A 12 year time series (1985-1996) of full resolution (~1.2 km) SST images derived from NOAA-9, NOAA-11, and NOAA-14 were remapped to a 1024 by 1024 pixel region stretching from Cape Hatteras to Nova Scotia. After declouding, fronts in each image were found using the multi-image edge detection algorithm described by Cayula and Cornillon (1995). An example of a typical image with the fronts superimposed is shown in Figure 1.
Mean
The probability of detecting a front at a particular pixel was computed by summing the number of times the pixel was an edge pixel and dividing by the number of times the pixel was clear. The probability map computed using the entire 12 year series of images (Figure 2) shows several preferred regions for fronts. The shelf-slope front is prominent as are the tidal mixing fronts over Georges Bank and Nantucket Shoals. Bands of relatively high probability occur along the western edges of the Gulf of Maine, the Southwestern Scotian shelf, and over the shelf in the Mid-Atlantic Bight.
Seasonal Cycle
The seasonal variability in frontal occurrence is illustrated by computing the probability of finding a front during each month of the year. The mean probability for each month represents an average during that month over the entire 12 year period. Monthly images are shown in Figure 3.
The late winter months (January-March) are characterized by bands of high probability over the entire length of the inner shelf with multiple bands occurring from New Jersey south. North of Cape Cod there appears to be a single band stretching uninterrupted all the way to the head of the Bay of Fundy. Many of these bands appear to follow bottom topography (note the band curving north and west towards the New York Bight which follows the 50m isobath). These high probability bands weaken with time, and by April fronts are unlikely to be detected on the inner shelf south of Penobscot Bay except in the vicinity of the mouths of the major estuaries (Long Island Sound, Hudson, Delaware, Chesapeake) and the Nantucket shoals region. In late spring and summer, the tidal mixing fronts on Georges Bank and Nantucket shoals are frequently detected. On the shelf south of Cape Cod, fronts are most prevalent in several localized regions, notably nearshore off the eastern end of Long Island and near the mouth of Delaware Bay. During the early fall months, most inner shelf frontal activity is concentrated near the estuary mouths. Late fall/early winter (December) sees the redevelopment of the winter nearshore frontal bands.
The shelf slope front undergoes a distinct seasonal cycle. South of Hudson Canyon it appears most frequently in late winter/early spring and again in fall. In summer, it is detected much less frequently. North of Hudson Canyon, the shelf slope front exhibits less seasonal variability and persists throughout the summer.
