Paper: Chapman, C.C., Lea, M., Meyer, A. et al. Defining Southern Ocean fronts and their influence on biological and physical processes in a changing climateNat. Clim. Chang. (2020).

This paper is a review article that stemmed from a debate within the Southern Ocean community. The paper explains how “fronts”, sharp boundaries between water masses, are defined, and what their effects might be on the biology of the Southern Ocean. New methods and observations over the last decade or so have changed the way many physical oceanographers think about them, but many of these advances in the physical oceanography are yet to trickle through the community.

We review the ways that fronts are defined and the implications of various definitions, particularly for biologists. There is no “correct” definition, only a correct definition for the problem at hand and we provide a “user guide” to help practitioners to choose a definition to suit their problem. We also review the response of Southern Ocean fronts to climate change. Although the Southern Ocean is warming, its fronts have not been observed to change their position, and current climate models do not project a strong shift of fronts in the future. This is a change from previous thinking, that predicted fronts would move south with climate change;

Although some of the key questions concerning the physics of fronts are becoming better understood, the role of fronts in the ecology of the Southern Ocean is still unclear. Only greater cross-disciplinary collaboration can overcome these knowledge gaps.

Article Summary

The Southern Ocean, which lies between Australia and Antarctica, is a unique oceanographic environment, home to a wide variety of animals life, from microscopic plankton through to some of the largest marine animals on Earth, such as blue whales. As all Southern Ocean researchers know, one of the principal environmental features of the region is the presence of large scale “fronts”, sharp boundaries between waters with different characteristics, such as temperature and salinity. George Deacon, an early Antarctic scientist, described crossing a Southern Ocean front as “…like passing at one step from winter into spring”. Fronts are vitally important for the climate system, as they regulate the transport of heat from the warmer north to the frigid south. They are also “hotspots” of biological activity since numerous animals are forage for food in and around fronts.

However, over the last decade or so, there has been a robust debate in the physical oceanographic community about the exact nature of Southern Ocean fronts. Prior to the era of widely available satellite data, scientists were forced to rely on sparse observational databases, assembled over decades. Using these observations, physical oceanographers developed the “traditional” view that fronts were smooth, unbroken “lines in the sea”. However, as more and more satellite observations of the ocean became available, our view of Southern Ocean fronts became more complicated: instead of smooth, unbroken lines, satellites revealed a complex “spaghetti” of braided, broken and intertwined fronts. Reconciliation of these differing views has, thus far, eluded satisfactory closure.

In this review article, we take a step back and assess how frontal definitions have changed over the years, and the implications of a particular definition for the study of the Southern Ocean, particularly its biology and ecology. We argue that there is no single “correct” definition, only the correct definition for the problem, and data, at hand. To facilitate future research, we include a “user guide” to help practitioners choose the right definition for their problem.

We also review the response of the Southern Ocean’s fronts to climate change. We note that, even though the Southern Ocean is both warming and freshening, there is a general (but far from unanimous) consensus within the literature that fronts have shown no strong shift in their position, and climate models generally project no clear shifts in the fronts’ position into the future.

The role of fronts within Southern Ocean ecosystems and the broader climate system is still an active area of research, and a large number of questions remain unanswered. In our article, we argue that greater cross-disciplinary collaboration is required to begin untangling these thorny questions.