Meridional Heat and Salinity Budgets of the Sargasso Sea Inferred from Two Decades of Ocean Time-series and Transect Observations
Meridional Heat and Salinity Budgets of the Sargasso Sea Inferred from Two Decades of Ocean Time-series and Transect Observations
Abstract:
Significant long-term trends in temperature and salinity for the upper and deep ocean at the Bermuda Atlantic Time-series Study (BATS) and Hydrostation ‘S’ sites have been documented whereby the upper ocean shows a long term increase in temperature and salinity, while in contrast deep observations suggest a cooling and freshening. However, clear multi year shifts in these trends are apparent throughout the BATS (1988-2019) and Hydrostation ‘S’ (1955-2019) records which for the upper ocean appears to be partly correlated with the North Atlantic Oscillation. More recently upper ocean warming has accelerated with the most recent pentad being the warmest on record, which is particularly evident in the properties of Subtropical Mode Water (STMW). A significant reduction in the thickness of STMW has occurred to the extent that this water mass is often not detectable on time series cruises and further, the core STMW temperature has increased by almost 1°C. To facilitate a broader spatial context for interpreting interannual variability and emerging trends observed at the Bermuda time-series sites, yearly meridional transect cruises, namely, BATS Validation (BVAL) were initiated in 2001. BVAL cruises have been conducted typically in late September to help capture maximum heat content of the upper ocean and consist of CTD stations every one degree of latitude from 35°N to 19°N along 66°W similar to the WOCE A22 section. Here we review this nearly two decades of transect data focusing on the depth integrated heat and salinity content over fixed depth intervals and for specific water masses as defined by neutral density surfaces. Specifically, we determine the temporal meridional covariance through the full water column and estimate decorrelation length scales while quantifying multi-year variability for the broader region in relation to the variability observed at the BATS and Hydrostation ‘S’ sites.