Notice

Estimates of Meridional Atmosphere and Ocean Heat Transports

Kevin E. Trenberth and Julie M. Caron


National Center for Atmospheric Research¹
P. O. Box 3000
Boulder, CO 80307

¹The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Kevin E. Trenberth may be contacted via the above Corresponding Author Address or via:

email: trenbert@cgd.ucar.edu
voice: (303) 497 1318
fax: (303) 497 1333

Abstract

New estimates of the poleward energy transport based on atmospheric reanalyses from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and European Centre for Medium Range Weather Forecasts (ECMWF) are presented. The analysis focuses on the period from February 1985 to April 1989 when there are reliable top-of-the-atmosphere (TOA) radiation data from Earth Radiation Budget Experiment (ERBE). Annual mean poleward transports of atmospheric energy peak at 5 PW, much larger than previous estimates, and with variability from 1979 to 1998 mostly less than 0.15 PW (1 to 3%). Results are evaluated by computing the implied ocean heat transports, utilizing physical constraints, and comparing them with direct oceanographic estimates and those from successful stable coupled climate models that have been run without artificial flux adjustments for several centuries. Reasonable agreement among ocean transports is obtained with the disparate methods when the results from NCEP/NCAR reanalyses are used, and this suggests that improvements have occurred and convergence is to the true values. Adjusted atmospheric transports are inferred, compensating for spurious subterranean transports over land areas, to show that poleward ocean heat transports are dominant only between 0 and 17° N. At 35° latitude, where the peak total poleward transport in each hemisphere occurs, the atmospheric transport accounts for 78% of the total in the Northern Hemisphere and 92% in the Southern Hemisphere. Generally a much greater portion of the required poleward transport is contributed by the atmosphere, than the ocean, compared with previous estimates.

1. Introduction

Radiative processes continually act to cool the high latitudes and warm the low latitudes of planet Earth, and it is only the poleward energy transport by the atmosphere and the oceans that serves to offset this. Early studies that tried to apportion how much each component contributed, first estimated the required poleward heat transport from satellite measurements, then computed the atmospheric transports from observations, and finally computed the ocean transports as residuals. Moreover, this was done using zonal means (Vonder Haar and Oort 1973, Oort and Vonder Haar 1976, Trenberth 1979, Masuda 1988, Carissimo et al. 1985, Savijärvi 1988, Michaud and Derome 1991). This procedure not only assumes that the atmospheric transports are correct, it also assumes they are correct over both land and ocean, yet subsequent analyses (e.g., Trenberth and Solomon 1994) have found that there are implied subterranean transports in land areas, whereas physical constraints ensure that any such transports must be tiny, as they can arise only from surface and ground water flows plus conduction. As estimates of direct global ocean heat transports became available (Bryden 1993), it became apparent that the atmospheric transports were likely to have been underestimated.

The studies of Vonder Haar and Oort (1973), Oort and Vonder Haar (1976) for the Northern Hemisphere (NH) and Trenberth (1979) for the Southern Hemisphere (SH), as well as those from Carissimo et al.(1985) and Savijärvi (1988) made use of radiosonde data, but the uncertainties in the atmospheric heat transports are substantial because of lack of observations over the oceans. The uncertainties are apparent at 70° S in the Carissimo et al. and Savijärvi results, for instance, where there is no ocean but their residuals imply a large polewards heat transport by the ocean. Moreover, use of global analyses (Masuda 1988) indicated larger estimates of poleward atmospheric transport apparently because radiosondes fail to pick up the substantial heat transports over the oceans. However, there has been a steady trend of increases in the magnitude of the poleward energy transports in both hemispheres as atmospheric analyses have improved, and this has continued with the recent reanalyses. Thus the poleward ocean transports inferred using residual methods have decreased over time.

In this paper we present new results of meridional ocean and atmosphere heat transports which are also based upon energy balance computations of the atmosphere, adjusted to fit physical constraints, using the reanalyses. The approach used is to estimate the atmospheric energy transports directly from analyses of measurements within the atmosphere. The analyses are produced using four dimensional data assimilation, and the results have become more reliable as reanalyses of atmospheric observations to remove spurious influences of the changes in the analysis system have been carried out. We make use of the reanalyses from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) (Kalnay et al. 1996) (referred to as NCEP) and European Centre for Medium Range Weather Forecasts (ECMWF) (Gibson et al. 1997). For ECMWF the reanalyses are available for 1979 to 1993, while the NCEP reanalyses are extended using the Climate Data Assimilation System (CDAS), and we use the interval 1979 to 1998 to document the variability.

Given the top-of-the-atmosphere (TOA) radiation and the atmospheric energy divergence, the net surface heat flux can be estimated locally, and this was done for the reanalyses in Trenberth et al. (2000a). We refer to these as the ``derived'' surface fluxes. The TOA radiation budget from satellite measurements provides quite reliable estimates of the total required meridional transports of heat, and we make use of data from the Earth Radiation Budget Experiment (ERBE) which has allowed the radiation errors to be much better quantified. The ERBE data have been adjusted for discontinuities when the NOAA-9 satellite was lost and missing data, which is pervasive near the delimiter of the solar radiation is filled in Trenberth (1997); see http://www.cgd.ucar.edu/cas/catalog/satellite/erbe/.

Hence the analysis focuses on a subperiod from February 1985 to April 1989 when ERBE data are available. The approach is to compute monthly means of all quantities, and combine the average monthly means over the ERBE period to produce an annualized mean. By averaging over more than a four year period we can reasonably ignore most changes in ocean heat storage, although we do allow for systematic warming trends which alter the heat storage. Therefore, we make several adjustments to the fluxes so that physical constraints are satisfied in order to provide the best estimates of values in the real world. The constraints are the estimates of long-term changes in heat storage, the transports at the northern and southern limits of our integration and the requirement that the TOA radiation balance the divergence of atmospheric energy over land.

To evaluate the results for the ocean, we compare the meridional transports of heat with alternative estimates from successful stable coupled climate models that have been run without artificial flux adjustments for several centuries (Boville and Gent, 1998, Gordon et al., 2000) and multiple analyses of direct ocean measurements to determine the extent to which independent means of obtaining these quantities has converged. The results show further increases in the poleward atmospheric transports of energy compared with previous estimates, but results are now at a point where there is almost no scope for further changes, as the inferred ocean transports would be reduced to values outside the error bars of the direct measurements. Hence there is a convergence of the ocean transport values to be mostly within error bars, which are typically in the range of ± 0.3 PW (1 PetaWatt is 10^15 W). Results of the zonal means of the quantities from this study are available online at http://www.cgd.ucar.edu/cas/catalog/ohts/.

Section 2 outlines the datasets used and the processing and evaluation which has already been carried out, and presents results for the atmospheric transports. The implied ocean heat transports are also presented along with the adjustments made to allow for heat storage changes and to satisfy the physical constraints. Section 3 presents the ocean transports from direct ocean measurements and the coupled models, and comparisons among the three datasets. It also compares the best estimate of the atmosphere and ocean energy transports with each other. Section 4 presents concluding remarks.

2. Transports derived from atmospheric energy budgets

a. The data and processing

The NCEP/NCAR reanalyses and CDAS are based on assimilation with an numerical weather prediction (NWP) model with T62 spectral resolution and 28 sigma levels in the vertical with five of those levels in the atmospheric boundary layer. There was a problem with the NCEP/NCAR reanalyses which affects the quality of the reanalyses over the Southern Hemisphere (SH). The problem arose from the assimilation of the Australian Surface Pressure Bogus Data for the Southern Hemisphere (PAOBS) (see http://wesley.wwb.noaa.gov/paobs/paobs.html) in which the observations were erroneously shifted by 180° in longitude affecting 1979-1992 (14 years). PAOBS are produced for the data poor southern oceans and are the product of human analysts who estimate sea-level-pressure based on satellite data, conventional data, and time continuity. Tests run on the impact of the problem indicate (i) the SH middle and high latitudes (40° --60° S) are most affected, (ii) the SH winter months are affected more than the SH summer months; (iii) the differences decrease rapidly as the time-scale increases from synoptic to monthly. This problem is likely to affect results over the southern oceans.

The ECMWF reanalyses (ERA-15) are at T106 resolution with 31 levels in the vertical and a hybrid coordinate that transitions to a pressure coordinate above about 100 mb. However, there are continuity problems with the ECMWF reanalyses arising from the positive reinforcement of biases in satellite radiances with those of the assimilating model first guess (Trenberth et al. 2000b). Two spurious discontinuities are present in tropical temperatures with jumps to warmer values throughout the tropics below 500 mb in late 1986 and early 1989, and further spurious interannual variability is also present. These features are also reflected in the specific humidity fields. The temperature discrepancies, which were identified initially using microwave sounder unit data, have a complex vertical structure with height (warming below 500 mb but cooling in the layer above) and these problems affect moist static energy profiles and therefore poleward heat transports. The time series of tropical temperatures from the NCEP/NCAR reanalyses are more consistent than those from ECMWF, and so only the NCEP results are used to examine the time series of variability.

The divergence of the monthly mean vertically integrated atmospheric energy transports from the two centers were compared for 1979-1993 in Trenberth et al. (2000a). Full maps of the spatial structure of the atmospheric energy divergence, the TOA fluxes, the derived surface fluxes, and the correlations and rms differences of the monthly means were also given. For the ERBE period net surface fluxes from the NCEP and ECMWF products were compared with each other and those from short-term (6 to 12 hour) integrations of the assimilating NWP models and from the Comprehensive Ocean Atmosphere Data Set (COADS) (da Silva et al. 1994).

Recent global air-sea flux climatologies based on ship data (COADS) and bulk formulae (da Silva et al., 1994, Josey et al. 1999) exhibit an overall global imbalance; on average the ocean gains heat at a rate of about 30 W m^-2. This was adjusted by da Silva et al. (1994) by globally scaling their long-term flux estimates, but the surface fluxes are not in balance for the ERBE subperiod. As Josey et al. (1999) found good agreement with buoy measurements in their unadjusted flux estimates, the evidence suggests that spatially uniform corrections are not appropriate but should be done locally. Time series of monthly COADS surface fluxes are shown by Trenberth et al. (2000a) to be unreliable south of about 20° N where there are fewer than 25 observations per 5° square per month. In addition, TOA biases in absorbed shortwave, outgoing longwave and net radiation from both reanalysis NWP models are substantial (› 20 W m^-2 in the tropics) and indicate that clouds are a primary source of problems in the NWP model fluxes, both at the surface and the TOA. Consequently, although time series of monthly bulk flux anomalies from the two NWP models and COADS agree very well over the northern extratropical oceans, these products were all found to contain large systematic biases which make them unsuitable for determining net ocean heat transports.

The surface fluxes can then in turn be integrated meridionally to give the implied ocean northward heat transports (see Trenberth et al. 2000a). Of the products examined in that study (two derived, two NWP model, and COADS, but not including the coupled models dealt with here) only the derived surface fluxes give reasonable implied northward ocean heat transports as the other three were corrupted by the large systematic biases.

b. The atmospheric energy transports

The zonal mean TOA energy budget from the ERBE data (Fig. 1) is used to compute the required poleward heat transport (RT), and this is presented along with the estimated atmospheric transports (AT) from both reanalyses for the same period (Fig. 2). Peak values in the NH of about 5.0 PW (see also Fig. 6) at 43° N greatly exceed the 3.1 PW of Oort and Vonder Haar (1976) and also those from the Global Weather Experiment ECMWF analyses of 4.0 PW (Masuda 1988). In Fig. 3 we present the mean northward atmospheric energy transports from NCEP as a function of month, as this allows a comparison with those for the Northern Hemisphere of Oort and Vonder Haar (1976). The latter featured peak northward transports of 5.0 PW in December at 63° N, values exceeded 4 PW from about mid November to the end of February and were less than 2 PW in summer. Figure 3 shows the maximum poleward transports occur in winter of both hemispheres and exceed 8 PW in the Northern Hemisphere, with values much greater throughout the year than those in Oort and Vonder Haar (1976). The peak poleward transport in the Southern Hemisphere is not quite as large but the annual cycle is much smaller.

To assess the representativeness of the ERBE period and the confidence in the estimates from an individual year, Fig. 4 presents latitude-time series for 1979 to 1998 of the 12-month running mean departures from the mean annual cycle. At right the panel shows the standard deviation as a function of latitude and it reveals values of almost 0.1 PW from 30 to 60° N but with larger variability farther south. Larger variability is not surprising in the tropics because of El Niño--Southern Oscillation interannual variations, and these may even be underestimated in the NCEP reanalyses (Trenberth and Guillemot 1998). However, variability in Fig. 4 is greatest from about 15 to 70° S and generally exceeds 0.15 PW. Moreover there is a distinctive pattern to the variability, with smaller southward energy transports in 1980-1985 and larger values from 1988 to 1998. However, comparison with ERA-15 AT zonal mean time series (not shown) reveals that this pattern is not reproducible. Instead from ERA-15 reanalyses, larger southward atmospheric transports occur from 1983 to 1988 than the mean for 1979 to 1993 in the Southern Hemisphere. In addition, ERA-15 annual mean standard deviations are less than 0.15 PW at all latitudes. The lack of reproducibility is consistent with the very small size of the interannual variability (1 to 3% of the actual transports) and the main message is that temporal sampling is not a major factor.

For the ERBE period, the bias in the northward atmospheric energy transport compared with the mean for the entire 1979 to 1998 period is slightly negative by ˜0.05 PW from 10 to 50° N and positive by ˜0.05 PW from 10 to 35° S. This is consistent with the error bar estimates provided by the panel on the right of Fig. 4, with values divided by 2 (square root of the number of ERBE years), in terms of sampling.

c. The derived ocean heat transports

In Fig. 2, the difference (RT-AT) gives an implied ocean heat transport and for NCEP, in particular, the implication is that there is almost no ocean contribution north of 45° N. However, such an ocean estimate assumes that the long-term surface heat budget over land is in balance as internal heat transport is negligible. Instead such a balance does not typically exist over land and so such an ocean estimate is contaminated by the considerable problems over land. This constraint allows the errors in atmospheric transports and surface fluxes over land to be quantified and they are found to be largest over complex and high topography (Trenberth et al. 2000a). Therefore it is desirable to recompute the ocean transport separately based upon the implied surface fluxes over just the ocean, and in this way we can also (somewhat arbitrarily south of 35° S) partition the transports into those from the individual ocean basins.

The implied zonal mean ocean transports are computed from the residually-derived surface fluxes (Fig. 5) starting at 65° N where there is a minimum of ocean available to transport heat northward, and estimates are that the transport through the Bering Strait is 0.2 x 10^13 W, and in the North Atlantic 1.4 x 10^14 W (Aagaard and Greisman 1975). Therefore we use 0.14 PW at 65° N as the starting point of our integration in the Atlantic. We set the dividing line between the Atlantic and Indian Ocean at 25° E, directly south of Africa. The Atlantic and the Pacific are separated at 70° W, south of South America. For the Pacific and Indian Oceans, we use 130° E from 5° S to south of Australia and 100° E north of 5° S. While integration of the surface fluxes readly partitions the contributions by basin, the result can not necessarily be interpreted as a heat transport unless the mass budget is closed. The Indian and Pacific Ocean partition is confounded by the Indonesian Throughflow, so that ocean mass flow in each basin is not closed, and only their sum is meaningful as a heat transport. The computations were done on a T42 grid (2.8°) as a compromise between the requirement for high resolution to resolve islands and the ocean basin configurations, and the need to smooth the analysis fields to suppress the small scale noise. The same domains and procedures were used for each product.

An additional consideration is systematic changes in heat storage in the ocean and, in particular, the evidence that a systematic warming of the global oceans is occurring (Levitus et al. 2000) with a magnitude of 0.3 W m^-2 overall. For 1985-1988 in the upper 300 m, where there is annual resolution, the warming appears to be 0.50 ±0.35 W m^-2. We take the first value as more representative of the overall changes because of uncertainties from sampling in the high frequency variability. Integrated over the oceans this would contribute to an apparent transport at 68° S of -0.1 PW which also provides the magnitude of this adjustment on the implied heat transports. To treat it as a change in heat storage, we subtract the 0.3 W m^-2 from the surface fluxes uniformly throughout the ocean, consistent with the global nature of the changes shown in Levitus et al. (2000).

Because we began the integration from the north and the global oceans are a closed system, any accumulated bias shows up as the imbalance at the southern-most latitude, taken as 68° S owing to seasonal ice cover. Adjustments for this imbalance were made as follows. The coupled ocean models (Boville and Gent, 1998, Gordon et al., 2000) have northward ocean heat transports at 68° S which average -0.1±0.05 PW. Hence we require that the total northward heat transport must go to -0.1 PW at 68° S. Further we assume that the main errors arise over the sparsely observed southern oceans south of 30° S, compatible with the problems revealed in COADS, the analysis of Josey et al. (1999), and the problems in the NCEP/NCAR reanalyses with the PAOBS.

Thus small adjustments are made to the surface heat flux over one or more of the southern oceans depending upon the sign of the residual versus the residual in the oceans south of 30° S. The northward heat transport by the Atlantic and Indian Oceans were combined because of the strong interbasin transports south of Africa and the role of Agulhas rings (Saunders and King 1995, Macdonald and Wunsch 1996). The computed total northward heat transport at 68° S has imbalances of -0.1 PW for NCEP-derived and -0.7 PW for ECMWF-derived products. These correspond to global surface flux imbalances of 0.3 and 2.3 W m^-2, respectively. As the imbalance is negative for both NCEP- and ECMWF-derived fields and the Atlantic Ocean transport is positive (northward), the adjustments were imposed on the Pacific Ocean of about 2 W m^-2 for NCEP and 16 W m^-2 for ECMWF from 30 to 68° S. This is the result plotted in Fig. 5. Note that the magnitude of the adjustments that we have applied to satisfy the global physical constraints are very small for NCEP.

Based upon computed root mean square differences and correlations between monthly fields of the ECMWF and NCEP derived surface fluxes, random errors of 25-30 W m^-2 for T31 resolution (scales of about 500 km) over the oceans are estimated (Trenberth et al. 2000a). The spatial power spectrum of the atmospheric energy divergence is slightly red, suggesting some spatial coherence. Here, the errors are accumulated by integrating from each northern and southern limit, which will somewhat underestimate southern ocean error bars, because they are not constrained except for the total for all oceans, and because of the adjustments. Rather than performing a formal error analysis, we use the empirical estimate of errors and we have plotted error bars in Fig. 5 assuming random errors of 30 W m^-2 over 1000 km scales, which approximately takes the spatial coherence into account, but which is probably a slight overestimate in the Northern Hemisphere.

3. Comparison among estimates

a. Direct ocean observations

The direct method for estimating ocean heat transport is where the products of ocean velocity and temperature measured over the boundaries of a closed volume are integrated to determine the ocean heat transport divergence for the volume. The advantage is that it deals with ocean circulation and the mechanisms of ocean heat transport while its disadvantages have been that estimates could only be made at a few locations where high quality observations were available and several assumptions, such as using geostrophic velocity estimates, are usually employed. As noted below, another substantial disadvantage is inadequate temporal sampling and resolution of the annual cycle.

Making direct estimates of heat transport from transoceanic sections in each ocean basin was a primary objective in designing the World Ocean Circulation Experiment (WOCE) field programme. Several WOCE section results are now also available and a synthesis of the WOCE results is underway. Macdonald and Wunsch (1996) and Macdonald (1998) made a synthesis of selected high-quality hydrographic sections covering all ocean basins, many taken prior to the WOCE observational period, to produce consistent meridional heat transports using a global inverse analysis. Another recent example is that by Holfort and Siedler (2000) for the South Atlantic which uses an inverse model to best fit observational constraints based upon measurements of mass, heat, salt, oxygen, nitrate, phosphate, silica and carbon. For the heat flux, the greatest uncertainties arise from the uncertainties in the surface wind stress and temporal variability. At 30° S they estimate for the Atlantic a mean northward transport of 0.29 ± 0.24 PW.

Koltermann et al. (1999) reexamined hydrographic data taken over the past 40 years in the North Atlantic and found physically consistent evidence for low frequency decadal changes within the ocean that produced changes in meridional ocean heat transport from 0.5 PW (1957-59) to 1.3 PW (1981-82) at 36° N and smaller changes of several tenths PW at 24° N and 45° N. However, independent analysis of all available data at 36° N by Sato and Rossby (2000) show that aliasing can occur because of variations with the seasons and spatial and temporal sampling. They find a mean annual northward heat transport of 1.2 ± 0.3 PW but with a range in the annual cycle of 0.6 ± 0.1 PW. Once the annual cycle is accounted for, they find interannual variations within 0.1 PW. However, individual sections have a standard deviation of 0.3 PW owing largely to eddy variability. Similar conclusions are arrived at by Roemmich et al. (2000) for the subtropical North Pacific (mean latitude 22° N) where they find a northward ocean heat transport of 0.77 ±0.12 PW based upon 25 transects, but with an interannual variability of 0.3 PW. It is apparent that many previous error bar estimates are conservative as they do not bridge the range of values found in different analyses of the same data.

In the Atlantic (Fig. 6, top panel), where there are the most abundant ocean observations, there is northward ocean heat transport at all latitudes mainly associated with the thermohaline circulation in which North Atlantic Deep Water is formed in the polar and subpolar North Atlantic and subsequently flows southward throughout the Atlantic. The most reliable single value is regarded as that at 24.5° N across the Atlantic where a recent reassessment that included the WOCE section in 1992 by Lavin et al. (1998) gave the best value of 1.27± 0.26 PW, somewhat larger than the 1.07 ± 0.26 PW estimated by MacDonald (1998). The error uncertainty range quoted here includes uncertainty in variability in the Bering Strait and other regions. The picture is rather different in the Pacific where a northward heat flux in the North Pacific is due principally to a shallow Ekman-upper thermocline cell (Wijffels et al., 1996) and values at 24° N are estimated as 0.8 PW (Bryden et al. 1991) or 0.5 PW (MacDonald 1998).

Some other estimates plotted in Fig. 6 with the error bars from the authors include Bacon (1997) at ˜55° N of 0.28± 0.06 PW, Klein et al. (1995) at 14.5° N of 1.22± 0.42 PW, Speer et al. (1996) at 11° S of 0.60± 0.17 PW, and Saunders and King (1995) at about 40° S of 0.53± 0.1 PW.

For the world ocean (Fig. 6, bottom panel) at 24° N, values range from 1.5± 0.3 PW (MacDonald and Wunsch 1996) to 2.0± 0.3 PW from the sum of the Lavin et al. and Bryden et al. values. For the South Pacific (Tsimplis et al. 1998, Koshlyakov and Sazhina 1995) and Indian oceans (Robbins and Toole 1997) the determination of ocean heat transport is clouded by uncertainties in the size of the Indonesian Throughflow and in the Agulhas Current transports (Beal and Bryden, 1997), and the estimates of northward heat transport across 30° S in the Indian Ocean range from -0.4 to -1.3 PW (Robbins and Toole, 1997, Macdonald, 1998). At 30° S, the total MacDonald and Wunsch (1996) estimate is -0.9± 0.3 PW.

b. Coupled models

Recent coupled ocean-atmosphere models from the Climate System Model (CSM) at NCAR (Boville and Gent, 1998), and the Hadley Centre, HADCM3 (Gordon et al. 2000) have been successfully run for several hundred years without artificial adjustments to the modeled surface heat fluxes, often called ``flux adjustment''. This implies that the surface air-sea heat and water fluxes calculated by the model are reasonably consistent with the simulated ocean heat and fresh water transports. Improvements in both the atmospheric (Hack 1998) and oceanic (Doney et al. 1998, Large and Gent 1999) modules of the climate models are believed to be responsible for this. Long term drifts in the sub-surface ocean indicate that some problems still exist, however, and these are most apparent in the fresh water budgets. For instance the InterTropical Convergence Zone (ITCZ) may spuriously migrate from one hemisphere to the other, seriously distorting the precipitation fields (e.g., Boville and Gent 1998). Routing of runoff from land precipitation has also been a problem but is being improved in the CSM. Nevertheless, this achievement by two groups is in effect a reconciliation of the atmospheric and oceanic modules as to what the meridional heat transport by the atmosphere and the ocean should be, and so this provides measures of these vital quantities which have heretofore been quite uncertain. However, the question of how well they agree with those in the real world remains, but is addressed here.

Fig. 6 shows the coupled model results in the center panel for the Atlantic and the bottom panel for the world ocean. For the Atlantic the two models agree quite well with each other in the SH while CSM values are as much as 0.3 PW larger in the North Atlantic. These differences are also reflected in the world ocean. At 24° N values are 1.67 PW (world) and 1.14 PW (Atlantic) for HADCM3 vs 2.02 PW and 1.31 PW for CSM, while at 30° S differences of 0.07 PW are smaller (-0.68 for HADCM3 vs -0.61 for CSM).

c. Comparisons with derived values

Figure 6 facilitates the comparisons among these different estimates. We compare the world ocean and Atlantic Ocean values in that order. For the direct ocean estimates at 24° N, NCEP-derived values (1.8 and 1.1 PW) fall within the observational uncertainties. In contrast both ECMWF-derived values (1.3, 0.8 PW) are too low and, as this is also true in the comparison with the coupled model results, it suggests that there are deficiencies in the ECMWF result. At 30° S for the world ocean the values of -0.8 PW for NCEP-derived and -1.1 PW for ECMWF-derived are within error bars of the direct estimate. The CSM (-0.6 PW) and HADCM3 (-0.7 PW) models suggest slightly smaller southward transports but the differences are within the error bars. In the Atlantic at 30° S the ECMWF derived values are much lower than for NCEP and the scatter in direct estimates does not discriminate which may be right. But given the ECMWF bias at 68° S, which required the adjustment of 0.7 PW, the NCEP result is preferred there too.

In the North Atlantic, the northward ocean heat transports by both coupled models are high relative to both the derived results and the direct observations by Bacon (1997) at 55° N. The CSM, in particular, seems to have ocean transports that are too large in the North Atlantic. There is otherwise quite good agreement between the two models and the NCEP estimates. Moreover, the agreement of the direct ocean estimates as a whole is very good with the NCEP estimates with the only exceptions being the estimates derived from the inverse modeling (labelled M and H on the figure), which depend on use of a model.

d. Relative transports

To more clearly show the relative roles of the atmosphere and ocean transports, Fig. 7 presents the required transport RT, as in Fig. 2, along with the adjusted derived ocean transports (OT) from the NCEP reanalyses, and the atmospheric transport (AT) as their difference. The latter can be compared with the raw estimates given in Fig. 2, and the differences are -0.2 PW near 40° N for AT, growing to ˜ -0.4 PW from 0 to 60° S for NCEP. For ECMWF (not shown) the changes are greater and, in particular, AT has to increase by 0.2 PW at 65° N, where the ocean role is tiny, and by 0.4 PW at 68° S, where the ocean role vanishes and thus is well defined. At 24° N the ECMWF AT increases by 0.6 PW. Aside from the changes at the northern and southern boundaries, which are small for NCEP, the differences exist mainly from the implied adjustments over land, plus the small ocean adjustments south of 30° S. For the NCEP results, a spurious downward surface flux into the land in the Northern Hemisphere (Trenberth et al. 2000a) results in lower adjusted northward AT throughout the globe. There is greater symmetry about the equator in the ECMWF atmospheric and oceanic transports (not shown), which is belied by the evidence of northward transport throughout the Atlantic Ocean.

There is now excellent agreement between NCEP and ECMWF AT north of 45° N where the inferred OT is quite small. The peak value is now 5.0± 0.14 PW at 43° N. However, in the Southern Hemisphere south of 40° S, the level of agreement between the two atmospheric transports evident in Fig. 2 is no longer as good. The peak southward atmospheric transport in Fig. 7 is -5.3±0.2 PW at 40° S, compared with -4.9 PW in Fig. 2. The magnitude of this change suggests that the true error bar may be somewhat larger than the 0.2 PW assigned.

At 35° latitude, which is very close to where the peak total poleward transport in each hemisphere occurs, the total atmospheric transport accounts for 78% of the total in the Northern Hemisphere and 92% in the Southern Hemisphere. Generally a much greater portion of the required poleward transport is contributed by the atmosphere, than the ocean, compared with previous estimates. It is only from the equator to 17° N that the poleward ocean transports exceed those from the atmosphere.

4. Concluding remarks

There is overall very good agreement between the NCEP-derived ocean heat transport and those from the coupled models and direct ocean measurements, while the ECMWF-derived values appear to be somewhat deficient. This is especially so at 24° N where no adjustments have been applied to either derived estimate. Further, the adjustments applied south of 30° S are miniscule for NCEP but amount to 0.7 PW at 68° S for ECMWF OT or 0.4 PW for ECMWF AT (the difference being integrated effects from the north versus the south), suggesting also that the latter are less reliable in absolute values. In the tropics, the problems with changes in the observing system, particularly satellite data, adversely influence the ECMWF results (Trenberth et al. 2000b) which are not within the error bounds of the other estimates at several latitudes.

Aside from the North Atlantic, where the coupled model results are high, the largest discrepancy among the results is in the Southern Hemisphere tropics where the NCEP-derived values imply a larger southward transport than the direct ocean estimates or the coupled climate models. This is in a region where Ekman transports play a key role and surface wind specifications are quite uncertain. For instance at 11° S a change in wind climatology alters the direct estimate from 0.48 PW to 0.63 PW in the Atlantic (Holfort and Seidler 2000), while tropical convergence zones in coupled model simulations are often dislocated in some seasons when a spurious ITCZ forms in the Southern Hemisphere, potentially corrupting values in the tropics (Boville and Gent 1998).

We have inferred the surface fluxes, and thus the ocean heat transports assuming no changes in ocean heat storage, except for those associated with global warming. However, the ocean heat storage change is not neglible from year to year (Sun and Trenberth 1998), although it is a reasonable assumption for the four years or so we used here provided that the global warming trend is factored in, as we have done. Variability in AT from sampling this particular interval is mostly less than 0.05 PW and is not a major factor, although interannual variability is not very reproducible between ECMWF and NCEP reanalyses.

It is important to note that while the ocean heat transports and surface fluxes derived from the TOA radiation plus the atmospheric transports (the indirect method) have improved substantially and mostly agree with the independent estimates, the same can not be said for the atmospheric NWP model surface fluxes computed with bulk parameterizations, which contain substantial biases. The NWP models have not yet been improved to satisfy the global energy budgets in the same way that the best coupled climate models have, highlighting the fact that weather prediction is constrained by the specification of the sea surface temperatures (SSTs) and does not have to get the SST tendencies correct to produce excellent weather forecasts.

Shortcomings in the hydrological cycle in the NCEP/NCAR reanalyses in the tropics (Trenberth and Guillemot 1998) suggest that they have limitations, although because there is huge compensation between the budgets for dry static energy and the moist component, the total energy transport is more robustly computed (Trenberth and Solomon 1994). The discrepancies between the atmospheric transports in the two reanalyses suggest that further revisions will occur, especially regionally. Nevertheless, the results on the ocean heat transports derived from the NCEP/NCAR reanalyses are in good agreement with those from the other approaches, suggesting that the coupled models, the atmospheric transports and the independently estimated ocean transports are converging to the correct values.

Acknowledgments.

This research was sponsored by NOAA Office of Global Programs grant NA56GP0247 and the joint NOAA/NASA grant NA87GP0105. Many thanks to Dave Stepaniak for computing the atmospheric energy transports.

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