S. D. Patil

Extremes in total ozone content over northern India

Using daily station total ozone data from TOMS on Nimbus‐7 (1979–1993) and Earth Probe (1997–2005) satellites of National Aeronautics and Space Administration (NASA)/Goddard Space Flight Center (GSFC) during the period 1979–2005, the characteristic features of extremes in the total ozone content and the frequency of the low/high ozone days have been carried out over the northern parts of India in the winter season. Stations located in the north of 20° N latitude regions of India have been chosen for the study. To determine whether the day is a low, high or normal ozone day, the statistical percentile thresholds are computed based on the daily data during winter months (January and February). It has been observed that the trends in the frequency of low ozone days are increasing and for the high ozone days are decreasing during 1979–1993. Similarly the trends in the highest total ozone reaching during January and February are decreasing. The recent period (1997–2005) shows opposite trends that are not statistically significant or stable during the period. Even the mean total ozone during January and February for the period 1979–1993 show the decreasing trends. Overall the trends in the total ozone extremes and the frequency of low/high ozone days are found to be decreasing over the northern parts of India during the winter season.

Total ozone in the most humid monsoon region

SummarySimple analysis of about fifteen years of monthly total ozone data in the light of associated meteorological processes has been carried out in the tropical and subtropical regions of northern hemisphere. A well marked trough in total ozone has been observed in July and August over the most humid part of the region, which comes under the influence of monsoon circulation. It appears that the occurrence of the trough may be due to the lifting of air mass by the intense convective activity over areas of the low level convergence, which is fed normally from the equatorial belt. Also, the possible occurrence of stratospheric fountain due to the existence of the coldest air around the tropopause over the entire Southeast Asia and adjacent west Pacific ocean, may be playing some role to produce this feature.

On the variation of the tropospheric ozone over Indian region in relation to the meteorological parameters

Using monthly mean satellite measurements of TOMS/SBUV tropospheric ozone residual (TOR) data and meteorological parameters (tropopause height (TPH), 200 hPa geopotential height (GPH) and outgoing longwave radiation (OLR)) during 1979–2001, seasonal variability of TOR data and their association with meteorological parameters are outlined over the Indian region. Prominent higher values of TOR (44–48 DU, which is higher than the globally averaged 31.5 DU) are observed over the northern parts of the country during the summer monsoon season (June–September). Similar to the TOR variation, meteorological parameters (tropopause height, 200 hPa geopotential height and outgoing longwave radiation) also show higher values during the summer monsoon season, suggesting an in phase relationship and strong association between them because of deep convection present during summer monsoon time. The monthly trends in TOR values are found to be positive over the region. TOR has significant positive correlations (5% level) with GPH, and negative correlations with OLR and TPH for the month of September. The oxidation chains initiated by CH4 and CO show the enhanced photochemical production of ozone that would certainly become hazardous to the ecological system. Interestingly, greenhouse gases (GHG) emissions were found to have continuously increased over the Indian region during the period 1990–2000, indicating more anthropogenic production of ozone precursor gases causing higher level of tropospheric ozone during this period.

Variation of total column ozone along the monsoon trough region over north India

This study examined the total column ozone (TCO) variations over New Delhi (28.65° N, 77.217° E) and Varanasi (25.32° N, 83.03° E), which lie along the monsoon trough region, and over the tropical station Kodaikanal (10.23° N, 77.46° E), which lies outside the monsoon trough. Monthly, seasonal and annual TCO variations were determined using data from ground-based Dobson spectrophotometers during 2000–2008, Brewer spectrophotometers during 2000–2005 and the satellite-based Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) during 2002–2008. We found that Dobson, Brewer and SCIAMACHY TCO variations showed negative trends, indicating a decreasing tendency during the period studied at all three stations. Over Varanasi, the trend decreased further by about 3 DU year−1. Quasi-Biennial Oscillation (QBO) influences were seen in the time series of TCO over New Delhi and Varanasi, and weaker QBO signals over Kodaikanal. Comparisons were made between ground-based Dobson and Brewer spectrophotometer and SCIAMACHY satellite monthly mean TCO values. The differences between SCIAMACHY and Dobson TCO were 0.4–4.2% for New Delhi and 2.3–6.2% for Varanasi. The differences between SCIAMACHY and Brewer TCO values were 2.0–6.4% for Kodaikanal. In the peak monsoon months (July and August), decreases in TCO values over New Delhi and Varanasi (the monsoon trough region) may be due to the deep convection present during the monsoon season. During the monsoon season, several intense cyclonic systems appear over the monsoon trough region and may cause lowering of the TCO. Kodaikanal shows opposite features, with high values being observed during the peak monsoon months. TCO values over New Delhi were found to be higher than those over Varanasi and Kodaikanal, and TCO values over Varanasi were higher than over Kodaikanal. It was concluded that TCO values increase with increasing latitude.

Association between ENSO and extremes in total ozone content over northern India

Using daily station total ozone column (TOC) data from the Total Ozone Mapping Spectrometer (TOMS) onboard the Nimbus-7 satellite, an association between the El Niño–Southern Oscillation (ENSO) and extremes in TOC content has been revealed during the period 1979–1993 over northern India in the winter season. From lag-simultaneous correlations of extremes in ozone with Niño 3.4 sea surface temperatures (SSTs), it is seen that, during this season, the highest TOC values show a strong positive relationship at the beginning of the preceding year with the occurrence of the highest values for all the stations. A weak relationship is observed up to the month of July and its sign is then reversed. The negative but weak relationship continues until the occurrence of the event, becoming positive again afterwards. On the contrary, the occurrence of the lowest values shows opposite features. The analysis indicates that the increase in SSTs during the first half of the preceding year is favourable for an increase in the highest values occurring over different stations while the increase in SSTs during the latter half of the preceding year is favourable for an increase in the lowest values of ozone. The lag-simultaneous correlations of the low/high ozone days and the mean TOC values occurring during the winter season also suggest a significant positive relationship for the frequency of the high ozone days at the beginning of the preceding year, becoming weaker as time progresses. Although both features show that the relationship is statistically significant for only a few months of the preceding year, it gives a broad indication of the association between ENSO and the extremes in the TOC amount in addition to local/geographical factors.

Global features of upper-tropospheric zonal wind and thermal fields during anomalous monsoon situations

Global analyses of mean monthly zonal wind component and temperature at 200, 150 and 100 mb levels have been made for the region between 60°N and 60°S, for the months May through September during two poor monsoon years (1972 and 1979) and a good monsoon year (1975). Prominent and consistent contrasting features of the zonal wind and thermal fields have been identified, with reference to the monsoon performance over India. It has been noticed that the areal spreading of easterlies over the tropics and extratropics is significantly more during a good monsoon year. Shifting of the axis of the tropical easterly jet stream to a higher level and generally stronger easterlies also characterize good monsoon activity. The upper troposphere has been found to be considerably cooler during poor monsoon years.

Seasonal behaviour of NCEP-NCAR longwave cloud radiative forcing and its relationship with all-India summer monsoon rainfall

Using National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) longwave cloud radiative forcing (LWCRF) reanalysis at the top of the atmosphere (TOA) for the period 1949–2006, the seasonal behaviour of the LWCRF and its relationship with the all-India summer monsoon rainfall (AISMR) during the winter (December–January–February, DJF), pre-monsoon (March–April–May, MAM) and summer monsoon (June– July–August–September, JJAS) seasons has been examined. The LWCRF over the Bay of Bengal region (15–20° N and 87.5–92.5° E) during the pre-monsoon season (MAM) is found to be significantly related to AISMR. The correlation coefficient (CC) between AISMR and LWCRF is 0.419, significant at the 1% level. The composite anomalies for excess minus deficient rainfall years of the LWCRF during MAM over the same region strongly support a significant relationship between LWCRF and AISMR. Thus, the LWCRF over the Bay of Bengal region in the pre-monsoon season appears to be a good indicator of the forthcoming monsoon rainfall.