The contrasting circulation and cloudiness over the tropics during two extremes of the zonally oriented Walker circulation are described. During February 1971 the trade winds over the Pacific Ocean were very strong with fast upper tropospheric westerlies superimposed. This was associated with well-developed, high-level oceanic troughs over the east central Pacific both north and south of the equator. Tropical convection was largely confined to the three tropical continental areas, while the ITCZ over the Pacific was very weak.
During February 1969 the Walker circulation was considerably weaker. Tropical convection was more extensive over the central and eastern Pacific. The high-level mid-oceanic troughs were also weaker and even replaced by weak anticyclonic flow over the convective regions. Associated with this the sub-tropical jet stream was very much stronger over the eastern Pacific, North America and the Atlantic, but weaker near Japan.
It is suggested that dynamic instability in the westerlies of the subtropical jet stream plays an important role in regulating the intensity of the tropical circulation.