Strengthening Marine Heat Wave NEP22A Helping to Shape U.S. Pattern

Discussion: Since 2013 a large mass of shallow and deep layer anomalous warm ocean temperatures has oscillated between the Dateline and northeastern Pacific. These warm ocean zones are referred to as marine heatwaves and sometimes the “warm blob”. The latest marine heat wave (NEP22A) is farther west than the 2013-2022 normal located well north and northeast of Hawaii. NEP22A has strengthened dramatically during the past 14 days. To compensate for the dramatic warming north of Hawaii, up-welling north-to-northwest wind has regenerated to cool the West North America coastal waters. In June, waters off the West Coast also warmed above normal. The cooling of the past 2 weeks has likely released latent heat into the atmosphere to inspire the super-hot upper-level high-pressure ridge over the Northwest U.S. this week. However, the cooling waters off the West Coast foreshadow another pattern change ahead. The Pacific North America (PNA) index is forecast to flip to negative phase next week which supports an upper trough over the U.S. West Coast. The cool upper trough is compensated for by an amplified upper ridge over the Midwest U.S. in the 11-15-day period made more intense by arrival of the residual heat from this week’s Northwest U.S. heat wave and the generally very dry soils in the Great Plains.

Fig. 1: Northeast Pacific daily SSTA analysis and recent trend.

Fig. 2: The 2-week SSTA change across the Northeast Pacific.

Fig. 3: The 15-day Pacific North America (PNA) index is forecast to shift to negative phase next week which supports a cooling trough in the Northwest U.S.

Fig. 4: In the 11-15-day period, GFS ENS forecasts an upper trough along the U.S. West Coast compensated for by a very strong upper ridge across the Midwest States.

Fig. 5: The GFS ENS risk of >100F in the 11-15-day period.