Widespread droughts can have a significant impact on western United States streamflow, but the causes of these events are not fully understood. This dissertation examines streamflow from multiple western US basins and establishes the robust, leading modes of variability in interannual streamflow throughout the past century. I show that approximately 50% of this variability is associated with spatially widespread streamflow anomalies that are statistically independent from streamflow's response to the El Niño-Southern Oscillation (ENSO). The ENSO-teleconnection accounts for approximately 25% of the interannual variability in streamflow, across this network. These atmospheric circulation anomalies associated with the most spatially widespread variability are associated with the Aleutian low and the persistent coastal atmospheric ridge in the Pacific Northwest. I use a watershed segmentation algorithm to explicitly track the position and intensity of these features and compare their variability to the multi-basin streamflow variability. Results show that latitudinal shifts in the coastal atmospheric ridge are more strongly associated with streamflow's north-south dipole response to ENSO variability while more spatially widespread anomalies in streamflow most strongly relate to seasonal changes in the coastal ridge intensity. This likely reflects persistent coastal ridge blocking of cool-season precipitation into western US river basins. I utilize the 35 model runs of the Community Earth System Model Large Ensemble (CESMLE) to determine whether the model ensemble simulates the anomalously strong coastal ridges and extreme widespread wintertime precipitation anomalies found in the observation record. Though there is considerable bias in the CESMLE, the CESMLE runs simulate extremely widespread dry precipitation anomalies with a frequency of approximately one extreme event per century during the historical simulations (1920 - 2005). These extremely widespread dry events correspond significantly with anomalously intense coastal atmospheric ridges. The results from these three papers connect widespread interannual streamflow anomalies in the western US - and especially extremely widespread streamflow droughts - with semi-permanent atmospheric ridge anomalies near the coastal Pacific Northwest. This is important to western US water managers because these widespread events appear to have been a robust feature of the past century. The semi-permanent atmospheric features associated with these widespread dry streamflow anomalies are projected to change position significantly in the next century as a response to global climate change. This may change widespread streamflow anomaly characteristic in the western US, though my results do not show evidence of these changes within the instrument record of last century.

Dissertation submitted to the Faculty of Science, University of the Witwatersraild, Johannesburg for the Degree of Master of Science. / Six general circulation model simulations of present-day southern African climate are assessed, Each of these models are early-generation equilibrium climate models linked to simple mixed-slab oceans. Simulations of surface air temperature over the subcontinent are sensitive to the grid-scale parameterisation of convection in summer. At high latitudes, large simulation errors are caused by errors in the specification of sea-ice albedo feedbacks. Increased spatial resolution and the inclusion of a gravity wave drag term in the momentum equations results in a markedly-improved simulated mean sea level pressure distribution. Tho models successfully simulate the pattern of rainfall seasonality over the Subcontinent, although grid-point simulation of precipitation is unreliable. Treatment of convection, cloud radiative feedbacks and the oceans by this generation of models is simplistic, and consequently there is a large degree of uncertainty associated with predictions of future climate under doubled-carbon dioxide conditions. For this reason, more reliable estimates of future conditions will be achieved using only those models which reproduce present climate most accurately. Early-generation general circulation models suggest a warming of 4°C to 5°C for the southern African region as a whole throughout the year. Over the subcontinent, warming is expected to be least in the tropics, and greatest in the dry subtropical regions in winter. Estimated changes in mean sea level pressure indicate a southward shlft of all pressure systems, with a weakening of the subtropical high pressure belt and mid-latitude westerlies. Little agreement exists between the models concerning predictions of regional precipitation change. However, broad scale changes in precipitation patterns are in accordance with predicted circulation changes over the subcontinent. Generally wetter conditions may be expected in the tropics throughout the year and over the summer rainfall region during summer. Decreased winter rainfall may be expected over the winter rainfall region of the south-western Cape. However, estimated precipitation changes are grid-point specific and therefore must riot be over-interpreted. The present climate validation has resulted in more reliable estimates of future conditions for the southern African region. This approach should be extended to recent slrnulations which include more comprehensive treatment of important physical processes. / Andrew Chakane 2018

Due to the wide range of physical scales involved, clouds cannot be fully resolved in models of the global climate, and so are parameterized. The resultant model deficiencies in simulating important cloud processes within the current climate are strongly implicated in the large uncertainty in model predictions of future climate changes. Previous work has highlighted the uncertainties in predictions of future climate related to thermodynamic cloud changes, understanding of which requires detailed observations of small-scale cloud microphysics. In this thesis, we argue that understanding the linkages between mid-latitude clouds and the general circulation of the atmosphere can advance efforts to constrain their response to climate forcing. We make this argument with three main methods of analysis: 1) observations, 2) state-of-the-art general circulation models, and 3) experiments with an idealized model of the global climate. First, we perform a comprehensive investigation of the observed inter-annual relationships between clouds, their radiative effects, and key indices of the large-scale atmospheric circulation. Using reanalysis data and satellite retrievals, we find a relationship between the edge of the Hadley circulation (HC) and the high cloud field that is largely robust against season and ocean basin. In contrast, shifts of the mid-latitude eddy-driven jet latitude, which had been the focus of previous work on the coupling between mid-latitude clouds and circulation, only correlate with the high cloud field in the wintertime North Atlantic. In that season and basin, poleward shifts of the circulation are associated with anomalous shortwave cloud radiative warming. During all seasons in the Southern Hemisphere, however, poleward shifts of the circulation are associated with anomalous shortwave cloud radiative cooling. Second, we examine Coupled Model Intercomparison Project phase 5 (CMIP5) model output to evaluate the models' simulation of the inter-annual co-variability between the Southern Hemisphere HC extent and the shortwave cloud radiative effect. In the control climate runs, during years when the HC edge is anomalously poleward, most models reduce their cloud cover in the lower mid-latitudes (approximately 30$^\circ$S - 45$^\circ$S) and allow more sunlight to warm the region, although we find no such shortwave radiative warming in observations. We correlate these biases in the co-variability between the HC extent and shortwave cloud radiative anomalies with model biases in the climatological HC extent. Models whose climatological HCs are unrealistically equatorward compared to the observations exhibit weaker climatological subsidence in the lower mid-latitudes and exhibit larger increases in subsidence there with poleward HC extent shifts than models with more realistic climatological HCs. This behavior, based on control climate variability, has important implications for the model response to forcing. In 4$\times$CO$_2$-forced runs, models with unrealistically equatorward HCs in the control climatology exhibit a stronger shortwave cloud radiative warming response in the lower mid-latitudes and tend to have larger values of equilibrium climate sensitivity than models with more realistic HCs in the control climatology. The above correlative analyses suggest that uncertainty in the linkages between mid-latitude clouds and the general circulation of the atmosphere contributes to uncertainty in the model response to forcing. Finally, we use simulations of the global climate in an idealized aquaplanet model to show that the biases in the climatological Southern Hemisphere circulation do indeed contribute to much of the model spread in the cloud-circulation coupling. We find that for the same 1$^\circ$ latitude poleward shift, simulations with narrower climatological HCs exhibit stronger mid-latitude shortwave cloud radiative warming anomalies than simulations with wider climatological HCs. The shortwave cloud radiative warming anomalies result predominantly from a subsidence warming of the planetary boundary layer, which decreases low-level cloud fraction and is stronger for narrower HCs because of a tighter mean meridional circulation. A comparison of the spread across aquaplanet simulations with that across CMIP5 models suggests that about half of the model uncertainty in the mid-latitude cloud-circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the boundary layer, and thus can be removed by improving the representation of the climatological circulation in models. Therefore, a more realistic representation of the Hadley circulation in models can improve their representation of the linkage between mid-latitude clouds and the atmospheric circulation in the current climate and increase overall confidence in predictions of future climate.

Mineral dust both influences and is influenced by climate on many timescales, from seconds to epochs. Its complex interactions with the climate system are still being unraveled. For example, dust fluxes change in tandem with other records of past changes in climate, and dust source is often presumed to change as well, in response to shifts in climate conditions in source regions; changes in wind regimes; or changes in atmospheric transport pathways. In this work, I investigate dust records from the Southern Hemisphere from ice core and marine sediment core climate archives, looking at both flux and provenance in order elucidate the conditions that allowed for those particles to travel from source to sink. Using multiple radiogenic isotope systems as tracers (87Sr/86Sr, εNd(0), 206Pb/207Pb, and 208Pb/207Pb), I geochemically “fingerprint” of dust particles from Southern Hemisphere climate archives over the last glacial cycle. I compare the dust fingerprints to potential source areas (PSA’s) from across the Southern Hemisphere in order to identify the sources of dust found in the WAIS Divide and Taylor Glacier ice cores from West Antarctica, as well as from marine sediment core ELT39.75 in the Tasman Sea. I use endmember mixing theory to determine the relative contribution of different sources to the climate archives over time. In West Antarctica, I geochemically identify specific local volcanoes from Marie Byrd Land as significant particle contributors to the WAIS Divide ice core during the previous glacial period. In the Tasman Sea, I identify a specific region of southeastern Australia as primary the dust source over the past glacial cycle, with the source remaining constant across glacial-interglacial climate transitions. This clarifies that the “fingerprint” of Australian dust is relatively invariant over time and allows a single Australian signature to be used as an endmember for identifying dust provenance in climate archives downwind. I also identify the dust sources in the WAIS Divide during the Last Glacial Maximum and through the early deglacial, identifying southern South America as the predominant source during cold stages. WAIS Divide and Taylor Glacier dust records do not record dust source changes across millennial-scale climate events, suggesting that a) the source regions did not change, b) the transport pathways remained pinned, or c) the proxy is not sensitive to changes in these variables. Contributions from local volcanoes are also inferred from the WAIS dust record using mixing theory. In summary, I find that the radiogenic isotope fingerprint of dust samples from the archives analyzed show subtle or no changes in source over climate transitions, and therefore the strategy of dust particles as a tracer of past atmospheric circulation pathways should be approached cautiously.

The relationship between the outgoing long-wave radiation (OLR) field over the tropical Pacific and wintertime monthly precipitation for the western U. S. is investigated, using the mid-latitude upper-air circulation as an intermediary. Principal components (PC's) of the 500mb monthly averaged height field over the NE Pacific and western North America are compared with those of the monthly tropical Pacific OLR field. It is found that, of the first 6 PC's of the height field, five are correlated significantly with the first 3 OLR field PC's at lags of between two and six months. Canonical correlations between the two sets of PC's are greatest at a lag of four months and are highly significant. When stratified by different levels of the OLR field PC's, the separations between means of the height field PC's are highly significant as well. Differing distributions of the height field PC ensemble are also found to be associated with different OLR field PC levels. The relationship between the 500mb height field and concurrent western district precipitation is examined. Using a hybrid model including both linear statistical and non-linear physical components it is found that considerably more of the variance in the precipitation can be explained by that of the height field alone than when the precipitation is inferred directly from a linear statistical model. A set of reconstructed height field PC's is predicted from OLR values based on the height field/OLR stratification associations compiled for a period separate from that of the forecast. Applied to the precipitation model, this results in predicted western district precipitation which is better correlated with the observations than is the equivalent precipitation forecast from the linear statistical relationship of precipitation to the Southern Oscillation Index. / Graduation date: 1995

Data from the GARP Atlantic Tropical Experiment (GATE) was analysed in an exploratory manner to discover the characteristics of mesoscale organization of convection and it's relationship to large-scale wind profiles. Automated methods were developed to identify the convective cells and their linear organization. These automated methods use a median high-pass filter to identify enhanced cells and a simple pattern recognition technique to ascertain the linear organization between them. Due to the simplified nature of the algorithm, the whole data set of the 21 day period from the phase 3 of GATE was processed in an economical manner obtaining a large data base which was used in the investigation of clusters and other associated phenomena. The mesoscale organization of convective cells and the widespread areas of lighter precipitation associated with them showed expected characteristics and compared satisfactorily with previous results. A large fraction of the rainfall (64%) fell from the widespread area. The total precipitation had a correlation of 0.94 with the fractional area of the widespread and a correlation of 0.89 with the fractional area of the clusters. The widespread precipitation had a correlation index of 0.97 with it's fractional area and the cluster precipitation had a strong linear relationship to it's area with a correlation of 0.99. These factors argue well for the parameterization of rainfall rate in tropical regions to a high accuracy by the area covered by organized convective cells and widespread areas. It was also seen that there was a good correlation with the number of clusters and number of cores with the total precipitation rate in a given area. These factors create a strong argument for identifying mesoscale systems consisting of convective cells and widespread precipitation as basic units of precipitation in tropical regions, having a characteristic life cycle of their own. The widespread and total precipitation showed very good correlation with upper-level vertical motion. Clusters tended to align parallel with the horizontal low-level wind shear and the degree of alignment appears to depend on the strength of the wind shear. / Graduation date: 1991

In order to understand the atmospheric branch of the earth's hydrologic cycle on the global scale, an atmospheric moisture balance is diagnostically analyzed from the January and July data of the OSU atmospheric general circulation model, which has been integrated for thirty-nine months of simulation with seasonally-varying sea-surface temperature and solar insolation. The model hydrologic processes analyzed for the balance include the surface evaporation, the precipitation by large-scale and cumulus condensation, the vertical transport by large-scale and cumulus mass fluxes, and the horizontal transport of water vapor. The large-scale transports include the contributions from the standing and transient components of motion. Also analyzed are the potential and stream functions of horizontal transport, and the statistics of seasonal and interannual variabilities of the global and hemispheric effects of the hydrologic processes. As a result of these analyses, the hydrologic cycle is constructed and understood for both January and July of the model. Large-scale vertical transport moistens the upper layer; the standing and transient motions contribute mostly in the tropics and higher latitudes, respectively. Large-scale horizontal transport moistens the continental atmosphere except for the relatively small transport from the continents to the oceans by the standing motion in the upper layer; the runoff occurs in the model to balance the marine transport but seasonal trends exist such that snow assumulates during January and melts during July on the global average. Cumulus convection drys not only the lower layer but also the upper layer of the model, and the penetrating cumuli are a major mechanism of maritime precipitation, whereas the large-scale condensation and penetrating cumuli have the dominating effect on the continental precipitation during January and July, respectively. The seasonal precipitation over the Northern Hemisphere continents concurs with strong surface evaporation in summer and also with strong cyclonic activity in winter. Comparison with other models and observational data indicates that the model reproduced some basic features of the atmospheric branch of the hydrologic cycle and its seasonal variation. The intense evaporation (≥ 5 mm day⁻¹) over the Pacific and Atlantic oceans and the rain belts in the tropics are well simulated for both January and July. The poleward transport in the northern middle and high latitudes is in good agreement with observations. The maximum toward-thermal-equator transport in the tropics occurs, however, at the geographic equator for both January and July, indicating that these maxima are about 5 degrees of latitude closer to the seasonal thermal equator than the observed maxima. Nevertheless the global statistics of the model atmosphere are not significantly different from that of the real atmosphere. Among others, we mention the following common features of the January and July moisture balances in the present model. Most precipitation of penetrating convection occurs in regions of strong surface evaporation even though some occurs in the moisture convergence zones where most of heavy mid-level convection is located. In the regions of intense penetrating convection, however, the standing part of surface evaporation is much larger in magnitude than the negative transient part which is essentially due to the positive correlation between the turbulence intensity and surface humidity over wet surfaces. Moreover, the horizontal structure of the standing part conforms to that of the standing vapor pressure difference between the air and the underlying surface. A strategy for further studies is recommended on the basis of our understanding of these features. / Graduation date: 1982

Using rotated principal component analysis, 200mb wind data from the First GARP Global Experiment are analyzed in the East Asia winter monsoon region. It is found that there exist slowly eastward-moving disturbances with a wind-speed maximum embedded in the jet stream (jet streak). The implied vertical circulation that is transverse to the jet streak is thermally direct in the entrance region of jet streak and thermally indirect in the exit region. The Lagrangian momentum equation for transient part of the flow indicates that the magnitude of westerly acceleration of an air parcel moving through the jet streak is related to the Coriolis acceleration of the transient ageostrophic flow, the advection of mean momentum by transient flow and momentum conversion between mean and transient flow due to transient eddies. Estimates of terms in this equation in the entrance region of jet streak suggest that the increase of westerly momentum due to the Coriolis acceleration of the ageostrophic flow is mostly balanced by the decrease due to the mean easterly momentum advection by the transient jet-streak flow. Thus, the confluence theory of Namias and Clapp (1949) appears to be valid for the slowly moving jet streaks in the East Asia jet stream. / Graduation date: 1989

Graduation date: 1982

Graduation date: 1989