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29<br />
Arabian Sea extends upto 700 mb. Higher up, a broad easterly current prevails<br />
which increases in speed with height. As the seasonal trough shifts to the<br />
south of Lat. 8°N or so during the second half of the season and also gradually<br />
becomes less marked, a nearly zonal easterly flow prevails over South Bay,<br />
south Peninsula and south Arabian Sea. In this zonal flow, wave like perturbations<br />
may form and move from east to west.<br />
12.2.13 The main difference between a low pressure area and a low pressure<br />
wave is, that in the low pressure area there is a complete cyclonic circulation,<br />
with west winds in the south, while a wave is a sinusoidal perturbation without<br />
any west winds in the south. The low pressure area is embedded in the seasonal<br />
eastwest oriented trough, while the wave is a perturbation in a deep zonal<br />
current. With these differences, it should, therefore, be possible to clearly<br />
differentiate between a low pressure area and a low pressure wave and use the<br />
appropriate terminology.<br />
12.2.14 Even during the earlier part of the northeast monsoon season, as the<br />
upper air pattern above 700 mb is a deep zonal flow, wave motion is possible in<br />
the mid— and upper tropospheric easterlies. Such waves may be expected to be<br />
associated with low pressure areas in the surface chart and in the lower troposphere<br />
(i.e.) a surface and lower tropospheric low pressure area may extend as a<br />
wave trough in the mid—tropospheric levels.<br />
12.3 East—west oriented trough across the Peninsula<br />
12.3.1 The seasonal trough (in surface and upper air) passes across the south<br />
Peninsula during the month of October. By <strong>No</strong>vember, the trough line shifts well<br />
to the south and is over the extreme south Peninsula and Sri Lanka. By December,<br />
the trough shifts still further south and the air over the south Peninsula,<br />
except along the Tamil Nadu coast and extreme south Peninsula is mainly of continental<br />
origin.<br />
12.3.2 The seasonal southward progression of the trough line across the<br />
Peninsula (with low pressure systems forming and moving westwards along the<br />
trough) agrees well with the mean rainfall distribution. In contrast to the<br />
seasonal trough over the northern India during the southwest monsoon where the
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