The El Niño – Southern Oscillation (ENSO) has far-reaching impacts across the globe and provides the most reliable source of seasonal to interannual climate prediction over North America. Though numerous studies have discussed the impacts of ENSO teleconnections on North America during boreal winter, it is becoming more and more apparent that the regional impacts of ENSO teleconnections are highly sensitive to the seasonal evolution of ENSO events. Also, the significant impacts of ENSO are not limited to the boreal winter seasons. To address these knowledge gaps, this thesis examines the seasonal dependence of ENSO teleconnections and impacts on North American surface climate, focusing on two examples. Chapter 1 examines the relationship between El Niño – California winter precipitation. Results show that the probability of the anomalous statewide-wetness increases as El Niño intensity increases. Also, the influences of El Niño on California winter precipitation are statistically significant in late winter (Feb-Apr), but not in early winter even though that is when El Niño usually reaches its peak intensity. Chapter 2 further investigates why the strong 2015/16 El Niño failed to bring above normal winter precipitation to California, focusing on the role of westward shifted equatorial Pacific sea surface temperature anomalies (SSTAs) based on two reasons: the maximum equatorial Pacific SSTAs was located westward during the 2015/16 winter compared to those during the 1982/83 and 1997/98 winters, both of which brought extremely wet late winters to California. Also, the North American Multi-Model Ensemble (NMME) forecasts overestimated the eastern tropical Pacific SSTAs and California precipitation in the 2015/16 late winter, compared to observations. The Atmospheric General Circulation Model (AGCM) experiments suggested that the SST forecast error in NMME contributed partially to the wet bias in California precipitation forecast in the 2015/16 late winter. However, the atmospheric internal variability could have also played a large role in the dry California winter during the event. ENSO also exerts significant impacts on agricultural production over the Midwest during boreal summer. Chapter 3 examines the physical processes of the ENSO summer teleconnection, focusing on the summer when a La Niña is either transitioning from an earlier El Niño winter or persisting from an existing La Niña winter. The results demonstrate that the impacts are most significant during the summer when El Niño is transitioning to La Niña compared to that when La Niña is persisting, even though both can loosely be defined as developing La Niña summer. During the transitioning summer, both the decaying El Niño and the developing La Niña induce suppressed deep convection over the tropical Pacific and thereby the corresponding Rossby wave propagations toward North America, resulting in a statistically significant anomalous anticyclone over northeastern North America and, therefore, a robust warming signal over the Midwest. These features are unique to the developing La Niña transitioning from El Niño, but not the persistent La Niña. In Chapter 4, we further evaluate the performance of NCAR CAM5 forced with historical SSTA in terms of the La Niña summer teleconnections. Though the model ensemble mean well reproduces the features in the preceding El Niño/La Niña winters, the model ensemble mean has very limited skill in simulating the tropical convection and extratropical teleconnections during both the transitioning and persisting summers. The weak responses in the model ensemble mean are attributed to large variability in both the tropical precipitation, especially over the western Pacific, and atmospheric circulation during summer season. This thesis synthesizes the physical processes and assessments of climate models in different seasons to establish the sensitivity of regional climate to the seasonal dependence of ENSO teleconnections. We demonstrate that the strongest impacts of ENSO on North American regional climate might not be necessarily simultaneous with maximum tropical Pacific SST anomalies. We also emphasize the importance of the multi-year ENSO evolutions when addressing the seasonal impacts on North American summertime climate. The findings in this thesis could benefit the improvement of seasonal hydroclimate forecasting skills in the future.

Coupled shape oscillations and translational motion of an incompressible gas bubble in a liquid container in response to forced vibration is studied numerically. Bond number (Bo) and the ratio of the vibration amplitude to the bubble diameter (A/D) are found to be the governing non-dimensional numbers. Bubble response is studied in both 2D and 3D. Different schemes are used for 2D and 3D simulations. In 2D, the flow solver is coupled to a Volume of Fluid (VOF) algorithm to capture the interface between the two phases while in 3D the interface is captured using a level set algorithm. The oscillation outcome ranges from small amplitude and regular oscillations for small Bo and A/D to large amplitude, nonlinear, and chaotic oscillations for large Bo and A/D. Chaotic behavior occurs due to the coupling between the nonlinear shape oscillations and large amplitude oscillatory translational motion. By further increase of the forcing, the inertia of the liquid results in the formation of a liquid jet which penetrates within the bubble core and pierces the bubble and a toroidal bubble shape is formed. The toroidal bubble shape then goes through large amplitude shape oscillations and smaller bubbles are formed. A summary of the 3D simulations provides a map which shows the bubble oscillation outcome as a function of Bo and A/D. The interaction between two bubbles is studied in 2D as well and the effect of vibration amplitude, frequency and liquid to gas density ratio on the interaction force is investigated.

Recent changes in climate have increased public attention and scientific evaluation of climate impacts on wild animals and plants. Variation in local weather and regional climate may affect breeding success in birds. Migratory species may be sensitive to these changes as breeding and wintering areas may experience different climate variations; some insectivorous species may be unable to alter timing of migration or laying dates and experience a mismatch between timing of nesting and peak insect availability for their nestlings. Therefore, I investigated the influence of local weather variables and regional climate on breeding performance of an insectivorous migrant songbird, the Tree Swallow (<i>Tachycineta bicolor</i>), and tried to examine effects of a mismatch between the timing of breeding and food availability.<p>I used a 14 year data set from St. Denis, Saskatchewan, Canada, 1991-2004, to evaluate correlations among local weather, wetland conditions, aerial insects and regional climate indices and their relationships with variation in clutch initiation date, clutch size, and fledging success. Swallows returned to the study site in late April each year. Annual variation in median clutch initiation date was best explained by mean minimum temperatures during 1-15 May. Larger clutches were laid in years with higher pond water levels (possibly an indication of increased insect availability) and when the Southern Oscillation Index (SOI) was positive (representing La Niña conditions). Fledging success was not influenced greatly by any explanatory variable; however, fledging success tended to increase in years with higher average temperatures. Individual variation in clutch initiation date was examined using path analysis. I found high correlations between initiation date and both local environmental variables and regional climate indices; earlier nesting was associated with warmer temperatures (increased local temperatures, more positive North Atlantic Oscillation Index (NAOI) values and more negative SOI values) and decreased moisture (more positive NAOI values). Two reduced data sets, including female age or insect abundance, were also examined. Clutches were initiated earlier by older females and during springs with higher abundance of aerial insects.<p>I applied two heating treatments to nest boxes used by pre-laying swallows and compared reproductive measures (timing of nesting, laying sequence, clutch size, egg weight and fledging success) of birds using heated boxes to those of females attending unheated control boxes. However, I was unable to directly examine the predictions of the mismatch hypothesis because nest box heating failed to advance laying dates. Furthermore, no increases in clutch size, egg weight and fledging success were detected between treatment and control nests. Although box heating increased nest temperatures an average of 6.1C (+ 0.8 SE) over controls, length of time females spent in heated boxes may have been too short to alleviate energetic constraints on egg production, or energy savings associated with box use were insufficient to supersede the influence of ambient environmental conditions that control food availability and energy expenditure of foraging swallows. My results demonstrated that local and regional climate variation strongly affected timing of nesting in swallows, likely via their effects on food supply.

Coupled shape oscillations and translational motion of an incompressible gas bubble in a liquid container in response to forced vibration is studied numerically. Bond number (Bo) and the ratio of the vibration amplitude to the bubble diameter (A/D) are found to be the governing non-dimensional numbers. Bubble response is studied in both 2D and 3D. Different schemes are used for 2D and 3D simulations. In 2D, the flow solver is coupled to a Volume of Fluid (VOF) algorithm to capture the interface between the two phases while in 3D the interface is captured using a level set algorithm. The oscillation outcome ranges from small amplitude and regular oscillations for small Bo and A/D to large amplitude, nonlinear, and chaotic oscillations for large Bo and A/D. Chaotic behavior occurs due to the coupling between the nonlinear shape oscillations and large amplitude oscillatory translational motion. By further increase of the forcing, the inertia of the liquid results in the formation of a liquid jet which penetrates within the bubble core and pierces the bubble and a toroidal bubble shape is formed. The toroidal bubble shape then goes through large amplitude shape oscillations and smaller bubbles are formed. A summary of the 3D simulations provides a map which shows the bubble oscillation outcome as a function of Bo and A/D. The interaction between two bubbles is studied in 2D as well and the effect of vibration amplitude, frequency and liquid to gas density ratio on the interaction force is investigated.

Recent changes in climate have increased public attention and scientific evaluation of climate impacts on wild animals and plants. Variation in local weather and regional climate may affect breeding success in birds. Migratory species may be sensitive to these changes as breeding and wintering areas may experience different climate variations; some insectivorous species may be unable to alter timing of migration or laying dates and experience a mismatch between timing of nesting and peak insect availability for their nestlings. Therefore, I investigated the influence of local weather variables and regional climate on breeding performance of an insectivorous migrant songbird, the Tree Swallow (<i>Tachycineta bicolor</i>), and tried to examine effects of a mismatch between the timing of breeding and food availability.<p>I used a 14 year data set from St. Denis, Saskatchewan, Canada, 1991-2004, to evaluate correlations among local weather, wetland conditions, aerial insects and regional climate indices and their relationships with variation in clutch initiation date, clutch size, and fledging success. Swallows returned to the study site in late April each year. Annual variation in median clutch initiation date was best explained by mean minimum temperatures during 1-15 May. Larger clutches were laid in years with higher pond water levels (possibly an indication of increased insect availability) and when the Southern Oscillation Index (SOI) was positive (representing La Niña conditions). Fledging success was not influenced greatly by any explanatory variable; however, fledging success tended to increase in years with higher average temperatures. Individual variation in clutch initiation date was examined using path analysis. I found high correlations between initiation date and both local environmental variables and regional climate indices; earlier nesting was associated with warmer temperatures (increased local temperatures, more positive North Atlantic Oscillation Index (NAOI) values and more negative SOI values) and decreased moisture (more positive NAOI values). Two reduced data sets, including female age or insect abundance, were also examined. Clutches were initiated earlier by older females and during springs with higher abundance of aerial insects.<p>I applied two heating treatments to nest boxes used by pre-laying swallows and compared reproductive measures (timing of nesting, laying sequence, clutch size, egg weight and fledging success) of birds using heated boxes to those of females attending unheated control boxes. However, I was unable to directly examine the predictions of the mismatch hypothesis because nest box heating failed to advance laying dates. Furthermore, no increases in clutch size, egg weight and fledging success were detected between treatment and control nests. Although box heating increased nest temperatures an average of 6.1C (+ 0.8 SE) over controls, length of time females spent in heated boxes may have been too short to alleviate energetic constraints on egg production, or energy savings associated with box use were insufficient to supersede the influence of ambient environmental conditions that control food availability and energy expenditure of foraging swallows. My results demonstrated that local and regional climate variation strongly affected timing of nesting in swallows, likely via their effects on food supply.

The intraseasonal Oscillation (ISO) is a very strong and coherent mode of variability observed in the Earths climate. Rainfall variability in the intraseasonal timescale is particularly strong in the Tropics and it directly interacts with the South Asian monsoon during boreal summer and with the Australian monsoon during winter. A detailed analysis of the horizontal and vertical structure of the ISO during both summer and winter is presented in this work considering the coupled ocean-atmosphere system. In addition, the role of the intraseasonal variability of the Southeast Asian monsoon is studied in detail. From the applications point of view, the intraseasonal time scale is arguably the most important period of variability. However, extended forecasting of intraseasonal activity has proven to be a difficult task for the state of the art numerical models. In order to improve the forecasts of the ISO activity over the Southeast Asian monsoon region, a physically based empirical scheme was designed. The scheme uses wavelet banding to separate the predictand and predictors into physically significant bands where linear regression followed by recombination of the bands is used to generate the forecast. Results of the empirical scheme suggest that isolating the evolution of the intraseasonal signal from higher frequency variability and noise improve the skill of the prediction. The hypothesis is that a similar phenomenon occurs in numerical models: The strong intraseasonal signal is eroded by high frequency errors due to the model parameterizations, especially in convection. To evaluate the hypothesis, a coupled ocean-atmosphere model was run in ensemble mode for 30 day periods initialized daily for 20 days before to 20 days after major intraseasonal oscillations, allowing the examination of the skill of the model relative to the phase of the oscillation. The results, which confirm the previous hypothesis, represent well the observations for about 7 days after which the magnitude of the errors is greater than the signal itself. An integration scheme was developed for the coupled ocean-atmosphere general circulation model in order to mimic the philosophy of the empirical scheme and use for 30-day forecasts. The propagation features associated to ISO activity are improved.

Intraseasonal oscillations (ISOs) are important elements of the tropical climate with time-scales of 20-80 day. The ISO is poorly simulated and predicted by numerical models. This work presents a joint diagnostic and modeling study of the ISO that examines the hypothesis that local coupling between the ocean and the atmosphere is essential to the existence and evolution of the ISO in the Indo-Pacific warm pool region. Low-level moistening during the transition phase preconditions the atmosphere for deep convection. The vertical structure of ISO from the ECMWF coupled model during different phases of the oscillation as well as the skill of the model in simulating the processes that occur during the transition phase were studied. The forecast skill of the vertical structure associated with the ISO is greater for winter than for summer events. Predictability of the convective period is poor when initialized before the transitional phase. When initialized within the transition period including lower tropospheric moistening, predictability increases substantially, although the model parameterizations appears to trigger convection quickly without allowing an adequate buildup of CAPE during the transition. The model tends to simulate a more stable atmosphere compared to data, limiting the production of deep convective events. Two different one-dimensional coupled models are used to analyze the role of local ocean-atmosphere coupling in generating ISO. The ocean component is a one-dimensional mixed layer model. In the first model the atmospheric component corresponds to the SCCM. Results suggest that convection in the model tends to be "overactive," inhibiting development of lower frequency oscillations in the atmosphere. In the second case, the atmospheric component is a semi-empirical model that allows reproducing the coupled ISO over long integration periods including only local mechanisms. In the semi-empirical scheme the rate of change of atmospheric variables is statistically related to changes in SST. The stable state of this model is a quasi-periodic oscillation with a time scale between 25 and 80 days that matches well the observed ISO. Results suggest that the period of the oscillation depends on the characteristics of the ocean mixed layer, with a higher frequency oscillation for a shallow mixed layer.

This thesis is a short survey for the comparison theorems and oscillation theorems for the second order half-linear equation [c(x)u'^{(p-1)}]'+a(x)u^{(p-1)}=0, where u^{(p-1)}=|u|^{p-2}u. Some examples are also given. The above equation is said to be oscillatory ( O ) if there exists a nontrivial solution having an infinite number of zeros in (0,¡Û); and non-oscillatory ( NO ) if otherwise. Oscillation theorems help to determine whether an equation is ( O ) or ( NO ). These comparison theorem and oscillation theorems give information for the number and position of zeros in (0,¡Û) for a nontrivial solution of the above equation. Materials in this thesis originate from the papers of Li-Yeh and the monograph of Dosly and Rehak. But Reid type comparison theorem is new.

Thesis (Ph.D.)--University of Delaware, 2009. / Principal faculty advisor: Daniel J. Leathers, Dept. of Geography. Includes bibliographical references.

Over the 1951-2009 time period, 47% of all tropical systems in the Atlantic Basin transitioned to post-tropical storms. These storms are capable of producing hurricaneforce winds, torrential, flooding rains and storm surge that floods coastal areas. This study adds to previous climatological work by completing a case-study of Hurricane Ike (2008) and examining how teleconnections such as the El Niño Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO) contribute to the strength of a transitioning post-tropical storm. T-tests performed show strong statistical relationships between an increase (decrease) in post-tropical storm frequency and warm PDO – La Niña (cold PDO – La Niña), cold PDO – ENSO neutral (warm PDO – ENSO neutral), and warm (cold) AMO conditions. Moreover, nearly significant results were found for the same increase (decrease) and La Niña seasons since (pre) 1980 and for cold (warm) PDO conditions. Modeling the MJO suggests that increased (decreased) relative humidity associated with the wet (dry) phase could increase (decrease) precipitation output from the storm and decrease (increase) forward speed of the storm, decreasing (increasing) wind speeds observed at the surface.