Atmospheric & Oceanic Science

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Formerly known as the Department of Meteorology.

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End date: 2009Subject: search.filters.subject.Physical Oceanography

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    MADDEN-JULIAN OSCILLATION AND SEA SURFACE TEMPERATURE INTERACTIONS IN A MULTI-SCALE FRAMEWORK
    (2009) Zhou, Lei; Murtugudde, Raghu; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The ocean-atmosphere coupling can play a role in initiating and sustaining the Madden-Julian Oscillations (MJOs), which are the major intraseasonal oscillations in the atmosphere. In this thesis, the oceanic influence on MJOs is studied with reanalysis products, numerical models, and idealized theoretical models. The energy sources for MJOs are calculated with NCEP reanalysis. The perturbed potential energy is found to be the most important energy source for most MJO events. In some MJO events, the sea surface is warmed due to the reduced latent heat flux during the suppressed phase of MJOs. As a result, warm sea surface temperature anomalies (SSTAs) occur, which appear to prolong the life time of these MJO events. In a minority of the MJO events, warm SSTAs can drive the atmosphere actively and trigger MJO events. In these events, the warm SSTAs are attributable to the internal oceanic processes influenced by the warm Indonesian Throughflow (ITF), which spreads from the southeastern Indian Ocean to the western Indian Ocean and modifies the subtle balance between stratification and mixing in the western Indian Ocean. In addition, during the transit period between monsoon seasons, a few MJO events are sustained by the energy obtained from the mean kinetic energy. Since the MJO events have different energy sources, their mechanisms should be considered in the context of these energy sources. While the spatial scale of the SSTAs in the Indian Ocean is only of order 100 km, the scale of MJOs is of order 1000 km, raising the potential for interactions between the oceanic and the atmospheric oscillations with different scales and this is demonstrated to be possible with analytical solutions to idealized linear governing equations. With a reasonable choice of parameters, the meso-scale oceanic and the large-scale atmospheric oscillations can interact with each other and lead to unstable waves in the intraseasonal band in this linear coupled model. The coupling and frequency shifts between oscillations with different scales and the atmospheric/oceanic responses to small variations in the external forcing are also tested with numerical models. Incorporating the oceanic influence on MJOs and the multi-scale interaction appropriately in a numerical model is supposed to help improve the simulation and forecast of MJOs. The hypothesis of multi-scale interaction is also expected to have wide applications in other studies, in addition to the MJO-SST interaction. The theoretical and numerical approach adopted here should also serve as a prototype for enhancing the process understanding of intraseasonal variability and lead to improved predictive understanding.
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    Interannual variability of sea surface temperature in the eastern tropical Pacific Ocean and Central American rainfall
    (2007-11-26) Karnauskas, Kristopher Benson; Busalacchi, Antonio J; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Sea surface temperature (SST) in the east Pacific warm pool (EPWP) plays a potentially important role in Central American rainfall, tropical cyclogenesis, ocean biology, large-scale tropical heating, and the El Niño-Southern Oscillation (ENSO). The first part of this dissertation is aimed at understanding what processes govern the interannual variability of SST in the EPWP. Interannual wind stress, shortwave radiation, and precipitation were used as forcing to an ocean general circulation model. Shortwave heating was identified as the primary driver of the interannual SST tendency in the EPWP. The high correlation between the EPWP and the equatorial Pacific Ocean is explained by the fact that equatorial SST anomalies modify the distribution of atmospheric vertical motions and therefore cloud cover and shortwave heating. In a parallel set of experiments, the low-frequency variability of the Tehuantepec gap winds was also shown to have a considerable effect on that of SST in the EPWP. Motivated by the results of the first part of this dissertation, the second part offers significant improvements to the mean state of the equatorial Pacific Ocean in a climatology ocean model experiment by including the Galápagos Islands in the model topography. In this context, the equatorial cold bias is reduced. Furthermore, when the ocean model is coupled to the atmosphere through zonal wind stress, the problem of an excessively regular and biennial ENSO is also reduced. The change in ENSO timescale is a result of the same dynamics operating on a different mean state. The third part of this dissertation is aimed at understanding the role of the interannual variability of SST in the EPWP in that of Central American rainfall. An anomalously warm EPWP can trigger a rapid enhancement of the east Pacific intertropical convergence zone (ITCZ) in rainy seasons following peak ENSO events, which leads to a rainfall anomaly over Central America. Moreover, the timing and amplitude of the SST-enhanced ITCZ depends on the persistence of the ENSO event. The longer the equatorial SST anomaly persists, the longer the EPWP is subject to anomalous shortwave heating and thus the greater the subsequent SST enhancement of the ITCZ.
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    BRED VECTORS IN THE NASA NSIPP GLOBAL COUPLED MODEL AND THEIR APPLICATION TO COUPLED ENSEMBLE PREDICTIONS AND DATA ASSIMILATION
    (2005-04-27) Yang, Shu-Chih; Yang, Shu-Chih; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The theme of my thesis research is to perform breeding experiments with NASA/NSIPP coupled general circulation model (CGCM) in order to obtain ENSO-related growing modes for ensemble perturbations. We show for the first time that the breeding method is an effective diagnostic tool for studying the coupled ENSO-related instabilities in a coupled ocean-atmosphere general circulation model that includes physical and dynamical processes of many different time scales. We also show for the first time that it is feasible to utilize the coupled bred vectors (BV) as a way to construct perturbations for ensemble forecasts for ENSO prediction using an operational coupled climate prediction model. The results of the thesis research show that coupled breeding can detect a precursor signal associated with ENSO events. Bred vectors are characterized by air-sea coupled features and they are very sensitive to ENSO phases and background season. This indicates that bred vectors can effectively project on the seasonal-to-interannual instabilities by growing upon the slowly varying coupled instability. These results are robust: bred vectors obtained from both the NASA and NCEP coupled systems exhibit similarities in many fields, even in atmospheric teleconnected regions. We show that bred vectors have a structure similar to the one-month forecast error (analysis increment). The BV growth rate and the one-month forecast error show similar low frequency variations. Both of their subsurface temperatures have large-scale variability near the depth of thermocline. Evidence shows that bred vectors capture the eastern movement of the analysis increment (one-month forecast error) along the equatorial Pacific during 1997-1998 El Niño evolution. The results suggest that one-month forecast error in NSIPP CGCM is dominated by dynamical errors whose shape can be captured by bred vectors, especially when the BV growth rate is large. These results suggest that bred vectors should be effective coupled perturbations for ensemble ENSO predictions, compensating for the lack of coupled ENSO-related perturbations in current operational ensembles. The similarity between the bred vectors and the one month forecast errors suggests that bred vectors can capture "errors of the month" and could also be applied to improve oceanic data assimilation by providing information on the month-to-month background variability.
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    MEASUREMENTS AND CHARACTERIZATION OF OPTICAL PROPERTIES IN THE CHESAPEAKE BAY'S ESTUARINE WATERS USING IN-SITU MEASUREMENTS, MODIS SATELLITE OBSERVATIONS, AND RADIATIVE TRANSFER MODELING
    (2004-01-27) Tzortziou, Maria; Hudson, Robert; Meteorology
    The core subject of this thesis is the development of coordinated atmospheric, in-water, and laboratory measurements leading to characterization of in-water optical properties in the estuarine environment of northern Chesapeake Bay, where natural and human-induced processes strongly interact. One of the main objectives is obtaining a sufficiently complete suite of measurements, combined with detailed radiative transfer calculations, so as to produce a closure experiment for the underwater inherent and apparent optical properties. The in-situ results are applied to the interpretation of satellite (MODIS) water leaving radiance data and their validation. The applicability of bio-optical models and parameterizations currently used in satellite algorithms are examined for the case of the optically complex Chesapeake Bay waters. Relationships between remotely sensed water leaving radiances and properties of optically active components in these waters are investigated. The resulting techniques and analysis should be broadly applicable to other coastal areas of the world. The results from this thesis, and other future work, will contribute to our ability to obtain more accurate information from remotely measured optical characteristics of estuarine and coastal regions. The combined use of in-situ measurements and detailed radiative transfer modeling enables the improvement of both the theoretical models and satellite remote sensing algorithms needed to a better understanding of biotic responses to environmental forcing.