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Emily Bercos-Hickey, Christina M. Patricola, and William A. Gallus Jr

Abstract

The impact of climate change on severe storms and tornadoes remains uncertain, largely owing to inconsistencies in observational data and limitations of climate models. We performed ensembles of convection-permitting climate model simulations to examine how three tornadic storms would change if similar events were to occur in pre-industrial and future climates. The choice of events includes winter, nocturnal, and spring tornadic storms to provide insight into how the timing and seasonality of storms may affect their response to climate change. Updraft helicity (UH), convective available potential energy (CAPE), storm-relative helicity (SRH), and convective inhibition (CIN) were used to determine the favorability for the three tornadic storm events in the different climate states. We found that from the pre-industrial period to the present, the potential for tornadic storms decreased for the winter event and increased for the nocturnal and spring events. With future climate change, the potential for tornadic storms increased for the winter and nocturnal events in association with increased CAPE, and decreased for the spring event despite greater CAPE.

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Tilla Roy, Jean Baptiste Sallée, Laurent Bopp, and Nicolas Metzl

Abstract

Anthropogenic CO2-emission-induced feedbacks between the carbon cycle and the climate system perturb the efficiency of atmospheric CO2 uptake by land and ocean carbon reservoirs. The Southern Ocean is a region where these feedbacks can be largest and differ most among Earth system model projections of twenty-first-century climate change. To improve our mechanistic understanding of these feedbacks, we develop an automated procedure that tracks changes in the positions of Southern Ocean water masses and their carbon uptake. In an idealized ensemble of climate change projections, we diagnose two carbon–concentration feedbacks driven by atmospheric CO2 (due to increasing air–sea CO2 partial pressure difference, dpCO2, and reducing carbonate buffering capacity) and two carbon–climate feedbacks driven by climate change (due to changes in the water mass surface outcrop areas and local climate impacts). Collectively these feedbacks increase the CO2 uptake by the Southern Ocean and account for almost one-fourth of the global uptake of CO2 emissions. The increase in CO2 uptake is primarily dpCO2 driven, with Antarctic Intermediate Waters making the largest contribution; the remaining three feedbacks partially offset this increase (by ~25%), with maximum reductions in Subantarctic Mode Waters. The process dominating the decrease in CO2 uptake is water mass dependent: reduction in carbonate buffering capacity in Subtropical and Subantarctic Mode Waters, local climate impacts in Antarctic Intermediate Waters, and reduction in outcrop areas in Circumpolar Deep Waters and Antarctic Bottom Waters. Intermodel variability in the feedbacks is predominately dpCO2 driven and should be a focus of efforts to constrain projection uncertainty.

Open access
Lei Zhang, Weiqing Han, Gerald A. Meehl, Aixue Hu, Nan Rosenbloom, Toshiaki Shinoda, and Michael J. McPhaden

Abstract

Understanding the impact of the Indian Ocean dipole (IOD) on El Niño–Southern Oscillation (ENSO) is important for climate prediction. By analyzing observational data and performing Indian and Pacific Ocean pacemaker experiments using a state-of-the-art climate model, we find that a positive IOD (pIOD) can favor both cold and warm sea surface temperature anomalies (SSTA) in the tropical Pacific, in contrast to the previously identified pIOD–El Niño connection. The diverse impacts of the pIOD on ENSO are related to SSTA in the Seychelles–Chagos thermocline ridge (SCTR; 60°–85°E, 7°–15°S) as part of the warm pole of the pIOD. Specifically, a pIOD with SCTR warming can cause warm SSTA in the southeastern Indian Ocean, which induces La Niña–like conditions in the tropical Pacific through interbasin interaction processes associated with a recently identified climate phenomenon dubbed the “warm pool dipole.” This study identifies a new pIOD–ENSO relationship and examines the associated mechanisms.

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Dafydd Stephenson and F. Sévellec

Abstract

Unpredictable variations in the ocean originate from both external atmospheric forcing and chaotic processes internal to the ocean itself, and are a crucial sink of predictability on interdecadal time scales. In a global ocean model, we present (i) an optimization framework to compute the most efficient noise patterns to generate uncertainty and (ii) a computationally inexpensive, dynamical method for attributing sources of ocean uncertainty to internal (mesoscale eddy-driven) and external (atmospherically driven) origins, sidestepping the more typical ensemble approach. These two methods are then applied to a range of metrics (heat content, volume transport, and heat transport) and time averages (monthly, yearly, and decadal) in the subtropical and subpolar North Atlantic. The optimal noise patterns create variability in integrated quantities of interest through features of the underlying circulation such as the North Atlantic Current and deep water formation regions. Meanwhile, noise forcing diagnosed from model representations of the actual climate system stimulates these theoretical patterns with various degrees of efficiency, ultimately leading to the growth of error. We reaffirm that higher-frequency variations in meridional transports are primarily wind driven, while surface buoyancy forcing is the ultimately dominant source of uncertainty at lower frequencies. For year-averaged quantities in the subtropics, it is mesoscale eddies that contribute the most to oceanic uncertainty, accounting for up to 60% after 60 years of growth for volume transport at 25°N. The impact of eddies is greatly reduced in the subpolar region, which we suggest may be explained by overall lower sensitivity to small-scale noise there.

Open access
Ryosuke Shibuya, Yukari Takayabu, and Hirotaka Kamahori

Abstract

This study examines disastrous historical precipitation cases that generate extreme precipitation simultaneously over a wide area in Japan (as in July 2018), defined as widespread extreme precipitation events. A statistically significant large-scale environment conducive for widespread extreme precipitation events over western Japan is investigated based on composite analysis. During a widespread precipitation event, a zonally elongated positive anomaly of the column-integrated water vapor extends from East China to western Japan. In the lower troposphere, a dipole of a geopotential height anomaly exists with positive and negative values at the east and west of the precipitation area, respectively. It is found that the negative geopotential anomaly is enhanced over East China at 2 days before the event and moves toward the precipitating area mainly due to the potential vorticity (PV) production term by diabatic heating, analogous to a diabatic Rossby wave. The temporal evolution of the dynamical forced vertical velocity is well in phase with that the PV production term, suggesting the importance of the coupling between the dynamical forced motion and diabatic heating. This result provides a physical understanding of the reason why both the background moisture and the baroclinicity are essential in the composited atmospheric fields and another view to the importance of the feedback parameter between the dynamical motion and diabatic heating.

Open access
Qiaoling Ren, Xingwen Jiang, Yang Zhang, Zhenning Li, and Song Yang

Abstract

It is known that the Tibetan Plateau (TP) can weaken the transient eddies (TEs) transported along the westerly jet stream. This study investigates the effects of the persistently suppressed TEs by the TP on the East Asian summer monsoon and the associated mechanisms using the NCAR Community Earth System Model. A nudging method is used to modify the suppression of the TEs without changing the steady dynamic and thermodynamic effects of the TP. The suppressed TEs by the TP weaken the East Asian westerly jet stream through the weakened poleward TE vorticity flux. On the one hand, the weakened jet stream leads to less (more) rainfall in northern (southern) East Asia by inducing anomalous moisture convergence, midtropospheric warm advection, and upper-level divergence, particularly in early summer when the eastward propagation of TE suppression by the TP is strong. On the other hand, the precipitation anomalies can shift the East Asian westerly jet stream southward and promote the moisture convergence in southern East Asia through latent heat release. Therefore, the persistent suppression of the TEs leads to a southward shift of the East Asian rain belt by a convective feedback, as it was previously found that the steady thermodynamic and dynamic forcings of the TP favored a northward shift of the rain belt. This study suggests that the anomalously weak TEs can lead to a rainfall change (more in the south, less in the north) over East Asia.

Open access
Ying Li, Chenghao Wang, and Fengge Su

Abstract

Reliable simulations of historical and future climate are critical to assessing ecological and hydrological responses over the Third Pole region (TP; the area including the Himalaya–Hindu Kush mountain range and the Tibetan Plateau). In this study, we evaluate the historical and future temperature and precipitation simulations of 18 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in southeastern TP (SETP) and the upstream area of the Amu Darya and Syr Darya (UAS) regions, two typical TP subregions dominated by the Indian summer monsoon system and westerlies, respectively. Comparison against station observations suggests that CMIP6 models generally capture the intra-annual variability and spatial pattern of historical climate over both subregions. However, the wetting and cold biases observed in CMIP5 still persist in CMIP6; annual temperature is underestimated by most models and annual precipitation is overestimated by all models. Multimodel average cold biases in SETP and UAS are 1.18° and 0.32°C, respectively, and wet biases in SETP and UAS are 119% and 46%, respectively. We further analyze climate projections under SSP1–2.6, SSP2–4.5, and SSP5–8.5 scenarios. Both SETP and UAS subregions are projected to experience significant warming in 2015–2100, with warming trends 34%–42% and 40%–50% higher than the global trend, respectively. Model projections suggest that the warming trend will slow down under SSP1–2.6 and SSP2–4.5 but further intensify under SSP5–8.5 in 2050–2100. Monsoon-dominated SETP is projected to experience a significant wetting trend stronger than UAS over the entire future period, especially in summer (cf. winter in westerlies-dominated UAS). Concurrently, a significant drying trend in summer is found in UAS during 2050–2100 under SSP5–8.5, suggesting the intensified uneven distributions of seasonal precipitation based on projections.

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Rodrigo J. Bombardi and William R. Boos

Abstract

This study examines the annual cycle of monsoon precipitation simulated by models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), then uses moist energy diagnostics to explain globally inhomogeneous projected future changes. Rainy season characteristics are quantified using a consistent method across the globe. Model bias is shown to include rainy season onsets tens of days later than observed in some monsoon regions (India, Australia, and North America) and overly large summer precipitation in others (North America, South America, and southern Africa). Projected next-century changes include rainy season lengthening in the two largest Northern Hemisphere monsoon regions (South Asia and central Sahel) and shortening in the two largest Southern Hemisphere regions (South America and southern Africa). Changes in the North American and Australian monsoons are less coherent across models. To understand these changes, relative moist static energy (MSE) is defined as the difference between local and tropical-mean surface air MSE. Future changes in relative MSE in each region correlate well with onset and demise date changes. Furthermore, Southern Hemisphere regions projected to undergo rainy season shortening are spanned by an increasing equator-to-pole MSE gradient, suggesting their rainfall will be increasingly inhibited by fluxes of dry extratropical air; Northern Hemisphere regions with projected lengthening of rainy seasons undergo little change in equator-to-pole MSE gradient. Thus, although model biases raise questions as to the reliability of some projections, these results suggest that globally inhomogeneous future changes in monsoon timing may be understood through simple measures of surface air MSE.

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Helga Kristin Olafsdottir, Holger Rootzén, and David Bolin

Abstract

Both the intensities of individual extreme rainfall events and the frequency of such events are important for infrastructure planning. We develop a new statistical extreme value model, the PGEV model, which makes it possible to use high-quality annual maximum series data instead of less well-checked daily data to estimate trends in intensity and frequency separately. The method is applied to annual maximum data from Vol. 10 of NOAA Atlas 14, dating from approximately 1900 to 2014, showing that in the majority of 333 rain gauge stations in the northeastern United States the frequency of extreme rainfall events increases as mean temperature increases, but that there is little evidence of trends in the distribution of the intensities of individual extreme rainfall events. The median of the frequency trends corresponds to extreme rainfall becoming 83% more frequent for each 1°C of temperature increase. Naturally, increasing trends in frequency also increase the yearly or decadal risks of very extreme rainfall events. Three other large areas in the contiguous United States, the Midwest, the Southeast, and Texas, are also studied, and show similar but weaker trends than those in the Northeast.

Open access
Bin Tang, Wenting Hu, and Anmin Duan

Abstract

A future projection of four extreme precipitation indices over the Indochina Peninsula and South China (INCSC) region with reference to the period 1958–2014 is conducted through the application of a multimodel ensemble approach and a rank-based weighting method. The weight of each model from phase 6 of the Coupled Model Intercomparison Project (CMIP6) is calculated depending on its historical simulation skill. Then, the weighted and unweighted ensembles are used for future projections. The results show that all four extreme precipitation indices are expected to increase over the INCSC region, both in the middle (2041–60) and at the end (2081–2100) of the twenty-first century, under three Shared Socioeconomic Pathway (SSP) scenarios. The increases in total extreme precipitation (R95p), extreme precipitation days (R95d), and the fraction of total rainfall from events exceeding the extreme precipitation threshold (R95pT) in the Indochina Peninsula are more significant than those in South China. The occurrence of extreme rainfall events may become more frequent in the future over the INCSC region, since the probability that R95pT increases is larger than 0.7 in the whole INCSC region. A comparison between the weighted and unweighted ensemble means shows that the uncertainty over South China is almost always reduced after applying the weighted scheme to future probabilistic projection, while the reductions in uncertainty over the Indochina Peninsula may depend on SSPs. The more extreme precipitation over the INCSC region in the future may be related to the larger water vapor supply and the more unstable local atmospheric stratification.

Open access