Dirk Slawinski

Retention and dispersal of shelf waters influenced by interactions of ocean boundary current and coastal geography

Retention and dispersal of shelf waters under the influence of ocean boundary currents is crucial to recruitment processes of many coastal species. In this study, a Lagrangian particle tracking method based on an eddy-resolving, data-assimilating, hydrodynamic model is used to study spatial variations of local retention rates and alongshore dispersal of surface waters on the continental shelf off the west coast of Australia. The circulation on the shelf off the west coast of Australia is dominated by the southward-flowing eastern boundary current, the Leeuwin Current, which is interrupted by episodic wind-driven, northward, inshore surface transport during the austral summer, and by mesoscale eddy formations during the austral winter. Low-retention shelf regions tend to experience high alongshore currents, owing to the near-shore influence of the Leeuwin Current, protruding coastal geography, or formation of mesoscale eddies, whereas high-retention regions are sheltered from the direct influence of the Leeuwin Current by coastal geographic features. Alongshore dispersal also exhibits spatial as well as seasonal heterogeneity, with predominantly southward dispersal during the austral winter, and more symmetrical dispersal during the austral summer. Shelf retention and seasonal dispersal are linked with recruitment processes of invertebrate and fish species off the west coast of Australia.

Freshening anomalies in the Indonesian throughflow and impacts on the Leeuwin Current during 2010–2011

During the 2010–2011 La Nina and Ningaloo Nino, excessive precipitations in the Maritime Continent and Indonesian-Australian Basin caused surface waters to freshen by 0.3 practical salinity unit in the southeast Indian Ocean. The low-salinity anomalies are observed to be carried westward by the Indonesian throughflow and the South Equatorial Current and transmitted into the poleward flowing eastern boundary current, the Leeuwin Current, along the Western Australian coast. Low-salinity anomalies contribute to about 30% of the anomalous increase of the southward Leeuwin Current transport during the evolution of the 2010–2011 Ningaloo Nino, resulting in unprecedented warming off the coast of Western Australia. Episodical freshening of the Leeuwin Current has been observed at the Rottnest coastal reference station of Western Australia during extended La Nina conditions over the past several decades; low-salinity anomalies at the station during the 2010–2011 Ningaloo Nino are comparable with strong historical events.

Observational insights into chlorophyll distributions of subtropical South Indian Ocean eddies

The South Indian Ocean subtropical gyre has been described as a unique environment where anticyclonic ocean eddies highlight enhanced surface chlorophyll in winter. The processes responsible for this chlorophyll increase in anticyclones have remained elusive, primarily because previous studies investigating this unusual behavior were mostly based on satellite data, which only views the ocean surface. Here we present in situ data from an oceanographic voyage focusing on the mesoscale variability of biogeochemical variables across the subtropical gyre. During this voyage an autonomous biogeochemical profiling float transected an anticyclonic eddy, recording its physical and biological state over a period of 6 weeks. We show that several processes might be responsible for the eddy/chlorophyll relationship, including horizontal advection of productive waters and deeper convective mixing in anticyclonic eddies. While a deep chlorophyll maximum is present in the subtropical Indian Ocean outside anticyclonic eddies, mixing reaches deeper in anticyclonic eddy cores, resulting in increased surface chlorophyll due to the stirring of the deep chlorophyll maximum and possibly resulting in new production from nitrate injection below the deep chlorophyll maximum.

Quantitative Quality Control (QC) procedures for the Australian National reference stations: Sensor data

The National Reference Station (NRS) network, part of Australias Integrated Marine Observing System (IMOS), is designed to provide the baseline multi decadal time series required to understand how large-scale, long-term change and variability in the global ocean are affecting Australias coastal ocean ecosystems. High temporal resolution observations of core variables are taken across the network using a standard instrument, the Wetlabs Water Quality Monitors (WQMs). These data are freely available to the scientific community and the general public and thus need to be controlled by means of a system that will ensure their consistency and fitness-for-use. This document provides insights into the Quality Control (QC) guidelines proposed for these data, towards a system that incorporates uncertainty into the standard qualitative flag system, making it quantitative.

Inferring and Removing a Spurious Response in the Optical Backscattering Signal from an Autonomous Profiling Float

AbstractDevelopment of autonomous profiling floats, allowing for long-term continuous measurement of bio-optical variables, promises to significantly increase our knowledge of the variability of th...