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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Fig. A. Topographical Map Gulf of Aden/Arabian Sea Region | Fig. B. Political Boundaries Southwest Asia Region |
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| GULF OF ADEN/WESTERN ARABIAN SEA FALL TRANSITION The winds of the Gulf of Aden and Western Arabian Sea are of a monsoonal nature in that they reverse in direction from summer to winter. They are controlled by the seasonal changes of surface pressure patterns. The pressure patterns are in turn the result of the annual variation of temperature that is much larger over land areas compared with neighboring ocean surfaces, causing high pressure over land during the cold winters and low pressure during the hot summers. Short transition seasons are experienced between the summer southwesterly and the winter northeasterly regimes. The fall transition season (September to November), during the change from large scale southwesterly flow to northeasterly flow, commences first in the Gulf of Aden starting in September, and over the Western Arabian Sea in early October. During the transition season, periods of wind reversals are mixed with periods of light and variable winds. Over the Gulf of Aden the transition from Southwest Monsoon to the Northeast Monsoon regimes commences in September. Over the greater part of the Gulf, the winds become light and variable during September, then light northeasterly in October, and increase only slightly throughout the Northeast Monsoon season (December to March). Near the eastern entrance to the Gulf, the Southwest Monsoon winds continue through September and into early October, with the Northeast Monsoon becoming fully developed by November. In the western entrance, through Bab el Mandeb Strait, the light winds of September are replaced by light to moderate southerly winds in October, with increasing speeds in November. The winds in the Strait are typically stronger than those over the Gulf. North of the Bab el Mandeb Strait, over the northern and central Red Sea, northwesterly winds prevail. The opposing low level flows over the southern Red Sea result in a climatological cloud feature referred to as the convergence zone cloud band (CZCB). The CZCB forms over the extreme Southern Red Sea in September and typically advances northward to about 18-20 N by October, remaining in that general location through February, after which it begins to move southward. Over the Western Arabian Sea and the rest of the open Sea the major transition takes place in October. The zone of transition progresses southward during October and November, but south of 5N the Northeast Monsoon is not fully established until December. | |
| This tutorial illustrates the progression of the fall transition wind
patterns over the Gulf of Aden and Western Arabian Sea region. Satellite
wind data and global numerical model data are used to show the transition.
Starting from the well established Southerly Monsoon wind pattern of mid-
August, through the September transition to easterly flow over the Gulf of
Aden, the following transition to general northerly flow over the Arabian
Sea takes place during October, and ends with the generally established
Northeast Monsoon wind pattern. Additional weather information and
forecast aids for this region can be found in the following
references: Additional tutorials for this region and others can be found on the NRL Satellite Training web page: http://199.9.2.108/sat_training.html |
| Fig. 1. QSCATT Winds Mid August | Fig. 2. QSCATT Winds Mid August |
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| The QSCATT satellite wind images, shown in Figs. 1 and 2, illustrate the Southwest Monsoon wind patterns over the Gulf of Aden and Arabian Sea prior to the start of the 2001 fall transition season. Note that southwesterly winds prevail throughout the region. The wind conditions during the Southwest Monsoon are addressed in other tutorials (see NRL Training Web Site). | |
| Fig. 3. QSCATT Winds Mid September | Fig. 4. QSCATT Winds Mid September | Fig. 5. QSCATT Winds Mid September |
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| Figs. 3, 4, and 5 are QSCATT wind images that illustrate the commencement of the fall transition during mid-September 2001. The 13 September QSCATT (Fig. 3) is quite similar to the earlier patterns (Fig. 1 and 2) of the Southwest Monsoon. Note however, that while the winds over the Gulf of Aden remain southwesterly, they are quite light. Another feature to note, but not addressed in detail here, are the fairly strong northwesterlies over the Persian Gulf. This is in contrast to the typical light winds (see Fig. 2) over the Persian Gulf during the Southwest Monsoon season. This Persian Gulf wind pattern is another indicator of the beginning of the fall transition season of the Southwest Asian area. Fig. 4 shows that one day later, on 14 September, the Gulf of Aden winds have shifted to light easterlies, a clear signal of the commencement of the fall transition season. Fig. 5 shows the continuation on 16 September of easterlies over the Gulf plus the transition to northerlies over parts of the northern and eastern Arabian Sea. | ||
| Fig. 6. QSCATT Winds Mid September | Fig. 7. SSM/I Winds Mid September | Fig. 8. QSCATT Winds Mid September |
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| Three additional satellite wind images for 16 and 17 September are provided in Figs. 6, 7, and 8. QSCATT wind images each including three satellite passes over the region (Fig. 6 and 8) further illustrate the transition features mentioned above and other regional wind features. The overall reduction in wind speeds as well as the direction changes typical of the fall transition season is clearly recognizable when these two images are compared with the mid-August views shown in Figs. 1 and 2. The light wind pattern over the Gulf of Aden and outside its eastern entrance is shown in the SSM/I wind speed image (Fig. 7). Note the loss of the large wind speed increase through the eastern entrance that prevails throughout the Southwest Monsoon season. | ||
| Fig. 9. QSCATT Winds Mid October | Fig. 10. QSCATT Winds Early November | Fig. 11. QSCATT Winds Early November |
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| The continued fall transition through mid-October and early November over the region is shown in QSCATT images for 17 October (Fig. 9), 3 November (Fig. 10), and 5 November (Fig. 11). The mid-October image (Fig. 9) shows the transition in progress, and the two November images show the transition generally completed, at least southward to near 5N over the Western Arabian Sea. | ||
| Fig. 12. GMS-5 Cloud Wind Vectors Mid October | Fig. 13. GMS-5 Cloud Wind Vectors Mid October |
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| The upper level wind patterns during the fall transition season over the region are illustrated by satellite cloud vector patterns in Figs. 12 and 13. The mid and upper level mid-latitude westerlies are shown to have progressed southward to over the Northern Arabian Sea in these 17 October 2001 images. Sub-tropical easterlies prevail at these levels over the Gulf of Aden and most of the Arabian Sea south of about 17-20N. | |
| Fig. 14. Meteo-5 IR NOGAPS SLP Overlay 22 October 2001 | Fig. 15. Meteo-5 IR NOGAPS 500mb Heights Overlay 22 October 2001 | Fig. 16. Meteo-5 IR NOGAPS 300mb Winds Overlay 22 October 2001 |
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| Satellite IR cloud images with NOGAPS overlays of SLP, 500 mb height contours, and 300 mb winds for 22 October 2001 are shown in Figs. 14,15, and 16. Fig. 14 illustrates the surface pressure pattern of the transition season. Note the change to high-pressure over the Southwest Asian land area and lower pressure over the oceanic area, a reverse from the Southwest Monsoon season pattern. The 500 mb (Fig. 15) and 300 mb (Fig. 16) show high pressure patterns over the Northern Arabian Sea resulting in the generally light winds throughout the atmosphere and clear skies over the Arabian Sea region. Jetstream wind speeds (60 kt) are shown to be nearing 30N over the land area north of the Arabian Sea in these later October images. | ||
| Fig. 17. Meteo-5 IR NOGAPS SLP Overlay 6 November 2001 | Fig. 18. Meteo-5 IR NOGAPS 500mb Overlay 6 November 2001 | Fig. 19. Meteo-5 IR NOGAPS 300mb Winds Overlay 6 November 2001 |
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| Figs. 17, 18, and 19 show the same set of charts as those in Figs. 14, 15, and 16 above, but about two weeks later on 6 November 2001. By this time, the transition from summer Southwest to winter Northeast Monsoon conditions is basically completed. The higher sea-level-pressure (SLP) over the land areas north of the region, compared to the SLP over the oceanic areas is clearly shown, the general wind flow from land to sea of the Northeast Monsoon regime. The upper level ridgeline and light wind zone has continued to migrate southward over the Arabian Sea. This upper level high-pressure pattern and associated general atmospheric subsidence results in the nearly cloud free conditions over the Northern Arabian Sea region during the Northeast Monsoon season. Further south, the convective activity associated with the north and south equatorial troughs are seen to be becoming well organized. Note the near circular convective cloud shapes with little or no cirrus outflow, an indication of near calm winds through the vertical layer of the convective cloud cells. | ||
| Fig. 20. QSCATT Winds Early November | Fig. 21. QSCATT Winds Early November |
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| The surface wind pattern over the oceanic areas during the early Northeast Monsoon season is illustrated in two QSCATT images (Fig. 20 and 21) from 6 November 2001. The images in combination clearly show the well- established northerly winds throughout the Arabian Sea to near the equator, and the light easterlies over the Gulf of Aden. The tendency for strong southerly winds over the southern Red Sea and through the western entrance to the Gulf of Aden is illustrated by the 15 to 25 kt winds seen over that area in Fig. 20. Note the contrast in wind speeds at this time for this area from the light winds shown in Figs. 1, 2 and 4 for mid August and September. Comparing Fig. 20 (local morning) to Fig. 21 (local evening) implies that the strongest winds in this area will be experienced in the morning period. | |
Author: Ron Englebretson Last Updated: Mon Dec 9 10:44:58 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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