A 'northern oscillation' relating northern hemispheric pressure anomalies and the Indian summer monsoon?

Analysis of the causes of the Indian monsoon have concentrated on climatological phenomena in the southern hemisphere, such as El Niño and the Southern Oscillation. By contrast, we have examined meteorological records for the northern hemisphere (0°-180°) spanning much of this century. We find that zonally-integrated mean sea-level pressure anomalies across Eurasia during January to April exhibit a statistically significant negative correlation between values for higher and sub-tropical latitudes. We further find that there is an association between above-normal activity of the Indian monsoon and a steep poleward-directed pressure anomaly gradient, which tends to persist through the summer and may indicate a strong zonal flow in the circumpolar westerlies. On the other hand, when this pressure anomaly gradient reverses, zonality is disturbed and the Indian monsoon tends to be drier than usual. Under these conditions, the circumpolar flow becomes persistently meridional as a result of a predisposition to blocking and splitting of the jet. © 1985 Nature Publishing Group