K. P. Sooraj

State-dependent atmospheric noise associated with ENSO

[1]xa0A strong relationship between ENSO (El Nino/Southern Oscillation) and atmospheric short-term variability in the near-surface zonal wind is reported in the present study. On one hand, anomalies in the variance of the short-term atmospheric variability over the western Pacific tend to lead El Nino development. On the other hand, the activity of the fast atmospheric variability over the central Pacific is simultaneously correlated to the NINO3.4 SST. The significant correlation exists over the broad range of atmospheric variability from the synoptic time scale to the inter-seasonal time scale. This finding supports the notion that the ENSO–state dependence of the atmospheric variability serves as a source of stochastic forcing for ENSO. Furthermore, it is demonstrated that there is a significant interdecadal change in this dependence. The state-dependent noise becomes much stronger in the recent period, which possibly explains the recent increase of the ENSO activities, consistent with the recent theory of noise-induced destabilization effect for ENSO.

Global warming and the weakening of the Asian summer monsoon circulation: assessments from the CMIP5 models

The evolution of the Asian summer monsoon (ASM) in a global warming environment is a serious scientific and socio-economic concern since many recent studies have demonstrated the weakening nature of large-scale tropical circulation under anthropogenic forcing. But, how such processes affect the ASM circulation and rainfall is still a matter of debate. This study examines the climate model projections from a selected set of Coupled Model Inter-comparison Project 5 (CMIP5) models to provide a unified perspective on the future ASM response. The results indicate a robust reduction in the large-scale meridional gradient of temperature (MGT) at upper levels (200xa0hPa) over the ASM region, associated with enhanced ascendance and deep tropospheric heating over the equatorial Pacific in the future climate. The differential heating in the upper troposphere, with concomitant increase (decrease) in atmospheric stability (MGT), weakens the ASM circulation, promotes a northward shift of the monsoon circulation and a widening of the local Hadley cell in the eastern Indian sector. An examination of the water vapour budget indicates the competing effects of the thermodynamic (moisture convergence) and dynamics processes (monsoon circulation) on future ASM rainfall changes. The former component wins out over the later one and leads to the intensification of Indian monsoon rainfall in the CMIP5 projections. However, the diagnostics further show a considerable offset due to the dynamic component.

Impacts of Indian and Atlantic oceans on ENSO in a comprehensive modeling framework

AbstractnThe impact of the Indian and Atlantic oceans variability on El Niño–Southern-Oscillation (ENSO) phenomenon is investigated through sensitivity experiments with the SINTEX-F2 coupled model. For each experiment, we suppressed the sea surface temperature (SST) variability in either the Indian or Atlantic oceans by applying a strong nudging of the SST toward a SST climatology computed either from a control experiment or observations. In the sensitivity experiments where the nudging is done toward a control SST climatology, the Pacific mean state and seasonal cycle are not changed. Conversely, nudging toward an observed SST climatology in the Indian or Atlantic domain significantly improves the mean state and seasonal cycle, not only in the nudged domain, but also in the whole tropics. These experiments also demonstrate that decoupling the Indian or Atlantic variability modifies the phase-locking of ENSO to the annual cycle, influences significantly the timing and processes of ENSO onset and termination stages, and, finally, shifts to lower frequencies the main ENSO periodicities. Overall, these results suggest that both the Indian and Atlantic SSTs have a significant damping effect on ENSO variability and promote a shorter ENSO cycle. The reduction of ENSO amplitude is particularly significant when the Indian Ocean is decoupled, but the shift of ENSO to lower frequencies is more pronounced in the Atlantic decoupled experiments. These changes of ENSO statistical properties are related to stronger Bjerknes and thermocline feedbacks in the nudged experiments. During the mature phase of El Niño events, warm SST anomalies are found over the Indian and Atlantic oceans in observations or the control run. Consistent with previous studies, the nudged experiments demonstrate that these warm SSTs induce easterly surface wind anomalies over the far western equatorial Pacific, which fasten the transition from El Niño to La Niña and promote a shorter ENSO cycle in the control run. These results may be explained by modulations of the Walker circulation induced directly or indirectly by the Indian and Atlantic SSTs. Another interesting result is that decoupling the Atlantic or Indian oceans change the timing of ENSO onset and the relative role of other ENSO atmospheric precursors such as the extra-tropical Pacific Meridional Modes or the Western North Pacific SSTs.

A Comprehensive Assessment of CFS Seasonal Forecasts over the Tropics

AbstractThe 15-member ensemble hindcasts performed with the National Centers for Environmental Prediction Climate Forecast System (CFS) for the period 1981–2005, as well as real-time forecasts for the period 2006–09, are assessed for seasonal prediction skills over the tropics, from deterministic (anomaly correlation), categorical (Heidke skill score), and probabilistic (rank probability skill score) perspectives. Further, persistence, signal-to-noise ratio, and root-mean-square error analyses are also performed. The CFS demonstrates high skill in forecasting El Nino–Southern Oscillation (ENSO) related sea surface temperature (SST) anomalies during developing and mature phases, including that of different types of El Nino. During ENSO, the space–time evolution of anomalous SST, 850-hPa wind, and rainfall along the equatorial Pacific, as well as the mechanisms involved in the teleconnection to the tropical Indian Ocean, are also well represented. During ENSO phase transition and in the summer, the skill of...

Precursors of the El Niño/La Niña onset and their interrelationship

[1]xa0In this study, the equatorial sea level, western Pacific zonal wind, and high-frequency (2∼90 day) wind variability are investigated as precursors of the onset of El Nino/La Nina. To a large extent, it is shown that each variable is a good indicator of El Nino–Southern Oscillation (ENSO) onset by exhibiting significant correlation to the ENSO index with 9 month lag. However, it is demonstrated here that the three precursors are remarkably correlated with each other. Based on the statistical analysis, we suggest here that the three precursors are governed by strong coupling processes: (1) scale interaction within the atmosphere between the low-frequency (LF, i.e., interannual) and high-frequency (HF, i.e., intraseasonal) winds and (2) positive air-sea coupled feedback among the western Pacific westerly, SST, and zonal mean thermocline. Our observational analysis and statistical prediction experiments suggest that the three precursors are not independent and they reflect the different aspects of the same coupling process during the ENSO onset phase.

Anomalous convective activity over sub-tropical east Pacific during 2015 and associated boreal summer monsoon teleconnections

The eastern Pacific Ocean received a record highest number of sub-tropical convective activities during boreal summer (June–September) of 2015, since last four decades. The associated rainfall distribution was also atypical with anomalously enhanced rainfall extending from equator to sub-tropical central-eastern Pacific. The present analysis reveals a pronounced meridional sea surface temperature (SST) gradient across central-eastern Pacific, with the mean SST exceeding 28xa0°C over sub-tropical north Pacific, setting up favorable conditions for these enhanced convective activities. It is found that these anomalous features promoted northward spanning of westerly anomalies and drastically modified the east–west circulation over sub-tropical north Pacific. This seems to induce large-scale subsidence over the off-equatorial monsoon regions of south and south-east Asia, thus constituting an east–west asymmetry over sub-tropical Indo-Pacific region. Based on our observational study, it can be concluded that the sub-tropical convective activities over east Pacific may play a pivotal role in mediating the Pacific-monsoon teleconnection through the unexplored meridional SST gradient across Pacific.

Sub-seasonal behaviour of Asian summer monsoon under a changing climate: assessments using CMIP5 models

AbstractnNumerous global warming studies show the anticipated increase in mean precipitation with the rising levels of carbon dioxide concentration. However, apart from the changes in mean precipitation, the finer details of daily precipitation distribution, such as its intensity and frequency (so called daily rainfall extremes), need to be accounted for while determining the impacts of climate changes in future precipitation regimes. Here we examine the climate model projections from a large set of Coupled Model Inter-comparison Project 5 models, to assess these future aspects of rainfall distribution over Asian summer monsoon (ASM) region. Our assessment unravels a north–south rainfall dipole pattern, with increased rainfall over Indian subcontinent extending into the western Pacific region (north ASM region, NASM) and decreased rainfall over equatorial oceanic convergence zone over eastern Indian Ocean region (south ASM region, SASM). This robust future pattern is well conspicuous at both seasonal and sub-seasonal time scales. Subsequent analysis, using daily rainfall events defined using percentile thresholds, demonstrates that mean rainfall changes over NASM region are mainly associated with more intense and more frequent extreme rainfall events (i.e. above 95th percentile). The inference is that there are significant future changes in rainfall probability distributions and not only a uniform shift in the mean rainfall over the NASM region. Rainfall suppression over SASM seems to be associated with changes involving multiple rainfall events and shows a larger model spread, thus making its interpretation more complex compared to NASM. Moisture budget diagnostics generally show that the low-level moisture convergence, due to stronger increase of water vapour in the atmosphere, acts positively to future rainfall changes, especially for heaviest rainfall events. However, it seems that the dynamic component of moisture convergence, associated with vertical motion, shows a strong spatial and rainfall category dependency, sometimes offsetting the effect of the water vapour increase. Additionally, we found that the moisture convergence is mainly dominated by the climatological vertical motion acting on the humidity changes and the interplay between all these processes proves to play a pivotal role for regulating the intensities of various rainfall events in the two domains.

Indian Ocean and Indian summer monsoon: relationships without ENSO in ocean–atmosphere coupled simulations

The relationship between the Indian Ocean and the Indian summer monsoon (ISM) and their respective influence over the Indo-Western North Pacific (WNP) region are examined in the absence of El Niño Southern Oscillation (ENSO) in two partially decoupled global experiments. ENSO is removed by nudging the tropical Pacific simulated sea surface temperature (SST) toward SST climatology from either observations or a fully coupled control run. The control reasonably captures the observed relationships between ENSO, ISM and the Indian Ocean Dipole (IOD). Despite weaker amplitude, IODs do exist in the absence of ENSO and are triggered by a boreal spring ocean–atmosphere coupled mode over the South-East Indian Ocean similar to that found in the presence of ENSO. These pure IODs significantly affect the tropical Indian Ocean throughout boreal summer, inducing a significant modulation of both the local Walker and Hadley cells. This meridional circulation is masked in the presence of ENSO. However, these pure IODs do not significantly influence the Indian subcontinent rainfall despite overestimated SST variability in the eastern equatorial Indian Ocean compared to observations. On the other hand, they promote a late summer cross-equatorial quadrupole rainfall pattern linking the tropical Indian Ocean with the WNP, inducing important zonal shifts of the Walker circulation despite the absence of ENSO. Surprisingly, the interannual ISM rainfall variability is barely modified and the Indian Ocean does not force the monsoon circulation when ENSO is removed. On the contrary, the monsoon circulation significantly forces the Arabian Sea and Bay of Bengal SSTs, while its connection with the western tropical Indian Ocean is clearly driven by ENSO in our numerical framework. Convection and diabatic heating associated with above-normal ISM induce a strong response over the WNP, even in the absence of ENSO, favoring moisture convergence over India.

Towards a realistic simulation of boreal summer tropical rainfall climatology in state-of-the-art coupled models: role of the background snow-free land albedo

State-of-the-art global coupled models used in seasonal prediction systems and climate projections still have important deficiencies in representing the boreal summer tropical rainfall climatology. These errors include prominently a severe dry bias over all the Northern Hemisphere monsoon regions, excessive rainfall over the ocean and an unrealistic double inter-tropical convergence zone (ITCZ) structure in the tropical Pacific. While these systematic errors can be partly reduced by increasing the horizontal atmospheric resolution of the models, they also illustrate our incomplete understanding of the key mechanisms controlling the position of the ITCZ during boreal summer. Using a large collection of coupled models and dedicated coupled experiments, we show that these tropical rainfall errors are partly associated with insufficient surface thermal forcing and incorrect representation of the surface albedo over the Northern Hemisphere continents. Improving the parameterization of the land albedo in two global coupled models leads to a large reduction of these systematic errors and further demonstrates that the Northern Hemisphere subtropical deserts play a seminal role in these improvements through a heat low mechanism.

Modulations of the Indian summer monsoon by the hot subtropical deserts: insights from coupled sensitivity experiments

This study revisits the role of subtropical deserts in the Indian Summer Monsoon (ISM) system by perturbing surface albedo over the subtropical deserts, to the west of the ISM domain in different ways, using a state-of-the-art coupled model. The analysis of up-to-date satellite datasets, atmospheric re-analyses and our control coupled simulation suggests that the model broadly reproduces the radiation budgets close to re-analyses and observed datasets. However, there are large uncertainties in the top-of-atmosphere radiation budget over the Northern Hemishere (NH) subtropical desert region during boreal summer; while the model has a rather neutral radiation budget during boreal summer over the Sahara Desert, the European Centre for Medium Range Weather Forecasts Interim reanalysis show in contrast a radiative excess throughout the NH desert region and the up-to-date satellite dataset has a clear negative radiation budget over north-eastern Sahara region and over Arabian Peninsula. Taking into account these incertitudes, our key finding is that by darkening the deserts and arid regions to the west of ISM through a negative albedo perturbation in our coupled model, the length and intensity of the rainy season over the Indian region are both significantly increased with two well-defined rainfall anomaly maxima in May–June and September–October. The ISM onset is advanced by 1xa0month and is characterized by a rapid northward propagation of the rainfall band over the Indian domain. Reversing the sign of our artificial albedo perturbation over the deserts in the model gives an opposite response, highlighting the robust role of the subtropical deserts in the ISM system, but the amplitude of the ISM response is also significantly larger, demonstrating nonlinearity in the monsoon-desert relationship. Additional albedo perturbation experiments further demonstrate that the whole hot subtropical deserts extending across Afro-Asian continents, and including the Sahara, plays a key-role in the ISM response. Finally, the modulations of the meridional tropospheric temperature gradient along with stronger equatorial asymmetry of mean easterly shear and moisture distribution over the Indian domain are key-factors for explaining the ISM response and its nonlinearity to the albedo perturbations over the NH subtropical deserts. Further insights from moisture budget show that the nonlinearity in advection moisture tendencies manifests in nonlinearity of the ISM response.