Annual to Decadal Variability in the Labrador Sea

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1.  Changes of Water Mass Properties in the Central Labrador Sea, 1928-1997

Hydrographic observations in the Labrador Sea from the 1930s to the 1990s reveal large annual and decadal variations in water mass properties. Values of potential temperature, salinity, pressure and potential density used in the analyses were computed as medians of all available measurements in density-time bins.

Figure 1.1.  Potential temperature (colours and white contour lines) and potential density (dashed lines) in pressure-time coordinates.

Figure 1.2.  Salinity (colours and white contour lines) and potential density (dashed lines) in pressure-time coordinates.

1.1.  The Entire Column

In the late 1960s - early 1970s, the intermediate a nd deep waters of the Labrador Sea were at their warmest and saltiest since the 1930s. It took only two decades for all the waters to reach the lowest ever observed temperature and salinity of the entire water column. This change resulted in a reduction of the dynamic heights of the 0-2000 db layer, which opposes a raise of the dynamic heights in the Central Newfoundland Basin.

Figure 1.3.  Potential temperature of the major water masses of the Labrador Sea.

Figure 1.4.  Salinity of the major water masses of the Labrador Sea.

1.2.  Cores of the Major Water Masses

Temperature and salinity variations in the three main water masses are similar over decadal scales, but sometimes interestingly different over interannual scales. For example, the rapid cooling and freshening of the LSW observed in the 1970s was not found in the NEADW. The lowest q and S of the 1990s were observed in the DSOW 2-4 years later than those in the LSW, but haven't been seen in the NEADW yet.

1.3.  Properties of the Labrador Sea Water: Potential Temperature - Salinity - Time Diagram

Figure 1.5.  Characteristics of the Labrador Sea water in potential temperature - salinity coordinates.

2.   WOCE Section AR7W: the 1990s under a Magnifying Glass

Figure 2.1.  1990-1997: potential temperature - salinity diagrams, distributions of potential temperature, salinity, potential density and CFC-11 and vertical profiles of CFC-11 along the AR7W line. Click on the image to view and compare up to four images at once. (Frames capability is required to use the image viewer.)

The LSW, formed by deep convection in the central part of the Sea, has become denser and more widespread due to a series of severe winters between 1988 and 1993.

Since 1993, when the densest water was found, convection has become gradually weaker leading to the coexistence of three versions of the LSW. In 1996, the deepest LSW formed the deep salinity minima at 1700-2400 m. This water, present from the previous years, was fresher than 34.84 and colder than 2.8°C. The intermediate LSW was up to 0.02 saltier and 0.1-0.2°C warmer than the deepest LSW. The uppermost "generation" of the LSW was the freshest.

The DSOW was coldest and freshest in the spring of 1996, afterwards getting saltier and warmer. In 1993 the cold and dense core of the DSOW was not seen on the AR7W, pointing to the significance of interannual variability of interactions between the Labrador Sea and the North Atlantic in the deep layers.

Unlike the LSW and DSOW, the NEADW has continued to cool and freshen since 1990 and shown weak interannual variability. The rate of cooling was the highest in the LSW, resulting in the top part of the NEADW being warmer than the entire LSW, and thus forming a narrow deep temperature minima, that has been decaying or diminishing since 1995.

Changes in the depth and degree of deep convection are also revealed by the transient tracers. In the central Labrador Sea, CFC-11 concentrations in the LSW generally increase over time, but do show a decrease when the older NEADW is incorporated into the LSW by deep convection.

CFC-11 concentrations in the NEADW and DSOW tended to increase monotonically each year with greater annual variability in DSOW than in NEADW.

3.  WOCE Section AR7W: Changes in Dynamic heights

In 1994 the dynamic heights in the central part of the Sea were 6 to 8 dyn. cm below the heights computed for the late 1960s.

A raise of the heights between 1995 and 1997 was associated with the warming of the LSW and reduction of its density. In 1997 the heights were still lower than those in the late 1960s, but not more than by 3 dyn. cm.

Figure 3.1.  Dynamic heights (cm) along the AR7W line. Distance is measured from the Labrador coast.

4.  List of Figures

Researcher: Igor Yashayaev - yashayaevi@mar.dfo-mpo.gc.ca
Web Designer: Dave Kellow - kellowd@mar.dfo-mpo.gc.ca