North American 2008 Cooling Attributed to Natural Causes



Oceans and Drought

Martin Hoerling

NOAA Earth System Research Laboratory, Boulder Colorado

SUMMARY

Three features of the relationship between oceans and drought are highlighted. The first is the link between the seasonal pulse of monsoons and the seasonality of sea surface temperatures (SST). It is shown that the north-south migration of monsoon rains is strongly tied to the ocean’s response to the annual march of the Sun. The second is the link between interannual-to-decadal droughts and anomalous ocean states. It is shown that SST variability enhances drought variability over land areas within 40°N-40°S. The effect of ocean states on drought is demonstrated for select historical events, and the ocean observing system needs for drought early warning are discussed. A third feature concerns the link between the oceans and drought in a warming climate. Ocean temperatures have risen over the warmest portions of the tropical oceans, likely due to the effect of increasing greenhouse gases. The effect of such sea surface temperature trends on regional rainfall trends, including recent droughts over southwest North America and Australia, are discussed.

Earth’s Dry and Wet Seasons

The annually recurring dry and wet seasons represent the most dramatic pulse of Earth’s climate. Notable are the tropical monsoons with wet phases developing in tandem with the poleward migration of solar radiation. The global pattern of boreal summer minus winter rainfall captures the essence of this pulse, with wet conditions prevailing over the Sahel, India, Southeast Asia, and Central America, while southern Africa, northern Australia and the Amazon simultaneously experience their dry phases (Fig. 1).

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Figure 1. The seasonal cycle of rainfall (July/Aug/Sept- Jan/Feb/March) based on simulations of a climate model using realistic observed seasonally varying sea surface temperatures. Green (red) indicates regions of wetter (drier) boreal summer (winter) climate.

The ocean’s response to the march of the seasons is a key factor driving the monsoons. During boreal summer, the NH oceans warm compared to the SH oceans, and monsoon rains develop over the NH continents. The seasonal cycle of rainfall simulated in a climate model in which specified sea surface temperatures have no seasonal cycle is compared to that occurring in a parallel run using realistic seasonally varying SSTs (Fig. 2). A large component of the monsoon rainfall cycle is attributable to the seasonal cycle of SSTs according to these experiments (cf. Fig. 1 and 2). These experiments further indicate that the oceans play a major role in the monsoon’s pulse, and that the annually recurring wet and dry seasons and not mere slaves to the seasonal cycle of the warming and cooling of land temperatures.

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Figure 2. The difference in seasonal cycle of rainfall (July/Aug/Sept- Jan/Feb/March) between simulations of a climate model using realistic observed seasonally varying sea surface temperatures and parallel simulations in which SSTs have no seasonal cycle.. Green (red) indicates regions where the SST seasonal cycle drives a wetter (drier) boreal summer (winter) climate compared to runs having no SST seasonal cycle.

Interannual and Decadal Droughts

Interannual to decadal drought events, particularly over tropical and subtropical latitudes, are sensitive to anomalous sea surface temperatures. Climate model simulations indicate that the effect of SST variability (i.e., fluctuations relative to the climatological seasonal cycle) significantly enhance the variance of precipitation, and thus contribute to the occurrence of severe droughts. For instance, precipitation variance is more than doubled in such regions as the Maritime Continent, equatorial South America and portions of India (Fig. 3). These are among the regions that have particularly strong sensitivity to the El Niño/Southern Oscillation (ENSO) phenomenon. Likewise, many other continental regions between 40°N-40°S experience an increase in precipitation variability on the order of 10%-25% due to the SST variability.

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Figure 3. The ratio of variance of annual standardized precipitation in a climate simulation forced with observed interannual SST variations of 1901-2000 to that occurring in the same model using climatological seasonally varying SSTs only. Colored areas indicate where precipitation variance is enhanced, with red shades denoting a more than a doubling of variance.

In light of the increased rainfall variance resulting from sea surface temperature variability, a key question is whether ocean observing systems can harvest the potential predictability implied in Fig. 3. Towards addressing this question we consider several major droughts and explore the ocean’s role more specifically.

Two iconic U.S. drought events of the last century are the Dust Bowl during the 1930s and the comparably severe and sustained drought during the 1950s. These affected different geographical regions (Fig. 4), and the question is whether early warning capability for such droughts now exists given current ocean observing and prediction systems (e.g., such as TAO/TRITON and ENSO prediction). [pic]

Figure 4. The observed standardized precipitation index (SPI) averaged for 1932-1939 (left) and for 1946-1956 (right). Red (blue) shaded regions indicate reduced (increased) precipitation during these epochs. Note that the Great Plains droughts of the 1930s and 1950s occurred over different geographical regions.

There is compelling evidence that ocean observations over the tropical Pacific and associated predictions of ENSO can forewarn of southern U.S. Plains drought, such as occurred during the 1950s This is indicated by the high statistical correlation between an index of annual precipitation over the southern Plains and annual sea surface temperatures during the 20th Century (Fig. 5, right panel). Yet, that same statistical analysis fails to indicate a coherent relation between drought over the Northern Plains, the epicenter for 1930s drought, and the state of global ocean temperatures (Fig. 5, left panel).

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Figure 5. The observed correlation between annual sea surface temperatures and the standardized precipitation index (SPI) averaged over the Northern Plains (left) and the Southern Plains (right) during 1895-2007. Geographic regions are shown by gray hatching. Note that Southern Plains drought correlates with cold phases of ENSO, but that there is no appreciable correlation of tropical SSTs with Northern Plains drought.

The correlations themselves are not proof (or lack thereof) that ocean conditions caused the major 20th Century US droughts. A common strategy to test the efficacy of the ocean’s role, and to verify a cause-effect link, is to force atmospheric climate models with the observed SSTs that occurred during the drought period. Such experiments conducted using SSTs spanning the 1930s and 1950s are unable to generate the extensive dry conditions over the central and northern Plains of the 1930s, but yield a realistic simulation of the 1950s drought pattern (Fig. 6). The Southern Great Plains thus resides within an epicenter of potentially predictable drought for which an ocean observing system would appear to also constitute a drought early warning system.

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Figure 6. The simulated standardized precipitation index (SPI) averaged for 1932-1939 (left) and for 1946-1956 (right). Red (blue) shaded regions indicate reduced (increased) precipitation during these epochs. Results are based on a 40-run average of atmospheric GCM simulations using three different models. Note the very similar patterns of Great Plains droughts of the 1930s and 1950s occurring in the GCM simulations.

As another case study, Sahelian drought events of seasonal duration often occur in concert with anomalous sea surface temperature conditions. Yet, the prolonged multi-decadal drying of the Sahel that commenced in the 1970s and persisted until the mid-1990s raises new questions whether long-lived ocean conditions can likewise be important factors. Can the existing global ocean observing system forewarn of protracted Sahelian drought in the future?

Statistical analysis of the relation between Sahel rainfall and an index of the interhemispheric contrast in SSTs reveals a strong relation on annual to decadal time scales. The prolonged drying of the Sahel occurred in unison with a cooling of NH SSTs compared to the SH SSTs, and subsequent climate model simulations using observed SSTs have confirmed a causal link. In particular, a prolonged drying trend of the Sahel during the 1950-1999 period was forced by cooling of the subtropical North Atlantic together with a warming of the Indian Ocean. An open question is the predictability of such ocean conditions, and whether existing ocean observations are adequate to meet the requirements to harvest predictability.

Drought in a Warming Climate

According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, surface temperatures averaged over the NH during the second half of the 20th Century were very likely higher than during any other 50-year period since 1500. Most of the warming of both land and sea surface temperatures during the last half-century is very likely the consequence of human-induced emissions of greenhouse gases and anthropogenic aerosols. Since 1977, virtually all oceans have warmed, with the notable exception of the tropical and subtropical east Pacific (Fig. 7, top). It is in this latter region where the simulated SST change due to GHG and aerosol forcing deviates most appreciably from observations.

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Figure 7. The 1977-2006 trend in annual observed sea surface temperatures (top) and CMIP3 simulated sea surface temperatures. Trend toward warmer (colder) SSTs are indicated by red (blue) shades. The difference in 30-yr trends (OBS minus CMIP) is shown in the lower panel. Simulations are based on the average of 21 CMIP3 models that were forced by the observed GHG and aerosol variations from 1977-1999, and by the SRES A1B emissions scenario for 2000-2006.

A key challenge in the emerging area of decadal prediction is to understand the relationship between slow and sustained changes in oceans and regional climates. One example was already discussed regarding the ocean’s role in Sahel drying. To what extent are trends in observed SSTs since 1977 contributing to regional trends in rainfall, and to what extent is that relationship indicative of an anthropogenic influence?

Most notable have been the drying trends during 1977-2006 observed over the Maury Darling Basin and over a broad region of the eastern Pacific and adjacent continents including southwestern North America (Fig, 8, top). The latter features are consistent with an influence of the oceans, as revealed by the results of atmospheric GCM simulations forced by the observed SSTs (Fig. 8, middle). However, this ocean influence is evidently not a feature of human-induced climate change, in so far as coupled ocean-atmosphere GCMs forced by GHG and aerosols fail to generate this wide spread-dryness.

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Figure 8. The observed (top), AMIP ensemble mean simulated (middle), and CMIP ensemble mean simulated (bottom) 30-year trends in annual precipitation during 1977-2006. AMIP simulations are based on an ensemble of 36 integrations of 4 different atmospheric models forced by the observed monthly varying SSTs, but using climatological GHGs. CMIP simulations are based on the average of 21 CMIP3 models that were forced by the observed GHG and aerosol variations from 1977-1999, and by the SRES A1B emissions scenario for 2000-2006.

Oceans Observations and Drought

Drought as a natural hazard often has severe, and long lasting societal impacts. This presentation has given several examples of the impact of oceans on drought. It has attempted to show where ocean information and related observing systems could be used for societal benefit relating to early warning for and possible mitigation of drought impacts. It is not an exhaustive survey of the potential benefits of ocean information, and the talk focused solely on the sea surface temperature information.

It was shown that ocean observations in the tropical east Pacific, information that is vital to support predictions of ENSO, are tantamount to drought early warning over vast areas of the tropics and select midlatitude regions. Regarding the latter, the talk emphasized that ocean information related to ENSO would provide early warning for drought over the U.S. Southern Great Plains, although there was less indication that ocean information, nor skillful predictions of SSTs, would serve to provide advance warning for drought in the U.S. Central to Northern Plains, a region devastated by drought during the 1930s.

Sahel drought of multi-decadal duration in the latter half of the 20th Century is now known to have been strongly forced by global ocean conditions. Research is only beginning to explore the capability to predict the relevant ocean states and their impacts on such long time scales. Regardless of the outcome of the prediction enterprise, there is considerable societal benefit in knowing the causes for drought conditions. A particular example was given of the trends in rainfall during 1977-2006, and the evidence that drying over the tropical Pacific and the adjacent Americas has resulted in part from sea surface temperature trends, though ones apparently unrelated to the anthropogenic forcing of the oceans.

Climate-quality ocean observations are essential for distinguishing natural from human-induced ocean conditions, and therefore are key in the accurate attribution of the causes of climate trends. Of particular importance is ensuring that natural variability, when occurring, is not misunderstood to indicate that climate change is either not happening or is happening more intensely than the true human influence.

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