Karumuri Ashok

Impact of the Indian Ocean dipole on the relationship between the Indian monsoon rainfall and ENSO

The influence of the recently discovered Indian Ocean Dipole (IOD) on the interannual variability of the Indian summer monsoon rainfall (ISMR) has been investigated for the period 1958-1997. The IOD and the El Nino/Southern Oscillation (ENSO) have complementarily affected the ISMR during the last four decades. Whenever the ENSO-ISMR correlation is low (high), the IOD-ISMR correlation is high (low). The IOD plays an important role as a modulator of the Indian monsoon rainfall, and influences the correlation between the ISMR and ENSO. We have discovered that the ENSO-induced anomalous circulation over the Indian region is either countered or supported by the IOD-induced anomalous meridional circulation cell, depending upon the phase and amplitude of the two major tropical phenomena in the Indo-Pacific sector.

Individual and Combined Influences of ENSO and the Indian Ocean Dipole on the Indian Summer Monsoon

The relative influences of the ENSO and Indian Ocean Dipole (IOD) events on the Indian summer rainfall were studied using observational data and an atmospheric general circulation model (AGCM). The composite analysis of rainfall anomalies demonstrates that the IOD, while significantly influencing the Indian summer monsoon rainfall, also significantly reduces the impact of ENSO on the Indian summer rainfall whenever these events with the same phase co-occur. The AGCM experiments have shown that during an El Nino event, the Walker circulation over the tropical Indo-Pacific region is modulated; a low-level anomalous divergence center over the western Pacific and an anomalous convergence zone over the equatorial Indian Ocean are induced. Furthermore, an anomalous zone of convergence over the Myanmar and south China regions is induced during an El Nino event. These zones of anomalous convergence are complemented by anomalous divergence over the Indian region, causing anomalous subsidence and weakened rainfall. When a strong positive IOD event simultaneously occurs with El Nino, the latters influence on the Indian monsoon is reduced by both poles of the IOD through the following mechanism: an anomalous divergence center, as compared to the summers when an El Nino alone occurs, is introduced in the eastern tropical Indian Ocean. From this center, the anomalous divergent flow crosses the equator, and this air, while weakening the El Nino-induced divergence over the western Pacific, also leads to convergence over the Indian monsoon region. This results in the reduction of the ENSO-induced subsidence and the related rainfall deficit over the eastern flank of the Indian monsoon trough region and adjoining regions to the east. On the other hand, over the western part of the tropical Indian Ocean sector, part of the anomalous ascending motion from the warm pole of the positive IOD event subsides just to the north of the equator, moves northward, ascends, and causes surplus rainfall. This reduces the ENSO-induced rainfall deficit over western India, the western part of the monsoon trough, and parts of Pakistan. The AGCM experiments also demonstrate that positive IOD events amplify the ENSO-induced dryness over the Indonesian region.

Climate change: The El Niño with a difference

Patterns of sea-surface warming and cooling in the tropical Pacific seem to be changing, as do the associated atmospheric effects. Increased global warming is implicated in these shifts in El Nino phenomena.

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

There are large uncertainties looming over the status and fate of the South Asian summer monsoon, with several studies debating whether the monsoon is weakening or strengthening in a changing climate. Our analysis using multiple observed datasets demonstrates a significant weakening trend in summer rainfall during 1901-2012 over the central-east and northern regions of India, along the Ganges-Brahmaputra-Meghna basins and the Himalayan foothills, where agriculture is still largely rain-fed. Earlier studies have suggested an increase in moisture availability and land-sea thermal gradient in the tropics due to anthropogenic warming, favouring an increase in tropical rainfall. Here we show that the land-sea thermal gradient over South Asia has been decreasing, due to rapid warming in the Indian Ocean and a relatively subdued warming over the subcontinent. Using long-term observations and coupled model experiments, we provide compelling evidence that the enhanced Indian Ocean warming potentially weakens the land-sea thermal contrast, dampens the summer monsoon Hadley circulation, and thereby reduces the rainfall over parts of South Asia.

Increased frequency of extreme Indian Ocean Dipole events due to greenhouse warming

The Indian Ocean dipole is a prominent mode of coupled ocean–atmosphere variability, affecting the lives of millions of people in Indian Ocean rim countries. In its positive phase, sea surface temperatures are lower than normal off the Sumatra–Java coast, but higher in the western tropical Indian Ocean. During the extreme positive-IOD (pIOD) events of 1961, 1994 and 1997, the eastern cooling strengthened and extended westward along the equatorial Indian Ocean through strong reversal of both the mean westerly winds and the associated eastward-flowing upper ocean currents. This created anomalously dry conditions from the eastern to the central Indian Ocean along the Equator and atmospheric convergence farther west, leading to catastrophic floods in eastern tropical African countries but devastating droughts in eastern Indian Ocean rim countries. Despite these serious consequences, the response of pIOD events to greenhouse warming is unknown. Here, using an ensemble of climate models forced by a scenario of high greenhouse gas emissions (Representative Concentration Pathway 8.5), we project that the frequency of extreme pIOD events will increase by almost a factor of three, from one event every 17.3 years over the twentieth century to one event every 6.3 years over the twenty-first century. We find that a mean state change—with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean—facilitates more frequent occurrences of wind and oceanic current reversal. This leads to more frequent extreme pIOD events, suggesting an increasing frequency of extreme climate and weather events in regions affected by the pIOD.

Impacts of ENSO and Indian Ocean Dipole Events on the Southern Hemisphere Storm-Track Activity during Austral Winter

Impacts of the ENSO and Indian Ocean dipole (IOD) phenomena on winter storm-track activity over the Southern Hemisphere are examined on the basis of the observed and reanalysis data for 1979–2003. The partial correlation technique is utilized to distinguish the impact of one phenomenon from that of the other. During an El Nino event, the subtropical jet stream tends to strengthen substantially, enhancing the jet bifurcation and thereby reducing storm-track activity over the midlatitude South Pacific and to the south of Australia. During a positive IOD event, the westerlies and storm-track activity also tend to weaken over southern Australia and portions of New Zealand. Thus both the positive IOD and, to a lesser extent, El Nino events act to reduce winter rainfall significantly over some portions of South Australia and New Zealand. Precipitation over the southeastern portion of the continent and over the northern portions of the two main islands of New Zealand is more sensitive to IOD. Significant reduction in precipitation associated with an El Nino event is seen over Tasmania. Over midlatitude South America, in contrast, the enhancement of the westerlies and storm-track activity tends to be more significant in a positive IOD event than in an El Nino event. It is demonstrated that despite the dominant influence of the Southern Hemispheric Annular Mode from a hemispheric viewpoint, the remote influence of ENSO and/or IOD on local stormtrack activity can be detected in winter as a significant signal in particular midlatitude regions, including South Australia and New Zealand.

Assessment of the APCC coupled MME suite in predicting the distinctive climate impacts of two flavors of ENSO during boreal winter

Forecast skill of the APEC Climate Center (APCC) Multi-Model Ensemble (MME) seasonal forecast system in predicting two main types of El Niño-Southern Oscillation (ENSO), namely canonical (or cold tongue) and Modoki ENSO, and their regional climate impacts is assessed for boreal winter. The APCC MME is constructed by simple composite of ensemble forecasts from five independent coupled ocean-atmosphere climate models. Based on a hindcast set targeting boreal winter prediction for the period 1982–2004, we show that the MME can predict and discern the important differences in the patterns of tropical Pacific sea surface temperature anomaly between the canonical and Modoki ENSO one and four month ahead. Importantly, the four month lead MME beats the persistent forecast. The MME reasonably predicts the distinct impacts of the canonical ENSO, including the strong winter monsoon rainfall over East Asia, the below normal rainfall and above normal temperature over Australia, the anomalously wet conditions across the south and cold conditions over the whole area of USA, and the anomalously dry conditions over South America. However, there are some limitations in capturing its regional impacts, especially, over Australasia and tropical South America at a lead time of one and four months. Nonetheless, forecast skills for rainfall and temperature over East Asia and North America during ENSO Modoki are comparable to or slightly higher than those during canonical ENSO events.

Statistical Downscaling of Precipitation in Korea Using Multimodel Output Variables as Predictors

Abstract A pattern projection downscaling method is applied to predict summer precipitation at 60 stations over Korea. The predictors are multiple variables from the output of six operational dynamical models. The hindcast datasets span a period of 21 yr from 1983 to 2003. A downscaled prediction was made for each model separately within a leave-one-out cross-validation framework. The pattern projection method uses a moving window, which scans globally, in order to seek the most optimal predictor for each station. The final forecast is the average of six model downscaled precipitation forecasts using the best predictors and will be referred to as “DMME.” It is found that DMME significantly improves the prediction skill by correcting the erroneous signs of the rainfall anomalies in coarse-resolution predictions of general circulation models. Although Korea’s precipitation is strongly influenced by local mountainous terrain, DMME performs well at 59 stations with correlation skill significant at the 95% con...

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.

Falling monsoon depression frequency: A Gray-Sikka conditions perspective

In this study, we show that the annual monsoon depression (MD) frequency making landfall on the east coast of India shows a statistically significant decreasing trend for the period 1979–2010. Importantly, about 80% of this fall is confined to the south of 20°N. To understand the plausible reason(s) for the weakening frequency of MDs in the southern Bay of Bengal in recent decades, we examine some of the seasonal average in-situ atmospheric parameters important for tropical cyclogenesis; we use various observational data from the IMD, and three atmospheric climate reanalysis datasets to account for possible quality constraints in them. Our findings suggest that the observed weakening of MD frequency south of 20°N in the Bay of Bengal since 1950s is likely due to a declining trend in the mid-tropospheric relative humidity over the Indian region. Our numerical sensitivity experiments support this finding.