Callum Thompson

Farmworkers are particularly vulnerable to heat stress, which can escalate into heat‐related illnesses such as heat exhaustion or heat stroke, and can also impact productivity loss. Heat exposure varies considerably depending on season, work‐shift timing, acclimatization, and workload. We examine the frequency of heat stress exceedance in the Imperial and Coachella Valleys of southern California using wet bulb globe temperature (WBGT) calculated using the outputs from Weather Research and Forecasting (WRF) model for a reference year of 2020. The critical WBGT threshold of 80℉ is exceeded for more than 500 hr in August, with considerable exceedance in all key harvesting months of April, May, and June. The threshold is exceeded most frequently in August, with daytime (nighttime) exceedance of 96.3% (38.0%) in the Imperial and 72.5% (11.9%) in the Coachella Valleys. Occupational WBGT limits are also exceeded in most crop environments for unacclimatized workers, and even for acclimatized workers during date and sugarcane harvests. Adjusting the work schedule to an early morning or late evening shift can reduce workers' productivity loss by up to 40%. Based on our findings, we propose several strategies to mitigate heat risks among farmworkers: (a) adjusting work hours to include cooler morning or evening hours (b) using WBGT instead of air temperature to monitor heat exposure (c) adopting variable rest‐break schedules (d) reducing nighttime heat exposure during summer to allow recovery from daytime heat (e) taking extra precaution in high‐risk crop zones (f) avoiding nighttime irrigation during summer, and (g) revising thresholds and definitions used to identify heatwaves.

Surface winds over California can compound fire risk during autumn, yet their long-term trends in the face of decadal warming are less clear compared to other climate variables like temperature, drought, and snowmelt. To determine where and how surface winds are changing most, this article uses multiple reanalyses and Remote Automated Weather Stations (RAWS) to calculate autumn 10 m wind speed trends during 1979–2020. Reanalysis trends show statistically significant increases in autumn night-time easterlies on the western slopes of the Sierra Nevada. Although downslope windstorms are frequent to this region, trends instead appear to result from elevated gradients in warming between California and the interior continent. The result is a sharper horizontal temperature gradient over the Sierra crest and adjacent free atmosphere above the foothills, strengthening the climatological nocturnal katabatic wind. While RAWS records show broad agreement, their trend is likely influenced by year-to-year changes in the number of observations.

Inertial instability is a hydrodynamic instability that occurs in strong anticyclonic flow and is typically diagnosed by negative absolute vorticity in the Northern Hemisphere. As such, inertial instability is often observed on the anticyclonic‐shear side of jet streams, yet the release of the instability in this environment is still poorly understood. We construct an idealized midlatitude zonal jet and perform two experiments: one control simulation with no inertial instability and one experiment with inertial instability simulating its release. We find that the release of the instability results in meridional wind perturbations of up to 7 m s⁻¹ over 200 km that persist for several days, in addition to radiating inertia–gravity waves several hundreds of kilometers away from the unstable region. Furthermore, these perturbations instigate light–moderate occurrences of clear‐air turbulence around the unstable region that persist for up to 12 h.

Surface winds are an important factor in wildfire growth and the decision-making process of when utility companies shut off power to suppress fire ignitions. However, long-term trends in surface winds and their implications for fire weather have received less attention compared to trends in temperature, humidity, and precipitation. This article uses the ERA5 reanalysis to calculate surface wind trends over California during 1979--2019. We find statistically significant increases in surface easterlies during autumn on the western slopes of the Sierra Nevada Mountains and increases in Hazardous Wind Events of heightened wind-related fire risk. Using the Canadian Fire Weather Index, we also show that wildfire risk has mainly increased over the Sierra Nevada Mountains, indicating that strengthening winds has contributed to a growing risk of wind-driven wildfires in this region compared to 40 years ago.

The ReNovRisk-Cyclone program aimed at developing an observation network in the south-west Indian ocean (SWIO) in close synergy with the implementation of numerical tools to model and analyze the impacts of tropical cyclones (TC) in the present and in a context of climate change. This paper addresses the modeling part of the program. First, a unique coupled system to simulate TCs in the SWIO is developed. The ocean–wave–atmosphere coupling is considered along with a coherent coupling between sea surface state, wind field, aerosol, microphysics, and radiation. This coupled system is illustrated through several simulations of TCs: the impact of air–sea flux parameterizations on the evolution of TC Fantala is examined, the full coupling developed during the program is illustrated on TC Idai, and the potential of novel observations like space-borne synthetic aperture radar and sea turtles to validate the atmosphere and ocean models is presented with TC Herold. Secondly, the evolution of cyclonic activity in the SWIO during the second half of the 21st century is assessed. It was addressed both using climate simulation and through the implementation of a pseudo global warming method in the high-resolution coupled modeling platform. Our results suggest that the Mascarene Archipelago should experience an increase of TC related hazards in the medium term.

Inertial instability is a hydrodynamic instability that occurs in strong anticyclonic flow and is typically diagnosed by negative absolute vorticity in the Northern Hemisphere. As such, inertial instability is often observed on the anticyclonic-shear side of jet streams, yet the release of the instability in this environment is still poorly understood. We simulate the release of inertial instability near an idealized midlatitude zonal jet compared to a control simulation with no instability. We find that the release of the instability results in flat meridional wind perturbations of up to 7 m s−1 over 200 km that persist for several days, in addition to radiating inertia-gravity waves several hundreds of kilometers away from the unstable region. Furthermore, these perturbations instigate light–moderate occurrences of clear-air turbulence around the unstable region that persist for up to 12 hours.

The international research program “ReNovRisk-CYCLONE” (RNR-CYC, 2017–2021) directly involves 20 partners from 5 countries of the south-west Indian-Ocean. It aims at improving the observation and modelling of tropical cyclones in the south-west Indian Ocean, as well as to foster regional cooperation and improve public policies adapted to present and future tropical cyclones risk in this cyclonic basin. This paper describes the structure and main objectives of this ambitious research project, with emphasis on its observing components, which allowed integrating numbers of innovative atmospheric and oceanic observations (sea-turtle borne and seismic data, unmanned airborne system, ocean gliders), as well as combining standard and original methods (radiosoundings and global navigation satellite system (GNSS) atmospheric soundings, seismic and in-situ swell sampling, drone and satellite imaging) to support research on tropical cyclones from the local to the basin-scale.

Today, resilience in the face of cyclone risks has become a crucial issue for our societies. With climate change, the risk of strong cyclones occurring is expected to intensify significantly and to impact the way of life in many countries. To meet some of the associated challenges, the interdisciplinary ReNovRisk programme aims to study tropical cyclones and their impacts on the South-West Indian Ocean basin. This article is a presentation of the ReNovRisk programme, which is divided into four areas: study of cyclonic hazards, study of erosion and solid transport processes, study of water transfer and swell impacts on the coast, and studies of socio-economic impacts. The first transdisciplinary results of the programme are presented together with the database, which will be open access from mid-2021.

Inertial instability is a hydrodynamic instability that occurs in strong anticyclonic flow and is typically diagnosed by negative absolute vorticity in the Northern Hemisphere. As such, inertial instability is often observed on the anticyclonic-shear side of jet streams, yet the release of the instability in this environment is still poorly understood. We simulate the release of inertial instability near an idealized midlatitude zonal jet compared to a control simulation with no instability. We find that the release of the instability results in flat meridional wind perturbations of up to 7 m s−1 over 200 km that persist for several days, in addition to radiating inertia-gravity waves several hundreds of kilometers away from the unstable region. Furthermore, these perturbations instigate light–moderate occurrences of clear-air turbulence around the unstable region that persist for up to 12 hours.