Decadal Changes in Water Mass Characteristics at 24°N in the Subtropical North Atlantic Ocean

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.