The Long-Term Hydrographic Record at Bermuda

A long, by oceanographic standards, time series of hydrographic observations at Bermuda was begun in 1954 and continues to the present. Analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer (0–200 dbar), where integral timescales for salinity are 50% longer than for temperature and surface salinity changes are correlated with decadal changes in the 18°C Water production. Temperature/salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of ±50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of ±300 km. Salinity changes in the deepest layer observed at Bermuda station "S" (1500–2500 dbar) are uncorrelated with temperature, masked by measurement errors in the early years of the time series. Inclusion of earlier data of opportunity near Bermuda has permitted the time series of temperature to be extended backward 32 years to 1922. Over the entire 73-yr period, temperature is characterized by decadal timescale changes at all depths, but notably the deepest layer (1500–2500 dbar) also shows a long-term, secular warming trend with a rate of increase of about 0.5°C/century. This rate of temperature rise amounts to a sequestration of 0.7 W m−2 of "excess" heat flux in the deep water and, due to thermal expansion of seawater, should produce a local rise in steric sea level of approximately 7 cm/century. A comparison of two meridional sections taken through the western subtropical gyre passing just to the west of Bermuda during the IGY and 1985 shows that this long-term temperature increase is widespread, with spatially averaged values comparable to the long-term climatological trend at Bermuda, and is associated with a salinity increase in the upper deep water.