SST Anomalies, Seasonality and Phenology Changes on the NE U.S. and Scotian Shelf

SST anomalies calculated from the OISST 0.25^o spatial resolution data set over a 34-year period (1982 – present) show that the NE US and Scotian shelf is warming, minimum in regions south of Cape Cod (~ 0.2 ^oC / decade), and maximum in the Gulf of Maine and Scotian Shelf (up to 0.6 ^oC / decade). The warming trend is even stronger over the most recent decade. This warming, however, is not balanced over the entire year. Summer months (June-September) are warming 2-6 times faster than winter (January – March), and in some locations, winter is cooling, despite overall positive trends. This means the nature of the SST annual cycle is changing. A series of metrics characterizing the phenology of various aspects of the satellite-measured SST seasonal cycle are defined. Some metrics, such as dates of the seasonal maximum, seasonal minimum, or spring SST thresholds, show either only localized or weaker trends. Among those metrics of SST phenology that show the most persistent trends in time and space, the strongest is for a lengthening of the duration of the summer warm period (0.6 – 1.8 days / year). This trend is primarily driven by changes (delays) in the timing of the fall cooling, rather than (earlier) spring warming, most strongly so over the Gulf of Maine and southern Scotian shelf. Relationships between these phenology and anomaly trends and various indices of forcing, such as wind mixing and heat flux, and larger-scale variability, such as the Gulf Stream position, the AMO and NAO, are quantified.