ORCID Profile
0000-0003-2397-5364
Current Organisation
University of Southampton
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Publisher: American Meteorological Society
Date: 02-2022
Publisher: Informa UK Limited
Date: 07-2013
Publisher: Frontiers Media SA
Date: 07-03-2016
Publisher: Frontiers Media SA
Date: 20-01-2022
DOI: 10.3389/FMARS.2021.771973
Abstract: We investigate the spatial distribution of diapycnal mixing and its drivers in the central South Atlantic thermocline between the Rio-Grande Rise to the Mid-Atlantic Ridge. Diapycnal mixing in the ocean interior influences the slowly evolving meridional circulation, yet there are few observations of its variability with space and time or its drivers. To overcome this gap, seismic reflection data are spectrally analyzed to produce a 1,600 km long full-thermocline vertical section of diapycnal diffusivity, that has a vertical and horizontal resolution of O (10) m and spans a period of 4 weeks. We compare seismic-derived diffusivities with CTD-derived diffusivities and direct observations from 1996, 2003, and 2011. In the mean and on decadal scales, we find that thermocline diffusivities have changed little in this region, retaining a background value of 1 × 10 –5 m 2 s –1 . Imprinted upon the background rates, mixing is heterogeneous at mesoscales. Enhanced mixing, exceeding 10 × 10 –5 m 2 s –1 and spreading between 200 and 700 m depth, is found above the Mid-Atlantic Ridge suggesting the ridge enhances diffusivity by at least one order of magnitude across the entire water column. Rapid decay of diffusivities within 30 km of the ridge implies local dissipation of tidal energy. Above smooth topography, patches of enhanced mixing are possibly caused by a recent storm that injects near-inertial energy into the water column and elevates mixing from 3 × 10 –5 m 2 s –1 to 50 × 10 –5 m 2 s –1 down to depths of more than 600 m. The propagation speed of near-inertial energy varies substantially from 17 to 27 m/day. Faster speed, and therefore greater penetration depths of 800 m, are probably facilitated by an eddy. Together, these data extend the observational record of central South Atlantic thermocline mixing and provide insights into drivers of mesoscale variability.
Publisher: American Meteorological Society
Date: 12-2020
Abstract: For the first time, the temperature transport of the Agulhas Current is quantified in a time series. Over a 25-month mooring deployment at 34°S, seven tall moorings were instrumented to measure current velocity, temperature, and salinity. Current- and pressure-recording inverted echosounders were used to extend geostrophic velocity, temperature, and salinity records to 300 km offshore. In the mean, the current transports 3.8 PW of heat southward relative to 0°C: −76 Sv (1 Sv ≡ 10 6 m 3 s −1 ) at a transport-weighted temperature of 12.3°C. A 0.9-PW standard deviation in temperature transport is due to variability in both volume transport and the temperature field. Meandering of the current core dominates variability in the temperature field by warming temperatures offshore and cooling temperatures near the coast. However, meandering has a limited impact on the temperature transport, which varies more closely with a deepening and broadening of the current associated with an inshore isotherm shoaling and an offshore isotherm deepening. Stronger southward temperature transports correspond to a deeper current transporting more volume, yet at a cooler transport-weighted temperature. Seasonality is not observed in the temperature transport time series, possibly because of the offsetting effects of cooler temperatures during times of seasonally stronger volume transports. Although volume transport and temperature transport are highly correlated, the large variability in transport-weighted temperature means that using volume transport alone to infer temperature transport results in an error that could be as large as 24% of the southern Indian Ocean heat transport.
Publisher: Frontiers Media SA
Date: 21-06-2022
DOI: 10.3389/FMARS.2022.736693
Abstract: Seismic reflection profiling of thermohaline structure has the potential to transform our understanding of oceanic mixing and circulation. This profiling, which is known as seismic oceanography, yields acoustic images that extend from the sea surface to the sea bed and which span horizontal distances of hundreds of kilometers. Changes in temperature and salinity are detected in two, and sometimes three, dimensions at spatial resolutions of ~ O (10) m. Due to its unique combination of extensive coverage and high spatial resolution, seismic oceanography is ideally placed to characterize the processes that sustain oceanic circulation by transferring energy between basin-scale currents and turbulent flow. To date, more than one hundred research papers have exploited seismic oceanographic data to gain insight into phenomena as varied as eddy formation, internal waves, and turbulent mixing. However, despite its promise, seismic oceanography suffers from three practical disadvantages that have slowed its development into a widely accepted tool. First, acquisition of high-quality data is expensive and logistically challenging. Second, it has proven difficult to obtain independent observational constraints that can be used to benchmark seismic oceanographic results. Third, computational workflows have not been standardized and made widely available. In addition to these practical challenges, the field has struggled to identify pressing scientific questions that it can systematically address. It thus remains a curiosity to many oceanographers. We suggest ways in which the practical challenges can be addressed through development of shared resources, and outline how these resources can be used to tackle important problems in physical oceanography. With this collaborative approach, seismic oceanography can become a key member of the next generation of methods for observing the ocean.
Publisher: Springer Science and Business Media LLC
Date: 27-03-2023
Publisher: American Meteorological Society
Date: 12-2020
Abstract: The Agulhas Current, like all western boundary currents, transports salt from the subtropics toward the poles and, on average, acts as a barrier to exchange between the open ocean and continental seas. Uniquely, the Agulhas jet also feeds a leakage of relatively salty waters from the Indian Ocean into the Atlantic Ocean. Despite its significance, the signals and drivers of water mass variability within the Agulhas Current are not well known. To bridge this gap, we use 26 months of moored observations to determine how and why salinity—a water mass tracer—varies across the Agulhas Current. 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. Shifting of the current causes heave and diapycnal mixing of subtropical, central, and intermediate waters. Diapycnal mixing between central and intermediate waters explains most of the variability, creating salinity anomalies between −0.4 and +0.1 psu. Pulsing of the current drives heave and, to a lesser extent, along-isopycnal mixing within the halocline. This cross-stream mixing results in salinity anomalies of up to 0.3 psu. The mean and standard deviation of Agulhas Current volume and salt transports are −76 and 22 Sv (1 Sv ≡ 10 6 m 3 s −1 ) and −2650 and 770 Sv psu. Transport-weighted salinity has a standard deviation of 0.05 psu. We estimate that O (10 13 ) kg yr −1 of the salt transported southwestward leaks into the fresher Atlantic Ocean. On the basis of our observations, the variability of the Agulhas Current could alter this salt leakage by an order of magnitude.
Publisher: American Meteorological Society
Date: 03-2022
Abstract: Since 2000, the Indian Ocean has warmed more rapidly than the Atlantic or Pacific Oceans. Air–sea fluxes alone cannot explain the rapid Indian Ocean warming, which has so far been linked to an increase in temperature transport into the basin through the Indonesian Throughflow (ITF). Here, we investigate the role that the heat transport out of the basin at 36°S plays in the warming. Adding the heat transport out of the basin to the ITF temperature transport into the basin, we calculate the decadal mean Indian Ocean heat budget over the 2010s. We find that heat convergence increased within the Indian Ocean over 2000–19. The heat convergence over the 2010s is of the same order as the warming rate, and thus the net air–sea fluxes are near zero. This is a significant change from previous analyses using transbasin hydrographic sections from 1987, 2002, and 2009, which all found ergences of heat. A 2-yr time series shows that seasonal aliasing is not responsible for the decadal change. The anomalous ocean heat convergence over the 2010s in comparison with previous estimates is due to changes in ocean currents at both the southern boundary (33%) and the ITF (67%). We hypothesize that the changes at the southern boundary are linked to an observed broadening of the Agulhas Current, implying that temperature and velocity data at the western boundary are crucial to constrain heat budget changes.
Publisher: Frontiers Media SA
Date: 05-10-2021
DOI: 10.3389/FMARS.2021.697179
Abstract: The southwest Atlantic gyre connects several distinct water masses, which means that this oceanic region is characterized by a complex frontal system and enhanced water mass modification. Despite its significance, the distribution and variability of vertical mixing rates have yet to be determined for this system. Specifically, potential conditioning of mixing rates by frontal structures, in this location and elsewhere, is poorly understood. Here, we analyze vertical seismic (i.e., acoustic) sections from a three-dimensional survey that straddles a major front along the northern portion of the Brazil-Falkland Confluence. Hydrographic analyses constrain the structure and properties of water masses. By spectrally analyzing seismic reflectivity, we calculate spatial and temporal distributions of the dissipation rate of turbulent kinetic energy, ε, of diapycnal mixing rate, K , and of vertical diffusive heat flux, F H . We show that estimates of ε, K , and F H are elevated compared to regional and global mean values. Notably, cross-sectional mean estimates vary little over a 6 week period whilst smaller scale thermohaline structures appear to have a spatially localized effect upon ε, K , and F H . In contrast, a mesoscale front modifies ε and K to a depth of 1 km, across a region of O (100) km. This front clearly enhances mixing rates, both adjacent to its surface outcrop and beneath the mixed layer, whilst also locally suppressing ε and K to a depth of 1 km. As a result, estimates of F H increase by a factor of two in the vicinity of the surface outcrop of the front. Our results yield estimates of ε, K and F H that can be attributed to identifiable thermohaline structures and they show that fronts can play a significant role in water mass modification to depths of 1 km.
Publisher: American Geophysical Union (AGU)
Date: 04-2018
DOI: 10.1029/2018JC013833
Publisher: American Meteorological Society
Date: 10-2022
Abstract: The global freshwater cycle is intensifying: wet regions are prone to more rainfall, while dry regions experience more drought. Indian Ocean rim countries are especially vulnerable to these changes, but its oceanic freshwater budget—which records the basinwide balance between evaporation, precipitation, and runoff—has only been quantified at three points in time (1987, 2002, 2009). Due to this paucity of observations and large model biases, we cannot yet be sure how the Indian Ocean’s freshwater cycle has responded to climate change, nor by how much it varies at seasonal and monthly time scales. To bridge this gap, we estimate the magnitude and variability of the Indian Ocean’s freshwater budget using monthly varying oceanic data from May 2016 through April 2018. Freshwater converged into the basin with a mean rate and standard error of 0.35 ± 0.07 Sv (1 Sv ≡ 10 6 m 3 s −1 ), indicating that basinwide air–sea fluxes are net evaporative. This balance is maintained by salty waters leaving the basin via the Agulhas Current and fresher waters entering northward across the southern boundary and via the Indonesian Throughflow. For the first time, we quantify seasonal and monthly variability in Indian Ocean freshwater convergence to find litudes of 0.33 and 0.16 Sv, respectively, where monthly changes reflect variability in oceanic, rather than air–sea, fluxes. Compared with the range of previous estimates plus independent measurements from a reanalysis product, we conclude that the Indian Ocean has remained net evaporative since the 1980s, in contrast to long-term changes in its heat budget. When disentangling anthropogenic-driven changes, these observations of decadal and intra-annual natural variability should be taken into account.
Publisher: Frontiers Media SA
Date: 11-07-2022
Publisher: Elsevier BV
Date: 12-2014
Publisher: Copernicus GmbH
Date: 03-02-2016
Abstract: Abstract. Flooding is a key driver of floodplain vegetation productivity. Adaptive cycles provide a model for examining the productivity of semi-arid floodplain vegetation in response to hydrology. We examined the response of vegetation productivity (measured as NDVI) through a hypothesised adaptive cycle to determine whether the cycle repeats over time and how it is affected by differently sized flood events. The area of floodplain inundation was associated with an adaptive cycle that repeated in four flood events through the following phases: wetting (exploitation phase), wet (conservation phase), drying (release phase) and dry (reorganisation phase). Vegetation productivity responses corresponded to these phases. The area and quality of floodplain vegetation productivity followed the hypothesised pattern of higher-quality vegetation vigour in the wetting and wet phases, lower vigour in the drying phase and lowest vigour in the dry phase. There were more transitions between NDVI classes in the wet phase, which was dominated by two-way transitions. Overall, the wetting, wet and drying phases were dominated by smaller-probability class changes, whereas in the dry phase, higher-probability class changes were more prominent. Although the four flood events exhibited an adaptive cycle the duration of the adaptive-cycle phases, and the nature of vegetation productivity response, differed with the character of the flood event. Vegetation response in two of the adaptive-cycle phases – the release and reorganisation phases – were as hypothesised, but in the exploitation and conservation phases, changes in vegetation productivity were more dynamic. The character of vegetation response through the adaptive cycle also indicates that semi-arid floodplain vegetation productivity is more vulnerable to changing state during the conservation and release phases and not during the exploitation and reorganisation phases as resilience theory suggests. Overall, the adaptive cycle represents a new model to improve our understanding of the complexity of change in semi-arid floodplain vegetation productivity through cycles of flooding and drying. Changes in vegetation productivity could initiate structural changes in floodplain vegetation communities, with commensurate influences on floodplain sediment dynamics.
Publisher: Springer Science and Business Media LLC
Date: 18-08-2016
Publisher: Springer Science and Business Media LLC
Date: 03-2013
Publisher: Springer Science and Business Media LLC
Date: 25-05-2023
DOI: 10.1038/S41558-023-01667-8
Abstract: Dense water formed near Antarctica, known as Antarctic bottom water (AABW), drives deep ocean circulation and supplies oxygen to the abyssal ocean. Observations show that AABW has freshened and contracted since the 1960s, yet the drivers of these changes and their impact remain uncertain. Here, using observations from the Australian Antarctic Basin, we show that AABW transport reduced by 4.0 Sv between 1994 and 2009, during a period of strong freshening on the continental shelf. An increase in shelf water salinity between 2009 and 2018, previously linked to transient climate variability, drove a partial recovery (2.2 Sv) of AABW transport. Over the full period (1994 to 2017), the net slowdown of −0.8 ± 0.5 Sv decade −1 thinned well-oxygenated layers, driving deoxygenation of −3 ± 2 μmol kg −1 decade −1 . These findings demonstrate that freshening of Antarctic shelf waters weakens the lower limb of the abyssal overturning circulation and reduces deep ocean oxygen content.
Publisher: Wiley
Date: 11-03-2016
Publisher: American Meteorological Society
Date: 2022
Publisher: American Geophysical Union (AGU)
Date: 07-2020
DOI: 10.1029/2020JC016293
Publisher: Wiley
Date: 13-06-2016
DOI: 10.1111/JVS.12426
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Kathryn Gunn.