Influence of Bay o Bengal on winter monsoon rainfall

ABSTRACT. Utilizing marine meteorological data the values of 1 latent heat flux, sea surface temperature (SST) and sea minus air temperature have been computed on a grid mesh of 5° over the Bay of Bengal during September month of the contrasting Winter monsoon years 1987 and 1988. It has been found that the good winter monsoon of 1987 followed (I) higher SSTs over western Bay of Bengal; (ii) very high evaporation rate over the sea area bounded by 10°.20°N. 80°.90oE and (iii) instability in the surface layer over north and adjoining central Bay of Bengal, whereas, the bad winter monsoon of 1988 followed (i) lower SSTs over western Bay of Bengal; (ii) very low evaporation rate over the area I0°.20oN, 80°.90oE and (iii) stability in the surface layer over north and adjoining central Bay of Bengal.

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.

Asian monsoon projection with a new large-scale monsoon definition

This paper focuses on Asian monsoon projection with CMIP5 multi-model outputs. A large-scale monsoon herewith is defined as a vector field of vertically integrated moisture flux from the surface to 500 hPa. Results demonstrate that the model ensemble mean underestimated the summer monsoon and overestimated slightly the winter monsoon over South Asia in both CMIP5 historical climate simulation and the monsoon projection for 2006–2015. The major of the bias is the model climate drift (MCD), which is removed in the monsoon projection for 2016–2045 under scenarios RCP4.5 for reducing the uncertainty. The projection shows that two increased moisture flows northward appeared across the Equator of Indian Ocean, the first is nearby Somalia coast toward northwestern part of South Asia, leading to excess rainfall in where the wet jet could reach, and the second starts from the equatorial Sect. (80°E–100°E) toward northeastern Bay of Bengal, leading to more rainfall spreading over the northwestern coast of Indochina Peninsula. In addition, a westward monsoon flow is intensified over the Peninsula leading to local climate moisture transport belt shifted onto South China Sea, which would reduce moisture transport toward Southwest China on one hand, and transport more moisture onto the southeast coast of the China mainland. The anomalous monsoon would result in a dry climate in Northwest China and wet climate in the coast belt during summer monsoon season for the period. Besides, the Asian winter monsoon would be seemingly intensified slightly over South Asia, which would bring a dry winter climate to Indian subcontinent, Northwest China, but would be more rainfall in southeast part of Arabian Peninsula with global climate warming.

Zonal shift in the cold airmass stream of the East Asian winter monsoon

The East Asian winter monsoon exhibits long-term variations in intensity and spatial pattern, though the latter one is less understood. To investigate the long-term spatial variations of the EAWM and their possible causes, we propose a new position index of the EAWM by quantifying the low-level East Asian stream (EAS) of cold airmass in the Lagrangian sense. Based on the new-defined index, we find that the EAS undergoes an evident zonal shift between two channels over the land and coast. At interdecadal timescale, the peak location of the EAS is displaced eastward, with an increasing southward cold airmass flux at the coast since the mid-1960s. The interannual shift of the EAS presents not only the zonal oscillation of peak location between two channels but also the width changes of coastal channel over the northwestern Pacific. These shifts in the EAS are related to the strength changes of two source cold airmass streams from Siberia or Bering Sea, which are associated with the phase changes in the upper-tropospheric atmospheric teleconnections. At interdecadal timescale, the phase change in the North Atlantic Oscillation modulates the zonal shift in the EAS via the East Atlantic-West Russia teleconnection. At interannual timescale, the Pacific/North American teleconnection becomes the dominant factor altering the zonal shift and width change of the EAS.

Asian Monsoon Projection With a New Large-Scale Monsoon Definition

This paper focuses on Asian monsoon projection with CMIP5 multi-model outputs. A large-scale monsoon herewith is defined as a vector field of vertically integrated moisture flux from the surface to 500 hPa. Results demonstrate that the model ensemble mean underestimated the summer monsoon and overestimated slightly the winter monsoon over South Asia in both CMIP5 historical climate simulation and the monsoon projection for 2006-2015. The major of the bias is the model climate drift (MCD), which is removed in the monsoon projection for 2016 -2045 under scenarios RCP4.5 for reducing the uncertainty. The projection shows that two increased moisture flows northward appeared across the Equator of Indian Ocean, the first is nearby Somalia coast toward northwestern part of South Asia, leading to excess rainfall in where the wet jet could reach, and the second starts from the equatorial section (80°E-100°E) toward northeastern Bay of Bengal, leading to more rainfall spreading over the northwestern coast of Indochina Peninsula. In addition, a westward monsoon flow is intensified over the Peninsula leading to local climate moisture transport belt shifted onto South China Sea, which would reduce moisture transport toward Southwest China on one hand, and transport more moisture onto the southeast coast of the China mainland. The anomalous monsoon would result in a dry climate in Northwest China and wet climate in the coast belt during summer monsoon season for the period. Besides, the Asian winter monsoon would be seemingly intensified slightly over South Asia, which would bring a dry winter climate to Indian subcontinent, Northwest China, but would be more rainfall in southeast part of Arabian peninsula with global climate warming.

Co-evolutions of terrestrial temperature and seasonal precipitation from the latest Pleistocene to the mid-Holocene in Japan: carbonate clumped isotope record of a stalagmite

Quantitative paleotemperature reconstruction is a challenging and important issue in terrestrial paleoenvironmental studies, for which carbonate clumped isotope (Δ47) thermometry is a promising approach. Here we analyzed Δ47 values from 68 layers of OT02 stalagmite from Ohtaki Cave in central Japan, covering two separate time intervals (2.6–8.8 and 34.8–63.5 ka) to reconstruct temperature and meteoric d18O records. The average Δ47 temperature of the Holocene portion of this stalagmite was 16.3℃ ± 5.6℃, 6.6℃ ± 7.2℃ higher than the average of the latest Pleistocene portion, which was 9.7℃ ± 4.6℃. Δ47 thermometry also revealed that the coldest intervals (5℃–10℃) correspond to the Heinrich cooling events H4–6, and the warmest interval (up to 19.9℃ ± 6.0℃) in middle Holocene (approximately 6–5 ka) accompanied by the Hypsithermal climate optimum. We also reconstructed past meteoric δ18O by subtracting the temperature effect from stalagmite δ18O. Average meteoric δ18O was less negative in the Holocene (8.22‰ ± 0.99‰ VSMOW) than in the latest Pleistocene (8.81‰ ± 0.84‰). Over centennial timescales, meteoric δ18O was more negative during colder periods, such as Heinrich cooling events and the cooling event around 7 ka, and less negative in warmer periods, such as Hypsithermal warming. These relations indicated co-evolution of terrestrial paleotemperature and paleoprecipitation. A temperature dependency of 18O fractionation from water to vapor is a likely reason for the negative correlation between temperature and meteoric δ18O. Additionally, it is possible that increasing lower δ18O precipitation from East Asian winter monsoon (EAWM) has decreased the averaged meteoric δ18O in colder periods. These temperature effects on meteoric δ18O occur in opposite directions to fractionation between water and the stalagmite δ18O, explaining the small amplitudes of changes observed in the δ18O of Japanese stalagmites.