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PreprintRecent changes in the Labrador Sea Water within the Deep Western Boundary Current southeast of Cape Cod( 2011-07-20) Pena-Molino, Beatriz ; Joyce, Terrence M. ; Toole, John M.Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500m, where it was originally found, to 700m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record, versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the western boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid 1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport.
ArticleVariability in the Deep Western Boundary Current : local versus remote forcing(American Geophysical Union, 2012-12-22) Pena-Molino, Beatriz ; Joyce, Terrence M. ; Toole, John M.Horizontal velocity, temperature and salinity measurements from the Line W array for the period 2004–2008 show large changes in the water mass structure and circulation of the Deep Western Boundary Current (DWBC). Fluctuations in the flow with periods from 10 to 60 days are bottom intensified: signals most likely associated with topographic Rossby waves (TRW). A fraction (∼15%) of the DWBC transport variability is caused by Gulf Stream rings and meanders. These flow anomalies are surface intensified and fluctuate at frequencies lower than the TRW. Interannual variability in the velocity field appears to be related to changes in the hydrographic properties. The dominant mode of variability is characterized by an overall freshening, cooling, a potential vorticity (PV) increase in the deep Labrador Sea Water (dLSW) and a PV decrease in the Overflow Water (OW). The variability in the flow associated with these property changes is not spatially homogeneous. Offshore (water depths larger than 3500 m) changes in the velocity are in phase with PV changes in the OW: a decrease in the OW PV is accompanied by an increase in the southward (negative) transport. Conversely, variations of the inshore flow are in phase with changes in the dLSW PV (increasing PV and decreasing transport). This trend, true for most of the record, reverses after the winter of 2007–2008. A sudden decrease of the dLSW PV is observed, with a corresponding intensification of the flow in the inner DWBC as well as a northward shift in the Gulf Stream axis.