Summertime Bottom Temperatures on the Southeast Shoal of the Grand Bank, and Implications for Exchange Rates

Abstract

From spring to fall, the Southeast Shoal usually has the warmest bottom water on the entire Grand Bank of Newfoundland. Historical hydrographic data indicate that the elevated lower-layer temperatures are derived from the seasonal surface heat input, through vertical exchange across the developing thermocline in combination with the shallow depths of the Shoal. However, in some months, the surface fluxes required by the monthly changes in depth-integrated heat content over the Shoal exceed climatological flux estimates computed using bulk formulae, suggesting that the climatological estimates may be low. The seasonal evolution of the lower-layer temperature provides information on the rates of vertical exchange across the thermocline and horizontal exchange in the lower layer. A seasonal variation in the vertical exchange coefficient, with an approximately inverse relation to the vertical density difference, and significant horizontal exchange in summer are suggested. The associated rates are quantified with a crude axisymmetric model for the lower layer's monthly heat budget.