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R. Krishnan and P. Swapna

Abstract

A majority of positive Indian Ocean dipole (IOD) events in the last 50 years were accompanied by enhanced summer monsoon circulation and above-normal precipitation over central-north India. Given that IODs peak during boreal autumn following the summer monsoon season, this study examines the role of the summer monsoon flow on the Indian Ocean (IO) response using a suite of ocean model experiments and supplementary data diagnostics. The present results indicate that, if the summer monsoon Hadley-type circulation strengthens during positive IOD events, then the strong off-equatorial southeasterly winds over the northern flanks of the intensified Australian high can effectively promote upwelling in the southeastern tropical Indian Ocean and amplify the zonal gradient of the IO heat content response. While it is noted that a strong monsoon cross-equatorial flow by itself may not generate a dipolelike response, a strengthening (weakening) of monsoon easterlies to the south of the equator during positive IOD events tends to reinforce (hinder) the zonal gradient of the upper-ocean heat content response. The findings show that an intensification of monsoonal winds during positive IOD periods produces nonlinear amplification of easterly wind stress anomalies to the south of the equator because of the nonlinear dependence of wind stress on wind speed. It is noted that such an off-equatorial intensification of easterlies over the SH enhances upwelling in the eastern IO off Sumatra–Java, and the thermocline shoaling provides a zonal pressure gradient, which drives anomalous eastward equatorial undercurrents (EUC) in the subsurface. Furthermore, the combination of positive IOD and stronger-than-normal monsoonal flow favors intensification of shallow transient meridional overturning circulation in the eastern IO and enhances the feed of cold subsurface off-equatorial waters to the EUC.

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Ayantika Dey Choudhury and R. Krishnan

Abstract

Simulation experiments using a simplified atmospheric GCM and supplementary diagnostic analyses of observations are performed to understand how the South Asian monsoon trough (MT) responds dynamically to latent heating from mesoscale convective systems (MCSs). Observations reveal that the MT during active monsoons is characterized by a deep cyclonic vorticity extending from the surface to 350 hPa and organized MCSs covering over 3500–4000 km along the Indo-Gangetic plains. The MCSs during active monsoons are composed of a relatively higher abundance of stratiform-type precipitation (mostly nimbostratus) as compared to the convective type. The results suggest that a stratiform-type heating profile is very effective in promoting upward development of continental-scale cyclonic circulation well above the midtroposphere over the MT region. The vertical development involves a dynamical uplift of the layer of cyclonic circulation and is induced by midlevel (600–500 hPa) convergence and vorticity stretching above 500 hPa. By varying the population of stratiform and convective rain types in the simulation, the horizontal scale of midlevel vorticity response is shown to increase significantly with stratiform population; in contrast, the midlevel response is more localized when the MCS is dominated by deep convective clouds. For large stratiform populations, the midlevel response is found to extend far westward up to the northern flanks of the African ITCZ, indicative of Rossby wave dispersion of PV anomalies that are generated near the level of maximum heating gradient. From the present findings, one can conclude that the vertical deepening of MT during active monsoons is not merely a localized phenomenon; instead it represents a large-scale dynamical response to organized MCSs that exert pivotal influence on the upward development of cyclonic circulation well above the midtroposphere.

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R. Krishnan, C. Zhang, and M. Sugi

Abstract

In this paper the authors present results of diagnostic analysis of observations and complementary experiments with a simple numerical model that enable them to synthesize the morphology and dynamics of “breaks” in the Indian summer monsoon (ISM). Almost one week ahead of the onset of a break spell over India, a monotonically decreasing trend in convective activity is found to occur over the Bay of Bengal in response to a steady eastward spreading of dry convectively stable anomalies from the equatorial Indian Ocean. A major intensification of the convectively stable anomalies over the Bay of Bengal is seen about 2–3 days prior to commencement of a monsoon break. Both observations and modeling experiments reveal that rapid northwest propagating Rossby waves are triggered in response to such a large strengthening of the convectively stable anomalies. It is shown that an abrupt movement of anomalous Rossby waves from the Bay of Bengal into northwest and central India marks the initiation of a break monsoon spell. Typically the Rossby waves are found to traverse from the central Bay of Bengal to northwest India in about 2–3 days’ time. With the establishment of a break phase, the eastward spreading low-latitude anomaly decouples from the rapid northwest propagating anomaly. This decoupling effect paves the way for the emergence of a convectively unstable anomaly over the equatorial Indian Ocean. It is proposed that the dynamics of the rapid northwest propagating anomalous Rossby waves from the central Bay of Bengal toward northwest India and decoupling of the eastward propagating anomaly are two extremely vital elements that determine the transition from an above normal phase to a break phase of the ISM and also help maintain the mutual competition between convection over the Indian subcontinent and that over the equatorial Indian Ocean. Through modeling experiments it is demonstrated that low-latitude Rossby wave dynamics in the presence of a monsoon basic flow, which is driven by a steady north–south differential heating, is a primary physical mechanism that controls the so-called monsoon breaks.

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R. Krishnan, Vinay Kumar, M. Sugi, and J. Yoshimura

Abstract

Results from a 20-yr simulation of a high-resolution AGCM forced with climatological SST, along with simplified model experiments and supplementary data diagnostics, are used to investigate internal feedbacks arising from monsoon–midlatitude interactions during droughts in the Indian summer monsoon. The AGCM simulation not only shows a fairly realistic mean monsoon rainfall distribution and large-scale circulation features but also exhibits remarkable interannual variations of precipitation over the subcontinent, with the 20-yr run showing incidence of four “monsoon droughts.”

The present findings indicate that the internally forced droughts in the AGCM emanate largely from prolonged “monsoon breaks” that occur on subseasonal time scales and involve dynamical feedbacks between monsoon convection and extratropical circulation anomalies. In this feedback, the suppressed monsoon convection is shown to induce Rossby wave dispersion in the summertime subtropical westerlies and to set up an anomalous quasi-stationary circulation pattern extending across continental Eurasia in the middle and upper troposphere. This pattern is composed of a cyclonic anomaly over west central Asia and the Indo-Pakistan region, a meridionally deep anticyclonic anomaly over East Asia (∼100°E), and a cyclonic anomaly over the Far East. The results suggest that the anchoring of the west central Asia cyclonic anomaly by the stagnant ridge located downstream over East Asia induces anomalous cooling in the middle and upper troposphere through cold-air advection, which reduces the meridional thermal contrast over the subcontinent. Additionally, the intrusion of the dry extratropical winds into northwest India can decrease the convective instability, so that the suppressed convection can in turn weaken the monsoon flow. The sustenance of monsoon breaks through such monsoon–midlatitude feedbacks can generate droughtlike conditions over India.

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Parthasarathi Mukhopadhyay, R. Krishnan, Ravi S. Nanjundiah, and M. Mohapatra
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Ayantika Dey Choudhury, R. Krishnan, M. V. S. Ramarao, R. Vellore, M. Singh, and B. Mapes

Abstract

Midtropospheric cyclones (MTCs) are a distinct class of synoptic disturbances, characterized by quasi-stationary cyclonic circulation in midtropospheric levels, which often produce heavy rainfall and floods over western India during the summer monsoon. This study presents a composite and diagnostic process study of long-lived (>5 days) midtropospheric cyclonic circulation events identified by the India Meteorological Department (IMD). Reanalysis data confirm earlier studies in revealing that the MTC composite has its strongest circulation in the midtroposphere. Lagged composites show that these events co-occur with broader-scale monsoon evolution, including larger synoptic-scale low pressure systems over the Bay of Bengal (BoB) and east coast, and the active phase of regional-scale poleward-propagating intraseasonal rain belts, with associated drying ahead (north) of the convectively active area. Diabatic heating composites, in particular the TRMM latent heating and Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2)-derived radiative cooling in the dry inland areas of southwest Asia north of the rain belt, are used to drive a nonlinear multilayer dynamical model in a forced-damped reconstruction of the global circulation. Results show that the midlevel circulation is largely attributable to top-heavy latent heating, indicative of the prevalence of stratiform-type precipitation in mesoscale convective systems in these moist, active larger-scale settings. Both the west coast and BoB latent heating are important, while the radiative cooling over southwest Asia plays a modest role in sharpening some of the simulated features. A conceptual model encapsulates the paradigm based on this composite and diagnostic modeling, a diabatic update of early theoretical studies that emphasized hydrodynamic flow instabilities.

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P. Priya, Milind Mujumdar, T. P. Sabin, Pascal Terray, and R. Krishnan

Abstract

Quite a few studies have documented the evolution of monsoon synoptic systems and midlatitude atmospheric blocking associated with the recent heavy precipitation and floods over northwest Pakistan during 2010. This period also witnessed a very unusual Indo-Pacific sea surface temperature (SST) evolution with a strong La Niña event in the Pacific, substantial Indian Ocean warming, and a negative Indian Ocean dipole event, together with significant enhancement of precipitation over both the equatorial western Pacific Ocean and the eastern Indian Ocean.

Here, the authors perform a suite of high-resolution atmospheric general circulation model experiments to investigate the influence of Indo-Pacific SST anomalies on the South Asian monsoon circulation and heavy precipitation over Pakistan and adjoining northwest India during 2010. The realistic simulation of these rainfall anomalies using observed SSTs motivated the authors to explore the specific influence of Indian Ocean and Pacific SST anomalies through additional simulation experiments. The authors find that, in addition to strengthening of the Pacific Walker circulation, the anomalous intensification of east–west circulation over the Indian Ocean in 2010 was a key element in contributing to precipitation enhancement over the northwest India–Pakistan region. It is found that the subsiding branch of the east–west circulation over the Indian Ocean induced anomalous subsidence over the western tropical Indian Ocean and played a key role in inducing northward transport of moisture and promoting generation of strong upward motion and heavy precipitation events over the northwest India–Pakistan region.

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R. Krishnan, M. Mujumdar, V. Vaidya, K. V. Ramesh, and V. Satyan

Abstract

Diagnostic analysis of observations and a series of ensemble simulations using an atmospheric general circulation model (GCM) have been carried out with a view to understanding the processes responsible for the widespread suppression of the seasonal summer monsoon rainfall over the Indian subcontinent in 2000. During this period, the equatorial and southern tropical Indian Ocean (EQSIO) was characterized by persistent warmer than normal sea surface temperature (SST), increased atmospheric moisture convergence, and enhanced precipitation. These abnormal conditions not only offered an ideal prototype of the regional convective anomalies over the subcontinent and Indian Ocean, but also provided a basis for investigating the causes for the intensification and maintenance of the seasonal anomaly patterns.

The findings of this study reveal that the strengthening of the convective activity over the region of the southern equatorial trough played a key role in inducing anomalous subsidence over the subcontinent and thereby weakened the monsoon Hadley cell. The leading empirical orthogonal function (EOF) of the intraseasonal variability of observed rainfall was characterized by a north–south asymmetric pattern of negative anomaly over India and positive anomaly over the region of the EQSIO and accounted for about 21% of the total rainfall variance during 2000. The GCM-simulated response to forcing by SST anomalies during 2000 is found to be consistent with observations in reasonably capturing the seasonal monsoon anomalies and the intraseasonal variability. Further, it is shown from the GCM experiments that the warm Indian Ocean (IO) SST anomalies influenced the regional intraseasonal variability in a significant manner by favoring higher probability of occurrence of enhanced rainfall activity over the EQSIO region and, in turn, led to higher probability of occurrence of dry spells and prolonged break-monsoon conditions over the subcontinent. In particular, the simulated break-monsoon anomaly pattern of decreased rainfall over the subcontinent and increased rainfall over the EQSIO is shown to intensify and persist in response to the IO SST anomalies during 2000. These results clearly bring out the significance of the IO SST anomalies in altering the regional intraseasonal variability and thereby affecting the seasonal mean monsoon. Further studies will be required in order to investigate the detailed physical mechanisms that couple the variability of convection over the IO region with the local SST boundary forcing and the large-scale monsoon dynamics.

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G. Di Capua, M. Kretschmer, J. Runge, A. Alessandri, R. V. Donner, B. van den Hurk, R. Vellore, R. Krishnan, and D. Coumou

Abstract

Skillful forecasts of the Indian summer monsoon rainfall (ISMR) at long lead times (4–5 months in advance) pose great challenges due to strong internal variability of the monsoon system and nonstationarity of climatic drivers. Here, we use an advanced causal discovery algorithm coupled with a response-guided detection step to detect low-frequency, remote processes that provide sources of predictability for the ISMR. The algorithm identifies causal precursors without any a priori assumptions, apart from the selected variables and lead times. Using these causal precursors, a statistical hindcast model is formulated to predict seasonal ISMR that yields valuable skill with correlation coefficient (CC) ~0.8 at a 4-month lead time. The causal precursors identified are generally in agreement with statistical predictors conventionally used by the India Meteorological Department (IMD); however, our methodology provides precursors that are automatically updated, providing emerging new patterns. Analyzing ENSO-positive and ENSO-negative years separately helps to identify the different mechanisms at play during different years and may help to understand the strong nonstationarity of ISMR precursors over time. We construct operational forecasts for both shorter (2-month) and longer (4-month) lead times and show significant skill over the 1981–2004 period (CC ~0.4) for both lead times, comparable with that of IMD predictions (CC ~0.3). Our method is objective and automatized and can be trained for specific regions and time scales that are of interest to stakeholders, providing the potential to improve seasonal ISMR forecasts.

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P. Swapna, M. K. Roxy, K. Aparna, K. Kulkarni, A. G. Prajeesh, K. Ashok, R. Krishnan, S. Moorthi, A. Kumar, and B. N. Goswami

Abstract

With the goal of building an Earth system model appropriate for detection, attribution, and projection of changes in the South Asian monsoon, a state-of-the-art seasonal prediction model, namely the Climate Forecast System version 2 (CFSv2) has been adapted to a climate model suitable for extended climate simulations at the Indian Institute of Tropical Meteorology (IITM), Pune, India. While the CFSv2 model has been skillful in predicting the Indian summer monsoon (ISM) on seasonal time scales, a century-long simulation with it shows biases in the ocean mixed layer, resulting in a 1.5°C cold bias in the global mean surface air temperature, a cold bias in the sea surface temperature (SST), and a cooler-than-observed troposphere. These biases limit the utility of CFSv2 to study climate change issues. To address biases, and to develop an Indian Earth System Model (IITM ESMv1), the ocean component in CFSv2 was replaced at IITM with an improved version, having better physics and interactive ocean biogeochemistry. A 100-yr simulation with the new coupled model (with biogeochemistry switched off) shows substantial improvements, particularly in global mean surface temperature, tropical SST, and mixed layer depth. The model demonstrates fidelity in capturing the dominant modes of climate variability such as the ENSO and Pacific decadal oscillation. The ENSO–ISM teleconnections and the seasonal leads and lags are also well simulated. The model, a successful result of Indo–U.S. collaboration, will contribute to the IPCC’s Sixth Assessment Report (AR6) simulations, a first for India.

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