Tropical Cyclone Activity in the High‐Resolution Community Earth System Model and the Impact of Ocean Coupling

Abstract

High‐resolution Atmosphere General Circulation Models (AGCMs) are capable of directly simulating realistic tropical cyclone (TC) statistics, providing a promising approach for TC‐climate studies. Active air‐sea coupling in a coupled model framework is essential to capturing TC‐ocean interactions, which can influence TC‐climate connections on interannual to decadal time scales. Here we investigate how the choices of ocean coupling can affect the directly simulated TCs using high‐resolution configurations of the Community Earth System Model (CESM). We performed a suite of high‐resolution, multidecadal, global‐scale CESM simulations in which the atmosphere (∼0.25° grid spacing) is configured with three different levels of ocean coupling: prescribed climatological sea surface temperature (SST) (ATM), mixed layer ocean (SLAB), and dynamic ocean (CPL). We find that different levels of ocean coupling can influence simulated TC frequency, geographical distributions, and storm intensity. ATM simulates more storms and higher overall storm intensity than the coupled simulations. It also simulates higher TC track density over the eastern Pacific and the North Atlantic, while TC tracks are relatively sparse within CPL and SLAB for these regions. Storm intensification and the maximum wind speed are sensitive to the representations of local surface flux feedbacks in different coupling configurations. Key differences in storm number and distribution can be attributed to variations in the modeled large‐scale climate mean state and variability that arise from the combined effect of intrinsic model biases and air‐sea interactions. Results help to improve our understanding about the representation of TCs in high‐resolution coupled Earth system models, with important implications for TC‐climate applications.

Lagrangian Analysis of Tropical Cyclone Genesis Simulated by General Circulation Models Compared with Observations

“…From our results, the ERAI and all GCMs consistently exhibited the following flaws: the underestimation of observed maximum near-surface wind speeds, presumably due, in part, to their coarse horizontal resolutions (0.78 for ERAI and 0.58 for GCMs); the underestimation of TCs over the North Atlantic Ocean; the false simulation of TCs over the South Atlantic Ocean; and the inaccurate extension of TC tracks to the south in the South Pacific convergence zone (SPCZ) and the southern Indian Ocean, although this could be that observations stopped tracking storms at some latitudes due to low risk of weak TCs (e.g., the best track observations stop tracking TCs when the responsible agency determines that the storm intensity is below tropical storm strength (i.e., maximum wind speed less than 34 kt; 1 kt ' 0.51 m s 21 ) or that the candidate storm is no longer ''tropical'' after having completed the extratropical transition in the midlatitude regions). These biases are similar to ones in other climate models with a similar horizontal resolution (Shaevitz et al 2014;Walsh et al 2013;Wehner et al 2015;Reed et al 2015;Kim et al 2014;Scoccimarro et al 2017;Li and Sriver 2018;Wing et al 2019).…”

The Tropical Atlantic Observing System

“…Variability of SST and other climatic variables in the northern tropical Atlantic is of great interest because most hurricanes form there. Hurricane-permitting global climate model simulations are becoming increasingly feasible (Patricola et al, 2014;Wehner et al, 2014;Walsh et al, 2015;Haarsma et al, 2016), but tend to overpredict hurricane activity if oceanic feedbacks are neglected (Zarzycki, 2016;Li and Sriver, 2018). Coupled atmosphere-ocean simulations are therefore needed for hurricane projections.…”

Projected changes in tropical cyclones over the South West Indian Ocean under different extents of global warming

“…This can be ascribed to the reason that most models that use prescribed climatological sea surface temperatures tend to simulate tropical cyclones that are higher in both intensity and numbers e.g. Li and Sriver (2018) studied the impact of ocean coupling and found that compared to mixed layer coupling and dynamic ocean coupling, prescribed sea surface temperature coupling results in high intensity and number of cyclones. There is also an agreement with observations as With increasing global warming, there is a tendency for all downscalings to simulate lower tropical cyclone frequencies across the main tropical cyclone formation region of the South West Indian Ocean.…”