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RealClimate: The water south of Greenland has been cooling, so what causes that?

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Sea surface temperature trend 1993 – 2018, from European Atlas of the Seas

Let’s compare two possibilities by a back-of-envelope calculation.

(1) Is it due to a reduced heat transport of the Atlantic Meridional Overturning Circulation (AMOC)?

(2) Or is it simply due to the influx of cold meltwater as the Greenland Ice Sheet is losing ice?

The latter is often suggested. The meltwater also contributes indirectly to slowing the AMOC, but not because it is cold but because it is freshwater (not saline), which contributes to the first option (i.e. AMOC decline).

AMOC heat transport

For that we take the AMOC flow rate times the temperature difference of 15 °C between the northward upper branch and southward deep return flow to obtain the heat transport.

17,000,000 m3/s x 15 K x 1025 kg/m3 x 4 kJ/kgK = 1 PW (1)

(Here, 1 PW = 1015 Watt and 4 kJ/kgK is the heat capacity of water.)

An AMOC weakening by 15 % thus cools the region at a rate of 0.15 PW = 1.5 x 1014 W and according to model simulations can fully explain the observed cooling trend (2). Of…

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