publications

2024

1. Weakening of tropical free tropospheric temperature gradients with global warming

   Heng Quan, Yi Zhang, and Stephan Fueglistaler

   Journal of the Atmospheric Sciences, 2024

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   The weak temperature gradients in the tropical free troposphere due to the vanishing Coriolis force near the equator lead to a strong dynamical coupling over the entire tropics. Using theory and a suite of targeted model experiments, we show that the weak temperature gradients further weaken under global warming. We show that the temperature gradient is set by the circulation strength, with a weaker circulation being associated with weaker gradients. Thus, the known scaling difference between atmospheric radiative cooling and static stability that leads to a slow-down of the circulation under warming also leads to a weakening of the temperature gradients in the tropical free troposphere. The impact from the weakening circulation on the weakening of temperature gradients is shown to dominate over the impact of masked CO2 forcing and the El-Nino like tropical Pacific warming pattern in model projections. Key to the result is the non-linear zonal momentum advection term. Using the well-known Matsuno-Gill model with correct scaling of heating and static stability may give the correct sign of the response in the temperature gradients, but inaccurate scaling, due to the linear momentum damping in that model. The robust scaling of the magnitude of the tropical quasi-stationary structure with temperature opens possibilities for theoretical advances on questions of societal relevance, ranging from changes in tropical cloudiness to heat stress under climate change.

2. Forecasting Tropical Annual Maximum Wet-Bulb Temperatures Months in Advance from the Current State of El Niño

   Yi Zhang, William R Boos, Isaac Held, and 2 more authors

   Geophysical Research Letters, 2024

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   Humid heatwaves, characterized by high temperature and humidity combinations, challenge tropical societies. Extreme wet-bulb temperatures (TW) over tropical land are coupled to the warmest sea surface temperatures by atmospheric convection and wave dynamics. Here, we harness this coupling for seasonal forecasts of the annual maximum of daily maximum TW (TWmax). We develop a multiple linear regression model that explains 80% of variance in tropical mean TWmax and significant regional TWmax variances. The model considers warming trends and El Niño and Southern Oscillation indices. Looking ahead, the strong-to-very-strong El Niño at the end of 2023, with an Oceanic Niño Index of ∼2.0, suggests a 2024 tropical land mean TWmax of 26.2°C (25.9–26.4°C), and a 68% chance (24%–94%) of breaking existing records. This method also predicts regional TWmax in specific areas.

3. Theory and the future of land-climate science

   Michael P Byrne, Gabriele C Hegerl, Jacob Scheff, and 8 more authors

   Nature Geoscience, 2024

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   Climate over land—where humans live and the majority of food is produced—is changing rapidly, driving severe impacts through extreme heat, wildfires, drought and flooding. Our ability to monitor and model this changing climate is being transformed through new observational systems and increasingly complex Earth system models. But fundamental understanding of the processes governing land climate has not kept pace, weakening our ability to interpret and utilize data from these advanced tools. Here we argue that for land-climate science to accelerate forwards, an alternative approach is needed. We advocate a parallel scientific effort, one emphasizing robust theories, that aims to inspire current and future land-climate scientists to better comprehend the processes governing land climate, its variability and extremes and its sensitivity to global warming. Such an effort, we believe, is essential to better understand the risks people face, where they live, in an era of climate change.

2023

1. An upper bound for extreme temperatures over midlatitude land

   Yi Zhang, and William R Boos

   Proceedings of the National Academy of Sciences, 2023

   APS Physics Magaine–Predicting Heatwaves’ Highest Temperatures (a very clear technical summary)

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   Heatwaves damage societies worldwide and are intensifying with global warming. Several mechanistic drivers of heatwaves, such as atmospheric blocking and soil moisture-atmosphere feedback, are well-known for their ability to raise surface air temperature. However, what limits the maximum surface air temperature in heatwaves remains unclear; this became evident during recent Northern Hemisphere heatwaves which achieved temperatures far beyond the upper tail of the observed statistical distribution. Here, we present evidence for the hypothesis that convective instability limits annual maximum surface air temperatures (TXx) over midlatitude land. We provide a theory for the corresponding upper bound of midlatitude temperatures, which accurately describes the observed relationship between temperatures at the surface and in the midtroposphere. We show that known heatwave drivers shift the position of the atmospheric state in the phase space described by the theory, changing its proximity to the upper bound. This theory suggests that the upper bound for midlatitude TXx should increase 1.9 times as fast as 500-hPa temperatures at the time and location of TXx occurrences. Using empirical 500-hPa warming, we project that the upper bound of TXx over Northern Hemisphere midlatitude land (40°N to 65°N) will increase about twice as fast as global mean surface air temperature, and TXx will increase faster than this bound over regions that dry on the hottest days.

2. Maximal reachable temperatures for Western Europe in current climate

   Robin Noyelle, Yi Zhang, Pascal Yiou, and 1 more author

   Environmental Research Letters, 2023

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   Human bodies, ecosystems and infrastructures display a non-linear sensibility to extreme temperatures occurring during heatwave events. Preparing for such events entails to know how high surface air temperatures can go. Here we examine the maximal reachable temperatures in Western Europe. Taking the July 2019 record-breaking heatwave as a case study and employing a flow analogues methodology, we find that temperatures exceeding 50 ∘C cannot be ruled out in most urban areas, even under current climate conditions. We analyze changes in the upper bound of surface air temperatures between the past (1940–1980) and present (1981–2021) periods. Our results show that the significant increase in daily maximum temperatures in the present period is only partially explained by the increase of the upper bound. Our results suggest that most of the warming of daily maximum surface temperatures result from strengthened diabatic surface fluxes rather than free troposphere warming.

2021

1. Some Theoretical Thinking on the Changing Tropical Climate

   Yi Zhang

   Princeton University, 2021

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   This dissertation synthesizes long-standing theoretical ideas to develop quantitative constraints on several aspects of tropical climate, namely convection, heat stress, precipitation, and outgoing longwave radiation. A zeroth-order picture of the tropical troposphere is that deep convection maintains a moist adiabatic temperature profile in the vertical and gravity waves quickly smooth any temperature gradients in the horizontal, which is formally known as convective quasiequilibrium (QE) and the weak-temperature-gradient (WTG) assumption. We expect that strict QE-WTG should yield a uniform moist static energy (MSE) threshold for deep convection. Consistent with the theoretical expectation, we find that deep convection only occurs over the highest subcloud MSEs and that the convective subcloud MSE (daily-mean subcloud MSE weighted by precipitation) is roughly uniform between 20°S and 20°N. QEWTG forces the highest subcloud MSEs to be equal over land and ocean, not only in the present climate but also in much colder and warmer climates. The annual-maximum wet-bulb temperature, a metric for extreme heat stress, is also controlled by the QE-WTG dynamics due to the functional relationship of wet-bulb temperature with MSE. We provide a theoretical projection that the annual-maximum wetbulb temperature will increase roughly uniformly by about 1°C for each 1°C of tropical mean warming. This result suggests that limiting the mean surface warming also limits heat stress extremes in the tropics. QE-WTG controls the occurrence of deep convection and thus precipitation. Global climate models consistently predict that tropical precipitation will be distributed more unevenly in space with global warming. We show that the unevenness of precipitation can be traced back to the unevenness of subcloud MSE distribution. We then explain the change in sublcoud MSE distribution with a simple scaling accounting for the Clausius-Clapeyron increases of boundary-layer specific humidity under invariant relative humidities. The invariance of relative humidities also has implications for the linearity of outgoing longwave radiation with surface temperature.

2. Projections of tropical heat stress constrained by atmospheric dynamics

   Yi Zhang, Isaac Held, and Stephan Fueglistaler

   Nature Geoscience, 2021

   Princeton University News–Simple atmospheric dynamics foretell dangerously hot future for the tropics (contains a schematic of the physical mechanism)
   Youtube channel "Just Have a Think" by Dave Borlace–Wet Bulb Temperature. Life or death? (contains a demo of wet-bulb temperature measurement)
   New York Times–Global Warming’s Deadly Combination: Heat and Humidity (for general public)
   

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   Extreme heat under global warming is a concerning issue for the growing tropical population. However, model projections of extreme temperatures, a widely used metric for extreme heat, are uncertain on regional scales. In addition, humidity needs to be taken into account to estimate the health impact of extreme heat. Here we show that an integrated temperature–humidity metric for the health impact of heat, namely, the extreme wet-bulb temperature (TW), is controlled by established atmospheric dynamics and thus can be robustly projected on regional scales. For each 1 °C of tropical mean warming, global climate models project extreme TW (the annual maximum of daily mean or 3-hourly values) to increase roughly uniformly between 20° S and 20° N latitude by about 1 °C. This projection is consistent with theoretical expectation based on tropical atmospheric dynamics, and observations over the past 40 years, which gives confidence to the model projection. For a 1.5 °C warmer world, the probable (66% confidence interval) increase of regional extreme TW is projected to be 1.33–1.49 °C, whereas the uncertainty of projected extreme temperatures is 3.7 times as large. These results suggest that limiting global warming to 1.5 °C will prevent most of the tropics from reaching a TW of 35 °C, the limit of human adaptation.

3. On the controlling factors for globally extreme humid heat

   Colin Raymond, Tom Matthews, Radley M Horton, and 5 more authors

   Geophysical Research Letters, 2021

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2020

1. How tropical convection couples high moist static energy over land and ocean

   Yi Zhang, and Stephan Fueglistaler

   Geophysical Research Letters, 2020

   AGU Eos: Editor’s Highlight–How Does Convection Work Over the Tropics?

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   We show that in the tropics, tropical atmospheric dynamics force the subcloud moist static energy (MSE) over land and ocean to be very similar in, and only in, regions of deep convection. Using observed rainfall as a proxy for convection and reanalysis data to calculate MSE, we show that subcloud MSE in the nonconvective regions may differ substantially between land and ocean but is uniform across latitudes in convective regions even on a daily timescale. This result holds also in CMIP5 model simulations of past cold and future warm climates. Furthermore, the distribution of rainfall amount in subcloud MSE is very similar over land and ocean with the peak at 343 J/g and a half width at half maximum of 3 J/g. Our results demonstrate that the horizontally uniform free tropospheric temperature forces the highest subcloud MSE values to be similar over land and ocean.

2. Linearity of outgoing longwave radiation: From an atmospheric column to global climate models

   Yi Zhang, Nadir Jeevanjee, and Stephan Fueglistaler

   Geophysical Research Letters, 2020

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   The linearity of global-mean outgoing longwave radiation (OLR) with surface temperature is a basic assumption in climate dynamics. This linearity manifests in global climate models, which robustly produce a global-mean longwave clear-sky (LWCS) feedback of 1.9 W/m2/K, consistent with idealized single-column models (Koll & Cronin, 2018). However, there is considerable spatial variability in the LWCS feedback, including negative values over tropical oceans (known as the “super-greenhouse effect”) which are compensated for by larger values in the subtropics/extratropics. Therefore, it is unclear how the idealized single-column results are relevant for the global-mean LWCS feedback in comprehensive climate models. Here we show with a simple analytical theory and model output that the compensation of this spatial variability to produce a robust global-mean feedback can be explained by two facts: (1) When conditioned upon free-tropospheric column relative humidity (RH), the LWCS feedback is independent of RH, and (2) the global histogram of free-tropospheric column RH is largely invariant under warming.

2019

1. Mechanism for increasing tropical rainfall unevenness with global warming

   Yi Zhang, and Stephan Fueglistaler

   Geophysical Research Letters, 2019

   AGU Eos: Editor’s Highlight–Understanding Tropical Rainfall Projections Under Climate Change

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   Global climate models predict that tropical rainfall will be distributed more unevenly with global warming; that is, dry regions or months will get drier and wet regions or months will get wetter. Previous mechanisms such as “dry-get-drier, wet-get-wetter”; “rich-get-richer”; or “upped-ante” focus on the spatial pattern of rainfall changes rather than the changes in probability distribution. Here, we present a quantitative explanation of the warming-induced probability distribution change of rainfall: Subcloud moist static energy (MSE) gradients are amplified by Clausius-Clapeyron relationship given roughly uniform warming and constant relative humidity. Therefore, the present-day wet regions will become more competitive for convection in a warmer world. Though changes in the atmospheric circulation pattern can enhance rainfall in one place and suppress rainfall in another, our results show that the total effect should be a decrease in the area of active convection even with uniform warming.