The response of the Antarctic Subpolar Seas to climatic changes

Abstract

The Antarctic Subpolar Seas are a poorly observed and understood region with a disproportionate importance for global ocean-climate processes. The region has been subject, over recent decades, to many dramatic climatic forcing perturbations. In particular, it has seen a substantial increase in surface wind stress, and an increase in freshwater forcing from the Antarctic Ice sheet.
The impact of increased freshwater forcing is investigated here using austral summer satellite altimetry measurements. Between 1992 and 2011, an anomalous circumpolar rise in sea level is observed across the Antarctic Subpolar Seas of 1–5 mm yr-1 above the global mean. Several lines of evidence suggest that the observed Subpolar Sea SSH anomaly is a steric response to a recent decadal increase in glacial runoff. An ocean GCM is used to simulate the response of the Antarctic Subpolar Seas to a realistic increase in glacial run-off. The resulting response agrees well with observations and provides insight into the ocean’s adjustment. In particular, the model suggests that approximately half of the steric change in sea level is driven directly by freshening in the upper 800 meters, and that deep ocean warming drives the remainder.
The response of the Subpolar Seas to an increase in surface wind forcing is then investigated using an idealised regional configuration of the MITgcm model, referred to as the gyre model. The gyre model is a simplified, Bousinesq, primitive equation model, designed to explore the dynamics of a southern subpolar gyre’s adjustment to wind forcing perturbations. The gyre model has four sub-configurations of ranging complexity. Each configuration is examined for its response to a 20% step-increase in surface wind forcing. The gyre model’s response varies between sub-configurations, however some common features are found in all runs. The density surfaces of the southern gyre are shown to dome in all sub-configurations following adjustment. Waves and advective features that move cold anomalies northwards dominate the western boundary adjustment. Two distinct types of boundary propagations occur in the gyre model: comparatively fast numerical short boundary waves (NSBW), which move at around 0.05 to 0.5 m s-1; and slower moving advective features, which propagate at between 0.01 and 0.05 m s-1. Analysis suggests that viscous parameters play an important role in the NSBW balance of terms. Further, the pathlines traced by neutrally buoyant particles seeded into the western boundary flow field suggest that the advective features are by the advection of temperature anomalies. The depth, magnitude and propagation velocity of the boundary features vary considerably following minor adjustments in model parameters, such as viscosity, diffusivity and background GM diffusivity. Further work is required to understand the role of boundary waves and boundary propagations, both in observations and models.

The research presented in this thesis highlights that the recent climatic perturbations in the forcing of the Antarctic subpolar Seas are changing the regions circulation in significant and far reaching ways.

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Identifiers

ORCID for Alberto Naveira Garabato: orcid.org/0000-0001-6071-605X

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