The frequency and cause of shallow winter mixed layers in the Gulf of Maine

In the Gulf of Maine, regional differences in wintertime stratification have important biological and physical implications. Phytoplankton blooms linked with shallow stratification events during the winter are important because they can provide an additional food source for zooplankton and larval fish populations. Regional differences in stratification and mixed layer depth may also affect rates of air-sea gas exchange. On an annual basis, variability in wintertime air-sea CO2 exchange is significant since it can affect the entire region\u27s role as a sink or source of atmospheric carbon. Before examining how patterns in stratification affect biological and physical systems in the Gulf of Maine, it is necessary to understand the spatial and temporal variability in wintertime mixed layer depths. The cause and frequency of shallow winter mixed layers in the Gulf of Maine is investigated using salinity, temperature, and wind data. Salinity and temperature data comes from hydrographic profiles and moored, autonomous data collection buoys. Hydrographic profile data are available from the Bedford Institute of Oceanography (BIO) and Coastal Ocean Observing and Analysis (COOA) databanks. Time-series salinity and temperature data are taken from Gulf of Maine Ocean Observing System (GOMOOS) moorings. Wind data are available from the National Center for Environmental Prediction (NCEP) Reanalysis II model. The roles of salinity, temperature, and wind stress are quantified to identify the cause of temporal and spatial patterns in stratification. To examine spatial variability, the Gulf of Maine is divided into 5 zones; Coastal, Western, Eastern, Northern, and Georges Bank. Inter-zonal comparison of mixed layer depth reveals distinct regional differences. Cast data shows considerable changes in mixed layer depth can occur over short distances. GOMOOS mooring data show that shallow mixed layers often occur and persist through the entire winter in the coastal and eastern Gulf of Maine. In these areas upper water-column (0--20m) stratification is governed by salinity. Cast data indicate deeper mixing over Wilkinson Basin. In this area, stratification in the upper 20m is weak and often governed by temperature decrease with depth. However, in all regions salinity increase with depth is responsible for the majority of shallow winter mixed layers. Comparison of wind stress and stratification at the GOMOOS moorings shows that winter wind events do not break down salinity driven stratification