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.

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Large-scale climate modes such as the El Niño/Southern Oscillation (ENSO), the Asian monsoon, and the Arctic Oscillation are responsible for much of the Earth’s climate variability. Despite the importance of these modes, we have limited understanding of how they vary on long (multidecadal to millennial) timescales due to the short length of instrumental climate records. Fortunately, climate information stored in natural archives can provide us with information on how these modes varied in the more distant past. Lake sediments are an ideal climate archive since they are continuous, have high temporal resolution, and contain many potential climate proxies. In the present study, I use lake sediment records to assess past climate and environmental changes associated with the El Niño/Southern Oscillation, the Asian monsoon, and the Arctic Oscillation. Exploring modern precipitation variability across the Asian monsoon region, I found that precipitation within this broad area is not coherent, which holds implications for paleorecords that are hypothesized to represent monsoon variability, including many lake sediment records on the Tibetan Plateau. Monsoon precipitation in the Arabian Sea is distinct from precipitation in India and China, and increased precipitation in the Arabian Sea coincides with decreased precipitation in the western North Pacific. Furthermore, only precipitation in southwestern Tibet responds to the Southwest monsoon, whereas precipitation in southeastern Tibet responds to the western North Pacific monsoon. In southwestern Tibet, I have reconstructed dust variability over the last millennium using the lake sediment record from Kiang Co. The sediment record shows a trend toward increasing dust over the 20th century, and our hypothesized dust proxy is positively correlated with the June-November Arctic Oscillation Index. A trend toward more positive Arctic Oscillation Index values as well as higher temperatures over the 20th century likely drove increased dustiness in southwestern Tibet, due the influence of temperature on glaciofluvial sediment availability in the Himalayas. Sediment trap, sediment core data, and modern measurements of local climate and lake water variables at Genovesa Crater Lake, Galápagos, indicate the lake and its sediments respond to local climate variability, with carbonate-rich sediments forming during prolonged dry periods (La Niña events), and organic-rich sediment forming during the warm season and El Niño events. The ratios of silica to calcium and strontium to calcium also reflect cool season SST. Thus, this lake sediment record has potential to provide a record of both seasonal and ENSO variability spanning the Holocene.

8 pp. / This publication provides an depth look at the North American Monsoon system and its impact on summer weather in Arizona.

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This M. Sc. thesis studies the statistical characteristics of convective storms in a monsoon regime in Darwin, northern Australia. It has been conducted with the use of radar. Enhanced knowledge of tropical convection is essential in studies of the global climate, and this study aims to bring light on some special characteristics of storms in a tropical environment. The observed behaviour of convective storms can be implemented in the parameterisation of these in cloud-resolving regional and global models. The wet season was subdivided into three regimes; build-up and breaks, the monsoon and the dry monsoon. Using a cell tracking system called TITAN, these regimes were shown to support different storm characteristics in terms of their temporal, spatial and height distributions. The build-up and break storms were seen to be more vigorous and particularly modulated diurnally by sea breezes. The monsoon was dominated by frequent but less intense and vertically less extensive convective cores. The explanation for this could be found in the atmospheric environment, with monsoonal convection having oceanic origins together with a mean upward motion of air through the depth of the troposphere. The dry monsoon was characterised by suppressed convection due to the presence of dry mid-level air. The effects of wind shear on convective line orientations were examined. The results show a diurnal evolution from low-level shear parallel orientations of convective lines to low-level shear perpendicular during build-up and breaks. The monsoon was dominated by complex orientations of convective lines. The thesis includes a study of merged and splitted cells, which have been separated from other storms, and mergers were shown to support more vigorous convection in terms of height distribution and reflectivity profiles. They were also seen to be the most long-lived category of storms as well as the most common type. Split storms were generally weaker, indicative of their general tendency to decay shortly after the split occurred. / En statistisk studie av konvektiva celler i en miljö som präglas av monsunförhållanden har utförts i Darwin, norra Australien, med hjälp av radar. En ökad förståelse for tropisk konvektion är nödvändig for att kunna studera klimatet globalt. Denna studie har bidragit till denna kunskapsbas genom att studera några viktiga parametrar hos konvektiva celler i en tropisk miljö. De observerade egenskaperna hos dessa celler kan implementeras i parametriseringen av högupplösta regionala och globala modeller. Regnperioden delades upp i tre olika regimer; uppbyggnad och avbrott, monsun och torr monsun. Genom att använda ett cellsökande system kallat TITAN, visade sig dessa regimer uppvisa olika karakteristika vad gäller tids- och rumsmässig samt vertikal distribution av konvektionsceller. Uppbyggnad- och avbrottsregimen dominerades av mäktiga och intensiva konvektionsceller, och modulerades av sjöbrisar på en daglig basis. Monsunen dominerades av talrika men mindre intensiva celler. Anledningen till detta kan finnas i atmosfäriska förhållanden, dar monsunen dominerades av konvektionsceller med oceanisk härkomst och allmän hävning genom större delen av troposfären. Den torra monsunen präglades av försvagad konvektion på grund av närvaron av mycket torr luft på medelhöga nivåer. Effekten av vindskjuvning på orienteringen av bylinjer undersöktes. Resultaten visar att en daglig övergång från en orientering som var parallell med vindskjuvningsvektorn till en vinkelrät orientering dominerade under uppbyggnad och avbrott. Monsunen präglades av komplexa orienteringar av bylinjer. Sammanväxande och splittrande celler separerades fran andra celler och undersöktes speciellt. De sammanväxande cellerna uppvisade mer intensiv konvektion och större vertikal maktighet. Denna kategori av celler, som var den vanligaste typen av ickeisolerade celler, levde också längre än andra celltyper. Splittrande celler var generellt svagare än andra celler, vilket indikerade den generella tendensen för denna celltyp att brytas ner strax efter det att en splittring ägt rum.

In this study, a one-dimensional model based on the Mellor and Yamada level 2.5 turbulence closure model was coupled with a biogeochemical model to investigate the inter-annual variation of biogeochemistry at the South-East Asian Time-series Study (SEATS) Site in the northern South China Sea (SCS) from 1997 to 2003. During the study period there were two El Niño Events and two La Niña Events. This study was focused on the hydrographic and biogeochemical conditions during these events. In order to better understand the model performance in the physical and biogeochemical aspects, numerical experiments were conducted to investigate the key processes. Numerical experiments by using idealized forcing conditions revealed that stronger wind stresses resulted in stronger cooling and deeper mixed layer. The model results were as sensitive to the initial density structure of the water column as to wind stress. Numerical experiments with the coupled model revealed that the biogeochemical results are insensitive to the initial biogeochemical conditions except the nitrate profile. Sensitivity tests indicated that primary production was sensitive to the remineralization rate constant for the detritus and parameters related to zooplankton, such as growth rate, grazing constant and mortality rate constant. It is less sensitive to iv aggregation constant of phytoplankton. The SEATS project of the National Center for Ocean Research provided data of sea surface chlorophyll-a (S-chl) concentrations, which were derived from SeaWiFS data for the period from Jan. 1997 to Dec. 2003 by calibrating against shipboard observations. The time-series showed decreases of mean S-chl by 35% and 9% below the climatological mean in the winter months (DJF) of the two El Niño Events. The negative S-chl anomalies corresponded to elevated sea surface temperature (SST) by 1.4oC and 0.4oC above the climatological mean, while the mean wind speed (WS) was reduced by 20% and 13%, and the surface heat exchange reverted from net loss to net gain or null. It is hypothesized that the anomalously low S-chl may have been caused by the weakened wind mixing and strengthened stratification. A 1-dimensional coupled physical-biogeochemical model was developed to test this hypothesis. The model driven by wind stress and surface heat fluxes successfully reproduced the seasonal cycles of S-chl and integrated primary productivity (IPP) as compared to shipboard observations and SeaWiFS data derived values, and predicted the negative anomalies of S-chl and IPP under the 1997-98 and 2002-03 El Niño conditions. However, the model-predicted strong positive anomalies of S-chl and IPP under the 1998-99 and 1999-2000 La Niña conditions, which brought v stronger wind and heat loss, were not substantiated by observations. Hydrographic conditions at the SEATS station indicated that, under La Niña conditions, unusual accumulation of warm and nutrient-depleted water occurred in the upper water column cancelled out the effect of stronger mixing. Therefore, the biogeochemical responses of the northern SCS to surface forcing during the recent El Niño/La Niña conditions displayed in a highly asymmetrical manner.

Instrumental and observational records of climate in West Africa suggest that this region may be susceptible to abrupt, decades-long drought events, with potentially catastrophic impacts for the people living in this region. However, because of the dearth of long, continuous and high quality climate records from sub-Saharan Africa, little is known about the long-term frequency and persistence of drought events in this region. It is also unclear whether observed 20th century droughts are natural or due to human impacts. In the present study, we use several complementary approaches to develop a high-resolution record of paleoclimatic changes in West Africa from the geological record preserved at Lake Bosumtwi, Ghana.Our results suggest that West Africa has undergone significant hydrologic variations over the last ca. 10,000 years. The dominant influence on hydrologic changes over this interval was changes in northern hemisphere summer insolation and the associated feedback processes acting in the oceans and on land. This led to a more northerly position of the Intertropical Convergence Zone (ITCZ) and increased precipitation during the early to mid-Holocene. In the late Holocene, a second increase in precipitation occurred along the Guinea coast as a result of the southward migration of the ITCZ from its northern position. This maximum was followed by an abrupt decrease in precipitation at ca. 2.5-3 kyr.The West African monsoon also varies on timescales from millennia to decades. Millennial and century-scale variations appear to be partly paced by changes in solar irradiance, either directly or indirectly. On decadal timescales, variability appears to be dominated by changes in Atlantic sea surface temperatures. The dominant mode is a ca. 40 year oscillation, which in strongly coherent and in phase with the Atlantic Multidecadal Oscillation (AMO). It is unclear from this study, however, if drought conditions over the last century are related to this multidecadal oscillation, or if they are forced by anthropogenic changes.

The East Asian Summer Monsoon (EASM) is a major component in Asian climate. It is largely driven by climatic factors such as humidity, solar insolation and temperature. For at least 50 years the EASM has been studied extensively by scientists regarding its current strength. Models have been recreating past monsoon intensity as well as attempted to predict future intensity. As the monsoon undergoes changes, the climatic shifts responsible for them leave various traces behind; geochemical as well as biological, and these have been preserved and recorded in various locales on the planet. The most significant climatic change is the variation between glacial and interglacial periods which have been alternating for the last 2.6 million years and the EASM has changed in tune with the climate during this time. The EASM follows the δ18O-record in speleothems found in Eastern Asia as well as in ice cores from Greenland. Various geochemical and biological tracers seem to reflect these fluctuations in climate locally as well as globally over a 200 kyr period. The current intensity of the EASM seems to be one of decreasing strength, a phase that has persisted since the Holocene climatic optimum 8.5 kyr ago. Recently however a decrease in the East Asian Winter Monsoon has been confirmed, indicating an increase in EASM intensity. During the Holocene the EASM reached peak intensity during the Holocene climatic optimum but has fluctuated largely in tune with solar insolation. This is also true for the Eemian period although some events such as the mid-Eemian cooling show that factors other than solar insolation regulate monsoon intensity over large time periods. The future of the EASM seems to be one of increased strength due to climate change and models predict both increased wind speeds and an increasing occurrence of extreme precipitation despite decreasing solar insolation.