Will A Returning Warm Phase Pacific Decadal Oscillation Contribute to U.S. Summertime Drought?

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Discussion: Every now and then the (climate) research community produces information and related schematics that carry a lot of (market) weight. One example is research done by Gregory McCabe (U.S. Geological Survey), Michael Palecki (Midwest Regional Climate Center) and Julio Betancourt (U.S. Geological Survey) regarding influences on U.S. multi-decadal drought frequency caused by the Pacific decadal oscillation (PDO) and Atlantic multi-decadal oscillation (AMO). Their research was published in 2004 and identified the influence of the Pacific and Atlantic Ocean(s) sea surface temperature anomaly (SSTA) patterns outside of El Nino southern oscillation (ENSO) on U.S. drought frequency. “More than half (52%) of the spatial and temporal variance in multi-decadal drought frequency over the conterminous United States is attributable to the PDO and AMO.”

Industry is increasingly realizing that ENSO is not the be-all-end-all toward predicting seasonal climate. Most climate scientists, meteorologists and industry have noticed ENSO is a less reliable predictor of climate over the past 1-2 decades. McCabe, Palecki and Betancourt have made a major contribution to realizing other reliable climate diagnostics to make better climate forecasts especially for high impact climate regimes such as drought.

Their research produced a very popular schematic which may apply to the warm season 2018 climate pattern in the U.S. (Fig. 1). When ENSO is weak as forecast for the middle third of 2018 the influence of the PDO and AMO is quite strong on the U.S. regarding drought frequency. There is a tendency for the U.S. to observe diminished drought risk when the North Atlantic is cooler than normal (-AMO) regardless of the Pacific (PDO) regime. Conversely, drought risk increases when the North Atlantic is warmer than normal (+AMO). The PDO Phase (combined with AMO) determines whether that drought risk is biased toward the northern or southern U.S.

Fig. 1: Drought risk increases (red) or decreases (blue) depending on the combination of Atlantic multi-decadal oscillation and Pacific decadal oscillation phase.

Nathan Mantua of the NOAA Southwest Fisheries Science Center and a leading expert on the Pacific decadal oscillation issued the January index today which was +0.70 qualifying as warm phase of the PDO. The warm phase seems to be returning after a brief period of neutral phase late last year following one of the strongest 3-to-4 year warm cycles of PDO on record (Fig. 2).

Fig. 2: The 2014 to present monthly Pacific decadal oscillation index provided by Nathan Mantua of the NOAA Southwest Fisheries Science Center.

Meanwhile we await the official AMO value for January. In 2017 the AMO was very warm (Fig. 3), characteristic of the decadal warm cycle present since the late 1990’s. Warm AMO in 2017 was a leading contributor to the robust hurricane season.

Fig. 3: The monthly Atlantic multi-decadal oscillation index for 2017 provided by NOAA.

According to the NMME forecast model, robust warm PDO and AMO signatures are likely for JUN/JUL/AUG 2018 (Fig. 4). If so, the +PDO/+AMO climatology may apply to the warm season climate identifying increased drought risk for the Northwest U.S., Great Plains and Southeast U.S. (Fig. 5).

Fig. 4: The NMME global SSTA forecast for JUN/JUL/AUG 2018 projects a warm North Pacific/North Atlantic.

Fig. 5: Warm PDO and AMO are likely during the warm season in 2018. Based on research by McCabe, Palecki and Betancourt the climatological risk of drought increases in the Northwest U.S., Great Plains and Southeast given the projected PDO/AMO regime.