Sea Surface Temperatures and Extreme Heat and Drought in the United States

H. Wang, S. Schubert, R. Koster, Y.-G. Ham, M. Suarez August 2013

Past studies of observations (e.g. Ting and Wang 1997) and model results (e.g. Schubert et al. 2009) have highlighted the importance of the influence, through teleconnections, that sea surface temperatures (SSTs) have on anomalies of temperature and precipitation in the United States. Extreme heat and drought over much of the U.S. during the warm seasons of 2011 and 2012 afforded the GMAO an opportunity to investigate whether, and to what extent, SSTs in Earth's large ocean basins played a role in forcing the anomalously hot and dry weather experienced.

While the summers of 2011 and 2012 were both characterized by drought and heat waves in the U.S., key differences did exist between the spatial distributions and evolution of heat and dryness of the two years (see Figure 1). An analysis of MERRA and MERRA-Land data reveals that cold conditions across much of the U.S. in early 2011 gave way in summer to abnormally hot conditions over the southern U.S. and northern Mexico, with exceptionally hot conditions in Texas and Oklahoma peaking in June. Many of the southern states also experienced precipitation deficits and decreased soil wetness in the summer, though these appeared to be present in some southern states throughout the preceding winter and spring. In 2012, however, the MERRA data show that winter and spring were anomalously warm over much of the U.S., and the heat persisted throughout the summer, reaching its most intense point in the Central Plains in July. Drought and dry soil were also present in the Central Plains at this time.

figure 1
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Figure 1. MERRA-Land anomalies for temperature (K, left columns), precipitation (mm/day, middle columns), and surface soil moisture (right columns) that show the droughts and heat waves of 2011 and 2012. Anomalies are relative to the 1980 – 2010 climatology. Figure from Wang et al. (2013).

The abnormally hot and dry conditions of the summers of 2011 and 2012 did not occur independent of SST anomalies. According to data from the Hadley Center, both summers were characterized by cold phases of the Pacific Decadal Oscillation (PDO) and warm phases of the Atlantic Multi-decadal Oscillation (AMO), though the PDO was stronger in 2011 and the AMO was stronger in 2012. Cool SSTs over the equatorial Pacific (La Niña) were also present in the months leading up to the summer of 2011, and cool anomalies persisted over the central tropical Pacific in that season. While La Niña conditions were still present during the winter of 2012, they had mostly decayed by the summer of 2012, and the eastern tropical Pacific was beginning to warm.

To investigate whether these SST anomalies acted, in part, to force the droughts and heat waves over the U.S. during the summers of 2011 and 2012, we conducted a series of experiments with our GEOS-5 atmospheric general circulation model (AGCM). In this study, the model, which employs the GMAO Catchment land surface model (LSM, see Koster et al. 2000), was run at a 1° horizontal resolution with 72 vertical levels and forced with prescribed SSTs. The first series of simulations conducted (the "baseline") was a 12-member ensemble run from January 1979 to August 2012 and was forced with observed SSTs; monthly SST and ice fraction data from Hurrell et al. (2008) were used until February 2010, after which point the simulations were forced with the NOAA weekly 1° Optimum Interpolation SST (Reynolds et al. 2002). For the treatment of aerosols, ten ensemble members were run with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model, and the other two were run with parameterized aerosols.

The results from this baseline ensemble can be seen in Figure 2. MERRA-Land and the precipitation used for it generally fall within the model's ensemble spread over the Southern Plains in 2011, the exception being summertime precipitation. This suggests that the model, when forced with SSTs, is capable of reproducing the extreme heat waves and drought observed in summer 2011, though some characteristics of the events, like their intensification, may not be represented with great accuracy. The model was not able to reproduce the heat or drought experienced over the Central Plains in 2012. While model is in good agreement with MERRA-Land in the first months of 2012, large discrepancies are apparent in summer 2012.

figure 2
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Figure 2. Comparisons between MERRA-Land (red line), the ensemble mean (black line) and individual GEOS-5 ensemble members (grey lines) for anomalies of surface temperature (left, K), precipitation (middle, mm/day), and surface soil wetness (right) in the Southern Plains (top) and Central Plains (bottom) for January — August of 2011 and 2012. Figure from Wang et al. (2013).

In addition to the long-term baseline simulations performed, we also conducted a series of experiments with a 20-member ensemble for 2011 and 2012 that were initialized in November of the preceding year with atmospheric and land conditions taken from the baseline simulations. In order to investigate the role that SST anomalies in any given ocean basin may have had on forcing the abnormally hot and dry conditions of the summers of 2011 and 2012, the model was run multiple times using forcing from climatological SSTs (from the period 1980 – 2010), except in select regions, where observed SSTs were used: the tropical Pacific, the North Pacific, the tropical Atlantic, and the North Atlantic. Additional ensemble runs were performed to examine the wintertime response to SST anomalies in the Pacific, Atlantic, and Indian Ocean basins. To construct a useful climatology for the short-term experiments, one model run was conducted for every year from 1980 to 2010 in order to generate a climatology against which experimental results could be compared. The atmospheric and land initial conditions for each of these yearly runs were again taken from the baseline simulations.

Analyses of upper level height anomalies from short-term simulations (not shown) forced with observed SST anomalies revealed a negative North Atlantic Oscillation (NAO)-type response over the North Atlantic during the early months of 2011. Over the Pacific there is a strong tropical response, with weaker and smaller-scale north-south oriented anomalies in the North Pacific/North American region. In the winter of 2012, the height anomalies appeared to be a combination of a positive NAO and Pacific-North American (PNA)-type wave response. Model results from experiments that confined SST anomalies to a single basin at a time (see Figure 3) showed that the SST anomalies in the Pacific could not alone account for the inter-annual differences experienced across the U.S. in the months leading up to the summer 2011/2012 droughts and heat waves. In 2011, SST anomalies in the Atlantic and Indian Oceans acted to confine the warm surface temperatures to the southern Great Plains early in the year. In 2012, SST anomalies in the Indian Ocean appeared to reinforce the warming induced by Pacific SST anomalies while Atlantic SST anomalies played a lesser role.

figure 3
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Figure 3. January – March 2011 and 2012 anomalies of 2 m temperature (K, left) and 250 hPa height (m) for ensembles forced with Pacific SST anomalies (top row), Atlantic SST anomalies (middle row), and Indian Ocean SST anomalies (bottom row). Note the similarity between the two years when only Pacific SST anomalies are considered. Figure from Wang et al. (2013).

Several additional sets of simulations were conducted to examine the impact of regional SST anomalies during spring and summer when the impact of SST is typically fairly modest. In these simulations, the SST anomalies were specified globally (in the control ensemble), or confined to the tropical Pacific, North Pacific, tropical Atlantic, or North Atlantic, with climatological SST elsewhere. In 2011 (Figure 4), results indicate that the abnormally warm temperatures experienced by the southern states were mainly forced by SST anomalies in the tropical Pacific. In the later part of the summer, tropical Atlantic SST anomalies appeared to force warming over the western U.S., and North Atlantic SST anomalies were the main driver of warm surface anomalies over the Central and Northern Plains states. In 2012 (Figure 5), tropical Pacific SST anomalies play a predominant role in defining U.S. temperature anomalies from early 2012 to the early part of summer, whereas the Atlantic takes over in importance for the central and western Plains during the second half of the summer 2012.

figure 4
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Figure 4. 2011 surface temperature anomalies (K) for model runs forced with global SST anomalies, and with all SSTs set to climatological values excepting in specific regions. Figure from Wang et al. (2013).
figure 5
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Figure 5. Same as for Figure 4, but for 2012. Figure from Wang et al. (2013).

Though our GEOS-5 AGCM simulations show that SST anomalies played a role in both of these extreme heat events, SST anomalies alone can't explain the magnitude of the extreme heat and drought, especially in the summer of 2012. Analysis of soil wetness anomalies across the U.S. suggest that the dry conditions of 2011 were not a precursor for the excessive heat and drought in 2012. MERRA-Land data presented in Figure 1 show near normal values for precipitation and soil wetness in the central Plains in early 2012. In fact, GEOS-5 AOGCM forecasts, initialized in May, June, or July, 2012 from MERRA and the GEOS-5 ocean analysis, were unable to fully represent the scope and severity of the dry summer until the forecast initialized in July. This suggests that the drought and soil moisture deficits in 2011 played little to no role in the development of severe precipitation deficits in 2012. There is evidence, however, that soil moisture deficits in 2011 had an impact on 2012 temperature anomalies through soil moisture feedbacks.

The 2011 heat waves and drought across the Southern Plains appear to have been a classic response to cool tropical Pacific SSTs present for much of that year. The 2012 heat waves and drought, however, were not as typical, and some other explanation must be found. There is some evidence that summertime Rossby waves play an important role in the development and evolution of the main precipitation and temperature anomalies over the central Great Plains during summer 2012. Wind and height anomalies from the season (not shown) showed a well-defined wave train propagating eastward from the Pacific with some evidence of phase locking that led to persistent positive height anomalies over the central U.S. in June and July. A second wave train developed during August producing negative height anomalies and negative temperature anomalies that alleviated the central U.S. precipitation deficits in late August. Given this evidence, it appears that the 2012 extreme heat and drought were, in large part, due to internal atmospheric dynamics, though there is some evidence that SST anomalies served to make the large scale environment favorable for the development of phase-locked Rossby waves.

See Schubert et al. (2011) for more information about phase locked waves and heat waves over Eurasia.


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