En Asie du Sud-Est, la dynamique côtière est régie par un équilibre spécial entre les moussons du nord-est et sud-ouest. Dans un contexte de l'élévation du niveau de la mer et d’un changement climatique avéré, les régimes de la mousson ont commencé à évoluer et à modifier les équilibres hydro-morpho-sédimentaires sensibles au niveau des espaces littoraux. Cette situation engendre des phénomènes d'érosion le long du littoral, notamment le littoral NE de la Malaisie. Cependant, les autorités publiques ont tenté d'atténuer les problèmes d'érosion par la construction d’ouvrages de défense côtière. Ces derniers peuvent affecter le régime des houles, la circulation hydrodynamique et le transport des sédiments, réduisant ainsi la capacité du rivage à répondre aux facteurs du forçage naturels (régime de la double saison de mousson) et à fragmenter l'espace côtier. La présente thèse explore les problèmes des phénomènes d'érosion, l'évolution du littoral et la morphodynamique des plages le long du rivage de Kuala Terengganu sur la côte Est de la Malaisie péninsulaire par rapport aux facteurs naturels et anthropiques. Cette étude a été menée en trois phases pour aborder les sujets suivants: i) évolution long terme du trait de côte du littoral NE de la Malaisie de 2006 à 2014; ii) évolution moyen terme du littoral (suivi mensuel) de juillet 2013 à juin 2015 et; iii) suivi court terme à travers des campagnes de mesures hautes fréquences (mesures topographiques, hydrodynamiques et de transport des sédiments) sur quelques sites du littoral NE de la Malaisie pendant les moussons nord-est et sud-ouest. Sur la base d'un modèle morphodynamique pour simuler les processus saisonniers, il est apparu que l'érosion domine la plupart des stations suivies sur le littoral pendant la période de la mousson du nord-est, tandis que l'accrétion ou la recouvrement sédimentaire des plages sont observés pendant les moussons du sud-ouest. / In Southeast Asia, coastal dynamics are governed by a special equilibrium between northeast an southwest monsoons. In the context of sea-level rise and climate change, the monsoon regimes create an adaptation of the coastal dynamics. This situation gives rise to erosion phenomena along the coastline. However, public authorities have attempted to mitigate the problems of erosion by the construction of coastal defence structures. However, artificial structures can affect the wave regime, hydrodynamic circulation and sediment transport, thus reducing the ability of the shoreline to respond to natural forcing factors (such as with double monsoon season regimes) and also fragmenting the coastal space. The present thesis explores the problems of erosional phenomena, shoreline evolution and beach morphodynamics along the Kuala Terengganu shoreline on the East Coast of Peninsular Malaysia, with the aim of understanding the natural versus anthropogenic factors. This study was conducted in three phases to address the following topics: i) shoreline evolution from 2006 to 2014; ii) mid-term surveys (bi-monthly) involving data collection from July 2013 until June 2015 and; iii) short-term surveys (twice daily) with data collection (Topographic, hydrodyanamic and sediment transport measurments) during northeast and southwest monsoons. Based on a morphodynamic model for simulating seasonal processes, erosion is found to dominate most of the stations during northeast monsoons, while accretion or beach recovery is observed during southwest monsoons.

abstract: The North American Monsoon (NAM) is a late summer increase in precipitation fundamentally caused by a wind shift that is evident in the southwestern United States and northwest Mexico from approximately June-August. Increased precipitation during these months bring an increased regional threat from heavy rains, blowing dust, and damaging storms. (Adams and Comrie 1997). Researchers in Phoenix, AZ theorized that using surface dewpoint measurements was an objective way to officially mark the start of the NAM in Phoenix, AZ (and Tucson, AZ). Specifically, they used three consecutive days at or above a certain dewpoint temperature (Franjevic 2017). The justification for this method was developed by Reitan (1957) who established that 25.4mm (1.00”) of integrated precipitable water (IPW) was a sufficient threshold to create storm activity in the NAM region. He also determined (Reitan 1963) that a strong correlation existed between (IPW) and surface dewpoint (Td), whereas, Td could be used as a proxy to determine IPW. I hypothesize that the correlation coefficients between IPW and Td will be greatest when using seasonal mean averages of IPW and Td, and they will decrease with shortened mean timescales (from seasonal to three-days). Second, I hypothesize that there is a unique relationship between IPW/Td that may signal monsoon onset. To conduct this study, I used the North American Regional Reanalysis (NARR) dataset (1979-2015). For ten locations in the Southwest, I conducted a series of statistical analyses between IPW, Td, and accumulated precipitation. I determined that there is a correlation between the two as set forth by Reitan (1963) as well as (Benwell 1965; Smith 1966; Ojo 1970). However, from the results I concluded this relationship is highly variable, spatially and temporally. Additionally, when comparing the three-hour, three-day, and the weekly mean measurements, I can conclude that, for my study, timescale averaging did enhance the IPW/Td relationship from three-hour to weekly as expected. The temporal and spatial evolution of the IPW/Td correlation as presented in this thesis may provide a framework for future research that reevaluates the NAM’s domain and the associated methods for determining its onset. / Dissertation/Thesis / Masters Thesis Geography 2017

The representation of aerosol processes and the skill in simulating the Asian summer monsoon vary widely across climate models. Yet, for the second half of the twentieth century, the models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) show a robust decrease of average precipitation in the South and Southeast Asian (SSEA) continental region due to the increase of anthropogenic aerosols. When taking into account anthropogenic aerosols as well as greenhouse gases (GHGs), the 15 CMIP5 models considered in this study yield an average June–September precipitation least squares linear trend of −0.20 ± 0.20mm d−1 (50 years)−1, or −2.9%, for all land points in the SSEA region (taken from 75 to 120◦E and 5 to 30◦N) in the years from 1950 to 1999 (multimodel average ± one standard deviation) in spite of an increase in the water vapor path of +0.99 ± 0.65 kg m−2 (50 years)−1 (+2.5%). This negative precipitation trend differs markedly from the positive precipitation trend of +0.29 ± 0.14mm d−1 (50 years)−1, or +4.1%, which is computed for GHG forcing only. Taking into account aerosols both decreases the water vapor path and slows down the monsoon circulation as suggested by several previous studies. At smaller scales, however, internal variability makes attributing observed precipitation changes to anthropogenic aerosols more difficult. Over Northern Central India (NCI), the spread between precipitation trends from individual model realizations is generally comparable in magnitude to simulated changes due to aerosols, and the model results suggest that the observed drying in NCI might in part be explained by internal variability.

The Indian Summer Monsoon (ISM) plays a vital role in the livelihoods and economy of those living on the Indian subcontinent, including the small, mountainous country of Bhutan. The ISM fluctuates over varying temporal scales and its variability is related to many internal and external factors including the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). In 2015, a Super El Niño occurred in the tropical Pacific alongside a positive IOD in the Indian Ocean and was followed in 2016 by a simultaneous La Niña and negative IOD. These events had worldwide repercussions. However, it is unclear how the ISM was affected during this time, both at a regional scale over the whole ISM area and at a local scale over Bhutan. First, an evaluation of data products comparing ERA5 reanalysis, TRMM and GPM satellite, and GPCC precipitation products against weather station measurements from Bhutan, showed that ERA5 reanalysis was the most suitable product to investigate ISM change in these two years. Using the reanalysis datasets, it was shown that there was disruption to the ISM during this period, with a late onset of the monsoon in 2015, a shifted monsoon flow in July 2015 and in August 2016 and a late withdrawal in 2016. However, this resulted in neither a monsoon surplus nor deficit across both years but instead large spatial-temporal variability. It is possible to attribute some of the regional scale changes to the ENSO and IOD events, but the expected impact of a simultaneous ENSO and IOD events are not recognisable. This may be due to a supposed weakening of the ENSO/ISM relationship and it is likely that 2015/16 monsoon disruption was driven by a combination of factors alongside ENSO and the IOD, including varying boundary conditions, the Pacific Decadal Oscillation, the Atlantic Multi-decadal Oscillation, and more. At a local scale, the intricate topography and orographic processes ongoing within Bhutan further amplified or dampened the already altered ISM. Whilst ENSO and IOD driven monsoon variability can be recognised at a regional scale, a direct link between ENSO and IOD activity and changes to the monsoon at a local scale over Bhutan is hard to distinguish. It is unknown how the ISM, ENSO, and the IOD will evolve under a future changing climate and therefore this presents a concern to Bhutan with its inherent vulnerability to monsoon variability.

The Miocene Lincang leaf assemblage is used in this paper as proxy data to reconstruct the palaeoclimate of southwestern Yunnan (SW China) and the evolution of monsoon intensity. Three quantitative methods were chosen for this reconstruction, i.e. Leaf Margin Analysis (LMA), Climate Leaf Analysis Multivariate Program (CLAMP), and the Coexistence Approach (CA). These methods, however, yield inconsistent results, particularly for the precipitation, as also shown in European and other East Asian Cenozoic floras. The wide range of the reconstructed climatic parameters includes the Mean Annual Temperature (MAT) of 18.5-24.7°C and the Mean Annual Precipitation (MAP) of 1213-3711. mm. Compared with the modern Lincang climate (MAT, 17.3°C; MAP, 1178.7. mm), the Miocene climate is slightly warmer, wetter and has a higher temperature seasonality. A detailed comparison on the palaeoclimatic variables with the coeval Late Miocene Xiaolongtan flora from the eastern part of Yunnan allows us to investigate the development and interactions of both South Asian and East Asian monsoons during the Late Miocene in southwest China, now under strong influence of these monsoon systems. Our results suggest that the monsoon climate has already been established in southwest Yunnan during the Late Miocene. Furthermore, our results support that both Southeast Asian and East Asian monsoons co-occurred in Yunnan during the Late Miocene.

The North American Monsoon System (NAMS) is a dominant factor in climate in the southwestern United States and northwestern Mexico. Despite the influence of the NAMS and the intense research efforts it receives, its predictability, its variability, and the details of its influence on the environment are not well understood. This dissertation is comprised of three papers, which collectively address these three aspects of this complex climate phenomenon through an examination of various data and analyses at multiple spatial and temporal scales, while focusing on impacts in southern Arizona. In the first paper, a modified definition of the NAMS is established to delineate dates for monsoon onset, bursts, breaks, and retreat. The results are applied to an atmospheric compositing study in the second paper and to an applied study of monsoon-wildland fire relationships in the third paper. In the second paper, geopotential height patterns that affect moisture advection are identified. Onset, retreat, and break timing and duration are impacted by shifts in the latitude of the mid-level anticyclone and by lower-level gradients and contour orientation. Analyses in the third paper reveal the some of the complex effects of monsoon onset, variations in break timing and duration, and monsoon retreat on fire occurrence. This research contributes to the current knowledge of the NAMS in general and to the specific regional impacts of the monsoon. The results can (1) improve meteorological forecasts through the recognition of synoptic and sub-synoptic patterns related to the NAMS and (2) help fire managers by expanding the current understanding of the regional controls of wildland fire.

No description available.

Arctic geoengineering wherein sunlight absorption is reduced only in the Arctic has been suggested as a remedial measure to counteract the on-going rapid climate change in the Arctic. Several modelling studies have shown that Arctic geoengineering can minimize Arctic warming but will shift the Inter-tropical Convergence Zone (ITCZ) southward, unless offset by comparable geoengineering in the Southern Hemisphere. In this study, we investigate and quantify the implications of this ITCZ shift due to Arctic geoengineering for the global monsoon regions using the Community Atmosphere Model version 4 coupled to a slab ocean model. A doubling of CO2 from pre-industrial levels leads to a warming of ~ 6 K in the Arctic region and precipitation in the monsoon regions increases by up to ~15 %. In our Arctic geoengineering simulation which illustrates a plausible latitudinal distribution of the reduction in sunlight, an addition of sulfate aerosols (11 Mt) in the Arctic stratosphere nearly offsets the Arctic warming due to CO2 doubling but this shifts the ITCZ southward by ~1.5⁰ relative to the pre-industrial climate. The combined effect from this shift and the residual CO2-induced climate change in the tropics is a decrease/increase in annual mean precipitation in the Northern Hemisphere /Southern Hemisphere monsoon regions by up to -12/+17%. Polar geoengineering where sulfate aerosols are prescribed in both the Arctic (10 Mt) and Antarctic (8 Mt) nearly offsets the ITCZ shift due to Arctic geoengineering, but there is still a residual precipitation increase (up to 7 %) in most monsoon regions associated with the residual CO2 induced warming in the tropics. The ITCZ shift due to our Global geoengineering simulation, where aerosols (20 Mt) are prescribed uniformly around the globe, is much smaller and the precipitation changes in most monsoon regions are within ±2 % as the residual CO2-induced warming in the tropics is also much less than in Arctic and Polar geoengineering. Further, global geoengineering nearly offsets the Arctic warming. Based on our results we infer that Arctic geoengineering leads to ITCZ shift and leaves residual CO2 induced warming in the tropics resulting in substantial precipitation changes in the monsoon regions.

Summer monsoon current (SMC) in the north Indian Ocean (NIO) is an open ocean current that flows eastward and enter into Bay of Bengal (BoB) during southwest monsoon (June–September). South of Sri Lanka, the SMC turns north-eastward instead of following eastward course and feeds into the BoB. Understanding the dynamics of SMC is crucial to understanding the interaction between Arabian sea (AS) and BoB. In the year 2009, the current moved north-eastward, meandered and then finally terminated into the southeast BoB. The northward bend of the current southeast of Sri Lanka has been attributed to the interaction of the eastward SMC with Rossby waves radiated from eastern boundary. An anticyclonic vortex formed right of the north-eastward meandering current which was associated with significantly high speeds. Reasons behind the unusually high speeds of SMC in this region remain unknown. Processes involved in the interaction of eddies with eastward SMC, the meandering of the current and its ultimate termination in southeast BoB are also not understood. Our study investigates the evolution, intensfication and meandering of SMC around Sri Lanka using an Indian Ocean general circulation model (MOM4p1) simulation for the year 2009. The model simulation, when compared with observational data OSCAR, showed good agreement. The study also explores the role of local and remote forcing in modulating the dynamics of SMC in the region. An eddy kinetic energy budget analysis for the region was performed which indicates the region to be a zone of significant eddy activity. Both barotropic, baroclinic instabilities were found to be the dominant mechanisms behind the generation of eddies. Based on eddy energetic analysis, the evolution of SMC was classified into stages i ii of onset, intensification, anticyclonic bend, anticyclonic vortices formation and meandering. Effect of eddies on mean flow were studied with the help of a transformed Eulerian mean (TEM) approach under quasi-geostrophic approximation. Eddy potential vorticity fluxes appearing in the TEM momentum equation and eddy enstrophy decay, divergence of eddy enstrophy advection from eddy enstrophy equation, helped to un-derstand when, where and how the eddies tended to drive the mean flow. Rossby waves and other westward propagating eddies arriving from the east, energise the SMC in June and induce an acceleration tendency on the mean flow through regions associated with upgradient eddy potential vorticity flux. In addition to the eddies, local winds also play a crucial role in driving the mean flow. Wind power, surface mean ocean kinetic energy and available potential energy (APE) were computed, integrated over the region of interest and compared to each other. The effect of local winds appear to be predominant in driving the mean flows as it not only increases the surface mean kinetic energy of the SMC but also raises the isopycnals and builds up large amount of APE in the ocean. Baroclinic instability takes place in late July and early August associated with the release of APE which flattens the isopycnals and thus weakens the SMC. Consequently the SMC meanders in course of time and flows into southeast BoB.

The long-term climatic and environmental history of Southeast Asia is still fragmentary. This thesis therefore aims at studying lake sediment/peat sequences using a multi-proxy approach to reconstruct the environmental history and the impact of past changes in monsoon variability and intensity on lake ecosystems in Thailand. The study focuses on two lakes located in northeast Thailand: the larger Lake Kumphawapi and the smaller Lake Pa Kho. The comparison of multiple sediment sequences and their proxies from Kumphawapi suggests a strengthening of the summer monsoon between c. 10,000 and 7000 cal yr BP. Parts of the lake had been transformed into a wetland/peatland by c. 7000 cal yr BP, while the deeper part of the basin still contained areas of shallow water until c. 6600 cal yr BP. This gradual lowering of the lake level can point to a weakening of the summer monsoon. Paleoenvironmental information for the time interval between 6200 and 1800 cal yr BP is limited due to a several thousand-year long hiatus. This new investigation demonstrates that arguments using the phytolith and pollen record of Lake Kumphawapi to support claims of early rice agriculture in the region or an early start of the Bronze Age are not valid, because these were based upon the assumption of continuous deposition. The lithostratigraphy and multi-proxy reconstructions for Pa Kho support a strengthened summer monsoon between 2120-1580 cal yr BP, 1150-980 cal yr BP, and after 500 cal yr BP; and a weakening of the summer monsoon between 1580-1150 cal yr BP and between 650-500 cal yr BP. The increase in run-off and higher nutrient supply after AD 1700 can be linked to agricultural intensification in the region. Conclusively, the Holocene records from northeast Thailand add important paleoclimatic information for Southeast Asia and allow discussing past monsoon variability and movements of the Intertropical Convergence Zone in greater detail. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Accepted. Paper 5: Manuscript.</p>