ORCID Profile
0000-0001-7722-7252
Current Organisation
University of California, San Diego
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Publisher: American Meteorological Society
Date: 07-2019
Abstract: Using 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and litude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.
Publisher: Research Square Platform LLC
Date: 06-07-2022
DOI: 10.21203/RS.3.RS-1515141/V1
Abstract: During the southwest monsoon, seasonal storms bring torrential rainfall to the South Asian subcontinent and the northern Indian Ocean. Dense cloud cover limits the amount of sunlight that reaches the ocean surface, and sediment-laden river runoff limits the depths to which light can penetrate. Changing light availability should affect phytoplankton primary productivity and its dependent biogeochemical processes. Yet little is known about how subtropical weather is linked to ecosystem processes below the ocean’s surface. Here, using novel physical and bio-optical measurements from an array of free-drifting, autonomous systems, we show that the onset of cloudy, rainy conditions associated with stormy periods led to % reduction in subsurface gross primary productivity (GPP) relative to clear, sunny break periods. Simultaneous bioacoustic measurements collected onboard the autonomous platforms suggested that this intraseasonal variability in GPP generated a response in higher trophic levels. Long-term measurements from biogeochemical (BGC) Argo floats indicated that the magnitude of the days-to-weeks variability reported here was similar to that of the regional annual cycle in the region. Our findings demonstrate that intraseasonal subtropical air-sea variability modulates important regional biogeochemical ocean processes in the Northern Indian Ocean, and may help improve our ability to forecast the changing global climate system.
Publisher: Frontiers Media SA
Date: 08-08-2019
Publisher: Wiley
Date: 24-02-2022
DOI: 10.1002/LNO.12043
Abstract: The ability to forecast the biological productivity of the coastal ocean relies on the quantification of the physical processes that deliver nutrients to the euphotic zone. Here we explore these pathways using observations of the coupled biological and physical variability of waters offshore of the east coast of Tasmania in the summertime. The observations include an array of moored autonomous profilers deployed over an 18‐d period—providing continuous, full‐depth measurements of turbulent microstructure, temperature, velocity, and chlorophyll a (Chl a ) fluorescence, complemented by shipboard nutrient measurements. Local upwelling was driven by the encroaching East Australian Current (EAC) extension onto the shelf and to a lesser extent the local winds. The interaction of the local winds and the encroaching boundary current was reflected in the shelf nutrient budget and led to a rapid increase in subsurface Chl a . Diffusive vertical fluxes had minimal impact on subsurface Chl a in the mid‐shelf and outer‐shelf. Upwelling‐favorable winds were too weak to drive significant vertical mixing, and mixing associated with the current‐driven Ekman transport was too deep compared to the euphotic zone depth. The observed subsurface Chl a did not reflect the satellite estimates of productivity. Since the EAC extension transports warm, low‐nutrient surface waters from the subtropics, satellite chlorophyll measurements decreased during the same period the depth‐averaged Chl a increased. This seeming paradox illustrated how long duration, full water column s ling can elucidate the coupled biological and physical processes that aid our ongoing effort to forecast the biological state of the coastal ocean.
Publisher: American Geophysical Union (AGU)
Date: 08-2017
DOI: 10.1002/2016JC012583
Publisher: Proceedings of the National Academy of Sciences
Date: 28-08-2023
Abstract: Harmful algal blooms (HABs) are increasing globally, causing economic, human health, and ecosystem harm. In spite of the frequent occurrence of HABs, the mechanisms responsible for their exceptionally high biomass remain imperfectly understood. A 50-y-old hypothesis posits that some dense blooms derive from dinoflagellate motility: organisms swim upward during the day to photosynthesize and downward at night to access deep nutrients. This allows dinoflagellates to outgrow their nonmotile competitors. We tested this hypothesis with in situ data from an autonomous, ocean-wave-powered vertical profiling system. We showed that the dinoflagellate Lingulodinium polyedra ’s vertical migration led to depletion of deep nitrate during a 2020 red tide HAB event. Downward migration began at dusk, with the maximum migration depth determined by local nitrate concentrations. Losses of nitrate at depth were balanced by proportional increases in phytoplankton chlorophyll concentrations and suspended particle load, conclusively linking vertical migration to the access and assimilation of deep nitrate in the ocean environment. Vertical migration during the red tide created anomalous biogeochemical conditions compared to 70 y of climatological data, demonstrating the capacity of these events to temporarily reshape the coastal ocean’s ecosystem and biogeochemistry. Advances in the understanding of the physiological, behavioral, and metabolic dynamics of HAB-forming organisms from cutting-edge observational techniques will improve our ability to forecast HABs and mitigate their consequences in the future.
Publisher: American Meteorological Society
Date: 10-2019
Abstract: Near-inertial waves (NIWs) are often an energetic component of the internal wave field on windy continental shelves. The effect of baroclinic geostrophic currents, which introduce both relative vorticity and baroclinicity, on NIWs is not well understood. Relative vorticity affects the resonant frequency f eff , while both relative vorticity and baroclinicity modify the minimum wave frequency of freely propagating waves ω min . On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where f eff was less than the Coriolis frequency f . If everywhere f eff f then NIWs were generated where ω min f and f eff was smallest. The background current not only affected the location of generation, but also the NIWs’ propagation direction. The estimated NIW energy fluxes show that NIWs propagated predominantly toward the equator because ω min f on the continental slope for the entire s le period. In addition to being laterally trapped on the shelf, we observed vertically trapped and intensified NIWs that had a frequency ω within the anomalously low-frequency band (i.e., ω min ω f eff ), which only exists if the baroclinicity is nonzero. We observed two periods when ω min f on the shelf, but the relative vorticity was positive (i.e., f eff f ) for one of these periods. The process of NIW propagation remained consistent with the local ω min , and not f eff , emphasizing the importance of baroclinicity on the NIW dynamics. We conclude that windy shelves with baroclinic background currents are likely to have energetic NIWs, but the current and seabed will adjust the spatial distribution and energetics of these NIWs.
Location: United States of America
Location: United States of America
Location: United States of America
Location: United States of America
No related grants have been discovered for Andrew Lucas.