REPORT ON OCEANIC CIRCULATION. 
15 
Mean Annual Temperature of the Surface of the Ocean. (Map 2.) 
In this map, showing the mean annual temperature of the surface of the ocean, the 
deeper red shaded portions indicate those regions where the temperature is highest, or 
where it exceeds 80°. 
The area of high temperature does not quite circle the equatorial region of the globe, 
as it does not occur in the Pacific from long. 117° to 140° W. In the Atlantic and 
eastern portion of the Pacific, these areas of high temperature lie north of the equator, 
which is due to the circumstance that in these parts of the ocean the south-east trades 
protrude north of the equator, being the simple result of the geographical distribution 
of atmospheric pressure there. Quite different is it in the western division of the 
Pacific where this area of high surface temperature extends east of Australia as far south 
as lat. 20° S. This striking extension southward of the temperature is occasioned by 
the circumstance that, for eight months of the year, the line of lowest barometric pressure 
is there to the south of the equator, and, necessarily, accompanied by northerly winds, 
which propel into more southern regions the warmer waters of the surface. The point 
here insisted on is vital to the whole question of oceanic circulation. 
Another important result of the geographical distribution of atmospheric pressure 
in the Atlantic and Pacific is that in the Atlantic the north and south trades are 
stronger and more persistent than those of the Pacific, and the inter-space between 
them characterised by calms and very light variable winds is therefore much narrower 
in the Atlantic than in the Pacific. From this it follows that the region of highest 
surface temperature in the Atlantic is comparatively much contracted in width, whereas 
in the Pacific it occupies a breadth nearly four times greater. 
To the north of lat. 13° S. the area of maximum surface temperature overspreads 
the whole of the Indian Ocean, including the Arabian Sea and the Bay of Bengal, 
except the north-western portion of the Arabian Sea. This comparatively low tempera- 
ture over the western division of the Arabian Sea is brought about by the prevailing 
north-westerly winds there in the summer and also the winter months, which drive the 
warm surface water to the south-eastwards, and thus bring, by upwelling, the colder 
waters of lower depths to the surface. The effect is well seen in the mean monthly 
temperature of the surface of the sea ofi“ the Arabian coast about long. 57° E., which, 
during the spring months, is 82°‘8, but falls in the summer months to 76°'3 when 
the N.W. winds are strongest, rising again in the autumn to 79°' 4. This low summer 
temperature is the more remarkable since the north-west winds which then prevail come 
from the deserts of Arabia, and are therefore hot, dry and rainless till the}^ have 
traversed the sea a considerable way to the south-east. On the other hand, the pre- 
vailing winds at this season at the head of the Bay of Bengal are southerly and 
