Daily feature: Vast Cooldown of the North Pacific Leads to a Warm December in U.S.

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The heat release into the atmosphere by a vast cooldown of the North Pacific in December contributed to a very warm climate across the U.S. for the month.

North Pacific Cools in December, Heat Release Contributes to Warm U.S. Climate. Event shapes the JAN/FEB/MAR U.S. climate.

Fig. 1: 30-day SSTA change across the North Pacific identifies a vast cool-down. The heat release into the atmosphere contributed to a warm December 2018 climate in the North America.

Discussion: During December the sea surface temperature anomaly (SSTA) change across the North Pacific is vastly cooler including the northern ocean and areas southwest of California to south of Hawaii (Fig. 1). The cooler change is due to several factors lead by widening and deepening eastern Asia snow cover contributing to a colder offshore flow causing cooling of the northern Pacific Ocean. Some of that cooling extends south and southwest of the West Coast of the U.S. entrained the California Ocean Current.

The vast cooling North Pacific over a relatively short amount of time causes a buoyant contribution of heat release into the atmosphere affecting jet stream flow and causing downwind (mild) maritime influence leading to a very warm December 2018 climate across the U.S. (Fig. 2).

Fig. 2: The U.S. temperature anomalies in December 2018 (so far) are historically very warm.

The North Pacific continues to cool (Fig. 3) as wind flow off East/Northeast Asia grows colder following snowstorms and following arctic air. Gather anomalous cool waters in the northern Pacific Ocean foreshadow a prevailing semi-permanent low pressure trough near the Aleutian Islands as forecast by ECMWF for JAN, FEB and MAR 2019 (Fig. 4-6). Downstream implications for winter is a warm upper ridge over western North America and susceptibility to a weak cold trough in the East U.S.

Fig. 3: A cool pool of SSTA is developing in-between the Hawaiian Islands and the Aleutians based on current North Pacific SSTA analysis.

Fig. 4-6: The ECMWF “monthlies” forecast of 500 MB anomalies for JAN, FEB and MAR 2019.