The purpose of this dissertation is to study the interannual flows along the western and southern Australian coasts and along the northern coast of the Gulf of Mexico. Along the western and southern Australian coasts, sea levels are highly correlated with the El Ni˜no signal due to the leak in the gappy western equatorial Pacific Ocean boundary. Along the western Australian coast the coastline is nearly meridional and particle displacements near the coast undergo a change in Coriolis parameter. In order to keep the potential vorticity constant, this interannual coastal signal should propagate westward as Rossby waves with large zonal scale. TOPEX/Poseidon sea level data and coastal tide gauge measurements do show these large-scale waves off Australia's northwest coast. Along Australia's nearly zonal southern coast, particle displacements are nearly zonal near the coast and experience no planetary vorticity change. Consequently the Rossby wave mechanism fails and theory suggests that the signal should decay from the shelf edge with baroclinic Rossby radius of deformation scale. High-resolution along-track TOPEX/Poseidon sea level heights show that the interannual height signal does decay rapidly seaward of the shelf edge with this scale. The sharp fall in sea level and geostrophic balance imply strong (10 cm/sec) low frequency currents seaward of the shelf edge. On the shelf, inter annual flow is in the same direction as the shelf edge flow but much weaker. The anomalous flows tend to be eastward during La Ni˜na, when the western equatorial Pacific and Australian coastal sea levels are unusually high, and westward during El Ni˜no when coastal sea levels tend to be anomalously low. The anomalous low-frequency flows can transport larvae large distances, enhancing the recruitment of Australian salmon to nursery grounds in the eastern part of the southern coast when the coastal sea level is higher than normal and decreasing recruitment when it is lower than normal. Along the shelf edge south of 23oS of the western Australian coast, although the coastline is nearly meridional, high resolution satellite sea level estimates show that the inter annual sea level signal does not have the expected large spatial scale as it decreases rapidly seaward from the shelf edge. The drop in interannual sea level amplitude coincides with the mean position of the Leeuwin Current. Theory shows that a nearly meridional mean flow, as in the case of the Leeuwin Current, can induce this fall in interannual signal amplitude by altering the potential vorticity balance. The associated interannual shelf-edge flow tends to strengthen the Leeuwin Current during La Ni˜na, weaken it during El Ni˜no and may profoundly affect the recruitment of the western rock lobster. Past work has shown that the interannual wind stress curl in the North Atlantic generates Rossby waves that reach the eastern U.S. coast and affect coastal sea levels both there and along the northern coast of the Gulf of Mexico. Tide gauge and TOPEX/Poseidon satellite sea level height measurements show that this signal penetrates all the way around the Gulf shelf to the Yucatan Peninsula, local alongshore interannual wind stress increasing the signal amplitude between Pensacola and the Texas-Louisiana shelf. In accordance with theory, satellite observations show that the seaward spatial structure of the sea levels and the associated geostrophic flows depend on the angle of the coastline with respect to due north and the Loop Current mean shelf edge flow. Off the eastern boundary (small) formed by the west coast of Florida, the Loop Current distorts the potential vorticity balance and the sea level falls rapidly from the shelf edge with a scale of order the Loop Current width. Off the northern boundary ( 90o), the signal behaves as a coastal Kelvin wave, the sea level amplitude falling quickly away from the shelf edge with first baroclinic radius of deformation scale. Off the western boundary ( 180o), the interannual sea level amplitude falls rapidly seaward of the shelf edge consistent with short western boundary scales. Geostrophic shelf edge flow may reach amplitudes of order 10cms−1 but along shelf flow amplitudes are a few cm/s or less. Even so, weak shelf flows of low frequency can transport particles many hundreds of kilometers. / A Dissertation submitted to the Department of Oceanography in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester, 2004. / October 22, 2004. / Interannual Flows, Physical-biological Interaction, Remote Sensing, Continental Shelf Dynamics / Includes bibliographical references. / Allan J. Clarke, Professor Directing Dissertation; Fei-Fei Jin, Outside Committee Member; William Dewar, Committee Member; Doron Nof, Committee Member; Richard Iverson, Committee Member.

Understanding and predicting the inner-core structure and intensity change of tropical cyclones (TCs) remains one of the biggest challenges in tropical meteorology. This study addresses this challenge by investigating the formation, structure, and intensity changes resulting from localized strong updrafts in TCs known as convective bursts (CBs). The evolution of CBs are analyzed in high-resolution simulations of two hurricanes (Dean 2007 and Bill 2009) using the Weather Research and Forecasting (WRF) model. The simulations are able to capture the observed track and peak intensity of the TCs. With Dean, there is a slight lag between the simulated intensification and actual intensification, and the extreme rate of RI is not fully captured. However, the cycle of intensification, weakening, and re-intensification observed in both TCs is captured in the simulations, and appears to be due to a combination of internal dynamics and the surrounding environmental conditions. CBs are identified based on the 99th percentile of eyewall vertical velocity (over the layer from z = 6-12 km) in each simulation (8.4 m s-1 for Dean, 5.4 m s-1 for Bill). The highest density of CBs is found in the downshear-left quadrant, consistent with prior studies. The structure of the CBs is analyzed by comparing r-z composites of azimuths with CBs and azimuths without CBs, using composite figures and statistical comparisons. The CB composites show stronger radial inflow in the lowest 0-2 km, and stronger radial outflow from the eye to the eyewall in the 2-4 km layer. The CB composites also have stronger low-level vorticity than the non-CBs, potentially due to eyewall mesovortices. The analysis of individual CBs also confirms the importance of the eye-eyewall exchange in CB development, potentially by providing buoyancy, as parcel trajectories show that many parcels are flung outward from the eye and rapidly ascend in the CBs, with as much as 500 J/kg of CAPE along the parcel path. In addition, the location of radial convergence seems to play a key role in governing the radial location of CBs. Inner-core CBs seem to be associated with local convergence maxima in the eyewall, while CBs outside the radius of maximum winds (RMW) are associated with convergence maxima due to bands and/or secondary eyewalls. Analysis of intensity change in the simulations shows that there are more inner-core CBs during times when the TCs are intensifying, while weakening/steady times appear to be associated with more CBs outside the radius of maximum wind (RMW), consistent with observational studies and theoretical work. However, times when the TC has already been intensifying and continues to do so have more CBs than times when the TC has been weakening but then intensifies. This suggests that CB development may not always be predictive, but rather may sometimes occur as a result of ongoing intensification. On the other hand, rapid intensification (RI) in the simulations is found to be associated with an even higher density of CBs inside the RMW than slower intensification. Lag correlations between CBs and intensity are calculated to investigate the time of the intensity response to CB development. These calculations reveal a broad peak in correlation, with the CBs tending to lead pressure falls by 0-3 hours. These results confirm the notion that convective heating inside the RMW is favorable for intensification. The findings from this analysis show that eyewall CBs are driven by asymmetric dynamical processes in the inner-core region of TCs, both in and above the TC boundary layer. In addition, the relationship between CB development and intensity change is indeed positive, sometimes in a predictive sense, and at other times while intensity change is ongoing. / A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the Doctor of Philosophy. / Spring Semester 2016. / March 24, 2016. / Convection, Eye, Eyewall, Hurricane, Updraft, WRF / Includes bibliographical references. / Robert E. Hart, Professor Directing Dissertation; Irinel Chiorescu, University Representative; Mark Bourassa, Committee Member; Henry Fuelberg, Committee Member; Guosheng Liu, Committee Member; Robert Rogers, Committee Member.

The polar regions of the world constitute an important sector in the global energy balance. Among other effects responsible for the change in the sea-ice cover like ocean circulation and ice-albedo feedback, the cloud-radiation feedback also plays a vital role in modulation of the Arctic environment. However the annual cycle of the clouds is very poorly represented in current global circulation models. This study aimed to explore the atmospheric conditions in the Arctic on an unprecedented spatial coverage spanning 70°N to 80°N through the use of a merged data product, C3MData (derived from NASA's A-Train Series). The following three topics provide outline on how this dataset can be used to accomplish a detailed analysis of the Arctic environment and provide the modelling community with first information to update their models aimed at better forecasts. (1)The three properties of the Arctic climate system to be studied using the C3MData are sea-ice, clouds, and the atmospheric conditions. The first topic is to document the present states of the three properties and also their time evolutions or their seasonal cycles. (2)The second topic is aimed at the interactions or the feedbacks processes among the three properties. For example, the immediate alteration in the fluxes and the feedbacks arising from the change in the sea-ice cover is investigated. Seasonal and regional variations are also studied. (3)The third topics is aimed at the processes in native spatial resolution that drive or accompany with sea ice melting and sea ice growth. Using a composite approach based on a classification due to surface type, it is found that limitation of the water vapour influx from the surface due to change in phase at the surface featuring open oceans or marginal sea-ice cover to complete sea-ice cover is a major determinant in the modulation of the atmospheric moisture. The impact of the cloud-radiative effects in the Arctic is found to vary with sea-ice cover and seasonally. The effect of the marginal sea-ice cover becomes more and more pronounced in the winter. The seasonal variation of the dependence of the atmospheric moisture on the surface and the subsequent feedback effects is controlled by the atmospheric stability measured as a difference between the potential temperature at the surface and the 700hPa level. A regional analysis of the same suggests that most of the depiction of the variations observed is contributed from the North Atlantic region. / A Dissertation submitted to the Geophysical Fluid Dynamics Institute in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2016. / May 2, 2016. / C3MData, Cloud, Feedback, Interaction, Sea-Ice, Stabillity / Includes bibliographical references. / Ming Cai, Professor Directing Dissertation; Christopher Tam, University Representative; Allan Clarke, Committee Member; Guosheng Liu, Committee Member; Kevin Speer, Committee Member.

The sub-Antarctic Zone (SAZ) is a zone of vigorous vertical mixing in the Southern Ocean where it is difficult to obtain data for model validation on the turbulence conditions. In this study, a onedimensional configuration from the Nucleus for the European Modelling of the Ocean (NEMO) model was implemented in order to determine the sensitivity and turbulence response of an idealized SAZ water column. Various turbulence scheme parameterizations that are available for ocean models were tested. Furthermore, the number of vertical levels were varied in order to ascertain the sensitivity of the grid. The forcing data were obtained from various reanalyses (ERA-Interim, NASA, NCEP and JRA55) and were likewise tested. Different turbulence diagnostics and univariate indicators were chosen to ascertain the turbulence response and to analyse the energetics of the water column. It was found that using different reanalyses produced different tracer (salinity and temperature) results. Even though the results varied considerably, very high correlations were found for the potential energy anomaly between reanalyses and insignificant correlations were found for the other indicators. This suggested that it was a valuable descriptor which captured the buoyancy fluxes and wind stress information and can be efficiently used to assess the vertical turbulent state with data such as ARGO profiles. It was further found that for a single reanalysis, the turbulence schemes had produced similar results (with small variability and not to the extent as changing the reanalysis) for the turbulence diagnostics and univariate indicators. An important finding of an entrapped warm water parcel beneath cooler waters was found in simulation outputs as well as ARGO validation data. For realistic conditions observed from the ARGO floats, as the season progressed, there were no more instances of a warm water parcel. There was no reason however, to why there should not have been eddies passing by the region. In simulations, the warm water parcel persisted throughout the season for simulated data, likely causing the early stratification that affects ocean models in the SAZ. The stratification was found to have an approximate one month early onset observed from comparing the ARGO data profiles to simulated profiles. The Brunt Väisälä frequency, potential energy anomaly as well as the buoyancy flux were analysed and these diagnostics indicated that an approximate one month early stratification was found during November. It was likely that this false stratification signal may have influenced the summer stratification leading to a poor representation of the Mixed Layer Depth (MLD) and various other indicators. It was found that during the austral winter months, the model simulated comparable MLD's to the ARGO float data as well as theWinter Cruise data (obtained from the SA Agulhas II), capturing the winter dynamics well.

A combination of observational and numerical analyses is used to investigate hurricane boundary layer (BL) structure in the context of intensity change. These analyses refer to hurricanes in three modes of intensity change: intensifying (IN), steady-state (SS), and weakening (WE). Observations from global positioning system (GPS) dropwindsondes launched in Atlantic tropical cyclones between 1998 and 2015 are collected for compositing based on intensity change. After quality control and sorting, 3,091 dropwindsondes were composited---1,086 were released into IN hurricanes, 1,042 were released during WE phases, and the remaining 963 releases were associated with steady-state storms. In non-intensifying hurricanes, lower-tropospheric tangential winds were stronger than IN storms outside the radius of maximum winds (RMW), which suggests greater inertial stability I² outside the RMW in non-intensifying hurricanes. The BL radial inflow is of similar thickness across the three composites, and all composite groups have an inflow maximum situated at the RMW. Non-intensifying hurricanes are associated with stronger near-surface inflow outside the eyewall region, which implies more frictionally forced ascent out of the BL at radii outside the RMW. At greater radii, inflow layer Θₑ is relatively low in the WE composite, suggesting enhanced subsidence or downdrafts at those radii. High-resolution numerical case studies of Hurricane Irma in 2017 and Hurricane Earl in 2010 are used to check results found in the composite analysis and highlight BL azimuthal structure. The Weather Research and Forecasting Model for Advanced Research (WRF-ARW) is employed for these full-physics simulations. Irma's strong tangential winds were relatively confined to the RMW, leading to weak I² outside the eyewall. Aside from land interactions, Irma tended to steadily intensify, with an inflow maximum at the RMW and BL ascent isolated inward of the RMW. A brief WE period in Irma was associated with shear- and motion-induced asymmetry, whereby drier air was able to descend into the BL inflow near the RMW. Hurricane Earl had a broader tangential wind field, with high I² outside the eyewall. Earl's strong BL inflow spread over a large radial band, which was associated with widespread BL convergence and shallow ascent outside the RMW. During a prolonged and progressive decay in Earl's intensity, two regions of BL convergence became apparent: one inward of the RMW, and the other well outside the RMW. Descent of low-enthalpy air into the BL near the RMW occurred during Earl's WE phases. Despite shear and storm motion of comparable magnitude to Irma, asymmetries were more pronounced in Earl's BL. Earl's decline in intensity was also associated with strong low-level outflow in the upshear-right quadrant, which may have led to kinematic and thermodynamic evolution that promoted an outer region of BL convergence, as well as an inner-eyewall collapse and coincident secondary eyewall formation. / A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / 2019 / August 8, 2019. / Boundary, Cyclone, Hurricane, Intensity, Layer, Tropical / Includes bibliographical references. / Mark A. Bourassa, Professor Co-Directing Thesis; Robert E. Hart, Professor Co-Directing Thesis; Kristine Harper, University Representative; Henry E. Fuelberg, Committee Member; Vasubandhu Misra, Committee Member; Jeffrey Chagnon, Committee Member.

Droughts occurred over South Africa during the summer seasons of 2014/2015 and 2015/2016. At the same time, the Pacific Ocean was warmer than normal starting in 2014 and leading to the very strong 2015/2016 El Niño. The first objective of this study is to document the ocean and climate conditions that occurred during the summer seasons 2014/2015 and 2015/2016 in southern Africa. NCEP Reanalysis data is used to compute the monthly and seasonal scale composite mean and anomalies of large-scale circulations during the summer seasons of 2014/2015 and 2015/2016. Results obtained from the study suggest that some months of 2014/2015 and 2015/2016 were canonical to the effect of El Niño over southern Africa, but not all of them during the summer seasons were dry. The wetter than normal conditions in northeast South Africa, Botswana and Zimbabwe during December 2014 are unfamiliar for a canonical summer El Niño event over southern Africa. Anomalous cooler than normal Sea Surface Temperature (SST) occurred over the west coast and south coast during December 2014 and February 2015, while it is usually warmer during El Niño. Additionally, the colder than normal SST at the south coast during February 2016 and Namibian and West Coast during March 2016 does not mimic the canonical El Niño patterns. However, this did not influence the El Niño-like warmer seasonal SST average during 2015/2016. The lower than normal pressure anomalies over the subcontinent during December 2014 and January 2015 were not portraying a canonical El Niño pattern but the other months were. The seasonal larger than normal pressure at 500 hPa over the subcontinent was more typical of El Niño during summer 2015/2016 and acted to suppress rainfall. Secondly, the study uses the Standard Precipitation Index (SPI) at different time scales (3 months duration, 5 months duration and 17 months duration) to assess the severity of 2015/2016 summer drought compared to the other droughts of the 20th and 21st century (1921 to 2016) and to analyse the relationship between droughts and ENSO. The South African Weather Service (SAWS) rainfall data shows that KwaZulu-Natal was the only region within South Africa, to have the 2015/2016 as the strongest summer drought since 1921 but 2015/2016 was still one of the worst droughts on record in South Africa, especially at the 2 consecutive seasonal scales. In general, droughts are favored by El Niño and wetter conditions by La Niña, but the second strongest El Niño of 1997/1998 led to near normal rainfall over the north-eastern region at all time-scales. The SPI has proven to be very versatile, flexible and very effective to monitor the 2015/2016 summer drought in the complex South African rainfall regime. However, there was little difference between 3 months SPI at the end of February and 5 months SPI at the end of March. For South Africa, the summer rainfall 2015/2016 season had the fifth worst drought after El Niño related drought of 1982/1983 and 1991/1992 and the non-El Niño related droughts of 1967/1968 and 1944/1945. At the 17-month scale, an index that encompasses two summer seasons 2015/2016 was the third worst drought since summer 1921/1922 due to dry conditions in 2014/2015 and 2015/2016.

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