Publications

A Simple Framework for Likely Climate Projections Applied to Tropical Width

Published in Climate Dynamics, 2024

The increasing use of climate projections in adaptation necessitates a consistent method for producing estimates of likely future conditions from available climate model data. Many climate projections are produced using high emission scenarios and an evenly weighted ensemble of all available climate models despite substantial evidence that the continuously rising emissions in high emission scenarios are unrealistic, and that some models are more reliable than others. While high emission scenarios can be used to generate a more significant climate change signal and are often not intended to be interpreted as projections, a reader who is a non-expert on climate scenarios may not understand this nuance. As a result, unlikely climate projections could be inadvertently used to plan crucial adaptation efforts for future warming. Here, we present a simple and easy to use framework for creating projections of our likely future climate by combining existing methods. The framework involves three measures: selecting the most likely emission scenario, choosing the most reliable models, and debiasing against observational or reanalysis data. Each of these steps allows for a range of methods with varying complexity, precision, and utility. To demonstrate our framework and its components, we use the simplest applicable methods to estimate future changes in tropical width, a hydrologically important climate feature. Our projections show that the likely tropical expansion by the end of this century is roughly half of some previously reported estimates, largely due to the selected emission scenario. This simple framework can be easily applied to other climate features, allowing for better estimates of likely future conditions.

Large uncertainty in observed estimates of tropical width from the meridional stream function

Published in Weather and Climate Dynamics, 2023

Recent Hadley cell expansion rate estimates vary substantially, as a multitude of methods and reanalysis datasets yield conflicting results. Among the many methods of estimating the Hadley cell width, the meridional-stream-function 500 hPa zero crossing is the most widely used, as it is directly related to the poleward edge of the Hadley cell (HC). Other common metrics use atmospheric phenomena associated with the HC as a proxy, for instance the zonal-surface-wind zero crossing. As each of these metrics requires different reanalysis data, each with varying error, the level of data-driven uncertainty differs between each metric. While previous work has analyzed the statistical and dynamical relationships between metrics, to date no study has quantified and compared the uncertainty due to reanalysis data error in different HC metrics. In this study, we use ERA5 ensemble members, which include small perturbations in atmospheric variables based on the data error, to quantify the uncertainty associated with six commonly used HC metrics as well as the range of their trend estimates. In the Northern Hemisphere, the tropical expansion rate calculated by the stream function is roughly 0.05◦ per decade, while the Southern Hemisphere rate is 0.2◦ per decade over the period from 1979–2022. Of the six metrics, only the meridional stream function and precipitation minus evaporation have substantial uncertainties. The stream function errors are large due to uncertainty in the underlying meridional-wind data and the presence of large regions of near-neutral circulation at the poleward edge of the tropics. These errors have decreased in recent decades because of improvements in the assimilated observations. Despite these improvements, metrics which use well-observed and constrained quantities such as the zonal-surface-wind zero crossing have lower uncertainty, particularly in summer and fall in the Northern Hemisphere.