Low Level Jet stream of Asian Summer Monsoon and its Variability

The main objective of the of present study are to study the intraseasonal variability of LLJ and its relation with convective heating of the atmosphere, to establish whether LLJ splits into two branches over the Arabian sea as widely believed, the role of horizonatal wind shear of LLJ in the episodes of intense rainfall events observed over the west coast of India, to perform atmospheric modeling work to test whether small (meso) scale vortices form during intense rainfall events along the west coast; and to study the relation between LLJ and monsoon depression genesis. The results of a study on the evolution of Low Level Jetstream (LLJ) prior to the formation of monsoon depressions are presented. A synoptic model of the temporal evolution of monsoon depression has been produced. There is a systematic temporal evolution of the field of deep convection strength and position of the LLJ axis leading to the genesis of monsoon depression. One of the significant outcomes of the present thesis is that the LLJ plays an important role in the intraseasonal and the interannual variability of Indian monsoon activity. Convection and rainfall are dependent mainly on the cyclonic vorticity in the boundary layer associated with LLJ. Monsoon depression genesis and the episodes of very heavy rainfall along the west coast of India are closely related to the cyclonic shear of the LLJ in the boundary layer and the associated deep convection. Case studies by a mesoscale numerical model (MM5) have shown that the heavy rainfall episodes along the west coast of India are associated with generation of mesoscale cyclonic vortices in the boundary layer.

Causes and consequences of long-term climatic variability on the Australian continent

1. Ice-volume forced glacial-interglacial cyclicity is the major cause of global climate variation within the late Quaternary period. Within the Australian region, this variation is expressed predominantly as oscillations in moisture availability. Glacial periods were substantially drier than today with restricted distribution of mesic plant communities, shallow or ephemeral water bodies and extensive aeolian dune activity. 2. Superimposed on this cyclicity in Australia is a trend towards drier and/or more variable climates within the last 350 000 years. This trend may have been initiated by changes in atmospheric and ocean circulation resulting from Australia's continued movement into the Southeast Asian region and involving the onset or intensification of the El Nino-Southern Oscillation system and a reduction in summer monsoon activity. 3. Increased biomass burning, stemming originally from increased climatic variability and later enhanced by activities of indigenous people, resulted in a more open and sclerophyllous vegetation, increased salinity and a further reduction in water availability. 4. Past records combined with recent observations suggest that the degree of environmental variability will increase and the drying trend will be enhanced in the foreseeable future, regardless of the extent or nature of human intervention.

Late Quaternary Milankovitch-scale climatic change and variability and its impact on monsoonal Australasia

Four pollen and charcoal records derived from marine cores around the northern perimeter of Australia are examined to provide a regional picture of patterns, causes and impacts of climate change over the last 100-300 ka. The availability of radiocarbon dates and oxygen isotope records for the cores provides primary chronological control. Spectral analysis of components of these records demonstrates an overall importance of Milankovitch frequencies with clear glacial-interglacial cyclicity dominated by variation in precipitation. In addition, a number of pollen taxa, as well as charcoal particles, exhibit a 30 ka frequency that is considered, from its relationship with biomass burning and with results of past modelling, to reflect changes in the intensity of El Nino-Southern Oscillation (ENSO) variability. Pollen components of all records show a decline, frequently stepwise, in more fire-sensitive vegetation and its replacement with more fire-tolerant vegetation. There is some evidence that this trend is linked to an onset or general increase in ENSO activity and perhaps also to variation in monsoon activity dating from about 300 ka BP that was caused by changes to oceanic circulation within the Indonesian region. The trend may have accelerated within the last 45 ka due to burning by indigenous people. (C) 2003 Elsevier B.V. All rights reserved.

Stratosphere Troposphere Interactions Associated With the Dynamical Processes in the Atmosphere

The aim of the present study is to understand the biennial scale stratosphere-troposphere interactions over India, and synoptic to interannual timescale meridional stratosphere-troposhere exchanges caused by upper tropospheric/lower stratospheric longwaves using NCEP/NCAR reanalysis data and satellite measured total ozone data. The biennial timescale interaction between lower stratosphere and troposphere over Thumba is analysed using high-resolution radiosonde data. The results suggest that TBO and QBO are two different phenomena with nearly biennial periodicity. Interannual timescale meridional stratosphere-troposphere exchanges caused by the newly documented Asia Pacific Wave (APW) were analysed using ozone as tracer of atmospheric motion. Synoptic timescale meridional stratosphere-troposhere exchanges caused by subtropical upper tropospheric long waves over Asia were studied using global total ozone measurements from TOMS. This research work can be extended to study the influence of decadal scale epochal nature in Indian summer monsoon activity on the APW generated total ozone anomalies around the globe and the trend estimates in total ozone

AN INVESTIGATION ON AIR-SEA INTERACTION IN THE INDIAN OCEAN DURING CONTRASTING MONSOONS

The study is undertaken with an objective to investigate the linkage between air-sea fluxes in the Indian Ocean and monsoon forcing. Since the monsoon activity is linked to fluxes, the variability of surface marine meteorological fields under the variable monsoon conditions is also studied. The very objective of the present study is to document various sea surface parameters of the Indian Ocean and to examine the anomalies found in them. Hence it is attempted to relate the anomaly to the variability of monsoon over India, highlighting the occasion of contrasting monsoon periods. The analysis of anomalies of surface meteorological fields such as SST, wind speed and direction, sea level pressure and cloud cover for contrasting monsoons are also studied. During good monsoon years, the pressure anomalies are negative indicating a fall in SLP during pre-monsoon and monsoon months. The interaction of the marine atmosphere with tropical Indian Ocean and its influence on ISMR continue to be an area of active research.

Late Quaternary highstands at Lake Chilwa, Malawi: frequency, timing and possible forcing mechanisms in the last 44ka

The extensive shoreline deposits of Lake Chilwa, southern Malawi, a shallow water body today covering 600 km2 of a basin of 7500 km2, are investigated for their record of late Quaternary highstands. OSL dating, applied to 36 samples from five sediment cores from the northern and western marginal sand ridges, reveal a highstand record spanning 44 ka. Using two different grouping methods, highstand phases are identified at 43.7–33.3 ka, 26.2–21.0 ka and 17.9–12.0 ka (total error method) or 38.4–35.5 ka, 24.3–22.3 ka, 16.2–15.1 ka and 13.5–12.7 ka (Finite Mixture Model age components) with two further discrete events recorded at 11.01 ± 0.76 ka and 8.52 ± 0.56 ka. Highstands are comparable to the timing of wet phases from other basins in East and southern Africa, demonstrating wet conditions in the region before the LGM, which was dry, and a wet Lateglacial, which commenced earlier in the southern compared to northern hemisphere in East Africa. We find no evidence that wet phases are insolation driven, but analysis of the dataset and GCM modelling experiments suggest that Heinrich events may be associated with enhanced monsoon activity in East Africa in both timing and as a possible causal mechanism.

Orbitally driven east-west antiphasing of South American precipitation

The variations of tropical precipitation are antiphased between the hemispheres on orbital timescales. This antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres, which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the Hadley cells(1-4). Here we compare an oxygen isotopic record recovered from a speleothem from northeast Brazil for the past 26,000 years with existing reconstructions of precipitation in tropical South America(5-8). During the Holocene, we identify a similar, but zonally oriented, antiphasing of precipitation within the same hemisphere: northeast Brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high, whereas the rest of southern tropical South America shows opposite characteristics. Simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation-driven monsoon activity. Our results suggest that convective heating over tropical South America and associated adjustments in large-scale subsidence over northeast Brazil lead to a remote forcing of the South American monsoon, which determines most of the precipitation changes in the region on orbital timescales.

Physical properties of four profiles from the Triassic in Germany

The Norian Steinmergel-Keuper (SMK) represents a low-latitude cyclically-bedded playa system of the Mid-German Basin. We investigated a drilling site (core Morsleben) and sections from marginal positions. Dolomite/red mudstone beds form rhythmic alternations that were associated with varying monsoon activity. Hence, low K/Al ratios of dolomite beds suggest increased chemical weathering of the crystalline hinterland and therefore increased monsoonal rainfall. High K/Al ratios in red mudstone beds reflect increased physical weathering of the hinterlands during dryer periods. Dolomite layers reflect the lake stage (maximum monsoon) while red mudstones indicate the dry phase (minimum monsoon) of the playa cycle. We distinguished five major types of cyclic facies alternations, representing specific facies zones in the playa system. We have implemented spectrophotometry as a tool for high-resolution cyclostratigraphy. The dense sampling increment (up to 1 cm) allows for the recognition of all orbital frequencies. Sediment colour profiles reveal striking hierarchical cycles from semi-precession (SP, 99 kyr) over precession (P, 19.8 kyr) and obliquity (O, 36 kyr) to eccentricity (E1-2 109 kyr; E3, 413 kyr). A significant about 2 Myr-signal is attributed to the longer-term eccentricity E4. One monsoonal (precession) cycle includes two carbonate precipitation events. We propose that stratified mudstone and red mudstone are associated with maximum and minimum monsoon during the transition of the solstices in perihelion and aphelion, respectively. The two carbonate precipitation events were most likely created when equinoxes were in perihelion and aphelion, respectively. A sedimentary semi-precession response cycle is a novel finding for the Norian strata. The obliquity signal is attributed to incoming atmospheric moisture from the northeast of the SMK basin. The E4 cycle controls lake-level changes over long times. Apparently, E4 is responsible whether or not a threshold value is crossed. Bundles of 109 kyr and 413 kyr in red mudstones suggest a dry system with reduced monsoonal activity. In contrast, humid periods reveal thick layers of dolomite beds, indicating that during those intervals the monsoonal activity was strong enough to prevent the playa system from drying out completely.

Bulk sediment element analysis of sediment cores GeoB10065-9 and GeoB10065-7, offshore northwest Sumba Island, Indonesia

The Australian-Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ~6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ~3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO).

Age determination and stable isotope records of sediment core M78/1_235-1

Paleoenvironmental studies and climate models demonstrate that fluvial runoff and moisture availability in the Caribbean hinterland react very sensitively to climatic variations. Late Pleistocene and Holocene climate records document pronounced dry and wet periods over tropical South America mainly caused by shifts of the Intertropical Convergence Zone (ITCZ). However, forcing mechanisms for changes in the ITCZ position remain controversial. Here we present high-resolution foraminiferal Ba/Ca and d18Oseawater records from a core located within the Orinoco River outflow documenting abrupt hydrological changes in the Orinoco catchment area during the deglacial and Holocene. Our data, obtained from the surface-dwelling foraminifera Globigerinoides ruber (pink), show an abrupt increase in Ba/Ca ratios in the early Holocene, starting ~600 yr after the end of the Younger Dryas (YD) cold interval at ca. 10.8 ka and suggesting a massive reorganization of moisture sources in northern South America. In contrast, the salinity dependent d18Oseawater from the same samples shows a gradual decrease starting at the end of the YD. The offset of our Ba/Ca peak excludes meltwater release in conjunction with the northern Andean glacier retreat well before the end of the YD as a forcing mechanism. We suggest that the Ba/Ca record documents an abrupt increase in Ba-rich waters of a northern Andean source caused by the insolation-driven shift of the ITCZ and/or enhanced monsoon activity.

Radiocarbon ages, grain sizes, paleomagentic properties and XRF data of Lake Nam Co sediments, Tibetan Plateau

In September 2008 several cores (68 cm-115 cm length) (water depth: 93 m) were retrieved from Lake Nam Co (southern-central Tibetan Plateau; 4718 m a.s.l.). This study focuses on the interpretation of high-resolution (partly 0.2 cm) data from three gravity cores and the upper part of a 10.4 m long piston core, i.e., the past 4000 cal BP in terms of lake level changes, hydrological variations in the catchment area and consequently variations in monsoon strength. A wide spectrum of sedimentological, geochemical and mineralogical investigations was carried out. Results are presented for XRF core-scans, grain size distribution, XRD-measurements and SEM-image analyses. These data are complemented by an age-depth model using 210Pb and 137Cs analyses as well as eleven AMS-14C-ages. This model is supported by excellent agreement between secular variations determined on one of the gravity cores to geomagnetic field models. This is a significant improvement of the chronology as most catchments of lacustrine systems on the Tibetan Plateau contain carbonates resulting in an unknown reservoir effect for radiocarbon dates. The good correlation of our record to the geomagnetic field models confirms our age-depth model and indicates only insignificant changes in the reservoir effect throughout the last 4 ka. High (summer-) monsoonal activity, i.e. moist environmental conditions, was detected in our record between approximately 4000 and 1950 cal BP as well as between 1480 and 1200 cal BP. Accordingly, lower monsoon activity prevails in periods between the two intervals and thereafter. This pattern shows a good correlation to the variability of the Indian Ocean Summer Monsoon (IOSM) as recorded in a peat bog ~1000 km in NE direction from Lake Nam Co. This is the first time that such a supra regional homogenous monsoon activity is shown on the Tibetan Plateau and beyond. Finally our data show a significant lake level rise after the Little Ice Age (LIA) in Lake Nam Co which is suggested to be linked to glacier melting in consequence of rising temperatures occurring on the whole Tibetan Plateau during this time.

Winter sea surface temperature reconstruction from planktic foraminiferal transfer functions in the northeastern Arabian Sea over the last two millennia

The Indian monsoon system is an important climate feature of the northern Indian Ocean. Small variations of the wind and precipitation patterns have fundamental influence on the societal, agricultural, and economic development of India and its neighboring countries. To understand current trends, sensitivity to forcing, or natural variation, records beyond the instrumental period are needed. However, high-resolution archives of past winter monsoon variability are scarce. One potential archive of such records are marine sediments deposited on the continental slope in the NE Arabian Sea, an area where present-day conditions are dominated by the winter monsoon. In this region, winter monsoon conditions lead to distinctive changes in surface water properties, affecting marine plankton communities that are deposited in the sediment. Using planktic foraminifera as a sensitive and well-preserved plankton group, we first characterize the response of their species distribution on environmental gradients from a dataset of surface sediment samples in the tropical and sub-tropical Indian Ocean. Transfer functions for quantitative paleoenvironmental reconstructions were applied to a decadal-scale record of assemblage counts from the Pakistan Margin spanning the last 2000?years. The reconstructed temperature record reveals an intensification of winter monsoon intensity near the year 100 CE. Prior to this transition, winter temperatures were >1.5°C warmer than today. Conditions similar to the present seem to have established after 450 CE, interrupted by a singular event near 950 CE with warmer temperatures and accordingly weak winter monsoon. Frequency analysis revealed significant 75-, 40-, and 37-year cycles, which are known from decadal- to centennial-scale resolution records of Indian summer monsoon variability and interpreted as solar irradiance forcing. Our first independent record of Indian winter monsoon activity confirms that winter and summer monsoons were modulated on the same frequency bands and thus indicates that both monsoon systems are likely controlled by the same driving force.

Assessment of Convective Activity Using Stability Indices as Inferred from Radiosonde and MODIS Data

The combined use of both radiosonde data and three-dimensional satellite derived data over ocean and land is useful for a better understanding of atmospheric thermodynamics. Here, an attempt is made to study the ther-modynamic structure of convective atmosphere during pre-monsoon season over southwest peninsular India utilizing satellite derived data and radiosonde data. The stability indices were computed for the selected stations over southwest peninsular India viz: Thiruvananthapuram and Cochin, using the radiosonde data for five pre- monsoon seasons. The stability indices studied for the region are Showalter Index (SI), K Index (KI), Lifted In-dex (LI), Total Totals Index (TTI), Humidity Index (HI), Deep Convective Index (DCI) and thermodynamic pa-rameters such as Convective Available Potential Energy (CAPE) and Convective Inhibition Energy (CINE). The traditional Showalter Index has been modified to incorporate the thermodynamics over tropical region. MODIS data over South Peninsular India is also used for the study. When there is a convective system over south penin-sular India, the value of LI over the region is less than −4. On the other hand, the region where LI is more than 2 is comparatively stable without any convection. Similarly, when KI values are in the range 35 to 40, there is a possibility for convection. The threshold value for TTI is found to be between 50 and 55. Further, we found that prior to convection, dry bulb temperature at 1000, 850, 700 and 500 hPa is minimum and the dew point tem-perature is a maximum, which leads to increase in relative humidity. The total column water vapor is maximum in the convective region and minimum in the stable region. The threshold values for the different stability indices are found to be agreeing with that reported in literature.

The vertical cloud structure of the West African monsoon: a 4 year climatology using CloudSat and CALIPSO

The West African summer monsoon (WAM) is an important driver of the global climate and locally provides most of the annual rainfall. A solid climatological knowledge of the complex vertical cloud structure is invaluable to forecasters and modelers to improve the understanding of the WAM. In this paper, 4 years of data from the CloudSat profiling radar and CALIPSO are used to create a composite zonal mean vertical cloud and precipitation structure for the WAM. For the first time, the near-coincident vertical radar and lidar profiles allow for the identification of individual cloud types from optically thin cirrus and shallow cumulus to congestus and deep convection. A clear diurnal signal in zonal mean cloud structure is observed for the WAM, with deep convective activity enhanced at night producing extensive anvil and cirrus, while daytime observations show more shallow cloud and congestus. A layer of altocumulus is frequently observed over the Sahara at night and day, extending southward to the coastline, and the majority of this cloud is shown to contain supercooled liquid in the top. The occurrence of deep convective systems and congestus in relation to the position of the African easterly jet is studied, but only the daytime cumulonimbus distribution indicates some influence of the jet position.