Abstract
Abstract. The Gulf Stream and Kuroshio regions feature strong sea surface temperature (SST) gradients that influence cyclone development and the storm track. Previous studies showed that smoothing the SSTs in either the North Atlantic or North Pacific yields a reduction in cyclone activity, surface heat fluxes, and precipitation, as well as a southward shift of the storm track and the upper-level jet. To what extent these changes are attributable to changes in individual cyclone behaviour, however, remains unclear. Comparing simulations with realistic and smoothed SSTs in the atmospheric general circulation model AFES, we find that the intensification of individual cyclones in the Gulf Stream or Kuroshio region is only marginally affected by reducing the SST gradient. In contrast, we observe considerable changes in the climatological mean state as well as a reduced cyclone activity in the North Atlantic and North Pacific storm tracks that are shifted equatorward in both basins. The upper-level jet in the Atlantic also shifts equatorward, while the jet in the Pacific strengthens in its climatological position and extends further east. Surface heat fluxes, specific humidity, and precipitation also respond strongly to the smoothing of the SST, with a considerable decrease in their mean values on the warm side of the SST front. This decrease is more pronounced in the Gulf Stream than in the Kuroshio region, due to the larger decrease in SST along the Gulf Stream SST front. Considering the differences of the different variables occurring within/outside of a 750 km radius of any cyclone over their entire lifetime, we find that cyclones play only a secondary role in explaining the differences in the mean state between the smoothed and realistic SST experiments.
Highlights
The Gulf Stream and Kuroshio regions with their strong sea surface temperature (SST) gradients are preferential locations for cyclogenesis (e.g. Hoskins and Hodges, 2002; Nakamura et al, 2004) and are found to determine the location and structure of storm tracks (e.g. Chen et al, 2010; Ogawa et al, 2012; Ma et al, 2015; Yao et al, 2018)
Analysing the SST (Fig. 1a) and SST gradient (Fig. 2a) distribution in the Gulf Stream region for CNTL, we note a remarkable SST contrast across the Gulf Stream, which results in a strong SST gradient (Fig. 2a) and in locally-well-confined SST front detections (Fig. 3a), consistent with an oceanographic viewpoint (Meinen and Luther, 2016)
In the Kuroshio, we observe a similar but spatially less confined SST contrast compared to the Gulf Stream region, which results in a weaker SST gradient in the Kuroshio region (Fig. 2d)
Summary
The Gulf Stream and Kuroshio regions with their strong sea surface temperature (SST) gradients are preferential locations for cyclogenesis (e.g. Hoskins and Hodges, 2002; Nakamura et al, 2004) and are found to determine the location and structure of storm tracks (e.g. Chen et al, 2010; Ogawa et al, 2012; Ma et al, 2015; Yao et al, 2018). To shed light on these aforementioned issues, we assess the effect of a weak or strong SST gradient using an atmospheric general circulation model (AFES 3) based on simulations with realistic and smoothed SSTs in the Gulf Stream and Kuroshio regions. Our analysis of these simulations is twofold. This two-pronged approach allows us to establish a connection between structural changes in individual cyclones and changes in the time-mean winter climatology
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