Thermal evolution of the Greenland Sea Gyre in 1988–1989

Abstract

Slice inverses of temperature and heat content from the 1988–1989 Greenland Sea Tomography experiment and other observations, including standard conductivity‐temperature‐depth stations, moored thermistors, surface meteorological variables, and surface ice cover are combined to better understand the thermodynamics of the Greenland Sea Gyre. Thermal evolution of the gyre center seems to divide naturally into the following three periods: a preconditioning phase (November–January), during which surface salinity is increased by brine rejection from ice formation and by entrainment but in which the mixed layer deepens only slowly to a depth of some 150–200 m; a deep mixing phase (February–March) during which the surface mixed layer deepens rapidly to approximately 1500 m in the gyre center purely under the influence of local surface cooling; and a restratification phase during which the products of deep mixing are replaced by Arctic Intermediate Water flowing in from the gyre edges. The onset of the deep mixing phase occurs after ice formation in the gyre center stops, resulting in an area of open water where large heat fluxes can occur. In surrounding regions, including the Odden region to the south, ice is still being formed. To the north and west, closer to the steep topography of the continental shelf, the inverse results show significant variability due to advection, and large temperature and heat content fluctuations with a period of about 50 days are seen.

Preconditioning the Greenland Sea for deep convection: Ice formation and ice drift

“…Intermediate convection down to 1500 m depth occurred in early March [GSP Group, 1990], and later on, rapid restratification was seen, as a series of warm and cold water masses encountered the mooring with a general warming trend. Note that this general picture was also seen in the slice inversion from the tomographic array [Greenland Sea Tornography Group, 1993; Pawlowicz et al, 1995] and hence seems to be characteristic for a larger region within the center of the Greenland Sea gyre.…”

“…These signals sug gest that mesoscale eddies, maybe generated around the con vective region as a consequence of the deep mixing (Gascard, 1978), were penetrating into the region. Moreover, the heat budget considerations from Pawlowicz et al (1995], which show a significant mismatch between the tomographically measured heat content and the integrated surface heat loss during March-April 1989, suggest that strong lateral mixing was tak ing place after the intensive mixing phase of early March 1989. During the previous month both estimates track quite nicely, which is consistent with a one-dimensional evolution.…”

“…The two other moorings (T6 and TS; see Figure 1) were part of an acoustic tomographic array [Greenland Sea Tomography Group, 1993] [Roach et al, 1993]. See also a recent review of the tomographic results by Pawlowicz et al [1995]. [Carmack, 1986).…”

How to identify winter convection in the Greenland Sea from hydrographic summer data

“…In order to understand how the system presently works, it is necessary to analyze observational data carefully in order to determine basic parameters like the basin‐wide convection depths and to refer to clear criteria for these estimates. Although convection processes are studied by a variety of means (tomography arrays [ Worcester et al , 1993; Pawlowicz et al , 1995; Morawitz et al , 1996; Sutton et al , 1997], moorings equipped with ADCPs [ Schott et al , 1993], winter ship campaigns, and artificial tracer injection [ Watson et al , 1999]), a time series of winter convection relies on summer data as the majority of data are collected during summer. Winter cruises investigate the convection process itself more directly but normally do not detect the maximum ventilation depth unless one is certain to have observed the very last convection event.…”

Sound speed in the Mediterranean Sea: an analysis from a climatological data set

“…In the Western Mediterranean Sea, the ninemonths-long experiment THETIS-2 (Send et al, 1997) has estimated the seasonal heat budget, but this experience remains an unicum among tomographic Mediterranean basinscale studies. Convective events have been successfully obCorrespondence to: S. Salon (ssalon@ogs.trieste.it) served in the northwestern Mediterranean (Send et al, 1995) and in the Greenland Sea (Pawlowicz et al, 1995;Morawitz et al, 1996). Ocean acoustic tomography has also proved to be a methodology for the observation of currents and internal tides (Shang and Wang, 1994;Demoulin et al, 1997).…”