Response of the ITCZ to imposed sea-ice loss in the Arctic: exploring a hierarchy of simple ocean models in a coupled framework

Recent studies that use coupled climate models to address the response to Arctic sea-ice loss (with no other forcing), find that the ocean plays an important role in terms of amplifying the atmospheric response as a result of the coupling. Dynamical adjustment in the ocean is also important for the response. In particular, these studies have shown a pronounced response in the Intertropical Convergence Zone (ITCZ), both in terms of location (more equatorward concentrated) as well as in intensity (more rainfall) when using fully coupled climate models, while coupling to a slab ocean model (SOM) simply moves the ITCZ away from the equator into the warmer hemisphere. However, fully coupled models are expensive to run and it is difficult to isolate processes in the ocean that may be important to the full response. Here we develop a hierarchy of increasingly physically realistic ocean models that are also computationally cheap to run. We will test this hierarchy with imposed sea-ice loss as forcing to determine how well they capture the response of the full ocean model. Ultimately, we can quantify the performance of models in "complexity-of-physics axis" to gain knowledge of atmosphere-ocean dynamics in terms of imposed sea-ice loss.

To begin with, we start by improving the most simple SOM whose governing equation is shown in the first line of the figure: h is the ocean mixed-layer depth that is constant in time, c_p the heat capacity, ρ the density of seawater, T the sea-surface temperature, F the surface heat fluxes (radiative + sensible + latent), and Q the heat flux that includes all unresolved processes such as horizontal heat transport and entrainment. The first problem with this model is that the use of a constant h ignores the seasonal variation in the mixed-layer depth which can exceed 50 meters over much of the ocean. Strong entrainment associated with a deepening mixed-layer during fall and winter can produce memory effect on the SST anomalies. This mechanism is thought to be essential for the generation of the Pacific Decadal Oscillation. To explore the effect of a variable mixed-layer depth we use a Niiler-Kraus mixed-layer model as our first improvement. The resulting equation is shown in the second line of figure: w_e is the entrainment velocity driven by surface fluxes, T_d the entrainment temperature. Q’ will be the new unresolved flux. The next unresolved process is horizontal heat transport. As a first approximation we include the horizontal transport due to Ekman flows. The resulting model is given in the last line of figure: where v_ek is the wind-driven Ekman velocity and Q’’ is again the updated unresolved flux.

Other unresolved processes such as geostrophic flow transport, Ekman induced downwelling, wave dynamics and mesoscale systems which are still under development. Finally, full ocean model stands at the top of this hierarchy serving as the “observation” benchmark.