On the characteristics of subtropical fronts in the North Atlantic

Abstract

The characteristics of surface temperature (SST) fronts in the subtropical frontal zone (STCZ) in the North Atlantic were investigated using a time series of front data produced by an edge‐detection algorithm applied to 18 years of 9.3 km resolution NOAA/NASA AVHRR Pathfinder SST imagery. Front probability, averaged over subregions of the STCZ, varies seasonally with highest probability in early spring in the western part of the region. In the eastern part of the region, maximum probability occurs in late spring to early summer, and the annual cycle in front probability is far less robust, being nearly absent prior to 1991. The location and strength of the fronts across the entire STCZ varies strongly with season, moving southward (northward) and strengthening (weakening) in winter/spring (summer). From estimates of the cross‐frontal SST step and gradient, the cross‐frontal scale of SST variability at fronts is estimated to be approximately 30 km. The monthly mean total meridional SST step across the STCZ is partitioned into a component due to frontal pixels and a component due to the background SST gradient at nonfrontal pixels, with approximately 30% of the total step attributable to the SST step across fronts. While the total meridional step is largest in winter due to the meridional gradient in surface heat flux, the frontal contribution lags the total step by 2 months or more in the western part of the STCZ. This suggests a frontogenesis mechanism requiring the presence of a background meridional SST gradient rather than one arising from surface heat flux gradients alone.