GFI/BCCR Seminar: Interference between Forced and Unforced Climate Variability in the North Atlantic and the Arctic

Neil Tandon (Department of Physics, University of Toronto, Canada):

Interference between Forced and Unforced Climate Variability in the North Atlantic and the Arctic

Abstract
Numerous studies have suggested that variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC) may drive predictable variations in North Atlantic sea surface temperature (NASST). However, two recent studies (Medhaug and Furevik, 2011; Zhang and Wang, 2013) showed results suggesting that coupled models disagree on both the sign and the phasing of the correlation between AMOC and NASST indices. Their analyses were based on linearly detrended output from 20th century historical simulations in CMIP3 and CMIP5. I argue that the apparent disagreement among models arises from a commingling of two processes: 1) a ``bottom-up'' effect in which unforced AMOC changes lead to NASST changes of the same sign, and 2) a ``top-down'' effect in which forced NASST changes lead to AMOC changes of the opposite sign. Linear detrending is not appropriate for separating these two effects because the timescales of forced and unforced variations are not well-separated. I explore the implications of these findings to predictability of NASST, as well as the understanding of Arctic sea ice variations. This ends up raising basic questions about the role of the ocean circulation in Arctic sea ice variability and the relationship between the ocean and atmospheric circulations.