Fig: 1-4: The last two full-tilt El Nino summer climate patterns across the U.S.
Discussion: The last two El Nino summer seasons in the U.S. were in 2015 and 1997. Each El Nino rivaled the 1982-83 El Nino for the strongest on record. During summer 2015, the prevailing thermal climate was extremely hot in the Northwest and hotter than normal for California, the Southern U.S. into the Carolinas (Fig. 1). A moderately cool summer was observed across the Midwest States. The precipitation pattern featured excessive rains in the Midwest States (Fig. 2). The 2015 El Nino was influenced by a marine heat wave in the northeastern Pacific Ocean which caused an amplified upper ridge propelling the Northwest U.S. heat and downstream upper trough bringing heavy rain to the Ohio Valley and vicinity.
The 1997 summer El Nino brought widespread cool climate to the Central and East U.S. with limited anomalous warmth on the immediate West Coast and Gulf Coast (Fig. 3). The precipitation pattern featured streaks of heavy rain through the southern Great Plains to the Tennessee Valley and parts of the Upper Midwest to Ohio and dry in-between (Fig. 4). The 1997 El Nino was also accompanied by anomalous warm SST in the Northeast Pacific although closer to the coast and not nearly as widespread as 2015.
The Climate Impact Company ENSO forecast indicates a weak El Nino ahead for meteorological summer 2023. However, the subsurface equatorial Pacific is warming rapidly (Fig. 5) and the aggressive El Nino forecasts by dynamic models (Fig. 6) are slowly gaining credibility.
Currently, a large marine heat wave is located north and northeast of Hawaii. The marine heat wave is forecast to shift slightly eastward and not reach the West Coast during August when ECMWF projects presence of a strong El Nino. Based on Climate Impact Company ongoing studies of marine heat waves (to be presented at GRAINCOM in Geneva, Switzerland next month), the MHW will need to reach the West Coast of North America to produce the strong El Nino indicated by most forecast models. If so, MWH NEP22A would likely propel a summer climate similar to the 2015 analog.
Fig: 5: Suddenly, the subsurface eastern equatorial Pacific has warmed dramatically.
Fig: 6: ECMWF global SSTA forecast for August 2023 identifying a strong El Nino.