Xiaojie Zhu

Vertical wind shear over the tropical Atlantic Ocean plays an important role in mediating hurricane activity. The vertical shear variability over the main development region for Atlantic hurricanes is affected by local factors as well as by the remote influence of the El Niño–Southern Oscillation (ENSO) phenomenon, as indicated by observational and climate modeling analyses. Climate model simulations of the ENSO–shear relationship are compared with observations. It is shown that there is a strong influence of background mean flow on the ENSO–shear relationship, because of the inherently nonlinear nature of vertical wind shear. In particular, the simulation of zonal flow over the tropical Atlantic is shown to play a critical role in how the remote influence of ENSO modulates the shear. Even with realistic simulations of the ENSO-induced westerly anomaly in the upper troposphere, overestimated easterly background flow in the model simulations can alter the relationship between ENSO and vertical wind shear, resulting in decreased vertical wind shear during warm ENSO events. This nonlinear superposition has important implications for the assessment of trends in large-scale environmental factors affecting tropical cyclogenesis in climate change simulations.

The trend of sea surface temperature (SST) in the twentieth century is examined in observations and the Intergovernmental Panel on Climate Change (IPCC) twentieth-century simulations. The observed SST neither shows a clear signal of the enhanced equatorial response (EER) warming nor exhibits a clear trend of the El Niño–like warming in the last century. Similarly, the IPCC simulations show neither a clear EER warming nor an El Niño–like warming in the last century. Furthermore, the comparison of heat fluxes in model simulations of the global warming scenario and the twentieth century indicates that the aerosol cooling effect, opposite to the greenhouse gases warming effect, plays an important role in the twentieth century and explains the EER-like signal in the twentieth-century simulations. Therefore, a conclusion that the IPCC model simulations of the twentieth century are consistent with observations within the error bars as well as the future projection of the EER warming pattern in the global warming scenario are validated.