231 

 to the eastward. The effect of this cool water is to cool the lower atmos- / 



phere and thereby increase the surface pressure. Normally at this season a 



belt of high pressure lies along the 10 S latitude line at 500 millibars. 



With the cooling of the lower air below and to the north of this belt, the 



surface high develops a weak ridge extending northward along the African 



Coastal waters to about 10 N. The characteristics and semi -permanent nature 



of this ridge of high pressure are clearly shown on the IGY Tropical Zone 



Weather Maps published by the Seewetteramt. Deutscher Wetterdienst. Hamburg. 



Study of the maps for July, August, and September 1957, shows the ridge to 



be present on all maps with minor variations in amplitude and position. 



Adjacent to this ridge is a trough of low pressure lying to the west over 



East Africa. This trough is caused by the intense solar heating of the land 



and air. Its efgect is to greatly increase the pressure gradient in the 



region around 10 N, 50 E. The surface pressure map for August 30. 196U, ^ 



has been drawn from data made available by the International Meteorological 



Center. Bombay, and presented as Figure 3- 



(3) Local interactions . As a result of the steep pressure gradient 

 developed along the coast of northeast 'Africa, the air that has moved slowly 

 across the equator into the regions begins to accelerate and quickly attains 

 higher velocities. Once the air has attained relatively high velocities 

 north of about 5 N, the transport imposed upon the surface water is greatly / 

 increased. With this increase in transport the upwelling of bottom water is 

 greatly increased and the surface temperatures drop many degrees over an area 

 of more than 100 square degrees . Figure "^ presents a chart of surface water 

 temperatures drawn by H- Stommel and B- Warren of W. H.O.I. The data were 

 obtained from the research vessels. ARGO, of Scripps Institute of Oceanography 

 and DISCOVERY - of the National Institute of Oceanography during their cruises 

 in August 196'+, to the Somali Current region. It is seen that immediately 

 off the coast at 9 N, exceedingly cold water with temperatures down to I3 C, "^ 

 was observed. 



This colder water continues the interaction cycle by cooling the sir at 

 a greatly accelerated rate. The intensity of this cooling is shown in 

 Figure 5 which presents dropsonde and psychrograph data obtained from the 

 C-5^ aircraft on August 30, 196^. The figure is a cross section of the 

 atmosphere from the Gulf of Aden to ^4-° N, 56 E. Potential temperatures 

 were plotted on the diagram and isentropes drawn from the data points. The 

 cooling of the air by the water about 200 kilometers southeast of the mouth 

 of the Gulf of Aden is very apparent. This highly localized cooling Is 

 superimposed upon the pattern of a general warming of the air from south to 

 north . 



To explain the increase in wind speed with height to the jet maximum 

 and its subsequent decrease above this level, the temperature cross section 

 made on August 30, 196^, must be studied. It will be noted that the tempera- 

 ture increases to the right of the wind, indicating that the wind will in- 

 crease with height. Above the 900 mb level or the level of the jet maximum, 

 the air temperature decreases to the right of the wind and hence the wind 

 should decrease with height. Measurement of the horizontal temperature 

 gradient and application of the thermal wind shows that the wind should 



