Intraseasonal Precipitation Variability Research Articles

Modulation of the intraseasonal variability in early summer precipitation in eastern China by the Quasi-Biennial Oscillation and the Madden–Julian Oscillation

Abstract. Using the reanalysis and multiple observations, the possible impact of the Madden–Julian Oscillation (MJO) on early summer (June–July) rainfall in eastern China and its modulation by the Quasi-Biennial Oscillation (QBO) are examined. The composite results show that the suppressed (enhanced) convection anomalies for MJO phases 8–1 (4–5) are more concentrated over the maritime continent and the western Pacific during easterly QBO (EQBO). As a consequence, more significant wet (dry) anomalies develop in South (eastern) China during MJO phases 8–1 (4–5) configured with easterly (westerly) QBO. The enhancement and expansion of the anomalous tropical convection band do not necessarily correspond to enhancement of the extratropical circulation response to MJO phases 8–1 (4–5) configured with westerly (easterly) QBO. The anomalous high (low) over the maritime continent and western Pacific associated with MJO phases 8–1 (4–5) is intensified (deepened) during easterly (westerly) QBO, leading to large southwesterly (northeasterly) anomalies in South China and the coasts, carrying abundant (sparse) moisture. Two anomalous meridional circulation cells are observed for MJO phases 8–1 in the East Asia sector, with downwelling anomalies around 5–20∘ N, upwelling anomalies around 20–30∘ N, and another downwelling branch northward of 30∘ N, which are enhanced during easterly QBO. The anomalous meridional circulation cells are reversed for MJO phases 4–5, which are stronger during westerly QBO with the anomalous downwelling and dry anomalies covering eastern China. The combined impact of MJO phases 8–1 and easterly QBO on the early summer rainfall is noticeable in 1996, 2016, and 2020. The enormous rainfall amount appeared along the Yangtze River in 1996, 2016, and 2020 due to the extended period of MJO phases 8–1 under the background of the easterly QBO.

Interannual Variations in the Intraseasonal Variability of Spring Precipitation over Southern China and the Possible Mechanisms

Abstract This study first investigates the interannual variations in spring precipitation intraseasonal variability over southern China (SC). The results show that SC spring precipitation exhibits distinct intraseasonal variations with a period of 7–25 days. The first mode of 7–25-day precipitation intraseasonal variability (PIV) displays a monopole pattern over SC, and the PIV magnitude is largely determined by the upward motion intensity during intraseasonal precipitation events. Further analysis suggests that two atmospheric wave trains are observed during intraseasonal precipitation events, which propagate eastward from the North Atlantic along the northern and southern paths. In strong PIV years, the two wave trains can propagate to East Asia and show coordinated influences. The resultant low pressure to the west of SC causes strong upward motion and PIV over SC by bringing strong zonal vorticity and meridional temperature advection. In weak PIV years, the southern wave train can only propagate to the Bay of Bengal; therefore, the northern wave train plays a major role. The resultant low pressure is now over the upper to middle reaches of the Yangtze River, which causes relatively weak upward motion and PIV over SC by bringing weak meridional vorticity and temperature advection. Further analysis indicates that the sea surface temperature (SST) condition over the tropical Indian Ocean and the South China Sea is essential for southern wave train propagation. The warming SST over the regions can intensify westerlies to its north and consequently favors the propagation of the southern wave train to SC, eventually contributing to strong PIV over SC.

Is the Gridded Data Accurate? Evaluation of Precipitation and Historical Wet and Dry Periods from ERA5 Data for Canadian Prairies

Precipitation is crucial for the hydrological cycle and is directly related to many ecological processes. Historically, measurements of precipitation totals were made at weather stations, but spatial and temporal coverage suffered due to the lack of a robust network of weather stations and temporal gaps in observations. Several products have been proposed to identify the location of the occurrence of precipitation and measure its intensity from different types of estimates, based on alternative data sources, that have global (or quasi-global) coverage with long historical time series. However, there are concerns about the accuracy of these estimates. The objective of this study is to evaluate the accuracy of the ERA5 product for two ecoregions of the Canadian Prairies through comparison with monthly means measured from 1981–2019 at ten weather stations (in-situ), as well as to assess the intraseasonal variability of precipitation and identify dry and wet periods based on the annual Standardized Precipitation Index (SPI) derived from ERA5. A significant relationship between in-situ data and ERA5 data (with the R2 varying between 0.42 and 0.76) (p < 0.01)) was observed in nine of the ten weather stations analyzed, with lower RMSE in the Mixed Ecoregion. The Mean Absolute Percentage Error (MAPE) results showed greater agreement between the datasets in May (average R value of 0.84 and an average MAPE value of 32.33%), while greater divergences were observed in February (average R value of 0.57 and an average MAPE value of 50.40%). The analysis of wet and dry periods, based on the SPI derived from ERA5, and the comparison with events associated with the El Niño-Southern Oscillation (ENSO), showed that from the ERA5 data and the derivation of the SPI it is possible to identify anomalies in temporal series with consistent patterns that can be associated with historical events that have been highlighted in the literature. Therefore, our results show that ERA5 data has potential to be an alternative for estimating precipitation in regions with few in-situ stations or with gaps in the time series in the Canadian Prairies, especially at the beginning of the growing season.

Origins of Intraseasonal Precipitation Variability over North China in the Rainy Season

Abstract We investigated the characteristics and mechanisms of subseasonal precipitation variability in North China during the rainy season (June–September). Two dominant intraseasonal modes with periods of 8–20 and 30–60 days were identified via spectral analysis. Together, they explain 62.8% of the total precipitation variability. Nearly all persistent heavy rainfall events in North China were observed concurrently with the enhanced positive phases of biweekly or/and 30–60-day precipitation modes. To elucidate the origins of these two intraseasonal precipitation variabilities, we performed moisture and vertical motion analyses. The moisture diagnosis results show that the anomalous lower-level southerly perturbations, which transport the background summer-mean moisture from the tropical areas toward North China, are the key process causing abundant moisture for the anomalous precipitation occurrence. The local ascending motion anomalies associated with the occurrence of intraseasonal precipitation come mainly from the anomalous vorticity advection induced by summer-mean thermal wind. Although the key processes causing the precipitation anomalies of these two intraseasonal modes are similar, the geneses and evolutions of large-scale conditions associated with them are distinct. The biweekly circulation and convective anomalies are driven by the midlatitude wave train pattern propagating southeastward across the Eurasian continent, while the 30–60-day anomalies are determined by both upper-level perturbations along 40°N and northward-propagating intraseasonal convective activities from the tropics.

Satellite‐Observed Vegetation Responses to Intraseasonal Precipitation Variability

AbstractThere is limited understanding of how vegetation responds to intraseasonal modes of rainfall variability despite their importance in many tropical regions. We use observations of precipitation and X‐band Vegetation Optical Depth (VOD) from 2000 to 2018 to assess the relationships between rainfall and vegetation water content on 25–60‐day timescales. Cross‐spectral analysis identifies coherent intraseasonal relationships between precipitation and VOD, mostly in arid or semi‐arid regions where vegetation is water‐limited. Changes in VOD tend to lag anomalous rainfall, usually within 7 days. The fastest vegetation response is observed in sparsely vegetated areas (median 3 days). Following strong intraseasonal wet events, anomalously high VOD can persist for 2 months after the rainfall peak. This vegetation response can feed back onto the atmosphere, so improved representation of vegetation responses in models has the potential to improve subseasonal‐to‐seasonal forecasts.

Causes of Interannual Variability of Summer Precipitation Intraseasonal Oscillation Intensity over Southwest China

AbstractThe interannual variability in summer precipitation intraseasonal oscillation intensity over southwest China (SWC) is investigated in this study. The results indicate that the 7–20-day period dominates the intraseasonal variability in summer SWC precipitation. The leading mode of summer SWC precipitation 7–20-day oscillation intensity (SPOI) is a north–south dipole pattern with prominent interannual variability. The atmospheric circulation anomalies from both tropics and mid- to high latitudes are responsible for the interannual variability in the dipole pattern. In the tropics, an enhanced local Hadley cell and an anomalous anticyclone over southern China and the northwest Pacific contribute to the north-positive–south-negative pattern of SPOI by inducing moisture convergence (divergence) over northern (southern) SWC in the background state. In the mid- to high latitudes, the 7–20-day Rossby wave trains along the subtropical jet are crucial for the 7–20-day precipitation over northern SWC. Further analyses suggest that the sea surface temperature (SST) anomalies over the Maritime Continent (MC) and the North Atlantic (NA) are associated with the SPOI dipole pattern. The MC SST warming causes convection anomalies over the tropical Indo-Pacific, consequently triggering a Matsuno–Gill-type atmospheric response conducive to the north-positive–south-negative pattern of SPOI. The NA SST tripole triggers a Rossby wave train across Eurasia that strengthens the East Asian westerly jet and enhances 7–20-day atmospheric variability, consequently favoring the variability of 7–20-day precipitation over northern SWC. Diagnoses of moisture and vorticity budgets further indicate the importance of the interaction between intraseasonal fluctuations and atmospheric background in the formation of the north–south difference in 7–20-day precipitation variability over SWC.

Intraseasonal variability of global land monsoon precipitation and its recent trend

Accurate prediction of global land monsoon rainfall on a sub-seasonal (2–8 weeks) time scale has become a worldwide demand. Current forecasts of weekly-mean rainfall in most monsoon regions, however, have limited skills beyond two weeks, calling for a more profound understanding of monsoon intraseasonal variability (ISV). We show that the high-frequency (HF; 8–20 days) ISV, crucial for the Week 2 and Week 3 predictions, accounts for about 53–70% of the total (8–70 days) ISV, generally dominating the sub-seasonal predictability of various land monsoons, while the low-frequency (LF; 20–70 days)’s contribution is comparable to HF only over Australia (AU; 47%), South Asia (SA; 43%), and South America (SAM; 40%). The leading modes of HFISVs in Northern Hemisphere (NH) monsoons primarily originate from different convectively coupled equatorial waves, while from mid-latitude wave trains for Southern Hemisphere (SH) monsoons and East Asian (EA) monsoon. The Madden-Julian Oscillation (MJO) directly regulates LFISVs in Asian-Australian monsoon and affects American and African monsoons by exciting Kelvin waves and mid-latitude teleconnections. During the past four decades, the HF (LF) ISVs have considerably intensified over Asian (Asian-Australian) monsoon but weakened over American (SAM) monsoon. Sub-seasonal to seasonal (S2S) prediction models exhibit higher sub-seasonal prediction skills over AU, SA, and SAM monsoons that have larger LFISV contributions than other monsoons. These results suggest an urgent need to improve the simulation of convectively coupled equatorial waves and two-way interactions between regional monsoon ISVs and mid-latitude processes and between MJO and regional monsoons, especially under the global warming scenarios.

Contrasting the Role of Regional and Remote Circulation in Driving Asian Monsoon Biases in MetUM GA7.1

AbstractMonsoon precipitation affects nearly half of the world's population, but monsoon biases are a long‐standing problem in climate simulations. We apply dynamical nudging either globally or regionally to demonstrate the role of regional and remote circulation in generating Asian monsoon biases in an atmospheric general circulation model. Monsoon precipitation biases are substantially reduced in response to global nudging but may also be exacerbated over the warm oceanic equatorial areas because of unconstrained sub‐grid convection. Regional nudging over Asia appears to be more efficient than nudging outside Asia in reducing seasonal precipitation biases over eastern China and India. This suggests a predominant role of local circulation anomalies in generating monsoon precipitation errors in these regions. An exception is the summer precipitation bias over eastern China, which is more strongly controlled by remote circulation. Besides seasonal mean rainfall, nudging can also improve the simulated interannual and intraseasonal precipitation variability over the subtropics. This results in a better skill in reproducing the observed El Niño teleconnections to India and China and the monsoon onset date. Improved understanding of the origin of Asian monsoon biases and the contribution from regional and remote circulation advances our knowledge of the interplay between the Asian monsoon and large‐scale circulation, which can be beneficial to the simulation and interpretation of monsoon projections.

Intraseasonal Precipitation Variability over West Africa under 1.5 °C and 2.0 °C Global Warming Scenarios: Results from CORDEX RCMs

This study assessed the performance of 24 simulations, from five regional climate models (RCMs) participating in the Coordinated Regional Climate Downscaling Experiment (CORDEX), in representing spatiotemporal characteristics of precipitation over West Africa, compared to observations. The top five performing RCM simulations were used to assess future precipitation changes over West Africa, under 1.5 °C and 2.0 °C global warming levels (GWLs), following the representative concentration pathway (RCP) 8.5. The performance evaluation and future change assessment were done using a set of seven ‘descriptors’ of West African precipitation namely the simple precipitation intensity index (SDII), the consecutive wet days (CWD), the number of wet days index (R1MM), the number of wet days with moderate and heavy intensity precipitation (R10MM and R30MM, respectively), and annual and June to September daily mean precipitation (ANN and JJAS, respectively). The performance assessment and future change outlook were done for the CORDEX–Africa subdomains of north West Africa (WA-N), south West Africa (WA-S), and a combination of the two subdomains. While the performance of RCM runs was descriptor- and subregion- specific, five model runs emerged as top performers in representing precipitation characteristics over both WA-N and WA-S. The five model runs are CCLM4 forced by ICHEC-EC-EARTH (r12i1p1), RCA4 forced by CCCma-CanESM2 (r1i1p1), RACMO22T forced by MOHC-HadGEM2-ES (r1i1p1), and the ensemble means of simulations made by CCLM4 and RACMO22T. All precipitation descriptors recorded a reduction under the two warming levels, except the SDII which recorded an increase. Unlike the WA-N that showed less frequency and more intense precipitation, the WA-S showed increased frequency and intensity. Given the potential impact that these projected changes may have on West Africa’s socioeconomic activities, adjustments in investment may be required to take advantage of (and enhance system resilience against damage that may result from) the potential changes in precipitation.

Intraseasonal Soil Moisture–Atmosphere Feedbacks on the Tibetan Plateau Circulation

AbstractSubstantial intraseasonal precipitation variability is observed across the Tibetan Plateau (TP) during boreal summer associated with the subtropical jet location and the Silk Road pattern. Weather station data and satellite observations highlight a sensitivity of soil moisture and surface fluxes to this variability. During rain-free periods of two or more days, skin temperatures are shown to rise as the surface dries, signalling decreased evaporative fraction. Surface fluxes are further enhanced by relatively clear skies. In this study we use an atmospheric reanalysis to assess how this surface flux response across the TP influences local and remote conditions. Increased surface sensible heat flux induced by decreased soil moisture during a regional dry event leads to a deepening of the planetary boundary layer and the development of a heat low. Consistent with previous studies, heat low characteristics exhibit pronounced diurnal variability driven by anomalous daytime surface warming. For example, low-level horizontal winds are weakest during the afternoon and intensify overnight when boundary layer turbulence is minimal. The heat low favors an upper-tropospheric anticyclone that induces an upper-level Rossby wave and leads to negative upper-level temperature anomalies across southern China. The Rossby wave intensifies the upper-level cyclonic circulation across central China, while upper-level negative temperature anomalies across south China extend the west Pacific subtropical high westward. These circulation anomalies influence temperature and precipitation anomalies across much of China. The association between land–atmosphere interactions across the TP, large-scale atmospheric circulation characteristics, and precipitation in East Asia highlights the importance of intraseasonal soil moisture dynamics on the TP.

Modes of coastal precipitation over southwest India and their relationship with intraseasonal variability

AbstractThe west coast of India, dominated by the Western Ghats mountain range, is among the rainiest places in the Tropics. The interaction between the land–sea contrast of the coast, the monsoonal westerlies, and the oblique mountains is subject to complex intraseasonal variability, which has not previously been explored in depth. This study investigates that variability from the perspective of the land–sea contrast, using empirical orthogonal function analysis to discern regimes of onshore and offshore rainfall over southwest India and the eastern Indian Ocean. Locally, it is found that the rainfall is most sensitive to midtropospheric humidity: when this is anomalously high, deep convection is encouraged; when this is anomalously low, it is suppressed. A moisture‐tracking algorithm is employed to determine the primary sources of the anomalously wet and dry midtropospheric air. There are important secondary contributions from low‐level vorticity and cross‐shore moisture flux. The dominant control on intraseasonal variability in coastal precipitation is found to be the Boreal Summer Intraseasonal Oscillation (BSISO): over 75% of the strongest offshore events occur during phases 3 and 4; and about 40% of the strongest onshore events occur during phases 5 and 6. The location of monsoon low‐pressure systems is also shown to be important in determining the magnitude and location of coastal rainfall.

Recent weakening in MJO-related convective activity over the equatorial Indian Ocean and Maritime Continent

We document the observed changes in the intraseasonal variability of precipitation and outgoing longwave radiation (OLR) associated with the Madden-Julian Oscillation (MJO) from 1998 to 2015 when reliable and simultaneous satellite-derived daily precipitation and OLR are available. Tropical Rainfall Measuring Mission (TRMM) observations show a statistically significant weakening trend in variance of convective activity associated with the MJO across the equatorial Indian Ocean (EQIO) and Maritime Continent (MC) in boreal summer and winter seasons. During northern summer, the MJO variability in the POST period (2007–2015) displays a remarkable reduction in convection at all wavenumbers compared with that in the PRE period (1998–2006). During northern winter, along with a reduction in variance, the MJO-related activity is shifted from lower to higher wavenumbers by breaking down the associated convection into smaller scales. EQIO and MC experience a shift from a higher probability of stronger events to weaker events during both seasons in the multivariate Wheeler-Hendon Index. Moreover, there is a southward shift in the location of maximum variance, increasing (reducing) meridional symmetry in MJO variance during boreal summer (winter). This is manifested by a significant increase in boreal winter MJO variance over northern Australia. We also observed that over western EQIO, OLR anomalies associated with dry (wet) phase of MJO are higher (lower) in POST as compared with the PRE period indicating warmer surface temperatures and taller clouds during dry and wet phases of MJO respectively, which is consistent with observation. The exact opposite behavior is seen over the MC and Western Pacific Oceans. These variations in MJO can have important implications for the local and remote climate systems across the globe.

Non‐stationary characteristics of intraseasonal precipitation variability in Northeast Asia during the boreal summer

AbstractIn this study, we examined the non‐stationary characteristics of precipitation variability in Korea on intraseasonal timescales during the boreal summer (June–July–August) for 1979–2017 by analysing the European Center for Medium‐Range Weather Forecasts ERA‐interim reanalysis data set. We reconstructed the Korean precipitation variability on intraseasonal timescales in each year using three empirical orthogonal function modes of precipitation anomaly representing northward propagating intraseasonal oscillation from the tropics to subtropics. Then, we compared the reconstructed precipitation variability with the total precipitation variability in Korea. We found that the correlation coefficients between two precipitation variabilities change from year to year, indicating non‐stationary characteristics of intraseasonal precipitation variability contributing to the total precipitation variability. Years, when reconstructed precipitation is highly (less) correlated with total precipitation variability, are characterized by a large (small) amount of total precipitation, an early (late) start of the rainy season and enhanced (reduced) precipitation variability on an intraseasonal timescale. We further examined the reason why the relationship between two variabilities is non‐stationary. A large convective forcing in the western tropical Pacific and the associated atmospheric conditions, including the weakening of Western North Pacific subtropical high with its eastward shift and vertical shear of zonal wind affect the characteristic change of intraseasonal precipitation variability in Korea.

The CNRM Global Atmosphere Model ARPEGE‐Climat 6.3: Description and Evaluation

AbstractThe present study describes the atmospheric component of the sixth‐generation climate models of the Centre National de Recherches Météorologiques (CNRM), namely, ARPEGE‐Climat 6.3. It builds up on more than a decade of model development and tuning efforts, which led to major updates of its moist physics. The vertical resolution has also been significantly increased, both in the boundary layer and in the stratosphere. ARPEGE‐Climat 6.3 is now coupled to the new version (8.0) of the SURFace EXternalisée (SURFEX) surface model, in which several new features (e.g., floodplains, aquifers, and snow processes) improve the water cycle realism. The model calibration is discussed in depth. An amip‐type experiment, in which the sea surface temperatures and sea ice concentrations are prescribed, and following the CMIP6 protocol, is extensively evaluated, in terms of climate mean state and variability. ARPEGE‐Climat 6.3 is shown to improve over its previous version (5.1) by many climate features. Major improvements include the top‐of‐atmosphere and surface energy budgets in their various components (shortwave and longwave, total and clear sky), cloud cover, near‐surface temperature, precipitation climatology and daily‐mean distribution, and water discharges at the outlet of major rivers. In contrast, clouds over subtropical stratocumulus decks, several dynamical variables (sea level pressure, 500‐hPa geopotential height), are still significantly biased. The tropical intraseasonal variability and diurnal cycle of precipitation, though improved, remained area of concerns for further model improvement. New biases also emerge, such as a lack of precipitation over several tropical continental areas. Within the CMIP6 context, ARPEGE‐Climat 6.3 is the atmospheric component of CNRM‐CM6‐1 and CNRM‐ESM2‐1.

Seasonal evolution of the intraseasonal variability of China summer precipitation

Seasonal cycle of China summer precipitation has significant impacts on its subseasonal predictability; yet current understanding of seasonal evolution of the intraseasonal variability (ISV) remains limited. Here, we show that the ISV of China summer precipitation features a distinct three-stage evolution during early summer, Meiyu season, and late summer. There are two common leading ISV modes: a uniform mode (UM) over southeastern China and a dipole mode (DM) between coastal southeastern China and mid-lower reaches of the Yangtze River Basin, which occur during all three stages with a dominant period of 8–15 days in the early and late summers and 8–25 days during Meiyu season. These two modes show southward propagation in early summer, but they are independent from each other in the other two stages. In early summer, both UM and DM are only related to mid-latitude wave train, and no preceding signal is found in the tropics due to the weak western North Pacific (WNP) monsoon trough. In contrast, during the Meiyu season and late summer, preceding tropical signals are observed when the WNP monsoon trough becomes strong. In the late summer, the effect of mid-latitude wave train is weak as the westerly jet-induced wave guide is far away from southeastern China. An improved subseasonal prediction system is expected to be benefited from consideration of the seasonal evolution of China summer precipitation ISV.

Interactions between intraseasonal and diurnal variability of precipitation in the South Central Pacific: The case of a small high island, Tahiti, French Polynesia

Intraseasonal and diurnal variability of precipitation over Tahiti, French Polynesia, are investigated with the use of wind regimes as intermediary tools. Four wind regimes have been determined over a large domain around Tahiti in the wet season. It has been shown that some phases of the Madden–Julian Oscillation (MJO) trigger some regimes and undermine some others. The diurnal cycle of precipitation in Tahiti is composed of two rainfall maxima. A frequent maximum at midnight but with a low value. An afternoon maximum, less frequent but with a higher value. We have noticed that the afternoon rainfall maximum, which is a reasonable consequence of the sea breeze circulation, is smoothen on the windward side when vigorous wind regimes obstruct the development of the sea breeze. We have also found that daily mean precipitation is significantly increased during MJO Phases 7 and 8, and reduced during Phases 2 and 3. Phase 8 is the wettest phase on Tahiti, suggesting the convective envelope is overlooking the island. Lastly, we have provided evidence that the diurnal cycle of rainfall is significantly affected by the MJO. Indeed, both the midnight and afternoon maxima increase during MJO Phase 7. The most quiescent wind regimes are triggered during Phase 7, as a result from the propagation of the westerly wind anomaly, which allow the development of the sea breeze circulation and the increase of the afternoon peak. For MJO Phase 8, however, the above relationships break down, as the diurnal cycle is not enhanced by the predominant quiescent wind regimes. We assume that the MJO convective envelope covers the island and stays several days, preventing the solar heating of the surface, hence the development of the land/sea breeze circulation, and foremost leads to higher amounts of rainfall as the Phase 8 is the wettest phase on Tahiti.

Intraseasonal summer rainfall variability over China in the MetUM GA6 and GC2 configurations

Abstract. The simulation of intraseasonal precipitation variability over China in extended summer (May–October) is evaluated based on six climate simulations of the Met Office Unified Model. Two simulations use the Global Atmosphere 6.0 (GA6) and four the Global Coupled 2.0 (GC2) configuration. Model biases are large such that mean precipitation and intraseasonal variability reach twice their observed values, particularly in southern China. To test the impact of air–sea coupling and horizontal resolution, GA6 and GC2 at horizontal resolutions corresponding to ∼25, 60, and 135 km at 50∘ N are analyzed. Increasing the horizontal resolution and adding air–sea coupling have little effect on these biases. Pre-monsoon rainfall in the Yangtze River basin is too strong in all simulations. Simulated rainfall amounts in June are too high along the southern coast and persist in the coastal region through July, with only a weak northward progression. The observed northward propagation of the Meiyu–Baiu–Changma rainband from spring to late summer is poor in all GA6 and GC2 simulations. To assess how well the MetUM simulates spatial patterns of temporally coherent precipitation, empirical orthogonal teleconnection (EOT) analysis is applied to pentad-mean precipitation. Patterns are connected to large-scale processes by regressing atmospheric fields onto the EOT pentad time series. Most observed patterns of intraseasonal rainfall variability are found in all simulations, including the associated observed mechanisms. This suggests that GA6 and GC2 may provide useful predictions of summer intraseasonal variability despite their substantial biases in mean precipitation and overall intraseasonal variance.

Groundwater depletion limits the scope for adaptation to increased rainfall variability in India

Recent studies have found that increasing intra-seasonal precipitation variability will lead to substantial reductions in rice production in India by 2050, independently of the effect of rising temperatures. However, these projections do not account for the possibility of adaptations, of which the expansion of irrigation is the primary candidate. Using historical data on irrigation, rice yields, and precipitation, I show that irrigated locations experience much lower damages from increasing precipitation variability, suggesting that the expansion of irrigation could protect Indian agriculture from this future threat. However, accounting for physical water availability shows that under current irrigation practices, sustainable use of irrigation water can mitigate less than a tenth of the climate change impact. Moreover, if India continues to deplete its groundwater resources, the impacts of increased variability are likely to increase by half.

Sensitivity of Indian summer monsoon simulation to physical parameterization schemes in the WRF model

A set of 17 experiments, using various combinations of physical parameterization schemes in the Weather Research and Forecasting (WRF) model, were carried out to choose a combination suitable for simulating the Indian summer monsoon. The model experiments, forced with the ERA-Interim reanalysis data, were at 30 km horizontal resolution. The WRF model experiments were initialized on 1 May of each year and integrated until 30 September to cover the entire monsoon season for the years 1982 to 2013. The results indicate that the simulated Indian summer monsoon precipitation and 2 m air temperature are sensitive to the physical parameterization schemes in the WRF model and that choosing the correct combination of physical parameterization schemes is essential for simulating the Indian summer monsoon realistically. Our analysis shows that a model setup with the Kain-Fritsch cumulus scheme, a radiation package with the Dudhia shortwave and Rapid Radiative Transfer Model longwave schemes, the Yonsei State University planetary boundary layer scheme, the WRF Single-Moment 3-class microphysics scheme, the revised MM5 Monin-Obukhov surface layer scheme, and the Unified Noah land surface model is suitable for simulating the precipitation realistically. The model setup with a combination of these physical parameterization schemes was found to have smaller biases and root mean square errors in the simulated precipitation, along with a realistic simulation of intraseasonal and interannual variability of precipitation. The results of this study will be useful to researchers and forecasters using the WRF model to improve the Indian summer monsoon simulations/forecasts over the Indian region.

Intraseasonal variability of winter precipitation over central asia and the western tibetan plateau from 1979 to 2013 and its relationship with the North Atlantic Oscillation

Winter precipitation over Central Asia and the western Tibetan Plateau (CAWTP) is mainly a result of the interaction between the westerly circulation and the high mountains around the plateau. Empirical Orthogonal Functions (EOFs), Singular Value Decomposition (SVD), linear regression and composite analysis were used to analyze winter daily precipitation and other meteorological elements in this region from 1979 to 2013, in order to understand how interactions between the regional circulation and topography affect the intraseasonal variability in precipitation. The SVD analysis shows that the winter daily precipitation variability distribution is characterized by a dipole pattern with opposite signs over the northern Pamir Plateau and over the Karakoram Himalaya, similar to the second mode of EOF analysis. This dipole pattern of precipitation anomaly is associated with local anomalies in both the 700hPa moisture transport and the 500hPa geopotential height and is probably caused by oscillations in the regional and large-scale circulations, which can influence the westerly disturbance tracks and water vapor transport. The linear regression shows that the anomalous mid-tropospheric circulation over CAWTP corresponds to an anti-phase variation of the 500hPa geopotential height anomalies over the southern and northern North Atlantic 10days earlier (at 95% significance level), that bears a similarity to the North Atlantic Oscillation (NAO). The composite analysis reveals that the NAO impacts the downstream regions including CAWTP by controlling south-north two branches of the middle latitude westerly circulation around the Eurasian border. During the positive phases of the NAO, the northern branch of the westerly circulation goes around the northwest Tibetan Plateau, whereas the southern branch encounters the southwest Tibetan Plateau, which leads to reduced precipitation over the northern Pamir Plateau and increased precipitation over the Karakoram Himalaya, and vice versa.