This thesis examines how early warning signals perform when tested on climate systems thought to exhibit future tipping point behaviour. A tipping point in a dynamical system is a large and sudden change to the state of the system, usually caused by changes in external forcing. This is due to the state the system occupies becoming unstable, causing the system to settle to a new stable state. In many cases, there is a degree of irreversibility once the tipping point has been passed, preventing the system from reverting back to its original state without a large reversal in forcing. Passing tipping points in climate systems, such as the Amazon rainforest or the Atlantic Meridional Overturning Circulation, is particularly dangerous as the effects of this will be globally felt. Fortunately there is potential for early warning signals, designed to warn that the system is approaching a tipping point. Generally, these early warning signals are based on analysis of the time series of the system, such as searching for ‘critical slowing down’, usually estimated by an increasing lag-1 autocorrelation (AR(1)). The idea here is that as a system’s state becomes less stable, it will start to react more sluggishly to short term perturbations. While early warning signals have been tested extensively in simple models and on palaeoclimate data, there has been very little research into how these behave in complex models and observed data. Here, early warning signals are tested on climate systems that show tipping point behaviour in general circulation models. Furthermore, it examines why early warning signals might fail in certain cases and provides prospect for more ‘system specific indicators’ based on properties of individual tipping elements. The thesis also examines how slowing down in a system might affect ecosystems that are being driven by it.

The eastern North Pacific Ocean has the highest density of tropical cyclone genesis events of any tropical basin in the world, and many of these systems form near land before moving westward. However, despite the level of tropical cyclone activity in this basin, and the proximity of the main genesis region to land, tropical cyclone behavior in the eastern North Pacific has been relatively unexplored. When synoptic conditions are favorable, moisture from northward-moving tropical cyclones can be advected into northern Mexico and the southwestern United States, often leading to the development of summertime thunderstorms during the North American monsoon season. An interaction with a mid-latitude trough produces the most rainfall, and the spatial variability of precipitation is greatly affected by the complex topography of the region. Moisture can be advected from a tropical cyclone around the subtropical ridge in place for much of the eastern North Pacific hurricane season and contribute to precipitation. This ridge, when it extends westward over the Pacific Ocean, can also prevent tropical cyclone moisture from impacting the southwestern United States. Northward-moving tropical cyclones often enter an environment with decreasing sea surface temperatures, increasing vertical wind shear, and meridional air temperature and moisture gradients. These key ingredients for extratropical transition are generally present in the eastern North Pacific, but the subtropical ridge prevents many named systems from moving northward, and only 9% of eastern North Pacific tropical cyclones from 1970 to 2011 complete ET according to cyclone phase space. However, over half of the systems that do not complete ET dissipate as cold core cyclones, a structural change that has yet to be explored in other tropical basins. It is difficult to estimate tropical cyclone intensity in a vast ocean area with few direct measurements available. The deviation angle variance technique, an objective method independent of the current techniques widely used today, was successfully applied to seven years of eastern North Pacific tropical cyclones. The RMS error of 13.5 kt for all seven years is comparable to the RMS errors found for other basins.

Approved for public release, distribution is unlimited / This thesis investigated the hypothesis that variations in tropical cyclone (TC) activity in the western North Pacific (WNP) may affect the teleconnection between the tropical WNP and North America. The teleconnection patterns of the 500 hPa geopotential height between a base point in the WNP (20 N 115 E) and a domain over North America (30 - 45 N, 70 -90 W) from 1951-2001 were examined. The 25 most active and the 25 least active TC years for two regions with the highest climatological average of TC activity, near the Philippines and Taiwan, respectively, were compared to determine if stronger teleconnection patterns occur during the more active years. For both regions, the correlation pattern is significant during active years and insignificant during inactive years, with the results based on TC activity in the Philippines region showing a larger difference. An analysis of 500 hPa mean winds showed weaker winds in the midlatitudes during active TC years when the teleconnection is stronger, which suggests that the teleconnection may consist mainly of Lau and Weng's (2000) zonally-elongated mode (Mode 1). Further cross correlations of the geopotential height and TC frequency parameters with the tropical eastern and western Pacific sea-surface temperatures (SST's) showed a significant correlation between TC activity and tropical eastern Pacific SST's, but the North America-WNP correlation is unlikely to be a result of a direct influence of SST's on the two regions. / Captain, United States Air Force

When a tropical cyclone (TC) is within 360 nautical miles of Kadena AB, the Air Force's Typhoon Determination (TYDET) program is used to estimate TC-induced winds expected at the base. Best-track data and Joint Typhoon Warning Center (JTWC) forecasts are used to evaluate systematic errors in TYDET. The largest contributors to errors in TYDET are a systematic error by which wind speeds are too large and the lack of size and symmetry parameters. To examine these parameters, best-track and forecasts are used to classify TCs as small or large and symmetric or asymmetric. A linear regression technique is then used to adjust TYDET forecasts based on the best-track and forecast position, size, and symmetry categories. Using independent data, over 65 percent of the overall cross-wind forecasts were improved and more than 60 percent of the cross-wind forecasts were improved when verifying conditions noted a cross-wind of 20 knots or greater. The effectiveness of the corrections and implications for TYDET forecasts are examined in relation to errors in forecast data used to initialize TYDET. A similar approach as developed here for the TYDET model at Kadena AB is proposed for other bases within the Pacific theater.

Sea level and current observations made over the Oregon continental shelf exhibit wavelike characteristics in a frequency band from approximately 0.15 to 0.45 cpd. In a narrow band around 0.22 cpd the current-sea level relationship is consistent with the predicted values for the first mode of Robinson's continental shelf waves. In addition, an interesting relationship exists between the form of the sea level-current coherency spectra and the arrangement of the maximum frequencies for the first three shelf wave modes. The currents were measured in 100 m of water about seven nautical miles off Depoe Bay, Oregon. Current meters were placed at 25, 50 and 75 m depth. The duration of the experiment was from 18 April 1968 until 11 September 1968. Due to some instrument failures a complete current data set for this period was not obtained. Simultaneous and continuous measurements of surface elevation and atmospheric pressure were also obtained at Newport, Oregon, a nearby coastal station. Shelf wave dispersion curves and eigenfunctions for the Oregon coastal profile are computed using a new numerical technique. These are compared with a low frequency (about 0.03 cpd to 0.75 cpd) spectral analysis of the current, sea level and atmospheric pressure records. The relative vertical uniformity of the currents, as a function of frequency, is examined. The longshore component of the current appears to be substantially more barotropic than the onshore-offshore component. / Graduation date: 1972

Graduation date: 1963

In this thesis I present the results of a comprehensive assessment of the Pacific-North American (PNA) teleconnection pattern in general circulation models (GCMs) and a regional climate model (RCM). The PNA teleconnection pattern is a quasi-stationary wave field over the North Pacific and North America that has long been recognized as a robust feature of Northern Hemisphere atmospheric circulation, and directly affects the interannual variability of North American temperature and precipitation. The teleconnection is evaluated under present (1950-2000) and future (2050-2100) climate in a coupled GCM (MPI/ECHAM5) and a high-resolution regional climate model (RegCM3). I further assess the PNA in 27 atmosphere-ocean GCMs and earth system models (ESMs) from the ongoing fifth phase of the Coupled Model Intercomparison Project (CMIP5). The National Centers for Environmental Prediction and Atmospheric Research (NCEP/NCAR) Reanalysis serves a quasi-observational baseline against which the models are evaluated. For each analysis, changes in the spatial and temporal patterns of the PNA spatial are assessed for both the present and future climates, and these changes are then related to changes in climate and surface hydrology in North America. Coupling the NCEP and ECHAM5 GCMs with RegCM3 is very successful in that the PNA is resolved in both models with little loss of information between the GCMs and RegCM3, thereby allowing an assessment of high-resolution climate with an inherent skill comparable to that of the global models. The value of the PNA index is generally independent of the method used to calculate it: three- and four-point modified linear pointwise calculations for both the RegCM3 and ECHAM5 model simulations produce very similar indices compared with each other, and compared with those extracted from a rotated principle component analysis (RPCA) which is also used to determine the PNA spatial pattern. The spatial pattern of the PNA teleconnection emerges as a leading mode of variability from the RPCA, although the strength of the teleconnections are consistently weaker than NCEP as defined by four main "centers of action". This discrepancy translates into the strength of the controls of the PNA on surface climate. Maps of the correlations between the GCM PNA indices and RCM surface climate variables are compared to the results from the NCEP/NCAR Reanalysis. I find that correlation patterns with temperature and precipitation are directly related to the positioning of the Aleutian low and Canadian high, the two main drivers of upper-atmospheric circulation in the PNA sector. The CMIP5 models vary significantly in their ability to simulate the quasi-observed features of the PNA teleconnections. The behavior of the models relative to NCEP is more definite than the trends within the models. Most models are unable to resolve the temporal variability of NCEP; however, on the other hand most of the models are able to capture the PNA as a low-frequency quasi-oscillation. Many of the models are unable to simulate the barotropic instability that initiates wave energy propagation through the 500-hPa geopotential height field, thereby leading to phase-locking and thus the positive and negative modes of PNA are indistinguishable. The behavior and the spatial patterns of the PNA throughout the 21st century are consistent with other projections of future climate change in that most models exhibit a lengthening of the eddy length scale and a poleward shift of the mid-latitude jet stream associated with polar amplification of greenhouse-gas driven global warming. Finally, my analyses underscore the robustness of multi-model means, suggesting that the cumulative results of multiple climate models outperform the results from individual models because ensemble means effectively cancel discrepancies and hereby expose only the most robust common features of the model runs. While ensembles provide better representation of the average climate, they potentially mask climate dynamics associated with inter-annual and longer time scales. Relying on ensemble means to limit model spread and uncertainties remains a necessity in using models to project future climate. / Graduation date: 2013

<p>This dissertation identifies and quantifies the impact of processes driving two biogeochemical phenomena of interest by considering them in the context of the large-scale circulation in which they occur. Both processes occur in the North Pacific transition zone (NPTZ), a basin-wide region near the subpolar-subtropical gyre boundary. </p><p>First, this work investigates the physical forcing behind the large seasonal variability in the location of the Transition zone chlorophyll front (TZCF). The TZCF is a persistent gradient in sea surface color that separates high chlorophyll waters to the north from low chlorophyll waters to the south. The chlorophyll front shifts seasonally by about 1000 km, oscillating between its southernmost winter latitude and its northernmost summer latitude. The forcing behind this seasonal signal is of interest because a number of migratory marine animals, both commercial and endangered, appear to track it.</p><p>This first study finds that vertical processes, traditionally viewed as controlling the dynamical supply of nutrients to surface waters, are insufficient to explain seasonal variations in nutrient supply to the transition zone. Instead, the wind-driven horizontal Ekman transport of nutrients, moving southward from the subpolar gyre into the northern reaches of the subtropical gyre, drives the southward migration of the TZCF. Such lateral transport of nitrate supports up to 40% of new primary productivity in the region annually, and nearly all of new primary productivity in the winter. </p><p>Second, this work investigates why the North Pacific transition zone waters are a notably strong sink for atmospheric carbon dioxide (CO2) on a mean annual basis, while seasonally they vary from a sink in the winter to a neutral to weak source in the summer. As the partial pressure of carbon dioxide (pCO2) in the surface mixed layer exerts primary control on the direction and magnitude of air-sea carbon exchange, this study quantifies the impact of processes regulating seawater pCO2: temperature, salinity, advection of dissolved inorganic carbon (DIC) and alkalinity (ALK), mixing of DIC and ALK, biology, and air-sea carbon flux.</p><p>Seasonal controls on pCO2 in the North Pacific transition zone differ from annual controls. Temperature effects dominate the seasonal signal, but are partially countered throughout the year by opposing processes. In spring and summer, biological drawdown partially offsets the increase in pCO2 due to warming waters; in fall and winter, the vertical entrainment of carbon moderates the decrease in pCO2 due to cooling waters. On a mean annual basis, air-sea carbon flux, biology, mixing, and advection all have a net impact on seawater pCO2. Though important seasonally, temperature has a small impact on pCO2 and air-sea carbon flux annually, accounting for only about 15-20% of oceanic carbon uptake through temperature-driven solubility changes.</p><p>This second study again finds an important role for lateral processes to play in regulating biogeochemical phenomena in the North Pacific transition zone. The ability of the region to uptake atmospheric carbon year after year is maintained by those processes exporting carbon from its surface waters: the vertical export of organic carbon to depth, and the lateral geostrophic advection of carbon out of the region. This lateral advection alone determines the location of the sink region: of the processes impacting seawater pCO2 on a mean annual basis, only the geostrophic divergence of DIC disproportionately lowers pCO2 in the transition zone latitudes, supporting greater atmospheric CO2 uptake here than in surrounding regions.</p><p>This dissertation identifies and quantifies processes driving biogeochemical features in the North Pacific transition zone, finding the large-scale circulation in the region plays a significant role in regulating these processes. The unique physical oceanographic characteristics of the NPTZ, and in particular the lateral transport, support biological and chemical attributes notably distinct from adjacent waters.</p> / Dissertation

A comprehensive analysis of midlatitude intraseasonal variability in extended integrations of General Circulation Models (GCMs) developed at the National Aeronautics and Space Administration Goddard Space Flight Center is conducted in the first part of this thesis. The model statistics, three-dimensional structure, and dynamical characteristics of storm tracks and large-scale weather regimes are diagnosed and directly compared to parallel observational analyses. Results indicate that several aspects of simulated intraseasonal phenomenon are linked to errors in the zonally asymmetric circulation. During the boreal winter, anomalously large meridional temperature gradients and enhanced zonal winds in the simulations are linked to suppressions in baroclinic wave activity in the models. As a result lower-frequency modes in the GCMs are more strongly driven by baroclinic dynamics. Nonetheless, the GCMs successfully reproduce the North Pacific midwinter suppression phenomenon. In an effort to provide insight into the North Pacific midwinter suppression, the second part of this work stratifies the boreal cool season into three stages, where the early and late (middle) stages approximately correspond to the seasonal maxima (minimum) in synoptic eddy activity that occurs over the North Pacific. Analyses using this unique approach reveal that during the midwinter suppression period, cyclonic perturbations entering the North Pacific storm track core from Asia are already deficient in magnitude compared to early and late winter stages. In both observations and model simulations, the North Pacific midwinter suppression feature is discovered to have a clear organized extension upstream into Siberia. Thus, the final portion of this thesis examines the causes for the midwinter suppression of upper tropospheric Rossby wave packets propagating across Asia prior to entering the North Pacific storm track. The study unveils an increased sensitivity of synoptic-scale wave packets with a large-scale, upper-tropospheric flow pattern over Siberia during midwinter. This interaction with the large-scale pattern over Siberia results in a general dampening of synoptic eddy amplitudes over Asia, which is proposed to be the contributing factor to the North Pacific midwinter suppression phenomenon.

In the North Pacific Subtropical Gyre, a nearly-annual phytoplankton bloom forms near the subtropical front at ~30° N. Mixed communities of nitrogen-fixing diatom symbioses (diatom-diazotroph associations) increase 10²-10³ fold in these blooms. In July 2008 (31.46˚N 140.49˚W) and August 2009 (25.18 °N 154 °W), two blooms were sampled to determine diatom-diazotroph association species composition, physical, and chemical characteristics of the water column. In both 2008 and 2009, the dominant diatom-diazotroph association was the Hemiaulus hauckii-Richelia intracellularis symbiosis. The 2009 subtropical front bloom was missed; however, another bloom closer to Hawaii was sampled where diatom-diazotroph association abundance was 10-fold lower (10² cells Lˉ¹) than 2008 despite surface chlorophyll a values that were 3 times greater. Both blooms showed substantial changes in phytoplankton size structure with the >10 μm size chlorophyll a fraction increasing from 10 to 40 % in 2008. In the 2009 bloom, the non-symbiotic pennate diatom Mastogloia woodiana numerically dominated (>150,000 cells Lˉ¹) and formed aggregates that resulted in substantially higher % of netplankton chlorophyll a fractions. Summer open ocean blooms from the two years share a common trend of Hemiaulus dominance of the diatom-diazotroph association population and size structure changes. However, non-symbiotic species can dominate the overall bloom, and diatom-diazotroph association species may not be responsible for the chlorophyll a increase. These two years may represent different types of blooms or temporal changes within summer diatom blooms. The increased biomass in the larger-size fraction suggests these blooms are potential sites for carbon export from the surface layer. / text