Thursday, May 21, 2009

Coming soon: Southwest Monsoon, Gulf of Aden and Western Arabian Sea

The monsoon is coming, the monsoon is coming! With which should come a hiatus in Somali pirate attacks because of the sea being too rough and the winds being too high for easy small boat operations.

From a U.S. Navy meteorological tutorial:
Over the Gulf of Aden the Southwest Monsoon usually sets in towards the end of May or early in June, shortly after it has become fully established over the western Arabian Sea. Once established, conditions persist throughout June, July and August. Near the eastern entrance SSW winds prevail and the wind speed increases very rapidly as the entrance is approached from the west. In July typical conditions are: 11-16 kt over the Gulf and eastward to about 52E, becoming 22-27 kt in the area of 52-54E, and further increasing to 28-33 kt in the vicinity of 56-60E. While gale force winds are infrequent in the Gulf, gales of 34 to 40 kt are experienced on about 11 days per month in the 52-54E zone. A marked increase in wave and swell heights are also experienced as one passes eastward out of the Gulf of Aden into the western Arabian Sea.

The region in which the Southwest Monsoon winds are strongest is in a belt running northeastward from about 7N on the African coast, passing close to the eastward of Ras Hafun and Socatra and onward to about 16-18N, 60E. During July in this area winds average 22-33 kt and are greater than 34 kt about 20% of the time. This belt of persistent strong southwesterly flow is referred to as the Somalia Low Level Jet (LLJ). Over the open water of the southwestern Arabian Sea there is very little diurnal variation in winds.

Fog and mist occur frequently along coastal Somalia and Oman during the Southwest Monsoon and may extend some 200 nm offshore. This weather condition is in response to the persistent along shore low level jet southwesterly winds, the induced Ekman Spiral offshore water transport, resulting upwelled subsurface cool water, lowered SST's and, marine boundary layer saturation and fog/stratus development. This pattern of wind, upwelling, cold SST's and low clouds and fog is similar to that found off the west coast of the US during the summer season when the dominant eastern Pacific high results in persistent northwesterly winds along that coast.

Opposite the mouth of the Gulf of Aden there are some minor diurnal wind variations. To the north of Ras Asir, including the Socotra Island area, the maximum is at midnight and the minimum is in the early afternoon, while to the east and south of Socotra there is a maximum in the early afternoon and minimum in the early morning. As is typical over the open seas, the speed range of the diurnal variation is relatively small being on the order of 5 kt or less.

The diurnal variation of wind speed over the open Gulf of Aden is similar in timing to that east of Socotra, but the speed range is larger. The midday maximum tends to more than double the 2000 LST (1700Z) minimum. The diurnal variation is larger yet on both the northern and southern shores and over the near coastal waters.

The "Somali Jet" is a relatively narrow wind stream along the East African Coast and

is part of the larger Southwest Monsoon circulation pattern. The Somali Jet is one of the strongest and most sustained low-level wind systems on earth. It is normally strongest in July and August when core maximum speeds up to 100 kt have been observed. The core is usually centered at an elevation of about 5000 ft. Figure 1 shows a typical July cloud pattern over the Arabian Sea during a moderately strong Southwest Monsoon situation. A relatively intense Somali Jet would be expected under this situation. The low-level wind speed maximum just east of Socotra Island usually appears as a nearly cloud free area bounded on the north and east by diverging cloud lines. Figure 2 shows a monthly mean airflow chart at 3500 ft for July. Notice the three local speed maxima, north of Madagasgar, off the coast of Kenya, and to the east of Socotra Island. These are semi- permanent low-level wind features during the Southwest Monsoon.
SST=Sea Surface Temperature. LST= Local Standard Time

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