Atmospheric response to Arctic sea ice loss moderated by (multi-) decadal ocean variability

Abstract:

Increasing evidence suggests that ongoing reductions of Arctic sea ice may affect various aspects of Northern Hemisphere weather and climate. Many of these linkages have been hypothesized based on statistical associations found in observations; however, it is difficult to unambiguously assign causality and to separate the influences of multiple interconnected processes in the climate system using observations alone. Modeling studies offer a way forward for understanding and isolating the physical processes underlying observed relationships. The atmospheric response to Arctic sea ice loss is often estimated through atmospheric general circulation model (AGCM) simulations with prescribed sea ice and sea surface temperature (SST) conditions. Typically, global SSTs are held to climatological-mean values. It is well known however, that (multi-) decadal ocean variability has a strong influence on the mean atmospheric state and thus, the atmospheric response to sea ice loss may be sensitive to the phase of (multi-) decadal ocean variability (i.e., be state dependent). Here we explore the atmospheric response to Arctic sea ice loss under different phases of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), using 8 ensembles of AGCM simulations; with extensive and reduced Arctic sea ice applied alongside PDO-, PDO+, AMO- and AMO+ SST anomalies. Preliminary analyses suggest, amongst other things: 1) an enhanced Arctic winter warming response during PDO- compared to PDO+; and 2) during AMO- sea ice loss forces a wintertime ridge-trough pattern over North America, with warmer conditions over the west and colder conditions over the east, compared to the response to identical sea ice loss during AMO+. The largest observed losses of Arctic sea ice have occurred since ~2000 during predominantly PDO- and AMO+. The possible implications of the recent switch to PDO+ in 2014, and an eventual return to AMO+ in coming decades, will be discussed.