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An Observational / Modeling Study of Wintertime Convection
and Water Mass Formation
Wendell S. Brown and Frank L. Bub
Ocean Process Analysis Laboratory and Department of Earth Sciences
University of New Hampshire
Durham, NH 03824
Pacific Marine Environmental Laboratory
Seattle, WA 98115
Wintertime atmospherically forced convective overturning (and subsequent mixing) is responsible for significant deep water formation in the Greenland Sea, the Labrador Sea, the western Mediterranean, and the Arctic / Antarctic polar regions. Observational evidence suggests that related water mass formation processes are also active in the Gulf of Maine (GOM) during wintertime.
In this project, the GOM is being used as a convenient environmental laboratory to explore the physics of overturning and mixing with an integrated program of observation and modeling. The research addresses basic questions concerning the kinematics and energetics of mixed layer formation in winter, with an emphasis on water column convective instability, sinking, and mixing in the presence of strong wind stress and surface cooling.
As the observations cannot resolve all the scales of this process, they will be integrated with the results of a numerical non-hydrostatic Ocean Large Eddy Model (OLEM) to provide further insight into the role of small-scale convection in wintertime water mass formation.
Field experiments, consisting of moored array and hydrographic / ADCP observations and augmented by hydrographic surveys, operational meteorological data and satellite imagery, were conducted during the winters of 1996-97 and 1997-98.
The first year's experiment was in shallower coastal water (near Jeffreys Basin - JB) where the convective plumes interact with the bottom. During the first and second year, experiments were also conducted in deeper offshore water (central Wilkinson Basin - WB) where convective activity does not reach the bottom. A central temperature / conductivity chain, two adjacent temperature chains, bottom pressure sensors, and ADCP vertical velocity measurements will provide both temporal and spatial information on the form and frequency of small scale convective plumes and mixed layer structure changes. Additional basin-scale information will be derived from hydrographic surveys, regional land and buoy weather observations, and satellite SST imagery.
These data are being used to assess the fidelity of the results from a series of OLEM experiments, designed to (a) model "typical" convective plumes and (b) determine the relative importance of convection and wind mixing in the water mass formation process. The observations will also be used to relate the patterns and intensity of convective overturning to winter mixed layer structure and water mass formation.
The 1996-7 CONVEX field observation program consisted of the wintertime deployment of a moored current / conductivity / temperature / bottom pressure instrumentation (Figure 1) and two series of hydrographic transects during JAN 97 (Figure 2) and FEB 97 (Figure 3).
During January 1997, a moored array was deployed in the center of Wilkinson Basin (Figure 4). A coastal pressure sensor was also deployed at the Coast Guard pier on Newcastle Island in Portsmouth Harbor, NH. These pressure sensors provide across-shelf pressure difference fluctuations which are proportional to along-shelf geostrophic currents. The velocity / temperature / conductivity measurements will resolve the hourly changes in the physical properties of the water at several depths at the moorings.
The 1997-98 field program was based on the deployment of an array of oceanographic instruments in the central Wilkinson Basin (Figure 5) between October 1997 and May 1998. The main instrument suite (Buoy "A") was deployed January-May 1998. Four hydrographic surveys documented the evolution of water mass structure during OCT 97 (Figure 6), JAN 98 (Figure 7), FEB 98 (Figure 8), and MAY 98 (Figure 9).
Hydrographic profile measurements (Figure 10) provide relatively high (spatial) resolution water property information along transects through the Wilkinson Basin, including the WB and JB mooring sites (Figure 8).
The combination of the time series data from the moorings and the spatial data from the hydrographic cruises will help us to understand the characteristics of wintertime water mass formation in the vicinity of the moorings and the along-coast currents in the western GOM.