NRL Monterey, Marine Meteorology Division
|Fig. 1. Mean Monthly Trough Position for June (Findlater, 1971)|
|The Southwest Monsoon|
The low-level circulation pattern for the Indian Ocean consists of southwest flow during the summer period and northeast flow during the winter period. The basic cause of such a wind pattern is the differential heating and cooling of adjacent large land and sea areas. The landmasses are warmer than the ocean areas in summer and cooler in winter, resulting in relatively lower pressure over the land in summer and higher in winter. These pressure differences cause winds to blow primarily onshore over India in summer and offshore of India during winter. The summer onshore flow is called the Southwest Monsoon and the winter offshore flow is referred to as the Northeast Monsoon.
Two major synoptic patterns associated with the Southwest Monsoon are the Southern Equatorial Trough (SET) and the Northern Equatorial Trough (NET). The SET is a region near the equator where winds turn in a clockwise fashion from southeasterly, south of the equator, to southwesterly, north of the equator. The SET is a minimum wind speed zone and may sometimes be located in visible satellite imagery through sunglint effects off of the nearly calm sea surface within the trough (Fett, et al, 1985). This study will show for the first time that Special Sensor Microwave Imager (SSM/I) data can also be usefully employed, at times, to locate the SET as a minimum wind speed zone.
The NET is a counterclockwise wind region to the north where winds turn from south southwesterly of the trough to north northeasterly of the trough. By June the NET begins to merge with the thermal trough over land and becomes indistinguishable from this feature. Fig. 1 shows the general position of these troughs during the month of June, the normal time for the commencement of the Southwest Monsoon.
|Fig 2. METEO-5 Visible Image. 21 June 2001, 0600Z|
|Fig. 2 shows a fully illuminated view of the Indian Ocean region shortly
after the onset of the SW Monsoon. Convective clusters associated with
convergence effects along the SET are apparent near the equator. Sunglint
is evident in the clear area separating the two clusters of convection.
The dark streak connecting the two clusters is a calm region, indicating
the position of the SET in that region (Fett, et al., 1985).|
The fact that the SW Monsoon is in "full swing" can be verified by the unusual amount of convective activity in the Bay of Bengal and by convective cloudiness, most prominent on the west coast, as opposed to the east coast of India, which often is quite clear during intense phases of the SW Monsoon. Fig. 3, below, is a DMSP visible image that shows this effect very clearly.
|Fig. 3. DMSP Visible Imagery. 27 June 1979, 0522Z|
|This is a classic view of SW Monsoon effects over India, acquired during
the Summer Monsoon Experiment (MONEX) during 1979. Note especially the
enhanced convection along the west coastal region of India, caused by
frictional convergence, as strong southwesterly flow from the ocean makes
landfall. The eastern coast of India at the same time is remarkably
clear. Convection is again induced on the east side of the Bay of Bengal
where land is encountered. Wave cloud formation associated with
mountainous terrain due to strong westerly flow is evident over
Fig. 4 below is another DMSP image acquired during MONEX, which depicts conditions off the coast of Somalia.
|Fig. 4. DMSP Visible Image. 27 June 1979, 0704Z|
|Characteristically, the waters off the coast of Somalia are clear of middle and upper level cloudiness. This is due to two effects. First, surface winds accelerate from south to north along the coast of Somalia (Somalia Jet). The surface acceleration produces low-level divergence that is compensated for by subsidence from aloft, which restricts higher- level cloud development. Tightly spaced low-level cloud streets, as shown in the above DMSP image, however, are commonly seen, aligned with the strong low level winds). Second, the strong southerly surface winds induce upwelling of cold water immediately adjacent to the Somalia coast, which also serves to restrict cloud development in this area. Dust, as shown in this DMSP image, is commonly seen over Somalia and the Gulf of Aden. The dust is raised as a portion of the strong southwesterly flow impinges on the dry desert area of Somalia. The accelerating wind effect from south to north can be seen quite clearly in the QSCAT data below.|
|Fig. 5. QSCAT Surface Wind Vectors. 21 June 2001, 0601Z||Fig. 6 . QSCAT Surface Wind Vectors (Southern Equatorial Trough Superimposed)|
|Fig. 5 shows two swaths of QSCAT surface wind vector data over the Indian
Ocean coincident in time with the METEO-5 visible image of Fig. 1. Winds
coming in from the southwest turn westerly as they flow over India,
striking the coast with speeds of 20-30 kt. Along the coast of Africa
winds accelerate from 10-15 kt near the equator to 20 -25 kt south of 10¸
N, and finally accelerate to 25-30 kt in the region east and northeast of
the northern tip of Somalia. The turning of winds from southeast, south
of the SET, to southwest and west, north of the SET, is quite evident.|
Fig. 6 locates the position of the SET as evidenced by this wind shift. The positon of the SET as derived from the QSCAT data is shown in Fig. 6 above. Note that the SET, appropriately, is very near the equator.
|Fig. 7. METEO-5 Visible Image (SET Position Superimposed)|
|In Fig. 7 the position of the SET has been superimposed on the METEO-5
visible image (Fig. 1) to show relationships of convective cloud cluster
location to this feature. The convection appears to be due to convergence
effects on either side of the SET. This convergence is restricted to a
region south of the SET in the easternmost cloud cluster as strong winds
decelerate approaching the trough position. Convective cloudiness, on the
other hand, appears on both sides of the western position of the SET. In
this region wind speed deceleration is less pronounced, and the turning of
winds is less abrupt than in the easternmost portion.|
Fig. 7 is an enhanced version of Fig. 1. Enhancement emphasizes the dark streak effect in the sunglint region between the two convective cloud clusters. The dark streak in sunglint is caused by a reflection of the sun's rays off of calm seas in a direction away from the satellite visible sensor. Had this region been near the satellite Primary Specular Point (PSP), where the sun's rays are reflected directly into the visible sensor, a brilliant rather than dark sunglint effect would have been observed (Fett et al., 1985). By combining the QSCAT indication of SET position with that obtained through sunglint analysis a continuous depiction of SET location can be derived, as shown in the above image.
|Fig.8. Special Sensor Microwave/Imager (SSM/I), 4 Aug 2001, 1653Z|
|Fig. 8 is an SSM/I image showing surface wind speed indications southwest of India. A minimum wind speed axis is revealed which should correlate with the position of the SET at this time. Note that the axis slopes northwestward as it passes the equator at 70¸E.|
|Fig. 9. QSCAT Surface Winds. 4 August 2001, 1725Z|
|Fig. 9 is scatterometer data taken at very nearly the same time as the
SSM/I data in Fig. 8, which can be used to test the hypothesis. QSCAT
data reveal a wind shift coinciding with the position of the SET, as
deduced from the minimum wind speed zone shown in the SSM/I data. Note
that the minimum wind speed axis slopes northwestward past the equator at
70¸E, similar to the indication in the SSM/I data.|
1. The Southwest Monsoon is a summer season phenomena caused by intense heating of the continental region of the Near East and Southwest Asia.
2. The NET and SET are important synoptic features of the Southwest Monsoon and can be located through a combination of real time scatterometer data, Special Sensor Microwave (SSM/I) data and through analysis of visible sensor satellite imagery. The SSM/I data reveal the position of the SET as a minimum wind speed zone near the equator.
3. Cloudiness and rainfall effects of the Southwest Monsoon are emphasized along the coast and western portion of India while the eastern portion remains quite clear and dry.
4. The near-coastal waters of Somalia are especially clear due to wind acceleration and upwelling effects that occur in that region. Narrow cloud streets aligned with the Somalia Jet are often apparent offshore, over warmer water regions.
2. Fett, R. W., W. A. Bohan and R. E. Englebretson, 1985: Navy Tactical Applications Guide, Vol. 5, Part 2, Indian Ocean (Arabian Sea/Bay of Bengal) Weather Analysis and Forecast Applications, Naval Research Laboratory, Monterey, CA., 93940, 202 pp.
Author: Bob Fett
Last Updated: Mon Dec 9 10:43:34 2002
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