Kathy Gunn

The ocean's meridional overturning circulation describes global-scale north-to-south transport of water masses. It has a large influence on the mean climate state and pace of climate change, as it stores and transfers heat around the globe. Antarctic Bottom Water (AABW) makes up the deepest branch of this circulation. Despite only forming in four locations, AABW fills the deepest 40% of the ocean. But, we have little information about the strength and variability of AABW due to its remote formation locations and deep depths. I combine legacy and state-of-the-art datasets to measure the strength and variability of AABW export.

• Around East Antarctica, we calculate the transport of AABW using a new method combining hydrographic, moored, and model data. We show that a 30% decrease in transport is driven by freshening of Antarctic shelf water. Read the paper here. Read The Conversation article here.

Cross-slope exchange describes the transfer of water from the ocean into continental seas and vice versa. In general, these exchanges are restricted. But, in some areas they make a significant contribution to the physical and biological conditions of the region. I use high-resolution seismic and moored datasets to observe and characterize cross-slope exchanges.

• We observed and characterized intrusions of warm water eddies onto the continental shelf of the Bellinghausen Sea, west Antarctica. These warm waters drive mass loss of the ice shelves which then raises global sea levels. We found that the eddy frequency is much greater (i.e., ~20-50 per month) than previously believed (i.e., ~3-5 per month). Read the paper here.
• This piece of work analyses a marine seismic reflection survey using the techniques of Seismic Oceanography. We can use these data to provide time-lapse imagery of the ocean, extract information about its physical properties, and estimate vertical mixing rates. To find out more about this field of science and its potential, see this Wikipedia page , and this outlook article.

(a) Seismic imagery from west Antarctica showing 4 eddies (white circles) and the Antarctic Slope Front (black arrows). N.B. seismic imagery can be considered a map of changes in temperature. (b) Temperature field inverted from seismic data. (c) Temperature anomaly field highlighting warm-cores of the eddies.

Mixing sets the capacity of the ocean to store heat and carbon and thereby influence climate. But, direct measurements of mixing are scarce. This paucity has left uncertainties in how to best represent ocean mixing, contributing to the large spread in climate model projections. I use indirect methods to estimate large- and small-scale mixing in the ocean.

• We determined the drivers of significant water mass changes in the western boundary current of the Indian Ocean. We find that salinity variability is driven by both shifting (i.e., changes in location) and pulsing (i.e., changes in strength) of the current. These upstream changes could alter the amount of salty water leaked from the Indian to the Atlantic Ocean by a factor of 10. Read the paper here .



Amount of days per year that the Agulhas Current is affected by changes in jet position (i.e., shifting) and strength (i.e., pulsing) (black bars). Small 10 km shifts in jet position affect the current for almost a third of the year (grey bar).

• We show how an ocean front modifies mixing rates up to 1 km below the sea-surface. The front enhances mixing rates near the surface whilst also suppressing it at depth. Read the paper here .