Introduction to Major Decadal Climate Variability Phenomena Part I: The Tropical Atlantic Gradient Variability and the Pacific Decadal Oscillation

Following the general background on decadal climate variability (DCV) in the Autumn 2007 issue of DroughtScape, we now look at some major DCV phenomena, hypothesized causes, and known impacts on global climate and society. In this issue, two DCV phenomena are described: the tropical Atlantic sea-surface temperature gradient variability (TAG hereafter for brevity) and the Pacific Decadal Oscillation (PDO).

Figure 1a: The dominant pattern of tropical-subtropical Atlantic sea-surface temperature (SST) variability between 1881 and 1990 shows positive (solid line) and negative (dashed line) SST anomalies  in North and South Atlantic, with maximum variability at 15ºN and 15ºS.

Research on the TAG dates back to the 1960s, when researchers first found associations between variations in the TAG pattern and rainfall variability in northeast Brazil and west Africa. Since then, as more and better ocean and atmosphere observations have become available, it has been found that variability of many atmosphere and ocean variables  are associated with the SST variability shown in Figures 1a and 1b, such as winds in the lower troposphere, heat transferred between the Atlantic Ocean and the overlying atmosphere, cloudiness, rainfall in northeast Brazil and west Africa, Atlantic hurricanes, and water vapor influx and rainfall in the southern, central, and midwestern U.S.

Sir Gilbert Walker of the India Meteorological Department first discovered a phenomenon he termed the North Pacific Oscillation (NPO) in the late 1920s. Sir Gilbert wanted to find precursor signals to predict the Indian monsoon rainfall and the NPO was an atmospheric pressure seesaw he found during his studies using worldwide atmospheric pressure measurements. Subsequently, when long-term SST data in the Pacific Ocean became available in the 1990s, a number of researchers found that the dominant pattern of SST variability in the extratropical Pacific varied at time scales of one or more decades and that this SST pattern corresponded to the NPO in the atmosphere. This SST pattern, shown in Figure 2a, is called the PDO and the time series modulating this SST pattern is shown in Figure 2b.    

Figure 1b: The 110-year-long SST time series (bars) modulating this pattern shows decadal-to-multidecadal variability. Rainfall in northeast Brazil (solid line) shows opposite phase variability with respect to the SST time series.

Among the phenomena associated with the PDO are winds in the lower troposphere, heat transferred between the Pacific Ocean and the overlying atmosphere, cloudiness, Pacific typhoons, and droughts and floods in the western U.S. and the Missouri River Basin. Major changes in northeast Pacific marine ecosystems have been correlated with phase changes in the PDO; warm eras have seen enhanced coastal ocean biological productivity in Alaska and inhibited productivity off the west coast of the contiguous United States, while cold PDO eras have seen the opposite north-south pattern of marine ecosystem productivity.

Scientists hypothesize that the principal cause of the TAG and the PDO is the variability of heat transported by currents and slow-moving waves in the Atlantic and Pacific Oceans, as a result of their interactions with the atmosphere. Both these phenomena are associated with decadal droughts, floods, and associated variability of crop yields in the Missouri River Basin.

Figure 2a: SST departures from average conditions in the PDO in the positive phase (left) and the negative phase (right). Courtesy Nathan Mantua and Stephen Hare, University of Washington.

Figure 2b: Time series of SST monthly departures associated with the PDO. Courtesy Nathan Mantua and Stephen Hare, University of Washington.

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