Atmospheric & Oceanic Science Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2747
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Item OCEAN HEAT CONTENT CALCULATION IMPROVEMENTS FOR EARTH’S ENERGY IMBALANCE QUANTIFICATION(2024) Boyer, Tim; Carton, James; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Earth’s Energy Imbalance, the difference between incoming and outgoing radiation at the top of the atmosphere, is stored in the atmosphere, land surface, cryosphere, and ocean, but is stored overwhelmingly (~90%) in the ocean on interannual and longer time scales. This imbalance, which is reflected in ocean heat uptake, is a primary indicator of the magnitude of change in energy the Earth’s system as well as an essential variable for understanding short-term variations and their effects on long-term regional and global climate change. The primary methods for calculating ocean heat content all depend on situ measurements of ocean subsurface temperature. The ocean subsurface temperature observing system as it is currently configured, with a substantial but not exclusive contribution from autonomous Argo profiling floats, is shown here to allow estimation of annual global ocean heat uptake with an uncertainty well below that possible with earlier ocean observing systems. It is also shown that maintenance and improvement of a global best quality ocean temperature profile database will lower uncertainty, both historically and for the current observing system and compensate to some extent for areas of sparse data in both direct calculation from observation and in data assimilation models. It is also shown that improvements to the methods used for mapping the inhomogeneous and anisotropic observations onto a regular grid spanning the global ocean will reduce uncertainty historically, currently, and into the future. On shorter monthly timescales regional changes in the Earth’s Energy Imbalance requires tracking the storage within the atmosphere, land, and cryosphere, and the heat transport within the ocean especially to depths where the energy is stored on longer time scales, in addition to ocean heat uptake. Monthly heat uptake estimates discussed here can be utilized with additional terms from atmosphere/land and ocean/sea ice reanalyses to provide Earth's Energy Imbalance estimates on these shorter time-scales in the future.Item RETRIEVALS OF ANTARCTIC SEA ICE PHYSICAL PROPERTIES FROM SATELLITE RADAR ALTIMETRY(2021) Fons, Steven William; Carton, James; Kurtz, Nathan; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Satellite observations have been used in sea ice research throughout the last 40+ years and have brought to light substantial changes in the global sea ice coverage. More recently, satellite altimetry has become a valuable tool to estimate the thickness of sea ice - a parameter that plays an important role in the Earth System by moderating heat and moisture fluxes between the polar ocean and atmosphere. While radar altimetry has been effective in providing estimates of Arctic sea ice thickness, the complex snow stratigraphy and uncertain snow depth on Antarctic sea ice have precluded sea ice thickness retrievals in the Southern Ocean, leading to a decade-long gap in the thickness record spanning the lifetime of ESA’s CryoSat-2 satellite. This dissertation will address the need for Antarctic sea ice thickness estimates from CryoSat-2 through the development and assessment of new retrievals of sea ice physical properties that enable the estimation of sea ice thickness.The first part of this dissertation is aimed at developing a CryoSat-2 retrieval algorithm that is less dependent on uncertain returns from the snow-ice interface of Antarctic sea ice. This method exploits observed scattering of Ku-band radar pulses from the snow surface and snow volume atop sea ice and uses a physical waveform model and optimization approach to retrieve the air-snow interface elevation and snow freeboard. Building off the initial development, the second part of this work offers improvements to – and assessments of – the retrieval process though comparisons with coincident snow freeboard measurements from NASA’s ICESat-2 laser altimeter. The final part of this dissertation uses the retrieval process to estimate snow depth and ice freeboard, enabling first estimates of Antarctic sea ice thickness that span the CryoSat-2 mission. Potential applications for use of this method over Arctic sea ice are also explored. The studies within this dissertation represent new possibilities for CryoSat-2 data and lay a foundation for the development of a combined laser-radar altimetric record of Antarctic sea ice thickness.Item EVALUATING OCEANOGRAPHIC HYPOTHESES: THREE METHODS FOR TESTING IDEAS(2020) Johnson, Benjamin K; Kalnay, Eugenia E; Wenegrat, Jacob O; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The disciplines of meteorology and oceanography are both vital to understanding the earth system. Throughout most of the last half century, meteorology has largely been a prognostic discipline. Forecasts made by meteorologists have been widely used and scrutinized, allowing for countless opportunities to test and improve ideas about atmospheric circulation and physics. Since weather forecasts involve integrating numerical models and updating the model state via data assimilation, forecasting demands frequent use of the principles of Bayesian inference. This requirement essentially confronts the physics contained within numerical models at recurring intervals and can reveal systematic model bias. In contrast, prognostic applications have been less prevalent in oceanography. Oceanographic forecasts are much rarer than atmospheric forecasts and, perhaps as a consequence of this disparity, many ideas concerning oceanic circulation have not been tested to the same degree as ideas concerning atmospheric circulation. This dissertation presents three methods for testing oceanographic ideas: applying common methodologies to analogous regions of different ocean basins; creating synthetic time series to mimic the properties of oceanographic time series in order to construct null distributions for hypothesis testing; and using water mass census information to interpret the results of water mass transformation analysis.Item Strongly Coupled Ocean-Atmosphere Data Assimilation with the Local Ensemble Transform Kalman Filter(2018) Sluka, Travis Cole; Kalnay, Eugenia; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Current state-of-the-art coupled data assimilation systems handle the ocean and atmosphere separately when generating an analysis, even though ocean atmosphere models are subsequently run as a coupled system for forecasting. Previous research using simple 1-dimensional coupled models has shown that strongly coupled data assimilation (SCDA), whereby a coupled system is treated as a single entity when creating the analysis, reduces errors for both domains when using an ensemble Kalman filter. A prototype method for SCDA is developed with the local ensemble transform Kalman filter (LETKF). This system is able to use the cross-domain background error covariance from the coupled model ensemble to enable assimilation of atmospheric observations directly into the ocean. This system is tested first with the intermediate complexity SPEEDYNEMO model in an observing system simulation experiment (OSSE), and then with real observations and an operational coupled model, the Climate Forecasting System v2 (CFSv2). Finally, the development of a major upgrade to ocean data assimilation used at NCEP (the Hybrid-GODAS) is presented, and shown how this new system could help present a path forward to operational strongly coupled DA.Item The role of the Indian Ocean sector and sea surface salinity for prediction of the coupled Indo-Pacific system(2016) Hackert, Eric C.; Busalacchi, Antonio J.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this dissertation is to evaluate the potential downstream influence of the Indian Ocean (IO) on El Niño/Southern Oscillation (ENSO) forecasts through the oceanic pathway of the Indonesian Throughflow (ITF), atmospheric teleconnections between the IO and Pacific, and assimilation of IO observations. Also the impact of sea surface salinity (SSS) in the Indo-Pacific region is assessed to try to address known problems with operational coupled model precipitation forecasts. The ITF normally drains warm fresh water from the Pacific reducing the mixed layer depths (MLD). A shallower MLD amplifies large-scale oceanic Kelvin/Rossby waves thus giving ~10% larger response and more realistic ENSO sea surface temperature (SST) variability compared to observed when the ITF is open. In order to isolate the impact of the IO sector atmospheric teleconnections to ENSO, experiments are contrasted that selectively couple/decouple the interannual forcing in the IO. The interannual variability of IO SST forcing is responsible for 3 month lagged widespread downwelling in the Pacific, assisted by off-equatorial curl, leading to warmer NINO3 SST anomaly and improved ENSO validation (significant from 3-9 months). Isolating the impact of observations in the IO sector using regional assimilation identifies large-scale warming in the IO that acts to intensify the easterlies of the Walker circulation and increases pervasive upwelling across the Pacific, cooling the eastern Pacific, and improving ENSO validation (r ~ 0.05, RMS~0.08C). Lastly, the positive impact of more accurate fresh water forcing is demonstrated to address inadequate precipitation forecasts in operational coupled models. Aquarius SSS assimilation improves the mixed layer density and enhances mixing, setting off upwelling that eventually cools the eastern Pacific after 6 months, counteracting the pervasive warming of most coupled models and significantly improving ENSO validation from 5-11 months. In summary, the ITF oceanic pathway, the atmospheric teleconnection, the impact of observations in the IO, and improved Indo-Pacific SSS are all responsible for ENSO forecast improvements, and so each aspect of this study contributes to a better overall understanding of ENSO. Therefore, the upstream influence of the IO should be thought of as integral to the functioning of ENSO phenomenon.Item Climate-scale chlorophyll patterns in the tropical Pacific from a 51 year statistical reconstruction(2015) Uz, Stephanie Schollaert; Busalacchi, Antonio J; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding large, slow biological changes in the oceans has been hindered by a lack of spatial coverage by direct measurements and a lack of temporal coverage by satellite remote-sensing observations. Global ocean surface chlorophyll, a proxy for phytoplankton standing stock, has been derived from satellites for over a decade. With these measurements, the strong connection between ocean physics and biology has become clear and provided new insights about what drives seasonal and interannual biological processes. At longer time scales, however, there are many unanswered questions about the variability of phytoplankton in the ocean that plays a critical role in the carbon cycle as well as the marine food web. Statistical reconstructions have been used by others to extend physical climate variables in space and time. Taking advantage of the fact that physical forcing has been found to be the primary driver of biological primary production in the tropical Pacific, especially during El Niño, the most closely correlated physical variables are used as predictors in a statistical reconstruction to extend monthly chlorophyll anomalies from just over a decade to just over five decades between 1958-2008. The reconstructed chlorophyll is evaluated through leave-one-out-cross-validation, compared to several independent data sets: in situ samples, another ocean color satellite data set, model output from a dynamic, fully-coupled ocean circulation-biogeochemistry model. Highest skill in the tropical Pacific reconstruction is away from the coast and within 10o of the equator, including areas known as Niño 3/3.4/4. Over the half-century of chlorophyll anomalies, the most dominant climate pattern apparent in the reconstruction is associated with the interannual El Niño followed by the Pacific Decadal Oscillation. Biological distinctions emerged between the east Pacific El Niño events and those that only extend to the central Pacific. Chlorophyll anomalies were compared between regimes to ascribe physical forcing mechanisms. While the overall patterns were consistent with what is known about the impact of ENSO on biology, with the PDO primarily serving to amplify or damp ENSO, a narrow equatorial band consistently displayed an inverse response to the rest of the equatorial cold tongue: lower values during the PDO cool phase between 1958-1976, higher values during the PDO warm phase between 1977-1995. A likely explanation for this anomaly is linked to variability in the depth of the Equatorial Undercurrent that transports iron to the high-nutrient, low-chlorophyll east Pacific. These and other ideas are explored to demonstrate the feasibility and utility of reconstructing ocean color chlorophyll to address open questions about large-scale, low frequency primary production that forms the base of the marine food web and plays an important role in Earth’s climate system.Item OCEAN VARIABILITY IN CMIP5 (COUPLED MODEL INTERCOMPARISON PROJECT PHASE 5) HISTORICAL SIMULATIONS(2014) Ding, Yanni; Carton, James A.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The oceans play a key role in the global climate variability. This dissertation examines climate variability in historical simulations from fourteen CMIP5 (Coupled Model Intercomparison Project Phase 5) coupled models on different time scales. Responses of oceans to the external volcanic eruption, green house gas forcing, and internally generated variability are investigated with emphasis on higher latitudes. Chapter 2 addresses the oceanic response to tropical volcanic eruptions. Previous modeling studies have provided conflicting high latitude climate responses to volcanic eruptions, including the ocean's role. This controversy happens mainly because the response varies widely from model to model, and even varies among ensemble members of a single model. The increase in Atlantic Meridional Overturning Circulation (AMOC) after the volcanic eruption is closely linked with its internal variability. Chapter 3 addresses the seasonal and centennial trends in the Arctic Ocean. The Arctic warming is apparent in all models, although there is considerable variability especially its seasonal cycle. Both the surface heat flux and the oceanic heat convergence contribute to the Arctic warming on centennial time scale. Meanwhile, the seasonal variation of oceanic warming is largely determined by the atmospheric heating. In models presenting a clear seasonal cycle of surface net flux increases, there is a notable retreat of sea ice extent in winter, which allows more heat loss from the ocean through turbulent fluxes. Chapter 4 discusses the internally generated variability of high latitude water masses. Both the magnitude and the time scale of subarctic decadal variability are strikingly similar to observations. The analysis of the more realistic models provides constraints on relative roles of the oceanic heat transport and the atmospheric heat flux. One possible factor that could give rise to the different origins of ocean variability is the blocking of mid-latitude jet stream. The oceanic heat transport is more important to the decadal variability of the high latitude ocean in models where winter-time atmospheric blocking events over the Euro-Atlantic sector are more frequent.