Connecting ocean circulation with ocean heat storage: experiments with an idealised aquaplanet model

Abstract

Global warming is really 'ocean warming' as ~93% of the excess heat energy entering the climate system is taken up by the oceans. The heat storage distribution is highly nonuniform in space, affected by a multitude of ocean heat uptake (OHU) processes across many spatial scales. These processes contribute to a substantial spread in climate model projections of future heat storage, sea-level rise, and surface warming. A better understanding of OHU processes is essential in order to reduce our uncertainty in climate projections.

I probe OHU processes using a coupled climate model with idealised geometry ('Double-Drake') under an abrupt CO2-doubling. By projecting the Double-Drake circulation onto depth-temperature coordinates, I find that the ocean circulation response leads to an increase in the downward vertical heat transport of ~0.2 W/m^2, enabling more interior heat storage. This supports the idea that changes in ocean circulation may further reinforce the ocean's role as a buffer of surface warming.

I examine in more detail the circulation response of an analogous Atlantic meridional overturning circulation (AMOC) in Double-Drake. Many models project a weakening of the AMOC in the future, where the implications for ocean warming are highly uncertain. I find that the link between the AMOC and heat storage rate is counterintuitive in Double-Drake and that AMOC weakening may not be as important as traditionally thought.

Finally, I explore the representation of OHU in two-layer energy-balance models (EBMs) and find that increasing/decreasing the sensitivity of OHU to the stratification in the EBM improves/worsens the representation for Double-Drake and a suite of other coupled climate models. This suggests that improving OHU representation might be achieved via a simple change to the two-layer EBM.

The results reaffirm that an understanding of OHU is incomplete without a grasp of the underlying ocean circulation processes.