Smoother versus sharper Gulf Stream and Kuroshio sea surface temperature fronts: effects on cyclones and climatology

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.

Figure S11: Composite evolution of cyclone-centred SST (blue-red shading, K), temperature at 850 hPa (purple contours, interval: 5 K), wind speed at 925 hPa (black contours, interval: 3 m s −1 ), SST front density (yellow shading, in 10 −5 km −1 ), and probability of being over land (grey shading, 60-100%) for the CNTL experiment.Left, middle, and right panels at 12 h prior to maximum intensification, maximum intensification, and 12 h after maximum intensification, respectively.Top, middle, and bottom panels show the categories C1, C2, and C3, respectively.Numbers in the top right of each panel represent the temperature difference at 850 hPa in the composite domain.Figure S12: Composite evolution of cyclone-centred SST (blue-red shading, K), temperature at 850 hPa (purple contours, interval: 5 K), wind speed at 925 hPa (black contours, interval: 3 m s −1 ), SST front density (yellow shading, in 10 −5 km −1 ) and probability of being over land (grey shading, 60-100%) for the SMTHG experiment.Left, middle, and right panels at 12 h prior to maximum intensification, maximum intensification, and 12 h after maximum intensification, respectively.Top, middle, and bottom panels show the categories C1, C2, and C3, respectively.Numbers in the top right of each panel represent the temperature difference at 850 hPa in the composite domain.

FigureFigure
Figure S1: (a) ERA-Interim DJF climatology for the period 1982-2000 for (a) SST (K), (b) SST gradient (K (100 km) −1 ), and (c) latent (shading, W m −2 ) and sensible heat fluxes (contours, W m −2 ) for the North Atlantic.(d-f) As (a-c), but for the North Pacific.The Gulf Stream and Kuroshio regions are marked with a black box, respectively.

Figure S3 :
Figure S3: Locations of C3 cyclones in the North Atlantic, (a) 12 hours prior to maximum intensification (blue crosses), (b) at the time of maximum intensification (dots coloured depending on their pressure tendency (hPa h −1 )), and (c) 12 hours after maximum intensification (green crosses) in the CNTL experiment.(d-f) As (a-c), but for the SMTHG experiment.The Gulf Stream region is marked with a black box.

Figure S4 :
Figure S4: Locations of C3 cyclones in the North Pacific, (a) 12 hours prior to maximum intensification (blue crosses), (b) at the time of maximum intensification (dots coloured depending on their pressure tendency (hPa h −1 )), and (c) 12 hours after maximum intensification (green crosses) in the CNTL experiment.(d-f) As (a-c), but for the SMTHK experiment.The Kuroshio region is marked with a black box.

Figure
Figure S13: (a) Cyclone frequency (% of time) within a radius of 750 km around a cyclone centre for the winter seasons in 1982-2000 for the North Atlantic for the CNTL experiment, (b), as (a) but for the SMTHG experiment.(c) As (a) but for the North Pacific, (d) as (b) but for the SMTHK experiment.