NRL Monterey, Marine Meteorology Division
|Fig. A. Topographical Map of the Northern Arabian Sea Region||Fig. B. Gulf of Aden|
|WESTERN ARABIAN SEA SOMALIA LOW LEVEL JET|
The Introduction materials presented in this tutorial are duplicated from the "GulfAdenSWMonsoon" tutorial. The material is repeated here to allow for stand-alone use. Satellite images and numerical model data are presented in the Example Sections to illustrate various issues and aspects of the Southwest Monsoon regime.
GULF OF ADEN AND WESTERN ARABIAN SEA:
The following weather pattern discussions and data are in part extracted from "Part 2 The Gulf of Aden and West Arabian Sea to Longitude 60E", NAVENVPREDRSCHFAC TB 80-02 of " Volume II" of "Weather in the Indian Ocean to Latitude 30S and Longitude 95E including the Red Sea and Persian Gulf", originally published by the Meteorological Office, Air Ministry, London, England, 1940-44. Present day satellite data and, NOGAPS and COAMPS forecast data are used in a series of tutorials to illustrate these weather pattern discussions.
The area of the Gulf of Aden (time zone - 3) and western Arabian Sea (time zones -3 and -4) addressed here extends from near the equator to 20N, and from 43E to 60E. The Gulf of Aden is located between 10-15N and 43-52E. It connects to the Red Sea via Bab el Mandeb Strait on the western end and opens to the western Arabian Sea north of Ras Asir (Cape Guardafin) on the eastern end. Socatra Island is located about 120 nm east-northeast of Ras Asir. The Gulf is approximately 480 nm in length and varies in width from 13 nm at Bab el Mandeb to about 174 nm at Ras Asir. The Gulf is bordered by 6000 ft mountains with peaks to 10000 ft on the north, while along the southern shore a coastal plain, narrowing from west to east, is backed by 6000 ft plus mountains and high plateaus. Significant ports and coastal locations of interest include Aden on the northwest coast, Djibouti at the western end, and Berbera on the southwest coast.
The annual weather patterns
for this region can be divided into four seasons:
The climate of the Gulf of Aden is exceptionally dry and, especially from May to September, very hot. Very little rain falls over the Gulf and coastal areas, on average of 2-4 inches per year, generally during the early part of the year and in the form of showers. Violent local thunderstorms occur over the coastal mountains and high plateaus. The climate of the western Arabian Sea is generally cooler than that of the Gulf, but still quite hot. Very little rain falls over the northern portion of the area of the western Arabian Sea addressed here, but near the equator annual amounts are about 25 inches, mostly falling during the Southwest Monsoon and transition seasons. This seasonal rainfall pattern is the reverse of the eastern Arabian Sea where the Southwest Monsoon brings extremely heavy rainfall to eastern India and offshore areas.
SOUTHWEST MONSOON, GULF OF ADEN AND WESTERN ARABIAN SEA
This is one of the three additonal tutorials provided to address specific local wind conditions during the Southwest Monsoon season for locations within the Gulf of Aden and Western Arabian Sea.
The regional and seasonal characteristics of the Southwest Monsoon are provided in the NRL training tutorial titled, "Regional Winds/Indian Ocean/Southwest Monsoon/training.
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 SSTús 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 use of the maximum winds within the boundary layer to forecast surface gusts has been addressed by Brasseur (2001). The approach is based on the concept that the surface wind gust speed is a function of the large-scale wind, the turbulent kinetic energy, and the stability of the boundary layer. A factor within this approach is that the maximum possible non-convective surface wind gust is limited to the maximum wind speed within the boundary layer. A modified version of the Brasseur approach is currently under development and testing at NRL Monterey (Burk and Thompson, 2001), with plans to incorporate it into the COAMPS model. The comments in this tutorial regarding the use of COAMPS low level wind data as an upper limit on non- convective surface wind gust speeds is based on the premise that the approach addressed by Brasseur will provide representative gust values.
Comments on local area coastal wind characteristics:
Local wind conditions at the Port of Aden (12.7N, 45.2E) have a very well marked diurnal land/sea breeze regime; early morning light and variable, increasing with the day to southerly 11- 16 kt, with a northeast land breeze setting in by 1700 LST (1400Z). The onset of the land breeze is often sudden with gusts of 15-25 kt. A similar sudden gusty onset of the southerly sea breeze about dawn is also characteristic of this area. Note that the onset gusty period typically only lasts for 15 to 30 minutes before the steadier land/sea breeze winds set in.
Near Djibouti (11.6N, 43.1E) on the western shore of the Gulf land breeze winds persist throughout the day and night during the Southwest Monsoon season. A light southerly land breeze occurs during the night and early morning. By 1000 LST (0700Z) increasing southwesterlies have set in reaching a maximum of 22-27 kt between 1200 and 1400 LST (0900 and 1100Z). NOTE: The present time COAMPS model forecast data does not totally agree with this climatological statement. The model data shows a less dominant Southwest Monsoon pattern. Details are provided in the Western Gulf of Aden tutorial.
The area around Berbera (10.4N, 45.0E) has a significant diurnal pattern. Persistent strong southwesterlies, known locally as the Kharif, blow throughout the night and early morning. Generally the speed maximum averaging 25 to 30 kt occurs around 0900 LST (0600Z). After this time the wind decreases. Between 1300 LST (1000Z) and 1500 LST (1200Z) a rapid veering to a northwest 7 to 12 kt sea breeze sets in and lasts until about 1600 LST (1300Z) when the wind drops off. The wind at sunset is usually light and variable and remains so until about 2100 LST (1800Z), when the southwesterly land breeze sets in with gradually increasing speed until its maximum is reached around 0900 LST (0600Z) the next morning. Land breeze gusts frequently reach 50 kt during the early morning period. The contrast of the gustiness of the southwesterlies compared with the steady northwesterly sea breeze is a wind characteristic of this area. There are periods of 1 to 4 days during the strongest Southwest Monsoon flow when the southwesterlies persist throughout the 24 hr period; that is, there are days without a period of sea breeze. The land breeze winds decrease rapidly as they move off shore, but may be felt up to 30 nm to seaward. This offshore wind (Kharif) is very hot (over 100F) and dry, and generally raises a great deal of sand and dust which may reduce coastal visibility's to less than 1/2 nm. Seaward of the land breeze direct influence, over the open waters of the southern Gulf, a speed pattern similar to that at Berbera is experienced. The exception is that the speeds are reduced and the time of maximum is near noon, about three hours later than onshore.
Near Ras Asir (Cape Guardafui) (about 11.5N, 51.0E) the winds blow steadily from the southsoutheast. This local veering to southsoutheast from the persistent southwesterlies is likely due to cornering effects and shape of the coastline in that area. Offshore, 20 to 30 nm the winds frequently reach gale force (28 to 47 kt).
Brasseur, O., 2001: Development and application
or a physical approach to estimating wind gusts. Mon. Wea. Rev.,
Additional weather information and forecast aids for this region can be found in the following references:
1. Hubert, W.E., A.N. Hull, D.R. Morford, and
R.E. Englebretson, 1983: Forecasters Handbook for the Middle East/Arabian
Sea, NAVENVPREDRSCHFAC CR83-06.
|Fig. 1. DMSP VIS|
5 July 1979
|Fig. 2. Mean July Winds|
|SOMALI LOW-LEVEL JET |
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.
1. When the Somali Jet intensifies, the
Southwest Monsoon flow over the Arabian Sea (and clouds/rain over western
India) intensifies 1 to 2 days later.
Fig. 3. SSM/I Winds Early August
Fig. 4. SSM/I Winds Late July
|Wind speed patterns of the Southwest Monsoon season over the Gulf of Aden and western Arabian Sea are illustrated by SSM/I data for late July and early August (Fig 3 and 4). Of primary interest for this Somalia Low Level Jet (LLJ) tutorial are the high wind areas, speeds approaching 30kt in the SSM/I data, seen off the coast of Somalia. Note that in both figures the local maximums are located south of Socrata Island. Also note that the numerical digits on the speed/color bar are incorrectly spaced for values of 10 and greater. The values for 10 and greater are shown as 1 01 21 41 6 etc, rather than as 10 12 14 16 etc.|
Fig. 5. QSCATT Winds Mid August
Fig. 6. QSCATT Winds Mid August
|Larger area wind direction and speed patterns are illustrated by QSCATT images (Fig 5 and 6) for the mid-August period. In combination, these two figures provide a representative view of the Somalia LLJ showing the pattern of maximum surface winds extending northeastward from the coast of Somalia near 5N to beyond Socotra Island, and then turning eastward across the Arabian Sea. Other Southwest Monsoon and LLJ features that can be seen in these figures are the South Equatorial Trough (SET), shown by the area of light counter-clockwise turning winds near and just north of the equator, and the southern hemisphere LLJ local maximum just north of Madagascar (Fig 5). It has been noted elsewhere that surges in the LLJ and Southwest Monsoon winds in general are responsive to surges originating south of the equator and then propogating northward and eastward across the Arabian Sea over a 2 to 3 day period. The close monitoring of real time satellite wind data over the regions shown in these figures may provide advance warnings on daily changes in the strength of the Somalia LLJ and overall intensity of the Southwest Monsoon.|
|COAMPS METEOGRAMS for LLJ|
Fig. 8. LLJ 11N,55E
|COAMPS meteograms for locations near the core of the LLJ are shown for
7N,49E (Fig 7), 11N, 55E (Fig 8), and 16N, 60E (Fig 9). On the date of
these forecasts, 8 August 2001, the LLJ maximum is shown to be near 11N
(Fig 8). Note the differences in 24hr maximum temperatures in the
overland (over 90F) versus the overwater (about 78F) meteograms. SST and
surface air temperatures, according to climatology, may be even lower in
these near offshore locations with readings near 70F. Readers should be
aware of the changing temperature scales and other parameters in the
meteogram presentations. For example in the overland data (Fig 7) the 2m
temperature scale range is about 50 degrees of F, while in Fig 8 overwater
the range is less than 20 degrees of F. |
Differences between the overland and overwater meteorgram data that are considered representative of this area and season include:
1. Very hot and dry conditions
over land, temperatures greater than 90F with about a 30 degree spread in
temperature and dew point.
|COAMPS SURFACE STREAMLINES|
Color Coded Speed
Fig. 11. Surface Streamlines
Tau 12, Afternoon
|COAMPS 6, 12, and 18hr forecast wind streamlines, color coded for speed, illustrate representative local morning (Fig 10), afternoon (Fig 11), and evening (Fig 12) surface wind patterns for the Somalia LLJ and general Southwest Monsoon. Close inspection of the color patterns shows a slightly weaker LLJ over water during the afternoon (Fig 11), in comparison with the morning and evening (Fig 10 and 12), but stronger winds over land in the morning period (Fig 10). The Gulf of Aden duirnal patterns, discussed in the tutorials for that area, are also evident in these figures.|
|COAMPS 500m Wind Speeds|
Fig. 14. 500m Wind Speed(m/s)
|The Somalia LLJ and general Southwest Monsoon wind speeds at 500m are shown, contoured at 5 m/s intervals, in Fig 13, 14, and 15 at times corresponding to the above surface wind streamline figures. The variations of the LLJ speeds are in phase with the above surface streamline speeds, and as expected the variations over water areas are relatively small. The spacial pattern of decreasing speeds beyond about 200nm offshore, and the local maximum just south of Socotra Island are clearly illustrated.|
|COAMPS 925, 850, and 500mb Wind Forecasts|
Fig. 17. 850mb Winds
Tau 12, Local Night
|COAMPS 12hr forecast winds for 925mb (Fig 16), 850mb (Fig 17), and 500mb (Fig 18) are presented to illustrate the changing wind patterns aloft of the Southwest Monsoon season. As previously noted the maximum winds of the Somalia LLJ and Southwest Monsoon in general, typically occur below 5000ft. In this sequence the strongest winds and largest areal coverage of strong winds are seen at the 925mb level (Fig 16). Note that the 500mb winds have little or no directional relationship to the lower levels, that is for the area shown the Southwest Monsoon flow does not extend to the 500mb level.|
Author: Ron Englebretson
Last Updated: Mon Dec 9 10:37:24 2002
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