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 IndoPakistan region, a meridionally deep anticyclonic anomaly over East Asia (;1008E), 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.

The year 2009 was a major drought year for the Indian summer monsoon with a seasonal deficit of rainfall by 21.6%. Standard oceanic predictors such as ENSO and the Indian Ocean dipole are not consistent for these dry spells. There are a host of other parameters such as the Himalayan ice cover, the Eurasian snow cover, the passage of intraseasonal waves, and even accumulated effects of Asian pollution that have been considered for analysis of the dry spells of the monsoon. This paper presents another factor, the western Asian desert air incursions toward central India, and emphasizes the formation of a blocking high over western Asia as an important feature for these dry spells. The blocking high advects descending very dry air toward central India, portrayed using swaths of three-dimensional trajectories. This is a robust indicator for dry spells of the monsoon during the last several decades. This dry air above the 3-km level over central India strongly inhibits the vertical growth of deep convection. Some of the interesting antecedents of the formation of the blocking high include an eastward and somewhat northward extension of the ITCZ over North Africa, a stronger than normal local Hadley cell over North Africa, a strong subtropical jet stream over the southern Mediterranean, and strong conversions of anticyclonic shear vorticity to anticyclonic curvature vorticity. The dynamical antecedents of the aforementioned scenario in this study are related to many aspects of North African weather features. They are portrayed using both reanalysis datasets and ensemble modeling using a suite of coupled atmosphere-ocean models.

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This review provides a summary of work in the area of ensemble forecasts for weather, climate, oceans, and hurricanes. This includes a combination of multiple forecast model results that does not dwell on the ensemble mean but uses a unique collective bias reduction procedure. A theoretical framework for this procedure is provided, utilizing a suite of models that is constructed from the well-known Lorenz low-order nonlinear system. A tutorial that includes a walk-through table and illustrates the inner workings of the multimodel superensemble's principle is provided. Systematic errors in a single deterministic model arise from a host of features that range from the model's initial state (data assimilation), resolution, representation of physics, dynamics, and ocean processes, local aspects of orography, water bodies, and details of the land surface. Models, in their diversity of representation of such features, end up leaving unique signatures of systematic errors. The multimodel superensemble utilizes as many as 10 million weights to take into account the bias errors arising from these diverse features of multimodels. The design of a single deterministic forecast models that utilizes multiple features from the use of the large volume of weights is provided here. This has led to a better understanding of the error growths and the collective bias reductions for several of the physical parameterizations within diverse models, such as cumulus convection, planetary boundary layer physics, and radiative transfer. A number of examples for weather, seasonal climate, hurricanes and sub surface oceanic forecast skills of member models, the ensemble mean, and the superensemble are provided.A suite of models, each of which carry somewhat different representation of the above processes, can be combined to reduce the collective local biases in space, time, and for different variables from the different models. That is the theme of the multimodel superensemble. The multimodel superensemble (which is not an ensemble mean) utilizes as many as ten million weights toward the reduction of such systematic errors. This figure of 10 million comes from the products of the three-dimensional grid points, the number of dependent variables, the number of models, and the number of forecast intervals in time.The notion of the multimodel superensemble was first described in Krishnamurti et al. [1999]. This utilizes a training and a forecast phase. The training phase learns from the recent past performances of models and is used to determine statistical weights from a least square minimization via a simple multiple regression. That regression is carried out with respect to analyzed (assimilated) values. Given a number of grid locations, base variables, forecast intervals, and a suite of models, the number of statistical weights can be as high as 10 7 . That many coefficients are needed because of different responses to physical parameterizations of local features such as water bodies, local mountain features, and land surface details within d...

A highlight of the 2006 boreal summer monsoon season was the enhanced activity of long-lived monsoon depressions and low-pressure systems (LPS) over the Indian region. Another important phenomenon during this period was the evolution of a positive Indian Ocean Dipole (PIOD) event. Although previous studies have investigated the impact of PIOD on the large-scale monsoon response, their influence on monsoon LPS activity is not well understood. Based on detailed diagnostic analyses of monsoon LPS during 2006, as well as those associated with other PIOD events during 1958-2007, the present work addresses two specific issues concerning the roles of (a) PIOD-induced large-scale circulation changes and (b) internal feedbacks between latent heating and dynamics, in sustaining the monsoon LPS activity. The results show that PIOD conditions generally favour increased propensity of long-lived (>5 days) LPS with long westward tracks extending into northwest India. The average contribution of long-lived monsoon LPS to the total is found to be approximately 12% higher during PIOD episodes as compared to non-PIOD. The PIOD events showed two important large-scale elements conducive for enhancement of LPS activity: (a) strengthening of cross-equatorial moisture transport from south-eastern tropical Indian Ocean into the Bay of Bengal and (b) enrichment of barotropic instability of monsoon flow. Estimates of latent-heating profiles from TRMM-satellite products during the 2006 LPS showed heating in the mesoscale updrafts above 600 hPa with maximum approximately 400 hPa; while cooling prevailed in lower levels. Stratiform precipitation covered approximately 70-85% of rain area during the prolonged LPS; and the large-scale monsoon Hadley-type circulation was found to be intensified with strong mid-level inflows entering the stratiform rain region. The overall findings suggest that the PIOD-induced background circulation together with internal feedbacks between mesoscale convective systems and large-scale circulation can effectively enhance the longevity of monsoon LPS. These results should serve as important inputs for numerical weather forecasting of extreme rainfall events associated with the regional monsoon phenomenon.