Effects of fluctuating daily surface fluxes on the time-mean oceanic circulation

Abstract

The effect of fluctuating daily surface fluxes on the time-mean oceanic circulation is studied using an empirical flux model. The model produces fluctuating fluxes resulting from atmospheric variability and includes oceanic feedbacks on the fluxes. Numerical experiments were carried out by driving an ocean general circulation model with three different versions of the empirical model. It is found that fluctuating daily fluxes lead to an increase in the meridional overturning circulation (MOC) of the Atlantic of about 1 Sv and a decrease in the Antarctic circumpolar current (ACC) of about 32 Sv. The changes are approximately 7% of the MOC and 16% of the ACC obtained without fluctuating daily fluxes. The fluctuating fluxes change the intensity and the depth of vertical mixing. This, in turn, changes the density field and thus the circulation. Fluctuating buoyancy fluxes change the vertical mixing in a non-linear way: they tend to increase the convective mixing in mostly stable regions and to decrease the convective mixing in mostly unstable regions. The ACC changes are related to the enhanced mixing in the subtropical and the mid-latitude Southern Ocean and reduced mixing in the high-latitude Southern Ocean. The enhanced mixing is related to an increase in the frequency and the depth of convective events. As these events bring more dense water downward, the mixing changes lead to a reduction in meridional gradient of the depth-integrated density in the Southern Ocean and hence the strength of the ACC. The MOC changes are related to more subtle density changes. It is found that the vertical mixing in a latitudinal strip in the northern North Atlantic is more strongly enhanced due to fluctuating fluxes than the mixing in a latitudinal strip in the South Atlantic. This leads to an increase in the density difference between the two strips, which can be responsible for the increase in the Atlantic MOC.

Climatic impacts of stochastic fluctuations in air–sea fluxes

“…First, because of the above mechanism, the mixed layer is expected to deepen systematically, consistent with Balan Sarojini and von Storch [2009]. Then, contact with deeper water is expected to cool the mixed layer, especially on the thermal equator where the mixed layer is shallowest.…”

“…[21] In this study, stochastic fluctuations have been applied to the air-sea buoyancy fluxes in a comprehensive climate model. Unlike related previous work, which has employed an ocean general circulation model coupled only to a simple empirical model of atmospheric dynamics [Balan Sarojini and von Storch, 2009], the present work has employed a full coupled atmosphere-ocean general circulation model. This advance allows the feedbacks in the coupled system to be captured as comprehensively as is permitted by contemporary high-performance computing, and it allows the impacts on the atmospheric circulation to be studied.…”

“…In atmosphere models, stochastic perturbations are applied routinely in many operational numerical weather prediction centers [e.g., Buizza et al, 1999Buizza et al, , 2005. In ocean models, stochastic air-sea fluxes derived from an empirical model of atmospheric dynamics deepen the mixed layer [Balan Sarojini and von Storch, 2009] and stochastic forcing prolongs the duration of deep convection compared to deterministic forcing [Kuhlbrodt and Monahan, 2003]. Stochastic perturbations are absent from most coupled atmosphere-ocean general circulation models used for climate simulation and prediction, however.…”

Stochastic Parameterization: Toward a New View of Weather and Climate Models

“…Mechanisms for how Gaussian zero-mean fluctuations can change the mean state (see Figure 1) have been discussed e.g. in Tompkins and Berner (2008) and Beena and von Storch (2009). Tompkins and Berner (2008) introduce perturbations to the humidity field and find that positive perturbations are more likely to trigger a convective event than negative perturbations can suppress convection.…”

The Impacts of the Oceans on Climate Change

“…Density contrasts caused by spatially differing air-sea heat exchange are one of the three driving forces behind the MOC; others are density contrasts due to spatially differing freshwater exchange (haline forcing) (Saenko et al, 2002) and surface flux of momentum (wind stress forcing) (Beena and von Storch, 2009;Chelton et al, 2001;Delworth and Greatbatch, 2000). Using models to help determine the relative importance of these three driving forces is an important research area for Climate Change.…”