Summer sea breeze, under suppressive synoptic forcing, in a hyper-arid city: Eilat, Israel

Abstract

Eilat is a hyper-arid city, located at the northern end of the Gulf of Eilat. The summer weather conditions over the region are highly persistent, with offshore northerly winds of 5 to 7 m s -1 and a daily maximum temperature of 39 to 40°C. The relatively cool (~26°C) sea surface temperature (SST) to the south suggests that a southerly sea breeze would exist in spite of the opposing prevail- ing northerly winds and the small scale of the sea (10 to 20 km width). A 6 yr study (1994 to 1999) indicates that the sea breeze develops in only 25% of the days in July and August. The breeze typi- cally lasts for about 4 to 6 h, with wind speeds of only 1 to 3 m s -1 . A synoptic analysis indicates that the prevailing pattern, the Persian Trough, exists on 99% of the days, but on the 'breeze' days the synoptic-scale suppressive pressure gradient was 38% weaker. In 4 cases for which the synoptic scale gradient was negligible, the speed of the sea breeze reached 6 to 7.2 m s -1 , indicating the effec- tiveness of the Gulf of Eilat as a source of sea breeze. A seasonal decrease of 49% found in breeze occurrence between July and August is explained by both a seasonal increase in the pressure gradi- ent and a decrease in solar radiation. The daily maximum temperature on the 'breeze' days was 1.2°C lower and the minimum relative humidity was 4% higher, whereas the heat stress regime did not dif- fer significantly. An increase in the wet bulb temperature that was found at the same time implies that the efficiency of evaporative cooling devices is reduced when sea breeze exists. Our study sug- gests that in coastal areas subjected to hot and dry conditions, where the prevailing synoptic winds oppose the sea breeze, the effectiveness of the sea breeze depends strongly on the inter-diurnal vari- ations of the synoptic pressure gradient.