Title: Isolating the Atmospheric Circulation Response to Arctic Sea-Ice Loss in the Coupled Climate System
Abstract: Arctic sea ice loss amplifies greenhouse warming, but it’s difficult to determine how specific changes in the climate system might be attributed to sea ice loss or to other drivers like low-latitude warming. In this talk we explore a way to attribute the atmospheric circulation response to sea-ice loss in earth-system model (ESM) simulations of greenhouse warming. We use many centuries of output from coupled ocean/atmosphere/land/sea-ice ESM simulations driven separately by sea-ice albedo reduction and by projected greenhouse-dominated radiative forcing. Compared to greenhouse forcing, sea-ice albedo reduction drives a lot of sea ice loss but relatively little low-latitude warming in the coupled system. This means that greenhouse forcing and sea-ice albedo reduction drive partially independent signals. We further isolate these signals using a technique of “pattern scaling”, in which we assume that part of the circulation response is proportional to low-latitude warming with no sea ice loss and the rest to sea ice loss with no low-latitude warming. We find that sea ice loss often cancels the circulation response driven from lower latitudes, but it can also act to reinforce the response, or drive changes that are structurally different. We will mostly focus on the NCAR CESM1 model but, in NCAR CCSM4 and ECCC’s CanESM2, many of the coupled responses to sea ice loss are similar. We find that internal variability can easily contaminate the estimated signals if not enough independent climate states are used to construct them. Time permitting, we will see how some of the projected changes in the midlatitude and SubArctic ocean surface temperatures brought about by sea ice loss might in turn warm up the Arctic free troposphere through atmospheric poleward heat transport.