Physical oceanography on yearly cruises to the central Greenland Sea between 1993 and 1996

From observations on yearly cruises to the central Greenland Sea between 1993 and 1996, conclusions are drawn with respect to winter convection and bottom water renewal. The data indicate that winter convection was extremely weak after 1993, not even ventilating the intermediate waters. This is remarkable, since the salinities in the upper layers increased considerably between 1993 and 1995, thus providing presumably favorable conditions for winter convection. With the absence of deep reaching winter convective events, the temperatures in the deeper waters of the Greenland Gyre increased steadily by about 0.03 K between 1993 and 1996. We conclude from the development of mainly the thermal structure on a zonal transect that an explanation for the temperature increase can be given by a large-scale downward water movement of about 150 m/yr in the central Greenland Sea. The data indicate that this process is independent of changes in the dynamically induced density distribution. It is therefore possible that a downward movement, perhaps masked by other processes, may continue for many years. If this is the case, resulting flushing times would be of the order of 20-30 years only. The presence of a large-scale circulation cell with downward movement in the central Greenland Gyre would explain the observed warming of the bottom waters without the demand for an actually active heat source. It is also in accordance with the observed increase of chemical tracer concentrations in the deep waters.