Climatic and mesoscale eddy modulation of the upper ocean at the Bermuda Time-series sites

Johnson, Rodney John (2003) Climatic and mesoscale eddy modulation of the upper ocean at the Bermuda Time-series sites. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Time-series observations from Hydrostation 'S' (20km SE of Bermuda; 1954-2001) and the Bermuda Atlantic Time-series Study site (BATS; 80km SE of Bermuda 1988-2001) reveal substantial variability in the upper ocean heat, salinity and nutrient budgets over monthly to decadal time-scales. The research presented here investigates the role of local surface fluxes and mesoscale eddies on driving these short and long-term patterns of upper ocean physics at the Bermuda Time-series Sites (BTS) with implications for the consequential flux of new nutrients to the euphotic zone.

A 1-dimensional model of upper ocean physics forced with hourly meteorological observations from Bermuda is implemented for Hydrostation 'S' (1955-2001) and the BATS site (1989-2001). Modulation of the underlying stratification by mesoscale eddies is parameterised as an eddy heaving term which displaces isopycnals relative to the rate of change in the dynamics height field. The model solutions capture much of the observed variability in SST and MLD with good quantitative skill yielding a mean difference (model-observed) of -0.3 ± 0.68 ^oC for SST and 17 ± 42m for MLD. Variability in the upper ocean heat content over seasonal to decadal time-scales is well produced by the model with the model explaining 75% of the variance in the heat content for the upper 400m and 88% for the upper 100m. The long-term mean surface heat and freshwater flux are determined to be insignificantly different to zero at 0.5 Wm^-2 and 2.5x10^-4 yr^-1. Model experiments using spatial site data for initial conditions suggest advection of waters from the immediate mesoscale field plays an important role in the upper ocean heat and salinity budgets over monthly time-scales.

The inclusion of a nitrate (NO₃^-) uptake model in the physical model gives good results in reproducing the winter mixed layer NO₃^- profiles at the BATS site. Models estimates of annual new production are highly variable (0.09 to 1.6 mol N m^-2 yr^-1) with mean annual rates for Hydrostation 'S' and BATS equal to 0.5 ± 0.36 and 0.39 ± 0.35 mol N m^-2 yr^-1, respectively, consistent with geochemical estimates for this region.