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Current Research

Simulating sub-ice shelf cavity circulation and dense water production in a global ocean model

Dense water produced in Antarctica via air-sea-ice interactions is one of the major drivers of the Meridional Overturning Circulation (MOC), also known as the global ocean conveyor belt. The impacts of ice shelves and the water circulation within their cavities on deep-ocean circulation and water mass properties around Antarctica are, however, poorly understood. Currently, none of the global ocean-climate coupled models used for the Coupled Model Intercomparison Project (CMIP), which informs the Intergovernmental Panel on Climate Change (IPCC), simulate sub-ice shelf cavity circulation. This circulation plays a critical role in global ocean overturning as it transforms salty water formed at the surface in Antarctica into the parent waters of Antarctic Bottom Water (AABW), the world’s deepest and heaviest water mass. The NEMO ocean model is used for 6 of the climate groups participating in CMIP. In most NEMO-based climate models, a “wall” is located at the mouth of the ice shelf cavities and the absence of explicit circulation under ice shelves is addressed via an input of freshwater at this wall. This means that the feedback of ocean conditions onto the ice shelf and dense water formation on the continental shelf is a missing puzzle piece in the models that are used to help inform climate policy and adaptation strategies. To address this I have been awarded a Marie Skłodowska-Curie Actions Grant to develop global ocean configurations using the NEMO ocean model where circulation beneath the major Antarctic the sub-ice shelf cavities is explicitly simulated. To find out more, checkout Project OPEN.

Published Research

Water mass mixing and transformation adjacent to Larsen C ice shelf

In 2019 I participated in the Weddell Sea Expedition, a voyage to a remote area of the Weddell Sea in Antarctica, as PI of physical oceanography. The data collected during this expedition has enabled a diagnosis of the water mass characteristics adjacent to Larsen C Ice Shelf (LCIS). This region is of interest due to its contribution to the properties of Antarctic Bottom Water (AABW), the heaviest and densest water in the world’s oceans. AABW constitutes a vital limb of the global ocean overturning system and its properties are thought to be sensitive to the effects of climate change. LCIS has recently undergone rapid thinning along with disintegrations of sections of the ice shelf, yet the role of the ocean in driving or contributing to these observed changes remains unclear. It is, therefore, timely that we improve our understanding of the ocean region adjacent to LCIS.  Hydrographic results revealed the presence of a warm water mass in the area which has its origins in the Antarctic Circumpolar Current and is modified in the Weddell Sea. This water mass, termed Modified Weddell Deep Water (MWDW), appears to thoroughly mix with the local shelf waters. Of specific interest is the high level of mixing observed between MWDW and Ice Shelf Water (ISW). ISW is a precursor of AABW, and thus an alteration of its properties on the continental shelf may impact AABW characteristics. Given the projected continuation of Southern Ocean warming, such mixing and transformation could have important implications for the global overturning circulation in future. Click here for the published article.

Principal forces driving Agulhas Current Seasonality

My PhD research focused on the Agulhas Current which flows along the east coast of South Africa, transporting warm Indian Ocean water southward. This current plays an important role in both global and local ocean circulation and climate regulation, acting as a vital limb of the global ocean conveyor belt by facilitating an exchange between the Indian and Atlantic oceans, and influencing local rainfall and climate by providing latent heat for evaporation. My PhD research looked at the seasonal cycle of the Agulhas Current and used idealized models to explore how winds over the Southern Indian Ocean may influence this seasonality. Mooring observations revealed that the current is 25% stronger in summertime, yet previous publications based on model simulations predicted the opposite seasonality and thus could not explain the variability observed. I found using idealized models and satellite observations that baroclinic processes communicating the wind stress curl variability from near-field winds have a dominant contribution to the seasonal phasing. Wind signals from further afield were found to die out during their journey west and so have little effect on the seasonal cycle of the Agulhas Current. Click here for the published article.

Investigating the Southern Ocean South of Africa

My masters' research used a proxy technique known as a Gravest Empirical Mode (GEM) to investigate changes in temperature and salinity that took place in the upper 2000m of the Atlantic sector of the Southern Ocean from 1992 to 2012. The Southern Ocean plays a critical role in the global circulation, as it links the three major ocean basins, and connects the deep sea to the surface. Due to harsh in situ sampling conditions, however, we have limited data for this region of the ocean. I therefore used the GEM proxy technique to establish a relationship between what measurements we do have and the overlying satellite altimetry data and obtained 20 years of proxy temperature and salinity profiles. I used this data to look specifically at the alteration of properties of Antarctic Intermediate Water and found significant trends and an interesting separation of changes by frontal zone. Click here for the published article.

 

Quality control of instrumentation that measures temperature in the Southern Ocean

My honours' research assessed the accuracy of an instrument called the Expendable Bathythermograph (XBT), which measures temperature in the upper 1000m of the ocean. It is widely used by the oceanographic community and XBT data makes up the majority of the historical subsurface temperature data archive. I found that this instrument gives biased results for the region of the ocean south of Africa. My research shows that the XBT probes fall slower that predicted the fall rate equation due to the cold viscous waters of the Southern Ocean. A publication describing this work can be found by clicking here.

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The XBT network is a key component of the global ocean observaing system. XBT data constitute a large portion of historic ocean temperature measurements and some XBT transects have been in operation for more than 30 years, thereby providing unique and valuable climate records. I co-authored a review article presenting the current state of the Global XBT Network, major scientific advances resulting from the decades-long XBT record, and synergy between the Global XBT Network and other components of the observing system. The publication on this work can be found here.

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