Changes in atmospheric circulation over Northern Hemisphere oceans associated with the rapid warming of the 1920s

Global mean surface temperature has increased since the late 19th century. The warming occurred largely during two periods: 1920-1940, and since the m id-1970s. Although most recent studies have focused on the latter period, i t is of interest to analyse the earlier period and compare its major featur es to the recent warming episode. The warming during 1920-1940 occurred mos t rapidly during the 1920s. It was strongest at high northern latitudes in winter, a pattern now believed to be characteristic of 'greenhouse warming' . This warming of the Arctic was much discussed during the 1930s and 1940s, but the data available at that time were mostly derived from land areas. I n this paper, we use the COADS marine data set and recent compilations of l and surface temperature data sets to examine the behaviour of the surface f ields over the ocean during this event. Considering the thermal and atmospheric fields at the surface, the stronges t signal occurs in the North Atlantic Ocean during winter, being distinct b ut more gradual in the other oceans and seasons. The Northern Hemisphere co ntinental record shows that both middle and high latitudes experienced rapi d warming in the early 20th century warming interval (the 1920s and 1930s, hereafter referred to as ETCW). Temperature data for northern tropics, whil e displaying similar general characteristics, exhibit some differences with regard to timing and rates of change. There is a suggestion of weakening of the westerlies and the trade wind sys tem in the 1930s, following an intensification of the westerlies across the North Atlantic during the previous two decades. This weakening may be rela ted to a lessening of atmospheric baroclinicity in association with the fac t that the amplitude of warming at high latitudes was much greater than tha t in low latitudes, reducing the mean meridional thermal gradient, and ther efore the geostrophic pressure gradient. There is some indication that the North Atlantic and North Pacific high-pre ssure systems shifted northward. Coincident with this northward shift of th e subtropical highs, typhoons in the Northwest Pacific and hurricanes in th e North Atlantic became more numerous in this period of rising temperature, which we suggest is linked to a northward shift of the respective near-equ atorial convergence zones. Concomitant to the weakening of the westerlies a nd trade wind systems, the Asian monsoon troughs deepened substantially, a situation generally favourable to the development of active monsoons. It is thought that the combination of these two features-enhanced continental mo nsoons and implied lowered vertical wind shear over the oceans-would tend t o enhance the release of latent heat in the tropics, representing strengthe ned Hadley and Walker circulations, which may have been at least partly res ponsible for greater aridity in subtropical land areas of both hemispheres during this period. The latter is also consistent with an expansion and/or strengthening of the subtropical high-pressure belt into the continents. Co pyright (C) 1999 Royal Meteorological Society.