Synoptic typing of atmospheric circulation patterns associated with coral reef high water temperature events, Gulf of Eilat (Aqaba), Israel.

Previous research on the energy balance of coral reefs on the Great Barrier Reef (Australia) and the Gulf of Eilat (Aqaba) (GoE, Israel) has shown that synoptic weather patterns that cause light winds, high temperatures and relative humidity, and clear skies increase the risk of high to extreme water temperatures. Here we present results of an automated k-means clustering method that uses ERA5 reanalysis data of mean sea level pressure, relative humidity at 1000 hPa and 850 hPa, winds at 10 m and 500 hPa, air temperature at 2 m and 850 hPa, and atmospheric thickness to identify synoptic weather patterns associated with summer high water temperature (> 29 °C) events for three consecutive days in the GoE. These conditions may trigger thermal coral bleaching events (TCBEs).

Three synoptic circulation patterns (Type 3, 2,1) were identified characterized by the advection of hot and humid air from the eastern Mediterranean toward the GoE beneath a region of atmospheric subsidence. This subsidence contributes further warming while inhibiting cloud development, thereby increasing the potential for strong radiative heating of the coral reefs in the GoE. The three synoptic patterns were found to follow the sequences of Type 3→2→1, Type 3→2 or Type 3→1 for 90% of events, almost always commencing with the Type 3 synoptic circulation pattern. The Type 3 synoptic circulation pattern is characterized by the axis of the Persian Trough running from central Iraq southeast to the United Arab Emirates and Oman. Winds at 10 m are generally light and below 10 ms^-1 from the north and northwest over the eastern Mediterranean and north Africa. Air temperatures at 10 m over land are mostly > 35 °C with terrestrial 1000 hPa relative humidity around 30% or less with higher values in the north and south over Ethiopia, South Sudan and the Central African Republic. 

Identifying the synoptic meteorology of TCBEs provides essential baseline data to monitor change in the weather patterns we identify. These resemble the previously documented “High to west” synoptic pattern. This pattern is linked to several large-scale global circulations that include the descending limb of the meridional Hadley Circulation and the zonal Walker Circulation which is linked to the Indian monsoon. Future warming and its predicted impact on the Hadley Cell and Walker Cell atmospheric circulations may therefore cause change to the synoptic patterns we have identified and their impact of the thermal environment of coral reefs in the GoE.