Arabian Sea/Gulf of Aden Winds - SW Monsoon Western Gulf Tutorial

	NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil

Arabian Sea/Gulf of Aden Winds - SW Monsoon Western Gulf Tutorial

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Introduction

Fig. A. Topographical Map of the Northern Arabian Sea Region	Fig. B. Gulf of Aden

WESTERN GULF OF ADEN 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. 2. Transition season, April and May. 3. Southwest Monsoon, June to September. 4. Transition season, October and November. 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.

Fig. A. Topographical Map of the Northern Arabian Sea Region

Fig. B. Gulf of Aden

WESTERN GULF OF ADEN

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.
2. Transition season, April and May.
3. Southwest Monsoon, June to September.
4. Transition season, October and November.

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.

Background

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.
Burk, S.D., and W.T. Thompson, 2001: Comments on "Development and Application of a Physical Approach to Estimating Wind Gusts". Submitted Mon. Wea. Rev., 8 pp.

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.

2. Fett, R.W., W.A. Bohan, and R.E. Englebretson, 1983: NTAG Vol 5, Part 1, Indian Ocean (Red Sea/Persian Gulf) Weather Analysis and Forecast Applications, NRL, Monterey, CA., 93940.

3. Fett, R.W., W.A. Bohan, and R.E. Englebretson, 1984: NTAG Vol 5, Part 2, Indian Ocean (Arabian Sea/Bay of Bengal) Weather Analysis and Forecast Applications, NRL, Monterey, CA., 93940.

Examples

Fig. 1. SSM/I Winds 08/03/1450Z Local Evening	Fig. 2. SSM/I Winds 07/30/1724Z Local Evening	Fig.3. SSM/I Winds 08/07/0248 Local Early Morning

Typical evening and morning Southwest Monsoon wind patterns over the Gulf of Aden and portions of the western Arabian Sea are illustrated in SSM/I images in Figs. 1 through 3. The general pattern of lower wind speeds over the Gulf of Aden and higher speeds over the Western Arabian Sea are evident in all three figures. Within the Gulf both the diurnal local changes (Fig. 1 and 3) and the day-to-day variations (Fig. 1 and 2) are evident. The diurnal and spatial patterns depicted in these figures are considered to be representative of the Southwest Monsoon seasonal conditions. Wind pattern features of interest that can be seen in these SSM/I images include: 1. Evening minimum winds in the Berbera area (Fig. 1 and 2). 2. Enhanced evening southwesterly wind speeds in the Aden area (Fig. 1 and 2). 3. Enhanced early morning wind speeds in the coastal areas eastward of Berbera (Fig. 3). 4. Speed increases exiting the eastern entrance of the Gulf (Fig. 1, 2, and 3). 5. Wind speed maximums off Somalia that decrease to seaward (Fig. 1, 2, and 3). 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 larger are shown as 1, 01, 21, 41, 61, etc, rather than as 10, 12, 14, 16, etc.

Fig. 1. SSM/I Winds
08/03/1450Z
Local Evening

Fig. 2. SSM/I Winds
07/30/1724Z
Local Evening

Fig.3. SSM/I Winds
08/07/0248
Local Early Morning

Typical evening and morning Southwest Monsoon wind patterns over the Gulf of Aden and portions of the western Arabian Sea are illustrated in SSM/I images in Figs. 1 through 3. The general pattern of lower wind speeds over the Gulf of Aden and higher speeds over the Western Arabian Sea are evident in all three figures. Within the Gulf both the diurnal local changes (Fig. 1 and 3) and the day-to-day variations (Fig. 1 and 2) are evident. The diurnal and spatial patterns depicted in these figures are considered to be representative of the Southwest Monsoon seasonal conditions.

Wind pattern features of interest that can be seen in these SSM/I images include:

1. Evening minimum winds in the Berbera area (Fig. 1 and 2).
2. Enhanced evening southwesterly wind speeds in the Aden area (Fig. 1 and 2).
3. Enhanced early morning wind speeds in the coastal areas eastward of Berbera (Fig. 3).
4. Speed increases exiting the eastern entrance of the Gulf (Fig. 1, 2, and 3).
5. Wind speed maximums off Somalia that decrease to seaward (Fig. 1, 2, and 3). 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 larger are shown as 1, 01, 21, 41, 61, etc, rather than as 10, 12, 14, 16, etc.

Fig. 4. Aden Meteogram Basetime 08/08/12Z	Fig. C. Wind Schematic	Fig. 5. North Coastal Zone Meteogram Basetime 08/08/12Z

Local diurnal patterns for the over water area near the Port of Aden (Fig. 4) and the North Coastal Zone about 12 nm seaward of the Port (Fig. 5) are depicted in COAMPS Meteogram format. Schematic of north/south coastal sea/land breeze interactions with large-scale southwesterly flow is depicted in Fig. C. The daytime north coast sea breeze enhances (increases) the southwesterlies while the south coast sea breeze counters (decreases) the southwesterlies. The nighttime land breeze interaction is reversed. This pattern of enhancing and countering result in the surface wind patterns addressed in the following discussions. Local wind features of interest that can be seen in Fig. 4 and 5 include: 1. The evening land breeze and light winds through early morning with increasing southwesterlies during the day for the Port of Aden (Fig. 4), 2. The vertical extent of the evening/night light land breeze and the abrupt change to strong southwesterlies about 0200LST (2300Z) (Fig. 4), 3. The potential for 30 kt gusts during the afternoon based on the winds near 950 mb (Fig. 4), 4. The lower level upper limit of the general southwesterly winds to about 6000ft (800 mb) over this western Gulf area (Fig. 4 and 5), in contrast to near 15000 ft addressed in other tutorials for the vicinity of the eastern entrance to the Gulf and over the Somalia LLJ region, 5. The limited distance offshore of the north coast that the land breeze overcomes the prevailing southwesterly winds (Fig. 5), 6. The high relative humidity values shown in Figs. 4 and 5, below about 8000 ft (750 mb), likely reflects the advection of moisture onshore and over the local terrain by the returning sea breeze enhanced southwesterlies. The more representative area wide humidity pattern over the Gulf can be seen in the following figures (6, 7, 8, and 9). The COAMPS forecast of precipitation indicated in Figs. 4 and 5 correspond time wise with the modelœs indication of increased low-level relative humidity. A reasonable interpretation and forecast extension of these model data would be to expect convective cloud development over the coastal area with the potential for local rain showers.

Fig. 4. Aden Meteogram
Basetime 08/08/12Z

Fig. C. Wind Schematic

Fig. 5. North Coastal Zone Meteogram
Basetime 08/08/12Z

Local diurnal patterns for the over water area near the Port of Aden (Fig. 4) and the North Coastal Zone about 12 nm seaward of the Port (Fig. 5) are depicted in COAMPS Meteogram format. Schematic of north/south coastal sea/land breeze interactions with large-scale southwesterly flow is depicted in Fig. C. The daytime north coast sea breeze enhances (increases) the southwesterlies while the south coast sea breeze counters (decreases) the southwesterlies. The nighttime land breeze interaction is reversed. This pattern of enhancing and countering result in the surface wind patterns addressed in the following discussions.

Local wind features of interest that can be seen in Fig. 4 and 5 include:

1. The evening land breeze and light winds through early morning with increasing southwesterlies during the day for the Port of Aden (Fig. 4),
2. The vertical extent of the evening/night light land breeze and the abrupt change to strong southwesterlies about 0200LST (2300Z) (Fig. 4),
3. The potential for 30 kt gusts during the afternoon based on the winds near 950 mb (Fig. 4),
4. The lower level upper limit of the general southwesterly winds to about 6000ft (800 mb) over this western Gulf area (Fig. 4 and 5), in contrast to near 15000 ft addressed in other tutorials for the vicinity of the eastern entrance to the Gulf and over the Somalia LLJ region,
5. The limited distance offshore of the north coast that the land breeze overcomes the prevailing southwesterly winds (Fig. 5),
6. The high relative humidity values shown in Figs. 4 and 5, below about 8000 ft (750 mb), likely reflects the advection of moisture onshore and over the local terrain by the returning sea breeze enhanced southwesterlies. The more representative area wide humidity pattern over the Gulf can be seen in the following figures (6, 7, 8, and 9). The COAMPS forecast of precipitation indicated in Figs. 4 and 5 correspond time wise with the modelœs indication of increased low-level relative humidity. A reasonable interpretation and forecast extension of these model data would be to expect convective cloud development over the coastal area with the potential for local rain showers.

Fig. 6. Djibouti Meteogram Basetime 08/08/12Z	Fig. 7. Western Mid-Gulf Meteogram Basetime 08/08/12Z

The wind patterns for Djibouti, near 11.6N 43.1E, on the extreme western coast of the Gulf are shown in Fig. 6, and the wind pattern about 100 nm to the east over the open western Gulf is shown in Fig. 7. The area around Djibouti is near the western limit of the Southwest Monsoon regional wind pattern. It was noted that throughout a sequence of about a week of COAMPS model forecast runs for late July early August 2001, the Djibouti vertical wind profile below about 20000 ft alternated between northwesterly, westerly, and southwesterly. These model results are not totally in agreement with the climatological data provided in the Background section. Of importance to Navy forecasters is the fact that the surface and upper air wind patterns over the Red Sea will be dominated by northwesterly flow in contrast to the southwesterly flow over the Gulf of Aden and Arabian Sea. The prevailing wind direction change will occur near the western extreme of the Gulf of Aden. See Fig. 15 and 16 below for an illustration of this wind pattern change at the 850 mb level. Local wind features of interest shown in Figs. 6 and 7 include: 1. A shift from northwesterly to southwesterly low level flow at Djibouti (Fig. 6) near 2100Z, in contrast to prevailing southwesterlies throughout the 24 hr period at the Fig. 7 location over the open Gulf about 100 nm east of Djibouti, 2. The local time period (0300 to 1000LST) wind maximum (sustained 25 kt) over the open Gulf (Fig. 7), 3. The relatively shallow layer of southwesterlies, less than 5000 ft at both locations, even when the surface southwesterlies are quite strong.

Fig. 6. Djibouti Meteogram
Basetime 08/08/12Z

Fig. 7. Western Mid-Gulf Meteogram
Basetime 08/08/12Z

The wind patterns for Djibouti, near 11.6N 43.1E, on the extreme western coast of the Gulf are shown in Fig. 6, and the wind pattern about 100 nm to the east over the open western Gulf is shown in Fig. 7. The area around Djibouti is near the western limit of the Southwest Monsoon regional wind pattern. It was noted that throughout a sequence of about a week of COAMPS model forecast runs for late July early August 2001, the Djibouti vertical wind profile below about 20000 ft alternated between northwesterly, westerly, and southwesterly. These model results are not totally in agreement with the climatological data provided in the Background section. Of importance to Navy forecasters is the fact that the surface and upper air wind patterns over the Red Sea will be dominated by northwesterly flow in contrast to the southwesterly flow over the Gulf of Aden and Arabian Sea. The prevailing wind direction change will occur near the western extreme of the Gulf of Aden. See Fig. 15 and 16 below for an illustration of this wind pattern change at the 850 mb level.

Local wind features of interest shown in Figs. 6 and 7 include:

1. A shift from northwesterly to southwesterly low level flow at Djibouti (Fig. 6) near 2100Z, in contrast to prevailing southwesterlies throughout the 24 hr period at the Fig. 7 location over the open Gulf about 100 nm east of Djibouti,
2. The local time period (0300 to 1000LST) wind maximum (sustained 25 kt) over the open Gulf (Fig. 7),
3. The relatively shallow layer of southwesterlies, less than 5000 ft at both locations, even when the surface southwesterlies are quite strong.

Fig. 8. Berbera Meteogram Basetime 08/08/12Z	Fig. 9. South Coastal Zone Meteogram Basetime 08/08/12Z

The significant diurnal wind pattern mentioned in the Background section for the Berbera area is quite evident in the COAMPS Meteograms for the Berbera area (Figs. 8 and 9). The pattern of strong southwesterlies throughout the night with an early morning 30 kt maximum around 0900LST (0600Z) is closely matched by the model data shown here. The climatological pattern of switching to a sea breeze between 1300 and 1500LST (1000 and 1200Z) with light northwesterlies of L/V conditions lasting until about 2100LST (1800Z) is also closely duplicated in Fig. 8. Note that for this date, using the model low-level winds as an upper limit, the maximum surface gusts would be about 45 kt and would occur in the early morning period. Fig. 9 shows the conditions about 12 nm seaward of the Berbera figure. Both the sea breeze and land breeze patterns of the Port conditions are closely matched. Comparing these South Coast conditions (Fig. 9) with the West Mid Gulf conditions (Fig. 7), which is about 84 nm due north, shows the drop off of coastal influence at the Mid Gulf location. That is, southwesterlies prevail at the Mid Gulf throughout the 24 hr period, but maximums are weaker by about 5 kt, and the coastal sea breeze period is reflected by about a 50% reduction in speed of the southwesterly flow. Local wind features of interest seen in Figs. 8 and 9 include: 1. A marked land/sea breeze regime on the southern coast with the light sea breeze dominant during 1500 to 2100LST (1200 to 1800Z), and a strong land breeze reinforced prevailing southwesterlies dominating from late evening through morning (2200 to 1400LST). This wind pattern of maximum winds at a Port during night and morning, with minimum winds in the afternoon and evening, is the reverse of the coastal wind pattern most typically experienced throughout the world. Forecasters should be aware of this time reversal pattern when supporting wind sensitive operations such as scheduling ship movements within port or small boat scheduling. 2. The onshore land/sea breeze regime that extends seaward beyond 12 nm in this COAMPS model data, and according to the climatology information, out to about 30 nm. 3. The southwesterly pattern that only extends vertically to about 5000 ft in this area. 4. The sea breeze regime that extends vertically throughout the normal southwesterly layer, resulting in generally L/V conditions up to 4-5000 ft.

Fig. 8. Berbera Meteogram
Basetime 08/08/12Z

Fig. 9. South Coastal Zone Meteogram
Basetime 08/08/12Z

The significant diurnal wind pattern mentioned in the Background section for the Berbera area is quite evident in the COAMPS Meteograms for the Berbera area (Figs. 8 and 9). The pattern of strong southwesterlies throughout the night with an early morning 30 kt maximum around 0900LST (0600Z) is closely matched by the model data shown here. The climatological pattern of switching to a sea breeze between 1300 and 1500LST (1000 and 1200Z) with light northwesterlies of L/V conditions lasting until about 2100LST (1800Z) is also closely duplicated in Fig. 8. Note that for this date, using the model low-level winds as an upper limit, the maximum surface gusts would be about 45 kt and would occur in the early morning period. Fig. 9 shows the conditions about 12 nm seaward of the Berbera figure. Both the sea breeze and land breeze patterns of the Port conditions are closely matched. Comparing these South Coast conditions (Fig. 9) with the West Mid Gulf conditions (Fig. 7), which is about 84 nm due north, shows the drop off of coastal influence at the Mid Gulf location. That is, southwesterlies prevail at the Mid Gulf throughout the 24 hr period, but maximums are weaker by about 5 kt, and the coastal sea breeze period is reflected by about a 50% reduction in speed of the southwesterly flow.

Local wind features of interest seen in Figs. 8 and 9 include:

1. A marked land/sea breeze regime on the southern coast with the light sea breeze dominant during 1500 to 2100LST (1200 to 1800Z), and a strong land breeze reinforced prevailing southwesterlies dominating from late evening through morning (2200 to 1400LST). This wind pattern of maximum winds at a Port during night and morning, with minimum winds in the afternoon and evening, is the reverse of the coastal wind pattern most typically experienced throughout the world. Forecasters should be aware of this time reversal pattern when supporting wind sensitive operations such as scheduling ship movements within port or small boat scheduling.
2. The onshore land/sea breeze regime that extends seaward beyond 12 nm in this COAMPS model data, and according to the climatology information, out to about 30 nm.
3. The southwesterly pattern that only extends vertically to about 5000 ft in this area.
4. The sea breeze regime that extends vertically throughout the normal southwesterly layer, resulting in generally L/V conditions up to 4-5000 ft.

Fig. 10. Aden Meteogram Basetime 08/08/12Z	Fig. B. Gulf of Aden	Fig. 11. Berbera Meteogram Basetime 08/08/12Z

The wind contrast in Southwest Monsoon season wind patterns for northern and southern port and coastal areas of the Gulf of Aden are illustrated by comparing features of the COAMPS meteorgrams for Aden (Fig. 10) and Berbera (Fig. 11). As previously noted, the Berbera area (Fig 11), and south coastal areas in general, exhibits a non-standard diurnal wind pattern of minimums during the afternoon and evening and maximums throughout the night. In contrast, the Aden area (Fig. 10), and north coastal areas in general, show more typical daytime maximums and nighttime minimums. These local patterns are the result of interaction between the regional southwesterlies and the local land/sea breeze forcing. Along the south coast, the regional and local forcing are in phase (both from the south) at night, that is the local land breeze re-enforces the regional Southwest Monsoon winds, and therefore maximum winds are experienced during that time period. During the day, the south coastal area northerly sea breeze forcing counteracts the prevailing southwesterlies. In contrast, along the northern coast the land/sea breeze interaction with the prevailing southwesterlies is reversed. That is, the night land breeze northerlies counter the regional southwesterlies while the daytime sea breeze enhances the regional southwesterlies. Local wind features of interest that are illustrated in Fig. 10 and 11 include: 1.The well established southwesterly winds at Berbera from about 1800Z (2100LST) through the night with early morning maximums around 0300-0600Z (0600- 0900LST) and the dropping off by mid-day 1000-1200Z (1300-1500LST). 2. The Aden area afternoon maximums around 1100-1200Z (1400-1500LST) and then the rapid dropping off by 1300Z (1600LST). 3. The relatively shallow layer of southwesterly flow for these western Gulf of Aden locations compared to the deeper southwesterly flow pattern over the Eastern Gulf and Western Arabian Sea. 4. The general disruption and L/V winds through the entire vertical layer of regional pattern southwesterlies during the periods of surface L/V winds.

Fig. 10. Aden Meteogram
Basetime 08/08/12Z

Fig. B. Gulf of Aden

Fig. 11. Berbera Meteogram
Basetime 08/08/12Z

The wind contrast in Southwest Monsoon season wind patterns for northern and southern port and coastal areas of the Gulf of Aden are illustrated by comparing features of the COAMPS meteorgrams for Aden (Fig. 10) and Berbera (Fig. 11). As previously noted, the Berbera area (Fig 11), and south coastal areas in general, exhibits a non-standard diurnal wind pattern of minimums during the afternoon and evening and maximums throughout the night. In contrast, the Aden area (Fig. 10), and north coastal areas in general, show more typical daytime maximums and nighttime minimums. These local patterns are the result of interaction between the regional southwesterlies and the local land/sea breeze forcing. Along the south coast, the regional and local forcing are in phase (both from the south) at night, that is the local land breeze re-enforces the regional Southwest Monsoon winds, and therefore maximum winds are experienced during that time period. During the day, the south coastal area northerly sea breeze forcing counteracts the prevailing southwesterlies. In contrast, along the northern coast the land/sea breeze interaction with the prevailing southwesterlies is reversed. That is, the night land breeze northerlies counter the regional southwesterlies while the daytime sea breeze enhances the regional southwesterlies.

Local wind features of interest that are illustrated in Fig. 10 and 11 include:

1.The well established southwesterly winds at Berbera from about 1800Z (2100LST) through the night with early morning maximums around 0300-0600Z (0600- 0900LST) and the dropping off by mid-day 1000-1200Z (1300-1500LST).
2. The Aden area afternoon maximums around 1100-1200Z (1400-1500LST) and then the rapid dropping off by 1300Z (1600LST).
3. The relatively shallow layer of southwesterly flow for these western Gulf of Aden locations compared to the deeper southwesterly flow pattern over the Eastern Gulf and Western Arabian Sea.
4. The general disruption and L/V winds through the entire vertical layer of regional pattern southwesterlies during the periods of surface L/V winds.

Fig. 12. COAMPS 500m Winds Tau 0, VT 08/07/00Z	Fig. 13. COAMPS 500m Winds Tau 12, VT 08/07/12Z	Fig. 14. COAMPS 500m Winds Tau 24, VT 08/08/00Z

The Southwest Monsoon winds have been noted to be at their maximum speeds in the lower few thousands of feet above the surface. This feature is quite evident in all the meteograms shown in this tutorial, plus those in other tutorials for this region and season. The regional wind speeds at 500m, without regards to direction, are shown in Figures 12, 13, and 14. Note that the speed units in these three figures are in meters per second (m/s), while all the other winds show in this tutorial are in knots. The figures show model Tau 0 (Fig. 12), Tau 12 (Fig. 13) and Tau 24 (Fig. 14) forecast wind speeds at 500m. The winds at this level are likely near the maximum southwesterly speeds and are close enough to the surface to act as a good forecast value for maximum non-convective surface gust forecasts. The pattern over the Gulf illustrates the typical stronger south coast winds at night, seen in Fig. 12 and 14, which have VT's of 0000Z (0300- 0400LST over the area shown) and lighter winds along the north coast at this time. In contrast, Fig. 13, with a VT of 1200Z (1500LST), shows lighter winds along the south coast and higher speeds along the north, as was discussed earlier. The pattern over the Gulf reflects the stronger winds over the west central part, weaker over the eastern Gulf but with a rapid increase just outside the eastern entrance. Also of interest is the pattern of maximums off the Somalia and Oman coasts, reflecting the so- called Somalia Low-Level Jet. Note not only the extent of this feature along the coasts, but also the limited extent to seaward. The wind patterns noted in these numerical model data are in general agreement with the SSM/I data shown in Fig. 3. Features of interest illustrated in these COAMPS 500m wind speed forecasts include: 1. These 500 m (approximately 1600 ft) winds that closely reflect the surface patterns discussed in this and other Gulf of Aden and Western Arabian Sea tutorials showing both local and regional areas and times of maximum and minimum winds. 2. The winds over the west central Gulf that are stronger than those over the eastern Gulf. 3. A rapid increase in wind speeds that will be experienced when exiting the Gulf through the eastern entrance. Winds within a couple thousands of feet (about 500m) of the surface will be in excess of 50kt (25m/s) near the core of Somalia LLJ, and over 40kt (20m/s) over areas of the western Gulf and over extensive areas off the coasts of Somalia and Oman. Surface wind gusts on the order of the speeds shown at 500m should be expected. 4. When entering the western Gulf of Aden from the Red Sea, low-level winds will significantly increase and as will be shown in the next figures have a major change in prevailing direction from northwesterly to southwesterly.

Fig. 12. COAMPS 500m Winds
Tau 0, VT 08/07/00Z

Fig. 13. COAMPS 500m Winds
Tau 12, VT 08/07/12Z

Fig. 14. COAMPS 500m Winds
Tau 24, VT 08/08/00Z

The Southwest Monsoon winds have been noted to be at their maximum speeds in the lower few thousands of feet above the surface. This feature is quite evident in all the meteograms shown in this tutorial, plus those in other tutorials for this region and season. The regional wind speeds at 500m, without regards to direction, are shown in Figures 12, 13, and 14. Note that the speed units in these three figures are in meters per second (m/s), while all the other winds show in this tutorial are in knots. The figures show model Tau 0 (Fig. 12), Tau 12 (Fig. 13) and Tau 24 (Fig. 14) forecast wind speeds at 500m. The winds at this level are likely near the maximum southwesterly speeds and are close enough to the surface to act as a good forecast value for maximum non-convective surface gust forecasts. The pattern over the Gulf illustrates the typical stronger south coast winds at night, seen in Fig. 12 and 14, which have VT's of 0000Z (0300- 0400LST over the area shown) and lighter winds along the north coast at this time. In contrast, Fig. 13, with a VT of 1200Z (1500LST), shows lighter winds along the south coast and higher speeds along the north, as was discussed earlier. The pattern over the Gulf reflects the stronger winds over the west central part, weaker over the eastern Gulf but with a rapid increase just outside the eastern entrance. Also of interest is the pattern of maximums off the Somalia and Oman coasts, reflecting the so- called Somalia Low-Level Jet. Note not only the extent of this feature along the coasts, but also the limited extent to seaward. The wind patterns noted in these numerical model data are in general agreement with the SSM/I data shown in Fig. 3.

Features of interest illustrated in these COAMPS 500m wind speed forecasts include:

1. These 500 m (approximately 1600 ft) winds that closely reflect the surface patterns discussed in this and other Gulf of Aden and Western Arabian Sea tutorials showing both local and regional areas and times of maximum and minimum winds.
2. The winds over the west central Gulf that are stronger than those over the eastern Gulf.
3. A rapid increase in wind speeds that will be experienced when exiting the Gulf through the eastern entrance.
Winds within a couple thousands of feet (about 500m) of the surface will be in excess of 50kt (25m/s) near the core of Somalia LLJ, and over 40kt (20m/s) over areas of the western Gulf and over extensive areas off the coasts of Somalia and Oman. Surface wind gusts on the order of the speeds shown at 500m should be expected.
4. When entering the western Gulf of Aden from the Red Sea, low-level winds will significantly increase and as will be shown in the next figures have a major change in prevailing direction from northwesterly to southwesterly.

Fig. 15. COAMPS 850mb Winds Tau 6, VT 08/08/0600Z	Fig. 16. COAMPS 850mb Winds Tau 18, VT 08/08/1800Z

The Southwest Monsoon season wind pattern at 850 mb, near 5000 ft (1500 m) is illustrated by COAMPS 6 and 18 hr forecasts (Fig. 15 and 16). These figures show that the western extent of the Southwest Monsoon regime lies near the western end of the Gulf of Aden. Thus, there is the variability previously noted for the Djibouti area between northwesterly and southwesterly flow regimes. Little if any change is shown at this level over the eastern entrance area. Note that even at this relatively low level, about 5000 ft (1500 m), the winds are becoming more westerly than southerly as they are at the surface. The diurnal influence, even at this level, appears to favor stronger winds over the Gulf and coastal Somalia in the morning 0600Z (0900LST) (Fig. 15) than in the evening 1800Z (2100LST) (Fig. 16). Features of interest illustrated in the 850 mb wind fields include: 1. The westward extent of the Southwest Monsoon season wind regime lies near the western end of the Gulf of Aden. Ships entering from the Red Sea should anticipate surface and low-level winds to be backing from northwesterly to southwesterly and increasing in speed. 2. Winds near the 850 mb level will be slightly stronger during the night and morning hours than during the day and evening hours. 3. There is little change in winds at the 850 mb level when exiting through the eastern entrance of the Gulf of Aden. 4. The general vertical wind profile of the Southwest Monsoon regime shows veering wind directions with increasing heights. A layer of light and variable winds typically overlays the southwesterlies. Above that the wind directions continue to veer to northeasterly and easterly. This general vertical wind profile is reflected in the various meteograms shown throughout this series of tutorials.

Fig. 15. COAMPS 850mb Winds
Tau 6, VT 08/08/0600Z

Fig. 16. COAMPS 850mb Winds
Tau 18, VT 08/08/1800Z

The Southwest Monsoon season wind pattern at 850 mb, near 5000 ft (1500 m) is illustrated by COAMPS 6 and 18 hr forecasts (Fig. 15 and 16). These figures show that the western extent of the Southwest Monsoon regime lies near the western end of the Gulf of Aden. Thus, there is the variability previously noted for the Djibouti area between northwesterly and southwesterly flow regimes. Little if any change is shown at this level over the eastern entrance area. Note that even at this relatively low level, about 5000 ft (1500 m), the winds are becoming more westerly than southerly as they are at the surface. The diurnal influence, even at this level, appears to favor stronger winds over the Gulf and coastal Somalia in the morning 0600Z (0900LST) (Fig. 15) than in the evening 1800Z (2100LST) (Fig. 16).

Features of interest illustrated in the 850 mb wind fields include:

1. The westward extent of the Southwest Monsoon season wind regime lies near the western end of the Gulf of Aden. Ships entering from the Red Sea should anticipate surface and low-level winds to be backing from northwesterly to southwesterly and increasing in speed.
2. Winds near the 850 mb level will be slightly stronger during the night and morning hours than during the day and evening hours.
3. There is little change in winds at the 850 mb level when exiting through the eastern entrance of the Gulf of Aden.
4. The general vertical wind profile of the Southwest Monsoon regime shows veering wind directions with increasing heights. A layer of light and variable winds typically overlays the southwesterlies. Above that the wind directions continue to veer to northeasterly and easterly. This general vertical wind profile is reflected in the various meteograms shown throughout this series of tutorials.

Author: Ron Englebretson
Last Updated: Mon Dec 9 10:45:53 2002
Produced by: The Composer (Ver: 1.1.2 )

NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil