Indian Summer Monsoon Region Research Articles

Asian Summer Monsoon Changes Inferred From a Stalagmite δ18O Record in Central China During the Last Glacial Period

The reconstruction of Asian summer monsoon (ASM) changes during the last glacial period is of great significance for better understanding monsoon dynamics. The phase relationship between the Indian summer monsoon (ISM) and East Asian summer monsoon (EASM) subsystems on different timescales is still unclear. The comparative analysis of speleothem records in the ISM region, EASM region, and central China helps to clarify the relationship between the ISM and EASM. Based on the well-dated isotope records of stalagmite DDH-B15 from the Didonghe (DDH) Cave in Hanzhong, Shaanxi, we reconstructed ASM changes during the past 34–13 thousand years before the present (kyr BP). The small average error (61 years) of 18 uranium-series ages enables a precise comparison of the stalagmite δ18O record with other well-dated records from the orbital to the millennial timescales. The δ18O signal of the DDH-B15 stalagmite is controlled by changes of the low latitude northern hemisphere summer insolation (NHSI) on the orbital timescale. It records cold Heinrich Stadial (HS) and Dansgaard–Oeschger (DO) cycles which are originated from the northern high latitude on the millennial time scale. The δ18O changes of stalagmites from the three regions are similar on the millennial and centennial timescales. But on the orbital-suborbital timescale, stalagmite δ18O changes during the last glacial cycle have different characteristics. The stalagmite δ18O values in eastern China became gradually negative, and the stalagmite δ18O values in the Indian monsoon domain showed a increasing trend, but the stalagmite δ18O values in Central China adopted an intermediate state between the EASM and ISM. Then we argued that the δ18O value of stalagmites in Central China is a mixed signal of the ISM and EASM, which indicates a change of the water vapor source as an important influence on the Chinese stalagmite δ18O record.

Land surface processes over Indian summer monsoon region: A review

Land surface processes over Indian summer monsoon region: A review

A 17 ka multi‐proxy paleoclimatic record on the northeastern Tibetan Plateau: implications for the northernmost boundary of the Asian summer monsoon during the Holocene

AbstractKnowledge of the paleoclimatic record of the northeastern Tibetan Plateau (NETP) can potentially improve our understanding of the evolution of the Asian summer monsoon (ASM). However, the history of climate change and inferred spatial extent of the ASM on the NETP since the last deglaciation remain unclear. Here, we use several environmental proxies from the sediments of Hala Lake (beyond the modern limit of ASM), including chironomids, loss‐on‐ignition, grain size and element data, to explore the climatic history of the NETP and the northern boundary of the ASM since the last deglaciation. The results document a series of climatic events during the deglaciation, including Heinrich Event 1, the Bølling–Allerød interstadial and the Younger Dryas event. The records also reveal the timing of the megathermal and precipitation maximum, the lake‐level maximum, and strongest chemical weathering, which occurred during ~10–7 ka. The inferred precipitation maximum during the early Holocene in the Hala Lake basin, which can be verified by the simulated precipitation change, is consistent with that in typical Indian summer monsoon (ISM) regions, suggesting that the ISM has penetrated into Hala Lake basin at that time. The monsoon‐dominated climate in the Hala Lake basin during the early Holocene and the westerlies‐dominated climate in the arid central Asia indicate that the maximum areal extent of the ASM on the NETP since the last deglaciation lay to the northwest of Hala Lake basin. In combination with other published records, the northernmost boundary of the ASM over China since the last deglaciation has been tentatively delineated, to shed some lights on this long‐standing debate.

Dynamical mechanisms linking Indian monsoon precipitation and the circumglobal teleconnection

The circumglobal teleconnection (CGT) is an important mode of circulation variability, with an influence across many parts of the northern hemisphere. Here, we examine the excitation mechanisms of the CGT in the ECMWF seasonal forecast model, and the relationship between the Indian summer monsoon (ISM), the CGT and the extratropical northern hemisphere circulation. Results from relaxation experiments, in which the model is corrected to reanalysis in specific regions, suggest that errors over northwest Europe are more important in inhibiting the model skill at representing the CGT, in addition to northern hemisphere skill more widely, than west-central Asia and the ISM region, although the link between ISM precipitation and the extratropical circulation is weak in all experiments. Thermal forcing experiments in the ECMWF model, in which a heating is applied over India, suggest that the ISM does force an extratropical Rossby wave train, with upper tropospheric anticyclonic anomalies over east Asia, the North Pacific and North America associated with increased ISM heating. However, this eastward-propagating branch of the wave train does not project into Europe, and the response there occurs largely through westward-propagating Rossby waves. Results from barotropic model experiments show a response that is highly consistent with the seasonal forecast model, with similar eastward- and westward-propagating Rossby waves. This westward-propagating response is shown to be important in the downstream reinforcement of the wave train between Asia and North America.

Organic carbon burial and quasi-periodic environmental changes in the Indian summer monsoon region during the Holocene

Organic carbon (OC) burial in lacustrine sediments represents an important sink in the global carbon cycle. However, limited knowledge is available on the spatial-temporal variation and driving factors in OC burial of lake sediment during the Holocene. Based on the environmental proxies, grain-size end-member modelling analysis (EMMA) and wavelet transform of the Fuxian Lake sediments, the key factors of the organic carbon accumulation rate (OCAR) and the driving mechanism of climate in the Indian summer monsoon (ISM) region have been discussed during the Holocene. The results show that there was significant correlation between the environmental proxies and the OCAR, and 1000-a, 500-a and 100-a quasi-periodic signals were present in the environment proxies. The ISM intensified with an increase in low-latitude solar insolation (LSI) during the early- to mid-Holocene. It brought the warm-humid climate, the increased biological productivity and the OC input of the lake catchment. Therefore, climate change was the primary influencing factor on OCAR at this stage. In the late-Holocene, the weakening LSI and frequent human activities of the Fuxian Lake catchment caused a decrease in vegetation coverage and an increase in soil erosion, and it indicated that the intense human activities and climate change had to work together to affect the OCAR of the lake catchment. The results would provide some references for predicting the carbon sequestration potential of a lake under the influence of climate change and human activities, and it can also provide theory guidance for controlling carbon storage of lake ecosystem under global warming.

Response of the Indian summer monsoon to global warming, solar geoengineering and its termination

The response of the Indian Summer Monsoon (ISM) to global warming, solar geoengineering and its termination is examined using the multi-model mean of seven global climate model simulations from G2 experiment of the Geoengineering Model Intercomparison Project. Under the global warming scenario, land–ocean temperature contrasts and low-level monsoon circulation progressively strengthen accompanied by enhanced precipitation over the Indian subcontinent. Notably, in the solar geoengineered scenario, marginal surface cooling is projected over the majority of the ISM region, and there is strengthening of both upper and lower level circulation. However, preferential precipitation near Western Ghats leads to dry bias over majority of Indian land. Upon the termination of the geoengineering, the climatic conditions—temperature, precipitation, winds and moisture would abruptly change to what it would have been under the global warming scenario. Thus, this may be important to note that such changes may need attention for the future mitigation and adaptation purposes if solar geoengineering is required to implement in future.

Prediction of dominant daily modes of the Indian summer monsoon in the NCEP GFS model

The prediction capability of dominant daily monsoon modes of Indian summer monsoon in the forecast of the Global Forecast System Version 2 (GFSv2) model is scrutinized. The dominant monsoon modes are procured by performing the multichannel singular spectrum analysis (MSSA) on daily precipitation anomalies of the Indian summer monsoon region (60–100°E, Eq.-35°N) during 2001–2014. The observation has one seasonally persistent mode and two intraseasonal oscillations with periods around 42 and 26 days, and the model has correctly simulated these modes. The spatial structure of the phase composites of the precipitation anomalies of the intraseasonal modes of the model is almost similar to the observed spatial pattern with slightly less magnitude of the precipitation anomalies over Western India and the core monsoon zone. The spatial structure of the 26-day mode is similar to the spatial structure of the 42-day mode with less magnitude of the precipitation anomalies all over the study domain. The lead forecast of the model demonstrates the robust predictive skill of intraseasonal modes. The variation of the active and break spells of monsoon precipitation over the Indian subcontinent is captured accurately by the contribution of both intraseasonal modes. The observed eastward and northward propagation features of the Indian summer monsoon have been accurately simulated by the model. The model has weak seasonally persistent signals over Western India, northeast India, and eastern land regions adjacent to the Western Ghats. The seasonally persistent mode shows a strong relationship with the equatorial central Pacific Ocean SSTs and a moderate correlation with the Indian and Atlantic Ocean SSTs. The seasonally persistent mode contributes largely to the seasonal precipitation anomalies over the Indian monsoon region.

Diurnal variation of deep convective clouds over Indian monsoon region and its association with rainfall

Diurnal cycle of deep convective clouds (DCC) over the Indian Summer Monsoon Region is studied. A harmonic analysis has been applied on the 3 hourly deep convective cloud fraction estimated from satellite observations to derive diurnal and semi-diurnal amplitudes of deep convective clouds. The diurnal cycle of deep convective clouds shows seasonal variations and amplitudes which are stronger during the northern hemispheric summer than winter and exhibits prominence over land as compared to ocean. Irrespective of seasons, deep convection peaks in the late evening hours over land whereas the peak is in the afternoon hours over majority of the oceanic regions considered in our study. The reason for the difference is attributed to the semi-diurnal oscillation which determines the oceanic diurnal cycle of deep convection. Various parameters which represent the tropospheric instability are equally affected by the semi-diurnal oscillation as in deep convective clouds over ocean. These results suggest that diurnal tides could play a key role in governing the daily cycle of deep convective clouds over oceanic regions. The relation between diurnal amplitudes of DCC and rainfall is also explored and it is shown that the strength of the daily cycle of DCC has a strong bearing on the amount of rainfall observed over the Indian summer monsoon region.

Understanding the development of systematic errors in the Asian summer monsoon

Abstract. Despite the importance of monsoon rainfall to over half of the world's population, many climate models of the current generation struggle to capture some of the major features of the various monsoon systems. Studies of the development of errors in several tropical regions have shown that they start to develop very quickly, within the first few days of a model simulation, and can then persist to climate timescales. Understanding the sources of such errors requires the combination of various modelling techniques and sensitivity experiments of varying complexity. Here, we demonstrate how such analysis can shed light on the way in which monsoon errors develop, their local and remote drivers and feedbacks. We make use of the seamless modelling approach adopted by the Met Office, whereby different applications of the Met Office Unified Model (MetUM) use essentially the same model configuration (dynamical core and physical parameterisations) across a range of spatial and temporal scales. Using the Asian summer monsoon (ASM) as an example, we show that error patterns in circulation and rainfall over the ASM region in the MetUM are similar between multidecadal climate simulations and seasonal hindcasts initialised in spring. Analysis of the development of these errors on both short-range and seasonal timescales following model initialisation suggests that both the Maritime Continent and the oceans around the Philippines play a role in the development of East Asian summer monsoon errors, with the Indian summer monsoon region providing an additional contribution, while the errors over the Indian summer monsoon region itself appear to arise locally. Regional modelling with various lateral boundary locations helps to separate local and remote contributions to the errors, while regional relaxation experiments shed light on the influence of errors developing within particular areas on the region as a whole.

Model Uncertainty in the Projected Indian Summer Monsoon Precipitation Change under Low-Emission Scenarios

The projected ISM precipitation changes under low-emission scenarios, Representative Concentration Pathway 2.6 (RCP2.6) and Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), are investigated by outputs from models participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). Based on the high-emission scenarios like RCP8.5, the Intergovernmental Panel on Climate Change Fifth Assessment Report suggests a wetter Indian summer monsoon (ISM) by the end of 21st century. Although the multi-model ensemble mean (MME) ISM precipitation under RCP2.6 and SSP1-2.6 is still projected to increase over 2050–2099 referenced to 1900–1949, the intermodel spread of the ISM precipitation change is tremendous in both CMIPs. Indeed, the signal-to-noise ratio (SNR) of ISM precipitation change, defined as the MME divided by its intermodel standard deviation, is even below 1 under the low-emission scenarios. This casts doubts on a future wetter ISM in a warmer climate. Moisture budget analyses further show that most of the model uncertainty in ISM precipitation change is caused by its dynamical component from the atmospheric circulation change. As expected, the interhemispheric surface warming contrast is essential in causing the intermodel differences in ISM circulation and precipitation changes under low-emission scenarios. In addition, the projected wetter ISM is prominently enhanced from CMIP5 to CMIP6, along with reduced model uncertainty. However, the resultant increased SNR in CMIP6 is still low in most ISM regions. The results imply that ISM precipitation change is highly uncertain under low-emission scenarios, which greatly challenges the decisions-making in adaptation policies for the densely populated South Asian countries.

New insights towards an integrated understanding of NE Asian monsoon during mid to late Holocene

Speleothem records from NE Asia are essential to understand the spatial patterns of the Asian Summer Monsoon (ASM) variability. In this paper, we present a new high-resolution and chronologically well-resolved speleothem record from Liu-li (LL) cave, NE China, through δ18O, δ13C and Sr/Ca ratio characterizing a dynamic ASM history over the past 6680 years. The LL δ18O record shows a long-term trend to higher values, following the North Hemisphere summer insolation (NHSI), with the exception of the last 2 kyr characterized by a negative δ18O excursion. The ‘2-kyr shift’ is evident in NE Asia over the past 2 kyr, originally inferred from the speleothem δ18O negative excursion in South China, as well as in many other proxy records from East China, but it merely manifests a flat trend in most of the western East Asian Summer Monsoon (EASM) and Indian Summer Monsoon (ISM) regions, rather than a negative excursion. On multi-centennial to millennial timescales, the intensification of the Tsushima Warm Current (TWC) can amplify rainfall on both sides of the Japan Sea. A close comparison between the monsoon reconstruction and the cultural changes in NE Asia suggests contemporary climatic and cultural regime-shifts, implying an important role of the ASM weakening in the collapse of Neolithic and Early Bronze cultures in this region.

The vertical structure of latent heating and its association with cloud types during the Indian summer monsoon

ABSTRACT In the present communication, using cloud-type distribution derived from CloudSat observations during the years 2006–2010 and vertical structure of LH derived from Tropical Rain Measuring Mission (TRMM) observations during the same period over the Indian summer monsoon (ISM) region, an attempt is made to relate the both. First, the regions where particular type of clouds forms predominantly are identified and then the LH profiles over those regions are associated with that particular cloud type. Using this approach, mean vertical structure of LH of Deep convective (DC) clouds over the Bay of Bengal (BoB) and Central India (CI) region, LH profile of Stratocumulus (Sc) and Cumulus (Cu) clouds over the Arabian Sea (ARB) and the Western Ghats (WG) region, respectively are constructed. The LH profile associated with DC clouds shows a broad distribution with a bimodal structure over the BoB and CI regions with peak value around 7 km. The LH profiles associated with Sc and Cu cloud types peak at lower height levels corresponding to shallow convection over ARB and WG regions. The significance of the present study lies in indirectly associating the observed vertical structure of LH with various types of clouds over the ISM region.

Reconstruction of temperature and precipitation spanning the past 28 kyr based on branched tetraether lipids from Qionghai Lake, southwestern China

The Indian summer monsoon (ISM) significantly influences the climate of southwestern China; however, continuous quantitative temperature reconstructions since the Last Glacial Maximum are rare, which limits our understanding of ISM evolution. In this study, we analyzed the branched glycerol dialkyl glycerol tetraethers (brGDGTs) from a lacustrine sediment core from Qionghai Lake to reconstruct the paleoclimatology of southwestern subtropical China over the past 27.7 kyr. Our results suggest that the brGDGTs in Qionghai Lake sediments were probably sourced from catchment soils, with some contributions from in situ lacustrine production. We found that the temperature of the Last Glacial Maximum and Holocene Climatic Optimum was ~3 °C cooler and ~ 2.5 °C warmer than present-day conditions, respectively, based on brGDGTs-reconstructed mean annual air temperatures (MATs). Generally, temperature and precipitation showed synchronous paleovariability in ISM regions. Moreover, Northern Hemisphere summer insolation was the dominant control on climate variability in ISM regions on the precessional scale, while Atlantic Meridional Overturning Circulation activity, Intertropical Convergence Zone positioning, and glacial boundary conditions (ice sheet extent and sea surface temperature) contributed to climate shifts on millennial timescales.

Reassessment of land–atmosphere interactions over India during summer monsoon using state-of-the-art regional climate models

The evaluation and characteristics of land-surface processes over the Indian Summer Monsoon (ISM) region are studied using Dirmeyer's two-legged land-atmosphere coupling metrics in regional climate modelling system. Different land-atmosphere coupling metrics are used for assessing the “hot-spots” of land atmosphere feedback over the Indian subcontinent. The model simulations are performed using regional climate model (RCM)—RegCMv4.0, 4.2 and 4.4.5.10 at different horizontal resolutions. The monthly soil wetness climatology over India is calculated using European Space Agency (ESA) datasets. Maximum soil moisture(SM) is found over the western part of central India during June–September. The results from RCM simulations (RegCMv4.0, 4.2 and 4.4.5.10) indicate more soil wetness, systematically (over-predicted) over North India, Indo-Gangetic plains and central India during June, July, August and September, implying that model soil wetness is driven by precipitation minus evapotranspiration (P-E). The role of aerosols in land–atmosphere interactions along with the impact of mixed convective parameterization schemes over ocean and land on modelling SM and land–atmosphere interactions is also addressed. The spatial and temporal variation of atmospheric and terrestrial coupling indices during ISM regime concludes that May, June, July and August are the prime months of land–atmosphere interactions over central and north-west India reiterating results from previous GLACE (The Global Land–Atmosphere Coupling Experiment) studies. Model simulations also indicate that during the ISM season the terrestrial segment of land–atmosphere coupling (i.e. terrestrial coupling index of soil moisture, SM) from SM to latent heat flux varies around 35–40 W m−2, while the atmospheric segment of coupling (i.e. atmospheric coupling index of precipitation) from latent heat flux to precipitation modulates around 4–6 mm/day. Aerosols are found to influence land-atmosphere interactions over north-west India by modulating net radiation. More in-situ observations of land surface variables are required for verification of land-atmospheric interactions (along with impact of aerosols) using RCMs.

The impact of modified rate of precipitation conversion parameter in the convective parameterization scheme of operational weather forecast model (GFS T1534) over Indian summer monsoon region

The performance of the present operational global forecast system (GFS) at T1534 (~12.5 km) horizontal resolution with modified fractional cloud condensate to precipitation conversion parameter in the revised simplified Arakawa-Schubert (RSAS) convection scheme is evaluated for the summer monsoon seasons of 2018 and 2019 over the Indian region. The modified parameter has the form of an exponentially decreasing function of temperature above the freezing level. In contrast, below the freezing level, it is constant and is similar to default conversion parameter. The results reveal that the GFS T1534 with modified conversion parameter (EXPT) shows better fidelity in forecasting the mean summer monsoon rainfall over the Indian subcontinent region as compared to default GFS T1534 (CTRL). The rainfall probability distribution function analysis indicates a notable improvement in forecasting moderate and heavier category rainfall in EXPT as compared to CTRL. The improved distribution of total rainfall is found to be contributed by the proper forecasting of convective and large-scale rainfall in EXPT. It is likely that the reduced rate of conversion of cloud condensate to convective precipitation above the freezing level leads to a reduction in the convective rainfall, which eventually increases the moisture in the upper level through detrainment and hence enhances the large-scale precipitation. Further, EXPT shows relative improvement in forecasting outgoing longwave radiation, wind circulation, cloud fraction, dynamical-thermodynamical processes, and moist-convective feedback through improved lower tropospheric moistening over the Indian region. Finally, various skill score analyses suggest that EXPT shows better skill in predicting moderate and heavier category rainfall with longer lead time over the Indian subcontinent region. Considering the large socioeconomic impact of heavy and extreme precipitation over India, the modified conversion parameter can be incorporated in the present operational GFS T1534 model.

Geochemical records of Qionghai Lake sediments in southwestern China linked to late Quaternary climate changes

Knowledge of the chemical weathering history under the subtropical monsoonal climate will help to resolve the discrepancies regarding how climate conditions constrain chemical weathering. Major element and rare earth element (REE) compositions of the Qionghai Lake sediments in Sichuan Province, southwestern China were analyzed, aiming to provide information on sediment source discrimination and history of chemical weathering intensities and its responses to variations of Indian summer monsoon (ISM) and paleoclimate conditions. As revealed by the A-CN-K diagram and other provenance proxies including TiO2/Zr ratio, Th/Sc vs. Zr/Sc and La/Th vs. Hf diagrams, as well as the REE characteristics, the source of Qionghai Lake sediments was felsic rocks. Additionally, these sediments were subjected to small changes in provenance, sedimentary sorting and recycling, and weak diagenesis after deposition and/or metasomatism, indicating original signals were maintained on the intensity of chemical weathering of source area. The chemical weathering intensities inferred from CIA (chemical index of alteration) values have significantly fluctuated over the past 28 kyr, reflecting three stages of paleoclimate evolution, which is consistent with grain size characteristics, carbonate contents, low-frequency magnetic susceptibilities, TOC and biomarker proxy (Paq). The CIA values changed significantly during the typical warm and cold climate events including the Last Glacial Maximum, Heinrich 1 cold event, Younger Dryas and Holocene Climatic Optimum, and generally exhibited an increasing trend in amplitude correspondingly, in concert with the climate shifting from cold and dry to warm and humid conditions. Generally warm and humid climate conditions suggest enhanced chemical weathering intensity, whereas weak chemical weathering denotes cold and dry climate in the ISM region. In addition, sequential variations of the CIA values during the past 28 kyr may reflect changing chemical weathering processes, in responses to the varying intensities of Indian summer monsoon and the climate conditions forced by the Northern Hemisphere solar insolation.

Archaeal lipid-inferred paleohydrology and paleotemperature of Lake Chenghai during the Pleistocene–Holocene transition

Abstract. Over the past decades, paleoenvironmental studies in the Indian summer monsoon region have mainly focused on precipitation change, with few published terrestrial temperature records from the region. We analysed the distribution of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) in the sediments of Lake Chenghai in southwest China across the Pleistocene–Holocene transition, to extract both regional hydrological and temperature signals for this important transition period. The lake level was reconstructed from the relative abundance of crenarchaeol in isoGDGTs (%cren) and the crenarchaeol′/crenarchaeol ratio. The %cren-inferred lake level identified a single lowstand (15.4–14.4 ka cal BP), while the crenarchaeol′/crenarchaeol ratio suggests a relatively lower lake level between 15.4–14.4 and 12.5–11.7 ka cal BP, corresponding to periods of weakened ISM during the Heinrich 1 and Younger Dryas cold event. A filtered TetraEther indeX consisting of 86 carbon atoms (TEX86 index) revealed that lake surface temperature was similar to present-day values during the last deglacial period and suggests a substantial warming of ∼4 ∘C from the early Holocene to the mid-Holocene. Our paleotemperature record is generally consistent with other records in southwest China, suggesting that the distribution of isoGDGTs in Lake Chenghai sediments has potential for quantitative paleotemperature reconstruction.

Deterministic Seasonal Quantitative Precipitation Forecasts: Benchmark Skill with a GCM

Many applications, from agricultural planning (such as crop choice) to estimation of hydro-power and surface water availability require quantitative precipitation forecasts (QPF). While a large number of studies have addressed the problem of forecasting seasonal Indian summer monsoon (ISM) rainfall over the decades, the emphasis generally has been on simulation and forecasting of rainfall anomalies from long-period mean. However, given the trends in monsoon rainfall, the procedure of considering anomalies has inherent errors; thus reconstruction of actual rainfall from the anomalies does not necessarily provide accurate information. Most QPF have been attempted at short-range forecasts, with a variety of techniques like model output statistics. This work represents evaluation of an atmospheric Variable Resolution General Circulation Model (VRGCM) for QPF during monsoon season. The VRGCM, with variable grid resolution provides relatively high (~ 50 km) resolution over the ISM region, has been validated over all India as well as in different regions like Central and North India, South India, North East India for seasonal forecast of monsoon rainfall. VRGCM simulated QPF appears to provide comparable information as that of anomaly forecasts of monsoon over different regions in India. The forecast skill is appreciable, with significant correlation at all India, South India and North East India regions. The Root Mean Square Error of VRGCM in forecasting quantitative rainfall overall India and Central North India is very low and bit high over South and North East region of India. The performance of VRGCM in forecasting the rainfall in extreme (deficit or excess) monsoon years is also very high with phase synchronisation of 89% in the inter annual variability of rainfall during monsoon at all India scale while the value is about 57%, 76% and 55% over Central North, South and North East India region, respectively.

Indian monsoon precipitation isotopes linked with high level cloud cover at local and regional scales

Precipitation stable isotopes preserve historic changes of evaporation in the source regions and precipitation processes, therefore, they can be used to reveal regional hydrological cycle dynamics and paleoclimate reconstructions. In monsoon regions, strong inverse impacts of convection on precipitation isotope ratios, have created a debate regarding the interpretation of isotope records as local climate proxies. The proportions of stratiform to convective precipitation on water isotopes, together with the influence mechanisms on seasonal and interannual scales remain highly uncertain. To further address the influence of precipitation patterns on water isotopes, we used 10 yrs of precipitation isotope data from the southern Tibetan Plateau (TP) to explore the effects of large-scale cloud cover and local climate on precipitation isotopes. Correlation analysis performed between local precipitation δ18O values and different level cloud data, indicated significant negative correlations between precipitation isotopes and high level cloud cover on both seasonal and interannual time scales. This result suggests that high-level convection in the upper moisture transport stream is a main control on precipitation isotopes in the southern TP. The clear and coherent variations of precipitation isotopes with the Southern Oscillation Index and outgoing longwave radiation confirmed that strong convection activity in the moisture source region and during transport significantly depleted heavy isotopes in vapor, producing substantially decreased precipitation δ18O in the study region. These results agree with earlier findings of tree ring cellulose isotope records that correlate with cloud cover, but we emphasized the important role of larger-scale regional cloud cover. We also delineated different maximum correlation zones for seasonal and interannual time scales, likely due to different mechanisms. These findings further improve the interpretation of paleoisotope records from the Indian summer monsoon region.

Impact of air\u2013sea coupling on Northern Hemisphere summer climate and the monsoon\u2013desert teleconnection

Previous studies have established the existence of a large-scale teleconnection between ascent in the South Asian Summer Monsoon and subsidence over the Mediterranean (known as “the monsoon–desert mechanism”). Improving the representation of this mechanism could potentially improve the skill of seasonal forecasts for European summer weather patterns. In this study, the impact of air–sea coupling on the NH summer climate and the representation of the monsoon–desert mechanism is analysed in two 45-year experiments with the Met-Office Unified Model. In the first coupled experiment, the atmosphere is allowed to freely interact with a high-vertical-resolution mixed-layer ocean model. The diagnosed daily SSTs from this experiment are then used to force an atmosphere-only uncoupled experiment. The two experiments have a similar mean state, but the coupled experiment has a substantially more realistic representation of interannual precipitation variability over the Indian Summer Monsoon region. The coupled experiment can capture the observed westward propagating Rossby-wave trains excited by the Indian Summer Monsoon, while in the uncoupled experiment the Rossby-wave response is more local. It is shown that in the coupled experiment more moisture is transported inland and monsoon precipitation reaches further north, which favours westward Rossby wave propagation. Finally, evidence is shown that the coupled experiment can capture the observed interannual relationship between the Indian Summer Monsoon precipitation and precipitation over the Balkans/Black Sea region.