Explaining The California Dryness and Why Drought Will Intensify

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Discussion: Large areas of a warmer than normal than normal ocean surface often cause amplified high pressure ridge patterns across the warm water zone and downwind. During the climate change era of the past 2+ decades these sprawling ridge patterns have become more common and are a partial reason for the increase in severe weather events and climate regimes. Beneath the associated high pressure zones dryness and anomalous heat is common while downstream from the amplified ridge a stormy low pressure trough is common causing excessive precipitation.

During 2017 somewhat independent of ENSO a large area of warmer than normal surface water emerged in the southeast North Pacific to the southwest of California. Subsurface temperatures in this region were also dramatically warm (Fig. 2). Signs of another “warm blob” event emerged.

Fig. 2:  Subsurface temperature anomalies across the East Pacific identifying emerging cool in the equatorial region as La Nina approaches and the unusual warmth in the North Pacific subtropics.

The subsequent effect on climate is to cause a large mass of dry and warm air to emerge across and downwind the warm ocean region. During the past 90 days a super-amplified upper ridge of high pressure (Fig. 3) developed over the Southwest U.S. and was a contributor to the record warm autumn of 2017 in the Southwest U.S.

Fig. 3:  500 MB anomalies for the past 90 days (Sep. 8 to Dec. 7, 2017) identify a super-amplified upper level high pressure ridge in the Southwest U.S.

During the past 90 days the Southwest U.S. including southern California have observed rainfall totals at <10% of normal (Fig. 4). The dryness is most harsh near and just southeast of the Los Angeles Basin and also across the southern half of Arizona.

Fig. 4:  Percent of normal precipitation across the U.S. during the past 90 days (through Dec. 8, 2017).

The warm ocean surface temperature pattern southwest of California is forecast to remain intact through quarter 1 of 2018 (Fig. 5). The anomalous warmth southwest of California during a La Nina event is VERY unusual. Normally, waters in this region are cool associated with the cool phase of the Pacific decadal oscillation (-PDO) common during La Nina events. Curiously, this does not happen with the 2017-18 La Nina.

The NMME forecast model indicates the warmth southwest of California weakens but does not dissipate during MAY/JUN/JUL of 2018 (Fig. 6).

Fig. 5:  The NMME forecast model indicates the warm pattern southwest of California continues through JAN/FEB/MAR 2018.

Fig. 6:  The NMME forecast model indicates the warm pattern southwest of California is weaker but still present MAY/JUN/JUL 2018.

Conclusions: Dryness across the Southwest U.S. most intense in southern California and the southern half of Arizona is attributed to a large “warm blob” of surface and subsurface anomalous warm water of the subtropical East Pacific southwest of California. The warm ocean surface pattern in this zone is unusual given the equatorial La Nina episode presence. Normally, a cool feed of the California Current toward the equator to help sustain La Nina is present. Forecast models indicate the warmth in the southeast equatorial Pacific is likely to last well into 2018.  The Southwest U.S. drought condition will worsen.