The simulation of tropical cyclones in the Western North Pacific by a climate system model FGOALS-f3-H and the effects of ocean coupling

In this study, the characteristics of Western North Pacific tropical cyclones (TCs) simulated in 1985 to 2014 by a climate system model FGOALS-f3-H that participated in the HighResMIP are evaluated. The simulations from the stand-alone atmospheric and fully coupled versions are inspected to explore the effects of ocean coupling. Results show that the model can capture the main TC characteristics in tracks, amounts, and genesis locations. The seasonal and interannual variation of TC genesis frequency (TCGF) are reasonably simulated, as well as the wind-pressure relationship (WPR), horizontal structures, and TC-induced precipitation. However, some obvious biases remain in both versions, mainly in the TC intensity and TCGF distributions. The simulation shows a much smaller number of super typhoons (SSTY) and tropical storms (TS), and a larger number of severe typhoons (STY), typhoons (TY), and severe tropical storms (STS). The TCGF shows underestimated biases in the west of 140E with less genesis of all categories, and overestimated biases in the east of 140E with more genesis of STY, TY, and STS, corresponding to the biases of intensity distributions. The overestimated TCGF biases are more obvious in coupled simulation which can be explained by the TC genesis potential index (GPI). The stronger biases of relative humidity, potential intensity and vertical wind shear contribute to the higher GPI biases together with warm sea surface temperature (SST) biases and weak Western Pacific subtropical high (WPSH). The model shows a stronger seasonal cycle in atmospheric simulation which is improved in coupled simulation but remains stronger. Meanwhile, the TCGF interannual variation in atmospheric simulation shows a better correlation with the observation (coefficient of 0.44 vs 0.14). The annual TCGF shows similar patterns in empirical orthogonal function (EOF) analysis, and the first principal component (PC1) is relevant to the Nino3.4 index, suggesting that TCGF is simultaneously modulated by the El Niño-Southern Oscillation (ENSO). Therefore, the poor interannual variation in coupled simulation can be attributed to a weak ENSO. For WPR, the simulation shows a larger wind speed with same sea level pressure, but the coupled simulation shows a closer WPR to the observation. The simulated horizontal structures of winds and pressure are similar in atmospheric and coupled simulation, while the warm core of intense TCs is stronger in coupled simulation. The simulated precipitation is overestimated which can be weakened in coupled simulation. The coupled simulation can also capture the observed SST cooling of TCs, while the atmospheric simulation cannot, showing more reasonable atmosphere-ocean interactions. Overall, ocean coupling can improve some details of the simulated TCs but not the climatology due to ocean biases and coupled interactions, the better TC simulation in a coupled model requires improvements on both atmospheric and ocean models.