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Jindong Jiang, Jiuxin Shi, and Fei Huang

Abstract

Argo data were used to investigate interannual variability in the subduction rate of the Indian Ocean Subtropical Mode Water (IOSTMW). The IOSTMW subduction rate in the southwestern part of the southern Indian Ocean subtropical circulation varies substantially from year to year without significant trend during the Argo period 2005–2019. The variability of the IOSTMW subduction rate is quasi-biennial and dominated by the lateral induction term associated with the wintertime mixed layer depth (MLD). The contribution of vertical pumping to the change of subduction rate is relatively small. This variability in the subduction rate directly contributes to IOSTMW volume with a quasi-biennial variation. The quasi-biennial variations in the IOSTMW subduction and volume reflect the variability of the overlying atmosphere. The wintertime Mascarene High (MH) modulates the winter MLD in the subduction area through changes in heat fluxes and wind forcing. The quasi-biennial variability in MH is associated with the Southern Annular Mode, but it was disturbed during two El Niño events in 2009 and 2015–2016. Variability of the wintertime MH plays a key role in driving the quasi-biennial variability of the IOSTMW subduction during the Argo period.

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Yi Shi, Zhihong Jiang, Zhengyu Liu, and Laurent Li

Abstract

The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) platform is used to simulate Lagrangian trajectories of air parcels in East China during the summer monsoon. The investigation includes four distinct stages of the East Asian summer monsoon (EASM) during its seasonal migration from south to north. Correspondingly, the main water vapor channel migrates from the west Pacific Ocean (PO) for the premonsoon in South China (SC) to the Indian Ocean (IO) for the monsoon in SC and in the Yangtze–Huaihe River basin, and finally back to the PO for the terminal stage of monsoon in North China. Further calculations permit us to determine water vapor source regions and water vapor contribution to precipitation in East China. To a large extent, moisture leading to precipitation does not come from the strongest water vapor pathways. For example, the proportions of trajectories from the IO are larger than 25% all of the time, but moisture contributions to actual precipitation are smaller than 10%. This can be explained by the large amount of water vapor lost in the pathways across moisture-losing areas such as the Indian and Indochina Peninsulas. Local water vapor recycling inside East China (EC) contributes significantly to regional precipitation, with contributions mostly over 30%, although the trajectory proportions from subregions in EC are all under 10%. This contribution rate can even exceed 55% for the terminal stage of the monsoon in North China. Such a result provides important guidance to understand the role of land surface conditions in modulating rainfall in North China.

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Nanxuan Jiang, Qing Yan, Zhiqing Xu, Jian Shi, and Ran Zhang

Abstract

To advance our knowledge of the response of midlatitude westerlies to various external forcings, we investigate the meridional shift of midlatitude westerlies over arid central Asia (ACA) during the past 21 000 years, which experienced more varied forcings than the present day based on a set of transient simulations. Our results suggest that the evolution of midlatitude westerlies over ACA and driving factors vary with time and across seasons. In spring, the location of midlatitude westerlies over ACA oscillates largely during the last deglaciation, driven by meltwater fluxes and continental ice sheets, and then shows a long-term equatorward shift during the Holocene controlled by orbital insolation. In summer, orbital insolation dominates the meridional shift of midlatitude westerlies, with poleward and equatorward migration during the last deglaciation and the Holocene, respectively. From a thermodynamic perspective, variations in zonal winds are linked with the meridional temperature gradient based on the thermal wind relationship. From a dynamic perspective, variations in midlatitude westerlies are mainly induced by anomalous sea surface temperatures over the Indian Ocean through the Matsuno–Gill response and over the North Atlantic Ocean by the propagation of Rossby waves, or both, but their relative importance varies across forcings. Additionally, the modeled meridional shift of midlatitude westerlies is broadly consistent with geological evidence, although model–data discrepancies still exist. Overall, our study provides a possible scenario for a meridional shift of midlatitude westerlies over ACA in response to various external forcings during the past 21 000 years and highlights important roles of both the Indian Ocean and the North Atlantic Ocean in regulating Asian westerlies, which may shed light on the behavior of westerlies in the future.

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Jiawen Shi, Dabang Jiang, Zhiping Tian, and Xianmei Lang

Abstract

Using all relevant climate experiments archived in the Paleoclimate Modelling Intercomparison Project Phase 3/4 (PMIP3/4), we examine the interannual variability change in global-scale surface air temperature and associated physical mechanisms during the Last Glacial Maximum (LGM). The results show that relative to the preindustrial period, the LGM interannual temperature variability increased by 20% globally, which was mainly attributed to the large-scale increase in the meridional temperature gradient, especially at midlatitudes. Larger magnitudes of change occurred in areas where the underlying surface properties, such as the surface altitude, land–sea distribution, and ice sheet extent, differed from those in the preindustrial period. In addition, the relationship between changes in the meridional temperature gradient and the interannual temperature variability became closer in the winter hemisphere. In the tropical land regions, changes in interannual temperature variability are mainly related to the adjustment of latent and sensible heat fluxes during the LGM.

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Lifen Jiang, Yaner Yan, Oleksandra Hararuk, Nathaniel Mikle, Jianyang Xia, Zheng Shi, Jerry Tjiputra, Tongwen Wu, and Yiqi Luo

Abstract

Model intercomparisons and evaluations against observations are essential for better understanding of models’ performance and for identifying the sources of uncertainty in their output. The terrestrial vegetation carbon simulated by 11 Earth system models (ESMs) involved in phase 5 of the Coupled Model Intercomparison Project (CMIP5) was evaluated in this study. The simulated vegetation carbon was compared at three distinct spatial scales (grid, biome, and global) among models and against the observations (an updated database from Olson et al.’s “Major World Ecosystem Complexes Ranked by Carbon in Live Vegetation: A Database”). Moreover, the underlying causes of the differences in the models’ predictions were explored. Model–data fit at the grid scale was poor but greatly improved at the biome scale. Large intermodel variability was pronounced in the tropical and boreal regions, where total vegetation carbon stocks were high. While 8 out of 11 ESMs reproduced the global vegetation carbon to within 20% uncertainty of the observational estimate (560 ± 112 Pg C), the simulated global totals varied nearly threefold between the models. The goodness of fit of ESMs in simulating vegetation carbon depended strongly on the spatial scales. Sixty-three percent of the variability in contemporary global vegetation carbon stocks across ESMs could be explained by differences in vegetation carbon residence time across ESMs (P < 0.01). The analysis indicated that ESMs’ performance of vegetation carbon predictions can be substantially improved through better representation of plant longevity (i.e., carbon residence time) and its respective spatial distributions.

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