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Summer Westerly Jet in Northern Hemisphere During the Mid-Holocene: a Multi-Model Study

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Summer Westerly Jet in Northern Hemisphere During the Mid-Holocene: a Multi-Model Study atmosphere Article Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study Chuchu Xu 1, Mi Yan 1,2,3,*, Liang Ning 1,2,3,4 and Jian Liu 1,2,5 1 Key Laboratory for Virtual Geographic Environment, Ministry of Education, State Key Laboratory Cultivation Base of Geographical Environment Evolution of Jiangsu Province, Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, School of Geography, Nanjing Normal University, Nanjing 210023, China; [email protected] (C.X.); [email protected] (L.N.); [email protected] (J.L.) 2 Open Studio for the Simulation of Ocean-Climate-Isotope, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China 3 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China 4 Climate System Research Center, Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA 5 Jiangsu Provincial Key Laboratory for Numerical Simulation of Large Scale Complex Systems, School of Mathematical Science, Nanjing Normal University, Nanjing 210023, China * Correspondence: [email protected]; Tel.: +86-1860-252-4134 Received: 17 August 2020; Accepted: 25 October 2020; Published: 3 November 2020 Abstract: The upper-level jet stream, a narrow band of maximum wind speed in the mid-latitude westerlies, exerts a considerable influence on the global climate by modulating the transport and distribution of momentum, heat and moisture. In this study by using four high-resolution models in the Paleoclimate Modelling Intercomparison Project phase 3, the changes of position and intensity of the northern hemisphere westerly jet at 200 hPa in summer during the mid-Holocene (MH), as well as the related mechanisms, are investigated. The four models show similar performance on the westerly jet. At the hemispheric scale, the simulated westerly jet has a poleward shift during the MH compared to the preindustrial period. The warming in arctic and cooling in the tropics during the MH are caused by the orbital changes of the earth and the precipitation changes, and it could lead to the weakened meridional temperature gradient and pressure gradient, which might account for the poleward shift of the westerly jet from the thermodynamic perspective. From the dynamic perspective, two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the north deviation, which could cause the northward movement of the westerly jet. The weakening of the jet stream is associated with the change of the Hadley cell and the meridional temperature gradient. The largest weakening is over the Pacific Ocean where both the dynamic and the thermodynamic processes have weakening effects. The smallest weakening is over the Atlantic Ocean, and it is induced by the offset effects of dynamic processes and thermodynamic processes. The weakening over the Eurasia is mainly caused by the dynamic processes. Keywords: mid-latitude westerly jet stream; mid-Holocene; multi-model simulation; mechanisms 1. Introduction The westerlies are the planetary wind belts between the subtropical high-pressure belts and the subpolar low-pressure belts in the northern and southern hemispheres. As an important part of the circulation system, it exerts a considerable effect on the global climate by modulating the transportation and distribution of momentum, heat and moisture [1–4]. The upper-level jet stream is a narrow band Atmosphere 2020, 11, 1193; doi:10.3390/atmos11111193 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 1193 2 of 19 of maximum wind speed near the tropopause in the westerlies. Even the location and intensity of the jet stream change slightly, it will have a great impact on the climate in the middle latitudes [5,6]. For example, the meridional position and zonal position of the jet stream could affect the intensity and seasonal variation of the East Asia monsoon [7–10]. Besides, the meridional movement of the jet stream is also closely related to the interannual variation of precipitation in China [5,8,9,11]. The North Atlantic jet stream, in conjunction with the midlatitude transient eddies, is closely associated with the North Atlantic Oscillation [12,13]. Moreover, recent studies have shown that the jet stream has a greater impact on the climate in the Asia Pacific region than that of the El Nino-Southern Oscillation (ENSO) [14,15]. As the jet stream is relevant to the location of the Hadley cell, the tropopause and the transient-eddy activity, a deep study on the westerly jet stream will improve the understanding of the dynamics of the atmospheric general circulation and the associated climate changes [16–18]. Many researchers used reanalysis datasets to study the changes of upper-level jet streams in the past decades, and found that the jet stream gradually moved poleward, causing some anomalous weather activities, such as rainfall, typhoon and hail [8,19]. Therefore, it is necessary to study the response of the jet stream in different climate states and the related mechanisms, not limited to the modern climate background. Due to the increased insolation, the mid-Holocene (ca. 6000 years ago, 6 ka) is a typical warm period during the Holocene in northern hemisphere, different from the glacial periods [20–24]. The Greenland Ice Sheet and Antarctic Ice Sheet in the mid-Holocene have melted, and the topography and coastlines are similar to the preindustrial period [25]. Zhang et al. pointed out that the future climate is also very similar to that in the mid-Holocene based on 13 ocean-atmosphere coupled models in Paleoclimate Modelling Intercomparison Project (PMIP) [25]. In the PMIP simulations, during the mid-Holocene, the earth’s orbital configurations differed from that of preindustrial period, with perihelion in boreal summer/autumn (implying greater seasonality of insolation in the northern hemisphere) and greater obliquity, implying higher summer (and annual) insolation in high latitudes [26]. The concentration of greenhouse gases and nitrogen dioxide was similar to that in the preindustrial period, but the methane gas had a quite different content. Compared with the preindustrial period, the mid-Holocene simulations are forced by altered astronomical parameters as well as prescribed greenhouse gases. Ice sheets have already melted to their preindustrial extents, making this a good period for exploring post-glacial climate changes. Previous studies show that there are essentially two physical processes which could affect the upper-level jet streams: the thermodynamic mechanism caused by the tropical Hadley circulation [27], and the eddy-driven forcing resulting from the mid-latitude baroclinicity [28]. We can regard them as a thermodynamic factor and a dynamic factor. The meridional temperature gradient is considered to be one of the primary thermodynamic factors that steers the westerlies jet stream through the thermal wind relationship [9,29–33]. Because temperature is a fundamental factor, the movement of westerlies is connected to various temperature anomalies induced by climate variabilities, such as ENSO, tropical heating and the cooling of the troposphere bottom [34–36]. Besides, the baroclinicity can also cause the shift of jet streams, and the barocline anomalies derived from the changes of the South Asia high and the western Pacific subtropical high can generate the anomalies of the jet stream [8,36–40]. The jet stream over the East Asia simulated by the LASG/IAP coupled climate system model is assessed, and the mean state bias can be explained by the synoptic-scale transient eddy activity [41]. Studying the westerly jet stream based on the reconstruction and simulation data in modern times is insufficient to provide a complete climate state. Therefore, many researchers began to study the characteristics of the westerly jet during the Holocene [42], so as to improve the prediction ability of the westerly jet in the future. Observation and reanalysis suggest that entire extratropical climate zones are moving towards poles under climate change, affecting the westerly jets, storm tracks, cloud, precipitation and ocean circulation patterns [43–50]. In particular, this phenomenon is more prominent and zonally symmetric in the Southern Hemisphere [51]. By using pollen records, stalagmite and other reconstructed data, it is found that in Northern Hemisphere the mid-latitude westerly jet was Atmosphere 2020, 11, 1193 3 of 19 intensified in the early Holocene and weakened since the mid-Holocene, and shifted southward compared with the early Holocene [8,31–42,52]. Besides, previous studies also focused on the westerlies during the last glacial maximum (LGM) [50,53–56], a typical period of cold climate, some suggested that the jet stream had a significant equatorward shift caused by the thermal wind and changes in the midlatitude baroclinic instability [12,57,58], while others found a poleward shift [56,59]. For the mid-Holocene, a typical warm period, some studies have shown that the temperature gradient is the main factor influencing the westerly jet. How the westerly jet performs during this period needs to be clarified. In this study, based on the model simulations in the framework of the Coupled Model Inter-comparison Project phase 5 (CMIP5) or PMIP phase 3 (PMIP3), the northern hemisphere (NH) westerly jet in summer during the mid-Holocene is investigated, and the related mechanisms are analyzed. The remainder of this paper is organized
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