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Ren-Jie Wu, Min-Hui Lo, and Bridget R. Scanlon

Abstract

The terrestrial water storage anomaly (TWSA) is a critical component of the global water cycle where improved spatiotemporal dynamics would enhance exploration of weather- and climate-linked processes. Thus, correctly simulating TWSA is essential not only for water-resource management but also for assessing feedbacks to climate through land–atmosphere interactions. Here we evaluate simulated TWSA from 25 climate models (from phase 6 of the Climate Model Intercomparison Project) through comparison with TWSA from GRACE satellite data (2003–14) in 14 river basins globally and assess causes of discrepancies by examining precipitation (P), evapotranspiration (ET), and runoff (R off) fluxes during recharge (increasing TWS) and discharge (decreasing TWS) cycles. Most models show consistent biases in seasonal amplitudes of TWS anomalies relative to GRACE output: higher modeled amplitudes in river basins in high northern latitudes and the Parana and Congo basins, and lower amplitudes in most midlatitude basins and other tropical basins. This TWSA systematic bias also exists in the previous CMIP5 simulations. Models overestimate P compared to observed P datasets in 7 out of 14 basins, which increases (decreases) seasonal storage amplitude relative to GRACE in the recharge (discharge) cycle. Overestimation (underestimation) of runoff is another common contributing factor in the discharge phase that increases (decreases) TWSA amplitudes relative to GRACE in five river basins. The results provide a comprehensive assessment of the reliability of the simulated annual range in TWSA through comparison with GRACE data that can be used to guide future model development.

Open access
Shasha Shang, Gaofeng Zhu, Jianhui Wei, Yan Li, Kun Zhang, Ruolin Li, Joël Arnault, Zhenyu Zhang, Patrick Laux, Qianya Yang, Ningpeng Dong, Lu Gao, and Harald Kunstmann

Abstract

Precipitation in the Three-River Headwater (TRH) region has undergone significant changes as a result of global warming, which can affect water resources in downstream regions of Asia. However, the underlying mechanisms of the precipitation variability during the cold season (October–April) are still not fully understood. In this study, the daily China gridded precipitation product CN05.1 as well as the NCEP–NCAR reanalysis are used to investigate the characteristics of the cold season precipitation variability over the TRH region and associated atmospheric mechanisms. The cold season precipitation shows an increasing trend (5.5 mm decade−1) from 1961 to 2014, with a dry-to-wet shift in around the late 1980s. The results indicate that the increased precipitation is associated with the enhanced easterly anomalies over the Tibetan Plateau (TP) and enhanced southeasterly water vapor transport. The enhanced Walker circulations, caused by the gradients of sea surface temperature between the equatorial central-eastern Pacific and Indo–western Pacific in tropical oceans, resulted in strengthened easterly anomalies over the TP and the westward expansion of the anticyclone in the western North Pacific. Meanwhile, the changed Walker circulation is accompanied by a strengthened local Hadley circulation, which leads to enhanced meridional water vapor transport from tropical oceans and the South China Sea toward the TRH region. Furthermore, the strengthened East Asia subtropical westerly jet may contribute to the enhanced divergence at upper levels and anomalous ascending motion above the TRH region, leading to more precipitation.

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Jiamin Wang, Xiaodan Guan, Yuping Guan, Kaiwei Zhu, Rui Shi, Xiangning Kong, and Shuyang Guo

Abstract

As a result of global warming, the lengths of the four seasons, which are always taken as constant values, have experienced significant variations with rising temperature. Such changes play different roles with regard to regional climate change, with the most significant effect on drylands. To guarantee local crop yields and preserve ecosystems, identification of the changes of the four seasons in drylands is important. Our results show that, relative to humid lands, changing trends in lengths of spring, summer, and autumn were particularly enhanced in drylands of the Northern Hemisphere midlatitudes during 1951–2020. In this period, summer length has increased by 0.51 days per year, while spring and autumn lengths have both contracted by 0.14 days per year. However, the enhanced changes in drylands did not appear in winter length. The winter has shortened by 0.23 days per year in drylands. Such changes of spring, summer, and autumn in drylands are dominated by internal variability over the entire study period, with a stronger external forcing effect on drylands than on humid lands. In drylands, the external forcing contributed to the lengths of spring, summer, and autumn by 30.1%, 42.2%, and 29.4%, respectively. The external forcing has become an increasingly important component since 1990, with the ability to dominate all seasons in drylands after 2010. Nevertheless, only 1 of the 16 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) used in this study can capture the enhanced changes in the lengths of spring, summer, and autumn in drylands. Further investigation on the local effects of changes in seasons on agriculture and ecosystem would be needed, especially for the fragile regions.

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Linyuan Sun, Xiu-Qun Yang, Lingfeng Tao, Jiabei Fang, and Xuguang Sun

Abstract

El Niño–Southern Oscillation (ENSO) events, which generally mature in winter, profoundly affect the following summer rainfall in eastern China (ECSR), but such an impact can change significantly with decadal background. This study examines how the impact has changed since the 1950s by running correlation and regression analyses. It is found that ENSO’s impact on ECSR has undergone two decadal shifts, one in the late 1970s and the other in the 1990s. Sequentially, three distinct ENSO-induced ECSR anomaly patterns are categorized, which exhibit both robust and changeable sides. The robust side manifests generally as more precipitation in the Yangtze River basin affected by the anomalous tropical western North Pacific anticyclone (WNPAC) in the post–El Niño summer. The changeable side is reflected in the more variable ENSO-induced rainfall anomalies north of the Yangtze River, due to the different ENSO-induced East Asian midlatitude circulation anomalies. Meanwhile, the El Niño–induced drought in South China has been enhanced since the late 1970s with the intensification of the anomalous WNPAC. ENSO’s changing impact on the ECSR stems from the changes of ENSO-induced tropical and midlatitude circulation anomalies over East Asia, which are associated with different zonal (from the tropical Pacific to the Indian Ocean) and meridional (from the tropical Pacific to the midlatitude North Pacific) teleconnections of ENSO-induced SST anomalies. The former affects the intensity and location of the anomalous WNPAC by affecting the Indian Ocean capacitor effect and convection anomalies over the tropical Indo-western Pacific. The latter modulates the ocean-to-atmosphere feedback in the midlatitude North Pacific, contributes to different local geopotential anomaly sources, and then directly or indirectly through the Rossby wave train affects the East Asian midlatitude circulation.

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Xinfeng Liang, Chao Liu, Rui M. Ponte, and Don P. Chambers

Abstract

Ocean heat content (OHC) is key to estimating the energy imbalance of the Earth system. Over the past two decades, an increasing number of OHC studies were conducted using oceanic objective analysis (OA) products. Here we perform an intercomparison of OHC from eight OA products with a focus on their robust features and significant differences over the Argo period (2005–19), when the most reliable global-scale oceanic measurements are available. For the global ocean, robust warming in the upper 2000 m is confirmed. The 0–300-m layer shows the highest warming rate but is heavily modulated by interannual variability, particularly El Niño–Southern Oscillation. The 300–700- and 700–2000-m layers, on the other hand, show unabated warming. Regionally, the Southern Ocean and midlatitude North Atlantic show a substantial OHC increase, and the subpolar North Atlantic displays an OHC decrease. A few apparent differences in OHC among the examined OA products were identified. In particular, temporal means of a few OA products that incorporated other ocean measurements besides Argo show a global-scale cooling difference, which is likely related to the baseline climatology fields used to generate those products. Large differences also appear in the interannual variability in the Southern Ocean and in the long-term trends in the subpolar North Atlantic. These differences remind us of the possibility of product-dependent conclusions on OHC variations. Caution is therefore warranted when using merely one OA product to conduct OHC studies, particularly in regions and on time scales that display significant differences.

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Yue Wu, Xiao-Tong Zheng, Qi-Wei Sun, Yu Zhang, Yan Du, and Lin Liu

Abstract

Ocean salinity plays a crucial role in the upper-ocean stratification and local marine ecosystem. This study reveals that ocean salinity presents notable decadal variability in upper 200 m over the southeast Indian Ocean (SEIO). Previous studies linked this salinity variability with precipitation anomalies over the Indo-Pacific region modulated by the tropical Pacific decadal variability. Here we conduct a quantitative salinity budget analysis and show that, in contrast, oceanic advection, especially the anomalous meridional advection, plays a dominant role in modulating the SEIO salinity on the decadal time scale. The anomalous meridional advection is mainly associated with a zonal dipole pattern of sea level anomaly (SLA) in the south Indian Ocean (SIO). Specifically, positive and negative SLAs in the east and west of the SIO correspond to anomalous southward oceanic current, which transports much fresher seawater from the warm pool into the SEIO and thereby decreases the local upper-ocean salinity, and vice versa. Further investigation reveals that the local anomalous wind stress curl associated with tropical Pacific forcing is responsible for generating the sea level dipole pattern via oceanic Rossby wave adjustment on decadal time scale. This study highlights that the local ocean–atmosphere dynamical adjustment is critical for the decadal salinity variability in the SEIO.

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Meng Zuo, Wenmin Man, and Tianjun Zhou

Abstract

Both proxy data and climate modeling show divergent responses of global monsoon precipitation to volcanic eruptions. The reason is, however, unknown. Here, based on analysis of the Community Earth System Model (CESM) Last Millennium Ensemble simulation, we show evidence that the divergent responses are dominated by the pre-eruption background oceanic states. We found that under El Niño–Southern Oscillation (ENSO) neutral and warm-phase initial conditions, the Pacific favors an El Niño–like anomaly after volcanic eruptions, whereas La Niña–like SST anomalies tend to occur following eruptions under ENSO cold-phase initial conditions, especially after southern eruptions. The cold initial condition is associated with stronger upper-ocean temperature stratification and a shallower thermocline over the eastern Pacific Ocean than is normal. The easterly anomalies triggered by surface cooling over the tropical South American continent can generate changes in SST through anomalous advection and the ocean subsurface upwelling more efficiently, causing La Niña–like SST anomalies. Under a warm initial condition, in contrast, the easterly anomalies fail to develop and the westerly anomalies still play a dominant role, thus forming an El Niño–like SST anomaly. This SST response further regulates the monsoon precipitation changes through atmospheric teleconnection. The contribution of direct radiative forcing and indirect SST response to precipitation changes shows regional differences that will further affect the intensity and sign of precipitation response in submonsoon regions. Our results imply that attention should be paid to the background oceanic state when predicting the global monsoon precipitation responses to volcanic eruptions.

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Jacob Coburn and S. C. Pryor

Abstract

This work quantitatively evaluates the fidelity with which the northern annular mode (NAM), southern annular mode (SAM), Pacific–North American pattern (PNA), El Niño–Southern Oscillation (ENSO), Pacific decadal oscillation (PDO), Atlantic multidecadal oscillation (AMO), and the first-order mode interactions are represented in Earth system model (ESM) output from the CMIP6 archive. Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families, and specific ESM realizations relative to ERA5. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climate modes. Substantial variability in skill scores manifests across realizations from individual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM–PNA first-order interaction and underestimate the NAM–AMO connection. These results suggest that the choice of ESM and ESM realizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near term.

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Ruhua Zhang, Wenshou Tian, Xin He, Kai Qie, Di Liu, and Hongying Tian

Abstract

Using observation, reanalysis, and model datasets, the impact of El Niño–Southern Oscillation (ENSO) on winter precipitation in southern China is re-examined. The results show that positive correlation between ENSO and winter precipitation in southern China after 1995 is significantly higher than that before 1995. Significant positive correlation is located mainly over the southern coastal areas of China before 1995, whereas the positive correlation extends northward to the Yangtze River basin after 1995. These changes in the relationship between ENSO and winter precipitation are related to the ENSO pattern and Philippine anticyclone changes. An increasing trend is observed in ENSO amplitude, and the area with cooler sea surface temperature (SST) in the Philippine Sea extends westward after 1995 compared with that before 1995, leading to an extension of the anticyclone from the east side to the west side of the Philippines. The westward extension of anticyclone after 1995 could enhance the winter precipitation over southern China by modifying water vapor fluxes and vertical motion. Model results support the observational analyses of the changes in the ENSO–precipitation relationship and the corresponding mechanism. The mean SST changes could also modify the ENSO–precipitation relationship.

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Maialen Martija-Díez, Belén Rodríguez-Fonseca, and Jorge López-Parages

Abstract

In certain regions, such as Europe, the increase in global air temperatures in the world is translated into more frequent extreme events. Recent studies suggest that the increasing intensity in heatwaves seems to be related to the interannual variability of the mean temperature, a finding that motivates the search for its possible predictability. El Niño–Southern Oscillation (ENSO) is the principal predictor of global climate variability at interannual time scales. Its impact on European climate has been deeply studied in relation to rainfall variability, but only a few studies exist that focus on its impact on temperature. In this work, we focus on the analysis of the interannual variability of maximum and minimum temperatures in order to find some predictability and trends. To that end, we choose the western European region, which has experienced intense heatwaves and is also the main region of air temperature interannual variability in Europe. Our results indicate that the ENSO impact on temperatures over this region is nonlinear and nonstationary. We have found the way in which, during the decades prior to 1980s, the increase in temperatures is related to La Niña in summer and to El Niño in fall during the decades after the 1980s, which shows a change in the seasonality of the impact. We study the dynamical mechanisms involved, which suggest a circumglobal response for summer and an arching-like teleconnection pattern in fall. The aforementioned warmer conditions in western European temperatures are found to be significantly correlated to ENSO characteristics of previous seasons, which suggests a potential source for improving the seasonal forecast.

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