Structure and variability of the West Greenland Current in Summer derived from 6 repeat standard sections
Introduction
Although known to be a gross over-simplification of reality, many discussions of the role of the ocean in the climate system start with the idea of the global ocean conveyor belt, based on a schematic used by Broecker (1987). Although there is evidence for a southern hemisphere driving role in the meridional overturning circulation (e.g. Toggweiler and Samuels, 1993), the most basic picture assumes a key role for the formation of deep water in the North Atlantic Ocean. Discussions of climate variability then focus on the stability of this overturning circulation related to changes in North Atlantic deep water formation and have been examined using approaches varying from simplified box models (e.g. Rooth, 1982), paleo-climatic studies (e.g. Spence et al., 2008) to coupled climate models (e.g. Wu et al., 2007). Many of these studies focus on the impacts of the input of low-salinity waters into the sub-polar North Atlantic ocean (e.g. Saenko et al., 2007).
Despite the importance of low-salinity water on the sub-polar North Atlantic and deep water formation, there is not, as of yet, a complete and quantitative observational picture of the circulation of fresh water in the high-latitude seas and its variability. The most complete and quantitative recent summary is presented in Dickson et al. (2007). For example, their Fig. 1 presents up to date estimates of freshwater inputs and fluxes through sub-arctic seas relative to a reference salinity of 34.8.
Focusing on the Labrador Sea, the formation region of Labrador Sea Water, one of the components of North Atlantic Deep Water, Dickson et al. (2007) provide estimates of the freshwater input from Baffin Bay (and thus the Canadian Arctic Archipelago and the Arctic Ocean), as well as Hudson Bay. We also note that a more recent estimate of the freshwater transport through Hudson Strait has been published (Straneo and Saucier, 2008). However, if we examine Fig. 1 in Dickson et al. (2007) in more detail focusing on the other route for freshwater into the Labrador Sea, through the East and West Greenland Currents, we see that the West Greenland Current is not represented at all in this schematic. This is despite observational (Straneo, 2006, Schmidt and Send, 2007) and modelling studies (Myers, 2005) that suggest that the West Greenland Current is the main source for freshwater for the interior of the Labrador Sea, as well as main provider of heat to help restratify the Labrador Sea post-convection (Straneo, 2006). Enhanced provision of freshwater associated with the Great Salinity Anomalies has also been shown to have played a role on the weakening of convection in the Labrador Sea (e.g. Houghton and Visbeck, 2002).
We thus focus on the West Greenland Current in this manuscript. We begin by providing a brief overview of what is known about the West Greenland Current based upon previous studies. We then discuss the 6 historical repeat summer sections that we use for our analysis in Section 3. Since we want to provide quantitative estimates of freshwater transport and its variability, we finish off Section 3 with a discussion of the frontal model used to improve our ability to capture the freshwater transport near the coast. Since we do not have any direct current measurements associated with the sections, Section 4 discusses the use of a series of Labrador Sea climatologies assimilated into an ocean general circulation model to provide us with an estimate of the barotropic velocity and thus absolute velocities and transports. We then present our results in Section 5, showing mean conditions at each section, as well as the temporal variability. Section 6 provides quantitative estimates of the volume, heat and freshwater transports across each section as well as the transport variability. Finally, we conclude and discuss the significance of our results.
Section snippets
Overview of knowledge of the West Greenland Current
The West Greenland Current flows northward in the eastern Labrador Sea along the west coast of Greenland (Fig. 1). It is part of the cyclonic circulation associated with the North Atlantic Ocean’s sub-polar gyre. Although dynamically represented as a jet with a single core at (or near) the shelf break, it is often divided into two components, a cold and fresh component on the shelf with a warmer and saltier Irminger Water component offshore (Fratantoni and Pickart, 2007).
The origin of the West
Data and methods
The data set we use is a set of standard sections handled by the Danish Meteorological Institute every year in June–July on behalf of the Greenland Institute of Natural Resources (Fig. 1). Surveys were normally run annually. However, each section has years when no survey was performed. The northern three sections were first performed back in the late 1950s while the southern three sections were first occupied in 1984. The years each section was occupied is shown in Fig. 2. The same 5 repeat
Diagnostic model and velocities
Since we do not have any direct observations of current speed associated with these sections, we have chosen another approach to provide a barotropic/correction velocity to add as a reference to our geostrophic velocities.
Kulan and Myers (in press) recently produced a detailed degree climatology of the Labrador Sea using an isopyncal framework. The combination of the isopyncal approach with small correlation length scales that took into account the bathymetry (and thus avoided smoothing
TS properties
Fig. 7 shows TS diagrams for five decades (50/60s, 70s, 80s, 90s and 00s; note that for the first 2 ‘decades’, only the 3 northern stations have data). Warm and salty pure ( and ) and modified Irminger Water ( and ) can be seen during the 1990s and 2000s, primarily at the southern sections but also to a lesser extent at the northern sections. Very little is seen during the 1980s, associated with lower salinities and cooler temperatures. This is consistent with
Transports
Here we examine the transport (volume, heat and freshwater) estimates given by the section data, as well as using the frontal model. As we want to focus on the low-salinity boundary current since the Irminger Water transports at these sections has been reported elsewhere (Myers et al., 2007), we use the 34.8 isohaline to represent the offshore edge of the current.
We provide purely baroclinic estimates, as well as total transports using the three different barotropic velocity fields discussed
Summary and discussion
Six historical sections across the West Greenland Current are examined. Three sections have been regularly occupied since the late 1950s, while the three southern ones have been taken since 1984. Significant variability is seen with respect to both the warm and saline Irminger Water component as well as the fresh and cold Polar Surface Water seen on the shelf. As well as varying over 3 salinity units between years, there is significant variability in the extent of this water mass. In some years
Acknowledgements
This work was funded by NSERC and CFCAS grants (the latter through the Canadian CLIVAR network) awarded to P.G.M. We also thank 3 anonymous reviewers for comments that significantly improved the manuscript. NCEP Reanalysis data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/. MHR was funded by Danish IPY grants in the project ECOGREEN.
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