Air-sea fluxes in a climate model using hourly coupling

Abstract:

The coupling frequency of once per day that is used in most CGCMs contributing to IPCC AR5 will become inadequate when increasing spatial resolutions of CGCMs contributing to the coming AR6. In this study, we analyze the changes in the air-sea fluxes of the momentum, heat and fresh water flux caused by increasing the coupling frequency from once per day to once per hour using the Max-Planck Institute Earth System Model. We focus on changes in the mean, the variance and the extremes of these fluxes. We diagnose the role of the averaging and high-frequency feedbacks on these changes and identify a feedback loop involving the fluxes.

We found that the mean of the momentum-flux magnitude is decreased up to -7% in the tropics and increased up to 10\% in the Southern Ocean. These changes result solely from feedbacks between atmosphere and ocean occurring on time scales shorter than a day. The variance and extremes of all the fluxes are intensified in most part of the oceans. Exceptions are found for the momentum and fresh water fluxes in the tropics. The increases result mainly from the average procedure, while the decreases in the tropics are caused by the high-frequency feedbacks. The variance increases are substantial, reaching up to 50% for the momentum flux, 100% for the fresh water flux, and factor 15 for the net heat flux. These diurnal and intra-diurnal variations account for up to 50% to 90% of the total variances and exhibit distinct seasonality.

The diurnal cycle of net heat flux accounted only in hourly coupled experiment increases the meridional gradient of SST of the Southern Ocean. The atmospheric circulation responds to the increased meridional gradient of SST by a increase in the westerly wind stress, which in turn cools the in situ SST. The feedback suggests that increasing coupling frequency can lead to systematic changes in the air-sea general circulation.