In the modeling community, there has been a general agreement that the tropical Walker circulation has gone a weakening, in response to the global warming caused by anthropogenic emission of greenhouse gases. Furthermore it has been argued that the weakening will continue throughout the 21st century (e.g., Vecchi et al. 2006). Contradictory to such assertion, recent study by Sohn and Park (2010) showed that the Walker circulation intensity has been strengthened during the last three decades. This talk presents results from recent exploratory study using long-term observations to question whether strengthened Walker circulation is also visible in other observational data.

Hypothesis is as follows. If there has been intensified Walker circulation over the tropics, the resultant changes may be detected in the deep convective clouds as well as in the upper tropospheric humidity (UTH) over the western Pacific warm pool region. At the same time the sea level pressure (SLP) gradient between the East Pacific and Western Pacific should be increased in response to the strengthened Walker circulation. It is shown that the long-term observations of deep convective clouds, UTH, and SLP all are much coherent with the hypothesized increased Walker circulation, strongly suggesting that the Walker circulation has been intensified in the last three decades. These are all appear to be associated with a decadal variation of ENSO, i.e. SST variations showing central/western Pacific warming and eastern Pacific cooling over the last three decades.