North Atlantic 20th century multidecadal variability in coupled climate models: sea surface temperature and ocean overturning circulation

I Medhaug,T Furevik

doi:10.5194/os-7-389-2011

Abstract

Abstract. Output from a total of 24 state-of-the-art Atmosphere-Ocean General Circulation Models is analyzed. The models were integrated with observed forcing for the period 1850–2000 as part of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. All models show enhanced variability at multi-decadal time scales in the North Atlantic sector similar to the observations, but with a large intermodel spread in amplitudes and frequencies for both the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The models, in general, are able to reproduce the observed geographical patterns of warm and cold episodes, but not the phasing such as the early warming (1930s–1950s) and the following colder period (1960s–1980s). This indicates that the observed 20th century extreme in temperatures are due to primarily a fortuitous phasing of intrinsic climate variability and not dominated by external forcing. Most models show a realistic structure in the overturning circulation, where more than half of the available models have a mean overturning transport within the observed estimated range of 13–24 Sverdrup. Associated with a stronger than normal AMOC, the surface temperature is increased and the sea ice extent slightly reduced in the North Atlantic. Individual models show potential for decadal prediction based on the relationship between the AMO and AMOC, but the models strongly disagree both in phasing and strength of the covariability. This makes it difficult to identify common mechanisms and to assess the applicability for predictions.

Highlights

The ocean currents transport vast amounts of heat from low to high latitudes by the horizontal wind-driven gyre circulation and the density-driven thermohaline circulation (THC), with a maximum of ∼2 PW at 17◦ N (Trenberth and Caron, 2001)
In the Atlantic these two components make up the Atlantic Meridional Overturning Circulation (AMOC), which in a zonal mean is characterized by a cell of northward flowing warm and saline water in the upper 1000 m, above a southward flowing colder and fresher water down to 3– 4000 m
For most models we find a clear link between the AMOC and Atlantic Multidecadal Oscillation (AMO) time scales, but no relation is found between the dominant time scales and the strength of the overturning

Summary

Introduction

The ocean currents transport vast amounts of heat from low to high latitudes by the horizontal wind-driven gyre circulation and the density-driven thermohaline circulation (THC), with a maximum of ∼2 PW at 17◦ N (Trenberth and Caron, 2001). The poleward transport of heat in the upper cell is an important driver for the climate system. At high latitudes the ocean is subjected to intense heat loss to the atmosphere and the water loses buoyancy and sinks. The potential energy lost in the sinking process is regained by wind- and tidal mixing across stable stratification further south, and the deep water gradually returns to the surface (Wunsch, 2002). On very long time scales (order of 1000 years) the AMOC is sustained by mechanical energy input through wind- and tidal mixing

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Discussion

Conclusion