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Current coupled climate models contain large biases in simulating tropical cyclogenesis, reducing the confidence in tropical cyclone (TC) projection. In this study, we investigated the influence of sea surface temperature (SST) biases on TC genesis in the Coupled Model Intercomparison Project phase 6 simulations from 1979 to 2014. Positive TC genesis biases were found over the tropical central North Pacific (CNP) in most of climate models, including the high-resolution models. Compared to coupled models, TC genesis density (TCGD) simulations over CNP in uncoupled models forced by observational SST improved obviously. A warm SST bias over the tropical CNP in the coupled models is the main cause of TC genesis biases. The SST bias–induced diabatic heating leads to an anomalous Gill-type atmospheric circulation response, which contributes to a series of favorable environmental conditions for TC formation over the CNP. Numerical experiments were also performed with HiRAM to demonstrate the influence of SST biases on the TCGD simulation, further confirming our conclusion. The current results highlight the importance of improving TC simulation in state-of-the-art climate models by reducing SST simulation bias. [ABSTRACT FROM AUTHOR]

Confidence and uncertainty issues of simulations were seldom evaluated in previous studies although the climate models are widely used. This study evaluates the performance of the CMIP6-HighResMIP simulations in presenting long-term variability of tropical cyclone (TC) genesis frequency (TCGF) and track density (TCTD) and quantifies the relative contributions of internal and external forcing to TC activities during the 1950–2014. There is overall poor model performance in simulating long-term changes in TC activities over the Northern Hemisphere, including interdecadal variabilities and long-term linear trends. The simulated long-term changes in TCGF and TCTD over the eastern North Pacific (ENP) in six high-resolution models show opposite characteristics to the observations. Moreover, most models cannot capture the variabilities of TCGF and TCTD over the western part of the western North Pacific (WNP) and northern part of the North Atlantic (NA). However, these models show a high degree of confidence in reproducing the interdecadal variabilities and linear trends of TCGF and TCTD over the eastern part of the WNP and the tropical NA. Quantitative evaluations further show that there are the opposite relative contributions of long-term climate variabilities to TCGF and TCTD changes over the ENP between the observations and the multimodel ensemble mean, followed by large model biases over the western WNP and the northern NA, but relatively consistent contributions over the southern NA and the Caribbean. These results help us cope with contrasting and consistent future TC changes among the model projections. Significance Statement: While climate models have been widely used to project future changes in tropical cyclone (TC) activity, few studies have examined to what extent we can trust these model projections. We used the CMIP6-HighResMIP simulations to quantify the model biases in presenting TC activity, and evaluate the relative contributions of internal and external forcing to TC activities. In general, the HighResMIP has large discrepancies in representing longer-term climate variability of TC activity. However, the models can capture well TC activity over the eastern part of the western North Pacific and tropical Atlantic, which is attributed to good performance of models in reproducing the relationship between long-term climate variabilities beyond interannual scale and TC activity. These results highlight confidence and uncertainty in future TC changes among the model projections. [ABSTRACT FROM AUTHOR]

The North Pacific Oscillation (NPO), a primary atmospheric mode over the North Pacific Ocean in boreal winter, is known to trigger El Niño–Southern Oscillation (ENSO) in the following winter, the process of which is recognized as the seasonal footprinting mechanism (SFM). On the basis of the analysis of model simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), we found that the SFM acts differently among models, and the correlation between the NPO and subsequent ENSO events, called the SFM efficiency, depends on the background mean state of the model. That is, SFM efficiency becomes stronger as the climatological position of the Pacific intertropical convergence zone (ITCZ) moves poleward, representing an intensification of the northern branch of the ITCZ. When the Pacific ITCZ is located poleward, the wind–evaporation–sea surface temperature (SST) feedback becomes stronger as the precipitation response to the SST anomaly is stronger in higher latitudes than that in lower latitudes. In addition, such active ocean–atmosphere interactions enhance NPO variability, favoring the SFM to operate efficiently and trigger an ENSO event. Consistent with the model results, the observed SFM efficiency increased during the decades in which the northern branch of the climatological ITCZ was intensified, supporting the importance of the tropical mean state of precipitation around the Pacific ITCZ. [ABSTRACT FROM AUTHOR]