P. Swapna

Indian Ocean and monsoon coupled interactions in a warming environment

Several studies have drawn attention to the steady warming of the equatorial and tropical Indian Ocean (IO) sea surface temperature (SST) observed during recent decades. An intriguing aspect of the IO SST warming trend is that it is has been accompanied by a pronounced weakening of the large-scale boreal summer monsoon circulation. Based on a detailed diagnostic analysis of observed datasets, reanalysis products and IPCC AR4 coupled model output, this study examines how the observed changes in the summer monsoon circulation could have contributed to this SST warming trend. The present results reveal that the weakening trend of the summer monsoon cross-equatorial flow has favored a reorientation of surface westerlies towards the equatorial IO during recent decades, relative to summer monsoons of earlier decades, which were dominated by stronger cross-equatorial flow. Our analysis suggests that the weakening of the summer monsoon cross-equatorial flow has in turn significantly accelerated the SST warming in the central equatorial IO. While the trend in the equatorial westerlies has promoted downwelling and thermocline deepening in the eastern equatorial IO, the central equatorial IO warming is attributed to reduced upwelling in response to a weakening trend of the wind-stress curl. The observed trends in Indian monsoon rainfall and the near-equatorial SST warming are shown to be closely related to variations in the meridional gradient of the monsoon zonal winds. An examination of the twentieth century simulations from 22 IPCC AR4 models, suggests that some models capture the recent equatorial IO warming associated with the weakened summer monsoon circulation reasonably well. The individual member models, however, exhibit significant inter-model variations in representing the observed response of the IO and monsoon coupled system.

Revisiting El Niño Modokis

The suggestion that there exist two types of El Niño in the tropical Pacific has generated a debate in the community. Applying various linear and non-linear approaches and composite analysis technique on observed and reanalyzed climate datasets primarily for the 1950–2010 period, we revisit the variability of the tropical Pacific in the light of this debate. Our objective is to examine whether the proposed El Niño Modokis need a classification distinct from canonical El Niños. Even if the distinction is subject to short data records, we demonstrate that the El Niño Modoki events indeed display a seasonal evolution and teleconnections different from the canonical El Niños, and that the distinction is not subject to inclusion of the two extreme El Niños 1982 and 1997 as canonical El Niños. We show that the El Niño Modoki events are not an artifact associated with the orthogonality constraint associated with the EOF technique. Our cluster analysis shows that evolutions of the canonical El Niño and El Niño Modokis through various seasons differ from one another. Importantly, the dynamic and thermodynamic air–sea coupling strength is distinctly different between the El Niño Modoki and the canonical El Niño events. We find that, dynamic feedback intensity is stronger for El Niño Modoki (canonical El Niño) during boreal summer (winter); though the air–sea coupling strength, a major contributor to Bjerknes feedback, is maximum for Modokis during the developing stages, it decreases thereafter. In case of thermodynamic feedback intensity, SST-wind-evaporation feedback is dominant for El Niños while SST-SHF feedback is important during El Niño Modokis. However, we find that the thermodynamic feedback values significantly differ across the flux datasets.

The IITM earth system model

AbstractWith 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 biogeo...

Multidecadal Weakening of Indian Summer Monsoon Circulation Induces an Increasing Northern Indian Ocean Sea Level

North Indian Ocean sea level has shown significant increase during last three to four decades. Analyses of long-term climate data sets and ocean model sensitivity experiments identify a mechanism for multidecadal sea level variability relative to global mean. Our results indicate that North Indian Ocean sea level rise is accompanied by a weakening summer monsoon circulation. Given that Indian Ocean meridional heat transport is primarily regulated by the annual cycle of monsoon winds, weakening of summer monsoon circulation has resulted in reduced upwelling off Arabia and Somalia and decreased southward heat transport, and corresponding increase of heat storage in the North Indian Ocean. These changes in turn lead to increased retention of heat and increased thermosteric sea level rise in the North Indian Ocean, especially in the Arabian Sea. These findings imply that rising North Indian Ocean sea level due to weakening of monsoon circulation demands adaptive strategies to enable a resilient South Asian population.

Long‐Term Climate Simulations Using the IITM Earth System Model (IITM‐ESMv2) With Focus on the South Asian Monsoon

Long-term climate simulations are performed using the IITM Earth System Model version 2 (IITM-ESMv2), a sequel to the earlier version (IITM-ESMv1), to assess climate variability and change with a special focus on the South Asian monsoon. Substantial improvements are incorporated in IITM-ESMv2 to obtain a radiatively balanced global climate modeling framework. The IITM-ESMv2 includes time-varying aerosol forcing and land-use land-cover changes. Multicentury simulations corresponding to preindustrial and present-day conditions show major improvements in capturing key aspects of time-mean atmosphere and ocean large-scale circulation. Representations of the Atlantic Meridional Overturning Circulation and poleward ocean heat transport, and major global climate drivers are superior to ESMv1. Teleconnections of the South Asian monsoon with climate drivers such as the El-Ni~ no–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are found to be more robust in IITM-ESMv2, thus leading to improved simulation of South Asian monsoon and its variability. The IITM-ESMv2 takes a novel pursuit from India to contribute in the Intergovernmental Panel for Climate Change (IPCC) sixth assessment report (AR6). Plain Language Summary This study details the capabilities of the IITM Earth System Model version 2 (IITM-ESMv2), developed at the Indian Institute of Tropical Meteorology, Pune, India, for investigating long-term climate variability and change with special focus on the South Asian monsoon.

Nonlinearities in the Evolutional Distinctions Between El Niño and La Niña Types

Using the HadISST, SODA reanalysis, and various other observed and reanalyzed data sets for the period 1950–2010, we explore nonlinearities in the subsurface evolutional distinctions between El Nino types and La Nina types from a few seasons before the onset. Cluster analysis carried out over both summer and winter suggests that while the warm-phased events of both types are distinguishable, several cold phased events are clustered together. Further, we apply a joint Self-Organizing Map (SOM) analysis using the monthly sea surface temperature anomaly (SSTA) and thermocline-depth anomalies in tropical Pacific (TP). Results reveal that the evolutionary paths of El Nino Modoki (EM) and El Nino (EL) are, broadly, different. Subsurface temperature composites of EL and EM show different onset characteristics. During an EL, warm anomaly in the west spreads eastward along the thermocline and reaches the surface in the east in March–May of year(0). During an EM, warm anomaly already exists in the central tropical Pacific and then reaches the surface in the east in September–November of year(0). Composited SSTAs during La Nina (LN) and La Nina Modoki (LM) are distinguishable only at 80% confidence level, but the composited subsurface temperature anomalies show differences in the location of the coldest anomaly as well as evolution at 90% confidence level. Thus, the El Nino flavor distinction is potentially predictable at longer leads.