Upper-ocean variability in Drake Passage and the Weddell Sea: Measuring the oceanic response to air-sea and ice-ocean interactions

• Main Author: Stephenson, Gordon Ronald, Jr.
• Format: Thesis
• Language: English
• Published: University of California, San Diego 2012
• Subjects: Physical oceanography; Drake Passage; Weddell; Weddell Sea
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=3544197

In the first part of this dissertation, reanalysis heat flux products and profiles from a 15 year time series of high-resolution, near-repeat expendable bathythermograph / expendable conductivity-temperature-depth (XBT/XCTD) sampling in Drake Passage are used to examine sources of upper-ocean variability, with a focus on the nature of MLD variations and their impact on a first-order, one-dimensional heat budget for the upper ocean in the regions north and south of the Polar Front. Results show that temperature and density criteria yield different MLD estimates, and that these estimates can be sensitive to the choice of threshold. The difficulty of defining MLD in low-stratification regions, the large amplitude of wintertime MLD (up to 700 m in Drake Passage), and the natural small-scale variability of the upper ocean result in considerable cast-to-cast variability in MLD, with changes of up to 200 m over 10 km horizontal distance. In contrast, the heat content over a fixed-depth interval of the upper ocean shows greater cast-to-cast stability and clearly measures the ocean response to surface heat fluxes. In particular, an annual cycle in upper ocean heat content is in good agreement with the annual cycle in heat flux forcing, which explains 24% of the variance in heat content above 400 m depth north of the Polar Front and 63% of the variance in heat content south of the Polar Front. At interannual timescales, the primary drivers of interannual variations in upper-ocean heat content in Drake Passage are advective processes; up to 40% of the variance of cross-Passage average upper-ocean heat content is due to meanders of the Polar Front, while 14% of the variability results from mesoscale eddies. Heat flux anomalies contribute less variance (5-10%) on interannual timescales. Teleconnections with ENSO and SAM contribute to anomalies in meridional winds and heat fluxes. As a result, ENSO and SAM contribute variability in upper ocean heat content at near-zero lags; ENSO and SAM are also correlated with upper ocean heat content anomalies on timescales of ~2-5 years. The second part of this dissertation explores a melting iceberg as a source of upper-ocean variability. Observations near a large tabular iceberg in the Weddell Sea in March and April 2009 show evidence that water from ice melting below the surface is dispersed in two distinct ways. Warm, salty anomalies in T-S diagrams suggest that water from the permanent thermocline is transported vertically as a result of turbulent entrainment of meltwater at the iceberg's base. Stepped profiles of temperature, salinity, and density in the seasonal thermocline are more characteristic of double-diffusive processes that transfer meltwater horizontally away from the vertical ice face. These processes contribute comparable amounts of meltwater--O(0.1 m3) to the upper 200 m of a 1 m2 water column--but only basal melting results in significant upwelling of water from below the Winter Water layer into the seasonal thermocline. This suggests that these two processes may have different effects on vertical nutrient transport near an iceberg.