Explore the vast range of titles available.

The seasonal evolution of the upper tropospheric South Asian high follows and influences underlying summer monsoon advancement. A strong connection between the South Asian high and westerly perturbation to the north suggests further planetary-scale dynamical control of the monsoon. In the mid-1990s, a clear location shift of the South Asian high in May–June was noted and was observed in fewer (more) frequencies of the high centers over the Indochina Peninsula (Iranian Plateau). Continental confinement of monsoonal circulation and precipitation was observed during 1995–2010, as opposed to larger-scale development in the Asia–Pacific region during 1979–1994. In view of early-summer monsoon evolution, a westward shifting and faster migration of the South Asian high may imply increased control of the midlatitude dynamics. By contrast, the convection over the tropical Western North Pacific (WNP) has an opposite and delayed contribution to monsoon advancement. After the mid-1990s than it had been previously, the midlatitude jet stream largely weakened over northern Africa and the East Asia–Pacific region, corresponding to an increase in the upper tropospheric geopotential heights north of the jet stream. Climate model experiments further reveal that the warming over Europe–Asia and temperature change in the North Atlantic can result in the change in midlatitude perturbations and the monsoon evolution in the mid-1990s, suggesting large-scale and dynamic impact on monsoon climatology.

The remote influence of west Pacific typhoons on the historic Kerala flood in the 2018 Indian summer monsoon (ISM) season is investigated using the weather research and forecasting (WRF-ARW) model. The flood occurred as a result of vigorous monsoon intra-seasonal activity with some districts receiving excess rainfall exceeding 405%. Observational data reveals that a deflection of the monsoon low level jet (LLJ) occurred from south westerly to north westerly direction together with a slow-down of cloud movement over the central Kerala region due to a split in the LLJ core nearby the Arabian sea coast. The split and deflection of LLJ core produced cyclonic vorticity along 10° N latitude which favored severe convection. Nevertheless, a detailed analysis of the flow pattern reveals that the simultaneous formation of a low-pressure system (near Orissa coast in the Bay of Bengal) and several typhoons in the west-Pacific Ocean had a crucial role in the deflection and hence the production of cyclonic vorticity. By employing the tropical cyclone bogussing scheme in WRF-ARW, the formation of a west-Pacific typhoon was suppressed and subsequently a comparison was made against the control run in order to quantify the typhoon’s effect on the Kerala rainfall. It is found that the presence of low-latitude typhoon was primarily responsible for the deflection of the monsoon LLJ and hence the production of cyclonic vorticity through remote forcing. The study thus provides an insight into the teleconnection of west-Pacific typhoons on the intraseasonal variation of ISM and associated extreme rainfall events.

Southwestern North America (SWNA), like many subtropical regions, is predicted to become drier in response to anthropogenic warming. However, during the Pliocene, when carbon dioxide was above pre‐industrial levels, multiple lines of evidence suggest that SWNA was much wetter. While existing explanations for a wet Pliocene invoke increases in winter rain, recent modeling studies hypothesize that summer rain may have also played an important role. Here, we present the first direct evidence for an intensified mid‐Pliocene monsoon in SWNA using leaf wax hydrogen isotopes. These new records provide evidence that the mid‐Pliocene featured an intensified and expanded North American Monsoon. Using proxies and isotope‐enabled model simulations, we show that monsoon intensification is linked to amplified warming on the southern California margin relative to the tropical Pacific. This mechanism has clear relevance for understanding present‐day monsoon variations, since we show that intervals of amplified subtropical warming on the California margin, as are seen during modern California margin heat waves, are associated with a stronger monsoon. Because marine heat waves are predicted to increase in frequency, the future may bring intervals of “Pliocene‐like” rainfall that co‐exist with intensifying megadrought in SWNA, with implications for ecosystems, human infrastructure, and water resources. Plain Language Summary The middle Pliocene, an interval approximately 3 million years ago, has long puzzled climate scientists. Despite having higher‐than‐preindustrial carbon dioxide levels, which should result in drier conditions in subtropical regions, some subtropical regions were wet during the Pliocene. In southwestern North America, there were large permanent lakes and plant and animal species that cannot exist in arid regions. We used measurements of hydrogen isotopes in ancient plant matter to show that wet conditions in the Pliocene southwest resulted from a stronger monsoon. This stronger monsoon was caused by changes in subtropical and tropical ocean temperatures in the eastern Pacific. This study presents the first direct evidence that monsoon changes caused wet conditions in the middle Pliocene. It also has relevance for the present, since we find evidence that present‐day changes in subtropical ocean temperatures can amplify the monsoon, via a mechanism that strongly resembles what happened in the Pliocene. Our study suggests that further studies of the Pliocene can shed light on how future monsoon changes may influence wildfire, landscapes, and water resources across the southwest. Key Points Leaf wax hydrogen isotopes preserved in ocean sediments reveal evidence of a stronger mid‐Pliocene monsoon in southwestern North America Isotope‐enabled simulations show that a stronger monsoon resulted from a diminished east Pacific subtropical‐tropical temperature gradient This mechanism is relevant to understanding present‐day monsoon variability in response to California margin marine heat waves

Clouds and aerosols contribute substantial uncertainty to the estimation of precipitation patterns at the global and regional scale. The present study focuses on understanding spatiotemporal and vertical variability in aerosol–cloud interactions over western India and the Arabian Sea. The study was conducted for the period 2000–2018 using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Tropical Rainfall Measuring Mission (TRMM) satellites and a regional climate model. In situ measurements of rainfall for western India from the India Meteorological Department (IMD) were also used. High values of aerosol optical depth (0.21–1.10) were observed for deficit rain, whereas low values (0.15–0.85) accounted for excess rain over western India. Over both regions, a negative linear correlation between aerosol optical depth and cloud effective radius and between aerosol optical depth and precipitation rate for excess, normal, and deficit rainfall conditions, with variations in correlation coefficient and slope values for the three monsoon conditions. In general, higher slope values were observed for excess rain over both regions, signifying a greater rate of change in cloud effective radius and precipitation rate with change in aerosol optical depth. Simulated cloud parameters including fractional cloud cover and the mass fraction of cloud liquid water content showed significant vertical variability for different monsoon conditions along different latitudes and longitudes for western India and the Arabian Sea, respectively.

Monsoons involve increases in dry static energy (DSE), with primary contributions from increased shortwave radiation and condensation of water vapor, compensated by DSE export via horizontal fluxes in monsoonal circulations. We introduce a simple box-model characterizing evolution of the DSE budget to study nonlinear dynamics of steady-state monsoons. Horizontal fluxes of DSE are stabilizing during monsoons, exporting DSE and hence weakening the monsoonal circulation. By contrast latent heat addition (LHA) due to condensation of water vapor destabilizes, by increasing the DSE budget. These two factors, horizontal DSE fluxes and LHA, are most strongly dependent on the contrast in tropospheric mean temperature between land and ocean. For the steady-state DSE in the box-model to be stable, the DSE flux should depend more strongly on the temperature contrast than LHA; stronger circulation then reduces DSE and thereby restores equilibrium. We present conditions for this to occur. The main focus of the paper is describing conditions for bifurcation behavior of simple models. Previous authors presented a minimal model of abrupt monsoon transitions and argued that such behavior can be related to a positive feedback called the ‘moisture advection feedback’. However, by accounting for the effect of vertical lapse rate of temperature on the DSE flux, we show that bifurcations are not a generic property of such models despite these fluxes being nonlinear in the temperature contrast. We explain the origin of this behavior and describe conditions for a bifurcation to occur. This is illustrated for the case of the July-mean monsoon over India. The default model with mean parameter estimates does not contain a bifurcation, but the model admits bifurcation as parameters are varied.

The 1998 extremely heavy rainfall events over East Asia are investigated through partial lateral forcing (PLF) experiments with the Weather Research and Forecasting model to determine the impacts of the synoptic (SY), intra-seasonal (IS), and inter-annual (IA) forcing across the lateral boundary on the extreme climate anomalies. The large-scale lateral boundary forcing was derived from an ensemble reanalysis dataset and decomposed into climatological, SY, IS, and IA components. The PLF experiments show that the IS forcing not only triggers the monsoon onset and drives two northward propagation events of the subtropical front but also has dominant contributions to the two heaviest rainfall events over the Yangtze River Basin (YRB) and South China, suggesting the critical role of the intra-seasonal variability in the devastating 1998 floods. Previous studies perceived that the northward propagating IS oscillation from the tropics regulates the extreme heavy rainfall of East Asia summer monsoon in 1998. However, we find that the IS forcing from the mid-latitude plays a more important role than the forcing from the tropics in generating the two extreme rainfall events in 1998. During the first extreme event in June, the IS forcing across the western boundary is the major cause of the northward advance of the subtropical front and the heavy rainfall over the YRB and South China, with the IS forcing across the northern boundary providing the second largest contribution. During the second extreme event (July 15–August 5), the IS forcing from the eastern boundary plays a dominant role in driving the southward retreat and northward advance of the subtropical front, causing another heavy rainfall over the YRB and South China. The western and northern IS forcing also has large contributions to the second extreme event. We have estimated the contributions to the seasonal anomalous rainfall by the three types of forcing. The SY forcing tends to have a moderate effect on the YRB rainfall but significant reduction of the rainfall in South China. The IS forcing has dominant contributions to the seasonal-mean rainfall anomalies over all three sub-regions of China (North China, the YRB, and South China). The IA forcing mainly enhances the rainfall in South China but reduces the precipitation in the YRB slightly. This study portends a promising application of regional climate models to identify key factors causing extreme climate events. The PLF methodology can be used to study a broad range of climate phenomena and to understand the effects of variety of dynamic and physical processes in climate variability and predictability.

The simulated response of the Indian summer monsoon to increasing greenhouse gases has been fairly robust, with a majority of the climate models showing an increase in the seasonal mean rainfall and a decrease in the intensity of the associated monsoon circulation. The background global warming signal produces an increased surface and lower tropospheric meridional temperature gradient over the Indian region, while also producing anomalously positive sea surface temperatures (SSTs) in the tropical Pacific. Our analysis reveals that the monsoon outflow associated with the local circulation driven by the differential (solar plus convective) heating explains more monsoon rainfall variance than that explained by the Walker circulation driven by the remote tropical Pacific SST anomalies under a warming scenario. In the simulated present‐day climate, Walker circulation explains more rainfall variance than that explained by the local circulation, consistent with the well‐documented conventional relationships between the monsoon rainfall and ENSO. Thus, remote and local circulations reverse their corresponding roles on influencing the monsoon rainfall. Climatologically, the mean ascending branch of the Walker circulation shifts eastward, resulting in an anomalous upper level convergence and corresponding anomalous lower level divergence over the eastern tropical Indian Ocean. The resulting pressure gradients at lower levels favor the northwestward displacement of low‐level monsoon flow. At upper levels an anomalous descending motion produces an anomalous adiabatic compression over the south of the equator, reducing the upper level meridional temperature gradient. Changes in the mean local circulation reinforce the northwestward displacement of the low‐level monsoon flow and also promote upper level meridional divergent flow toward the equator. Key Points Local processes play a more important role on the future monsoon rainfall Changes in the remote circulation promote northward shift in the monsoon Remote and local circulations reverse their roles on influencing the rainfall

The present study examines changes in the low-level summer monsoon circulation over the Arabian Sea and their impact on the ocean dynamics using reanalysis data. The study confirms intensification and northward migration of low-level jet during 1979 to 2015. Further during the study period, an increase in the Arabian Sea upper ocean heat content is found in spite of a decreasing trend in the net surface heat flux, indicating the possible role of ocean dynamics in the upper ocean warming. Increase in the anti-cyclonic wind stress curl associated with the change in the monsoon circulation induces downwelling over the central Arabian Sea, favoring upper ocean warming. The decreasing trend of southward Ekman transport, a mechanism transporting heat from the land-locked north Indian Ocean to southern latitudes, also supports increasing trend of the upper ocean heat content. To reinstate and quantify the role of changing monsoon circulation in increasing the heat content over the Arabian Sea, sensitivity experiment is carried out using ocean general circulation model. In this experiment, the model is forced by inter-annual momentum forcing while rest of the forcing is climatological. Experiment reveals that the changing monsoon circulation increases the upper ocean heat content, effectively by enhancing downwelling processes and reducing southward heat transport, which strongly endorses our hypothesis that changing ocean dynamics associated with low-level monsoon circulation is causing the increasing trend in the heat content of the Arabian Sea.

The authors present a reconstruction of summer (June-July-August) mean dynamic Indian monsoon index (DIMI) back to 1880 based on a large number of historical surface observation data as well as information from the upper air data. The reconstruction shows a satisfying skill in terms of both the value of reduction of error and an evaluation against other independent monsoon indices. The skill of reconstruction increases over time with more predictor data (in particular upper-level data) becoming available. A comparison with the observed all Indian summer monsoon rainfall index (AIRI) shows a high consistence in both inter-decadal and inter-annual variability. The reconstruction shows stronger than normal monsoon during the 1880s, 1915-1925 (around 1920) and 1930-1945 (around 1940) as the AIRI. The El Nino/Southern Oscillation (ENSO)—monsoon relationship is reasonably captured in the reconstruction. Powers concentrating within quasi-biennial band stand out in the reconstruction as well as in the AIRI. A comparison of the reconstruction against an atmospheric general circulation model simulation with specified SST and external forcing agents spanning 1901-1999 indicates a slightly higher reproducibility of monsoon circulation than monsoon rainfall in terms of interannual variability. The relationship between the Asian continent warming and the ENSO-monsoon connection is also discussed by using the new dynamic index.

Based on monthly ECMWF reanalysis-Interim （ERA-Interim） reanalysis data, along with monthly precipitation and temperature data, the Dynamic Plateau Monsoon Index （DPMI） is defined. The results of a contrast analysis of the DPMI versus the Traditional Plateau Monsoon Index （TPMI） are described. The response of general circulation to northern Qinghai-Xizang Plateau summer monsoon anomalies and the correlation of the DPMI with general circulation anomalies are investigated. The results show that, the DPMI reflected meteorological elements better and depicted climate variation more accurately than the TPMI. In years when the plateau summer monsoon is strong, the low over the plateau and the trough near the eastern coast of Asia are deeper and higher than normal over South China. This correlation corresponds to two anomalous cyclones over the plateau and the eastern coast of Asia and an anomalous anticyclone in South China. The plateau and its adjacent regions are affected by anomalous southwesterly winds that transport more moisture to South China and cause more precipitation. The lower reaches of the Yangtze River appear to receive more precipitation by means of the strong westerly water vapor flow transported from the ＂large triangle affecting the region＂. In years when the plateau summer monsoon is weak, these are opposite. The plateau monsoon is closely related to the intensity and position of the South Asian high, and the existence of a teleconnection pattern in the mid-upper levels suggests a possible linkage of the East Asian monsoon and the Indian monsoon to the plateau summer monsoon.