In search of greenhouse signals in the equatorial middle atmosphere

A series of Rocketsonde and Radiosonde data (balloon) were collected over the equatorial station Thumba (8°N, 76°E), India extended for about two solar cycle period (1971-1993). This is one of a few longest sets of rare records available from equatorial stations. These data were now analyzed using two independent and upto date new regression models, which are based on multiple function regression theory and also takes account of successive instrumental modifications and tidal effects. These experimental results are complemented with 2-D interactive model results simulated for the same duration and found to agree well. The model accounts for the actual measured growth rate of several greenhouse gases including CO2, CH4, N2O and CFCs for this period. Results of both balloon and rocket data analysis indicate a cooling of the order of 1°K /decade in the lower stratosphere which agrees well with low latitude data by other workers. A negative trend of 2 to 3°K/decade in the lower mesosphere and a rise in cooling up to 5.6°K/decade in the upper mesosphere has been observed. A loss of trend near the stratopause is noticed in annual trend analysis. However, on seasonal scales, trend structure gets modified and shows slightly stronger cooling in winter as compared to summer in the mesosphere

Association of the pre-monsoon thermal field over north India and the western Tibetan Plateau with summer monsoon rainfall over India

In this paper, interannual variability of tropospheric air temperatures over the Asian summer monsoon region during the pre-monsoon months is examined in relation to Indian summer monsoon rainfall (ISMR; June to September total rainfall). For this purpose, monthly grid-point temperatures in the entire troposphere over the Asian summer monsoon region and ISMR data for the period 1949–2012 have been used. Spatial correlation patterns are investigated between the temperature field in the lower tropospheric levels during May over the Asian summer monsoon region and ISMR. The results indicate a strong and significant northwest–southeast dipole structure in the spatial correlations over the Indian region, with highly significant positive (negative) correlations over the regions of north India and the western Tibetan Plateau region – region R1 (north Bay of Bengal: region R2). The observed dipole is seen significantly up to a level of 850 hPa and eventually disappears at 700 hPa. Thermal indices evaluated at 850 hPa level, based on average air temperatures over the north India and western Tibetan Plateau region (TI1) and the north Bay of Bengal region (TI2) during May, show a strong, significant relationship with the ISMR. The results are found to be consistent and robust, especially in the case of TI1 during the period of analysis. A physical mechanism for the relationship between these indices and ISMR is proposed. Finally the composite annual cycle of tropospheric air temperature over R1 during flood/drought years of ISMR is examined. The study brings out the importance of the TI1 in the prediction of flood/drought conditions over the Indian subcontinent

Atmospheric CO2 source and sink patterns over the Indian region

In this paper we examine CO2 emission hot spots and sink regions over India as identified from global model simulations during the period 2000–2009. CO2 emission hot spots overlap with locations of densely clustered thermal power plants, coal mines and other industrial and urban centres; CO2 sink regions coincide with the locations of dense forest. Fossil fuel CO2 emissions are compared with two bottom-up inventories: the Regional Emission inventories in ASia (REAS v1.11; 2000–2009) and the Emission Database for Global Atmospheric Research (EDGAR v4.2) (2000–2009). Estimated fossil fuel emissions over the hot spot region are  ∼  500–950 gC m−2 yr−1 as obtained from the global model simulation, EDGAR v4.2 and REAS v1.11 emission inventory. Simulated total fluxes show increasing trends, from 1.39 ± 1.01 % yr−1 (19.8 ± 1.9 TgC yr−1) to 6.7 ± 0.54 % yr−1 (97 ± 12 TgC yr−1) over the hot spot regions and decreasing trends of −0.95 ± 1.51 % yr−1 (−1 ± 2 TgC yr−1) to −5.7 ± 2.89 % yr−1 (−2.3 ± 2 TgC yr−1) over the sink regions. Model-simulated terrestrial ecosystem fluxes show decreasing trends (increasing CO2 uptake) over the sink regions. Decreasing trends in terrestrial ecosystem fluxes imply that forest cover is increasing, which is consistent with India State of Forest Report (2009). Fossil fuel emissions show statistically significant increasing trends in all the data sets considered in this study. Estimated trend in simulated total fluxes over the Indian region is  ∼  4.72 ± 2.25 % yr−1 (25.6 TgC yr−1) which is slightly higher than global growth rate  ∼  3.1 % yr−1 during 2000–2010

Influence of enhanced Asian NOx emissions on ozone in the upper troposphere and lower stratosphere in chemistry–climate model simulations

The Asian summer monsoon (ASM) anticyclone is the most pronounced circulation pattern in the upper troposphere and lower stratosphere (UTLS) during northern hemispheric summer. ASM convection plays an important role in efficient vertical transport from the surface to the upper-level anticyclone. In this paper we investigate the potential impact of enhanced anthropogenic nitrogen oxide (NOx) emissions on the distribution of ozone in the UTLS using the fully coupled aerosol–chemistry–climate model, ECHAM5-HAMMOZ. Ozone in the UTLS is influenced both by the convective uplift of ozone precursors and by the uplift of enhanced-NOx-induced tropospheric ozone anomalies. We performed anthropogenic NOx emission sensitivity experiments over India and China. In these simulations, covering the years 2000–2010, anthropogenic NOx emissions have been increased by 38 % over India and by 73 % over China with respect to the emission base year 2000. These emission increases are comparable to the observed linear trends of 3.8 % per year over India and 7.3 % per year over China during the period 2000 to 2010. Enhanced NOx emissions over India by 38 % and China by 73 % increase the ozone radiative forcing in the ASM anticyclone (15–40° N, 60–120° E) by 16.3 and 78.5 mW m−2 respectively. These elevated NOx emissions produce significant warming over the Tibetan Plateau and increase precipitation over India due to a strengthening of the monsoon Hadley circulation. However, increase in NOx emissions over India by 73 % (similar to the observed increase over China) results in large ozone production over the Indo-Gangetic Plain and Tibetan Plateau. The higher ozone concentrations, in turn, induce a reversed monsoon Hadley circulation and negative precipitation anomalies over India. The associated subsidence suppresses vertical transport of NOx and ozone into the ASM anticyclone

The mesospheric inversion layer and sprites

The vertical structure of temperature observed by SABER (Sounding of Atmosphere using Broadband Emission Radiometry) aboard TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) and sprites observations made during the Eurosprite 2003 to 2007 observational campaign were analyzed. Sprite observations were made at two locations in France, namely Puy de Dome in the French Massif Central and at the Pic du Midi in the French Pyrenees. It is observed that the vertical structure of temperature shows evidence for a Mesospheric Inversion Layer (MIL) on those days on which sprites were observed. A few events are also reported in which sprites were not recorded, although there is evidence of a MIL in the vertical structure of the temperature. It is proposed that breaking gravity waves produced by convective thunderstorms facilitate the production of (a) sprites by modulating the neutral air-density and (b) MILs via the deposition of energy. The same proposition has been used to explain observations of lightings as well as both MILs and lightning arising out of deep convections.Comment: 34 pages, 5figures. Accepted in Journal of Geophysical Research, US

The impact of COVID-19 lockdown measures on the Indian summer monsoon

Aerosol concentrations over Asia play a key role in modulating the Indian summer monsoon (ISM) rainfall. Lockdown measures imposed to prevent the spread of the COVID-19 pandemic led to substantial reductions in observed Asian aerosol loadings. Here, we use bottom-up estimates of anthropogenic emissions based on national mobility data from Google and Apple, along with simulations from the ECHAM6-HAMMOZ state-of-the-art aerosol-chemistry-climate model to investigate the impact of the reduced aerosol and gases pollution loadings on the ISM. We show that the decrease in anthropogenic emissions led to a 4 W m−2 increase in surface solar radiation over parts of South Asia, which resulted in a strengthening of the ISM. Simultaneously, while natural emission parameterizations are kept the same in all our simulations, the anthropogenic emission reduction led to changes in the atmospheric circulation, causing accumulation of dust over the Tibetan plateau (TP) during the pre-monsoon and monsoon seasons. This accumulated dust has intensified the warm core over the TP that reinforced the intensification of the Hadley circulation. The associated cross-equatorial moisture influx over the Indian landmass led to an enhanced amount of rainfall by 4% (0.2 mm d−1) over the Indian landmass and 5%–15% (0.8–3 mm d−1) over central India. These estimates may vary under the influence of large-scale coupled atmosphere–ocean oscillations (e.g. El Nino Southern Oscillation, Indian Ocean Dipole). Our study indicates that the reduced anthropogenic emissions caused by the unprecedented COVID-19 restrictions had a favourable effect on the hydrological cycle over South Asia, which has been facing water scarcity during the past decades. This emphasizes the need for stringent measures to limit future anthropogenic emissions in South Asia for protecting one of the world's most densely populated regions