The effect of ocean mixed layer depth on climate in slab ocean aquaplanet experiments

The effect of ocean mixed layer depth on climate is explored in a suite of slab ocean aquaplanet simulations with different mixed layer depths ranging from a globally uniform value of 50-2.4 m. In addition to the expected increase in the amplitude of the seasonal cycle in temperature with decreasing ocean mixed layer depth, the simulated climates differ in several less intuitive ways including fundamental changes in the annual mean climate. The phase of seasonal cycle in temperature differs non-monotonically with increasing ocean mixed layer depth, reaching a maximum in the 12 m slab depth simulation. This result is a consequence of the change in the source of the seasonal heating of the atmosphere across the suite of simulations. In the shallow ocean runs, the seasonal heating of the atmosphere is dominated by the surface energy fluxes whereas the seasonal heating is dominated by direct shortwave absorption within the atmospheric column in the deep ocean runs. The surface fluxes are increasingly lagged with respect to the insolation as the ocean deepens which accounts for the increase in phase lag from the shallow to mid-depth runs. The direct shortwave absorption is in phase with insolation, and thus the total heating comes back in phase with the insolation as the ocean deepens more and the direct shortwave absorption dominates the seasonal heating of the atmosphere. The intertropical convergence zone follows the seasonally varying insolation and maximum sea surface temperatures into the summer hemisphere in the shallow ocean runs whereas it stays fairly close to the equator in the deep ocean runs. As a consequence, the tropical precipitation and region of high planetary albedo is spread more broadly across the low latitudes in the shallow runs, resulting in an apparent expansion of the tropics relative to the deep ocean runs. As a result, the global and annual mean planetary albedo is substantially (20 %) higher in the shallow ocean simulations which results in a colder (7C) global and annual mean surface temperature. The increased tropical planetary albedo in the shallow ocean simulations also results in a decreased equator-to-pole gradient in absorbed shortwave radiation and drives a severely reduced (a parts per thousand 50 %) meridional energy transport relative to the deep ocean runs. As a result, the atmospheric eddies are weakened and shifted poleward (away from the high albedo tropics) and the eddy driven jet is also reduced and shifted poleward by 15A degrees relative to the deep ocean run.