Fig. 1-2: Comparing the Pacific decadal oscillation and Atlantic multi-decadal oscillation for the current 2020-22 regime to 2010-12. Long-tern cool PDO/warm AMO historically increase the risk of drought in the Southwest and Midwest U.S.
Discussion: The May 2022 Pacific decadal oscillation index (PDO) was released yesterday by Nate Mantua at the NOAA Southwest Fisheries Science Center located in Santa Cruz, CA. The result was a strengthening (already strong) cool phase signature at -1.92. The -PDO regime identifies cool SSTA on the northeast Pacific Basin while waters north of Hawaii are warmer than normal. The cooler waters off the West Coast of North America are correlated to below normal low-level atmospheric moisture limiting precipitation in the West U.S. often leading to drought as observed in mid-2022. The -PDO regime is about to enter a 3rd year dating back to late spring 2020 and is remarkably similar to the -PDO regime observed during 2010-12 (Fig. 1). Of course, the summer of 2012 featured a historic widespread U.S. drought.
The Atlantic multi-decadal oscillation (AMO) identifies the SSTA pattern across the North Atlantic basin. Since the late 1990’s, AMO has entered a long-term warm cycle. Most of the past almost 25 years has featured a warmer than normal AMO regime. The 2020-22 AMO regime has flirted with the warm phase similar to the 2010-12 pattern.
Frequently referred to research identifying PDO/AMO regimes combined influence on U.S. drought risk was published by McCabe, Palecki and Betancourt in 2004. The influence of long-term -PDO/+AMO regimes on U.S. drought risk was elevated for the Southwest and West-central U.S. plus Texas and also the Midwest States (Fig. 2). The Southwest U.S. to Texas drought is already locked-in. However, the Midwest U.S. has avoided drought so far.
The Climate Impact Company JUL/AUG/SEP forecast indicates the Midwest States gain a dry climate (Fig. 3) and at least a D1/D2 drought is likely to develop in that region during mid-to-late summer. Earlier today NOAA issued a seasonal drought outlook that now forecasts drought in parts of the Midwest U.S. (Fig. 4).
Fig. 3-4: The Climate Impact Company JUL/AUG/SEP 2022 precipitation anomaly forecast and the just-released NOAA seasonal drought outlook.