Tesi sul tema "Physical oceanography"

Coral reefs are greatly impacted by the physical characteristics of the water surrounding them. Incidence and severity of mass coral bleaching and mortality events are increasing worldwide due primarily to increased water temperature, but also in response to other stressors. This decline in reef health demands clearer understanding of the compounding effects of multiple stressors, as well as widespread assessment of coral reef health in near-real time.

Satellites offer a means by which some of the physical stressors on coral reefs can be measured. The synoptic spatial coverage and high repeat sampling frequency of such instruments allow for a quantity of data unattainable by in situ measurements. Unfortunately, errors in cloudmasking algorithms contaminate satellite derived sea surface temperature (SST) measurements, especially during anomalously cold events. Similarly, benthic interference of satellite-derived reflectance signals has resulted in large errors in derivations of water quality or clarity in coral reef environments.

This work provides solutions to these issues for the coral reef environments of the Florida Keys. Specifically, improved SST cloudmasking algorithms were developed for both Advanced Very High Resolution Radiometer (AVHRR; Appendix A) and Moderate Resolution Imaging Spectroradiometer (MODIS) data (Appendix B). Both of these improved algorithms were used to reveal the extent and severity of a January 2010 cold event that resulted in widespread mortality of Florida Keys corals. Applied to SST data from 2010, the improved MODIS cloudmasking algorithm also showed improved quantity of SST retrievals with minimal sacrifice in data quality.

Two separate algorithms to derive water clarity from MODIS measurements of optically shallow waters were developed and validated, one focusing on the diffuse downwelling attenuation coefficient (K_d, m^-1 ) in visible bands (Appendix C), the other on K_d in the ultraviolet (Appendix D). The former utilized a semi-analytical approach to remove bottom influence, modified from an existing algorithm. The latter relied on empirical relationships between an extensive in situ training dataset and variations in MODIS-derived spectral shape, determined using a stepwise principal components regression. Both of these algorithms showed satisfactory validation statistics, and were used to elucidate spatiotemporal patterns of water clarity in the Florida Keys. Finally, an approach was developed to use Landsat data to detect concurrent MODIS-derived reflectance anomalies with over 90% accuracy (Appendix E). Application of this approach to historical Landsat data allowed for long-term, synoptic assessment of the water environment of the Florida Keys ecosystem. Using this approach, shifts in seagrass density, turbidity increases, black water events, and phytoplankton blooms were detected using Landsat data and corroborated with known environmental events.

Many of these satellite data products were combined with in situ reports of coral bleaching to determine the specific environmental parameters individually and synergistically contributing to coral bleaching. As such, SST and visible light penetration were found to be parsimoniously explaining variance in bleaching intensity, as were the interactions between SST, wind and UV penetration. These relationships were subsequently used to create a predictive model for coral bleaching via canonical analysis of principal coordinates. Leave-one-out-cross-validation indicated that this model predicted `severe bleaching' and `no bleaching' conditions with 64% and 60% classification success, respectively, nearly 3 times greater than that predicted by chance. This model also showed improvement over similar models created using only temperature data, further indicating that satellite assessment of coral bleaching based only on SST data can be improved with other environmental data. Future work should further supplement the environmental parameters considered in this research with databases of other coral stressors, as well as improved quantification of the temperature at the depth of corals, in order to gain a more complete understanding of coral bleaching in response to environmental stress.

Overall, this dissertation presents five new algorithms to the field of satellite oceanography research. Although validated primarily in the Florida Keys region, most of these algorithms should be directly applicable for use in other coastal environments. Identification of the specific environmental factors contributing to coral bleaching enhances understanding of the interplay between multiple causes of reef decline, while the predictive model for coral bleaching may provide researchers and managers with widespread, near real-time assessments of coral reef health.

Ocean general circulation models of the IPCC class have biases even when simulating present-day conditions, which may bring into question their predictions of future conditions. This dissertation is about tools for, and results from assessing biases in the Community Earth System Model (CESM) ocean component, by itself and when combined with the Biological Ecosystem Cycling (BEC) model. Newly developed tools and their applications are listed. 1. An offline matrix tracer transport model for the ocean component of CESM. 2. A fast Newton-Krylov implicit tracer equilibrium solver for both the annually-averaged and the seasonally-varying circulation. 3. An effective preconditioner for the solver simulating radiocarbon. Application results: For a natural radiocarbon simulation, an equilibrium solution was obtained in 23 model-years, a dramatic decrease from the 4000 model-years reported for time-stepping. The modeled circulation in the deep Pacific Ocean produced radiocarbon ages twice those of observations. 4. A capability for computing the surface origin of water mass fractions as well as the age of the various water masses.

Application results: The North Atlantic was the major supplier of ventilated water to not only the Atlantic, but also the Pacific and Indian Oceans. A lack of formation of bottom water in the Southern Ocean was discovered. 5. A capability for restricting the tracer simulation domain to a limited region of the ocean while retaining the effectiveness of advection and diffusion fields on the boundary. This reduces computational costs and allows separating local versus remote impacts of tracer sources on the biogeochemical tracer concentrations. This capability has the potential to provide a platform for further biogeochemical studies.

Application results: The Indian Ocean region was isolated. Global versus regional circulation effects were determined using radiocarbon. Most of the bias within the region was eliminated by using observational, rather than globally calculated values, on the boundaries. Oxygen production and consumption from a CMIP5 BEC simulation were used to drive a regional oxygen model. Boundary values of oxygen from the CMIP5 BEC simulation were replaced with observations, resulting in less bias within the region. However, significant bias in the location of the Arabian Sea oxygen minimum zone remained.

The circulation of the Southern Ocean is unique due to the lack of meridional boundaries at the latitudes of Drake Passage. Westerly winds drive the Antarctic Circumpolar Current (ACC), linking the major ocean basins and facilitating inter-basin exchange of properties. Additionally, the steeply tilted isopycnals in the Southern Ocean allow interaction between the deep ocean and the atmosphere, and as a result the Southern Ocean has an outsized contribution to the global uptake and redistribution of heat, carbon and nutrients. Complex topography and eddies make this circulation fundamentally three-dimensional, but many features and associated mechanisms of this three-dimensional circulation are not well understood.

The objective of this thesis is to use the 1/6°, data-assimilating Southern Ocean State Estimate (SOSE), along with other high-resolution ocean models and available observations, to describe aspects of the three-dimensional structure of the upper cell of the Southern Ocean overturning circulation. First, we diagnose the upper ocean heat budget in the Southern Ocean (Chapter 2), and determine that a strong zonal asymmetry in the air-sea heat flux over the Southern Ocean is associated with large-scale meander the ACC mean path and associated asymmetry in geostrophic heat advection. Second, we use Lagrangian particle release experiments to show, for the first time, the full three-dimensional upwelling pathways of deep water from 30°S to the surface of the Southern Ocean (Chapter 3). We find that deep water moves south in narrow paths along the western and eastern boundaries of each ocean basin, then within the ACC upwelling is concentrated at hotspots associated with high eddy activity at major topographic features. Next, we quantify the water mass transformation along the upwelling pathways from Chapter 3, and find that although the upwelling in the ocean interior is largely along isopycnals, there is significant transformation just below the mixed layer and homogenization of deep water mass properties due to isopycnal mixing (Chapter 4). Finally, we highlight a newly identified poleward pathway of deep water along the eastern boundary of the Indian Ocean and describe the structure and variability of this pathway (Chapter 5).

Western Long Island Sound (WLIS) bottom waters experience low dissolved oxygen (DO) levels in the summertime. The seasonal hypoxia in the WLIS motivated the development of a coupled biogeochemistry/hydrodynamic model named the System Wide Eutrophication Model (SWEM). A critical assessment of the SWEM model, however, reveals that vertical transport rates are grossly underestimated, indicating a need for data-based estimates of these rates.

I present a novel approach to estimate vertical mixing coefficients from time-series of measurements made at two or more depths by measuring the attenuation and phase lag of scalar signals generated periodically at the surface as they propagate downwards and estimate mean summertime depth-averaged downward fluxes of DO and heat of 14±4 μM day^–1 and 17±10 Wm^–3, respectively. In order to assess the importance of horizontal transport in the bottom waters, I present an analysis of time-series of moored temperature, DO, and current observations in the hypoxic area of Long Island Sound and estimate mean near-bottom along-channel flux differences of DO and heat as 4±6 μM day^–1 and –5±6 Wm^–3, respectively. I conclude that vertical transport forms the bulk of the physical supply of both DO and heat to the hypoxic zone.

When WLIS moored instrument records are examined, it is evident that near-bottom increases in DO and heat and a decrease in salt occur during the middle of the flood tide; an analysis of water mass signatures indicates that the transport involved is vertical and not horizontal. Temperature data from a thermistor string deployed in the WLIS for 16 days in August 2009 clearly shows internal waves and a pycnocline depression of approximately 25% of the water depth occurring at mid-flood. Near-bottom internal wave energy is correlated with near-bottom DO and temperature changes at both supertidal and subtidal scales, and I conclude that internal mechanisms are potentially important to vertical transport in the WLIS region.

In the mid-1970s, an enormous open-ocean polynya developed in the Weddell Sea. Since the Weddell Polynya's occurrence, no polynya of similar size or duration has been observed in the region. A polynya of this magnitude could significantly affect global abyssal ocean properties via increased Antarctic Bottom Water (AABW) formation and large Weddell Sea water mass property perturbations. However, the scarcity of 1970s Weddell Sea observations, coupled with the sparseness of abyssal ocean observations, make it difficult to study this phenomenon's oceanic impact without models. This dissertation examines the influence of Weddell Polynyas on abyssal ocean water mass properties and circulation using the GFDL CM2G coupled climate model.

Abyssal ocean temperature, salinity, and water mass changes resulting from Weddell Polynyas are quantified in CM2G and compared to observations. The model polynyas initially cool the abyssal Southern Ocean and South Atlantic, but 2-3 decades after polynya cessation the same regions warm as they relax toward their mean state. Composites of multiple, spontaneously-occurring polynyas in CM2G reveal that up to 10% of recently observed warming in the abyssal Southern Ocean could be the result of the 1970s Weddell Polynya recovery.

Weddell Polynya transport mechanisms are also investigated. Polynya signal transport is governed by two processes acting on different timescales and spreading at different rates: 1) topographic and planetary waves that act on interannual-to-decadal timescales, and 2) advection that acts on decadal-to-centennial timescales. Both mechanisms generate property changes on isobaths. Despite different spreading rates, the advective and wave signals act contemporaneously in many Southern Hemisphere abyssal basins. The combined effect and relative magnitude of the two signals dictates the prevailing property changes.

During Weddell Polynyas, vigorous exchange occurs between the surface and deep waters, resulting in increased abyssal ventilation. In climate models, ideal age tracer is often used to investigate oceanic ventilation. This tracer suffers from several flaws that detract from its suitability as a ventilation diagnostic. We develop a new tracer, ?-age, that rectifies some of ideal age's problematic aspects and examine its utility in an offline tracer model.