Modulation of Sri Lankan Maha rainfall by the Indian Ocean Dipole

Investigating the September to December rainy season in Sri Lanka associated with the Maha rice growing season provides insights into the Asian monsoon during the boreal fall. Here, the modulation of the Maha rainfall by the tropical air-sea coupled phenomenon referred to as the Indian Ocean Dipole (IOD) is documented. The Maha rainfall has a strong and robust association with the IOD from 1869 to 2000. The anomalously warm sea surface in the western Indian Ocean associated with the positive IOD phase induces large scale convergence in the lower troposphere extending to Sri Lanka leading to the preponderant enhancement of Maha rainfall

Modulation of ISOs by land-atmosphere feedback and contribution to the interannual variability of Indian summer monsoon

A mechanism of internal variability of Indian summer monsoon through the modulation of intraseasonal oscillation (ISO) by land-atmosphere feedback is proposed. Evidence of feedback between surface soil moisture and ISOs is seen in the soil moisture data from GSWP-2 and rainfall data from observations. Using two sets of internal simulation by a regional climate model (RCM), it is shown that internally generated anomalous soil moisture interacts with the following ISO and generates interannual variability. To gain further insight, 27 years of sensitivity experiment by prescribing wet (dry) soil moisture condition during break (active) period along with a control simulation are carried out. The sensitivity experiment reveals the large-scale nature of soil moisture and ISO feedback which takes place through the changes in atmospheric stability by altering lower-level atmospheric conditions. The feedback is inherent to the monsoon system and a part of it acts through the intraseasonal varying memory of soil moisture. The RCM used to test the hypothesis is constrained by one-way interactions at the lateral boundary. Experiments with a much larger domain upheld the findings and hence suggest the true nature of soil moisture and ISO feedback present in the monsoon system

Meghdoot—A Mobile App to Access Location-Specific Weather-Based Agro-Advisories Pan India

Timely agricultural advisories to farmers can enhance their decision-making and reduce production risk under challenging weather conditions. To enhance access to relevant climate information services in India, a mobile application called Meghdoot was designed to deliver weather information and crop-specific advisories, as a joint initiative of the India Meteorological Department (IMD), Indian Institution for Tropical Meteorology (IITM), Indian Council for Agricultural Research (ICAR) and the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT). Building on IMD’s District Agrometeorological Advisory Service (DAAS) which issues crop-specific weather-based agro-advisories twice a week for all districts in India, the Meghdoot app makes available observed weather recordings, forecasts, and warnings generated by IMD and IITM. This working paper describes the concept design, the framework and a preliminary user analysis of the Meghdoot mobile application. Meghdoot mobile app is available on Google Play (Google Play Store) as well as Apple App Store. Since its inception more than two years ago, Meghdoot has received a good response with 200,000+ downloads/installs and an average rating of 3.3/5.0 by 863 app users (as of July 26, 2021) on Google Play(Google Play Store)

Modulation of ISOs by land-atmosphere feedback and contribution to the interannual variability of Indian summer monsoon

A mechanism of internal variability of Indian summer monsoon through the modulation of intraseasonal oscillation (ISO) by land-atmosphere feedback is proposed. Evidence of feedback between surface soil moisture and ISOs is seen in the soil moisture data from GSWP-2 and rainfall data from observations. Using two sets of internal simulation by a regional climate model (RCM), it is shown that internally generated anomalous soil moisture interacts with the following ISO and generates interannual variability. To gain further insight, 27 years of sensitivity experiment by prescribing wet (dry) soil moisture condition during break (active) period along with a control simulation are carried out. The sensitivity experiment reveals the large-scale nature of soil moisture and ISO feedback which takes place through the changes in atmospheric stability by altering lower-level atmospheric conditions. The feedback is inherent to the monsoon system and a part of it acts through the intraseasonal varying memory of soil moisture. The RCM used to test the hypothesis is constrained by one-way interactions at the lateral boundary. Experiments with a much larger domain upheld the findings and hence suggest the true nature of soil moisture and ISO feedback present in the monsoon system

Monsoons to Mixing in the Bay of Bengal Multiscale Air-Sea Interactions and Monsoon Predictability

Skillful prediction of the ``active'' and ``break'' spells of monsoon intraseasonal oscillations during the South Asian monsoon season is crucial for the socio-economic fate of one-sixth of the world's population, yet it remains a grand challenge problem. The limited skill of our coupled weather and climate models is largely due to our inability to represent the complex multiscale interactions of the north Indian Ocean and the atmosphere. Air-sea interactions are at the heart of not only the climatological mean annual cycle of the South Asian monsoon but also its synoptic, subseasonal, interannual, and decadal variability. With high local monsoon precipitation and discharge from major rivers (Ganges-Brahmaputra, Irrawaddy), the Bay of Bengal (BoB) exhibits the lowest surface salinities in the tropics as well as unique thermal stratification, making it a natural laboratory for studying multiscale interactions ranging from planetary-scale monsoons to submesoscale mixing in freshwater pools. The current ocean component of coupled models is inadequate for simulating the BoB's upper-ocean thermal structure with fidelity. To further improve monsoon forecasts on intraseasonal and interannual time scales, we need new high-resolution and high-frequency observations over the BoB to fill the gap in our understanding of how the ocean mixes in highly fresh regions, and we need modeling of processes that will convert this understanding to parameterizations of mixing that can be used to improve large-scale ocean models

Changing Relationship between the Tropical Easterly Jet and the Indian summer Monsoon Rainfall: Role of Indian Ocean Warming

1678-1683Using long-term observational and reanalysis products, we noticed that the strength of the Tropical Easterly Jet (TEJ) stream shows a weakening trend. Model sensitivity experiments with Atmospheric General Circulation Model (ECHAM5-AGCM) hypothesize that the weakening of the TEJ is mainly a result of the recent Indian Ocean warming and the associated with changes in the convection and circulation. Another notable feature is the weakening of the relationship between the TEJ strength and Indian summer monsoon rainfall (ISMR) in recent decades and sensitivity experiments with AGCM confirm the role of the recent Indian Ocean warming in weakening this relationship. Time series of both TEJ strength and ISMR shows a decreasing trend in recent decades. However, the rate of decrease in ISMR is less compared to the decrease in TEJ strength. This seems to be because of the increases in extreme rainfall events over the Indian landmass in recent decades which can compensate the decrease in total rainfall. It suggests that the local convective rainfall events become more frequent and hence, rainfall due to large scale circulation should decrease to maintain the decreasing trend of total rainfall over Indian landmass. In short, our results indicate that the unequal rate of decrease in the ISMR and the TEJ strength in the context of the recent Indian Ocean warming breaks up the relationship between the ISMR and TEJ

Unusual central Indian drought of summer monsoon - 2008: Role of Southern Tropical Indian Ocean warming

While many of the previous positive Indian Ocean dipole (IOD) years were associated with above (below)-normal monsoon rainfall over central (southern) India during summer monsoon months [June-September (JJAS)], the IOD event in 2008 is associated with below (above)-normal rainfall in many parts of central (southern peninsular) India. Because understanding such regional organization is a key for success in regional prediction, using different datasets and atmospheric model simulations, the reasons for this abnormal behavior of the monsoon in 2008 are explored. Compared to normal positive IOD events, sea surface temperature (SST) and rainfall in the southern tropical Indian Ocean (STIO) in JJAS 2008 were abnormally high. Downwelling Rossby waves and oceanic heat advection played an important role in warming SST abnormally in the STIO. It was also found that the combined influence of a linear warming trend in the tropical Indian Ocean and warming associated with the IOD have resulted in abnormal warming of the STIO. This abnormal SST warming resulted in enhancement of convection in the southwest tropical Indian Ocean and forced anticyclonic circulation anomalies over the Bay of Bengal and central India, leading to suppressed rainfall over this region in JJAS 2008. The above mechanism is tested by conducting several model sensitivity experiments with an atmospheric general circulation model (AGCM). These experiments confirmed that the subsidence over central India and the Bay of Bengal was forced mainly by the anomalous warming in the STIO region driven by coupled ocean-atmosphere processes. This study provides the first evidence of combined Indian Ocean warming, associated with global warming, and IOD-related warming influence on Indian summer monsoon rainfall. The combined influence may force below-normal rainfall over central India by inducing strong convection in the STIO region. The conventional seesaw in convection between the Indian subcontinent and the eastern equatorial Indian Ocean may shift to the central equatorial Indian Ocean and the Bay of Bengal if the central Indian Ocean consistently warms in the global warming scenario