Modeling 18° water variability

Abstract

Variability of 18° Water formation is investigated with an isopycnic-coordinate model of the North Atlantic. A 30-year spinup integration is used as a "control" experiment in which the upper water column in the Sargasso Sea is shown to be in approximate hydrographic equilibrium after 20 years, at which time the potential density of mode water formed in this area is 26.40, close to the typical value in observations. Sensitivity studies investigate the impact on the 18° Water of anomalously strong wintertime Gulf Stream cooling. Successive "cold" winters are applied over the Gulf Stream from year 20 of the control run. These cold winters attempt to mimic the likely integral effect of anomalously frequent or long-lasting outbreaks of cold, dry, continental air observed over the Gulf Stream region during some years. Denser varieties of mode water evolve in a manner similar to that observed during the relatively cool 1960s. Estimated water mass formation rates quantify the sensitivity of 18° Water formation to the intensity and spatial pattern of excess Gulf Stream cooling. The model Gulf Stream intensifies by up to 20% after the cold winters - evidence for the hypothesized "Worthington effect." In general, the paper demonstrates that variability of 18° Water observed during 1954-1978 was likely to have been driven by the variability in the wintertime cooling of the Gulf Stream region.