Abstract
Sea surface temperature (SST) fronts, generally defined as regions of enhanced surface temperature gradient, are of broad interest in oceanography both because of the role that they play in the dynamics of the upper ocean and because of the large volume of data available from satellite-borne sensors with which they can be studied. Gradients in the background surface temperature, surface wind stress, and cloud cover likely play a role in establishing and maintaining SST fronts. Furthermore, each of these characteristics is thought to be affected by changes in global climate. The objective of this study is to determine to what extent the probability of finding SST fronts in satellite-derived SST has changed in the recent past. To this end, front probability was determined from the output of an edge detection algorithm applied to the 30-year (1981-2011) time series of Pathfinder v5.2 SST data. Based on approximately 1°x1° squares that are 90% or more clear, front probability has been found to increase globally at a very nearly linear rate of approximately 0.25 %/decade; i.e., over the 30-year period the mean probability of finding a front has increased from approximately 5.58% to 6.30%. However, the trend in front probability is not globally uniform so the study also included a determination of regional trends in front probability. Requiring broad temporal coverage in each 1°x1° square, to reduce the uncertainty associated with the trend estimates, resulted in dense coverage only in an approximately 750 km wide ‘coastal’ band. In this region, clusters of predominantly positive trends that were significantly larger, 0.6 to 0.8%/decade, than the mean trend were observed; i.e., increases of 30 to 50% in the number of fronts over the past 30 years. The mean trend in the ‘coastal band’ is approximately 0.30%/decade, substantially higher than the global trend. This implies that the increase in front probability in coastal regions is significantly larger than in open ocean regions.
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