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

Two possibilities are compared by a back-of-envelope calculation

Is it due to a reduction in heat transport?

Is it because of the influx of cold meltwater as the ice sheet is melting?

Many people suggest the latter. The AMOC decline can be slowed by the meltwater but not because it is cold but because it is freshwater.

AMOC heat transports.

The heat transport can be obtained by taking the AMOC flow rate times the temperature difference between the north and south branches.

17,000,000 m 3 /s x 15 K x 1025 kg/m 3 x 4 kJ/kgK is the total.

The heat capacity of the water is 4 kJ/ kgK.

According to model simulations, the cooling trend can be explained by the AMOC being weakened by 15 %. The impact of Greenland melting is not negligible and other factors like increasing precipitation probably play a larger role in slowing down.

The ice melt in the island nation.

The temperature difference between the meltwater and the water it replaces is known as the ocean temperature. We are interested in the long-term temperature trend over the region with the meltwater properly mixed in, not at some temporary patches of meltwater floating locally.

For the last two decades, the total mass loss in Greenland has averaged 270 Gt.

Around 100 Gt/year is the average for the amount of ice and meltwater that ends up in the ocean.

The AMOC flow is 5000 times larger than 100 Gt per year.

The cooling rate is assumed to be 10 C in the summer and 0 C in the winter.

1.2 x 10 11 W is the number of km/s x 10 k.

The cooling effect of a 15 % AMOC slowdown is over 1000 times larger than the cooling effect of the meltwater.

To melt ice in the ocean requires a lot of energy. The fusion of water has a heat of 334 kJ/kg.

If we talk about the direct impact of stuff coming off Greenland than ice is the dominant factor and the energy that is required to melt the ice is not as important. Regardless of whether we take the numbers of Box et al. or other estimates, the effects of meltwater and icebergs entering the ocean aredwarfed by the weakened AMOC. The contribution to that is not because the meltwater is cold, but because it is fresh and has no salt.

The idea that it isn't due to a direct effect of cold meltwater influx is supported by this. In the image above, you can see the temperature change at the coast of the island.

Increased heat loss to the atmosphere may have been the cause of the cold blob. It's relevant for short-term weather variability, but I don't think it can explain the long-term trend.

There are references.

The Atlantic meridional heat transports are computed. Allow. The year 1919- 1927.

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G.

The twentieth-century slowdown in Atlantic Ocean overturning circulation has been reported. Nature Climate Change, 5, 475– 480.

NASA Vital signs can be found atclimate.nasa.gov/vital-signs/ice-sheets/

Climate disequilibrium and sea-level rise can be seen in the picture. There is a change in the 12 (9).