ALBERT

Description: Warm ocean waters drive rapid ice-shelf melting in the Amundsen Sea. The ocean heat transport toward the ice shelves is associated with the Amundsen Undercurrent, a near-bottom current that flows eastward along the shelf break and transports warm waters onto the continental shelf via troughs. Here we use a regional ice-ocean model to show that, on decadal time scales, the undercurrent's variability is baroclinic (depth-dependent). Decadal ocean surface cooling in the tropical Pacific results in cyclonic wind anomalies over the Amundsen Sea. These wind anomalies drive a westward perturbation of the shelf-break surface flow and an eastward anomaly (strengthening) of the undercurrent, leading to increased ice-shelf melting. This contrasts with shorter time scales, for which surface current and undercurrent covary, a barotropic (depth-independent) behavior previously assumed to apply at all time scales. This suggests that interior ocean processes mediate the decadal ice-shelf response in the Amundsen Sea to climate forcing.

Description: The ocean’s Meridional Overturning Circulation (MOC) plays a key role in the climate system, and thus monitoring its evolution is a scientific priority. However, monitoring arrays are limited to just a few latitudes in the Atlantic Ocean. Here we explore the possibility of inferring the MOC from globally-available satellite measurements via machine learning (ML) techniques, using the ECCOV4 state estimate as a test bed. The methodological advantages of the present approach include the use purely of available satellite data, its applicability to multiple basins within a single ML framework, and the ML model’s simplicity. The ML model exhibits high skill in reconstructing the overturning cells in the Southern Ocean, Indo-Pacific Ocean, and the Atlantic Ocean. In particular, the approach achieves a higher skill in predicting the Southern Ocean abyssal MOC and the AMOC at 26.5N than has previously been achieved via dynamically-based approaches. We quantify the skill of our ML-based MOC reconstructions as a function of latitude in each ocean basin, and as a function of the time scale of MOC variability. We further test which combinations of satellite-observable variables are optimal, and explore how spatial coarsening of the input variables influences the ML model skill. For example, we find via ML interpretability techniques that high reconstruction skill in the Southern Ocean is mainly due to bottom pressure variability at a few prominent bathymetric ridges. Finally, we discuss the potential for reconstructing MOC strength from real satellite measurements.

Description: Elevated ice shelf melt rates in the Amundsen Sea have been attributed to transport of warm Circumpolar Deep Water onto the continental shelf via bathymetric troughs. These inflows are supplied by an eastward, subsurface slope current that opposes the westward momentum input from local winds and tides, referred to as the Antarctic slope undercurrent. Previous studies have linked variations in the melt rates of the Amundsen Sea ice shelves to wind fluctuations. Yet the mechanism via which the undercurrent forms, and thus what controls the mean shoreward heat transport, remains unclear. In this study we investigate the dynamics of the undercurrent using a high-resolution ocean-sea ice process model coupled to a static ice shelf. We explore the sensitivities of the undercurrent strength, shoreward heat transport, and ice shelf melt rates to winds, tides, diapycnal mixing, and geometry. We find that the undercurrent forms with realistic strength provided that there is a trough allowing access to the continental shelf and ice shelf cavity, and that there is a cross-slope buoyancy gradient. The vorticity balance within the CDW layer reveals that the bathymetric steering of the undercurrent toward the ice shelf is related to diapycnal upwelling that occurs as CDW melts the ice. These findings imply that the mean flow of the Antarctic slope undercurrent is primarily established by buoyancy forcing on the continental shelf, and motivate a focus on processes that influence cross-shelf/slope buoyancy gradients to better understand future changes in shoreward heat transport.

Description: Amino acids were isolated from the muscle tissue of Potamocorbula amurensis, an invasive clam species, collected from two locations in the northern portion of the San Francisco Bay. Clam specimens were collected biannually in 1997, 2002, and from 2009-2017 at both locations. The carbon and nitrogen isotope values of individual amino acids were measured. Clam specimens were collected at USGS Sites 4.1 (Suisun Bay) and 8.1 (Carquinez Strait) in the San Francisco Bay and processed as described in Stewart et al. (2013; doi:10.3354/meps10503). Amino acids were hydrolyzed from P. amurensis, derivatized, and isolated following Vokhshoori et al. (2013; doi:10.3354/meps10746). Carbon and nitrogen isotope values of individual amino acids were measured following Vokhshoori et al., 2013 and Vokhshoori and McCarthy, 2013 (doi:10.1371/journal.pone.0098087), respectively. The purpose of this study was to assess long-term changes in the biogeochemistry of the San Francisco Bay estuary following the arrival of invasive P. amurensis. Sites were selected both due to species occurrence as well as significantly different salinity ranges. This design allowed for intraspecies and site-specific variations to be explored. Nitrogen isotopes of amino acids were used to isolate variations in nutrient baseline over the twenty-year period. Carbon isotopes of amino acid were utilized to understand long-term changes in dietary sources and/or changes in the baseline carbon isotope value of the estuary's food-web.

Description: Amino acids were isolated from the muscle tissue of Potamocorbula amurensis, an invasive clam species, collected from two locations in the northern portion of the San Francisco Bay. Clam specimens were collected biannually in 1997, 2002, and from 2009-2017 at both locations. The nitrogen isotope values of individual amino acids were measured. Clam specimens were collected at USGS Sites 4.1 (Suisun Bay) and 8.1 (Carquinez Strait) in the San Francisco Bay and processed as described in Stewart et al. (2013; doi:10.3354/meps10503). Amino acids were hydrolyzed from P. amurensis, derivatized, and isolated following Vokhshoori et al. (2013; doi:10.3354/meps10746). Nitrogen isotope values of individual amino acids were measured following Vokhshoori and McCarthy, 2013 (doi:10.1371/journal.pone.0098087). The purpose of this study was to isolate variations in nutrient baseline over the twenty-year period following the arrival of invasive P. amurensis.

Description: Amino acids were isolated from the muscle tissue of Potamocorbula amurensis, an invasive clam species, collected from two locations in the northern portion of the San Francisco Bay. Clam specimens were collected biannually in 1997, 2002, and from 2009-2017 at both locations. The carbon isotope values of individual amino acids were measured. Clam specimens were collected at USGS Sites 4.1 (Suisun Bay) and 8.1 (Carquinez Strait) in the San Francisco Bay and processed as described in Stewart et al. (2013; doi:10.3354/meps10503). Amino acids were hydrolyzed from P. amurensis, derivatized, isolated and the carbon isotope values were measured following Vokhshoori et al. (2013; doi:10.3354/meps10746). The purpose of this study was to assess long-term changes in dietary sources and/or changes in the baseline carbon isotope value of the estuary's food-web following the invasion of P. amurensis.

Description: Amino acids were isolated from the muscle tissue of Corbicula fluminea and Potamocorbula amurensis; two co-occurring invasive clams within the San Francisco Bay-Delta system. Clam specimens were collected near Montezuma Slough (Contra Costa County, California) at the confluence of the Sacramento and San Joaquin Rivers twice during the hydrological extremes of 2010 water year (November, 2009 and May, 2010). Carbon and nitrogen isotope values of individual amino acids were measured. Clam specimens were collected at USGS Sites 2.1 and processed following Stewart et al. (2013; doi:10.3354/meps10503). Amino acids were hydrolyzed from clam muscle tissue, derivatized, and isolated following Vokhshoori et al. (2013; doi:10.3354/meps10746). The measurement of the carbon and nitrogen isotope values of individual amino acids were conducted following Vokhshoori et al., 2013 and Vokhshoori and McCarthy, 2013 (doi:10.1371/journal.pone.0098087), respectively. The purpose of this study was to monitor biogeochemistry during 2010 water year and assess dietary difference between two co-occurring invasive species. This study focused on two species, C. fluminea and P. amurensis, benthic sessile primary consumers that can inhabit the same environment. USGS Site 2.1 was specifically selected as it exhibited the environmental conditions where both clam species co-existed during 2010 water year. This design allowed for interspecies variation to be explored. Nitrogen isotopes of amino acids were used to isolate variations in nutrient baseline from dietary changes across the season between the two species. Carbon isotopes of amino acid were utilized to understand the diet of the two species at two points in the season.