Seasonal variations of the cold intermediate layer on the Newfoundland and Labrador Shelf

Across the Newfoundland and Labrador (NL) continental shelf, sub-surface temperatures remain below 0°C throughout the summer, when the surface is much warmer. This oceanographic feature is known as a cold intermediate layer (CIL), and its properties are assessed annually in the region to support ocean climate research and fisheries management. Monitoring in this region is either too infrequent or too sparse spatially to provide a detailed picture of sub-annual variations, so the GLORYS12 global ocean reanalysis is used for this purpose. GLORYS12 is shown to reproduce both the signs and magnitudes of inter-annual variations in CIL area on NL Shelf transects, although the magnitude of the seasonal cycle may be under-estimated.

GLORYS12 indicates that sub-zero water volume is maximized across the region during March and decreases with time, beginning in the south and proceeding north. CIL minima are reached between November to December at most shelf locations, proceeding in the opposite direction. Stratification triggered by surface freshwater is an important contributor to CIL seasonal changes on all transects on the NL Shelf. While the CIL area tends to gradually erode throughout the summer, the downstream advection of CIL water from more northern transects leads to the development of secondary CIL area maxima during late August to early September on the Newfoundland component of the shelf. Onshore intrusions either at the shelf break or via cross-shelf troughs contribute to the erosion of CIL area from below at some transects, although this effect may be exaggerated in the model.