, 2000]. In an exceptional event such as the 1997-98 El
Niño the amount exceeds 0.2
C. Christy and McNider [1994] and
Angell [2000] show that the entire troposphere warms up with an overall
lag of 5 to 6 months, but the lag is slightly less in the tropics and greater
at higher latitudes. Consequently, the empirical evidence suggests a strong
diabatic component to ENSO. Either the atmospheric circulation and
cloudiness change with ENSO in such a way to allow this to happen directly
within the atmosphere, or following El Niño, the ocean gives up heat to the
atmosphere to produce the delayed warming. In fact both seem to occur. The
latter appears to be more likely in the tropics and subtropics, and was
confirmed for a limited period by Sun and Trenberth [1998] and
Sun [2000]. However, in the extratropics of the Northern Hemisphere, the
deeper Aleutian low that accompanies El Niño, advects warm moist air along
the west coast of North America bringing warmth to western Canada and Alaska
[ Trenberth and Hurrell, 1994]. Similarly, in most parts of the tropics,
through teleconnections and because the main precipitation shifts over the
central Pacific Ocean, drier and sunnier conditions favor higher temperatures
with El Niño [ Klein et al., 1999]. These examples suggest that
several mechanisms need to be considered to explain the mini global warming
following El Niño. Hence a primary purpose of this paper is to clarify how
the global atmospheric surface temperatures respond to ENSO and illuminate
the global heat budget associated with ENSO.
The sea surface temperatures (SSTs) are a key in the two-way communication
between the atmosphere and ocean but have to be supported by a substantial
heat content anomaly of the ocean mixed layer if they are to have a
continuing influence on the atmosphere. Such an influence typically means
that the anomalous heat is being drained from the ocean and thus a negative
feedback occurs, as seems to be the case generally in the tropics. This was
shown from observations by Trenberth et al. [2001b], who analyzed a new
dataset of the divergence of the vertically integrated energy transports in
the atmosphere, along with SSTs, precipitation, atmospheric diabatic heating,
and other fields to demonstrate how the atmosphere and ocean respond to ENSO,
the dominant large-scale coherent coupled mode of variability. ENSO was
dominant in the first two modes emerging from a singular value decomposition
(SVD) analysis of the temporal covariance of SST with the atmospheric
variables, explaining 62% and 12% of the covariance in the Pacific domain
and 39.5% and 15.4% globally, respectively. In the tropical Pacific during
major El Niño events, the anomalies in divergence of the atmospheric energy
transports exceed 50 W mover broad regions and primarily come from the
surface fluxes from the ocean to the atmosphere. High SSTs associated with
warm ENSO events are damped through surface heat fluxes into the atmosphere
which fuel teleconnections and atmospheric circulation changes that transport
the energy into higher latitudes and throughout the tropics, contributing to
loss of heat by the ocean, while the cold ENSO events correspond to a
recharge phase as heat enters the ocean.
The two coupled atmosphere-ocean modes can be thought of as reflecting
different aspects of the mean tropical Pacific SST characteristics.
The first measures the mean SSTs in the tropical central and eastern
Pacific and is best represented by SST anomalies in the Niño 3.4
region (170-120W, 5
N to 5
S); we refer to this index as
N34 to distinguish the SST index from area averages of other
quantities over the same region. The second measures the contrast in SST
differences across the Pacific from about the dateline to coastal South
America and can be represented by the normalized difference in SSTs in
regions Niño 1+2 (0-10
S, 90-80
W) minus Niño 4 (5
N-5
S,
160
E-150
W) that we call the Trans-Niño Index, TNI [
Trenberth and Stepaniak, 2001]. These two indices are roughly orthogonal at
zero lag, but have strong relationships that change with time at various
leads and lags up to a year. The evolution of ENSO and the implicit lag
relationships manifested in the N34 and TNI indices [ Trenberth et
al., 2001b; Trenberth and Stepaniak, 2001] suggest that a systematic
exploration of lead and lag relationships is warranted, and that the N34
index can be used as the key index.
The evolution of SST patterns with ENSO and its changes over time are highly
relevant to the question of how heat is redistributed, and is explored
further in this paper. Several of our datasets, especially those dependent
on satellite information, begin in 1979. Accordingly, we provide the most
complete analysis for the post-1979 period. But we also wish to know the
extent to which our results apply more generally in previous periods.
Therefore an important factor taken into account in our analysis is the
1976-77 climate shift of ENSO activity toward more warm phases after about
1976 [ Trenberth, 1990], which is very unusual given the record of
previous 100 years [ Trenberth and Hoar, 1996, 1997; Urban et al.,
2000], and has been linked to decadal changes in climate throughout the
Pacific basin [ Trenberth and Hurrell, 1994; Graham, 1994] and
changes in evolution of ENSO. Rasmusson and Carpenter [1982] presented
composites of the evolution of ENSO events for 6 warm ENSO events from 1951
to 1972 and showed the ``antecedent'', ``onset'', ``peak'', ``transition''
and ``mature'' phases of the composite event. These run from September in the
year before to January the year following the event and became known as the
``canonical'' El Niño, see Wallace et al. [1998] for the historical
discussion. Before 1976, ENSO events began along the west coast of South
America ( TNI positive) and developed westwards. However, after 1977 the
warming has developed from the west so that TNI with reversed sign
prevailed some 3 to 12 months before the main peak in N34 and was followed
by TNI itself some 3 to 12 months after the peak. Therefore, the evolution
of ENSO events changed abruptly about 1976/77 [ Wang, 1995; An and
Wang, 2000; Trenberth and Stepaniak, 2001]. We take 1979-98 as
representative of the post 1976/77 shift period. As well as the climate
shift, another reason why the more recent period may be anomalous in ENSO
evolution, especially with regard to effects on surface temperatures, is the
presence of two strong volcanic eruptions (El Chichon in April 1982 and
Pinatubo in June 1991). Wigley and Santer [2000] estimate a global mean
cooling from the two volcanoes peaking at 0.2
C for El Chichon and
0.5
C for Pinatubo some 30 months after the eruption. Hence, we also
explore the relationships for the 1950 to 1978 interval, whenever adequate
data are available, as a way to examine the impact of the 1976/77 shift and
volcanic influences and determine to the extent possible how reproducible the
results are.
In this paper, we are especially interested in what can be determined with regard to the major interannual variations in the tropical Pacific and how they evolve with time, as this will help determine the role of El Niño in the climate system. Often El Niño has been regarded as simply a superposed fluctuation which undergoes its own cycle driven by either a coupled ocean-atmosphere instability within the tropical Pacific or stochastic forcing (and the Madden-Julian intraseasonal oscillation in particular) [ Power and Kleeman, 1994; Flügel and Chang, 1996; Eckert and Latif, 1997; Blanke et al., 1997; Stone et al., 1998; and Moore and Kleeman, 1999]. The combination of the tropical air-sea instability and the delayed negative feedback due to subsurface ocean dynamics can give rise to oscillations [ Suarez and Schopf, 1988; Battisti and Hirst, 1989; Münnich et al., 1991; Tziperman et al., 1994; Neelin and Jin, 1993; Jin, 1997]. As noted above, some theories of El Niño based on the delayed oscillator paradigm move heat out of the equatorial region during El Niño but move heat back as part of the overall ENSO cycle and the process is essentially adiabatic. The same is true for the Cane-Zebiak model [ Cane and Zebiak, 1985]. Yet during the course of these changes, the amount of warm water in the tropical Pacific builds up prior to and is then depleted during ENSO (see also Wyrtki, 1985; Cane and Zebiak, 1985; Jin, 1997; Tourre and White, 1995; Giese and Carton, 1999; Meinen and McPhaden, 2000). Observed changes in subsurface ocean temperatures [ Zhang and Levitus, 1996, 1997] and sea level [ Smith, 2000] and how they evolve with ENSO support this view. We show that part of the ocean heat content buildup and depletion is through exchanges of heat with the atmosphere and involve diabatic processes.
Therefore we exploit new data sets to help deduce and clarify what can be said about the diabatic processes involved in ENSO, how they relate to SST variations and those of the subsurface ocean, and how the mini-global warming following El Niño arises when it appears that it can not be sustained by the atmosphere alone. Tropical precipitation variations are also explored as a key indicator of the latent heating of the atmosphere, and inferences are made about effects of changes in cloudiness and increased solar radiation, and surface wetness on sensible versus latent heating.
Section 2 outlines the data sets used and their processing. The presentation of results in section 3 proceeds from the relationships between ENSO and global mean temperature, to zonal means of various quantities, and subsequently to full geographic spatial structure, so that we can trace where the relationships are coming from. Results for 1950-78 are presented along with those for 1979-98 where available. In discussing the results in section 4, the relative roles of the links with ENSO through the Pacific Ocean and the teleconnections through the atmosphere are addressed. The conclusions are given in section 5.
2. Data and methods
Most of the in-depth analysis here is for the period 1979 to 1998, as
this is the time when high quality atmospheric reanalyses are
available from the National Centers for Environmental
Prediction/National Center for Atmospheric Research (NCEP/NCAR), and
global fields of precipitation and outgoing longwave radiation (OLR)
are also available. Prior to 1979, the absence of satellite data
adversely affects the quality of the reanalyses. For the period 1979
to 1998 we computed many quantities for each month including the
vertically integrated total atmospheric energy transports
and their divergence
and the vertically integrated diabatic
heating [ Trenberth et al., 2001a]. The total energy consists of
the potential and internal energy, the latent energy, and the kinetic
energy, while the transports also include a pressure-work term and can
be broken down into components from the dry static energy and the
moist (or latent) component, which together make up the moist static
energy, plus the kinetic energy. The energy tendencies are combined
with the computed divergence of the vertically integrated atmospheric
energy transports to give the net column change, which has to be
balanced by the top-of-the-atmosphere (TOA) radiation and/or the
surface fluxes, see Trenberth and Solomon [1994] and
Trenberth et al. [2001a] for details. Thus, ignoring the tendency
terms, which average to be very small over a few months,
Trenberth et al. [2001a] show that the variability of TOA fluxes is small
and so
is mainly balanced by the surface fluxes. Trenberth et
al. [2001b] exploit the large spatial and temporal scales of ENSO to bring
out the ENSO signal from the noise. Over the Niño 3.4 region, results imply
a random standard error of
6 W m
and suggests that signals greater
than 12 W m
are significant.
We also make use of the OLR data set from the NOAA series of satellites adjusted using results from Waliser and Zhou [1997], as in Trenberth et al. [2001b]. We utilize the precipitation dataset from Xie and Arkin [1996, 1997], called the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP). Over land these fields are mainly based on information from rain-gauge observations, while over the ocean they primarily use satellite estimates made with several different algorithms based on OLR, and scattering and emission of microwave radiation.
We use the SSTs from NCEP from the optimal interpolation (OI) SST analysis of
Reynolds and Smith [1994] after 1982 and the empirical orthogonal
function (EOF) reconstructed SST analysis of Smith et al. [1996] for the
period before then. The latter does not contain anomalies south of 40S.
However, these SSTs are preferred to those in the global surface temperature
dataset from the University of East Anglia (UEA) and the United Kingdom
Meteorological Office [ Hurrell and Trenberth, 1999], although the
latter is employed to examine values over land and to examine the longer
record and the global means.
To examine aspects of the subsurface ocean heat content changes, we have used the ocean analyses of the tropical Pacific from the Environmental Modeling Center at NCEP. These model-based analyses have developed over time and currently assimilate observed surface and subsurface ocean temperatures, as well as satellite altimetery sea-level data from TOPEX/POSEIDEN. We use a monthly mean analysis from 1980 to 1998 derived from weekly analyses using the RA6 schemes described by Behringer et al. [1998].
The RA6 wind forcing and surface heat flux have changed over time. For 1980-96 wind forcing was the Hellerman and Rosenstein [1983] climatological wind stress combined with monthly pseudostress fields from Florida State University. NCEP operational near-surface winds, four times daily, with a constant drag coefficient were used to determine wind stress for 1997 through mid 1998. From mid-1998, wind stress is determined directly from NCEP operational analysis. Transients resulting from the wind forcing changes are minimized by gradually applying a new forcing over six months. Surface heat flux for 1980-96 was the mean annual climatological cycle of Oberhuber [1988]. From 1997 onward the heat flux is taken from the NCEP operational analysis. The impact of these changes has not been quantified.
The heat content is computed using data for the upper 387.5 m (using model
levels down to and including 345m but excluding levels at 430 m and below),
based on the fact that most expendable bathythermographs sample this layer
but not deeper and there is inadequate information at greater depths to
determine interannual variability. However, it is apparent that the main
variability is above 300 m depth and contributions from deeper layers are
believed to be very small. We compute the heat content as
Because large natural variability on synoptic timescales appears as weather
noise in monthly means, and also spurious noise related to sampling is
present (especially for SSTs, OLR, subsurface ocean temperature, and
),
we have smoothed the monthly anomaly fields used in the analyses with a
binomial
filter which removes two month fluctuations.
We primarily employ correlation and regression analysis to bring out
relationships at various lags.
3. The tropical Pacific variability
3.1 Time series in the tropical Pacific
As noted in the introduction, we use N34 as a key index. For correlations
between variables with persistence related to ENSO [ Trenberth, 1984]
for the 240 months (1979 to 1998), there are about 80 degrees of freedom,
suggesting statistical significance at the 5% level if the correlations
exceed about 0.23. The N34 and TNI indices are given in Fig. 1 normalized
using the means and standard deviations for 1950 to 1979. Both the time
series and the spatial patterns of N34 and TNI are essentially
orthogonal. Cross correlations of N34 and the Southern Oscillation index
(SOI) based upon surface pressures at Tahiti and Darwin are a maximum at zero
lag (0.83) for 1950-98 and for the two subperiods. Trenberth et al.
[2001b] presented the regression of N34 with the global SST field (see
Fig. 9, later), and the correlation is also given later as a function of lag
in Fig. 8. Patterns of SST associated with TNI are given in Trenberth
and Stepaniak [2001] along with the relations of TNI and N34 with lag.
Figure 1 also presents the means before and after 1976/77 for N34 and the
global mean temperature to illustrate the shift that occurred in these
indices toward warmer and more El Niño-like conditions.
3.2 Links with global mean temperature
For the global mean temperatures for 1979-98 (Fig. 2), the cross correlations (Fig. 3) reveal a broad maximum with the global mean temperature lagging N34 by 4 months. This is less than found in several other studies for reasons that are not fully clear. For instance Wigley and Santer [2000] find a lag of 6 months for 1979-99 with an SOI, although their SOI is formulated differently and is not optimal [ Trenberth, 1984]. They attempt to remove the cooling effects of the two volcanic eruptions (El Chichon and Pinatubo) after which their cross correlation sharpens and increases in magnitude to about 0.7 with a 7 month lag. Their results highlight the convolution of the volcanic signal and ENSO event during this period with a blurring of the ENSO-related relationships as a consequence. This result reinforces the need in Fig. 3 and subsequent analysis to also explore the relationships for the 1950 to 1978 period.
Figure 3 shows that the lag correlations are higher and more sharply peaked
in the 1950-78 period than more recently, strongly suggesting either a
contaminating influence from the two major volcanic eruptions or an impact of
the 1976/77 climate shift, or both. The lag is sharply defined as 3 months
and with a peak correlation of 0.65, corresponding to a regression of 0.11
C per N34, compared with 0.08
C for 1979-98.
The 1976/77 climate shift, whether part of decadal variability or trend,
influences results when the two subperiods are combined. Therefore it is
worthwhile to include results for the period (1950-98), as the different
means in the two subperiods factor into the results. The correlations for
the entire period are intermediate between those of the two subperiods except
positive correlations last longer (10 to +16 months) and are centered at
+3 months lag. Negative correlations before and after, which signal the ENSO
quasiperiodicity, are less in evidence on the longer time scale, no doubt
reflecting the influence of the 1976/77 climate shift. Alternatively, there
is a significant linear trend to the global mean temperatures which accounts
for 41% of the total variance for 1950 to 1998.
Finally in this subsection, we examine how much variance of the global mean
temperature is accounted for by ENSO for the entire 1950-98 period using a
screening regression, and what the residual series looks like (Fig. 2). For
the entire 1950-98 period the maximum lag correlation of 0.53 (28% of the
variance) between N34 and global temperature is at +3 months. Surprisingly,
this is identical whether or not the two series are detrended. The
regression coefficient based on the detrended relationship is 0.094C per
N34. This is deemed the more appropriate one and the N34 contribution is
given in Fig. 2. It shows that for the 1997-98 El Niño, where N34 peaked
at about 2.5
C, the global mean temperature was elevated as much 0.24
C
(Fig. 2) although, averaged over the year centered on March 1998, the value
drops to about 0.17
C. Correlations of the residual with TNI are not
significant. The linear trend in N34 over 1950-98 accounts for 4.1% of the
N34 variance, and this accounts for 1.8% of the variance due to the trend in
global mean temperature. As this is based on the regressions using detrended
data, the relationship is determined by the interannual variations. It
means that 13.6% (
) of the linear trend in surface
temperature, or 0.06
C for 1950 to 1998, arises from the changes in ENSO
(out of 0.43
C).
3.3 Zonal mean results
Figure 4 presents cross correlations with N34 of zonal means at
leads and lags as a function of latitude. Positive values always refer to
N34 leading. The top panel shows results for 1979-98, the middle panel
is for 1950 to 1978, and bottom panel is for the full period 1950-98. For
SSTs (first column) the figure shows the maximum correlations exceeding 0.8
lasting from 2 months before to 3.5 months after N34, and reversing in sign
a year before and a year later for 1979-98. Correlations outside about
30
are not significant. However, of considerable interest are the
``wings'' or lobes of positive correlations extending from the equatorial
region into the subtropics a year later. For 1950-78 the leads and lags in
the equatorial region are more symmetric although the wings into the
subtropics are still present, and this pattern also exists for the entire
period, without diminution of the correlations, and thus there is an
enhancement in statistical significance.
The equivalent results for the full surface temperatures (not shown), which
thus include land portions, are quite similar overall, although the central
highest correlations are delayed by 2 months in the equatorial region and
positive values are slightly higher in the subtropics a year later. As with
SST, there is no evidence of any relationship extending to higher latitudes,
although such has been claimed by Angell [2000]. Therefore the second
column of panels (Fig. 4) shows the results just for land. Over land there is
less evidence for delays with increasing latitude, seen over the ocean, but
the whole pattern is delayed by 3 to 4 months, and the extension is to
slightly higher latitudes, especially in the Northern Hemisphere where
correlations 45-50N exceed 0.2 at 0 to 7 months lag, and is barely 0.2
at 65
N at 13 months lag. This fits better with Angell's result and
indicates that his result is probably biased by the distribution of stations
used.
Shown in Fig. 5 are three panels for 1979-1998 giving results for OLR,
precipitation and
relations with N34 for the ocean only. Because
the main precipitation algorithm over the oceans uses OLR -- low OLR from
high cloud tops corresponds to high convective rainfall -- very similar
results to precipitation come from OLR. We have examined the same results for
just the Pacific Ocean and also for the globe (including land). For
the land values are unreliable and not useful [ Trenberth et al.,
2001a], while the main relationship over the oceans clearly comes from the
Pacific, where the correlations exceed 0.8 just south of the equator near lag
0, but with a pattern quite similar to that in Fig. 4. For precipitation,
the patterns for the Pacific versus total ocean versus global zonal means
(not shown) are quite similar, but with values dropping in the near
equatorial positive correlation region, while the negative correlations in
the subtropics hold steady in all three domains. For both precipitation and
, the lagged relationship indicated by the wing into the Northern
Hemisphere is absent and only the wing in the Southern Hemisphere is
present. As seen later, this wing arises from the changes in the South
Pacific Convergence Zone (SPCZ).
Fig. 6 shows similar figures for the oceans for several other quantities, the
vertically-integrated diabatic atmospheric diabatic heating () and
from the moisture budget [ Trenberth et al., 2001a], and the
ocean heat content in the Pacific. The patterns are almost identical for the
land plus oceans combined. The
relationship is similar to but much
weaker than that for precipitation alone. The diabatic heating, computed
from the thermodynamic equation as a residual, is quite similar to that for
precipitation, as would be expected if latent heating is dominant, although
the fact that it is slightly weaker is probably a deficiency in the NCEP
reanalyses [ Trenberth and Guillemot, 1998].
The correlations with Pacific upper ocean heat content, on the other hand,
are quite different in character. Fig. 6 reveals strong correlations,
indicating the build up in equatorial Pacific heat from 12 to 6 months prior
to the peak in N34, and with the maximum correlation occurring 4 months
prior to N34. Meanwhile there is cooling from 5 to 20N that is a
maximum at 8
N 2 months before N34. Significant cooling also follows the
peak in the equatorial region culminating about 12 months after N34,
but the off-equatorial correlations are not as strong, suggesting some loss
of signal through diabatic effects. This pattern is consistent with that
depicted by Meinen and McPhaden [2000].
3.4 Evolution of spatial patterns
Fig. 7 shows a breakdown of Fig. 3 by ocean sector for 1950-78 and
1979-98. It seems obvious that the lag of surface temperatures in the Pacific
should be closely in phase with N34 because of the close proximity, and
indeed this is the case, with a 2 month lag in both subperiods. However,
peak correlations are almost double in the earlier period. The Atlantic
sector (defined as 90W-0
) lags by 4-5 months, while the Indian Ocean
sector (defined as 0-120
E) shows a lag of 7 months for 1979-98 but much
less lag, although skewed and centered around +5 months from 1950-78. The
corresponding regressions have peak values of 0.10
C in the Indian and
Atlantic sectors, versus 0.06
C for the Pacific for 1979-98 and
0.12-0.13
C for all three oceans for 1950-79. These results highlight
the need to examine the redistribution of heat spatially.
To explore where the heat in the oceans and its effects on the atmosphere
actually occur, we have computed correlation and regression maps for several
fields at various lags, a subset of which are given here. Generally the
regression maps are quite similar in overall patterns but the correlations
are presented as they allow significance to be roughly assessed. Figure 8
presents the time sequence for correlations with N34 of surface
temperatures (based on the NCEP SSTs over the ocean blended with the UEA land
data) for 1979-1998 (right column) at 8,
4, 0, 4, and 8 months, where,
again, positive values signify that N34 leads. This reveals the
mean evolution of ENSO. The sequence is quite coherent and traceable from
about
12 to +12 months lag with N34, but becomes quite weak at larger
lags, and this
4 month sequence is chosen as a compromise through
consideration of the interval between panels and their number. Figure 8
(left column) presents the same sequence for surface temperatures from
1950-1978 to allow us to explore the role of the climate shift in 1976/77 and
the possible effects of the two volcanoes in the more recent interval.
For 1979-98, the maximum warming in the central equatorial Pacific develops
from off-equatorial tropical regions and from the west, progressing
eastward. In contrast, for 1950-78, the anomalous warming begins strongly
along the coast of South America and appears to spread westward, as was found
by Rasmusson and Carpenter [1982]. This difference in development
continues, with the eastern Pacific south of the equator warming during the
+4 to +8 months for 1979-98 but cooling from 1950-78. In the Indian Ocean,
warming begins 4 months prior to the peak in N34 and is strongest from 0 to
+4 months for 1950-78 and at +4 months for 1979-98, and is still strongly in
evidence at +8 months. The Atlantic signal becomes strongest about +4 months
and is quite similar in the two periods, with warming mainly from 0 to 25
N, but also some warming about 20
S. These results are consistent with
those of Klein et al. [1999] for the tropical Atlantic and Indian
oceans.
Figure 9 (left) presents the same evolution but as a regression for the
entire period (1950-1998). It therefore highlights the surface temperature
signal that is the same between the two subperiods and converts the signal
into actual temperature anomalies, thereby sharpening the focus on the
tropical Pacific and extratropical North America, where the variance is
greater than over the rest of the oceans. Referring to the warm ENSO phase,
at lag 0, the warmth in the tropical central and eastern Pacific is
accompanied by substantial warmth along the west coast of the Americas,
especially in the western two thirds of Canada and Alaska, and is mirrored to
some extent in the South Pacific near 50-60S. Modest but widespread
warmth is also in evidence in the Indian Ocean, Africa and southern Asia, and
in bands from South America across the Atlantic near 20
N and 30
S.
The effects in the Atlantic and Indian Ocean sectors become strongest at
about +4 months and are still evident at +8 months. Australia also tends to
be warm from 0 to +8 months. The latter relates to the relatively dry, or
even drought, conditions during El Niño and so is at least partly a result
of changes in cloudiness and atmospheric circulation, and not just heat from
the ocean [ Klein et al., 1999]. Similarly, drier conditions over
southeast Asia (mainly prior to 1977, Kumar et al., 1999; see also the
differences in Fig. 8) and parts of Africa imply warmer temperatures in the
summer half year, as shown next.
Figure 9 (right) therefore also presents the regression sequence of N34 with
precipitation, but only for 1979-98. The pattern is, not surprisingly, almost
identical with OLR (not shown), except the units are different. Peak values
for one standard deviation of N34 of order 1 mm/day in precipitation
correspond to about 10 W min OLR and correlations over 0.5 in magnitude.
The strong east-west dipole/boomerang pattern in ENSO is familiar and has
very large cancellation when compiled into the zonal means featured in the
previous section. Because much of these patterns arise from movements
northeastward of the SPCZ and southward of the Intertropical Convergence Zone
(ITCZ), the dividing lines are sharp and do not match the SST changes,
although there is a general tendency for wetter and warmer conditions to
co-exist in the tropical oceans. Over land the relationship is rather
different, and warmer conditions more often than not go hand-in-hand with
drier conditions, symptomatic of the changes in monsoons that occur with
ENSO.
The relationships with the inferred vertically integrated diabatic heating of
the atmosphere and
(Fig. 10) are similar to those at zero lag, given
in Trenberth et al. [2001b]. The diabatic heating pattern tends to be
similar to that of precipitation, signifying the dominance of the latent
heating anomalies. It is also closely related to anomalies in mean vertical
motion in the mid-troposphere (not shown). The positive correlations between
and SST anomalies in the Pacific are reversed in middle and higher
latitudes and in the tropical Atlantic. Although a clear sequence of
anomalies can be discerned in Fig. 10, it is not as strong as
with some other fields and the pattern is strongest at 0 and +4
months, suggesting that it is mainly a response to ENSO.
The evolution of the Pacific Ocean heat content anomalies (Fig. 11) shows
patterns that have been partly documented elsewhere, with the buildup of heat
in the western Pacific before 8 months, progressing eastwards to reach the
west coast of the Americas before lag 0, subsequently spreading polewards
into both hemispheres (+4) and then westwards from 10 to 20
N at +8 months
while building up near 10
S but with little spread westward beyond about
150
W, near the location of the SPCZ. Figure 12 extends the time sequence
by focusing on what appear to be fairly coherent regional averages. This
shows that the buildup of heat in the western Pacific, west of 150
W,
first occurs off the equator (
20 to
18 months), most strongly south of
8
S. It progresses into the equatorial region (8
N to 8
S), peaking
at about
15 months. The equatorial and southern region west of 150
W
becomes negative at
3 and
5 months, and the maximum negative
correlation is at +4 or +5 months. Meanwhile the initial heat deficit from
8
N to 30
N, which peaks at
5 months, becomes positive at +4 months
and peaks at +14 months. The maximum heat content from 150
W to the
Americas in the
strip coincides with N34 and drops to zero at
+9 months. From 8-30
S, east of 150
W, the warming occurs from
7
to +18 months, peaking at +5 months.
While the changes in Pacific heat content lead and appear to determine the
SST evolution in the equatorial strip, this does not seem to be the case at
higher latitudes. This is shown by the local correlation between the monthly
anomalies of ocean heat content and the SST (Fig. 14), which has direct
implications for the relevance of a simple two layer ocean model in being
able to depict SST variations with variations in the local thermocline. Only
in the equatorial region, broadening from 20N to 10
S west of 170
E
and 10
N to 15
S along the Americas, is the correlation significantly
positive. Values are actually negative along most of the Pacific at 10-15
N, north of the ITCZ, and also just northeast of the SPCZ region from 10
S
160
W to 35
S 100
W. Giese and Carton [1999] show a somewhat
similar figure based on model assimilated data. This result directly
reflects the ability of the subsurface ocean heat to sustainably influence
the atmosphere.
4. Discussion
4.1 Role of the tropical Pacific Ocean
In the tropical Pacific, and the tropical Indian Ocean to some extent [
Tourre and White, 1995], heat loss from the ocean drives the atmospheric
circulation changes, and the redistribution of heat by ocean currents as ENSO
evolves, contributes directly to subsequent heating and increased
temperatures in the tropics and subtropics as El Niño fades. Magnitudes of
observed changes in surface fluxes with moderate El Niños are about 40 W m
[ Trenberth et al., 2001a] which would correspond to a 1
C
temperature change over a column of water of 130 m deep over 5 months. Zonal
means across the Pacific of the ocean heat content (not shown) reveal typical
anomalies of order 0.5
J m
over 5
latitude which
corresponds to a 1
C change over a layer of 122 m. Maximum anomalies in
1981-83 and 1997-98 are about 3 times this value but these are the main times
when there is also a meridional dipole of anomalies of opposite sign present,
signifying a significant meridional rearrangement of heat within the ocean.
Consequently, the estimated surface heat fluxes are a dominant factor in the
change in area-average subsurface ocean heat content and the heat shows up in
the atmosphere.
The evolution of ENSO in the tropical Pacific illustrated here supports much of that previously described by Barnett et al. [1991], Zhang and Levitus [1996], Tourre and White [1995], Giese and Carton [1999], Smith [2000], and Meinen and McPhaden [2000] in the way that anomalies of subsurface ocean heat content in the western Pacific develop as they progress eastward across the equatorial Pacific, often with a dipole pattern across the Pacific, and then with anomalies progressing off the equator to higher latitudes. Zhang and Levitus found links only to the North Pacific, perhaps reflecting the available data, while links are strong in our results into both hemispheres. The SST evolution lags somewhat behind that of the subsurface ocean and is damped by surface fluxes and transports out of the region by the atmosphere, emphasizing the dominant role of the surface wind stress and ocean dynamics and advection in producing the local ocean heat content and SST anomalies. This damping of the ocean signal, however, forces the atmospheric anomalies. Moreover, this aspect also emphasizes that in cold La Niña conditions, the surface fluxes of heat are going into the ocean relative to the mean and are warming the ocean, although not locally, as where the heat builds up is determined by the currents.
The evolution of the ocean heat content shows the eastwards progression in
the equatorial region, then spreading north and southwards (presumably as
Kelvin edge waves) along the west coast of the Americas to higher latitudes,
but also with a component spreading westwards at about 10 to 20 latitude
in both hemispheres. From 1979-98 the SSTs were strongly positive in the
Southern Hemisphere portion but remained negative in the Northern Hemisphere
12 months after the N34 index peaks. The SST development from 1950-78 is
rather different and results in strong positive anomalies in the Northern
Hemisphere near 10 to 20
N 12 months after the peak, and with some
residual heat also in the Southern Hemisphere in a region where it can
influence the SPCZ. The quite different evolution along the coast of South
America in the classical El Niño region has previously been emphasized,
with the warming at the surface often occurring first there before 1979, as
in the Rasmusson and Carpenter [1982] composites, but after the central
Pacific warming in more recent years [ Wang, 1995].
4.2 Role of the atmosphere
In dealing with a time mean heat budget over several months, tendencies are
necessarily small and all the terms must be in balance. As a result, the
cause of a temperature anomaly may not be easily deduced and the utility of
quantities like
are limited.
There is little to support the idea that the local ocean heat storage plays
much role at higher latitudes. Instead, there is a known strong link through
atmospheric teleconnections between ENSO events and changes elsewhere [
Trenberth et al., 1998]. Over the North Pacific, ENSO teleconnections are
well captured at the surface by the index of area-average sea level pressure
over the North Pacific, called the NP index [ Trenberth and Hurrell,
1994]. Cross correlations between N34 and NP (not shown) are negative for
5 to +10 months and peak at
0.20 at +2 months for 1979 to 1998. That
relationship is strongest in the winter half year. Composites of seasonal
anomalies of mean temperature, changes in storm tracks and tendencies in
temperature from divergence of transient eddy heat transport [ Trenberth
and Hurrell, 1994] show that the latter two are negatively spatially
correlated and act to destroy the temperature structures in the lower
troposphere. Instead the temperature perturbations are partly set up by
advection by the anomalous mean flow, and this is demonstrated by regressing
the 700 mb temperature and vector wind on N34 as a function of lag, with
N34 leading by 0, 4 and 8 months (Fig. 14).
Thus, the substantial warming with El Niño over Alaska and western Canada
is clearly related to the change in atmospheric circulation that is known to
be forced through teleconnections from the tropical Pacific. The change that
most obviously brings about the warming is warm advection, and this comes
about through an anomalous southerly component to the flow advecting the mean
temperature gradient. We split the temperature into the time mean, given
by the overbar, and the anomaly perturbation, given by the prime
, and similarly for other quantities. Thus it is the term
that gives rise to part of the
(see Fig. 14) while it is
offset by changes in storm tracks and the transient eddy temperature flux
that acts to destroy the temperature anomaly on about a two week time scale
[ Trenberth and Hurrell, 1994]. These counterbalancing effects are both
included within
and largely cancel. However, associated changes in
moisture advection and cloudiness occur, introducing some radiative anomalies
as well, and there is more to
than advection.
The North American warming at 700 mb extends eastwards from the maximum
southerly flow to the upstream anticyclonic flow (see also Wallace et
al., 1998). Over the North Pacific, Fig. 14 shows the link between the
cooling and the more northerly flow but also to the cyclonic circulation. An
approximate mirror image appears over the South Pacific, although the wave
train has a slightly shorter wavelength. Cooler conditions east of New
Zealand are associated with southwesterly flow anomalies around a cyclonic
circulation centered near 40S 150
W and a blocking high exists farther
downstream near 60
S 110
W.
The more complete picture therefore, is that there is a substantial equivalent barotropic component to the atmospheric anomalies. In other words there is a substantial temperature anomaly in phase with the geopotential height field, and surface geopotential height or pressure anomalies increase in magnitude with height, although with some westward slope caused by the baroclinic component seen in the advection. Within the atmosphere, this comes about through the potential vorticity dynamics of teleconnections [ Trenberth et al., 1998] in which upper tropospheric transports of vorticity and momentum by transient eddies are important along with the heat transports. To keep the system in geostrophic and hydrostatic balance, subsidence occurs in anticyclonic regions, producing warming, while rising motion in cyclonic regions results in cooling. At the surface, cyclonic flow is accompanied by Ekman transports that produce upwelling and a shoaling thermocline, and increased storminess that encourages increased ocean mixing and fluxes of heat from the ocean into the atmosphere (e.g., see Deser et al., 1996). Thus the SST signature is similar to the tropospheric signature.
The remarkable persistence of the surface and 700 mb temperature anomalies
extending from at least 0 through +8 months, and thus transitioning from
winter to summer seasons, is worth noting. Fig. 9 shows that the tropical
precipitation anomalies and thus anomalous diabatic heating and forcing of
the atmosphere in the tropics continues throughout this period. There is
some change in the anomalies with season, however; at +8 months the main
contrast is from the Aleutian Islands region to North America, north of 50
N (Fig. 14). It is also of interest to compare the tropical precipitation
(Fig. 9) and diabatic heating (Fig. 10) and the atmospheric response in
Figs. 8 and 9 for
4 with +8 months, which are for the same season but a
year apart. The differences in tropical regions are fairly subtle but
evidently important. At
4 months the tropical Pacific anomalies are
oriented more east-west while at +8 months they involve the SPCZ and the
subtropical high near Hawaii is more involved in the tropical overturning.
The response over North America is much greater in the latter case.
Hence the at 700 mb is quite similar to that in Fig. 8 at the surface in
the extratropics, except the cooling with El Niño over southern California
at 700 mb is not present at the surface. The latter relates to the changes
in storm tracks, cloudiness, and precipitation, and perhaps some direct
influence of the local warming of the subsurface ocean (Fig. 11).
Some nonlinear effects that come into play depend on whether the temperature anomalies occur over the ocean or land. Over the ocean, the magnitude of that surface temperature change is muted by the heat capacity of the underlying ocean, the mixing of heat through the mixed layer, and possible entrainment and deepening of the thermocline. Consequently, the net surface warming over land is typically much larger than an equivalent response over the ocean (Fig. 9) and this can influence the global mean temperature, although it is not equivalent to a net heat content anomaly. The changes in the North Pacific and North America are mirrored to some extent over the southeast South Pacific as anomalous blocking takes place in that region as part of the Pacific-South American teleconnection but the land-ocean rectification effect is not present.
In the tropics and subtropics over land, the predominant change is one of warming over continental land (Africa, northern South America, Australia, southern Asia), which is associated with less precipitation, less high cloud (OLR), and more solar radiation [ Klein et al., 1999]. The dominant precipitation center moves over the central tropical Pacific, with a southwards shift in the ITCZ and a northeastwards shift in the SPCZ. The result is overturning teleconnections in the atmosphere that create the drier or even drought conditions in these locations. Other studies, such as Dai et al. [1999], which used detailed measurements from the First International Satellite Land Surface Climatology Project Field Experiment (FIFE), have documented the processes involved. They show how increased precipitation and surface moisture results in more evaporation and cooler surface temperatures, whereas decreased cloud increases temperatures mainly through increased solar radiation during the day, and there is an increase in temperature with poleward compared with equatorward winds. Therefore, the associations shown in this paper are consistent with the physical processes known to be operating.
The delayed warming in the tropical Indian and Atlantic Oceans is also a
result of changes in atmospheric circulation [ Wallace et al., 1998;
Klein et al., 1999]. In the Atlantic, the relationship of
(Fig. 11) with local SST anomalies is the opposite to that in
the Pacific, showing warming of the Atlantic from the changes in the
atmosphere heat budget with El Niño. In the tropical Indian Ocean,
however, especially in the western part, the results are more like
those in the Pacific, suggesting that the changes in surface wind
stress are responsible for much of the surface temperature changes,
but then there is a feedback and forcing of the atmosphere through
local convection changes. This was especially noticeable in 1997-98,
although claims have been made of independence with ENSO [ Webster et
al., 1999; Saji et al., 1999].
These results highlight the fact that over most of the globe away from the tropical Pacific, it is the persistent change in atmospheric circulation, driven from the Pacific that results in the changes in temperature.
4.3 Individual events
To reveal how the correlations presented in the previous figures have eventuated from individual ENSO events, we have examined the latitude-time Hovmöller diagrams for several quantities (not shown, but see Wallace et al., 1998). While these show to some extent how the individual events contribute to the statistical relationships explored earlier, they also show considerable variability from event to event. Moreover, detailed comparisons of the evolution of each event suggest that the time scale is not necessarily fixed and the evolution in some events is stretched out relative to others, making relationships at fixed lags or leads not necessarily the best way to examine the evolution. Whether external influences such as the effects of volcanic eruptions is a factor in this or whether it relates more to the amplitude of each event and the way in which transients perhaps trigger and contribute to the development remain outstanding questions.
5. Conclusions
ENSO events contribute to coherent interannual and even decadal fluctuations in the global mean temperature and, as we have shown here, the nature of the ENSO contribution is quite complex. Part of it involves the recharge and discharge of heat from the tropical Pacific Ocean. During and following El Niño events the heat from the ocean is redistributed within the tropical Pacific and much of it is released to the atmosphere, creating local warming. However, a major part of the ocean heat loss is through evaporation, and the heat is realized in the atmosphere as latent heating in precipitation. This diabatic heating drives large-scale overturning that influences the response throughout the tropics and subtropics as well as other teleconnections within the atmosphere extratropics.
The main tool used in this study is correlation and regression analysis
which, through least squares fitting, tends to emphasize the larger events.
This seems appropriate as it is those events where the signal is clearly
larger than the noise. Moreover, the method properly weights each event
(unlike many composite analyses). Although it is possible to use regression
to eliminate the linear portion of the global mean temperature signal
associated with ENSO, the processes that contribute regionally to the global
mean differ considerably and the linear approach likely leaves an ENSO
residual. We have shown here that 0.06C of the warming from 1950 to 1998
can be accounted for by the increased El Niño phase of ENSO. The lag of
the global temperatures behind N34 is 3 months, somewhat less than found in
previous studies. In part this probably relates most to the key ENSO index
used, as the evolution of ENSO means that greater or lesser lags arise for
alternative indices and these vary across the 1976/77 climate shift.
We have shown how positive correlations of surface temperatures with N34 extend from the equatorial region into the subtropics a year later. Results are fairly similar for both 1979-98 and 1950-78 and this pattern also exists for the entire period, without diminution of the correlations, and thus with an enhancement in statistical significance. However, for 1979-98, the warming in the central equatorial Pacific develops from the west and progresses eastward. In contrast, for 1950-78, the anomalous warming begins along the coast of South America and spreads westward, as shown by Rasmusson and Carpenter [1982]. This difference in evolution continues as the eastern Pacific south of the equator warms during the +4 to +8 months for 1979-98 but cools from 1950-78.
Over the oceans, there is an evolution from the maximum anomaly in the
equatorial Pacific spreading polewards and to the other oceans over
several months. Many tropical land areas tend to be warm from 0 to +8
months. This warmth relates to the relatively sunny and dry, or even
drought, conditions during El Niño and so is mainly a result of
subsidence and changes in cloudiness and atmospheric circulation. Heat
from the ocean is mainly important for the tropical heat sources that
drive the teleconnections and tropical large-scale overturning. More
generally, over land there is less evidence for delays with increasing
latitude, seen over the ocean, but instead the whole pattern is
delayed by 3 to 4 months. Also the extension is to slightly higher
latitudes at 4 to 8 months, especially in the Northern Hemisphere over
North America from 45N to 65
N. This fits with Angell
[2000] but indicates that his result is probably biased by the
distribution of stations used and that it is not representative
of the land and ocean combined. Because the peak in N34 tends to
occur about November to December and is phase locked to the annual
cycle, these results mean that the maximum warmth occurs in the
following northern spring and extends well into the summer. The
processes involved are discussed in section 4 and are well established
in winter and spring [ Trenberth et al., 1998].
OLR is often used as a proxy for precipitation owing to the dominant changes that occur in cloudiness and high cloud tops. However, the algorithms for translating OLR to precipitation do not account for the real effects on OLR of surface temperature changes or small changes associated with heating of the atmosphere. High SSTs are associated with the atmospheric convergence, deep convection and thus low OLR, but also with a flux of latent energy into the atmosphere, condensation and heating of the atmosphere, and transport of heat to higher latitudes where it can be radiated to space. This increase in OLR, seen especially in the subtropics, may be interpreted erroneously by precipitation algorithms as less precipitation, and hence whereas the precipitation amounts should increase when integrated over the domain, they do not appear to and this is probably incorrect [ Soden, 2000]. We infer that the increase in tropospheric temperature in the tropics, shown in Fig. 14, mainly originates from the increase in precipitation and latent heating and the redistribution from overturning and subsidence in the atmosphere, but it is not present in the Xie-Arkin dataset.
There is no doubt that the subsurface ocean heat content in the Pacific leads
the SST and atmospheric changes. However, the much lower lag correlations
between N34 and temperatures in the Pacific for 1979-98 versus 1950-79
suggest differences in evolution, part of which may have been due to the two
volcanic eruptions that occurred, with the Pinatubo event in 1991 estimated
to cause a global cooling of 0.5
C, peaking 30 months after the
eruption [ Wigley and Santer, 2000]; see especially the residual
temperature signal in Fig. 2. But the climate shift in 1976/77 seems to have
been a major factor in fundamentally altering the evolution sequence of ENSO
events. It suggests that the subsurface ocean evolution since 1980, which
has become the paradigm for ENSO, may not be robust across all events, and/or
the links between the subsurface and the surface may have changed. Mechanisms
for warming in the tropical Pacific depend on different balance of terms
within the ocean [ Neelin et al., 1998] and this balance may have
shifted. It is known, for instance, that vertical temperature gradients and
upwelling in the eastern tropical Pacific play a key role in westward
development, while eastward development relies more on east-west temperature
gradients and advection in the central tropical Pacific. We noted in Fig. 13
how poorly the SSTs were related to the total ocean heat content anomalies
outside of the equatorial Pacific strip and that this will limit the ability
of two layer oceans, such as are often employed in intermediate models, to
simulate ENSO.
The negative feedback between SST and surface fluxes can be interpreted as showing the importance of the discharge of heat during El Niño events and the recharge of heat during La Niña events. Relatively clear skies in the central and eastern tropical Pacific allow solar radiation to enter the ocean, apparently offsetting the below normal SSTs, but the heat is carried away by ocean currents and adjustments through ocean Rossby and Kelvin waves, and the heat is stored in the western Pacific tropics. This is not simply a rearrangement of the ocean heat, but also a restoration of heat in the ocean. Similarly, during El Niño, the loss of heat, especially through evaporation, into the atmosphere is a discharge of the heat content, and both contribute to the life cycle of ENSO. These observations support the picture put forward by Barnett et al. [1991] based mainly on model results in which the SST anomalies are created by ocean dynamics and response to wind forcing, and not local surface fluxes. However, the role of the surface fluxes and the diabatic component of the ENSO cycle should not be underestimated as it has implications for the role of ENSO in climate.
An alternative view of the change in evolution of ENSO is one based upon time filtering of the data into interannual and interdecadal time scales (e.g., Zhang et al., 1997; Mantua et al., 1997; Giese and Carton, 1999) but because the patterns of SST on each time scale are not orthogonal and the processes are nonlinear, strong interactions are implied and results are difficult to interpret. The reasons why the change in evolution with the 1976/77 climate shift occurred are quite uncertain but appear to relate to changes in the thermocline [ Guilderson and Schrag, 1998; Giese and Carton, 1999] which could be linked to climate change and global warming [ Trenberth and Hoar, 1996; Meehl and Washington, 1996; Timmermann et al., 1999].
Acknowledgments.
This research was sponsored by NOAA Office of Global Programs grant NA56GP0247 and the joint NOAA/NASA grant NA87GP0105. We thank Clara Deser for comments.
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