A Water Mass Classification Approach to Tracking Variability in the East Australian Current

Lachlan R. Phillips,Ian Jonsen,Moninya Roughan,Robert Harcourt,Gemma Carroll

doi:10.3389/fmars.2020.00365

Abstract

The East Australian Current (EAC) is a southward flowing western boundary current that transports relatively warm and nutrient-depleted subtropical water along Australia's east coast. The EAC is a highly variable system that is formed by temporally-varying mixtures of water in the Coral Sea that do not form a linear density gradient or conform to a set range of temperature and salinity values. It can therefore be difficult to track EAC dynamics across both space and time using traditional analytical approaches. In order to more accurately quantify variability and trends in penetration of the EAC we develop a novel machine-learning classification approach to quantify variability in coastal EAC dynamics along a latitudinal gradient within the EAC extension zone in southeastern Australia. Applying our method to data from a 22-year free running regional hydrodynamic model revealed significant decadal-scale changes to EAC dynamics in the region. The annual period (generally in the austral summer) when the EAC is the dominant water mass in the region increased by approximately 2 months over the model time series. The encroachment of the EAC's traditional period of summer dominance into winter may have significant ecological implications through the acceleration of poleward range extensions by vagrant tropical species, facilitation of community phase shifts from temperate to tropical assemblages, and a phenological shift in the timing of major phytoplankton blooms. These results highlight the need to further understand the rapid changes occurring within western boundary current systems, and illustrates how classification approaches may assist in uncovering patterns in these highly variable systems.

Highlights

Western boundary currents are strong and persistent features of ocean circulation that form along the western margins of the world’s major ocean basins (Hogg and Johns, 1995; Imawaki et al, 2013)
The classification algorithm produced a metric of East Australian Current (EAC) presence for the study region and was able to track structural features such as EAC-derived eddies that were evident in the surface temperature and salinity profiles (Figure 4 and Video 1)
The method revealed latitudinal variability in the dynamics of the EAC extension, with differences being detected between the three study sites

Summary

Introduction

Western boundary currents are strong and persistent features of ocean circulation that form along the western margins of the world’s major ocean basins (Hogg and Johns, 1995; Imawaki et al, 2013). The current has intensified over the last eight decades with strong lines of evidence demonstrating a marked increase in the strength, duration, and frequency of southward incursions of EAC water (Ridgway, 2007; Johnson et al, 2011). These patterns are consistent with a “spin up” of the South Pacific sub-tropical gyre (Cai et al, 2005; Cai, 2006). This intensification has resulted in the EAC extension zone seeing rates of ocean warming that are 3– 4 times the global average, making southeast Australia one of the fastest warming regions in the southern hemisphere (Wu et al, 2012)

Objectives

Methods

Results

Conclusion