On the Influence of Sea Surface Temperatures on Cyclone Characteristics in the Gulf Stream and Kuroshio Region

Abstract

The Gulf Stream and the Kuroshio are the western boundary currents in the North Atlantic and North Pacific, respectively and are associated with maxima in midlatitude precipitation and air-sea heat exchange in the midlatitudes. Both regions are characterized by the strongest sea surface temperature (SST) gradients over the Northern Hemisphere’s midlatitudes, able to anchor the storm track and influence the development of individual cyclones. Other mechanisms were also found to contribute significantly to the intensification of cyclones. These include the absolute SST values, the land-sea temperature contrast, latent heat release, surface fluxes from the ocean, and strong upper-level forcing.The main goal of this thesis is to understand the characteristics of individual cyclones and the mechanisms leading to their intensification, as well as to document the changes on both individual cyclones and the storm track as a whole when a strong SST gradient is present/absent in the Gulf Stream and Kuroshio region. Individual cyclones were tracked using the University of Melbourne cyclone detection and tracking algorithm and categorised depending on their propagation relative to the Gulf Stream and Kuroshio SST fronts, which were detected automatically using an established algorithm that was originally used for detecting atmospheric fronts. Comparing results for the different categories we found that cyclones staying north of the Gulf Stream SST front and those crossing it northward were the ones with the maximum intensification,due to the increased low-level baroclinicity, arising primarily from the land-sea temperature contrast. Differently, in the Kuroshio region, we found cyclones crossing the SST front or remaining on its warm side, to intensify the most. We related this higher intensification to the propagation of cyclones near the left exit region of the jet stream,accounting also for higher precipitation. Even if the SST front contributes to the climatological low-level baroclinicity, no direct effect to the intensification of individual cyclones was found.The latter was confirmed for both the Gulf Stream and the Kuroshio region, by comparing simulations with realistic and smoothed SSTs in the atmospheric general circulation model AFES. The experiments with smoothed SSTs revealed that the intensification of individual cyclones in the two regions was only marginally affected by reducing the SST gradient. In contrast, cyclone activity was reduced in the North Atlantic and North Pacific storm tracks, which found to shift equatorward in both basins. The mean values of precipitation, specific humidity and surface heat fluxes were found to considerably decrease after the SST smoothing, particularly in the Gulf Stream region, due to the stronger decrease in SST along the SST front. Moreover, to clarify whether the individual cyclones’ behaviour is crucial for these changes, we subdivided the winter climatology into dates with/without cyclones in the Gulf Stream and Kuroshio regions and we highlighted overall the secondary role of cyclones in building the mean state’s differences between the SST experiments.In summary, the results presented in this thesis indicate that despite the similarities between the Gulf Stream and Kuroshio western boundary currents, different mechanisms act to enhance the development and affect the characteristics of individual cyclones, which follow different pathways in the two regions. A weaker SST gradient was found to marginally affect the intensification of individual cyclones, but consider-ably affect the mean state, confirmed by a reduction in cyclone activity, a shift in the storm track position, a differently changed upper-level jet in the two basins, and a de-crease in various meteorological parameters, such as humidity and surface heat fluxes.Considering the changes in the mean state between the experiments with realistic and smoothed SSTs, we found that SST fronts along the Gulf Stream and the Kuroshio region affect the winter climatology primarily in the absence of cyclones.