Past monsoons : A review of proxy data and modelling

Recent modelling studies have given insight into the role of internal feedback processes among components of the climate system on the evolution of monsoon strength since the Last Glacial Maximum (21,000 years ago). Here we present an overview of these modelling studies related to the summer monsoon over India and northern Africa. These studies indicate that the seasonal insolation changes alone do not explain the observed extent of hydrological changes during the early and middle Holocene over northern Africa. To simulate the extent of observed changes during this period incorporation of vegetation as an active component in climate models appears to be necessary. Over the Indian region, model results show that precipitation-soil moisture feedbacks play an important role in determining the response of the monsoon to changes in insolation and glacial-age surface boundary conditions. Indian monsoon strength from proxy records during the early and middle. Holocene have also been used in conjunction with coupled ocean atmosphere general circulation model experiments to refute the suggestion that semi-permanent warm surface conditions prevailed over equatorial Pacific ocean from 11 to 5ka.

Rainwater isotopes in central Vietnam controlled by two oceanic moisture sources and rainout effects

<p>Oxygen isotopes are commonly used proxies in paleoclimate research, however, a correct interpretation requires a detailed understanding of processes controlling isotope variability for a specific site.  A common interpretation for oxygen isotopes in precipitation across the Asian monsoon region, links the seasonal and interannual variability to changes in the summer monsoon strength.</p><p>However, some locations within tropical Asia do not receive rainfall during the summer monsoon season. In central Vietnam most of the annual rainfall falls during autumn instead of summer, making central Vietnam ideal to investigate processes controlling rainwater isotope variability, independent from the summer monsoon. By using rainwater isotopes, collected over five years, and moisture uptake simulations for these time periods, we investigate the seasonal cycle and interannual variability of hydrological processes in central Vietnam.</p><p>Our results show that the seasonal variability is dominated by a shift in moisture source from the Indian Ocean in summer to the South China Sea (western Pacific) from autumn to spring. The different source locations are reflected by an increase in δ<sup>18</sup>O values from around − 8 to − 10‰ during summer to values between 0 and − 3‰ during winter/spring. Further, we show that the amount effect and the occurrence of tropical cyclones, which are typical for the region, have no effect on a seasonal to interannual scale. Instead, we find that the timing of the seasonal ITCZ migration is a driving component of variability on these time scales.</p>

Dual response of Arabian Sea cyclones and strength of Indian monsoon to Southern Atlantic Ocean

Variability and trends of the south Asian monsoon at different time scales makes the region susceptible to climate-related natural disasters such as droughts and floods. Because of its importance, different studies have examined the climatic factors responsible for the recent changes in monsoon strength. Here, using observations and climate model experiments we show that monsoon strength is driven by the variations of south Atlantic Ocean sea surface temperature (SASST). The mechanism by which SASST is modulating the monsoon could be explained through the classical Matsuno-Gill response, leading to changes in the characteristics of vertical wind shear in the Arabian Sea. The decline in the vertical wind shear to the warming of SASST is associated with anomalous lower (upper)-level easterlies (westerlies). This further leads to a strong increase in the frequency of the Arabian Sea cyclones; and also prohibits the transport of moisture to the Indian landmass, which eventually reduces the strength of monsoon. The conditions in the SASST which drove these responses are aggravated by greenhouse gas emission, revealing the prominent role played by anthropogenic warming. If, with proper mitigation, these emissions are not prevented, further increases in the SASST is expected to result in increased Arabian sea cyclones and reduced monsoon strength.

Rainwater isotopes in central Vietnam controlled by two oceanic moisture sources and rainout effects

The interpretation of palaeoclimate archives based on oxygen isotopes depends critically on a detailed understanding of processes controlling the isotopic composition of precipitation. In the summer monsoonal realm, like Southeast Asia, seasonally and interannually depleted oxygen isotope ratios in precipitation have been linked to the summer monsoon strength. However, in some regions, such as central Vietnam, the majority of precipitation falls outside the summer monsoon period. We investigate processes controlling stable isotopes in precipitation from central Vietnam by combining moisture uptake calculations with monthly stable isotope data observed over five years. We find that the isotopic seasonal cycle in this region is driven by a shift in moisture source from the Indian Ocean to the South China Sea. This shift is reflected in oxygen isotope ratios with low values (− 8 to − 10‰) during summer and high values during spring/winter (0 to − 3‰), while 70% of the annual rainfall occurs during autumn. Interannual changes in precipitation isotopes in central Vietnam are governed by the timing of the seasonal onset and withdrawal of the Intertropical Convergence Zone, which controls the amount of vapour contributed from each source.

Weather Generator–Based Downscaling of EAWM Strength Prediction to the Climate of a Korean Basin

In this paper, we propose a downscaling method that statistically describes a local-scale climate from large-scale circulations using the case of a Korean basin during boreal winter. Specifically, since the East Asian winter monsoon (EAWM) affects the climate of the Korean Peninsula, we make a weather generator model describing the response of the basin climate to the monsoon strength. Moreover, it operates on the basis of a tercile probabilistic prediction of the EAWM strength to generate diverse scenarios of daily weather sequence during the season, which can be utilized in evaluation of the climate impact. We evaluate the prediction skills of operational hindcasts for several existing EAWM indices by applying a multinomial logistic regression method to choose the most suitable index for the downscaling. In the weather generator model, the precipitation model part is designed to be fully parametric. Its parameter values are allowed to vary according to the monsoon strength so that they can represent the climate variability of precipitation. In the temperature model part, the daily temporal variations of the temperature over the Korean basin are decomposed into several oscillations with different frequencies. Since the slowly varying oscillations significantly respond to the monsoon strength, the proposed downscaling scheme is based on the statistical simulation of oscillations according to the monsoon strength. The proposed downscaling scheme is evaluated in terms of the reproducibility of the climate characteristics for a given EAWM strength and the informativeness for predicting monthly climate characteristics.

Relationship between Asian monsoon strength and transport of surface aerosols to the Asian Tropopause Aerosol Layer (ATAL): interannual variability and decadal changes

In this study, we have investigated the interannual variability and the decadal trend of carbon monoxide (CO), carbonaceous aerosols (CA) and mineral dust in the Asian Tropopause Aerosol Layer (ATAL) in relation to varying strengths of the South Asian summer monsoon (SASM) using MERRA-2 reanalysis data (2001–2015). Results show that during this period, the aforementioned ATAL constituents exhibit strong interannual variability and rising trends connected to the variations of the strength of SASM. During strong monsoon years, the Asian monsoon anticyclone (AMA) is more expansive and shifted northward compared to weak years. In spite of the effect of quenching of biomass burning emissions of CO and CA by increased precipitation, as well as the removal of CA and dust by increased washout from the surface to the mid-troposphere in monsoon regions, all three constituents are found to be more abundant in an elongated accumulation zone in the ATAL, on the southern flank of the expanded AMA. Enhanced transport to the ATAL by overshooting deep convection is found over preferred pathways in the Himalayan-Gangetic Plain (HGP) and the Sichuan Basin (SB). The long-term positive trends of ATAL CO and CA are robust, while the ATAL dust trend is weak due to its large interannual variability. The ATAL trends are associated with increasing strength of the AMA, with earlier and enhanced vertical transport of ATAL constituents by enhanced overshooting convection over the HGP and SB regions, outweighing the strong reduction of CA and dust from the surface to the mid-troposphere.

Indian winter and summer monsoon strength over the 4.2 ka BP event in foraminifer isotope records from the Indus River delta in the Arabian Sea

The plains of northwest South Asia receive rainfall during both the Indian summer (June–September) and winter (December–March) monsoon. Researchers have long attempted to deconstruct the influence of these precipitation regimes in paleoclimate records, in order to better understand regional climatic drivers and their potential impact on human populations. The mid–late Holocene transition between 5.3 and 3.3 ka is of particular interest in this region because it spans the period of the Indus Civilization from its early development, through its urbanization, and onto eventual transformation into a rural society. An oxygen isotope record of the surface-dwelling planktonic foraminifer Globigerinoides ruber from the northeast Arabian Sea provided evidence for an abrupt decrease in rainfall and reduction in Indus River discharge at 4.2 ka, which the authors linked to the decline in the urban phase of the Indus Civilization (Staubwasser et al., 2003). Given the importance of this study, we used the same core (63KA) to measure the oxygen isotope profiles of two other foraminifer species at decadal resolution over the interval from 5.4 to 3.0 ka and to replicate a larger size fraction of G. ruber than measured previously. By selecting both thermocline-dwelling (Neogloboquadrina dutertrei) and shallow-dwelling (Globigerinoides sacculifer) species, we provide enhanced detail of the climatic changes that occurred over this crucial time interval. We found evidence for a period of increased surface water mixing, which we suggest was related to a strengthened winter monsoon with a peak intensity over 200 years from 4.5 to 4.3 ka. The time of greatest change occurred at 4.1 ka when both the summer and winter monsoon weakened, resulting in a reduction in rainfall in the Indus region. The earliest phase of the urban Mature Harappan period coincided with the period of inferred stronger winter monsoon between 4.5 and 4.3 ka, whereas the end of the urbanized phase occurred some time after the decrease in both the summer and winter monsoon strength by 4.1 ka. Our findings provide evidence that the initial growth of large Indus urban centers coincided with increased winter rainfall, whereas the contraction of urbanism and change in subsistence strategies followed a reduction in rainfall of both seasons.

Climate evolution across the Mid-Brunhes Transition

The Mid-Brunhes Transition (MBT) began ∼ 430 ka with an increase in the amplitude of the 100 kyr climate cycles of the past 800 000 years. The MBT has been identified in ice-core records, which indicate interglaciations became warmer with higher atmospheric CO2 levels after the MBT, and benthic oxygen isotope (δ18O) records, which suggest that post-MBT interglaciations had higher sea levels and warmer temperatures than pre-MBT interglaciations. It remains unclear, however, whether the MBT was a globally synchronous phenomenon that included other components of the climate system. Here, we further characterize changes in the climate system across the MBT through statistical analyses of ice-core and δ18O records as well as sea-surface temperature, benthic carbon isotope, and dust accumulation records. Our results demonstrate that the MBT was a global event with a significant increase in climate variance in most components of the climate system assessed here. However, our results indicate that the onset of high-amplitude variability in temperature, atmospheric CO2, and sea level at ∼430 ka was preceded by changes in the carbon cycle, ice sheets, and monsoon strength during Marine Isotope Stage (MIS) 14 and MIS 13.