Introduction

Mechanisms of Decadal Variability of the North Atlantic Tripole 1951-2000

Dr. Edwin Schneider 
Center for Ocean- Land- Atmosphere Studies, Institute of Global Environment and Society, USA

Abstract

The role of weather noise forcing of low frequency North Atlantic tripole variability is inferred for the observed 1951-2000 climate using the COLA CGCM. The atmospheric feedback to the 1951-2000 SST evolution is found from AMIP-type AGCM ensemble simulations forced by the observed 1951-2000 SST evolution. The weather noise surface fluxes are then estimated by subtracting the atmospheric feedbacks from the 1951-2000 surface fluxes as represented by the NCEP reanalysis. The weather noise surface fluxes are used to force the Interactive Ensemble (IE) CGCM, a model in which the weather noise forcing is eliminated by construction, but the feedbacks between the atmosphere and ocean are fully represented. The climate variability simulated by the IE is regressed against the North Atlantic Tripole index evaluated from the observed SST. This analysis shows that the weather noise forced IE approximately simulates the observed SST variability, and demonstrates that the tripole SST mode is weather noise forced rather than an unstable mode of the coupled system. The dominant component of the weather noise forcing for both modes is shown to be the heat flux, rather than the momentum or fresh water flux. The dynamical relationships between the tripole mode and the North Atlantic gyre and meridional overturning circulations are evaluated by lag regressions. The feedback parameters introduced in the simple model of Marshall et al (2001) are estimated from this analysis. Implications for decadal predictability are discussed.