Discussion: Sea surface temperature anomaly (SSTA) patterns are a reaction to internal ocean dynamics and the long-term climate pattern and help to explain current weather and project future climate to 1-3 months. There are many regional SSTA patterns present now affecting in a dramatic way the global climate. A few of the more dramatic signatures are indicated in the Aug. 18 global SSTA analysis provided by the University of Maine.
The most important weather event occurring now is the upgrade to category 5 of Major Hurricane Lane south-southeast of Hawaii forecast to move toward the western Hawaiian Islands late this week (Fig. 1). Previously, Hector passed just south of Hawaii reaching category 4 hurricane intensity. Hector and lane are the 2 strongest hurricanes to reach Hawaii, an island rarely affected by intense tropical cyclones. Causing the approach of these intense storms (and unusually hot mid-summer weather) is the El Nino Modoki-look to the central/east tropical Pacific Ocean whereas very warm water is near and southwest of Hawaii compared to the relatively cooler eastern equatorial Pacific SSTA.
Fig. 1: Satellite view of Category 5 Major Hurricane Lane south-southeast of the Hawaiian Islands.
The remnants of 5 tropical cyclones have passed into waters southwest of Baja California the past 1-2 months. Tropical cyclones up-well the cooler water beneath the surface and have caused the SSTA pattern southwest of California/Baja California too cool significantly mid-to-late summer. This zone has been exceptionally warm in recent years. The anomalous warmth has contributed to a warmer than normal atmosphere above and downwind the warm SSTA region amplifying high pressure ridging and enhancing long-term drought in California. However, the recent cooler SSTA pattern has likely knocked down the hot ridge pattern of JUN/JUL in California and a weak trough (aloft) is forecast much of the remainder of summer across California lowering the risk of additional excessive heat events.
Fig. 2: The 15-day mean 500 MB anomaly pattern across the U.S. forecast by the GFS ENS indicates a trough in the West and ridge in the East U.S.
East of the Northeast U.S. exceptionally warm SSTA will sustain a mean upper ridge pattern well into autumn causing the northeast sector of the U.S. to stay warmer-than-normal (Fig. 2). There is concern that while the deep tropics of the outer North Atlantic are cooler-than-normal and unlikely to allow hurricanes to develop and flourish, the warm SSTA off the U.S. East Coast may lead to tropical cyclone development in September very close to the coastline.
Warm SSTA surrounding Europe certainly inspired the anomalous hot/dry ridge pattern causing historic drought in Central Europe. The SSTA pattern remains very warm and the next 15 days will continue to maintain the mean upper ridge pattern over Eastern Europe to Western Russia (Fig. 3). Note the weak upper trough in the 500 MB anomaly forecast into Central Europe (bringing some needed showers) associated with cooler SSTA just west of Ireland/U.K.
Fig. 3: The 15-day mean 500 MB anomaly pattern across Europe forecast by the GFS ENS indicates the strong upper ridge pattern continues mainly over Eastern Europe to Western Russia.
Elsewhere very warm SSTA east of Japan lead to a heat wave across Japan to close August. Cool waters west of Australia inspire an upper trough the next 15 days across Australia (Fig. 4) that could bring needed rainfall to east and southeast portions of the continent.
Fig. 4: The 15-day mean 500 MB anomaly pattern across Australia forecast by the GFS ENS indicates a persistent cool trough capable of producing some needed rainfall in eastern and southeastern Australia.