Persian Gulf Response to a Wintertime Shamal Wind Event

The results from a approx. 1 km resolution HYbrid Coordinate Ocean Model HYCOM, forced by 12 deg Navy Operational Global Atmospheric Prediction System NOGAPS atmospheric data, were used in order to study the dynamic response of the Persian Gulf to wintertime shamal forcing. Shamal winds are strong northwesterly winds that occur in the Persian Gulf area behind southeast moving cold fronts. The period from 20 November to 5 December 2004 included a well defined shamal event that lasted 4-5 days. In addition to strong winds 16 m s-1 the winter shamal also brought cold dry air Taualpha20 dec C, qalpha10 g kg-1 which led to a net heat loss in excess of 1000 W m-2 by increasing the latent heat flux. This resulted in SST cooling of up to 10 deg C most notably in the northern and shallower shelf regions. A sensitivity experiment with a constant specific humidity of qalpha15 g kg-1 confirmed that about 38 of net heat loss was due to the air-sea humidity differences. The time integral of SST cooling closely followed the air-sea heat loss, indicating an approximate one-dimensional vertical heat balance. It was found that the shamal induced convective vertical mixing provided a direct mechanism for the erosion of stratification and deepening of the mixed layer by 30 m. The strong wind not only strengthened the circulation in the entire Persian Gulf but also established a northwestward flowing Iranian Coastal Current ICC, 25-30 cm s-1 from the Strait of Hormuz to about 52 deg E, where it veered offshore. The strongest negative sea level of 25-40 cm was generated in the northernmost portion of the Gulf while the wind setup against the coast of the United Arab Emirates established a positive sea level of 15-30 cm. The transport through the Strait of Hormuz aat 56.2 deg E indicated an enhanced outflow of 0.25 Sv Sv 106 m3 s-1 during 24 November followed by an equivalent inflow on the next day.