Mid-Late Holocene Sub-Millennial Scale Inverse Trends of South Asian Summer and Winter Monsoons in Sri Lanka

The South Asian Monsoon (SAM) brings precipitation crucial for agriculture across the densely populated region of South Asia. Identifying the key long-term drivers of the SAM is essential to improve the predictability of future monsoonal trends in the context of current global climate scenarios and increasingly frequent drought and flooding events in this part of the world. Here, we reconstruct ∼6000 years of climatic and environmental history of the South Asian summer monsoon-fed Bolgoda South Lake and the Horton Plains, and the winter monsoon-fed Panama lagoon, in Sri Lanka to better understand monsoonal operation over this island and its connection to broader climate systems. Multiple proxies (diagnostic biomarkers, hydrogen and carbon isotopes of individual n-alkane, grain size, and Zr/Rb elemental ratio) indicate a sub-millennial scale decreasing trend of summer monsoon rainfall in the wet zone of Sri Lanka alongside an increasing trend of winter monsoon rainfall in the dry zone during the last ∼6000 years. We also observed multi-centennial scale arid events in the Bolgoda South Lake and Horton Plains records at ∼3,500 and ∼1,000 cal years BP. Inverse monsoonal behavior during the mid- and late Holocene seems to be led by the southward migration of the mean latitudinal position of ITCZ, induced by varying solar energy distribution between the Northern and Southern hemispheres due to Earth’s processional cycle. Our observations are broadly supported by existing paleoclimatic records from the Indian sub-continent, but abrupt arid phases are asynchronous in the regional records. In addition, these short-term arid conditions do not show systematic correlations with the different modes of climate variables known to have teleconnections with the Indian Ocean monsoon.

Three types of positive Indian Ocean dipoles and their relationships with the South Asian summer monsoon

The relationship between the Indian Ocean dipole (IOD) and the South Asian summer monsoon (SASM), which remains a subject of controversy, was investigated using data analyses and numerical experiments. We categorized IOD events according to their sea surface temperature anomaly (SSTA) pattern: Type-W and Type-E are associated with stronger SSTA amplitudes in the western and eastern poles of the IOD, respectively, while Type-C has comparable SSTA amplitudes in both poles during boreal autumn. Type-W is associated with a weak SASM from May to summer, which contributes to substantial warming of the western pole in autumn; the east–west SST gradient linked to the warming of the western pole causes weak southeasterly wind anomalies off Sumatra and feeble and cold SSTAs in the eastern pole during the mature phase. Type-E is associated with a strong SASM and feeble warming of the western pole; interaction between the strong SASM and cold SSTAs in the eastern pole in summer results in strong southeasterly wind anomalies off Sumatra and substantial cooling of the eastern pole during the mature phase. For Type-C, warming of the western pole and cooling of the eastern pole develop synchronously without apparent SASM anomalies, and reach comparable intensities during the mature phase. Observations and numerical simulation results both indicate the role of disparate SASM anomalies in modulating SSTA patterns during the development of positive IODs. Warming of the tropical Indian Ocean becomes established in the winter and spring following Type-W and Type-C IODs, but not following Type-E events.

Differences in The Causes of Light And Extreme Precipitation For The Qaidam Basin In Summer

The Qaidam Basin (QB) locates over the northeast of the Tibetan Plateau (TP), where precipitation especially extreme precipitation possesses obvious local characteristics compared with that over the whole TP. This study tries to investigate cause of light (50% threshold) and extreme (95% threshold) precipitation in boreal summer in the QB, which is helpful to deepen understanding of the mechanism of precipitation formation in different regions of the TP. The extreme (light) precipitation thresholds in the eastern QB are greater than that in the western QB, with a value of 6~16mm (2mm) for most regions. There are two main moisture transport channels for light and extreme precipitation events. One is from the Eurasia and carried by the westerlies, which provides 48.2% and 55.8% of moisture for light and extreme precipitation events, respectively. The other moisture transport channel is from the Arabian Sea and the Bay of Bengal, which is transported toward the QB at the joint role of the South Asian summer monsoon and the plateau monsoon, contributing 51.8% and 44.2% of moisture for light and extreme precipitation events, respectively. The stronger moisture transport to precipitation mostly attributes to the enhanced moisture influxes from the western and southern boundaries. Additionally, the weaker moisture outflux across the eastern boundary is also responsible for the extreme precipitation. The circulation characteristics shows that, the precipitation in the QB has a closely relationship with the weak ridge over the Caspian Sea and Aral Sea, the enhanced South Asian summer monsoon and plateau monsoon, which are conducive to the moisture transport from the Eurasia and low-latitudes toward the QB. The meridional circulation enhances, meantime the westerly jet stream splits into east- and west-branch, and the south Asian high (SAH) strengthens, which are beneficial for the stronger convective motion. Especially, the trough in the northwest of the QB and the more significant east- and west-branch structure of westerly jet are the main circulation characteristics for the extreme precipitation events. Further analysis reveals that the apparent heat source over the QB is contributed to more synchronous moisture transport around the TP and its surrounding areas for light precipitation events, while the apparent heat source enhances 1 day prior to moisture transport from the east part region of the South Asian summer monsoon to around the eastern TP for extreme precipitation events. Meantime, the apparent heat source triggers an abnormal cyclone over the TP which can positively strength the local convective motion. Such abnormal configuration of atmospheric circulation and the influence of apparent heat source can explain the difference in cause of precipitation with different magnitude to a great extent in the QB.

Crucial Role of Mesoscale Convective Systems in the Vertical Mass, Water and Energy Transports of the South Asian Summer Monsoon

Convective vertical transport is critical in the monsoonal overturning but the relative roles of different convective systems are not well understood. This study used a cloud classification and tracking technique to decompose a convection-permitting simulation of the South Asian summer monsoon (SASM) into sub-regimes of mesoscale convective system (MCS), non-MCS deep convection (non-MCS), congestus, and shallow convection/clear-sky. Isentropic analysis is adopted to quantify the contributions of different convective systems to the total SASM vertical mass, water, and energy transports. The results underscore the crucial roles of MCSs in the SASM vertical transports. Compared to non-MCSs, the total mass and energy transports by MCSs are at least 1.5 times stronger throughout the troposphere, with a larger contributing fraction from convective updrafts compared to upward motion in stratiform regions. Occurrence frequency of non-MCSs is around 40 times higher than that of MCSs. However, per instantaneous convection feature, the vertical transports and net MSE export by MCSs are about 70-100 and 58 times stronger than that of non-MCSs. While these differences are dominantly contributed by differences in the per-feature MCS and non-MCS area coverage, MCSs also show stronger transport intensities than non-MCSs over both ocean and land. Oceanic MCSs and non-MCSs show more obvious top-heavy structures than their inland counterparts, which are closely related to the widespread stratiform over ocean. Compared to the monsoon break phase, MCSs occur more frequently (~1.6 times) but their vertical transport intensity slightly weakens (by ~10%) during the active phases. These results are useful for understanding the SASM and advancing the energetic framework.

Climate Precursors of Satellite Water Marker Index for Spring Cholera Outbreak in Northern Bay of Bengal Coastal Regions

Cholera is a water-borne infectious disease that affects 1.3 to 4 million people, with 21,000 to 143,000 reported fatalities each year worldwide. Outbreaks are devastating to affected communities and their prospects for development. The key to support preparedness and public health response is the ability to forecast cholera outbreaks with sufficient lead time. How Vibrio cholerae survives in the environment outside a human host is an important route of disease transmission (see a review of Racault et al. 2019). Thus, identifying the environmental and climate drivers of these pathogens is highly desirable. Here, we elucidate for the first time a mechanistic link between climate variability and cholera (Satellite Water Marker; SWM) index in the Bengal Delta, which allows us to predict cholera outbreaks up to two seasons earlier. High values of the SWM index in fall were associated with above-normal summer monsoon rainfalls over northern India. In turn, these correlated with the La Niña climate pattern that was traced back to the summer monsoon and previous spring seasons. We present a new multi-linear regression model that can explain 50% of the SWM variability over the Bengal Delta based on the relationship with climatic indices of the El Niño Southern Oscillation, Indian Ocean Dipole, and summer monsoon rainfall during the decades 1997–2016. Interestingly, we further found that these relationships were non-stationary over the multi-decadal period 1948–2018. These results bear novel implications for developing outbreak-risk forecasts, demonstrating a crucial need to account for multi-decadal variations in climate interactions and underscoring to better understand how the south Asian summer monsoon responds to climate variability.

Intra-seasonal contrasting trends in clouds due to warming induced circulation changes

Quantification of long term changes in cloud distribution and properties is critical for the proper assessment of future climate. We show contrasting trends in cloud properties and cloud radiative effects over Northwest Indian Ocean (NWIO) in south Asian summer monsoon. Cloud top height (CTH) decreases in June (− 69 ± 3 myr−1) and July (− 44 ± 3 myr−1), whereas it increases in August (106 ± 2 myr−1) and September (37 ± 1 myr−1). These contrasting trends are investigated to be due to the changes in upper tropospheric winds and atmospheric circulation pattern. Strengthening of upper tropospheric easterlies and changes in vertical wind dampen the vertical development of clouds in June and July. In contrast, weakening of upper tropospheric winds over NWIO and strengthening of updraft favour the vertical growth of clouds in August. Further, changes in horizontal winds at 450–350 hPa and strengthening of Indian Ocean Walker cell favour the westward spread of high level clouds, contributing to the increase in CTH over NWIO in August. Decrease of cloud cover and altitude in June and July and increase of the same in subsequent months would affect the monsoon rainfall over the Indian region. Proper representation of these intra-seasonal contrasting trends of clouds in climate models is important for the better prediction of regional weather.

Tracing the origin of the South Asian summer monsoon precipitation and its variability using a novel Lagrangian framework

The water sources and their variability responsible for the South Asian summer monsoon precipitation were analyzed using Lagrangian atmospheric water-mass trajectories. The results indicated that evaporated waters from the Central and South Indian Ocean are the major contributors to the South Asian summer monsoon rainfall, followed by the contribution from the local recycling (precipitated water that evapotranspirated from the South Asian landmass), the Arabian Sea, remote sources and the Bay of Bengal. It was also found that although the direct contribution originating from the Bay of Bengal is small, it still provides a pathway for the atmospheric water that come from other regions. This pathway is hence only crossing over the Bay of Bengal. The outcomes further revealed that the evaporated waters originating from the Central and South Indian Ocean are responsible for the net precipitation over the coastal regions of the Ganges-Brahmaputra-Meghna Delta, Northeast India, Myanmar, the foothills of the Himalayas and Central-East India. Evaporated waters from the Arabian sea are mainly contributing to the rainfall over the Western coast and West-Central India. Summer monsoon precipitation due to the local recycling is primarily restricted to the Indo-Gangetic plain. No recycled precipitation was observed over the mountain chain along the West coast of India (Western Ghats). The month-to-month precipitation variation over South Asia was analysed to be linked with the Somali Low Level jet variability. The inter-annual variability of the South Asian summer monsoon precipitation was found to be mainly controlled by the atmospheric waters that were sourced and travelled from the Central and South Indian Ocean.