A waveform skewness index for measuring time series nonlinearity and its applications to the ENSO–Indian monsoon relationship

Many geophysical time series possess nonlinear characteristics that reflect the underlying physics of the phenomena the time series describe. The nonlinear character of times series can change with time, so it is important to quantify time series nonlinearity without assuming stationarity. A common way of quantifying the time evolution of time series nonlinearity is to compute sliding skewness time series, but it is shown here that such an approach can be misleading when time series contain periodicities. To remedy this deficiency of skewness, a new waveform skewness index is proposed for quantifying local nonlinearities embedded in time series. A waveform skewness spectrum is proposed for determining the frequency components that are contributing to time series waveform skewness. The new methods are applied to the El Niño–Southern Oscillation (ENSO) and the Indian monsoon to test a recently proposed hypothesis that states that changes in the ENSO–Indian monsoon relationship are related to ENSO nonlinearity. We show that the ENSO–Indian rainfall relationship weakens during time periods of high ENSO waveform skewness. The results from two different analyses suggest that the breakdown of the ENSO–Indian monsoon relationship during time periods of high ENSO waveform skewness is related to the more frequent occurrence of strong central Pacific El Niño events, supporting arguments that changes in the ENSO–Indian rainfall relationship are not solely related to noise.

Dynamical-Empirical forecast for the Indian monsoon rainfall using the NCEP Coupled Modelling System – Application for real time monsoon forecast

The performance of the National Centre for Environmental Prediction’s (NCEP) operational coupled modeling system known as the Climate Forecast System (CFS) is evaluated for the prediction of all India summer monsoon rainfall (AISMR) during June to September (JJAS). The evaluation is based on the hindcast initialized during March, April and May with 15 ensemble members each for 25 years period from 1981 to 2005.The CFS’s hindcast climatology during JJAS of March (lag-3), April (lag-2) and May (lag-1) initial conditions show mostly an identical pattern of rainfall similar to that of observed climatology with both the rainfall maxima (over the west-coast of India and over the head Bay of Bengal region) well captured, with a signification correlation coefficient between the forecast and observed climatology over the Indian monsoon region (bounded by 50°E-110°E and 10°S-35°N) covering Indian land mass and adjoining oceanic region. Although the CFS forecast rainfall is overestimated over the Indian monsoon region, the land only rainfall amount is underestimated compared to observation. The skill of the prediction of monsoon rainfall over the Indian land mass is found to be relatively weak, although it is significant at 95% with a correlation coefficient (CC) of 0.44 with April ensembles.By using CFS predicted JJAS rainfall over the regions of significant CCs, a hybrid dynamical-empirical model is developed for the real time prediction of AISMR, whose skill is found to be much higher (CC significant above 99% level) than the raw CFS forecasts. The dynamical-empirical hybrid forecast applied on real time for 2009 and 2010 monsoons are found to be much closer to the observed AISMR. Thus, when the hybrid model is used there is a correction not only to the sign of the actual forecast as in the case of 2009 monsoon but also to its magnitude and hence can be used as a better tool for the real time prediction of AISMR.

Activity of southern Indiar1 Ocean convergence zone as seen in satellite cloud data during pre-monsoon months

The weekly mean cloud cover data for the pre-monsoon months of April and May over the Indian Ocean between20°S to 20°N latitudes and 40°E to 100" E longitudes have been studied for three good moon- soon years (1977, 1983, 1988) and three drought years (1972,1979, 1987). It is shown that while the characteristics of weekly mean cloud cover data during pre-monsoon months are similar for all the good monsoon years, they varied from one drought year to another. The study reveals some of the interesting features of southwest monsoon. An overall negative relationship between southern Indian Ocean convergence zone (SIOCZ) and monsoon activity is indicated. While at intraseasonal scale this may only be a simultaneous association, the pre-monsoon activity of SIOCZ may possibly have long-range predictive potential to some extent, for Indian monsoon rainfall.

The effect of Indian Ocean warming on the Indian Monsoon: An atmospheric model study

The results from an atmospheric modeling study using the Florida State University Global Spectral Model indicate that, in years such as 1997 when the Indian Ocean SSTs are large, the Indian monsoon exhibits a typical behaviour. During that year, an extended shift of the tropical convergence zone towards the north played a role in the regional Hadley cell anomalies. The local warm boundary conditions in the northwestern Indian Ocean aided the high rainfall anomaly in Western India during the model simulations. The upper level structure, exhibited in terms of the global velocity potential is slightly shifted east for 1997, but with the correct sign. This structure shows regions of convergence over Indonesia where severe drought had occurred. The performance of the model rainfall over the equatorial Indian Ocean was uncanny for most seasons studied. Overall, the model performed best over the oceanic regions.

Association between mid-latitude circulation and Indian monsoon rainfall

The statistical relationship between the summer monsoon rainfall over India and mid-latitude general calculation at the 500 hPa level was investigated for the period 1971-1989. The index used to characterise general circulation feature is the perturbation of the zonal flow (ratio of meridional to zonal index) for the latitudinal band 35°N - 70°N over different geographical area and the hemisphere. It was found that the perturbation of the zonal flow during preceding January over the geographical sector 1 (45°W - 90°E) shows significant relationship with the subsequent Indian summer monsoon rainfall in an inverse manner. Thus, the perturbation of the mid-latitude circulation during preceding January over the geographical sector seems to be a useful predictor of the subsequent Indian monsoon rainfall. Significant simultaneous inverse relationship also exists between perturbation of mid-latitude zonal flow during July to September over Sector 2 (90°E- 160°W) and summer monsoon rainfall over northwest India.

Relationship between tropospheric thickness anomalies and Indian summer monsoon rainfall

ABSTRACT .The tropospheric mean monthly thickness anomalies of northern Indian stations of selected layers for the months April to July for a 28 years (1968-95) period have been analysed. The thickness anomalies of April and May exhibit significant persistence through July. Also the thickness anomalies of different layers for the months May-July are found to have generally significant (5% to 0.1% level) linear correlations with the succeeding all India seasonal monsoon rainfall. Out of different layers and all the months analysed, the thickness anomalies of 850-300 and 850-100 hPa layers for May are found to have maximum correlations (significant at 0.1% level). From linear and multiple regression results, 850-300 hPa thickness anomaly is seen to be a useful predictor for long range prediction of Indian monsoon rainfall.

A linear model study of the mid-tropospheric ridge and its displacement

A linear model of the steady response of a stratified fluid to isolated heat sources is used to study the maintainence of the mean position of the mid-tropospheric ridge and its displacemerit. It is well known that the performance of the southwest Indian monsoon is intimately related to the latitudinal position of the April 500 hPa ridge along 75°E. Recent observational studies have demonstrated that the winter/spring snow cover over Eurasia are negatively related to the April 500 hPa ridge position. In this study we propose one possible physical mechanism of southward displacement of the mid-tropospheric ridge. The anomalous cooling associated with the increased snow cover in Eurasia may be considered as a heat sink north of the tropical heal sources. It is demonstrated that such a heat sink can result in significant southward displacement of the mid-tropospheric ridge.

Towards an improved representation of carbonaceous aerosols over the Indian monsoon region in a regional climate model RegCM4.6

Mitigation of carbonaceous aerosol emissions is expected to provide climate and health co-benefits. The accurate representation of carbonaceous aerosols in climate models is critical for reducing uncertainties in their climate feedbacks. In this regard, emission fluxes and aerosol life-cycle processes are the two primary sources of uncertainties. Here we demonstrate that incorporating a dynamic ageing scheme and emission estimates that are updated for the local sources improve the representation of carbonaceous aerosols over the Indian monsoon region in a regional climate model, RegCM, compared to its default configuration. The mean BC and OC surface concentrations in 2010 are estimated to be 4.25 and 10.35 μg m−3, respectively, over the Indo-Gangetic Plain (IGP), in the augmented model. The BC column burden over the polluted IGP is found to be 2.47 mg m−2, 69.95 % higher than in the default model configuration and much closer to available observations. The anthropogenic AOD increases by more than 19 % over the IGP due to the model enhancement, also leading to a better agreement with observed AOD. The top-of-the-atmosphere, surface, and atmospheric anthropogenic aerosol shortwave radiative forcing are estimated at −0.3, −9.3, and 9.0 W m−2, respectively, over the IGP and −0.89, −5.33, and 4.44 W m−2, respectively, over Peninsular India. Our results suggest that both the accurate estimates of emission fluxes and a better representation of aerosol processes are required to improve the aerosol life cycle representation in the climate model.