 |
NRL Monterey, Marine Meteorology Division
|
|---|
|
NRL Monterey, Marine Meteorology Division
|
|---|
| Fig. A. Topographical Map of the Northern Arabian Sea Region | Fig. B. Gulf of Aden |
|---|---|
) |
) |
| GULF OF ADEN EASTERN ENTRANCE The Introduction and Background 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 6,000 ft mountains with peaks to 10,000 ft on the north, while along the southern shore a coastal plain, narrowing from west to east, is backed by 6,000 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: 1. Northeast Monsoon,
December to March. 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 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 additional 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 consist of 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 and the induced Ekman Spiral offshore water transport, which results in upwelled subsurface cool water, lowered SSTœs 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 United States 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, 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), and there are 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 visibilities to less than 1/2 nm. Seaward of the land breeze's 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 south-southeast. This local veering to south-southeast 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). REFERENCES Brasseur, O., 2001: Development and application or a
physical approach to estimating wind gusts. Mon. Wea. Rev., 129, 5-
25. 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. SSM/I Winds 7/31/1711Z (2011/2111 LST) | Fig. 2. SSM/I Winds 8/5/1423Z (1723/1823 LST) |
|---|---|
) |
) |
| Satellite SSM/I wind data, for late July (Fig. 1) and early August (Fig. 2), over the Gulf of Aden and western Arabian Sea show representative local area wind patterns for the Southwest Monsoon season. Both of these evening images clearly show the rapid increase in wind speed as the eastern entrance to the Gulf of Aden is approached from the west. They show winds increasing from less than 10kt near 50E to about 25kt near 52E. The speed reduction (wind shadow) downwind (north-northeastward) from Socotra Island can be clearly seen in Fig 2. Note that the numerical digits on the speed/color bar are incorrectly spaced for values of 10 and greater. That is, the values for 10 and greater are shown as 1, 01, 21, 41, 61 etc., rather than as 10, 12, 14, 16, etc. | |
Fig. 3. Mid Gulf 12.5N,50E | COAMPS Meteograms Fig. 4. Mid Gulf 13N,53E | Fig. 5. Mid Gulf 14N,56E |
|---|---|---|
) |
) |
) |
| Figs. 3 through 5 present 24 hour time section meteograms along a mid-Gulf
of Aden transect through its eastern entrance. In these 0000Z base time
meteograms, the local morning hours are located on the left side of the
figures. The general trend of morning maximums and afternoon minimums in
wind speeds, discussed in the Background climatology above, over the open
Gulf can be seen in the 50E (Fig. 3) and 53E (Fig. 4) meteograms. Note
that on this date the 56E (Fig. 5) meteogram data reflects the maximum
near midnight with an afternoon minimum, as mentioned in the Background
section for the area north of Ras Asir, including the Socotra Island
area. A second set of meteograms along this transect, for a date a few
days after this set, illustrates the pattern of morning maximums and
afternoon minimums (see Figures 12 through 14 below). Other typical Southwest Monsoon season features of interest in these meteograms include: 1. The vertical wind pattern of the Southwest Monsoon showing a maximum at or below 900mb (about 2500ft). Note that the maximum wind speed occurring within the boundary layer is considered the limiting upper value for non-convective or terrain influenced surface wind gusts. Therefore the 900 mb wind speeds shown for these over water locations would be a good estimate of maximum local surface wind gusts. For example, using the 56E (Fig. 5) meteogram, this approach would indicate maximum surface gusts of 50-55 kt during the morning and mid-day period. 2. The light and variable wind between 10,000 and 20,000 ft and reversal of direction above about 500mb. 3. The drop in 10m air temperatures (greater than 85F to about 78F) and increase in low level relative humidity from 50E (Fig. 3) to 56E (Fig. 5). | ||
| Fig. 6. FNMOC WAM Wave Heights |
|---|
) |
| A typical Southwest Monsoon season wave height pattern for the Gulf of Aden and western Arabian Sea area is shown by the FNMOC WAM Wave Height chart (Fig. 6). Note the local wave height maximum from off Somalia extending north-northeast beyond Socotra Island. Ships exiting the Gulf into the western Arabian Sea will experience a marked increase in the periods and heights of seas over a relatively short distance. The increases will be reflected in both wind waves and swell, resulting from the increase in local southwesterly wind speeds (extension of the Somali low-level-jet (LLJ)) and the longer fetch (approximately 800nm) that extends from near the equator. |
| Fig. 7. Socotra Meteogram 13N,55E | Fig. 8. LLJ 16N. Meteogram 16N,60E |
|---|---|
) |
) |
| Figs. 7 and 8 provide additional illustrations of the Southwest Monsoon season wind patterns over the open western Arabian Sea, within a couple hundred miles of the eastern entrance to the Gulf of Aden. Fig. 7 illustrates the winds near 13N 55E, just off the northeast corner of the Island of Socotra. All of the Meteograms used in this tutorial, through Fig. 11 shown below, are from the same base time COAMPS model 0000Z run for 7 August 2001, labeled in standard Navy model format as 2001080700. Note the LST morning (0700 to 1100) surface sustained winds in Fig. 7 (0400 to 0800Z) are 30 kt, with 60 kt at 925 mb. With the potential of surface gusts approaching the 925mb speed, a fairly rough sea and turbulent low level condition should be expected in this area. Fig. 8 shows the conditions about 450 nm to the northeast, near the climatological northern extent of the LLJ. Note the reduced surface and low-level wind speeds in this area as compared to those for the area of Fig. 7. | |
Fig. 9. Ras Asir 11.5N,51E | COAMPS Meteograms Fig. 10. Offshore Ras Asir 11.5N,51.5E | Fig. 11. LLJ 11N,55E |
|---|---|---|
) |
) |
) |
| Figs. 9, 10, and 11 illustrate the change in Southwest Monsoon season winds between locations overland (Fig. 9), near coastal (Fig. 10) and farther offshore near the core of the LLJ (Fig. 11). The Ras Asir meteogram shows the surface winds becoming light and variable by late morning and remaining so until late in the evening. During this period there is potential for strong low level turbulence when passing over the coastal zone. Note that over the land, during the period of light and variable surface winds, the light and variable conditions extend to altitudes above 5,000 ft. Fig. 10 provides the vertical wind picture a short distance offshore. The influence of the overland wind pattern can be seen, especially above the surface in the marine boundary layer. Fig. 11, about 200 nm to seaward and near the core of the LLJ, is outside the land diurnal wind pattern. Note that the pattern of surface wind evening maximums and morning minimums mentioned for the areas south and east of Socotra is evident in Fig. 11 for the near LLJ core location. The vertical and horizontal wind patterns depicted in these figures should be representative of the patterns anywhere along the Somalia coast during the Southwest Monsoon season. The potential for low-level turbulence and the day to night change in wind patterns should be taken under consideration by forecasters when making low-level flight forecasts. The large change in air temperatures from greater than 90 over land to the mid 70's or lower for offshore locations may also create lift issues for aircraft. | ||
Fig. 12. Mid Gulf 12.5N,50E | COAMPS Meteograms Fig. 13. Mid Gulf 13N,53E | Fig. 14. Mid Gulf 14N,56E |
|---|---|---|
) |
) |
) |
| Figs. 12, 13, and 14 show the meteograms along the transect across the
eastern entrance to the Gulf of Aden. They are from the same locations as
show above in Figs. 3, 4, and 5, but from a 36 hr later 2001080812 base
time COAMPS model run. Note that for display purposes, the time line of
the meteograms from this 1200Z base time run are reversed. That is, time
wise, what is on the left half of Figs. 3, 4, and 5 is now on the right
half of Figs. 12, 13, and 14. Two characteristics of the Southwest Monsoon season winds are noted in this sequence of meteograms. First, the climatalogical tendency for morning maximum surface wind speeds and evening minimums are more faithfully depicted in this set. Second, the overall comparison of the two sets (Figs. 3,4, and 5 vs. Figs. 12, 13, and 14) is an example of the highly persistent nature of the wind patterns that occur throughout the Gulf of Aden and western Arabian Sea during the Southwest Monsoon season. | ||
Author: Ron Englebretson Last Updated: Mon Dec 9 10:46:43 2002 Produced by: The Composer (Ver: 1.1.2 ) |
|