Fig. 1-4: 500 MB anomalies across Europe/North Africa for Sep. 6-10, 2023 and the daily Mediterranean Sea SSTA analysis plus 14-day change and global SSTA for August 2023 identifying the Northwest Africa marine heatwave which advanced into the Mediterranean Sea during late summer.
Discussion: During MAR/APR 2023 a large and rapidly intensifying marine heatwave (MHW) formed off the Northwest Africa Coast. During MAY, the MHW expanded northward into West/Southwest Europe. In JUL/AUG, the impressive anomalous warmth expanded across the Mediterranean Sea.
Typical of vast areas of much warmer than normal oceans in the mid-latitudes, a semipermanent upper level high-pressure formed across and upwind the warm SSTA region. In this case, the summer 2023 semi-permanent upper-level high-pressure ridge shifted from Northwest Africa to Southern Europe and to the Black Sea region since June. Compensating for the amplified upper ridge is a deep low-pressure trough in the east-central Mediterranean Sea. Coupled with the warming Mediterranean Sea associated with the Northwest Africa MHW, inevitable was the entrainment by a cold upper trough of above normal low atmospheric water vapor associated with the warmer than normal sea surface to cause an explosive rainstorm.
This time around, a classic omega high-pressure block formed over Germany keeping potent upper troughs in-place west of Portugal and the heavy rainmaker (trough) just north of Libya (Fig. 1). The omega high-pressure block caused 5-day temperature anomalies >7C from U.K. across France and through Germany to Western Poland while excessive rains associated with the upper trough stretched from Northern Libya to Southern Italy to Turkey.
The daily SSTA analysis across the Mediterranean Sea (Fig. 2) reveals the cooling of the ocean centered on the past 1-2 weeks (Fig. 3) and the implied release of latent heat into the atmosphere to cause an excessive rainstorm. The global SSTA for August 2023 identifies the extent of the widening Northwest Africa MHW during late northern hemisphere summer (Fig. 4).