Regional Patterns of Tropical Indo - Pacific Climate Change : 1 Evidence of the Walker Circulation Weakening

17 18 Regional patterns of tropical Indo-Pacific climate change are investigated over the last six 19 decades based on a synthesis of in situ observations and ocean model simulations, with a 20 focus on physical consistency among sea surface temperature (SST), cloud, sea level 21 pressure (SLP), surface wind, and subsurface ocean temperature. A newly-developed 22 bias-corrected surface wind dataset displays westerly trends over the western tropical 23 Pacific and easterly trends over the tropical Indian Ocean, indicative of a slow down of 24 the Walker circulation. This pattern of wind change is consistent with that of observed 25 SLP change showing positive trends over the Maritime Continent and negative trends 26 over the central equatorial Pacific. Suppressed moisture convergence over the Maritime 27 Continent is largely due to surface wind changes, contributing to observed decreases in 28 marine cloudiness and land precipitation there. 29 Furthermore, observed ocean mixed-layer temperatures indicate a reduction in 30 zonal contrast in the tropical Indo-Pacific characterized by larger warming in the tropical 31 eastern Pacific and western Indian Ocean than in the tropical western Pacific and eastern 32 Indian Ocean. Similar changes are successfully simulated by an ocean general circulation 33 model forced with the bias-corrected wind stress. Whereas results from major SST 34 reconstructions show no significant change in zonal gradient in the tropical Indo-Pacific, 35 both bucket-sampled SSTs and nighttime marine-air temperatures (NMAT) show a 36 weakening of the zonal gradient consistent with the subsurface temperature changes. All 37 these findings from independent observations provide robust evidence for ocean38 atmosphere coupling associated with the reduction in the Walker circulation over the last 39 six decades. 40