Secular changes of the standing circulation

The evidence produced seems to show a secular intensification of the North Indian summer‐monsoon low. The effects upon rainfall in North India and the Peninsula are discussed qualitatively. The secular decrease of the mean summer pressures in North India was associated with an increase of winter pressure. Similar pressure changes, with an even larger seasonal amplitude, occurred throughout eastern Europe, Siberia and eastern Asia. The secular increase in the seasonal pressure oscillation over the interior of Eurasia may have been associated with variations in the wind field which should tend to cause colder winters in the Far East and warmer, shorter winters in Europe. In summer the opposite tendency should prevail. An analysis of records confirms this inference. An intensification of the standing circulation of the northern hemisphere may have caused an increased transport of warm air into the Arctic and hence contributed to the retreat of the arctic ice. The absence of large‐scale standing oscillations in the southern hemisphere, accounts, probably, for the apparent absence of marked climatic change in the Antarctic. The strength of the standing monsoon‐type circulations appears to be in a roughly inverse relation to the strength of the zonal circulation. The resulting interaction could cause a large amplification of small disturbances in the climatic equilibrium.