Structure of low-level Jet-stream over the Arabian Sea and the Peninsula as revealed by observations in June and July during the monsoon experiment (Monex) 1973 and its probable origin

Abstract

Surface and upper air observations recorded by the four Russian ships during Monsoon Experiment (MONEX) 1973 over the Arabian sea and the Indian Ocean upto the equator and by the obsrvatories over Kenya and Peninsular India, have been examined to study details about the jet-speed winds noticed over the area during active or strong monsoon conditions. The following points are brought out by the study:

(1) The low-level jet was noticed simultaneously over the Arabian Sea between Lats. 7° and 16° N east of about long. 53° E and over the Peninsula at (i) different levels over the same place, (ii) different latitudes at the same time, and (iii) different latitudes and levels at different times.

(2) The extent of the jet in the horizontal and vertical was generally larger over land than oversea. It may be at as high a level as 3.0 km over sea and 3.8 km over land.

(3) When a jet-stream persisted over an area for a day or two, the lowest jet over the sea would appear to be at a lower level during the morning than evening hours.

(4) The low-level jet over the Peninsula and the Arabian Sea would not appear to originate due to thermal conditions off Somalia an over the Arabian Sea. Jet-speed winds occurred over sea in the moist air and not near the boundary of moist and dry airmasses  they occurred below above or  in isothermal layer or in the and their airmasses with nearly moist adiabatic lapse and the other moist adiabiatic lapse.

(5) The low-level jet over the Peninsular India is a continuation of that over the Arabian Sea, which in turn,  would appear to be a coatinuation of that which crosses equator between Longs. 37° and 45°E and has its origin in the southern hemisphere  as seen from paper of Findlater (1969) and as discussed by Desai (1972); the cold fronts in the mid latitude of that  hemisphere moving eastwards initiate the low-level Jet, the topographical features to the west of the eastern coast of Africa and of Madagascar directing it northwards across the Mozambique channel and also enhancing its speed.

(6) The need for closer network of Rawin and Radiosonde observation over the equator between longitude of Nairobi and 45 degrees East in any future MONEX expeditions has been emphasized.