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CGD 2009 Profiles in Science: Dr. Gerald Meehl
Summary of achievements
The role of anthropogenic black carbon aerosols was studied to identify the role they have played in shaping climate trends we have observed over the past 50 years (Meehl et al. black carbon aerosol paper). An ensemble of 20th century simulations with CCSM3 that included only time evolving changes of black carbon aerosols was analyzed to show that the observed changes of seasonality of Indian monsoon rainfall, with greater rainfall in the pre-monsoon season and less rainfall during the monsoon season, have likely been due to increases in black carbon aerosols. These aerosols absorb solar radiation over India in the pre-monsoon season (thus heating the lower atmosphere) as well as shield the surface from solar radiation. The result is a cooler north Indian Ocean and Indian sub-continent, while the heated air is advected north over the Tibetan Plateau to contribute to an early elevated heat source that intensifies the tropospheric meridional temperature gradient, thus increasing pre-monsoon rainfall over India. However, during the monsoon season the cooler surface temperatures reduce the meridional temperature gradient and thus weaken monsoon season rainfall. This is the first time the role of black carbon aerosols has been isolated in a global coupled climate model to explain recent observed trends of monsoon rainfall.
The important role of the 11 year cycle of solar forcing is emerging in model simulations that have been analyzed, along with observations, to confirm earlier hypotheses (Meehl et al. 2003) and model results in that the effect of enhanced solar forcing is to strengthen the climatological precipitation regimes in the tropics. This produces a La Nina-like response in the tropical Pacific with teleconnections to the North Pacific that reduce precipitation with greater solar forcing in the Pacific northwest and northern California. A coupled air-sea mechanism, postulated in earlier work that examined the solar effects on early 20th century climate, is identified from 20th century simulations with PCM and CCSM3 that involves air-sea coupling in the tropical Pacific that produces the pattern in the observations (Meehl et al. 2008 solar). This response is different from La Nina events that also have a similar pattern (van Loon and Meehl, 2008). This is the first work that has identified a coupled air-sea mechanism in the Pacific that responds on the time scale of the 11 year solar cycle.
New work looking weather and climate extremes has shown that observed changes of heat extremes (e.g. reductions of frost days) are most likely due to increases of anthropogenic greenhouse gases. Additionally, projected future changes of El Niñ0 teleconnections over North America (Meehl et al., 2007) that involve an eastward and northward shift of the PNA pattern, also affect patterns of temperature and precipitation extremes over North America that show a similar projected shift. This work documents for the first time the connections between anthropogenic forcing of changes of extremes, changes of El Niñ0 teleconnections, and changes of the pattern of extremes associated with El Niñ0.
Publications
Figure 1: High resolution figure
Hurrell, J.W., T. Delworth, G. Danabasoglu, H. Drange, S. Griffies, N. Holbrook, B. Kirtman, N. Keenlyside, M. Latif, J. Marotzke, G.A. Meehl, T. Palmer, H. Pohlmann, T. Rosati, R. Seager, D. Smith, R. Sutton, A. Timmermann, K.E. Trenberth, and J. Tribbia, 2009: Decadal Climate Prediction: Opportunities and Challenges. Community White Paper, OceanObs '09. [article]
Introduction: The scientific understanding of Earth’s climate system is now sufficiently developed to show that climate change from anthropogenic greenhouse gas forcing is already upon us, and the rate of change as projected exceeds anything seen in nature in the past 10,000 years. Uncertainties remain, however, especially regarding how climate will change at regional and local scales where the signal of natural variability is large. Decision makers in diverse arenas, from water managers in the U.S. Southwest to public health experts in Asia, need to know the extent to which the climate events they are seeing are the product of natural variability, and hence can be expected to reverse at some point, or are the result of potentially irreversible anthropogenic climate change.
Figure caption: The global number of temperature observations per month as a function of depth. The data sources are XBTs, fixed tropical moorings (TAO (Pacific), TRITON (Pacific), PIRATA (Atlantic), and the developing Indian Ocean array) and ARGO floats. The apparent horizontal strata reflect the successive influence of 450 m XBTs, 750 m XBTs, 500 m TAO-class moorings and 1000 m and 2000 m Argo floats.
Meehl G.A. and J.M. Arblaster, 2009: A lagged warm event-like response to peaks in solar forcing in the Pacific region J. Climate, doi:10.1175/2009JCLI2619.1.
Figure 2: High resolution figure
Abstract: The forced response coincident with peaks in the 11 year decadal solar oscillation (DSO) has been shown to resemble a cold event or La Niña-like pattern during DJF in the Pacific region in observations and two global coupled climate models. Previous studies with filtered observational and model data have indicated that there could be a lagged warm-event or El Niño-like response following the peaks in the DSO forcing by a couple of years. Here we examine observations and two climate model simulations, and show that dynamical coupled processes initiated by the response in the tropical Pacific to peaks in solar forcing produce wind-forced ocean Rossby waves near 5N and 5S. These reflect off the western boundary, producing downwelling equatorial Kelvin waves that contribute to transitioning the tropical Pacific to a warm event or El Niño-like pattern that lags the peaks in solar forcing by a couple of years.
Figure caption: Composite differences, peak solar years (denoted year 0) minus climatology from the SODA ocean reanalysis data, as well as the year prior (year -1), and the years after (denoted year+1, year+2) for a) zonal wind stress at 10N, b) zonal wind stress at the equator, c) zonal wind stress at 10S, d) upper ocean heat content at 5N, e) upper ocean heat content at the equator, and f) upper ocean heat content at 5S. Units are on color bar and stippling indicates significance at the 90% level from a t test. Solid lines highlight ocean Rossby waves (slanting down from right to left), and equatorial Kelvin waves (slanting down from left to right).
Meehl, G.A., A. Hu, and B.D. Santer, 2009: The mid-1970s climate shift in the Pacific and the relative roles of forced versus inherent decadal variability. J. Climate, 22, 780-792, doi:10.1175/2008JCLI2552.1.
Figure 3: High resolution figure
Abstract: A significant shift from cooler to warmer tropical Pacific sea surface temperatures (SSTs), part of a pattern of basinwide SST anomalies involved with a transition to the positive phase of the Interdecadal Pacific Oscillation (IPO), occurred in the mid-1970s with effects that extended globally. One view is that this change was entirely natural and was a product of internally generated decadal variability of the Pacific climate system. However, during the mid-1970s there was also a significant increase of global temperature and changes to a number of other quantities that have been associated with changes in external forcings, particularly increases of greenhouse gases from the burning of fossil fuels. Analysis of observations, an unforced control run from a global coupled climate model, and twentieth-century simulations with changes in external forcings show that the observed 1970s climate shift had a contribution from changes in external forcing superimposed on what was likely an inherent decadal fluctuation of the Pacific climate system. Thus, this inherent decadal variability associated with the IPO delayed until the 1970s what likely would have been a forced climate shift in the 1960s from a negative to positive phase of the IPO.
Figure caption: (a) The second EOF from the low-pass-filtered SST observations (as in Fig. 2c ); (b) PC time series from the second EOF from the observations (solid, as in Fig. 2d ) and pattern correlations from projecting the first EOF from the control run (Fig. 3 ) onto the low-pass-filtered observed SST (dotted) and similarly from projecting the first (dash–dot) and second (dashed) EOFs from the ensemble-mean all-forcings experiment (Figs. 4a and 4c , respectively) onto the low-pass-filtered observed SST data. The left axis labels refer to the amplitude of the PC time series (dark solid line), and the right axis labels are for the amplitude of the pattern correlations (dash–dot, dashed, and dotted lines). Thin vertical lines denote 1960 and 1973 as discussed in text.
Hu, A., G.A. Meehl, W. Han and J. Yin. 2009: Transient response of the MOC and climate to potential melting of the Greenland Ice Sheet in the 21st century. Geophysical Research Letters, 36, L10707, doi:10.1029/2009GL037998.
Figure 4: High resolution figure
Abstract: The potential effects of Greenland Ice Sheet (GrIS) melting on the Atlantic meridional overturning circulation (MOC) and global climate in the 21st century are assessed using the Community Climate System Model version 3 with prescribed rates of GrIS melting. Only when GrIS melting flux is strong enough to be able to produce net freshwater gain in upper subpolar North Atlantic does the MOC weaken further in the 21st century. Otherwise this additional melting flux does not alter the MOC much relative to the simulation without this added flux. The weakened MOC doesn't make the late 21st century global climate cooler than the late 20th century, but does reduce the magnitude of the warming in the northern high latitudes by a few degrees. Moreover, the additional dynamic sea level rise due to this weakened MOC could potentially aggravate the sea level problem near the northeast North America coast.
Figure caption: Dynamic sea level anomalies: (a) the mean of 2080–2099 in A1Bexp minus the mean of the last 20 year of the 20th century; (b–d) are the mean of 2080–2099 in 1%exp, 3%exp, and 7%exp minus the mean of 2080–2099 in A1Bexp, respectively.
Hu, A., G. A. Meehl. 2009: Effect of the Atlantic hurricanes on the oceanic meridional overturning circulation and heat transport. Geophys. Res. Lett., 36, L03702, doi:10.1029/2008GL036680.
Figure 5: High resolution figure
Abstract: Hurricanes have traditionally been perceived as intense but relatively small scale phenomena, with little effect on the large scale climate system. However, recent evidence has suggested that hurricanes could play a much more significant role in global climate. Here we prescribe Atlantic hurricanes in a global coupled climate model to show that, climatically, the strong hurricane winds can strengthen the Atlantic meridional overturning circulation (MOC) that is responsible for an increased northward meridonal heat transport (MHT), and the hurricane rainfall tends to weaken the MOC and to reduce the MHT. The net effect of the hurricanes on the MOC and MHT depends on the outcome of these two competing processes. This result implies that hurricanes may indeed play an important role in the coupled climate system and need to be studied further in high resolution global coupled models.
Figure caption: (a) Century mean sea surface temperature anomalies (°C) from the hurricane winds-only experiment (HW) relative to the control run. (b) Same as Figure 2a except for the hurricane winds and precipitation experiment (HWR). (c) Same as Figure 2a except for salinity anomalies (psu). (d) Same as Figure 2b except for salinity anomalies (psu). Stippling indicates the anomalies are significant at greater than the 95% level based on a student t-test.
Washington, W.M., R. Knutti, G.A. Meehl, H. Teng, C. Tebaldi, D. Lawrence, L. Buja, and W.G. Strand, 2009: How much climate change can be avoided by mitigation? Geophys. Res. Lett., 36, L08703, doi:10.1029/2009GL037074.
Figure 6: High resolution figure
Abstract: Avoiding the most serious climate change impacts will require informed policy decisions. This in turn will require information regarding the reduction of greenhouse gas emissions required to stabilize climate in a state not too much warmer than today. A new low emission scenario is simulated in a global climate model to show how some of the impacts from climate change can be averted through mitigation. Compared to a non-intervention reference scenario, emission reductions of about 70% by 2100 are required to prevent roughly half the change in temperature and precipitation that would otherwise occur. By 2100, the resulting stabilized global climate would ensure preservation of considerable Arctic sea ice and permafrost areas. Future heat waves would be 55% less intense, and sea level rise from thermal expansion would be about 57% lower than if a non-mitigation scenario was followed.
Figure caption: (a) Surface temperature and (b) precipitation changes for the end of the 21st century (ensemble average for years 2080–2099) minus a reference period at the end of the 20th century (ensemble average for years 1980–1999) from 20th-century historical simulations with natural and anthropogenic forcings. (bottom) Non-mitigation minus mitigation to show "warming averted" (Figure 2a) and "% precipitation change averted" (Figure 2b).
Meehl, G.A., L. Goddard, J. Murphy, R.J. Stouffer, G. Boer, G. Danabasoglu, K. Dixon, M.A. Giorgetta, A.M. Greene, E. Hawkins, g. Hegerl, D. Karoly, N. Keenlyside, M. Kimoto, B. Kirtman, A. Navarra, R. Pulwarty, D. Smith and T. Stockdale. 2009: Decadal Prediction: Can it be skillful? Bulletin of the American Meteorological Society, 36, L08703, doi:10.1175/2009BAMS2778.1, (early online release).
Figure 7: High resolution figure
Capsule: A new field called "decadal prediction" will use initialized climate models to produce time-evolving predictions of regional climate that will bridge ENSO forecasting and future climate change projections.
Abstract: A new field of study, "decadal prediction", is emerging in climate science. Decadal prediction lies between seasonal/interannual forecasting and longer term climate change projections, and focuses on time-evolving regional climate conditions over the next 10–30 years. Numerous assessments of climate information user needs have identified this timescale as being important to infrastructure planners, water resource managers, and many others. It is central to the information portfolio required to adapt effectively to and through climatic changes. At least three factors influence time-evolving regional climate at the decadal timescale: 1) climate change commitment (further warming as the coupled climate system comes into adjustment with increases of greenhouse gases that have already occurred), 2) external forcing, particularly from future increases of greenhouse gases and recovery of the ozone hole, and 3) internally-generated variability. Some decadal prediction skill has been demonstrated to arise from the first two of these factors, and there is evidence that initialized coupled climate models can capture mechanisms of internally-generated decadal climate variations, thus increasing predictive skill globally and particularly regionally. Several methods have been proposed for initializing global coupled climate models for decadal predictions, all of which involve global time-evolving three-dimensional ocean data, including temperature and salinity. An experimental framework to address decadal predictability/prediction is described in this paper and has been incorporated into the coordinated CMIP5 experiments, some of which will be assessed for the IPCC AR5. These experiments will likely guide work in this emerging field over the next five years.
Figure caption: (An optimal perturbation for the Atlantic domain from the HadCM3 model, using a Linear Inverse Modelling approach (from Hawkins and Sutton 2009b). The panels show integrated temperature (left) and salinity (right) from the surface to a depth of 1800m. The coloured regions indicate where the ocean is sensitive to small anomalies, and are thus the optimal regions for initial condition perturbations, and for targeted observations to improve forecast skill.
Meehl, G.A., J.M. Arblaster, K. Matthes, F. Sassi and H. Van Loon. 2009: Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing. Science, 325, 1114-1118, doi:10.1126/science.1172872.
Figure 8: High resolution figure
Abstract: One of the mysteries regarding Earth's climate system response to variations in solar output is how the relatively small fluctuations of the 11-year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific associated with such solar variability. Two mechanisms, the top-down stratospheric response of ozone to fluctuations of shortwave solar forcing and the bottom-up coupled ocean-atmosphere surface response, are included in versions of three global climate models, with either mechanism acting alone or both acting together. We show that the two mechanisms act together to enhance the climatological off-equatorial tropical precipitation maxima in the Pacific, lower the eastern equatorial Pacific sea surface temperatures during peaks in the 11-year solar cycle, and reduce low-latitude clouds to amplify the solar forcing at the surface.
Figure caption: Composite averages for DJF for peak solar years. (A) Observed SSTs for 11 peak solar years (2) (°C). (B) Same as (A) except for precipitation for three available peak solar years (2) (mm day-1). (C) Same as (A) except for CCSM3 average of five ensemble members for 20th-century climate (19) (°C). (D) Same as (C) except for precipitation (19) (mm day-1). (E) Same as (A) except for WACCM with specified nonvarying SSTs, for 10 peak solar years. (F) Same as (E) except for precipitation (mm day-1). (G) Same as (A) except for WACCM coupled to the dynamical ocean, land, and sea ice components of CCSM3, for 11 peak solar years (°C). (H) Same as (G) except for precipitation (mm day-1). Stippling indicates significance at the 5% level, and dashed lines indicate position of climatological precipitation maxima.