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J. E. Jack Reeves Eyre and Xubin Zeng

Abstract

Global and regional water cycle includes precipitation, water vapor divergence, and change of column water vapor in the atmosphere, and land surface evapotranspiration, terrestrial water storage change, and river discharge, which is linked to ocean salinity near the river mouth. The water cycle is a crucial component of the Earth system, and numerous studies have addressed its individual components (e.g., precipitation). Here we assess, for the first time, if remote sensing and reanalysis datasets can accurately and self-consistently portray the Amazon water cycle. This is further assisted with satellite ocean salinity measurements near the mouth of the Amazon River. The widely used practice of taking the mean of an ensemble of datasets to represent water cycle components (e.g., precipitation) can produce large biases in water cycle closure. Closure is achieved with only a small subset of data combinations (e.g., ERA5 precipitation and evapotranspiration plus GRACE satellite terrestrial water storage), which rules out the lower precipitation and higher evapotranspiration estimates, providing valuable constraints on assessments of precipitation, evapotranspiration, and their ratio. The common approach of using the Óbidos stream gauge (located hundreds of kilometers from the river mouth) multiplied by a constant (1.25) to represent the entire Amazon discharge is found to misrepresent the seasonal cycle, and this can affect the apparent influence of Amazon discharge on tropical Atlantic salinity.

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Min-Gyu Seong, Seung-Ki Min, Yeon-Hee Kim, Xuebin Zhang, and Ying Sun

Abstract

This study conducted a detection and attribution analysis of the observed global and regional changes in extreme temperatures during 1951–2015. HadEX3 observations were compared with multimodel simulations from the Coupled Model Intercomparison Project phase 6 (CMIP6) using an optimal fingerprinting technique. Annual maximum daily maximum and minimum temperatures (TXx and TNx; warm extremes) and annual minimum daily maximum and minimum temperatures (TXn and TNn; cold extremes) over land were analyzed considering global, continental, and subcontinental scales. Response patterns (fingerprints) of extreme temperatures to anthropogenic (ANT), greenhouse gases (GHG), aerosols (AA), and natural (NAT) forcings were obtained from CMIP6 forced simulations. The internal variability ranges were estimated from preindustrial control simulations. A two-signal detection analysis where the observations are regressed onto ANT and NAT fingerprints simultaneously reveals that ANT signals are robustly detected in separation from NAT over global and all continental domains (North and South America, Europe, Asia, and Oceania) for most of the extreme indices. ANT signals are also detected over many subcontinental regions, particularly for warm extremes (more than 60% of 33 subregions). A three-signal detection analysis that considers GHG, AA, and NAT fingerprints simultaneously demonstrates that GHG signals are detected in isolation from other external forcings over global, continental, and several subcontinental domains especially for warm extremes, explaining most of the observed warming. Moreover, AA influences are detected for warm extremes over Europe and Asia, indicating significant offsetting cooling contributions. Overall, human influences are detected more frequently, compared to previous studies, particularly for cold extremes, due to the extended period and the improved spatial coverage of observations.

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Siyan Dong, Ying Sun, Chao Li, Xuebin Zhang, Seung-Ki Min, and Yeon-Hee Kim

Abstract

While the IPCC Fifth Assessment Working Group I report assessed observed changes in extreme precipitation on the basis of both absolute and percentile-based extreme indices, human influence on extreme precipitation has rarely been evaluated on the basis of percentile-based extreme indices. Here we conduct a formal detection and attribution analysis on changes in four percentile-based precipitation extreme indices. The indices include annual precipitation totals from days with precipitation exceeding the 99th and 95th percentiles of wet-day precipitation in 1961–90 (R99p and R95p) and their contributions to annual total precipitation (R99pTOT and R95pTOT). We compare these indices from a set of newly compiled observations during 1951–2014 with simulations from models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). We show that most land areas with observations experienced increases in these extreme indices with global warming during the historical period 1951–2014. The new CMIP6 models are able to reproduce these overall increases, although with considerable over- or underestimations in some regions. An optimal fingerprinting analysis reveals detectable anthropogenic signals in the observations of these indices averaged over the globe and over most continents. Furthermore, signals of greenhouse gases can be separately detected, taking other forcing into account, over the globe and over Asia in these indices except for R95p. In contrast, signals of anthropogenic aerosols and natural forcings cannot be detected in any of these indices at either global or continental scales.

Open access
Yunji Zhang, David J. Stensrud, and Eugene E. Clothiaux

Abstract

Recent studies have demonstrated advances in the analysis and prediction of severe thunderstorms and other weather hazards by assimilating infrared (IR) all-sky radiances into numerical weather prediction models using advanced ensemble-based techniques. It remains an open question how many of these advances are due to improvements in the radiance observations themselves, especially when compared with radiance observations from preceding satellite imagers. This study investigates the improvements gained by assimilation of IR all-sky radiances from the Advanced Baseline Imager (ABI) on board GOES-16 compared to those from its predecessor imager. Results show that all aspects of the improvements in ABI compared with its predecessor imager—finer spatial resolution, shorter scanning intervals, and more channels covering a wider range of the spectrum—contribute to more accurate ensemble analyses and forecasts of the targeted severe thunderstorm event, but in different ways. The clear-sky regions within the assimilated all-sky radiance fields have a particularly beneficial influence on the moisture fields. Results also show that assimilating different IR channels can lead to oppositely signed increments in the moisture fields, a by-product of inaccurate covariances at large distances resulting from sampling errors. These findings pose both challenges and opportunities in identifying appropriate vertical localizations and IR channel combinations to produce the best possible analyses in support of severe weather forecasting.

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Xianglin Dai, Yang Zhang, and Xiu-Qun Yang

Abstract

Low-frequency (LF) transient eddies (intraseasonal eddies with time scales longer than 10 days) are increasingly found to be important in large-scale atmospheric circulation, high-impact climate events, and subseasonal-to-seasonal forecasts. In this study, the features and maintenance of available potential energy of LF eddies (LF EAPE), which denote LF temperature fluctuations, have been investigated. Our study shows that wintertime LF EAPE, with greater amplitude than that of the extensively studied high-frequency (HF) eddies, exhibits distinct horizontal and vertical structures. Different from HF eddies, whose action centers are over midlatitude oceans, the LF EAPE is most active in the continents in the midlatitudes, as well as the subpolar region with shallower vertical structure. By diagnosing the derived energy budget of LF EAPE, we find that, with the strong background temperature gradient in mid- and high-latitude continents (e.g., coast regions along the Greenland, Barents, and Kara Seas), baroclinic generation is the major source of LF EAPE. The generated LF EAPE in the subpolar region is transported downstream and southward to midlatitude continents via background flow. The generated LF EAPE is also dissipated by HF eddies, damped by diabatic effects, and converted to LF EKE via vertical motions. The above energy budget, together with the barotropic dynamics revealed by previous works, suggests multiple energy sources and thus complicated dynamics of LF variabilities.

Open access
Xinxi Wang and Haiyan Jiang

Abstract

Based on 35-yr (1982–2016) best track and Statistical Hurricane Intensity Prediction Scheme data, this study examined climatology of rapidly intensifying (RI) and slowly intensifying (SI) events as well as their time evolutions of storm-related and environmental parameters for tropical cyclones (TCs) in both North Atlantic (AL) and eastern North Pacific (EP) basins. Major hurricanes were intensified mainly through RI while tropical depression and tropical storms were intensified through SI. The percentage of TCs that underwent RI peaks in the late hurricane season whereas the percentage of TCs that underwent SI peaks early. For the first time in the literature, this study found that RI events have significantly different storm-related and environmental characteristics than SI events for before-, during-, and after-event stages. In both AL and EP basins, RI events always intensify significantly faster during the previous 12 h, are located farther south, and have warmer sea surface and 200-hPa temperatures, greater ocean heat content, larger 200-hPa divergence, weaker vertical wind shear, and weaker 200-hPa westerly flow than SI events for all event-relative stages. In the AL basin, RI events have larger low-level and midlevel relative humidity and larger 850-hPa relative vorticity than SI events for all event-relative stages in the AL and most event-relative stages in the EP. RI events are associated with more convectively unstable atmosphere and are farther away from their maximum potential intensities than SI events for most event-relative stages in the AL and for all event-relative stages in the EP.

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Gareth J. Marshall

Abstract

The Arctic continues to warm at a much faster rate than the global average. One process contributing to “Arctic amplification” involves changes in low-frequency macroscale atmospheric circulation patterns and their consequent influence on regional climate. Here, using ERA5 data, we examine decadal changes in the impact of seven such patterns on winter near-surface temperature (SAT) and precipitation (PPN) in northern Russia and calculate the temporal consistency of any statistically significant relationships. We demonstrate that the 40-yr climatology hides considerable decadal variability in the spatial extent of such circulation pattern–climate relationships across the region, with few areas where their temporal consistency exceeds 60%. This is primarily a response to the pronounced decadal expansion/contraction and/or mobility of the circulation patterns’ centers of action. The North Atlantic Oscillation (NAO) is the dominant pattern (having the highest temporal consistency) affecting SAT west of the Urals. Farther east, the Scandinavian (SCA), Polar/Eurasian (POL), and West Pacific patterns are successively the dominant pattern influencing SAT across the West Siberian Plains, Central Siberian Plateau, and mountains of Far East Siberia, respectively. From west to east, the SCA, POL, and Pacific–North American patterns exert the most consistent decadal influence on PPN. The only temporally invariant significant decadal relationships occur between the NAO and SAT and the SCA and PPN in small areas of the North European Plain.

Open access
Liping Zhang, Thomas L. Delworth, William Cooke, Hugues Goosse, Mitchell Bushuk, Yushi Morioka, and Xiaosong Yang

Abstract

Previous studies have shown the existence of internal multidecadal variability in the Southern Ocean using multiple climate models. This variability, associated with deep ocean convection, can have significant climate impacts. In this work, we use sensitivity studies based on Geophysical Fluid Dynamics Laboratory (GFDL) models to investigate the linkage of this internal variability with the background ocean mean state. We find that mean ocean stratification in the subpolar region that is dominated by mean salinity influences whether this variability occurs, as well as its time scale. The weakening of background stratification favors the occurrence of deep convection. For background stratification states in which the low-frequency variability occurs, weaker ocean stratification corresponds to shorter periods of variability and vice versa. The amplitude of convection variability is largely determined by the amount of heat that can accumulate in the subsurface ocean during periods of the oscillation without deep convection. A larger accumulation of heat in the subsurface reservoir corresponds to a larger amplitude of variability. The subsurface heat buildup is a balance between advection that supplies heat to the reservoir and vertical mixing/convection that depletes it. Subsurface heat accumulation can be intensified both by an enhanced horizontal temperature advection by the Weddell Gyre and by an enhanced ocean stratification leading to reduced vertical mixing and surface heat loss. The paleoclimate records over Antarctica indicate that this multidecadal variability has very likely happened in past climates and that the period of this variability may shift with different climate background mean state.

Open access
Yang Chen, Wei Li, Xiaoling Jiang, Panmao Zhai, and Yali Luo

Abstract

Detecting long-term changes in precipitation extremes over monsoon regions remains challenging due to large observational uncertainty, high internal variability at the regional scale, and climate models’ deficiency in simulating monsoon physics. This is particularly true for Eastern China, as illustrated by limited yet controversial detection results for daily scale precipitation extremes and the lack of detection analysis on hourly scale extremes there. Relying on high-quality gauge observations, two complementary techniques are used to detect the footprint of anthropogenic forcings in observed changes in both hourly and daily scale precipitation extremes across Eastern China. Results show that, scaled with global-mean surface temperature during 1970–2017, the regional-scale intensification nearly doubles the Clausius–Clapeyron rate (C-C; ~6.5% °C−1) for the wettest 10 h in the period and almost triples the C-C rate for the top 10 heaviest daily precipitation extremes. The intensification at both time scales, as well as the resulting increase in frequency, is discernibly stronger and more widespread than expected due to random internal variability. This not only lends supports to the model-based detection of forced trends for daily scale precipitation extremes, but it also suggests that anthropogenic warming has already be intensifying hourly scale precipitation extremes in this monsoon region. The magnitude and detectability of observed changes arise primarily from systematic intensification of non-tropical-cyclone-related precipitation extremes in response to the past warming.

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Na-Yeon Shin, Jong-Seong Kug, F. S. McCormack, and Neil J. Holbrook

Abstract

Recently, El Niño diversity has been paid much attention because of its different global impacts. However, most studies have focused on a single warm peak in sea surface temperature anomalies (SSTAs), either in the central Pacific or the eastern Pacific Ocean. Here, we demonstrate from observational analyses that several recent El Niño events show double warm peaks in SSTA—called “double-peaked (DP) El Niño”—that have only been observed since 2000. The DP El Niño has two warm centers, which grow concurrently but separately, in both the central and eastern Pacific. In general, the atmospheric and oceanic patterns of the DP El Niño are similar to those of the warm-pool (WP) El Niño from the development phase, such that the central Pacific peak is developed by the zonal advective feedback and reduced wind speed anomalies. However, a distinctive difference exists in the eastern Pacific where the DP El Niño has a second SSTA peak. In addition, the DP El Niño shows more distinctive anomalous precipitation along the Pacific intertropical convergence zone (ITCZ) when compared with the WP El Niño. We demonstrate that the peculiar precipitation anomalies along the Pacific ITCZ play a critical role in enhancing the equatorial westerly wind stress anomalies, which help to develop the eastern SSTA peak by deepening the thermocline in the eastern Pacific.

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