ALBERT

Description: This chapter assesses scientific evidence for tipping points across circulations in the ocean and atmosphere. The warming of oceans, modified wind patterns and increasing freshwater influx from melting ice hold the potential to disrupt established circulation patterns. We find evidence for tipping points in the Atlantic Meridional Overturning Circulation (AMOC), the North Atlantic Subpolar Gyre (SPG), and the Antarctic Overturning Circulation, which may collapse under warmer and ‘fresher’ (i.e. less salty) conditions. A slowdown or collapse of these oceanic circulations would have far-reaching consequences for the rest of the climate system, such as shifts in the monsoons. There is evidence that this has happened in the past, having led to vastly different states of the Sahara following abrupt changes in the West African monsoon, which we also classify as a tipping system. Evidence about tipping of the monsoons over South America and Asia is limited, however large-scale deforestation or air pollution are considered as potential sources of destabilisation. Although theoretically possible, there is little indication for tipping points in tropical clouds or mid-latitude atmospheric circulations. Similarly, tipping towards a more extreme or persistent El Niño Southern Oscillation (ENSO) state is not sufficiently supported by models and observations. While the thresholds for many of these systems are uncertain, tipping could be devastating for many millions of people. Stabilising climate (along with minimising other pressures, like aerosol pollution and ecosystem degradation) is critical for reducing the likelihood of reaching tipping points in the ocean-atmosphere system.

Description: A positive matrix factorization model (US EPA PMF version 5.0) was applied for the source apportionment of the dataset of 37 non-methane volatile organic compounds (NMVOCs) measured from 19 December 2012 to 30 January 2013 during the SusKat-ABC international air pollution measurement campaign using a proton-transfer-reaction time-of-flight mass spectrometer in the Kathmandu Valley. In all, eight source categories were identified with the PMF model using the new constrained model operation mode. Unresolved industrial emissions and traffic source factors were the major contributors to the total measured NMVOC mass loading (17.9 and 16.8 %, respectively) followed by mixed industrial emissions (14.0 %), while the remainder of the source was split approximately evenly between residential biofuel use and waste disposal (10.9 %), solvent evaporation (10.8 %), biomass co-fired brick kilns (10.4 %), biogenic emissions (10.0 %) and mixed daytime factor (9.2 %). Conditional probability function (CPF) analyses were performed to identify the physical locations associated with different sources. Source contributions to individual NMVOCs showed that biomass co-fired brick kilns significantly contribute to the elevated concentrations of several health relevant NMVOCs such as benzene. Despite the highly polluted conditions, biogenic emissions had the largest contribution (24.2 %) to the total daytime ozone production potential, even in winter, followed by solvent evaporation (20.2 %), traffic (15.0 %) and unresolved industrial emissions (14.3 %). Secondary organic aerosol (SOA) production had approximately equal contributions from biomass co-fired brick kilns (28.9 %) and traffic (28.2 %). Comparison of PMF results based on the in situ data versus REAS v2.1 and EDGAR v4.2 emission inventories showed that both the inventories underestimate the contribution of traffic and do not take the contribution of brick kilns into account. In addition, the REAS inventory overestimates the contribution of residential biofuel use and underestimates the contribution of solvent use and industrial sources in the Kathmandu Valley. The quantitative source apportionment of major NMVOC sources in the Kathmandu Valley based on this study will aid in improving hitherto largely un-validated bottom-up NMVOC emission inventories, enabling more focused mitigation measures and improved parameterizations in chemical transport models.

Description: The North Atlantic Oscillation (NAO) plays a crucial role in the development of winter conditions and associated extreme weather in the North Atlantic basin. Although seasonal prediction skill for the NAO has seen recent improvement, the influence of the upper stratosphere, a region that has received relatively little attention, is yet to be clarified.Lu et al. (2021) proposed a flow regime index using early winter upper stratospheric information. It was found that the index characterises the seasonal development of the northern stratospheric polar vortex, the signal of which then projects onto the NAO in middle to late winter. In this study, we examine this stratospheric regime behaviour and subsequent response in the troposphere using ERA5 reanalysis data and will assess the associated bias of the CMIP6 model simulations. Zonal mean winds in the subtropical upper stratosphere are used as a proxy for the flow regime index. Our results based on ERA5 confirm a significant relationship between the strength of this proxy index and the late winter mean sea level pressure and near surface temperature. Stronger winds in the early winter subtropical upper stratosphere are associated with a stronger positive NAO phase in late winter. To test the hypothesis that this connection exists due to planetary wave-breaking feedback processes, we examine the behaviour of the stratospheric surf zone in which the strongest quasi-horizontal mixing occurs. Tropospheric precursors preceding the regime development are also investigated. Implications for improved process understanding of stratosphere-troposphere coupling and its role in seasonal weather forecasting are discussed.

Description: The anthroposphere over the Indo-Gangetic Plain has witnessed multi-fold growth in terms of agricultural production accompanied by rapid urbanization over the past six decades. This has ensured food security for more than billions of people, but the present scale of emissions associated with activities like post-harvest agricultural waste burning as well as open burning of biofuel for domestic heating, cooking and waste disposal frequently triggers extreme air pollution events every year, in particular during the post-monsoon and winter season. In this presentation I shall review research conducted over the past decade aimed at unravelling some of the complex interplays between emissions and meteorology leading to the build up of the severe air pollution episodes using tools developed by my laboratory. These include molecular chemical fingerprinting of air pollution sources for quantitative source apportionment, compilation of regionally representative emission inventories and the application of ambient mass spectrometry for chemical speciation of gases and fine mode aerosol. I show that it is not just the general increase in air pollutant concentrations but also increase in specific ambient air toxics that aggravates the health risks for the exposed population. Further, open burning of solid fuels for heating purposes results in a strong temperature induced emission feedback that aggravates the wintertime fog. Based on the spatio-temporal occurrence of the emissions, I shall also share evidence based mitigation suggestions for air quality improvement, highlighting intervention strategies for their potential impact with specific focus on the sectoral coupling between various sectors for maximizing air quality gains.

Description: It is important to understanding the mechanism behind multidecadal changes in North Atlantic ocean heat storage as these directly impact the climate of the surrounding continents. We construct a multidecadal upper ocean heat budget for the North Atlantic for the period 1950 to 2020 based on multiple observational datasets and a state of the art forced global ocean model. On multidecadal timescales ocean heat transport convergence is the dominant term in all regions of the North Atlantic. In the subpolar region (north of 45N) the heat transport convergence is largely explained by anomalous geostrophic currents acting on the mean temperature gradient. The timescale and spatial distribution of the anomalous geostrophic currents are consistent with basin scale ‘thermal’ Rossby waves propagating westwards/northwestwards in the subpolar gyre. Using a forced ocean model we link the ocean heat transport convergence with variations in the Atlantic Meridional Overturning Circulation.