Warming of Atlantic Ocean in mid-1990s due to natural ocean oscillations

A paper published last week in the Journal of Climate attributes warming of the North Atlantic ocean surface temperatures in the mid-1990s to a prolonged positive phase of the North Atlantic Oscillation (NAO), a natural ocean oscillation with no relation to 'greenhouse gas' concentrations. 

Journal of Climate 2012 ; e-View

doi: http://dx.doi.org/10.1175/JCLI-D-11-00443.1

Causes of the Rapid Warming of the North Atlantic ocean in the mid 1990s

Jon Robson* and Rowan Sutton

NCAS-Climate, Department of Meteorology, University of Reading, UK.

Katja Lohmann

Max Planck Institute for Meteorology, Hamburg, Germany.

Doug Smith and Matthew D. Palmer

Met Office Hadley Centre, Exeter, UK.

Abstract

In the mid-1990s the subpolar gyre of the North Atlantic underwent a remarkable rapid warming, with sea surface temperatures increasing by around 1C in just 2 years. This rapid warming followed a prolonged positive phase of the North Atlantic Oscillation (NAO), but also coincided with an unusually negative NAO index in the winter of 1995/96. By comparing ocean analyses and carefully designed model experiments we show that this rapid warming can be understood as a delayed response to the prolonged positive phase of the NAO, and not simply an instantaneous response to the negative NAO index of 1995/96. Furthermore, we infer that the warming was partly caused by a surge, and subsequent decline, in the Meridional Overturning Circulation and northward heat transport of the Atlantic Ocean. Our results provide persuasive evidence of significant oceanic memory on multi-annual timescales, and are therefore encouraging for the prospects of developing skillful predictions.