Antarctic Bottom Water response to Varying Surface Fluxes

Abstract

Antarctic Bottom Water (AABW) is one of the densest and most voluminous water masses of the global ocean. It forms the lower limb of the global overturning circulation and plays an important role in transporting carbon, heat and freshwater sequestered from the atmosphere to the deep ocean. Surface buoyancy fluxes modulate the production of AABW through the formation of Dense Shelf Water (DSW) on the Antarctic continental shelf. The DSW flows down the continental slope as an overflow, entraining ambient Circumpolar Deep Water (CDW), to form AABW. The AABW spreads through the abyssal ocean, influencing global deep stratification, water properties and circulation over centennial, and even millennial, time scales

While surface fluxes play a key role in defining AABW production rates, the role of varying surface fluxes in influencing AABW properties and variability remains uncertain. Broad scale observational analysis of AABW processes is hindered by the extreme conditions particular to the Southern Ocean and Antarctic regions, and climate models struggle to accurately represent AABW formation processes. The difficulty climate models have in representing AABW formation originates from challenges in simulating DSW formation and the resultant overflow. Through both observational analysis and novel model development, this thesis provides insight into the role of varying surface fluxes in controlling AABW responses and feedbacks, and the limitations of climate models in representing such responses.

A coarse resolution sector model of the Atlantic Ocean is developed to aid in testing the limitations of climate model representation of AABW formation. With realistic forcing and bathymetry, the sector model efficiently emulates climate model processes and allows AABW sensitivity to overflow parameterisations to be assessed. While AABW proves relatively insensitive to most current generation overflow parameterisations, understanding the importance of DSW formation in defining AABW's role in a changing climate remains an important challenge.

Increased horizontal and vertical resolution allows the sector model to maintain DSW as the dominate mode of AABW formation. Under such formation conditions, the influence of varying surface buoyancy fluxes on DSW sourced AABW is assessed. Increased buoyancy fluxes decrease the cross-shelf exchange of DSW and CDW. The reduced exchange cools DSW and propagates changes to the abyssal ocean, driving a decadal scale variability of AABW.

The role of surface buoyancy variations in driving the cross-shelf exchange and AABW production, is further revealed at seasonal time scales through an observational analysis of circulation on the Adelie Land continental shelf, East Antarctica. The seasonality of surface buoyancy fluxes leads to enhanced cross-shelf exchange of DSW and CDW in winter, at an order of magnitude larger than that in summer. The enhanced exchange sets up a cyclonic flow on the shelf and highlights the influence of buoyancy fluxes in controlling circulation on the continental shelf.

The influence of surface buoyancy fluxes on AABW formation, shelf circulation and cross-shelf exchange, occurs through inclusion of DSW sourced AABW, a process absent from most climate models. Without correct representation of AABW formation mechanisms, climate models are missing key responses and feedbacks driven from changes in surface fluxes. On-going work into climate model development of AABW formation processes is thus essential to develop an increased understanding of AABW dynamics, variability and response to climate change.