Response of Walker circulation to CO2 increases and its relation to tropical precipitation in CMIP5 models

Changes in Walker circulation induced by global warming are examined using concept of effective wind linked to water vapor transport, from outputs of 14 IPCC CMIP (5th Coupled Model Intercomparison Project) models under a 1% increase scenario of CO2 emissions. Then Walker circulation changes were examined, in conjunction with increases in rainfall over the western Pacific and ITCZ. Results show that the more rainfall shown in wet equatorial regions, pronounced in the Equatorial Pacific ITCZ, are not directly linked to weakened Walker circulation but rather to increased Hadley-type meridional circulation. On the other hand, increase in precipitation over the western Pacific is zonally associated more with the Indian Ocean, through the increase in water vapor transport from the Indian Ocean to the western Pacific, in response to CO2 increase. Analysis results strongly suggest that two epicenters of the Walker circulation cell are not zonally linked, but rather separated, responding differently to the CO2 warming. Caution should be taken when the pressure gradient between the western Pacific and eastern Pacific are used as a Walker circulation index because the changes in pressure gradient (often used for examining the Walker circulation change) are not due to the east-west coupled circulation.