Examining Arctic warming amplification linked to tropical convection in reanalysis and CMIP5 simulations

Abstract

This study explores the mechanism of Arctic amplification driven by tropical convection related to the Walker circulation (WC) change, through Rossby wave propagation in the Coupled Model Intercomparison Project Phase 5 (CMIP5) and European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) reanalysis data. Results from 38 CMIP5 model reveal that 7 models having increased trends of Walker circulation index (WCI) reproduce La Nina-like decadal difference pattern (1996-2012 minus 1979-1995) of water vapor transport similar to 6 different kind of reanalysis products, while 7 other models with decreased WCI trends seems to show a warming pattern across Pacific oceans sandwiched by disperse divergence regions. Linear trend of upper level stream function in ERA-Interim exhibits a clear poleward Rossby wave propagation, in the annual mean sense. It seems to be intensified during winter. Such behaviors are well simulated by the ensemble of positive WCI CMIP5 models but with much weaker signals. By contrast, the ensemble of negative WCI CMIP5 models shows a completely opposite wave phase. Positive WCI CMIP5 models also reproduce much weaker ERA-Interim North-western Pacific and North-eastern Canada-Greenland warming. Whilst, negative WCI CMIP5 models do not show the North-western Pacific warming at all and only give weaker trend in the North-eastern Canada region even when observed SST was subscribed into the representative model. In the Arctic, all results show downwelling longwave radiation, related to ice and liquid water content, contributing to the warming in the Arctic.