1,721,063 research outputs found
Primary Productivity in the Gulf of Alaska in a High-Resolution Global Climate Model
No full textThe Gulf of Alaska is a major fishery that plays a vital role in the economic
development and success of Alaska. The fishery’s success is dependent on several factors,
one being primary production. Primary producers, such as phytoplankton, form the base of
the oceanic food web and supply energy and materials to secondary consumers. The Gulf of
Alaska is a high nitrate low chlorophyll (HNLC) region often limited by iron. Phytoplankton
require iron to synthesize chlorophyll and reduce other nutrients. Sources of iron include
suspended sediments transported via rivers, dust deposition from drier lands, and sediments
released by melting glaciers transported from the coast to the interior of the gyre via Haida
Eddies. This study focuses on identifying the factors that control the seasonal succession of
phytoplankton at Ocean Station Papa in the Gulf of Alaska. Using a high-resolution global
climate model, the interannual variability of surface nutrients, mixed layer depth,
phytoplankton biomass, zooplankton biomass, and irradiance are examined in order to draw
conclusions regarding the cause of two distinct chlorophyll regimes that occur over a 15 year
period. Irradiance turns out to be the main culprit behind the diverging chlorophyll regimes.
Lastly, a preliminary investigation is conducted on the role that Haida Eddies play in iron
transport and primary production in the Gulf of Alaska. Elevated iron concentrations within
the eddy cause phytoplankton biomass to reach a maximum a month earlier than the
surrounding waters
Mesoscale Processes' Effects on Spatial and Temporal Variability in the Ocean's Biological Carbon Pump
No full textThe ocean is a significant carbon sink through both physical and biological processes. The biological carbon pump (BCP) injects organic carbon into the interior ocean via sinking organic matter, mixing, and subduction. Small scale processes in the ocean like eddies (scales < 100km) affect the BCP by altering the sinking, mixing, and subduction. Eddies introduce strong spatial variability in the export flux, making it difficult to estimate the global export of carbon by the BCP from in-situ sampling. The NASA EXPORTS project aims to better understand the BCP and its variability due to eddies. As part of this project we use an eddy-resolving ocean model to examine how the undersampling of spatial variability will impact the estimates of export and design a sampling strategy for the upcoming cruise part of the EXPORTS project. We confirm previous studies’ results that high-resolution sampling is necessary to capture the spatial variability in export. The EXPORTS cruise will benefit from having a second ship that is able to take high spatial-resolution export measurements throughout the sampling region. Although difficult to measure, subduction and mixing prove to have a significant impact on export and the associated sampling bias
Physical Controls of Coastal Hypoxia in the Indian Ocean Dipole
No full textThe Indian Ocean Dipole (IOD) is an understudied physical phenomenon that involves a currently unpredictable reversal of temperature gradient across the Indian Ocean. In addition to increasing the risk of natural disasters such as tropical cyclones, the IOD influences the frequency and severity of coastal hypoxia events. Hypoxia refers to depleted oxygen levels operationally defined as an oxygen concentration below 60μmol/kg (Breitburg et al., 2018). Recurring and widespread coastal hypoxic events have been directly implicating the productivity of fisheries that sustain more than 2.49 billion livelihoods in the Indian Ocean littoral (NIC, 2013).
The overall mechanism of IOD propagation is well-established. However, existing literature lacks sufficient study of specific impacts of the IOD on different regions within the Indian Ocean basin. In our research, we analyzed the differing nature and extent of the influence of positive and negative IOD phases on a variety of locations across the Indian Ocean. We found that positive IOD phases increase the risk of coastal hypoxia in the Bay of Bengal while decreasing it in the Arabian Sea. The converse is true for a negative IOD year. Combining a theoretical understanding of the IOD, data from the biogeochemical MOM6 ocean model, and the Dipole Mode Index (DMI), an oxygen budget analysis formed the crux of our research, enabling the study of physical and biological mechanisms that influence coastal hypoxia in the Indian Ocean
Datasets for 'Hydrological cycle amplification reshapes warming-driven oxygen loss in Atlantic Ocean'
No full textPhysical and biogeochemical variables from the NOAA-GFDL Earth System Model 2M experiments, and previously published observation-based datasets, used for the study 'Hydrological cycle amplification reshapes warming-driven oxygen loss in Atlantic Ocean'.National Science Foundation Career award 2042672; NSF Grant No. DGE-2039656; NASA Award 80NSSC20K0879;See readm
Data and code for 'Hydrological cycle amplification reshapes warming-driven oxygen loss in Atlantic Ocean'
No full textPhysical and biogeochemical variables from the NOAA-GFDL Earth System Model 2M experiments (pre-processed), previously published observation-based datasets, and code to reproduce figures from these datasets, used for the study 'Hydrological cycle amplification reshapes warming-driven oxygen loss in Atlantic Ocean'.National Science Foundation Career award 2042672; NSF Grant No. DGE-2039656; NASA Award 80NSSC20K0879hogikyanetal_sss_oxygen.py, oxygen_data.zip, README-hogikyan_oxygen.tx
Observation and model-based analyses of ocean biological carbon fluxes and ecosystem dynamics
No full textThis dissertation examines the ocean biological carbon pump, a complex coupling of biophysical processes that exports carbon from the surface to the deep (5-15 Pg C yr-1), by focusing on the largest component, the gravitational sinking flux (4-9 Pg C yr-1). In the first chapter, I examine the impact of interannual variability, specifically marine heat waves, on ecosystem and export production in the Northeast Pacific using a suite of observational data and an ocean biogeochemical model. The model shows that warming-induced strati- fication during marine heat waves relieves winter light limitation while decreasing nutrient supply increasing small phytoplankton production at the expense of large phytoplankton, in agreement with observations. This shift in the phytoplankton assemblage is propagated through the food web, leading to a smaller zooplankton assemblage, and weaker export. I also show that previous observation-based estimates of ecosystem production misattributed a spatial gradient in nutrients to a rapid decline in productivity during a recent marine heat wave. These results highlight the difficulty in disentangling spatial and temporal variability when interpreting sparse observations.
In Chapters two and three, I examine the export proxy, thorium-234, used to estimate particle fluxes in situ. I detail the collection of over 1500 individual water samples collected in the Northeast Pacific and the North Atlantic during the 2018 and 2022 NASA EX- PORTS field campaigns. These data were analyzed to produce dozens of particle sinking flux profiles using 1-dimensional (1D) thorium budgets that rely on the assumption that the influence of physical dynamics is negligible relative to the flux of thorium adsorbed on sinking particles. Using observed and simulated velocities, I find that physical processes could bias observed flux estimates by 30% in both regions (Chapter 2). In Chapter 3, using an idealized double gyre model to examine physical thorium transport in fine-scale struc- tures I show that coherent mesoscale eddies (lasting > 20 days) are well-suited for thorium sampling due to the decreased likelihood of strong physical transport. In contrast, frontal regions exhibit large vertical velocities that decouple the sinking flux and 1D thorium esti- mate, suggesting that thorium-based particle sinking fluxes across frontal regions should be interpreted cautiously
COMPARISON OF GRACE DERIVED GREENLAND MASS WASTING TO NEAR SURFACE TEMPERATURE FROM MERRA-2 REANALYSIS
No full textMelting ice from the Greenland Ice Sheet has accounted for an increasing percentage— now estimated at 25%—of rising global mean sea-level since the early 1990s. As recently as 2016, gravimetric and altimetric studies of Greenland melting rates found increasing rates of ice loss, which have not been borne out in GRACE gravimetric observations over the last few years (2015–2017). I investigate the correlations of atmospheric variables from MERRA-2 cli- mate model reanalysis to show the ways in which temperature over the Greenland Ice Sheet has changed over the MERRA-2 (1980–) and GRACE (2003–2017) records. Our results not only confirm that temporal and spatial changes in GRACE derived mass loss are coincident with changes in near surface temperature, but demonstrate some of the limitations in GRACE spatial resolution, and contextualize recent variability in ice loss within the variability and long term trend of Greenland temperature. As Greenland Ice Sheet melting continues to be more unpre- dictable than early GRACE studies may have foreseen, context is extremely important in both interpreting and communicating trends in ice loss
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Oxygen, Carbon, Heat: Explorations in Atmosphere-Ocean Interaction
No full textThis thesis describes three novel mechanisms of air-sea interaction. The first two chapters focus on the amplification of the hydrological (water) cycle as the climate system warms in response to a CO2 increase, which is realized as an amplification of freshwater flux (precipitation - evaporation) patterns. We find that this freshwater flux pattern leads to a redistribution of oxygen and carbon in the ocean, which modifies the previously recognized changes due to the atmospheric CO2 increase, warming, and circulation changes. The change in oxygen concentrations results from the change in sea surface salinity patterns which modifies the ocean circulation and heat uptake. The change in carbon concentrations results from the dilution or concentration of carbonate species (in parallel to salinity), and the same changes in heat uptake. The redistributions of oxygen and carbonate species in response to hydrological cycle amplification are comparable to the effect of global warming, which decreases both oxygen and carbon concentrations throughout the ocean.
The third chapter provides a causal mechanism, for the first time, to link the change in sea surface temperature that develops during El Niño events to the change in temperature of the tropical free troposphere. The temperature of the free troposphere is primarily determined by the temperature profile followed by moist convection over the ocean (rising air above high sea surface temperatures). We isolate the part of the sea surface associated with convection (which is determined on thermodynamic rather than geographic grounds) and demonstrate that the El Niño surface temperature increase is driven by a decrease in surface wind speed which damps the evaporation rate. This result suggests a possible relationship between the zonal symmetry of the tropical atmospheric circulation, temperature of the free troposphere, and top-of-atmosphere energy budget
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