Anomalous Moisture Convergence Research Articles

Subseasonal relationship between the zonal oscillation of the Western Pacific Subtropical High and the South Asia High

This study investigates relationship between the zonal oscillation of the western Pacific subtropical high (WPSH) and the South Asia High (SAH). As the WPSH extends westward, in upper-troposphere the SAH shows conspicuous eastward extension in 10–25-day period, while weak zonal movement in 25–50-day period. In 10–25-day period, the zonal oscillation of the SAH lags the movement of the WPSH by about one day. It is closely related with a westward moving anomalous high from the south of the Japan to the continent. Further analysis shows that the WPSH-related precipitation plays the significant role in the variation of this anomalous high. Accompanied with the westward extension of the WPSH, in lower-troposphere an anomalous anticyclonic circulation occupies the western Pacific. It enhances the moisture transportation to the east side of the SAH, causing above-normal precipitation here. With the enhancement of the precipitation, the anomalous diabatic heating induces divergence anomaly in upper-troposphere, and further stimulates anomalous high by the horizontal divergence effect, leading to the eastward extension of the SAH. In 25–50-day period, due to the weaker convective anomaly, the WPSH is unable to extend farther westward. As the result, less moisture is transported and the anomalous moisture convergence is restricted to the north of Yangtze River, and so is the precipitation. At the east side of the SAH it is dominated by the anomalous cooling, unfavorable for the stimulation of the anomalous high, and thus, the zonal oscillation of the SAH.

Uncertainty of the Simulated Mid-Pliocene Changes of Sahel Summer Rainfall in the PlioMIP2 Multimodel Ensemble

Abstract The mid-Pliocene (approximately 3.3–3.0 Ma) was one of the past geological warm periods. Since this past warmer climate was in many respects comparable to near future warming projections, how the regional monsoonal rainfall changed during the mid-Pliocene is an important scientific and socioeconomic concern. Based on phase 2 of the Pliocene Model Intercomparison Project (PlioMIP2), this work examines the simulated changes of Sahel summer rainfall during the mid-Pliocene. The results show the considerable intermodel uncertainty of the simulated mid-Pliocene Sahel rainfall changes in the PlioMIP2 multimodel ensemble, which is due to the uncertainties of both the simulated dynamic and thermodynamic responses to this past warmer climate. In particular, we find that the intermodel spread in the simulated northern North American warming is a major source of the uncertainty of the mid-Pliocene Sahel summer rainfall changes through two processes. One is a direct dynamic process: a stronger warming over northern North America could enhance the meridional temperature gradient between the extratropical and tropical regions, inducing an interhemisphere energy imbalance of the atmosphere. This could lead to a northward shift of the intertropical convergence zone, strengthening the Western African summer monsoon (WASM) circulation and Sahel summer rainfall. Another is an indirect thermodynamic process: the strengthened WASM circulation could further induce anomalous moisture convergence over the Sahel region, increasing local atmospheric moisture at the low-level troposphere, in favor of a wetter Sahel. Our results suggest that an improved warming simulation over northern North America is essential for the hydrological cycle simulation around the Sahel in the mid-Pliocene warmer climate. Significance Statement The Sahel summer rainfall change in a global warmer climate is a widespread scientific and socioeconomic issue because of its important impacts on regional agriculture, ecosystems, food security, water resources, and even cultural environment. However, the present study identifies a nonnegligible intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene (one of the most recent geological warm periods in Earth’s history) in the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) multimodel ensemble. In particular, such an intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene is attributed to that of the simulated northern North America warming via both the direct dynamic process and the indirect thermodynamic process. This is quite different from the uncertainty source of near-future projections of Sahel summer rainfall changes. The present results improve our understanding of the underlying physics of the hydrological cycle change around the Sahel region in the mid-Pliocene warmer climate, with implications for the future projections of regional monsoonal rainfall.

A contrast in biennial variability of rainfall between central India and the Western Ghats and its mechanisms

The Western Ghats (WG) of Peninsular India, an integral part of the Indian summer monsoon rainfall, receives three times the average of all India rainfall. The averaged rainfall over WG is characterized by an intense tropospheric biennial oscillation (TBO) of 2–3 years of periodicity. The rainfall anomalies over WG are almost uncorrelated to rainfall over Central India (CI) in the TBO window. This study characterizes the WG and CI biennial rainfall variability and their governing mechanisms using observation and reanalysis datasets for 1980–2020. A zonally symmetric build-up of heat anomalies from the Iranian Plateau (IP) to the Tibetan Plateau (TP) extending from the surface to the mid-troposphere governs the TBO rainfall of CI. On the other hand, localized heating (cooling) over the IP and Pak-Afghanistan region (PA) and cooling (heating) over the TP governs the phases of TBO rainfall over WG. An increase (decrease) in anomalous heat build-up in the vertical column causes an increase (decrease) in anomalous moist static energy during positive (negative) WG TBO years extending over the IP (TP) region. Increased heating (cooling) over the IP and PA (TP) during positive WG TBO years can shift the center of near-surface cyclonic circulation, anchored over the Indian subcontinent and surrounding areas during the positive CI TBO years, to move westward. This shift in rainfall anomalies and the center of cyclonic circulation is because of the westward shift in anomalous moisture convergence from CI and significant moisture loss along southeastern Peninsular India. Considering the growing number of extreme rainfall events over the WG regions recently, the present work is an attempt to understand the mechanism through which TBO modulates WG and CI rainfall.

Influences of Spring Land Surface Thermal Anomalies over West Asia on Indian Early Summer Monsoon Activity and Its Pathway

Abstract Exploring the premonsoonal land thermal predictor of the Indian summer monsoon is a hot topic under the background of global warming, and West Asia is one of the regions with the most significant warming in spring. In this study, we investigated the impact of anomalous spring land surface warming over West Asia on early summer (June) Indian monsoon precipitation as well as its possible mechanisms based on statistical analysis and numerical simulations. It has been found that spring land surface anomalous warming over West Asia corresponds to the enhancement of the leading mode of early summer precipitation in the Indian subcontinent, especially in its northern part. Further analysis indicates that an anomalously warm land surface over West Asia can advance the transition of atmospheric conditions toward the warm season by heating the atmosphere above. The increased land–sea meridional thermal contrast favors the intensification of the low-level jet and monsoon trough, further inducing anomalous moisture convergence and ascending motion over northern India. Additionally, the heat-driven anomalous upper-tropospheric anticyclone over West Asia favors the intensification of the tropical easterly jet and the northwestward development of the South Asian high (SAH). The enhanced SAH dynamically couples with the lower- to middle-level cyclonic circulation over northern India, resulting in a stronger monsoon and increased precipitation. These findings are helpful for better understanding and prediction of Indian early summer monsoon. Significance Statement The land surface thermal condition is critical to the monsoon activity and exploring the premonsoonal land thermal predictor of Indian summer monsoon remains a hot topic. The purpose of this study is to explore how spring land surface thermal anomalies over West Asia impact Indian monsoon activity in early summer (June). The anomalous land surface warming over West Asia can lead to a stronger Indian monsoon in early summer by heating and driving the atmosphere, which benefits the precipitation increase over northern India. Our results provide a further scientific basis for the prediction of early summer Indian precipitation.

Relative Contributions of Sea Surface Temperature and Atmospheric Nonlinearities to ENSO Asymmetrical Rainfall Response

Abstract Here, we investigate the relative roles of atmospheric nonlinearities and asymmetrical sea surface temperature (SST) forcing in the El Niño–Southern Oscillation (ENSO) asymmetrical rainfall response. Applying a vertically integrated water vapor budget to the ERA5 reanalysis leads to a simple analytical equation for precipitation anomalies. This formulation reveals that ENSO rainfall anomalies are dominated by the linear component of the dynamical term (i.e., the anomalous moisture convergence due to the effect of circulation anomalies on climatological humidity). Nonlinearities in this term and the linear thermodynamical term (i.e., the effect of climatological circulation on humidity anomalies) both strengthen central Pacific rainfall anomalies for both ENSO phases. In contrast, the nonlinear term associated with the effect of anomalous divergence on anomalous moisture (i.e., the mixed term) weakens La Niña dry and strengthens El Niño wet anomalies, in particular during extreme El Niño events when it contributes to about 40% of the eastern Pacific wet anomalies. Overall, atmospheric nonlinearities directly account for ∼70% of the positively skewed ENSO rainfall distribution east of the date line, and ∼50% of the negatively skewed rainfall distribution in the western Pacific. The remaining ENSO rainfall asymmetries are attributable to the asymmetrical ENSO SST pattern. This asymmetrical SST pattern also has contributions from atmospheric nonlinearities through the Bjerknes feedback loop, in addition to those from oceanic nonlinearities. Our estimates are thus likely a lower bound of the contribution of atmospheric nonlinearities to the overall ENSO rainfall asymmetry.

Factors determining the subseasonal prediction skill of summer extreme rainfall over southern China

The occurrence of summer extreme rainfall over southern China (SCER) is closely related to the boreal summer intraseasonal oscillation (BSISO), and whether operational models can reasonably predict the BSISO evolution and its modulation on SCER probability is crucial for disaster prevention and mitigation. Here, we find that the skill of subseasonal-to-seasonal (S2S) operational models in predicting the first component of BSISO (BSISO1) might determine the forecast skill of SCER. A systematic assessment is conducted on the reforecast data from two operational models that participated in the S2S project, i.e., the model of European Centre for Medium-Range Weather Forecasts (ECMWF) and the model of China Meteorological Administration (CMA). The results show that the ECMWF model can yield skillful prediction of the BSISO1 index up to 24 days in advance, while the skill of the CMA model is about 10 days. Accordingly, the SCER occurrence is correctly predicted by ECMWF (CMA) model at a forecast lead time of ~ 14 (7) days. The diagnostic results of modeled moisture processes further suggest that the anomalous moisture convergence (advection) induced by the BSISO1 activity serves as the primary (secondary) source of subseasonal predictability of SCER. With better prediction of the moisture convergence anomaly in the specific phases of BSISO1, higher skills can be obtained in the probability prediction of SCER. The present study implies that a further improvement in predicting the BSISO and its related moisture processes is crucial to promoting the subseasonal prediction skill of SCER probability.

Effects of Suppressed Transient Eddies by the Tibetan Plateau on the East Asian Summer Monsoon

AbstractIt is known that the Tibetan Plateau (TP) can weaken the transient eddies (TEs) transported along the westerly jet stream. This study investigates the effects of the persistently suppressed TEs by the TP on the East Asian summer monsoon and the associated mechanisms using the NCAR Community Earth System Model. A nudging method is used to modify the suppression of the TEs without changing the steady dynamic and thermodynamic effects of the TP. The suppressed TEs by the TP weaken the East Asian westerly jet stream through the weakened poleward TE vorticity flux. On the one hand, the weakened jet stream leads to less (more) rainfall in northern (southern) East Asia by inducing anomalous moisture convergence, midtropospheric warm advection, and upper-level divergence, particularly in early summer when the eastward propagation of TE suppression by the TP is strong. On the other hand, the precipitation anomalies can shift the East Asian westerly jet stream southward and promote the moisture convergence in southern East Asia through latent heat release. Therefore, the persistent suppression of the TEs leads to a southward shift of the East Asian rain belt by a convective feedback, as it was previously found that the steady thermodynamic and dynamic forcings of the TP favored a northward shift of the rain belt. This study suggests that the anomalously weak TEs can lead to a rainfall change (more in the south, less in the north) over East Asia.

Changes in the Relationship between Spring Precipitation in Southern China and Tropical Pacific–South Indian Ocean SST

AbstractThe present study explores the changed relationship between the interannual variations in spring (April–May) precipitation over southern China (SPSC) and sea surface temperature (SST) anomalies in the tropical Pacific and south Indian Oceans during the 1960–2017 period. Observational analysis shows that the relation between SPSC and El Niño–Southern Oscillation (ENSO) was significant before the mid-1980s (P1) and after the early 2000s (P3) but insignificant in between, from the mid-1980s to the early 2000s (P2). In P2, positive anomalous SPSC was significantly correlated with negative anomalous SST in the south Indian Ocean. During this period, an anomalous anticyclone and intensified southwesterly winds tended to appear over tropical India accompanied by a negative anomalous south Indian Ocean SST, which caused anomalous low-level convergence over the western Pacific. As a result, the western Pacific subtropical high (WPSH) tended to weaken and retreat eastward. This resulted in anomalous moisture convergence in southern China, favoring enhanced SPSC. Further analysis shows that the negative south Indian Ocean SST anomalies tended to induce anomalous cross-equatorial vertical circulation where the south Indian Ocean and southern China are controlled by descending and ascending airflow. The ascending motion may also contribute to positive anomalous SPSC. The observed contribution of the south Indian Ocean SST anomalies to the SPSC variation is confirmed by numerical experiments using an atmospheric model. The intensified variance of SST in the south Indian Ocean and the eastward shift of the ENSO-related circulation anomalies over the western tropical Pacific may partly account for the changes in the SST–SPSC relationship.

Different Impacts of Madden-Julian Oscillation on Winter Rainfall over Southern China

Winter rainfall over southern China is usually enhanced when Madden-Julian oscillation (MJO) is active over the Indian Ocean, but it can be weakened under certain conditions. Here, the diversity of MJO impacts on winter rainfall and its mechanisms are explored by using scenarios of enhanced and suppressed rainfall anomalies over southern China when MJO is active over the Indian Ocean. The combined effects of low-frequency background moisture and intraseasonal winds are the major contributors to the different rainfall anomalies. Anomalous circulation in mid-high latitudes, especially on intraseasonal timescales, is almost opposite in the two scenarios, which can modulate the response of extratropical atmosphere to MJO heating and then induces the different circulations over southern China. In the enhanced scenario, mid-high latitudes of Eurasia and southern China are dominated by positive and negative sea level pressure anomalies, respectively. The southerly over southern China and the South China Sea induced by MJO heating promotes the anomalous moisture convergence and ascending motion over southern China, resulting in the enhanced rainfall. In the suppressed scenario, however, the circulation in mid-high latitudes does not favor rainfall over southern China and leads to the northerly response to MJO heating over southern China, which enhances moisture divergence and weakens rainfall over southern China.

Global monsoon response to tropical and Arctic stratospheric aerosol injection

Stratospheric aerosol injection (SAI) is considered as a backup approach to mitigate global warming, and understanding its climate impact is of great societal concern. It remains unclear how differently global monsoon (GM) precipitation would change in response to tropical and Arctic SAI. Using the Community Earth System Model, a control experiment and a suite of 140-year experiments with CO2 increasing by 1% per year (1% CO2) are conducted, including ten tropical SAI and ten Arctic SAI experiments with different injecting intensity ranging from 10 to 100 Tg yr−1. For the same amount of injection, a larger reduction in global temperature occurs under tropical SAI compared with Arctic SAI. The simulated result in the last 40 years shows that, for a 10 Tg yr−1 injection, GM precipitation decreases by 1.1% (relative to the 1% CO2 experiment) under Arctic SAI, which is weaker than under tropical SAI (1.9%). Further, tropical SAI suppresses precipitation globally, but Arctic SAI reduces the Northern Hemisphere monsoon (NHM) precipitation by 2.3% and increases the Southern Hemisphere monsoon (SHM) precipitation by 0.7%. Under the effect of tropical SAI, the reduced GM precipitation is mainly due to the thermodynamic term associated with the tropical cooling-induced decreased moisture content. The hemispheric antisymmetric impact of Arctic SAI arises from the dynamic term related to anomalous moisture convergence influenced by the anomalous meridional temperature gradient.

Anomalous winter moisture transport associated with the recent surface warming over the Barents–Kara seas region since the mid‐2000s

AbstractObservational evidences have shown that the increased water vapour and accompanying enhancement of downward infrared radiation (IR) can explain the abrupt warming of surface air temperature (SAT) over the Barents–Kara Seas during boreal winter. This study investigates anomalous moisture transport associated with the abrupt change of SAT since the mid‐2000s over the Barents–Kara Seas, since moisture transport contributes to the anomalous water vapour and downward IR. Results show that anomalous stationary moisture flux induces anomalous moisture convergence over the Barents–Kara Seas region, whereas the anomalous transient flux opposes the anomalous moisture convergence there. Hence, the anomalous stationary moisture transport is suggested to be responsible to the increased water vapour over the Barents–Kara Seas region. Further decomposition of the stationary moisture flux indicates that the anomalous moisture convergence associated with the change in atmospheric mean flow plays an important role in the increased water vapour on both interannual and decadal time scales. In addition, decrease in the synoptic‐scale eddy activity observed over northern Siberia may contribute to generation of the anomalous atmospheric mean flow via eddy feedback processes.

Simulations of Monsoon Intraseasonal Oscillation Using Climate Forecast System Version 2: Insight for Horizontal Resolution and Moist Processes Parameterization

In the present study, we analyze the Climate Forecast System version 2 (CFSv2) model in three resolutions, T62, T126, and T382. We evaluated the performance of all three resolutions of CFSv2 in simulating the Monsoon Intraseasonal Oscillation (MISO) of the Indian summer monsoon (ISM) by analyzing a suite of dynamic and thermodynamic parameters. Results reveal a slower northward propagation of MISO in all models with the characteristic northwest–southeast tilted rain band missing over India. The anomalous moisture convergence and vorticity were collocated with the convection center instead of being northwards. This affected the northward propagation of MISO. The easterly shear to the north of the equator was better simulated by the coarser resolution models than CFS T382. The low level specific humidity showed improvement only in CFS T382 until ~15° N. The analyses of the vertical profiles of moisture and its relation to rainfall revealed that all CFSv2 resolutions had a lower level of moisture in the lower level (< 850 hPa) and a drier level above. This eventually hampered the growth of deep convection in the model. These model shortcomings indicate a possible need of improvement in moist process parameterization in the model in tune with the increase in resolution.

A statistical and dynamical characterization of large-scale circulation patterns associated with summer extreme precipitation over the middle reaches of Yangtze river

The large-scale circulation associated with extreme precipitation over the middle reaches of Yangtze river (MRYR) in early summer (June and July) are classified into three canonical patterns via hierarchical clustering. The clustering results reveal a clear connection between the MRYR extreme precipitation and anomalous moisture convergence over this region with the eastward expansion of South Asia High and intensified westerly jets providing additional forcing for local rising motion. In all three clusters, the anomalous moisture convergence results from anomalous low-level southwesterlies encountering anomalous northerlies from mid-high latitudes. The southwesterly anomaly is associated with the expansion of the Western Pacific Subtropical High (WPSH). However, the anomalous northerlies weakening the northward advance of the Mei-yu front are mainly driven by different extratropical circulation anomalies in the three clusters. These anomalies range from zonally-elongated barotropic disturbances to developing baroclinic disturbances that are potentially tied to upstream storm tracks. All three clusters are characterized by a meridional dipole in geopotential height anomaly over the tropical–subtropical East Asia. The northern and also more pronounced node of the dipole is located over the MRYR with a cyclonic (anti-cyclonic) height anomaly in the lower (upper) troposphere, suggesting the critical role played by anomalous latent heating of extreme MRYR rainfall in driving the formation of this dipole. This dipole anomaly projects effectively onto the negative phase of the Pacific–Japan teleconnection pattern and acts partly as a positive feedback to the westward expansion of the WPSH. Also discussed are the implications of the identified large-scale circulation patterns for model evaluation and operational forecasting of extreme precipitation events.

Projected changes in the annual cycle of precipitation over central Asia by CMIP5 models

Future changes in the annual cycle of the precipitation in central Asia (CA) were estimated based on the historical and representative concentration pathway 8.5 (RCP8.5) experiments from 25 models of the Coupled Model Intercomparison Project phase 5. Compared with the Global Precipitation Climatology Project observations, the historical (1979–1999) experiments showed that most models can capture the migration of rainfall centres, but remarkable discrepancies exist in the location and intensity of rainfall centres between simulations and observations. Considering the skill scores of precipitation and pattern correlations of circulations, which are closely related to the precipitation for each month, for the 25 models, the four best models (e.g., CanESM2, CMCC‐CMS, MIROC5 and MPI‐ESM‐LR) with relatively good performance were selected. The four models' ensemble mean indicated that the migration and location of the precipitation centres were better reproduced, except the intensity of the centres was overestimated, compared with the result that only took the precipitation into consideration. Based on the four best models' ensemble mean under RCP8.5 scenarios, precipitation was projected to increase dramatically over most of the CA region in the boreal cold seasons (November, December, January, February, March and April) and May, with the maximum in December at the end of 21st century (2079–2099), and several positive centres were located in the Pamirs Plateau, the Tianshan Mountains and the northern Himalayas. The precipitation changes were weak in the boreal warm seasons (June, July, August, September and October), with a dry centre located in the Pamirs Plateau and a wet centre in the northern Himalayas. These projected changes can be directly attributed to the overlay of anomalous moisture convergence and evaporation, and other possible mechanisms of dynamic and thermodynamic factors need to be further elucidated.

North-East monsoon rainfall extremes over the southern peninsular India and their association with El Niño

The present study investigates the relationship between extreme north-east (NE) monsoon rainfall (NEMR) over the Indian peninsula region and El Niño forcing. This turns out to be a critical science issue especially after the 2015 Chennai flood. The puzzle being while most El Niños favour good NE monsoon, some don’t. In fact some El Niño years witnessed deficit NE monsoon. Therefore two different cases (or classes) of El Niños are considered for analysis based on standardized NEMR index and Niño 3.4 index with case-1 being both Niño-3.4 and NEMR indices greater than +1 and case-2 being Niño-3.4 index greater than +1 and NEMR index less than −1. Composite analysis suggests that SST anomalies in the central and eastern Pacific are strong in both cases but large differences are noted in the spatial distribution of SST over the Indo-western Pacific region. This questions our understanding of NEMR as mirror image of El Niño conditions in the Pacific. It is noted that the favourable excess NEMR in case-1 is due to anomalous moisture transport from Bay of Bengal and equatorial Indian Ocean to southern peninsular India. Strong SST gradient between warm western Indian Ocean (and Bay of Bengal) and cool western Pacific induced strong easterly wind anomalies during NE monsoon season favour moisture transport towards the core NE monsoon region. Further anomalous moisture convergence and convection over the core NE monsoon region supported positive rainfall anomalies in case-1. While in case-2, weak SST gradients over the Indo-western Pacific and absence of local low level convergence over NE monsoon region are mainly responsible for deficit rainfall. The ocean dynamics in the Indian Ocean displayed large differences during case-1 and case-2, suggesting the key role of Rossby wave dynamics in the Indian Ocean on NE monsoon extremes. Apart from the large scale circulation differences the number of cyclonic systems land fall for case-1 and case-2 have also contributed for variations in NE monsoon rainfall extremes during El Niño years. This study indicates that despite having strong warming in the central and eastern Pacific, NE monsoon rainfall variations over the southern peninsular India is mostly determined by SST gradient over the Indo-western Pacific region and number of systems formation in the Bay of Bengal and their land fall. The paper concludes that though the favourable large scale circulation induced by Pacific is important in modulating the NE monsoon rainfall the local air sea interaction plays a key role in modulating or driving rainfall extremes associated with El Niño.

Covariability of climate and streamflow in the Upper Rio Grande from interannual to interdecadal timescales

Study regionThe Upper Rio Grande (URG) flows from its headwaters in Colorado, U.S., and provides an important source of water to millions of people in the U.S. states of Colorado, New Mexico, Texas, and also Mexico. Study focusWe reassess the explanatory power of the relationship of sea surface temperatures (SST) on URG streamflow variability on interannual to interdecadal timescales. We find a significant amount of the variance of spring-summer URG streamflow cannot be fully explained by SST. New hydrological insightsWe find that the interdecadal teleconnection between SST and streamflow is more clear than on interannual timescales. The highest ranked years tend to be clustered during positive phases of the Pacific Decadal Oscillation (PDO). During the periods of decadal high flow (1900–1920, and 1979–1995), Pacific SST resembles a positive PDO pattern and the Atlantic a negative Atlantic Multidecadal Oscillation (AMO) pattern; an interbasin pattern shown in prior studies to be conducive to high precipitation and streamflow. To account for the part of streamflow variance not explained by SST, we analyze atmospheric Reanalysis data for the months preceding the highest spring-summer streamflow events. A variety of atmospheric configurations are found to precede the highest flow years through anomalous moisture convergence. This lack of consistency suggests that, on interannual timescales, weather and not climate can dominate the generation of high streamflow events.

On the decreasing trend of the number of monsoon depressions in the Bay of Bengal

This study unravels the physical link between the weakening of the monsoon circulation and the decreasing trend in the frequency of monsoon depressions over the Bay of Bengal. Based on the analysis of the terms of Genesis Potential Index, an empirical index to quantify the relative contribution of large scale environmental variables responsible for the modulation of storms, it is shown here that the reduction in the mid-tropospheric relative humidity is the most important reason for the decrease in the number of monsoon depressions. The net reduction of relative humidity over the Bay of Bengal is primarily due to the decrease in the moisture flux convergence, which is attributed to the weakening of the low level jet, a characteristic feature of monsoon circulation. Further, the anomalous moisture convergence over the western equatorial Indian Ocean associated with the rapid warming of the sea surface, reduces the moisture advection into the Bay of Bengal and hence adversely affect the genesis/intensification of monsoon depressions. Hence, the reduction in the number of monsoon depression over the Bay of Bengal could be one of the manifestations of the differential rates in the observed warming trend of the Indian Ocean basin.

Influence of the large-scale climate variability on daily rainfall extremes over Argentina

A singular value decomposition (SVD) analysis was performed jointly on the daily intensity of extreme rainfall (DIER) over Argentina and sea surface temperature (SST) anomalies from 17.5°N–90°S to describe and understand the influence of the large-scale variability of the SSTs on the regional extreme rainfall events for spring summer, autumn and winter. Three main leading modes were identified in agreement with previous works. Mode 1 activity is strongly related to El Nino-Southern Oscillation (ENSO). Warm anomalies in the central-eastern tropical Pacific and western Indian Ocean induce circulation anomalies extended along the South Pacific and the development of a continental-scale circulation gyre in South America promoting moisture convergence, and in turn favouring DIER positive anomalies, in eastern Argentina. The combined influence of SST anomalies in the tropical Atlantic and western tropical Pacific characterizes Mode 2 activity, which induces an anticyclonic (cyclonic) circulation gyre in southeastern South America promoting anomalous moisture convergence (divergence) and thus positive (negative) DIER anomalies in eastern Argentina in spring and fall (summer and winter). Finally, Mode 3 activity is also influenced by SST anomalies in tropical central-eastern Pacific from winter to summer. The associated teleconnections contribute to the development of a cyclonic circulation mainly influencing southeastern South America (SESA) circulation to the north of 30°S from summer to winter, and further south in spring.

Relative Roles of Circumnavigating Waves and Extratropics on the MJO and Its Relationship with the Mean State*

Abstract A set of atmospheric general circulation model (GCM) experiments is designed to explore the relative roles of the circumnavigating waves and the extratropics on the Madden–Julian oscillation (MJO). In a “control” simulation, the model is forced by the climatological monthly sea surface temperature for 20 yr. In the first sensitivity experiment, model prognostic variables are relaxed in the tropical Atlantic region (20°S–20°N, 80°W–0°) toward the “controlled” climatological annual cycle to suppress the influences from the circumnavigating waves. In the second sensitivity experiment, model prognostic variables are relaxed in the 20°–30° latitude zones toward the controlled climatological annual cycle to suppress the influences from the extratropics (or the tropics–extratropics interactions). The numerical results demonstrate that the extratropics play a more important role in maintaining the MJO variance than the circumnavigating waves. The simulations further show that both the tropical mean precipitation and the intraseasonal precipitation variability are reduced when the extratropical influences are suppressed. The in-phase relationship is primarily attributed to the effect of the mean state on perturbations. A moisture budget analysis indicates that a positive feedback to the mean precipitation by the anomalous moisture convergence is offset by a negative feedback due to the anomalous moisture advection. The change in the mean precipitation in the absence of extratropical influences is primarily determined by the change in the mean moisture convergence, which in turn is due to the change in circulation. This study is the first attempt to quantitatively separate the effects of the circumnavigating waves and the extratropics on the MJO. Implications and limitations of this study are discussed.

Warm Season Response over North America to a Shutdown of the Atlantic Meridional Overturning Circulation and CO2 Increases

Abstract Paleo-proxy and modeling evidence suggest that a shutdown of the Atlantic meridional overturning circulation (AMOC) would decrease North Atlantic Ocean sea surface temperatures and have far-reaching climate impacts. The authors use a regional climate model to examine the warm season response over North America to a hypothetical late-twenty-first-century shutdown of the AMOC with increased atmospheric CO2. In the future simulation, precipitation decreases over the western and central United States by up to 40% and over eastern Mexico by up to 50%. Over the eastern United States rainfall generally increases except during July. Variations in the moisture convergence associated with large-scale circulation changes dominate the rainfall variations, while evaporation plays a critical role over the northeastern United States in spring and the north-central United States in summer. During April–June the westward extension of the North Atlantic subtropical high enhances southwesterly moisture fluxes from the Gulf of Mexico into the eastern and south-central United States. Increases in low-level moisture content reduce the stability of the atmosphere. Enhanced southerly winds promote convergence over the eastern United States through the Sverdrup vorticity balance and precipitation increases. In July–August anomalous anticyclonic moisture fluxes associated with an anomalous high over the Gulf of Mexico and eastern Pacific decrease the moisture supply into the United States and Mexico. Over the central United States decreases in evaporation support decreases in low-level moisture content and increases in atmospheric stability. Over the eastern United States the Sverdrup balance weakens in summer and anomalous moisture convergence is mainly located over the East Coast.