Dr. Chris Chapman

Research Scientist

CSIRO Oceans and Atmosphere, Hobart, Tasmania.

email: chris.chapman@csiro.au

Research interests: Southern Ocean turbulence, variability and change; near-term and decadal forecasting; data assimilation; causality and information flow in the climate system, geophysical fluid dynamics, ice-ocean-atmosphere interaction; Southern Ocean ecology; ocean instrumentation (principally XBTs)

Abstract:

The Southern Ocean hosts the eastward flowing Antarctic Circumpolar Current (ACC), one of the strongest current systems on Earth. The ACC is not, in fact, a single current, but a system of currents: observations dating back to the 1920s revealed that the strongest currents are concentrated into narrow “fronts" or “jets", generally oriented east-west, that have a profound influence on the on the physical and biogeochemical characteristic of the Southern Ocean. Most notably, the strong currents associated with fronts form barriers, greatly reducing to the north-south exchange of heat, salt, carbon and nutrients. Given that there has been substantial warming and freshening observed in the Southern Ocean, and that the strong westerly winds that blow over it have intensified, attention has naturally turned to how such trends might influence its fronts and the ecosystems that depend on them.

A number of studies, particularly in the early and mid-2000s, investigated the temporal variability of Southern Ocean fronts. During the period from around 2002 to about 2015, a tentative consensus emerged: there had been a southward displacement in the position of fronts of between 0.5 and 1 degrees over a period from 1993 (the year satellite altimetry became widely available) to about around 2010, and that fronts are sensitive to changes in wind forcing. Such studies attracted the attention of biologists, biogeochemists, and paleo-oceanographers, curious about such

important changes in the physical environment might impact Southern Ocean ecosystems, both in present, past and future climates.

However, starting from about 2008, a number of studies began to contradict the consensus that had begun to establish itself. Using both observations and new generation numerical ocean models, these studies observed little to no movement in the fronts over the observational period and found that their locations were relatively insensitive to changes in wind forcing. Why is there such a discrepancy? Why do researchers using identical datasets or numerical models arrive at such different conclusions? This presentation will dive into the controversy and show how our conception of Southern Ocean fronts has changed over the last decade, and what this means for study of the physics, climatology and biology of this vital region, and the impact on the climate system as a whole. Finally, I will discuss open research questions and identify several promising avenues for future research.