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

Abstract

[1] The identification of winter convection depths from later field observations is an often required but not trivial task. We show from field measurements that the effects of winter convection in the Greenland Sea can vary enormously, including such opposite changes as an increase or decrease of temperature, salinity, or stability. Also, small-scale fluctuations in the vertical can grow larger or cease. Commonly used indicators for past convection in the Greenland Sea like overall vertical homogeneity or a freshwater input to deeper layers alone are therefore not reliable to construct a time series of winter convection. We explain this variety of effects with the occurrence of two rather different ventilation scenarios possible in the Greenland Sea. They are exemplified by two winters, using mainly hydrographic observations of the adjoining summers. The observed differences are substantiated by the results of a one-dimensional model. Out of this a multicriteria catalogue is developed, which can be used to determine unambiguously convection depths in the Greenland Sea. The fulfillment of one of the criteria is sufficient to evidence convection, but the suitable criterion may differ from winter to winter. A straightforward indicator proving the absence of winter convection does not exist.Citation: Ronski, S., and G. , How to identify winter convection in the Greenland Sea from hydrographic summer data,

An Integral View of the Hydrographic Development in the Greenland Sea Over a Decade

“…However, convection is not always combined with a stability reduction. While a mixed-layer type of convection results in outstandingly homogeneous vertical conditions in the Greenland Sea (as possibly not observed elsewhere), the second type, namely plume convection, does not destroy the vertical stratification [26]. The thermobaric effect is important for the latter convection type and it might therefore be particularly effective in the Greenland Sea.…”

“…When AW are distributed vertically by winter convection, the salinity increase can occasionally be recognised to depths just above the salinity/density step. A mixed layer type ventilation is related to this [26]. In contrast, plume convection transfers fresher waters to greater depths and is effective when the PW input dominates.…”

“…It has been shown for the Greenland Sea that higher heat losses are not synonymous with deeper convection depths and that the occurrence of ice formation is not a good indicator for particularly strong convection [25,26]. Though evidently initially forced by the heat loss to the atmosphere, the convection depth and the affected volume are controlled mainly by the structure of the water column which is present before the convection phase.…”

“…The development from 1996 to 1997 is an example for the plume convection type, in this case reaching to about 1100 m. We see that despite this comparatively deep convection, the stratification is not destroyed. Naturally, such a statement cannot be based solely on the shown time series, as there could also occur a restratification phase between the time of convection and that of the cruise, but dense sampling in time, including cruises directly after the convection phase, document the intact stratification [26]. Below the annually varying depth of winter convection, the stability in the upper layer has a tendency Fig.…”

“…A criteria catalogue for convection depths is developed and discussed in detail by [26] and the time series of convection depth, as analysed from these criteria, is shown in fig. 6.…”