Classic -PDO/+AMO Late Summer Drought In Iowa

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Discussion: The tendency for oceans to warm in the middle latitudes separate from the ENSO System has created better-understood relationships/influences on global precipitation patterns. One relationship between the middle latitude East Pacific and North Atlantic sea surface temperature anomaly (SSTA) regimes is warm season rainfall (and attendant drought risk).

Fig. 1: The current global SSTA analysis reveals a developing La Nina, weak cool phase of the Pacific decadal oscillation and moderate warm phase of the Atlantic multidecadal oscillation.

Currently, very warm SSTA is north and northeast of Hawaii while waters off the West Canada coast are much cooler – near normal (Fig. 1). The regional SSTA relationship indicates weak cool phase Pacific decadal oscillation (-PDO) is present. Meanwhile most of the non-tropical North Atlantic basin is somewhat warmer than normal especially the western basin. The SSTA pattern indicates presence of the warm phase of the Atlantic multi-decadal oscillation (+AMO).

Research (Greg McCabe, Julie Betancourt and Michael Palecki) has shown that the -PDO/+AMO regime has a tendency to increase drought risk across the Southwest and Midwest U.S. during the warm season (Fig. 2). The current U.S. soil moisture regime certainly recognizes the -PDO/+AMO regime (Fig. 3).

Positive values are shaded red, and negative values are shaded blue. Darker shades indicate values >0.4 or <–0.4.

Fig. 2-3: Research by McCabe, Betancourt and Palecki has shown a drought risk relationship to the U.S. by the PDO/AMO regimes. Currently, the -PDO/+AMO regime is present and supports increased drought risk for the Southwest and Midwest U.S. which is present based on the current U.S. drought monitor.