This special issue arises from work conducted by the AMAP Expert Group on short-lived climate forcers for an assessment that will be finalized in 2021. It will include only invited papers.

In the vast low-nutrient—low-chlorophyll (LNLC) ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be 1 order of magnitude greater over short timescales. Recently, significant experimental, field and modeling work allowed us to better link atmospheric deposition with nutrient availability, ocean productivity, carbon cycling and marine emissions. By improving our understanding on how atmospheric inputs are impacting biological activity and the carbon balance in oligotrophic environments, their representation in biogeochemical models at different timescales and space scales will be better assessed. In particular, processes may be affected by ongoing environmental changes in the ocean such as a decrease in pH and an increase in temperature.

This special issue aims at gathering contributions from both experimental and modeling approaches showing how natural or anthropogenic inputs from the atmosphere can impact biogeochemical and ecological processes in the present and future oligotrophic ocean. It will present the results obtained during the PEACETIME (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea) cruise conducted in 2017 in the Mediterranean Sea, but the special issue is open to any submissions provided that the subject is consistent with the objectives defined above.

The Peruvian upwelling system is one of the most productive ecosystems in the ocean. Continuous upwelling of nutrient-rich water generates high primary production, supporting an efficient food web and leading to high fishery yields. An extensive oxygen minimum zone underlying the productive surface waters gives rise to high losses of fixed nitrogen via denitrification and anaerobic ammonium oxidation. This results in nutrient ratios of upwelled waters deviating strongly from Redfield stoichiometry, with a high N deficit and excess P. How this unusual nutrient stoichiometry affects plankton community production and succession and how this feeds back on biogeochemical cycling are still not well understood. To address these questions, an in situ mesocosm experiment was conducted in the coastal upwelling system off Peru in the framework of the Collaborative Research Center 754 "Climate-Biogeochemistry Interactions in the Tropical Ocean". The field experiment took place from January to April 2017 during a coastal El Niño, a rare event of El Niño conditions confined to Peruvian coastal waters. Key objectives of this mesocosm study, which involved 57 researchers from 10 nations, were to unravel the interplay of pelagic ecology and biogeochemistry under the unique environmental conditions of the Peruvian upwelling system. This special issue in Biogeosciences is intended as a collective outlet for publications resulting from this study, covering the broad range of research topics from seawater chemistry to plankton ecology and pelagic biogeochemistry.

Remote sensing observations in the microwave frequencies have been intensively used to map and investigate water dynamics at the land surface. Microwave observations are also increasingly used to observe changes in vegetation structure and function and to study interactions between water and vegetation. For example, microwave observables such as brightness temperature, radar backscatter, polarization, SAR interferometry, GNSS reflectometry, and variables retrieved from them such as vegetation optical depth and soil moisture allow one to map ecosystem states. All these variables can be used to study processes, states, and temporal changes in vegetation–water interactions such as in vegetation phenology, canopy structure, biomass, photosynthesis, evapotranspiration, interception, dew formation, vegetation water content, water stress, surface and root-zone soil moisture, and irrigation.

The special issue aims to advance our understanding on existing and new techniques, methods, and applications of microwave remote sensing for vegetation–water interactions. We welcome studies that use active or passive microwave satellite, airborne, tower, and field observations to monitor processes, states, and temporal dynamics of vegetation–water interactions by

1. developing and evaluating microwave radiative transfer models for vegetation and water;
2. retrieving land surface variables through radiative transfer model inversion, machine learning, or hybrid approaches;
3. evaluating and constraining land surface, ecosystem, crop, forest, and hydrological models with microwave observations; and
4. assessing field experiments on the impacts of plant water dynamics on microwave observations.

This special issue is an outcome of sessions BG3.20 (Global Earth observation for improved understanding of terrestrial ecosystem dynamics) and HS6.4 (Remote sensing of interactions between vegetation and hydrology) held at the General Assembly 2020 of the European Geoscience Union and of the related precursor sessions in previous years. However, the special issue is open for all submissions within its scope.

The Weddell Sea and the ocean off Dronning Maud Land, constituting the Weddell Gyre, are representative of the high-latitude Southern Ocean due to its pronounced seasonality, circumpolar currents, deep-water formation and seasonal sea-ice cover. The Weddell Gyre connects water masses from the Antarctic Circumpolar Current with those shaped by the large ice shelves and sea-ice formation in the southern perimeter of the Weddell Gyre. Biological and biogeochemical processes are strongly influenced by these conditions and contribute to a unique Weddell Sea ecosystem.

A portion of the western Weddell Sea is already experiencing consequences of climate change, including the acceleration of mass loss from ice shelves, enhanced ocean warming and freshening, rising air temperatures, and changes in wind patterns. This likely has an emerging influence on the local biological and biogeochemical processes. The eastern part of the Weddell Gyre, off Dronning Maud Land, however, appears relatively stable but is expected to experience warming, sea-ice retreat and ice-shelf loss in the course of this century. The re-emergence of the Maud Rise Polynya, with a potentially significant impact on regional water masses and biogeochemistry, underscores the importance of observing and understanding this region. Establishing knowledge of the present biological and biogeochemical processes and coupling with physics prior to major changes in this region is essential.

This special issue emerged from an October 2020 workshop organized by the Southern Ocean Observing System (SOOS) Weddell Sea and Dronning Maud Land Regional Working Group that brought together numerous scientists across multiple disciplines. However, the issue is open to anyone and we welcome additional contributions.

Fire is an essential feature of terrestrial ecosystems and plays an important role in the Earth system. Fires are regulated by climate, vegetation characteristics, and human activity and also feedback to them in multiple ways. For example, fires can shape vegetation composition and structure; adjust land carbon, nutrient, water, and energy cycles; change atmospheric composition, chemistry, and physics; and affect air quality and human health. Elevated fire activity in 2019 across the arctic, Amazon, and other regions underscores the urgency of a quantitative understanding of fire as an Earth system process that interacts with vegetation, climate, and humans. The EGU2020 B3.17 session on the role of fire in the Earth system received the most abstracts in the Biogeosciences division this year and was highly attended during the live chat. The abstracts cover several aspects of interactions between fire and the biosphere, atmosphere, and humans across various temporal and spatial scales using modelling, field and laboratory observations, and remote sensing. Given that the topic is highly interdisciplinary and overarching, we propose an inter-journal (ESD/BG/ACP/GMD/NHESS) special issue for this session with ESD as the lead journal, and we also encourage the submission of topic-related studies outside the EGU fire session. The special issue will bring together new advances in understanding the feedbacks and interactions between fire and other components of the Earth system at all temporal and spatial scales using various methods, including (1) impacts of fire on weather, climate, and atmospheric chemistry; (2) interactions between fire, the biogeochemical cycle, vegetation composition and structure, and land water and energy budgets; (3) influence of humans on fire and vice versa; (4) fire characteristics (e.g., fire duration, emission factor, emission height, smoke transport); (5) spatial and temporal changes of fires in the past, present, and future; (6) fire products and models, their validation, and error/bias assessment; and (7) analytical tools designed to enhance situational awareness among fire practitioners and early warning systems, addressing specific needs of operational fire behaviour modelling.

Compound weather and climate events refer to combinations of multiple weather and climate drivers and/or hazards that lead to potentially large impacts. Compound events encompass a highly diverse set of events including concurrent climate extremes but also an array of nonstandard high-impact events. Consequently, research on compound events requires expertise from a variety of disciplines such as climate science, hydrology, impact modelling, engineering, and statistics, among others. The European COST Action DAMOCLES (http://damocles.compoundevents.org) provides a platform for compound event research and for generating synergies between the different research domains. This inter-journal special issue will serve as an outlet for the work within DAMOCLES and aims to advance our understanding of different aspects of compound events, including present-day and future risk assessments of such events, modelling of individual compound events, bottom-up approaches to identify new types of compound events, and novel model evaluation techniques. It is open for all submissions within its scope.

The Weddell Sea and the ocean off Dronning Maud Land, constituting the Weddell Gyre, are representative of the high-latitude Southern Ocean due to its pronounced seasonality, circumpolar currents, deep-water formation and seasonal sea-ice cover. The Weddell Gyre connects water masses from the Antarctic Circumpolar Current with those shaped by the large ice shelves and sea-ice formation in the southern perimeter of the Weddell Gyre. Biological and biogeochemical processes are strongly influenced by these conditions and contribute to a unique Weddell Sea ecosystem.

A portion of the western Weddell Sea is already experiencing consequences of climate change, including the acceleration of mass loss from ice shelves, enhanced ocean warming and freshening, rising air temperatures, and changes in wind patterns. This likely has an emerging influence on the local biological and biogeochemical processes. The eastern part of the Weddell Gyre, off Dronning Maud Land, however, appears relatively stable but is expected to experience warming, sea-ice retreat and ice-shelf loss in the course of this century. The re-emergence of the Maud Rise Polynya, with a potentially significant impact on regional water masses and biogeochemistry, underscores the importance of observing and understanding this region. Establishing knowledge of the present biological and biogeochemical processes and coupling with physics prior to major changes in this region is essential.

This special issue emerged from an October 2020 workshop organized by the Southern Ocean Observing System (SOOS) Weddell Sea and Dronning Maud Land Regional Working Group that brought together numerous scientists across multiple disciplines. However, the issue is open to anyone and we welcome additional contributions.

This special issue arises from work conducted by the AMAP Expert Group on short-lived climate forcers for an assessment that will be finalized in 2021. It will include only invited papers.

Remote sensing observations in the microwave frequencies have been intensively used to map and investigate water dynamics at the land surface. Microwave observations are also increasingly used to observe changes in vegetation structure and function and to study interactions between water and vegetation. For example, microwave observables such as brightness temperature, radar backscatter, polarization, SAR interferometry, GNSS reflectometry, and variables retrieved from them such as vegetation optical depth and soil moisture allow one to map ecosystem states. All these variables can be used to study processes, states, and temporal changes in vegetation–water interactions such as in vegetation phenology, canopy structure, biomass, photosynthesis, evapotranspiration, interception, dew formation, vegetation water content, water stress, surface and root-zone soil moisture, and irrigation.

The special issue aims to advance our understanding on existing and new techniques, methods, and applications of microwave remote sensing for vegetation–water interactions. We welcome studies that use active or passive microwave satellite, airborne, tower, and field observations to monitor processes, states, and temporal dynamics of vegetation–water interactions by

1. developing and evaluating microwave radiative transfer models for vegetation and water;
2. retrieving land surface variables through radiative transfer model inversion, machine learning, or hybrid approaches;
3. evaluating and constraining land surface, ecosystem, crop, forest, and hydrological models with microwave observations; and
4. assessing field experiments on the impacts of plant water dynamics on microwave observations.

This special issue is an outcome of sessions BG3.20 (Global Earth observation for improved understanding of terrestrial ecosystem dynamics) and HS6.4 (Remote sensing of interactions between vegetation and hydrology) held at the General Assembly 2020 of the European Geoscience Union and of the related precursor sessions in previous years. However, the special issue is open for all submissions within its scope.

Fire is an essential feature of terrestrial ecosystems and plays an important role in the Earth system. Fires are regulated by climate, vegetation characteristics, and human activity and also feedback to them in multiple ways. For example, fires can shape vegetation composition and structure; adjust land carbon, nutrient, water, and energy cycles; change atmospheric composition, chemistry, and physics; and affect air quality and human health. Elevated fire activity in 2019 across the arctic, Amazon, and other regions underscores the urgency of a quantitative understanding of fire as an Earth system process that interacts with vegetation, climate, and humans. The EGU2020 B3.17 session on the role of fire in the Earth system received the most abstracts in the Biogeosciences division this year and was highly attended during the live chat. The abstracts cover several aspects of interactions between fire and the biosphere, atmosphere, and humans across various temporal and spatial scales using modelling, field and laboratory observations, and remote sensing. Given that the topic is highly interdisciplinary and overarching, we propose an inter-journal (ESD/BG/ACP/GMD/NHESS) special issue for this session with ESD as the lead journal, and we also encourage the submission of topic-related studies outside the EGU fire session. The special issue will bring together new advances in understanding the feedbacks and interactions between fire and other components of the Earth system at all temporal and spatial scales using various methods, including (1) impacts of fire on weather, climate, and atmospheric chemistry; (2) interactions between fire, the biogeochemical cycle, vegetation composition and structure, and land water and energy budgets; (3) influence of humans on fire and vice versa; (4) fire characteristics (e.g., fire duration, emission factor, emission height, smoke transport); (5) spatial and temporal changes of fires in the past, present, and future; (6) fire products and models, their validation, and error/bias assessment; and (7) analytical tools designed to enhance situational awareness among fire practitioners and early warning systems, addressing specific needs of operational fire behaviour modelling.

Compound weather and climate events refer to combinations of multiple weather and climate drivers and/or hazards that lead to potentially large impacts. Compound events encompass a highly diverse set of events including concurrent climate extremes but also an array of nonstandard high-impact events. Consequently, research on compound events requires expertise from a variety of disciplines such as climate science, hydrology, impact modelling, engineering, and statistics, among others. The European COST Action DAMOCLES (http://damocles.compoundevents.org) provides a platform for compound event research and for generating synergies between the different research domains. This inter-journal special issue will serve as an outlet for the work within DAMOCLES and aims to advance our understanding of different aspects of compound events, including present-day and future risk assessments of such events, modelling of individual compound events, bottom-up approaches to identify new types of compound events, and novel model evaluation techniques. It is open for all submissions within its scope.

The Peruvian upwelling system is one of the most productive ecosystems in the ocean. Continuous upwelling of nutrient-rich water generates high primary production, supporting an efficient food web and leading to high fishery yields. An extensive oxygen minimum zone underlying the productive surface waters gives rise to high losses of fixed nitrogen via denitrification and anaerobic ammonium oxidation. This results in nutrient ratios of upwelled waters deviating strongly from Redfield stoichiometry, with a high N deficit and excess P. How this unusual nutrient stoichiometry affects plankton community production and succession and how this feeds back on biogeochemical cycling are still not well understood. To address these questions, an in situ mesocosm experiment was conducted in the coastal upwelling system off Peru in the framework of the Collaborative Research Center 754 "Climate-Biogeochemistry Interactions in the Tropical Ocean". The field experiment took place from January to April 2017 during a coastal El Niño, a rare event of El Niño conditions confined to Peruvian coastal waters. Key objectives of this mesocosm study, which involved 57 researchers from 10 nations, were to unravel the interplay of pelagic ecology and biogeochemistry under the unique environmental conditions of the Peruvian upwelling system. This special issue in Biogeosciences is intended as a collective outlet for publications resulting from this study, covering the broad range of research topics from seawater chemistry to plankton ecology and pelagic biogeochemistry.

In the vast low-nutrient—low-chlorophyll (LNLC) ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be 1 order of magnitude greater over short timescales. Recently, significant experimental, field and modeling work allowed us to better link atmospheric deposition with nutrient availability, ocean productivity, carbon cycling and marine emissions. By improving our understanding on how atmospheric inputs are impacting biological activity and the carbon balance in oligotrophic environments, their representation in biogeochemical models at different timescales and space scales will be better assessed. In particular, processes may be affected by ongoing environmental changes in the ocean such as a decrease in pH and an increase in temperature.

This special issue aims at gathering contributions from both experimental and modeling approaches showing how natural or anthropogenic inputs from the atmosphere can impact biogeochemical and ecological processes in the present and future oligotrophic ocean. It will present the results obtained during the PEACETIME (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea) cruise conducted in 2017 in the Mediterranean Sea, but the special issue is open to any submissions provided that the subject is consistent with the objectives defined above.