Ice sheet melt and the Atlantic Ocean

Abstract

The ocean is nestled in between the continents and exchanges heat and water with the atmosphere. The wind pushes against the water and aids the heat exchange. The Atlantic part of the ocean stretches from the Arctic in the far north to the beginning of the Southern Ocean around Antarctica, connecting these two cold areas with the warmth of the equatorial region. The ocean is not uniformly salty. Currents, evaporation and precipitation, rivers and icebergs continually change the salinity at the ocean surface and to depth. One of these currents is the so called Atlantic Meridional Overturning Circulation (AMOC) and consists of a northward flowing shallow branch and a deeper southward branch and is part of the entire Atlantic Ocean.

The AMOC is the result of the effects of wind and heat loss to the atmosphere in the cold North. The upper ocean needs to have a sufficiently high salinity for the upper AMOC branch to feed the lower branch. Under climate change the polar ice sheets will melt at an increasing rate and more freshwater will be led to the ocean surface, which leads to a freshening of the upper ocean. There is a risk that the AMOC will decrease in strength or shuts down entirely.

Melting of the polar ice sheets can be easily mimicked in climate models by adding freshwater to the ocean surface. The salt distribution changes by the currents, like the AMOC, at the surface, and eventually spreads to depth and further from the poles. The Atlantic Ocean loses salt by adjustment in the ocean salt transports through Bering Strait and the interface with the Southern Ocean. The AMOC itself remains relatively unaffected, though the connection between the shallow branch and the deeper branch changes by shifting to the North, into the Arctic.

The absence of drastic changes in the AMOC strength is remarkable when compared against with the distant past when the AMOC oscillated between vigorous and weakened states. No clear picture has emerged how such abrupt transitions in AMOC strength came about, and whether current climate change AMOC stability is at risk. What is clear is that climate models represent the AMOC and its critical transitions in various ways. The response of the Atlantic Ocean is complicated and the future of the AMOC remains uncertain.