Abstract. The COordinated Regional Downscaling EXperiment (CORDEX) is a diagnostic model intercomparison project (MIP) in CMIP6. CORDEX builds on a foundation of previous downscaling intercomparison projects to provide a common framework for downscaling activities around the world. The CORDEX Regional Challenges provide a focus for downscaling research and a basis for making use of CMIP6 global climate model (GCM) output to produce downscaled projected changes in regional climates and assess sources of uncertainties in the projections, all of which can potentially be distilled into climate change information for vulnerability, impacts and adaptation studies. CORDEX Flagship Pilot Studies advance regional downscaling by targeting one or more of the CORDEX Regional Challenges. A CORDEX-CORE framework is planned that will produce a baseline set of homogeneous high-resolution, downscaled projections for regions worldwide. In CMIP6, CORDEX coordinates with ScenarioMIP and is structured to allow cross comparisons with HighResMIP and interaction with the CMIP6 VIACS Advisory Board.

Abstract. The southeastern tropical Indian Ocean (SE-TIO) was characterized by unusually cold sea surface temperature (SST) and strong northwestward alongshore surface winds during 1994. Using multi-source data sets including ocean model simulation, two key processes are identified for the SETIO cooling. Entrainment cooling produced most of the negative SST anomaly near the coast whereas evaporative cooling dominated the process away from the coast. Convection was anomalously suppressed over SETIO and the divergence of moist air from the region helped the local evaporative process. This also led to anomalous moisture transports that explain the enhanced convection over the central equatorial Indian Ocean, India and East Asia. The positive feedback between the enhanced and suppressed convection regions in turn helped maintain the surface wind anomalies. These evidences clearly indicate the existence of an ocean-atmosphere coupled phenomenon in the Indian Ocean during 1994.

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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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