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Harry L. Bryden, Michael J. Griffiths, Alicia M. Lavin, Robert C. Millard, Gregorio Parrilla, and William M. Smethie

Abstract

Three transatlantic hydrographic sections across 24°N through the center of the subtropical gyre are analyzed for evidence of decadal changes in water mass characteristics. In the main thermocline, there has been a steady increase in salinity from 1957 to 1981 to 1992. The salinity at a given potential temperature between 12°C and 17°C has increased at a nearly constant rate of 0.010 per decade; at constant potential density, salinity and temperature both increased at rates of 0.018 per decade and 0.065°C per decade. In the intermediate waters between 800 and 1500 dbar, salinity has increased as well, on average by as much as 0.025 from 1981 to 1992, either on constant potential temperature or constant density surfaces. These changes cannot be due to vertical heaving of a constant water mass structure and they cannot be easily explained in terms of horizontal shifts in the subtropical gyre water mass structure. Throughout the water column down to 2000 dbar and across the zonal extent of the 24°N section, salinity and temperature have broadly increased on density surfaces. Below 2000 dbar, there has been slight cooling and freshening on isopycnal surfaces since 1981.

Decadal changes in temperature and salinity from 1981 to 1992 are separated into a component due to vertical motion of the isopycnals and a component due to the changes in temperature and salinity on isopycnals. The remarkable interior warming at constant depth with a maximum increase in temperature at 1100 dbar of nearly 1°C per century reported from earlier analyses of these sections is due to both processes. From 1957 to 1981, the warming was principally due to downward displacement of isopycnals and isotherms by order 50 dbar with little change in water mass characteristics. From 1981 to 1992 there were only small changes in isopycnal depths of order 10 dbar in the thermocline and intermediate waters, however. Thus, the continued warming from 1981 to 1992 has principally been due to changes in water mass characteristics, higher salinities and higher temperatures on isopycnal surfaces. Because of possible problems with 1957 salinity determinations, the cause of the cooling of the deep waters from 1957 to 1981 is ambiguous. Since 1981, the continued cooling of the waters deeper than 2000 dbar has been due principally to changes in the water mass characteristics, with a slight cooling and freshening on isopycnal surfaces.

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Elaine L. McDonagh, Brian A. King, Harry L. Bryden, Peggy Courtois, Zoltan Szuts, Molly Baringer, Stuart A. Cunningham, Chris Atkinson, and Gerard McCarthy

Abstract

The first continuous estimates of freshwater flux across 26.5°N are calculated using observations from the RAPID–MOCHA–Western Boundary Time Series (WBTS) and Argo floats every 10 days between April 2004 and October 2012. The mean plus or minus the standard deviation of the freshwater flux (F W) is −1.17 ± 0.20 Sv (1 Sv ≡ 106 m3 s−1; negative flux is southward), implying a freshwater divergence of −0.37 ± 0.20 Sv between the Bering Strait and 26.5°N. This is in the sense of an input of 0.37 Sv of freshwater into the ocean, consistent with a region where precipitation dominates over evaporation. The sign and the variability of the freshwater divergence are dominated by the overturning component (−0.78 ± 0.21 Sv). The horizontal component of the freshwater divergence is smaller, associated with little variability and positive (0.35 ± 0.04 Sv). A linear relationship, describing 91% of the variance, exists between the strength of the meridional overturning circulation (MOC) and the freshwater flux (−0.37 − 0.047 Sv of F W per Sverdrups of MOC). The time series of the residual to this relationship shows a small (0.02 Sv in 8.5 yr) but detectable decrease in the freshwater flux (i.e., an increase in the southward freshwater flux) for a given MOC strength. Historical analyses of observations at 24.5°N are consistent with a more negative freshwater divergence from −0.03 to −0.37 Sv since 1974. This change is associated with an increased southward freshwater flux at this latitude due to an increase in the Florida Straits salinity (and therefore the northward salinity flux).

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