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Sulphur dioxide (SO2) is a hazardous air pollutant, which is mostly emitted from burning of fossil fuels, and has an adverse impact on the human health and ecosystem functioning. The COVID-19 natural anthropause (lockdown) provides a great opportunity to understand the changes in SO2 pollution across the globe, as there was a temporary standstill for most human activities. Therefore, we analyse the changes in global SO₂ pollution during lockdown compared to pre-lockdown and identify its hotspots driven by human activities using satellite measurements, reanalysis data and emission inventory. We observe a decline in SO₂ pollution of about 2.21 % in its global average, −21.05 % in Indo-Gangatic Plain, −16 % in East China, −7.67 % in East United States of America, −3.99 % in Western Europe and −3.85 % in Middle East owing to the halt in human activities such as industrial and transport operations, as found from the emissions inventory. There are point and aerial hotspots of SO₂ pollution across the globe (e.g. cities or industrial units), which also show a decrease (20–30 %) in SO₂ pollution during the anthropause. Fossil fuel burning in thermal power plants is a major source of SO2 pollution, and it has declined notably (1–12 %) during the lockdown in the major coal consuming countries such as the United States, China, Japan, Canada, Brazil, Australia, France, Germany, Spain, Italy and the United Kingdom. Therefore, lockdown provides a clear understanding of global human-driven hotspots of SO₂ pollution and their changes, which would help us to make better and effective air pollution mitigation strategies.

We present a detailed long-term (1997–2019) analysis of observed cyclone-induced surface cooling (or cold wake) in the Arabian Sea. Here, the analysis is performed for 33 cyclones that drove significant cooling at the sea surface in three different seasons: the pre-monsoon, monsoon and post-monsoon. Our study shows that cyclones can cool the sea surface up to 4.76° C after their passage, depending on the intensity, duration and other factors that contribute to cooling. The monsoon and pre-monsoon cyclones show stronger cooling, but the post-monsoon cyclones exhibit a longer duration (8–10 days) of cooling and slower recovery time (15 days). In general, the pre- and monsoon cyclones exhibit a strong positive correlation with the Latent Heat Flux, whereas the post-monsoon cyclones show a higher correlation with Ekman Pumping Velocity, wind stress and intensity of cyclones. The cold wake composite analysis for the El-Niño, La-Niña and Normal years shows that cyclone-induced cooling is similar in El-Niño and La-Niña years, and the cooling is more dominant during the negative Indian Ocean Dipole (IOD) than that in the positive IOD years. Co-occurrence of positive IOD and La Niña events has led to more intense cyclones in recent decades. The power dissipation index, accumulated cyclone energy and oceanic heat content also show an increasing trend in AS and favour rapid intensification of cyclones. Since the drop in SST normally impedes cyclones from intensification, our study is important and the findings of this study will aid in tropical cyclone predictions. In response to rapid warming of Indian Ocean in recent decades, extreme events such as cyclones are expected to increase in the context of climate change. [ABSTRACT FROM AUTHOR]

Nowcasting and short‐range predictions of severe weather events require frequent observations of atmospheric thermodynamic state. To meet these requirements, India has launched a series of advanced weather satellites in geostationary platform, with Indian National Satellite System (INSAT)‐3D and INSAT‐3DR as first two satellites placed in this orbit. Both satellites are designed for meteorological observations, particularly the vertical profiles of temperature and humidity at hourly intervals. As these satellites are relatively new, there is a need to validate and make accuracy assessment of their retrievals prior to scientific analyses. These sounders have a spatial resolution of 10 km, and measurements are available from 2014 to date. This study makes use of collocated measurements of INSAT‐3D and INSAT‐3DR with those from 18 radiosondes, Atmospheric Infrared Sounder (AIRS), and reanalysis data within the troposphere (1,000–100 hPa) to validate INSAT data. We analyzed the data for India land regions, and found an average bias of about 2–3 K (1–1.5 K) in most regions, when INSAT‐3D (3DR) is compared to AIRS measurements. Compared to the reanalysis data, INSAT‐3D (3DR) shows a bias within 2.5–5 K (2–3 K) in lower troposphere, 1.5–2.5 K (1–2 K) in mid‐troposphere and 1.5–2.5 (<2 K) in upper troposphere. Similar biases were also estimated with radiosonde measurements. In general, the validation shows that temperature profiles are generally accurate within 1–1.5 K, and INSAT‐3DR retrievals are better than that of INSAT‐3D. Nevertheless, a better cloud filter together with an advanced radiative transfer model can further improve the algorithm used for retrieval. Plain Language Summary: Monitoring temperature at different atmospheric levels is crucial for detecting changes in weather patterns, which can have detrimental effects on crops, especially during crucial growth stages, resulting in reduced yields. Additionally, monitoring temperature helps in predicting extreme weather events such as storms, heatwaves and droughts, which demand additional energy and resources. The Indian Space Research Organization has developed two advanced weather monitoring satellites, Indian National Satellite System (INSAT)‐3D and INSAT‐3DR, which provide high temporal resolution temperature profiles owing to its observation from a geostationary orbit. However, the satellite data are collected remotely, and accuracy of the data can be affected by various factors including atmospheric conditions and the sensor characteristics. Therefore, a thorough validation is necessary before the data to be used for scientific studies. Our study validates the temperature measurements from INSAT‐3D and INSAT‐3DR within the troposphere (1,000–100 hPa) using collocated measurements from ground‐based, satellite, and reanalysis data. The validation shows that the temperature profiles are accurate within 1–1.5 K and further improvements can be made by using a better cloud‐filtering algorithm together with an advanced radiative transfer model. [ABSTRACT FROM AUTHOR]

Anthropogenic emissions have produced significant amount of carbon dioxide (CO2) in the atmosphere since the beginning of the industrial revolution. High levels of atmospheric CO2 increases global temperature as CO2 absorbs outgoing longwave radiation and re-emits. Though a well-mixed greenhouse gas, CO2 concentration is not uniform in the atmosphere across different altitudes and latitudes. Here, we uncover a region of high CO2 concentration (i.e. CO2 pool) in the middle troposphere (500–300 hPa) over the Indo-Pacific Warm Pool (IPWP, 40° E–140° W, 25° S–25° N), in which the CO2 concentration is higher than that of other regions in the same latitude band (20° N–20° S), by using CO2 satellite measurements for the period 2002–2017. This CO2 pool extends from the western Pacific to the eastern Indian Ocean. Much of the CO2 pool is over the western Pacific Ocean (74.87%), and the remaining lies over the eastern Indian Ocean (25.13%). The rising branch of Walker circulation acts as a "CO2 Chimney" that constantly transports CO2 released from the natural, human-induced and ocean outgassing processes to the middle and upper troposphere. The CO2 pool evolves throughout the year with an average annual trend of about 2.17 ppm yr−1, as estimated for the period 2003–2016. Our analysis further reveals that La Niña (El Niño) events strengthen (weaken) the CO2 pool in the mid-troposphere. The radiative forcing for the CO2 pool suggests more warming in the region and is a grave concern for global warming and climate change. [ABSTRACT FROM AUTHOR]

Significant changes in the photosynthesis of terrestrial vegetation are observed in recent decades due to climate variability, land use and land cover changes, frequent disasters and anthropogenic forcing, but this situation in the Indian context remains largely unexplored. Here, therefore, we estimate the long–term changes in photosynthesis and its key climate drivers in India. The gross photosynthetic activity peaks (FPAR mean = 0.57) in post–monsoon (September, October and November) due to high soil moisture (87.26 kg/m² ), good precipitation (5.2 mm/day) and optimum temperature (21.46 C) conditions, whereas it is smallest (FPAR mean = 0.4) in summer (March, April and May) due to very high temperature (27.37 C) and lower soil moisture (55.45 kg/m2 ). The trend in FPAR over 2000–2019 reveals enhanced photosynthetic activity in India. In the northwest, both soil moisture and temperature have a positive influence on greening (i.e. moisture induced greening). Conversely, the unfavourable soil moisture and temperature conditions dominate in the peninsular region to trigger browning there (i.e. warming induced moisture stress). Our analyses suggest that the photosynthetic trend is primarily controlled by trend in the soil moisture (44.31%) and air temperature (32.54%). The warming induced moisture stress can be a serious threat in the future to drive browning in more areas. Extreme events such as droughts, landslides and fires are expected to increase in the hilly regions such as the northeast due to browning. Furthermore, as food production and forest resources influence the economy of a country like India, management and conservation of greenery are inevitable for a sustainable ecosystem. [ABSTRACT FROM AUTHOR]

Byline: S.J. Solomon (Institute of Environmental Physics, University of Bremen, Germany, juliet@iup.physik.uni-bremen.de); G.W. Schade (Institute of Environmental Physics, University of Bremen, Germany, Department of Atmospheric Sciences, Texas A&M University, College [...]

The discovery of causal structures behind a phenomenon under investigation has been at the heart of scientific inquiry since the beginning. Randomized control trials, the gold standard for causal analysis, may not always be feasible, such as in the domain of climate sciences. In the absence of interventional data, we are forced to depend only on observational data. This study demonstrates the application of one such causal discovery algorithm using a neural network for identifying the drivers of surface ozone variability in Antarctica. The analyses reveal the overarching influence of the stratosphere on the surface ozone variability in Antarctica, buttressed by the southern annular mode and tropospheric wave forcing in mid-latitudes. We find no significant and robust evidence for the influence of tropical teleconnection on the ground-level ozone in Antarctica. As the field of atmospheric science is now replete with a massive stock of observational data, both satellite and ground-based, this tool for automated causal structure discovery might prove to be invaluable for scientific investigation and flawless decision making. [ABSTRACT FROM AUTHOR]

The El Niño Southern Oscillation (ENSO) is an important coupled ocean–atmosphere phenomenon in the tropical Pacific and an important modulator of the spatio-temporal variability of Indian summer monsoon rainfall (ISMR). Here, we explore the impact of ENSO during onset (June), peak (July–August) and withdrawal (September) phases of ISMR for the period 1951–2015, by studying the changes in the ENSO-monsoon relationship from early decades (1951–1980) to recent decades (1986–2015). We observe noticeable changes in the ENSO-monsoon relationship from the early to recent decades during all the three phases. During El Niño events, rainfall over most of the Indian regions is significantly increased in recent decades during onset phase, but a significant decrease in rainfall is observed during peak and withdrawal phases. On the other hand, the rainfall during La Niña events is significantly decreased over the monsoon core zone of India during all the three phases. A significant increase (decrease) in sea surface temperature (SST) is observed over the central equatorial Pacific and Indian Ocean during El Niño (La Niña) events in recent decades. These changes differ from one phase to another and related to the observed rainfall patterns over the Indian region. Apart from these, the changes in the anomalies of low level circulation and Walker circulation over the Indo-Pacific domain are also linked to the changes in rainfall during the ENSO events, with most significant relationship during the onset phase. [ABSTRACT FROM AUTHOR]

The western boundary current in the Bay of Bengal, also known as East India Coastal Current, is northward (southward) and continuous during pre‐ (post‐) Indian Summer Monsoon (ISM) but discontinuous during ISM (June–September). This study investigates the features of this discontinuity and role of eddies, local winds, and southern open boundary lateral forcing using AVISO TOPEX Poseidon, ERS, and Jason1 combined sea surface height anomaly, Ocean Surface Current Analysis Real‐time (OSCAR) surface currents and high‐resolution Regional Ocean Modelling System simulations. The study shows that the discontinuity is governed by westward and southwestward propagating eddies between two opposite facing flows along the boundary, northward of 10°N and southward of 21°N. This northward flow of 10°N is wind driven with lateral influence during September. The southward flow of 21°N is influenced by the Summer Monsoon Current, but the local wind influence starts from July onward. During ISM, about 79% cyclonic and 77% anticyclonic eddies move westward, while 68% cyclonic and 69% anticyclonic eddies move southward. Three highly active eddy genesis areas, having more than 50% activities during ISM, are identified: northern and southwestern bay for cyclonic and off Visakhapatnam (17.68°N, 83.21°E) coast for anticyclonic eddies. The pre‐ISM (post‐ISM) is dominated by anticyclonic (cyclonic) eddies, which are found across the western Bay of Bengal and away from northward (southward) East India Coastal Current. The ISM is dominated by westward and southwestward moving both cyclonic and anticyclonic eddies, which are confined to the boundary, where discontinuity is generally observed. Plain Language Summary: The western boundary current in Bay of Bengal, which is also known as East India Coastal Current, flows northward during February–May and southward during October–December. During Indian Summer Monsoon period (June–September), the boundary current is not consistent and various cyclonic and anticyclonic eddies are generated in the western Bay of Bengal. This study investigates the features of this disorganized boundary currents and the role of eddies, local winds, and southern boundary in making the currents disorganized using measurements and high‐resolution numerical model simulations. The study is important to understand the changes in upper ocean dynamics associated with the inconsistent flow and to assess the contribution of the above said forcings to trigger such inconsistency. The study can further be used to understand the discontinuity induced ocean mixing, upwelling, sea level variability, and energy distribution along the western boundary of Bay of Bengal. Key Points: The discontinuity features are more pronounced along the western boundaryThe discontinuity is governed by westward and southwestward propagating eddies along the boundary between two opposite flows of 10°N and 21°NLocal winds and southern boundary lateral forcing have significant roles in the boundary current discontinuity [ABSTRACT FROM AUTHOR]