Modeled Sensitivity of the Upper-Ocean Response to Tropical Cyclones in the Northwestern Pacific Using a Fully-coupled Climate Model with Varying Ocean Resolution

Wednesday, 17 December 2014
Hui Li, University of Illinois at Urbana Champaign, Urbana, IL, United States and Ryan L Sriver, University of Illinois, Urbana, IL, United States
Tropical cyclones (TCs) actively contribute to Earth's climate by influencing oceanic mixing rates, surface fluxes, ocean heat budgets and transports, and large-scale circulations within the atmosphere and ocean. However, TC-climate effects are largely unexplored in fully-coupled Earth system models. Here we analyze results from century-scale pre-industrial control simulations using the high-resolution Community Climate System Model version 3.5 (CCSM3.5) (Kirtman et al., 2012). The modeling experiment consists of two simulations in which the 0.5 degree atmosphere component model is coupled to two different versions of the ocean model, with horizontal grid resolutions of 1 degree and 0.1 degree, respectively. In both configurations, realistic TCs are formed spontaneously within the model. We find that the atmosphere model simulates realistic TCs up to category 3 intensity for both configurations, and the modeled TCs’ mean intensity (i.e. power dissipation) is consistent with observation-based estimates for TCs up to category 3. Both model configurations produce realistic TC climatologies in the Northwest Pacific basin when compared against the observational record, and the model robustly reproduces the observed transient upper ocean surface responses following storm passage, from the perspective of individual storms as well as basin-scale budgets. We estimate TC-induced ocean heat convergence within both model configurations using multiple strategies accounting for mixing depths, and we find the heat convergence estimates are generally consistent across methods and insensitive to ocean model resolution. Using scaling arguments between heat convergence and power dissipation, we estimate that the model’s flux-adjusted TC-induced ocean heat convergence in the northwestern Pacific basin is ~0.20PW and ~0.23PW for the low and high resolution configurations, respectively, which is within the range of previous observation-based estimates. These results suggest that the basin-scale integrated TC effects within the coupled model are relatively insensitive to the choice of ocean grid resolution, when considering 1 degree versus 0.1 degree resolutions. Thus, relatively low resolution (non-eddy resolving) ocean models may sufficiently capture the large-scale ocean response to TC forcing.