A multi-decadal survey program undertaken at the Collaroy-Narrabeen embayment in SE Australia identifies medium-term (~2-7 year) cycles of both erosion and accretion across the entire embayment ('beach oscillation') and at its two extremities ('beach rotation'). These cycles have been observed to respond to phase shifts in the El Ni??o/Southern Oscillation (ENSO). To investigate wave and climate controls of embayment variability in finer detail, this study combines historical surveys with 45 years of wave data from the ERA-40 reanalysis and four years of high-resolution beach measurements using RTK-GPS and image-derived survey techniques. ENSO and Southern Annular Mode (SAM) controls of wave variability in the Sydney region are first explored. In general, wave heights increase/decrease and wave directions become more easterly/southerly during La Ni??a/El Ni??o phases. A positive correlation is observed between the SAM and summer wave heights, and a negative correlation between the SAM and winter wave directions. Storm variability is observed to be modified by the ENSO, but not the SAM. In particular, La Ni??a phases are generally associated with longer duration, higher energy events from a more easterly direction when compared to those during El Ni??o phases. Wave controls of embayment variability are subsequently investigated. In the short-term (days - months), beach oscillation/rotation is observed to be the most dominant process, accounting for approx. 60%/20% of overall embayment variability. Beach oscillation is related to changes in wave height and storms, whereas beach rotation is related to changes in wave direction and/or wave period. An empirical model that estimates the beach response to individual storm events is developed. In the longer-term (months - years), beach rotation is observed to respond to both wave heights and directions. Larger waves are sheltered somewhat at the southern end, creating an apparent clockwise rotation under energetic wave conditions. Clockwise/anticlockwise rotations are also observed to follow southerly/easterly wave shifts at lags of up to 12 months. Comparisons between the ENSO and beach oscillation/rotation agree with previous observations that El Ni??o/La Ni??a phases are associated with an overall accretion/erosion and clockwise/anticlockwise rotation of the embayment. In general, the SAM shows little influence on embayment variability. While it is clear that beach oscillation is driven by cross-shore processes, to what extent beach rotation is a longshore and/or cross-shore phenomena requires further investigation.

Linkages between influenza prevalence and climate (e.g. precipitation, temperatures, El Nino Southern Oscillation ENSO) have been suspected, but definitive evidence remains elusive. This analysis investigated a climatic relationship between influenza mortality (measured by multiple caused pneumonia and influenza deaths) and influenza morbidity (measured by isolates tested for influenza). Influenza-climate linkages were analyzed at multiple spatial scales (e.g. local analysis, and regional analysis) and multiple temporal scales (e.g. annualized mortality counts, and mortality counts based on cumulative percentiles). Influenza mortality and morbidity were found to have significant correlations to seasonal temperatures, precipitation, and ENSO. Influenza-climate associations varied spatially and temporally, and underscore the importance of considering geographic scale in investigative analyses of disease. Evidence for an influenza-climate relationship provides a greater understanding of the enviro-climatic factors that can contribute to an influenza epidemic, and provides an impetus for further studies that incorporate climatic factors in influenza risk modeling.

Large-scale circulation patterns have a significant modulating influence on local hydro-meteorological variables, and consequently on water availability. An understanding of the influence of these patterns on the hydrological cycle, and the ability to timely predict their impacts, is crucial for water resources planning and management. This dissertation focusses on the influence of two major large-scale circulation patterns, the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), on the Rio Grande basin and the state of Texas, US. Both study areas are subject to a varying climate, and are extremely vulnerable to droughts, which can have devastating socio-economic impacts. The strength and spatial correlation structure of the climate indices on gauged precipitation was first established. Precipitation is not linearly related to water availability; therefore a land surface model (LSM), with land use land cover constant, was used to create naturalized flow, as it incorporates all necessary hydro-meteorological factors. As not all ENSO events are created equal, the influence of individual El Niño and La Niña events, classified using four different metrics, on water availability was examined. A general increase (decrease) in runoff during El Niños (La Niñas) was noted, but some individual events actually caused a decrease (increase) in water availability. Long duration El Niños have more influence on water availability than short duration high intensity events. Positive PDO enhances the effect of El Niño, and dampens the negative effect of La Niña, but when it is in its neutral or transition phase, La Niña tends to dominate climatic conditions and reduce water availability. LSM derived runoffs were converted into 3-month Standardized Runoff Indices (SRI 3) from which water deficit durations and severities were extracted. Conditional probability models of duration and severity were developed and compared with that based on observed precipitations. It was found that model derived information can be used in regions having limited ground observation data, or can be used in tandem with observation driven conditional probabilities for more efficient water resources planning and management. Finally a multidimensional model was developed, using copulas, to predict precipitation based on the phase of ENSO and PDO. A bivariate model, with ENSO and precipitation, was compared to a trivariate model, which incorporates PDO, and it was found that information on the state of PDO is important for efficient precipitation predictions.

The ocean reanalysis Simple Ocean Data Assimilation (SODA) 2.2.4 is used to explore the changes in upwelling from normal conditions to either El Nino or La Nina conditions. Physical and thermodynamic variables from the reanalysis are used to explore the structure and behavior of El Nino Southern Oscillation (ENSO) events. The results of this analysis show that sea surface temperature (SST), entrainment velocity, wind stress, mixed layer depth, wind curl, and heat content anomalies are in general agreement with ENSO theory. Interestingly, the distribution of upwelling based on the entrainment velocity is very patchy, which led us to explore zonal and meridional sections of vertical velocity. We used three methods to compute changes in upwelling during ENSO events. The first method computes upwelling within the areas of SST anomalies during ENSO events. During El Nino events upwelling shows prominent decadal variability, while during La Nina the decadal variability is weaker. A new upwelling index is used for the second method, and upwelling is computed in the areas of strong upwelling anomalies. The variability of upwelling is higher in periods of reduced upwelling than in periods of strong upwelling. Despite the fact that the new index is computed independently, it agrees in the timing of the index used to define ENSO events for this research. The first and second methods show that the amplitude of SST anomalies and upwelling anomalies do not have a direct relationship, suggesting that upwelling does not explain all of the variance in SST. The last method used is to compute changes in upwelling in the Nino 1+2 region during ENSO events. In the east Pacific there is almost no correlation between upwelling and SST anomalies during ENSO, but this might be attributed to the fact that the Nino 1+2 region is a relatively small region compared to the Nino 3.4 region that is used to define ENSO events. In general, the time series of SST and upwelling anomalies agree well just in the cases when ENSO events are prominently in the eastern Pacific. A comparison between yearly fisheries data from Ecuador and Peru and monthly data of SST anomalies during ENSO years is presented showing that during El Nino events the fish catch decreases and during La Nina events the fish catch increases. We infer that the increase or decrease in fish catch is associated to changes in fish populations, and that these changes are mainly due to availability of nutrients and changes in temperature during ENSO events.

To investigate the tropical Atlantic response to the remote El Nino-Southern Oscillation (ENSO) forcing, a Reduced Physics – Coupled Global Circulation Model (RP-CGCM) is developed, and four experiments are carried out. The results show that the RP-CGCM is capable of capturing the major features of Tropical Atlantic Variability (TAV) and its response to ENSO forcing. The SST response to the remote influence of ENSO may be divided into two stages. In stage one, the ENSO influences the tropical Atlantic SST primarily through the Troposphere Temperature (TT) mechanism, which predicts a uniform warming in the tropical Atlantic following the mature phase of El Nino. In the north tropical Atlantic (NTA), the Walker mechanism and the Pacific-North-American (PNA) mechanism work in concert with the TT-induced warming, giving rise to a robust SST response during the boreal spring in this region. In the south tropical Atlantic (STA), the southeasterly wind anomaly and increased stratus clouds work against the TT-induced warming, resulting in a much weaker SST response in this region. At this stage, the response can be largely explained by the ocean mixed layer response to changes in surface heat fluxes induced by ENSO. In stage two, ocean dynamics play a more active role in determining the evolution of SST. The cross-equatorial wind anomaly in the western to central equatorial Atlantic can change the SST in the eastern equatorial Atlantic through Bjerknes feedback and the SST in the central equatorial Atlantic through Ekman feedback. These feedback result in a cooling of SST in the equatorial south Atlantic (ESA) region which is so overwhelming that it cancels the warming effect induced by the TT mechanism and reverses the sign of the warm SST anomaly that is formed during stage one in this region. In general, the horizontal advection of heat plays a secondary role in the SST response to the remote influence of ENSO, except in the regions where the North Equatorial Countercurrent (NECC) dominates and the SST variability is strong. Entrainment is particularly important in maintaining the correct SST structure during boreal summer.

In the first part of this dissertation we use an Intermediate Coupled Model to develop a quantitative test to validate the null hypothesis that low-frequency varia- tion of ENSO predictability may be caused by stochastic processes. Three "perfect model scenario" prediction experiments are carried out, where the model is forced ei- ther solely by stochastic forcing or additionally by decadal-varying backgrounds with different amplitudes. These experiments indicate that one can not simply reject the null hypothesis unless the decadal-varying backgrounds are unrealistically strong. The second part of this dissertation investigates the extent to which internal atmospheric variability (IAV) can influence ENSO variation, and examines the un- derlying physical mechanisms linking IAV to ENSO variability with the aid of a newly developed coupled model consisting of an atmospheric general circulation model and a Zebiak-Cane type of reduced gravity ocean model. A novel noise filter algorithm is developed to suppress IAV in the coupled model. A long control coupled simulation, where the filter is not employed, demonstrates that the coupled model captures many statistical properties of the observed ENSO behavior. It further shows that the development of El Ni~no is linked to a boreal spring phenomenon referred to as the Pacific Meridional Model (MM). The MM, character- ized by an anomalous north-south SST gradient and anomalous surface circulation in the northeasterly trade regime with maximum variance in boreal spring, is inherent to thermodynamic ocean-atmosphere coupling in the Intertropical Convergence Zone latitude. The Northern Pacific Oscillation provides one source of external forcing to excite it. This result supports the hypothesis that the MM works as a conduit for extratropical atmospheric influence on ENSO. A set of coupled simulations, where the filter is used to suppress IAV, indicate that reducing IAV in both wind stress and heat flux substantially weakens ENSO variance. Furthermore, the resultant ENSO cycle becomes more regular and no longer shows strong seasonal phase locking. The seasonal phase locking of ENSO is strongly tied to the IAV in surface heat flux. The ENSO cycle is strongly tied to IAV in surface wind stress.

Oxygen and hydrogen isotopes (δ18O, δD) are increasingly powerful tools for reconstructing past hydroclimate variability. The utility of δ18O- and δD-based paleoclimate records, however, depends on our understanding of how well these tracers reflect past climate conditions. The dynamics controlling the relationship between climate and water isotope variability are highly complex and often poorly constrained, especially in the tropics, where many key high-resolution paleoclimate records rely on past rainfall isotopes as proxies for hydroclimate. In this dissertation, I use multi-year timeseries of daily rainfall and biweekly dripwater δ18O from northern Borneo – a site for stalagmite δ18O-based paleoclimate reconstruction in the heart of the West Pacific Warm Pool – to track the cloud-to-calcite transformation of δ18O and its relationship to large-scale climate variability. Chapter 2 investigates the variability of rainfall δ18O variability from northern Borneo on diurnal to interannual timescales and its relationship with local and regional climate. Chapter 3 investigates the rainfall-to-dripwater transformation of climate-related isotopic signals following water transit through the Borneo cave system. Overall, this dissertation provides empirical support for the interpretation of northern Borneo stalagmite δ18O as a robust indicator of regional-scale hydroclimate variability, where higher δ18O reflects regional drying. More generally, this research provides a roadmap for obtaining more nuanced interpretations of speleothem δ18O records from multi-year, high-resolution, paired timeseries of rainfall and dripwater δ18O.

This research provides insight into changes in volumetric flushing of the Tampa Bay estuary caused by synoptic scale wind events. The two main studies of this dissertation involve 1) using wavelet analysis to investigate the link between the El Niño-Southern Oscillation (ENSO) and the frequency and strength of volumetric flushing driven by synoptic variability and 2) using a multi-decadal model simulation to examine how extratropical/winter storms and hurricanes affect the overall flushing rates for Tampa Bay, FL. In the first study, two analyses are performed on 55 years of observational data to investigate the effect of multiple small wind events on estuarine flushing. First I use subtidal observed water level as a proxy for mean tidal height to estimate the rate of volumetric bay outflow. Second, I use wavelet analysis on sea level and wind data to isolate the synoptic sea level and surface wind variance. For both analyses the long-term monthly climatology is removed to focus on the volumetric and wavelet variance anomalies. The overall correlation between the Oceanic Niño index and volumetric anomalies is small (r2=0.097) due to the seasonal dependence on the ENSO response. The mean monthly climatology between the synoptic wavelet variance of elevation and axial winds have similar seasonal behavior. During the winter, El Niño (La Niña) increases (decreases) the synoptic variability, but decreases (increases) it during the summer. The difference in winter El Niño/La Niña wavelet variances is about 20% of the climatological value. ENSO can swing the synoptic flushing of the bay by 0.22 bay volumes per month. These changes in circulation associated with synoptic variability have the potential to impact mixing and transport within the bay. In the second study, volumetric changes from large scale weather events are investigated using a numerical circulation model simulation (1975-2006) to find the cumulative impact of flushing on the bay by extreme events. The strong wind speeds, duration of high winds and wind direction during these events all affect the amount of water flushed in and out of the estuary. Normalized volume anomalies are largest when wind components blow up/down the estuary in the NE/SW direction. Wind induced normalized flushing rates for all 10 extratropical/winter storms range from 12% to 40% and from 14% to 40% for all 10 hurricanes. All storms discussed in this study caused winds greater than 15 m s-1 (~30 knots). The direction of the winds had an impact on the flushing rates during these extreme events. Storm9 (February 1998) and Hurricane Gabrielle (September 2001) experienced the smallest total volume changes (14% and 13%). Both storms experienced weak axial and co-axial winds causing volume changes to be small. The Storm of the Century (March 1993) and Hurricane Frances (September 2004) saw the largest total volume changes of 40%. They both had strong winds blowing in the NE direction. Hurricane Frances had two wind peaks and lingered in the area for approximately 48 hours, so both strength and duration of winds played a large role in the total volume change. Total inflow and outflow rates per year show that there is year to year variability of flushing in Tampa Bay.

A multi-decadal survey program undertaken at the Collaroy-Narrabeen embayment in SE Australia identifies medium-term (~2-7 year) cycles of both erosion and accretion across the entire embayment ('beach oscillation') and at its two extremities ('beach rotation'). These cycles have been observed to respond to phase shifts in the El Ni??o/Southern Oscillation (ENSO). To investigate wave and climate controls of embayment variability in finer detail, this study combines historical surveys with 45 years of wave data from the ERA-40 reanalysis and four years of high-resolution beach measurements using RTK-GPS and image-derived survey techniques. ENSO and Southern Annular Mode (SAM) controls of wave variability in the Sydney region are first explored. In general, wave heights increase/decrease and wave directions become more easterly/southerly during La Ni??a/El Ni??o phases. A positive correlation is observed between the SAM and summer wave heights, and a negative correlation between the SAM and winter wave directions. Storm variability is observed to be modified by the ENSO, but not the SAM. In particular, La Ni??a phases are generally associated with longer duration, higher energy events from a more easterly direction when compared to those during El Ni??o phases. Wave controls of embayment variability are subsequently investigated. In the short-term (days - months), beach oscillation/rotation is observed to be the most dominant process, accounting for approx. 60%/20% of overall embayment variability. Beach oscillation is related to changes in wave height and storms, whereas beach rotation is related to changes in wave direction and/or wave period. An empirical model that estimates the beach response to individual storm events is developed. In the longer-term (months - years), beach rotation is observed to respond to both wave heights and directions. Larger waves are sheltered somewhat at the southern end, creating an apparent clockwise rotation under energetic wave conditions. Clockwise/anticlockwise rotations are also observed to follow southerly/easterly wave shifts at lags of up to 12 months. Comparisons between the ENSO and beach oscillation/rotation agree with previous observations that El Ni??o/La Ni??a phases are associated with an overall accretion/erosion and clockwise/anticlockwise rotation of the embayment. In general, the SAM shows little influence on embayment variability. While it is clear that beach oscillation is driven by cross-shore processes, to what extent beach rotation is a longshore and/or cross-shore phenomena requires further investigation.

A multi-decadal survey program undertaken at the Collaroy-Narrabeen embayment in SE Australia identifies medium-term (~2-7 year) cycles of both erosion and accretion across the entire embayment ('beach oscillation') and at its two extremities ('beach rotation'). These cycles have been observed to respond to phase shifts in the El Ni??o/Southern Oscillation (ENSO). To investigate wave and climate controls of embayment variability in finer detail, this study combines historical surveys with 45 years of wave data from the ERA-40 reanalysis and four years of high-resolution beach measurements using RTK-GPS and image-derived survey techniques. ENSO and Southern Annular Mode (SAM) controls of wave variability in the Sydney region are first explored. In general, wave heights increase/decrease and wave directions become more easterly/southerly during La Ni??a/El Ni??o phases. A positive correlation is observed between the SAM and summer wave heights, and a negative correlation between the SAM and winter wave directions. Storm variability is observed to be modified by the ENSO, but not the SAM. In particular, La Ni??a phases are generally associated with longer duration, higher energy events from a more easterly direction when compared to those during El Ni??o phases. Wave controls of embayment variability are subsequently investigated. In the short-term (days - months), beach oscillation/rotation is observed to be the most dominant process, accounting for approx. 60%/20% of overall embayment variability. Beach oscillation is related to changes in wave height and storms, whereas beach rotation is related to changes in wave direction and/or wave period. An empirical model that estimates the beach response to individual storm events is developed. In the longer-term (months - years), beach rotation is observed to respond to both wave heights and directions. Larger waves are sheltered somewhat at the southern end, creating an apparent clockwise rotation under energetic wave conditions. Clockwise/anticlockwise rotations are also observed to follow southerly/easterly wave shifts at lags of up to 12 months. Comparisons between the ENSO and beach oscillation/rotation agree with previous observations that El Ni??o/La Ni??a phases are associated with an overall accretion/erosion and clockwise/anticlockwise rotation of the embayment. In general, the SAM shows little influence on embayment variability. While it is clear that beach oscillation is driven by cross-shore processes, to what extent beach rotation is a longshore and/or cross-shore phenomena requires further investigation.