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Extreme wet-bulb temperatures (T_w) are often used as indicators of heat stress. However, humid heat extremes are fundamentally compound events, and a givenT_wcan be generated by various combinations of temperature and humidity. Differentiating between extreme humid heat driven by temperature versus humidity is essential to identifying these extremes’ physical drivers and preparing for their distinct impacts. Here we explore the variety of combinations of temperature and humidity contributing to humid heat experienced across the globe. In addition to using traditional metrics, we derive a novel thermodynamic state variable named “stickiness.” Analogous to the oceanographic variable “spice” (which quantifies the relative contributions of temperature and salinity to a given water density), stickiness quantifies the relative contributions of temperature and specific humidity to a givenT_w. Consistent across metrics, we find that high magnitudes ofT_wtend to occur in the presence of anomalously high moisture, with temperature anomalies of secondary importance. This widespread humidity dependence is consistent with the nonlinear relationship between temperature and specific humidity as prescribed by the Clausius–Clapeyron relationship. Nonetheless, there is a range of stickiness observed at moderate-to-highT_wthresholds. Stickiness allows a more objective evaluation of spatial and temporal variability in the temperature versus humidity dependence of humid heat than traditional variables. In regions with high temporal variability in stickiness, predictive skill for humid heat-related impacts may improve by considering fluctuations in atmospheric humidity in addition to dry-bulb temperature.

Extreme humid heat increases the risk of heat stress through its influence over humans’ ability to cool down by sweating. Understanding whether humid heat extremes are generated more due to elevated temperature or humidity is important for identifying factors that may increase local risk, preparing for associated impacts, and developing targeted adaptation measures. Here we explore combinations of temperature and humidity across the globe using traditional metrics and by deriving a new variable called “stickiness.” We find that extreme humid heat at dangerous thresholds occurs primarily due to elevated humidity, but that stickiness allows for thorough analysis of the drivers of humid heat at lower thresholds, including identification of regions prone to low- or high-stickiness extremes.

 

Most current climate models predict that the equatorial Pacific will evolve under greenhouse gas–induced warming to a more El Niño-like state over the next several decades, with a reduced zonal sea surface temperature gradient and weakened atmospheric Walker circulation. Yet, observations over the last 50 y show the opposite trend, toward a more La Niña-like state. Recent research provides evidence that the discrepancy cannot be dismissed as due to internal variability but rather that the models are incorrectly simulating the equatorial Pacific response to greenhouse gas warming. This implies that projections of regional tropical cyclone activity may be incorrect as well, perhaps even in the direction of change, in ways that can be understood by analogy to historical El Niño and La Niña events: North Pacific tropical cyclone projections will be too active, North Atlantic ones not active enough, for example. Other perils, including severe convective storms and droughts, will also be projected erroneously. While it can be argued that these errors are transient, such that the models’ responses to greenhouse gases may be correct in equilibrium, the transient response is relevant for climate adaptation in the next several decades. Given the urgency of understanding regional patterns of climate risk in the near term, it would be desirable to develop projections that represent a broader range of possible future tropical Pacific warming scenarios—including some in which recent historical trends continue—even if such projections cannot currently be produced using existing coupled earth system models. 

The authors propose a new analytic approximation to aid understanding of the “moisture modes” that arise in idealized models of tropical intraseasonal oscillations. The approximation makes use of the separation between the time scale of the large scale flow and that of convective adjustment, but does not neglect the moisture tendency. The approximation filters out equatorial Kelvin waves, so that the only remaining mode is the intraseasonal moisture mode. Some physical insights are more transparent under the approximation: (a) vertical advection of moisture or temperature lead to a diffusion‐type operator that damps short waves, and instability is strongest at planetary scale. (b) A positive effective gross moist stability appears in a wave operator, while a negative effective gross moist stability leads to instabilities whose growth rates increase with zonal wavenumber. (c) Zonal moisture advection and surface flux feedbacks are crucial for both planetary instability and eastward propagation.

 

Abstract The Propagation of Intraseasonal Tropical Oscillations (PISTON) experiment conducted a field campaign inAugust-October 2018. The R/V Thomas G. Thompson made two cruises in thewestern North Pacific region north of Palau and east of the Philippines. Using select field observations and global observational and reanalysis data sets, this study describes the large-scale state and evolution of the atmosphere and ocean during these cruises. Intraseasonal variability was weak during the field program, except for a period of suppressed convection in October. Tropical cyclone activity, on the other hand, was strong. Variability at the ship location was characterized by periods of low-level easterly atmospheric flow with embedded westward propagating synoptic-scale atmospheric disturbances, punctuated by periods of strong low-level westerly winds that were both connected to the Asian monsoon westerlies and associated with tropical cyclones. In the most dramatic case, westerlies persisted for days during and after tropical cyclone Jebi had passed to the north of the ship. In these periods, the sea surface temperature was reduced by a couple of degrees by both wind mixing and net surface heat fluxes that were strongly (~200 Wm −2 ) out of the ocean, due to both large latent heat flux and cloud shading associated with widespread deep convection. Underway conductivity-temperature transects showed dramatic cooling and deepening of the ocean mixed layer and erosion of the barrier layer after the passage of Typhoon Mangkhut due to entrainment of cooler water from below. Strong zonal currents observed over at least the upper 400 meters were likely related to the generation and propagation of near-inertial currents. 

During the summer of 2016, a boreal summer intraseasonal oscillation (BSISO) event was observed over Southeast Asia and the South China and Philippine seas. Precipitation anomalies associated with this event propagated northward at a speed of 0.5–1° per day from July to August. To understand the mechanisms, a regional atmosphere‐ocean coupled system with the Weather Research and Forecasting (WRF) model and the Hybrid Coordinate Ocean Model (HYCOM) is used to study this BSISO event. The 50‐day‐long coupled simulations reasonably capture large‐scale northward propagation of the event. Coupled simulations with altered air‐sea interaction and atmosphere‐only simulations with prescribed sea surface temperature illuminate the insignificant role of air‐sea interaction within the computation domain in the northward propagation the for this event. Diagnostics of the coupled simulation as well as the ECMWF‐Interim reanalyses indicate that convection and barotropic vorticity are largely in phase north of 5°N during the event. The BSISO convection is accompanied by moisture anomalies whose magnitudes increase as the BSISO propagates northward. Analysis of the moisture budget shows that positive horizontal advection leads positive moisture anomalies on the intraseasonal time scale north of 10°N. The vorticity‐convection relationship, the lead‐lag relationship between moisture and its horizontal advection, and the latitude dependence of each for this BSISO event are consistent with general features of BSISO events composited with ECWMF‐Interim reanalysis and satellite precipitation data sets.