Relationship between North Atlantic SST Front and Storm Track as well as Large-scale Atmosphere Anomalies

Temporal and spatial variations of the storm track and SST (Sea Surface Temperature) frontal zone in the North Atlantic, as well as climate variability of the SST frontal zone and its relationship with the storm track and large-scale atmospheric circulation anomaly, are analyzed by using the NCEP and HadISST (Hadley Centre Sea Ice and Sea Surface Temperature data set) reanalysis data and NAO (North Atlantic Oscillation) monthly index data. Results show that after the seasonal cycle is filtered out, the main variability of the SST frontal zone is shown by its south/north shift, which corresponds to the southwestward/northeastward moving of the storm track and negative/positive geopotential height anomaly. This negative/positive geopotential height anomaly can lead to anticyclonic/cyclonic circulation at high latitudes, which is favorable for the development of negative/positive eddy anomaly in the ocean and eventually weakens/strengthens the SST gradient. Furthermore, when the SST frontal zone near 40°N-45°N intensifies as a single zone, the meridional gradients of geopotential height and SLP (Sea Level Pressure) become sharper, corresponding to the NAO positive phase. Meanwhile, the storm track demonstrates a single-track pattern, and the SLP anomaly results in cyclonic wind stress curl near the Iceland and anticyclonic wind stress curl near the Azores. As a result, stronger oceanic circulation develops in the subtropics and sub-polar region, which has a positive feedback to the SST gradient and storm track located near 40°N-45°N. When the SST frontal zone near 40°N-45°N splits into two bands, the storm track also bifurcates into two branches and the SST gradient becomes weaker, corresponding to the NAO negative phase. Although the variance contribution of SST front in the middle of the ocean is small, it is still significantly correlated with NAO. Spectral analysis suggests that the North Atlantic SST gradient anomaly has a covariant cycle of 1-3 years with the storm track anomaly and large scale atmospheric anomaly. The SST gradient anomaly also has a decadal covariant cycle with the storm track anomaly. The quasi-annual cycle shows that there exist certain covariations between SST gradient anomaly and storm track and NAO. Atmospheric baroclinicity, air temperature gradient, and poleward and upward transport of eddy heat flux are strong near the SST frontal zone, which are favorable for the conversion from mean available potential energy to eddy kinetic energy, and eventually lead to the establishment and strengthening of the storm track and affect the relationship between synoptic turbulence and large-scale circulation anomaly.