Response of the North Atlantic dynamic sea level and circulation to Greenland meltwater and climate change in an eddy-permitting ocean model

Oleg A Saenko,Paul G Myers,Duo Yang

doi:10.1007/s00382-016-3495-7

Abstract

The response of the North Atlantic dynamic sea surface height (SSH) and ocean circulation to Greenland Ice Sheet (GrIS) meltwater fluxes is investigated using a high-resolution model. The model is forced with either present-day-like or projected warmer climate conditions. In general, the impact of meltwater on the North Atlantic SSH and ocean circulation depends on the surface climate. In the two major regions of deep water formation, the Labrador Sea and the Nordic Seas, the basin-mean SSH increases with the increase of the GrIS meltwater flux. This SSH increase correlates with the decline of the Atlantic meridional overturning circulation (AMOC). However, while in the Labrador Sea the warming forcing and GrIS meltwater input lead to sea level rise, in the Nordic Seas these two forcings have an opposite influence on the convective mixing and basin-mean SSH (relative to the global mean). The warming leads to less sea-ice cover in the Nordic Seas, which favours stronger surface heat loss and deep mixing, lowering the SSH and generally increasing the transport of the East Greenland Current. In the Labrador Sea, the increased SSH and weaker deep convection are reflected in the decreased transport of the Labrador Current (LC), which closes the subpolar gyre in the west. Among the two major components of the LC transport, the thermohaline and bottom transports, the former is less sensitive to the GrIS meltwater fluxes under the warmer climate. The SSH difference across the LC, which is a component of the bottom velocity, correlates with the long-term mean AMOC rate.

Highlights

The dynamic sea surface height (SSH), or sea level relative to geoid, reflects surface and, in many places, subsurface geostrophic circulation in the ocean (Wunsch 1997)
Mean SSH is considerably lower in the northern North Atlantic than in the North Pacific
It is thought that this is in part due to the formation of deep, dense water in the northern North Atlantic and the associated Atlantic meridional overturning circulation (AMOC)

Summary

Introduction

The dynamic sea surface height (SSH), or sea level relative to geoid, reflects surface and, in many places, subsurface geostrophic circulation in the ocean (Wunsch 1997). While in both these regions the SSH field is mostly negative (relative to the global mean), reflecting the predominantly cyclonic character of the large-scale ocean circulation, the mean SSH is considerably lower in the northern North Atlantic than in the North Pacific This SSH difference between the two basins is thought to be maintained, at least in part, by the formation of deep water in the northern North Atlantic, associated with the Atlantic meridional overturning circulation (AMOC). Scaling arguments (e.g., L05) do suggest a simple linear relationship between the AMOC rate and SSH change To confirm their scaling, L05 ran a set of freshwater hosing experiments under present-day climate conditions, aimed at simulating a set of ocean states with progressively weaker AMOC. L05 found, in particular, that in an extreme case of complete AMOC collapse, corresponding in their model to a freshwater flux of 0.35 Sv,

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