Executive Summary: For the first times since 2012, the cool “horse shoe” SSTA pattern stretching from the Gulf of Alaska to just-off the West Coast of North America and southwestward to the south of Hawaii toward the Dateline in the Pacific tropics has emerged. Presence of the cool “horse shoe” SSTA pattern coupled with La Nina normally increases the risk and amplitude of La Nina climate signals which includes heavy precipitation during NOV/DEC/JAN in the Northwest U.S., dryness leading to drought in the Mid-south States and a general warmer-than-normal national pattern. Projections beyond one season ahead are much less certain although notable is the 2012 U.S. climate (when the last “horse shoe” pattern was in-place while a long-term La Nina was slowly fading) which at that time was the warmest on record and featured historic Great plains drought.
Fig. 1: The Northeast Pacific “horse shoe” SSTA pattern is emerging.
Discussion: For the first time since 2012, a cool SSTA pattern in the Gulf of Alaska, most of the U.S. West Coast and southwestward toward the central tropical Pacific Ocean has emerged (Fig. 1). Commonly referred to as the North Pacific “horse shoe” pattern the presence of which when combined with cold ENSO causes an enhanced La Nina climate pattern. Since 2013 the Northeast Pacific has encountered the “warm blob” SSTA pattern preventing the Gulf of Alaska and off the West Coast of the U.S. from any significant cooling. The “warm blob” is still present but has shifted westward to north of Hawaii. The cool “horse shoe” pattern is also characteristic of a fortified cool phase of the Pacific decadal oscillation (PDO).
Implications: Long-term implications of the Northeast Pacific “horse shoe” SSTA pattern (coupled with La Nina) are not consistent. Although, the last time this climate pattern was in-place during 2012 the U.S. experienced the warmest year on record and historic Central U.S. drought. Researchers caution analysts on using more reliable season ahead projections based on the presence of this pattern. Based on NOAA/CPC La Nina climatology for NOV/DEC/JAN the risk of the anomalies indicated increases and the possibility of a stronger signature than indicated (due to -PDO/La Nina combination) also increases. The sensible risks include: Increased chance of a warmer than normal climate for much of the U.S. biased across the southern tier, heavy precipitation including above normal snow in the Northwest while the Mid-south States can be unusually dry. The northeast quadrant of the U.S. has the lowest climate correlation.
Fig. 2-4: NOAA/CPC La Nina climatology for NOV/DEC/JAN which when combined with the cool horse shoe pattern (-PDO) is not only more likely to occur but can be more extreme.
Conclusions: Forecast models have under-estimated the emergence and early intensity of the Northeast Pacific cool “horse shoe” pattern characteristic of the presence of the cool phase of the PDO. Since 2013, the “horse shoe” pattern has not occurred with La Nina events due to the emergence of the “warm blob” of SSTA in the northeast Pacific. The “warm blob” is still present but shifted westward to the north of Hawaii. Presence of the cool “horse shoe” SSTA pattern combined with La Nina should enhance a La Nina climate pattern for NOV/DEC/JAN. Included is a possible super-wet (and snowy) Northwest U.S., emergence of Mid-south U.S. drought and a warmer than normal late autumn to mid-winter. Not known, is the influence of the “warm blob” north of Hawaii on the cool “horse shoe” pattern to the east. Forecast models have a tendency to downplay the “horse shoe” pattern and emphasize the “warm blob” which would match the 2013-2020 climatology. However, forecast models did not predict the cool “horse shoe” pattern currently in-place.