Ocean conveyor and climate

The ocean overturning circulation is potentially sensitive to climate change. In the north and south alike, human influence is less pronounced than we thought, but that is no reason to relax our watchfulness.

A weakening of the Atlantic meridional overturning circulation has emerged from noise after years of painstaking measurements. Three independent lines of evidence suggest that an anthropogenic influence on this overturning is not yet detectable.

Conversion of Antarctic circumpolar upwelling waters to less dense water has mainly been attributed to surface heat fluxes. An analysis of water-mass transformation shows that the dominant process is the formation of sea ice near Antarctica and its melt offshore.

The North Atlantic Oscillation has varied markedly on multidecadal timescales. Analyses of climate simulations show that these variations have contributed to Arctic sea ice loss, Northern Hemisphere warming and tropical storm activity.

The mid-1990s’ warming of the North Atlantic subpolar gyre was probably related to strengthened overturning. Observations and numerical models suggest that a climate reversal to a cooling trend occurred around 2005.

The Atlantic meridional overturning circulation has weakened over the past decade. Examination of a global reanalysis that matches independent observations shows that the decline is consistent with recovery from an earlier invigoration.

Freshwater release from melting polar ice could weaken the Atlantic overturning circulation. Eddy-resolving ocean simulations reveal that the freshening has not yet significantly affected meridional overturning, but an effect may emerge soon.

Meltwater runoff from the Greenland ice sheet alters ocean surface salinity. Numerical simulations show that meltwater from southeastern Greenland is transported to the Labrador Sea more efficiently than that from southwestern Greenland.

Sea-ice formation is a key factor in the lower branch of the Southern Ocean overturning circulation. Observation-based data in conjunction with a water-mass transformation framework reveal that sea ice plays a central role in the upper branch too.

Unlike the Arctic, the Southern Ocean has shown little warming. An analysis of observations and numerical simulations suggests that Southern Ocean warming patterns are shaped by meridional overturning more than surface heating.

The meridional overturning circulation of the ocean plays a central role in the climate and its variability. This Review of recent studies emphasizes the importance of wind-driven upwelling in the Southern Ocean for global ocean circulation.

The exchange of water across the Antarctic continental shelf break brings warm waters towards ice shelves and glacier grounding lines. Ocean glider observations reveal that eddy-induced transport contributes significantly to this exchange.

In the Southern Ocean, deep-water masses of the world ocean upwell to the surface and subsequently sink to intermediate and abyssal depths in two overturning cells. Observational evidence relates changes in abyssal mixing—a key influence on the lower cell—to oceanic eddy variability.

In the tropics, substantially more rain falls just north of the Equator. An analysis of satellite observations, reanalysis data and model simulations suggests that the meridional ocean overturning circulation contributes significantly to the tropical rainfall peak north of the Equator.

Uptake of atmospheric carbon dioxide in the subpolar North Atlantic Ocean declined rapidly between 1990 and 2006. An analysis of oceanographic data suggests that the slowdown of the meridional overturning circulation was largely responsible.

Hydrographic properties of the North Atlantic Ocean have changed significantly over the past decades. A combination of changes in seawater density, calculated from observed properties of sea water and a numerical ocean model, reveals that the strength of the meridional overturning circulation has changed in different directions in the subpolar and subtropical basins, respectively.