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 as follows. The model and methods are described in Section2. The position and intensity of NH summer westerly jet in the mid-Holocene are shown in Section3. The thermodynamic and dynamic mechanisms of the westerly-jet changes are discussed in Section4. Conclusions are provided in Section5.

2. Model, Data and Methods

2.1. Model and Experiment The results derived from the high horizontal resolution models are shown to be more accurate than those from low resolution ones [60]. Therefore, four models participated in CMIP5/PMIP3 with the resolution higher than 2◦ are used in this study, including CCSM4 from the National Center for Atmospheric Research of United States, CNRM-CM5 from the National Centre for Meteorological Research of France, MPI-ESM-P from the Max Planck Institute for Meteorology of Germany, and MRI-CGCM3 from the Meteorological Research Institute of Japan (Table1). In order to obtain the multi-model mean results (MMM), we interpolate the results of the four models into the 0.5◦ (latitude) 0.5 (longitude) grids using bilinear interpolation. The variables of monthly mean zonal × ◦ and meridional wind, surface temperature, geopotential height, sea level pressure, and vertical velocity are used to analyze the changes of the NH summer westerly jet and the associated physical processes.

Table 1. Basic information about the four CMIP5/PMIP3 models and the variables used in this study.

AGCM Model Years Used in Years Used in Variables Used in Country Resolution Identiﬁer Analysis (0 ka) Analysis (6 ka) Analysis Lat Lon Grids × CCSM4 United States 288 192 Last 100 100 ua va tas zg tos wap × CNRM-CM5 France 256 128 Last 100 100 ua va tas zg tos wap × MPI-ESM-P Germany 196 98 Last 100 100 ua va tas zg tos wap × MRI-CGCM3 Japan 320 160 Last 100 100 ua va tas zg tos wap ×

Two experiments are used in this study, including the mid-Holocene experiment (MH) and the preindustrial control experiment (PI). In order to be consistent in the length of time, only the last 100 years of PI are used. The boundary conditions (the orbital parameters, the trace gases and so on) are listed in Table2. During the mid-Holocene, the orbital parameters are quite di ﬀerent from the preindustrial period, resulting in the changed insolation, particularly in mid-high latitudes in summer (Figure1). Atmosphere 2020, 5, x FOR PEER REVIEW 4 of 18

Table 2. Comparison of the boundary conditions for the CMIP5/PMIP3 preindustrial and mid- Holocene experiments.

Land- Angular CO2 CH4 N2O Experiment Obliquity Eccentricity Sea Precession(°) (ppmv) (ppbv) (ppbv) Mask AtmospherePreindustrial2020, 11 , 119323.446 0.016724 102.04 280 760 270 Modern4 of 19 Mid- Same as 24.105 0.018682 0.87 280 650 270 HoloceneTable 2. Comparison of the boundary conditions for the CMIP5/PMIP3 preindustrial andPI mid-Holocene experiments. Furthermore, compared between the mid-Holocene and the pre-industrial period, the methane Angular Precession CO CH N O Land-Sea concentrationExperiment has decrease Obliquityd slightly, Eccentricity but the concentrations of the carbon2 dioxide4 and2 the nitric oxide ( ) (ppmv) (ppbv) (ppbv) Mask have not changed. The topography and coastline◦ s are the same as those in the modern times. Preindustrial 23.446 0.016724 102.04 280 760 270 Modern Therefore, the MH provides the opportunity to test the effects of orbital-induced insolation on the Mid-Holocene 24.105 0.018682 0.87 280 650 270 Same as PI climate.

. Figure 1. Change of the insolation during the mid-Holocene compared to the preindustrial period. FigureThe insolation 1. Change during of the the insolation preindustrial during period the m isid plotted-Holocene in contours, compared and to the the di preindustrialfference between period the. The insolation during the preindustrial period is plotted in contours, and the2 difference between the mid-Holocene and the preindustrial period is color-shaded (units in W m− ). mid-Holocene and the preindustrial period is color-shaded (units in W m−2). More information about the two experiments can be found on the PMIP3 website (Figure1)[61] 2.2.and Method in Braconnot et al. [62–64]. TheFurthermore, definition comparedmethod of betweenRojas [65 the], although mid-Holocene it was initially and the pre-industrialused to define period,the axis the of westerly methane jetconcentration in Southern has Hemisphere decreased, slightly,is used but to definethe concentrations the axis of ofthe the westerly carbon jetdioxide and theand location the nitric of oxide the westerlyhave not jet, changed. i.e., the The locations topography of maximum and coastlines wind arespeed the sameat each as longitude those in the are modern linked times. up to Therefore,form the westerliesthe MH provides axis. However, the opportunity the westerly to test wind the einffects the ofNorthern orbital-induced Hemisphere insolation is less onzonally the climate. symmetric due to the land–sea distribution. To obtain the position and strength of the NH westerlies, we identify 2.2. Method the latitude/longitude of maximum wind speed after the meridional/zonal running average at each longitudeThe definition/latitude, and method the relative of Rojas maximum [65], although wind it speed, was initially as the position used to defineand intensity the axis of of westerly westerly jetjet stream in Southern, respectively Hemisphere,. is used to define the axis of the westerly jet and the location of the westerlySince jet, there i.e., are the prominent locations ofanomalies maximum over wind 140 speed° E and at 140 each° W, longitude we divide are the linked NH upinto to three form key the regions,westerlies i.e. axis., Eurasia However, (30° E– the120 westerly° E), Pacific wind (130 in° theE–120 Northern° W), and Hemisphere North America is less and zonally North symmetric Atlantic (1due20° to W the–30 land-sea° W). Because distribution. the jet axis To obtainis a line the with position directivity, and strength in order of to the better NH westerlies,describe the we variation identify ofthe the latitude westerly/longitude jet over of the maximum three regions, wind speed the variation after the ofmeridional the jet center/zonal is running used to average represent at eachthe variationlongitude of/latitude, the jet stream. and the relative maximum wind speed, as the position and intensity of westerly jet stream,In the respectively. mid-latitude regions, the prevailing westerlies are active in the troposphere, including the low-levelSince jet there stream are at prominent 850 hPa and anomalies the upper over-level 140 jet◦ E stream and 140 at◦ 200W, hPa. we divide Unlike the the NH low into-level three jet, the key regions, i.e., Eurasia (30◦ E–120◦ E), Pacific (130◦ E–120◦ W), and North America and North Atlantic (120◦ W–30◦ W). Because the jet axis is a line with directivity, in order to better describe the variation of the westerly jet over the three regions, the variation of the jet center is used to represent the variation of the jet stream. In the mid-latitude regions, the prevailing westerlies are active in the troposphere, including the low-level jet stream at 850 hPa and the upper-level jet stream at 200 hPa. Unlike the low-level jet, Atmosphere 2020, 11, 1193 5 of 19 Atmosphere 2020, 5, x FOR PEER REVIEW 5 of 18 upperthe upper-level-level jet is jet not is affected not affected by topographical by topographical factors. factors. Therefore, Therefore, we we will will focus focus on on the the upper upper-level-level westerlywesterly jet jet at at 200 200 hPa hPa in in this this study. study. InIn addition, addition, t too assess assess whether therethere areare significantsignificant di differencesfferences between between MH MH and and PI, PI, Student Student t-tests t- testweres were applied applied to the to variablesthe variables (ua, (ua, va, tas,va, zg,tas, wap,zg, wap ta and, ta and so on). so on). The The t-test t-test is applied is applied to not to onlynot only each eachindividual individual model model but also but MMM. also MMM. That is That to say, is the to resultssay, the derived results from derived MMM from is tested MMM individually is tested individuallyusing its own using 100-year its own time 100 series.-year time These series. statistically These significantstatistically values significant are presented values are as presented “dots” in as the “dotsfigures” in with the figures 95% confidence with 95% level. confidence level.

3.3. Results Results AccordingAccording to to the the simulated simulated results results shown shown in in Figure Figure 2,2, the the axis axis of of the the jet jet stream stream in in the the NH NH is is locatedlocated near near 40º 40 ºNN and and spanning spanning the the entire entire hemisphere. hemisphere. Figure Figure 2a2a shows shows the the simulation simulation results results based based onon the the ensemble ensemble mean mean of of the the four four models. models. Compared Compared with with the the PI PI period period (Figure (Figure 2f)2f),, the the jet jet axis axis during during thethe MH MH shifts shifts northward northward in in East East Asia Asia and and the the Pacific Pacific region region near near 30 30° ◦NN where where the the zonal zonal wind wind is is weakened.weakened. The The jet jet axis axis in in the the west west part part of of North North America America moves moves slightly slightly northwa northward,rd, but but the the jet jet axis axis inin the the east east North North America America and and North North Atlantic Atlantic region region features features a smaller a smaller degree degree of shifting. of shifting. During During the midthe-Holocene, mid-Holocene, the upper the upper-level-level zonal zonal wind wind is significantly is significantly weakened weakened in in Africa, Africa, the the middle middle-low-low latitudeslatitudes of of Eurasia Eurasia,, the the mid middle-latitudedle-latitude Pacific Pacific and andpart partof the of Atlantic the Atlantic regions, regions, while whilethe zonal the wind zonal iswind apparently is apparently strengthened strengthened in Europe, in Europe,the middle the and middle high andlatitudes high of latitudes the Asian of continent the Asian, the continent, north Pacificthe north and Pacific western and North western America. North It America. is thus clear It is thusthat clearin areas that where in areas the where jet axis the obvi jet axisously obviously moves northward,moves northward, the westerly the westerly wind on wind the on south the southside is side weakened, is weakened, whereas whereas that onthat the on north the north side side is enhanced.is enhanced. For For example, example, in Eurasia, in Eurasia, the the westerly westerly wind wind to tothe the south south of of the the jet jet axis axis is is significantly significantly weakened,weakened, whereas whereas that that to to the the north north of of the the jet jet axis axis is is significantly significantly strengthened, strengthened, especially especially over over the the EastEast Asia Asia (Figure (Figure 2a).2a).

(a) MMM (b) MRI

(e) CNRM (f) PI

FigureFigure 2. 2. ChangesChanges (color (color shading) shading) of the of the200 200-hPa-hPa zonal zonal wind wind (mid (mid-Holocene-Holocene minus minus preindustrial preindustrial);); (a) refers(a) refers to the to arithmetic the arithmetic multi multi-model-model mean mean;; (b– (eb)– corresponde) correspond to the to the four four models models,, respectively respectively.. The The red red dasheddashed line line and and the the green green solid line are thethe indicatorsindicators ofof thethe jet jet axis axis in in the the preindustrial preindustrial period period and and the themid-Holocene, mid-Holocene, respectively. respectively. (f) ( Thef) The climatology climatology of of zonal zonal wind win ind in PI PI at at 200-hPa. 200-hPa. The The stippled stippled areas areas in in(a (–ae)–)( indicatee) indicate that that the the diff differenceerence is statistically is statistically significant significant at the at the confidence confidence level level of 95%. of 95%.

The results from each individual model all show the negative anomalies of the zonal wind during the mid-Holocene over the western part of Africa, the southern part of Eurasia, the Pacific

Atmosphere 2020, 11, 1193 6 of 19

The results from each individual model all show the negative anomalies of the zonal wind during the mid-Holocene over the western part of Africa, the southern part of Eurasia, the Pacific region near 40◦ N, the eastern tropical Pacific and the northwestern Atlantic, whereas positive anomalies are found in middle-latitude Eurasia, the Pacific region near 30◦ N and the eastern Atlantic near 30◦ N (Figure2b–e). However, there are some di fferences among the four models. In MPI-ESM-P, the center of the positive anomaly is located in the western Eurasia; in the other three models, the center of the positive anomaly is located in the eastern Eurasia. The positive and negative anomalies over America and the Atlantic regions are smaller in CCSM4 and CNRM-CM5 than those in MRI-CGCM3 and MPI-ESM-P. Additionally, in CCSM4, there are no significant negative anomalies in North Africa (Figure2d). Regarding to the movement of the jet axis, the results of MMM indicate that the westerly jet stream generally has a poleward shift (Figure2a). The poleward shift over the Pacific region is the most obvious in all of the four models, while the shift over America and the Atlantic region is not so notable. The northward shift over Eurasia seems larger in MRI-CGCM3 than in the other three models. To quantitatively reflect the changes of the westerly jet stream during the mid-Holocene compared to the PI period, we have calculated the differences between the two periods in terms of latitudinal position, longitudinal position and intensity, and the results are shown in Figure3. The changes in the three regions are compared, i.e., Eurasia, Pacific, and North America and North Atlantic. Note that the changes derived from the MMM in Figure3 are the quantified results from Figure2a, instead of the arithmetic average of the changes derived from each individual model. Considering the change of the latitudinal position, the results of MMM show that the latitudinal deviation is the largest in the Pacific region, which is about 1.4◦ to the north, followed by Eurasia, where the westerly jet stream moves 1.2◦ northward. The westerly jet stream over the North Atlantic is characterized by the smallest change with only 0.5◦ northward shifting (Figure3a). Although there are some differences of amplitude among each individual model, the simulation results of all the models imply that the upper-level westerly jet stream has moved northward, with the deviation degree over the Pacific being relatively large and that over the North Atlantic being comparatively small. Over the Pacific, there are great differences between the simulation results of the four models. The changes of the latitudinal position of the westerly jet stream vary from the largest of 4.4◦ in MPI-ESM-P to the smallest of 0.8◦ in MRI-CGCM3. Over Eurasia, the differences between the models are relatively small. The changes range from 1.8◦ in MRI-CGCM3 to less than 0.1◦ in CNRM-CM5. For the upper-level westerly jet stream over the North Atlantic, the differences among the four models are smaller, with the deviation degree ranging from 0.8◦ to 0.4◦ (Figure3a). For the longitudinal position, the MMM of the simulated upper-level jet shows much large shift. The upper-level jet over Eurasia and the North Atlantic moves 8◦ and 6◦ westward, respectively. The jet stream over the Pacific region moves 4◦ to the east (Figure3b). Despite the di fferences of magnitude among the four models, the simulated results of all models indicate the shifts of the jet stream in the same direction. The largest model difference remains over the Pacific region. The shift is the largest in CCSM4 (12◦), whereas the difference among the other three models is relatively small (less than 2◦). Over Eurasia, the westward shift spans from 11◦ in MPI-ESM-P to 7.5◦ in CCSM4. In North America and North Atlantic, the largest deviation degree is found in CCSM4, the simulated result of which shows that the jet stream moves 7◦ westward, and the smallest deviation degree (3.5◦) is found in CNRM-CM5. Atmosphere 2020, 5, x FOR PEER REVIEW 6 of 18 region near 40° N, the eastern tropical Pacific and the northwestern Atlantic, whereas positive anomalies are found in middle-latitude Eurasia, the Pacific region near 30° N and the eastern Atlantic near 30° N (Figure 2b–2e). However, there are some differences among the four models. In MPI-ESM- P, the center of the positive anomaly is located in the western Eurasia; in the other three models, the center of the positive anomaly is located in the eastern Eurasia. The positive and negative anomalies over America and the Atlantic regions are smaller in CCSM4 and CNRM-CM5 than those in MRI- CGCM3 and MPI-ESM-P. Additionally, in CCSM4, there are no significant negative anomalies in North Africa (Figure 2d). Regarding to the movement of the jet axis, the results of MMM indicate that the westerly jet stream generally has a poleward shift (Figure 2a). The poleward shift over the Pacific region is the most obvious in all of the four models, while the shift over America and the Atlantic region is not so notable. The northward shift over Eurasia seems larger in MRI-CGCM3 than in the other three models. To quantitatively reflect the changes of the westerly jet stream during the mid-Holocene compared to the PI period, we have calculated the differences between the two periods in terms of latitudinal position, longitudinal position and intensity, and the results are shown in Figure 3. The changes in the three regions are compared, i.e., Eurasia, Pacific, and North America and North Atlantic.Atmosphere 2020Note, 11 that, 1193 the changes derived from the MMM in Figure 3 are the quantified results 7from of 19 Figure 2a, instead of the arithmetic average of the changes derived from each individual model.

(a) (b)

(c)

Figure 3. 3. (a(,ab,)b Location) Location (unit: (unit: degree) degree) and and(c) intensity (c) intensity (unit: (unit: m/s) anomalies m/s) anomalies of the westerly of the westerly jet at 200 jet hPa at over200 hPa three over key three regions key— regions—thethe Eurasia, Eurasia, the North the NorthPacific, Pacific, and the and North the North Atlantic, Atlantic, respectively. respectively. The positiveThe positive value valuess in (a in) refer (a) refer to the to northward the northward movement movement of the of thecenter center of the of thejet stream; jet stream; in ( inb) (referb) refer to theto the eastward eastward movement movement of the of thecenter center of the of jet the stream; jet stream; in (c in) refer (c) refer to the to intensified the intensified jet stream. jet stream. The valuesThe values which which are significant are significant at 95% at 95%confidence confidence level level are marked are marked with with dots dots..

In terms of intensity, according to the MMM, the upper-level westerly jet stream is weaker during the mid-Holocene than during the preindustrial period. The zonal wind speed shows the largest weakening in the Pacific region with 6 m/s, followed by that in the Eurasian region, which is 4 m/s. − − The zonal wind speed in the Atlantic, which is less than 1 m/s (Figure3c), is of the least anomaly. − The weakening of the upper-level westerly jet, with the largest attenuation in the Pacific and the smallest attenuation in the Atlantic, is similar to the changes in the latitudinal position of the jet stream. The Pacific region still shows large model differences. The weakening ranges from 7.5 m/s in MRI-CGCM3 to 3 m/s in CCSM4. The variation of the wind speed over the Eurasia is relatively small, which is ranging from 3.5 m/s in CCSM4 to 1.5 m/s in CNRM-CM5. For the upper-level westerly jet stream over the Atlantic, the difference among the four models is comparatively the smallest, with a variation range within 1 m/s.

4. Mechanism Analysis Previous studies have shown that the latitudinal position of the jet stream is affected by the meridional uneven heat transport [31,36,66], while the east-west movement of the jet is affected by the diabatic heating of the troposphere [6,10]. For the East Asian westerly jet, it has the westward movement due to the diabatic heating of the troposphere from the Tibetan plateau in summer [6,7]. It is also found that the northward shift of the westerly jet is accompanied by the westward movement [8]. Atmosphere 2020, 5, x FOR PEER REVIEW 8 of 18 oceanAtmosphere at the2020 mid, 11,dle 1193-high latitudes tends to rise by about 0.5° C. The changes of the temperature 8over of 19 the land is mainly found in the middle-latitude Eurasia and the middle-high latitudes of the North American continent, with the temperature rising by over 1.5° C, which is consist with the reconstructionsEarlier studies also[23,3 suggested1,68,69]. However, that the over westerly the mid jet is-low closely latitudes related such to as the the transport south China of momentum and most ofand India vorticity, and Africa, and itsthe formationtemperature is relatedis decreased to sea-land (Figure thermal4a). The contrastdecreased and temperature topography might [66, 67be]. causedThe simulation by the release results ofin latent this studyheat induced show that by the enhanced westerly jet monsoon moves northwardprecipitation. inthree Wu and key Liu regions. [69] confirmedOver the Eurasian that during continent the mid and-Holocene, the Atlantic the theprecipitation jet stream in moves the north westward, of East but Asia over was the decreasing, Pacific the whilewesterly that jet in stream the south moves was eastward. increasing. We will Sun discuss et al. the also underlying found the mechanisms precipitation in the increases following. most significantlySolar radiation in the North is the African direct cause[70]. Moreov of temperatureer, the reconstruction rise in the NH data during also indicated summer. a During decline theof temperaturemid-Holocene, in solar the radiation middle and increased low latitudes in the NH during because the of the mid changes-Holocene in earth [71,7 orbital2]. The parameters. 200-hPa geopotentialEspecially in height the middle anomaly and high(Figure latitudes 4b) also (Figure shows1), that the summerthere is a temperature significant difference increased significantly,between the southwith aand concurrent north regions, large pressure which is change consistent (Figure with4). the surface temperature anomaly.

(a)

(b)

FigureFigure 4. 4. TheThe difference differencess ofof thethe simulatedsimulated ( a()a surface) surface temperature temperature and and (b )(b geopotential) geopotential height height at 200 at 200 hPa hPabetween between MH MH and and PI. ThePI. The stippled stippled areas areas indicate indicate that that the the diff erencedifference is statistically is statistically significant significant at theat theconfidence confidence level level of 95%.of 95%.

ItIn is general, generally based found on that the MMM,there are during two physical the mid-Holocene, processes related the surface to the temperature change of the to thewesterly north jetof: 30the◦ Nthermal in the f NHorcing increases mechanism by more and thanthe eddy 1◦ C,-driven especially forcing over mechanis the land.m [2 The7,28 temperature]. The following over discussionthe ocean atof thethe middle-highmechanisms latitudesabout the tends change to of rise the by westerly about 0.5 jet◦ willC. The be changesbased on of these the temperaturetwo factors. Toverhe meridional the land is temperature mainly found gradient in the is middle-latitude considered as one Eurasia of the andprimary the middle-highthermodynamic latitudes factors of that the steersNorth the American westerly continent, jet. The dynamical with the effect temperature is related rising to the by anomaly over 1.5 of◦ transientC, which eddy is consist activit withies. Ren the hasreconstructions pointed out [23 that,31 ,the68, 69 dynamical]. However, connection over the mid-low between latitudes the zonal such wind as the and south the eddy China is and closely most connectedof India and with Africa, the location the temperature and intensity is decreasedvariations of (Figure the westerly4a). The jet decreased [72]. temperature might be caused by the release of latent heat induced by the enhanced monsoon precipitation. Wu and 4.1.Liu Positional [69] conﬁrmed Shifting that during the mid-Holocene, the precipitation in the north of East Asia was decreasing, while that in the south was increasing. Sun et al. also found the precipitation increases The changes in the position of the axis of the jet stream include north-south movement and east- most signiﬁcantly in the North African [70]. Moreover, the reconstruction data also indicated a west movement. The reasons of the east-west movement may be the small-scale events such as the decline of temperature in the middle and low latitudes during the mid-Holocene [71,72]. The 200-hPa

Atmosphere 2020, 11, 1193 9 of 19 geopotential height anomaly (Figure4b) also shows that there is a significant di fference between the south and north regions, which is consistent with the surface temperature anomaly. It is generally found that there are two physical processes related to the change of the westerly jet: the thermal forcing mechanism and the eddy-driven forcing mechanism [27,28]. The following discussion of the mechanisms about the change of the westerly jet will be based on these two factors. The meridional temperature gradient is considered as one of the primary thermodynamic factors that steers the westerly jet. The dynamical effect is related to the anomaly of transient eddy activities. Ren has pointed out that the dynamical connection between the zonal wind and the eddy is closely connected with the location and intensity variations of the westerly jet [72].

4.1. Positional Shifting The changes in the position of the axis of the jet stream include north-south movement and east-west movement. The reasons of the east-west movement may be the small-scale events such as the thermal eﬀect of the Qinghai Tibet Plateau and the onset time of the South China Sea summer monsoon. On the other hand, the northward movement of the jet stream is often accompanied by the westward movement, so we mainly focus on the mechanism of north-south movement.

4.1.1. Thermal Forcing Figure3a shows the northward shift of the westerly jet stream. The degree of deviation in the Pacific region is the largest, that in Eurasia is the second, and that in North America is the smallest. The jet stream has seasonal north-south shift following the seasonal movement of the general circulation, such as the Hadley circulation. According to the simulation results of the ensemble average, the associated stream function also presents significant northward-shift of the Hadley circulation in the middle and upper levels, which is consistent with the northward shift of the jet stream (figure omitted). Wang et al. [59] found that the NH upper-level westerly jet also moved northward during the LGM. They suggested that the upper tropospheric cooling in the tropics, possibly due to reduced latent heat release, was expected to account for the poleward shift of the 200-hPa jet through the thermal wind relationship. Figure5 displays the temperature anomalies between MH and PI at 850 hPa, 500 hPa and 200 hPa. It is shown that the low-latitude temperature in the upper level of the troposphere is declined while the high-latitude temperature is increased from lower to upper troposphere, and thus the meridional temperature gradient at high levels decreased. Therefore, the upper-level jet stream moved northward. Since the zonal wind connects temperature field through the thermal wind relationship, the westerly jet stream is located at the latitude where the thermal wind is at a maximum. The change of meridional temperature gradient directly affects the change of the meridional pressure, and thus affects the latitudinal position of the westerly jet. In this study, during the mid-Holocene, the low-latitude temperature is decreased while the high-latitude temperature is increased in three key regions at lower to upper levels (Figure5), leading to weakened meridional temperature gradient. The weakening of the temperature gradient in the upper-level might be an important factor leading to the northward shift of the upper-level westerly jet stream via pressure gradient. The distribution of meridional pressure gradient indicates that the NH westerly jet is located in the region with the largest meridional pressure gradient (Figure6a). According to the change of the meridional pressure gradient (Figure6c), with 50 ◦ N as the boundary, the pressure gradient on the south side of the mid-latitude Eurasia and the Central Pacific is characterized by a positive anomaly (i.e., the negative pressure gradient is weakened), and the pressure gradient on the north side is characterized by a negative anomaly (i.e., the negative pressure gradient is strengthened), driving the westerly jet stream over the Eurasia and the Pacific to move northward. The deviation over the Eurasia is larger than over the Pacific. Over North America and North Atlantic, although the negative meridional pressure gradient on the south side is weakened, the enhanced negative meridional pressure gradient on the north side is also conducive to the northward movement of the jet Atmosphere 2020, 5, x FOR PEER REVIEW 9 of 18 thermal effect of the Qinghai Tibet Plateau and the onset time of the South China Sea summer monsoon. On the other hand, the northward movement of the jet stream is often accompanied by the westward movement, so we mainly focus on the mechanism of north-south movement.

4.1.1. Thermal Forcing Figure 3a shows the northward shift of the westerly jet stream. The degree of deviation in the Pacific region is the largest, that in Eurasia is the second, and that in North America is the smallest. The jet stream has seasonal north-south shift following the seasonal movement of the general circulationAtmosphere 2020, such, 11, 1193 as the Hadley circulation. According to the simulation results of the ensemble10 of 19 average, the associated stream function also presents significant northward-shift of the Hadley circulation in the middle and upper levels, which is consistent with the northward shift of the jet streastream.m (figure However, omitted). because of the smaller negative meridional pressure gradient in the northeastern partWang of North et al. America [59] found and that North the Atlantic,NH upper the-level northward westerly movement jet also moved of the northward pressure gradientduring the is LGMsuppressed,. They suggested thereby reducing that the the upper northward tropospheric movement cooling of the in westerlythe tropics, jet stream possibly over due these to reduced regions latent(Figure heat3a). release, As a result, was theexpected simulated to account jet stream for the over poleward the North shift America of the has 200- thehPa least jet through northward the thermalmovement. wind For relationship. the individual Figure models, 5 displays all of whichthe temperatu show consistentre anomalies northward between shift MH of theand jet PI stream,at 850 hPa,also show 500 hPa similar and meridional200 hPa. It pressureis shown gradient that the changeslow-latitude but with temperature different amplitudes.in the upper The level di ffoferent the troposphereamplitudes haveis declined well explained while the the modelhigh-latitude difference temperature of position is changes. increased For from example, lower the to changes upper troposphereof the meridional, and thus pressure the meridional gradient over temperature the eastern gradient Eurasian at high andlevels the Pacific decreased regions. Therefore, show larger the upperamplitudes-level jet in MRI-CGCM3stream movedand northward. CCSM4 Since than thosethe zonal in MPI-ESM-P wind connects and temperature CNRM-CM5 field (Figures through not theshown). thermal This wind is consistent relationship, with the thewesterly changes jet of stream the latitudinalis located position at the latitude of the westerlywhere the jet thermal stream, windwhichisshow at a maximum. larger northwardThe change shift of overmeridional the Pacific temperature region in gradient MRI-CGCM3 directly and affects CCSM4 the change (Figures of thenotmeridional shown). pressure, and thus affects the latitudinal position of the westerly jet.

(a)

(b)

(c)

Figure 5. The temperature anomalies between MH and PI at (a) 850 hPa, (b) 500 hPa and (c) 200 hPa, respectively. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%. Atmosphere 2020, 5, x FOR PEER REVIEW 10 of 18

Figure 5. The temperature anomalies between MH and PI at (a) 850 hPa, (b) 500 hPa and (c) 200 hPa, respectively. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.

In this study, during the mid-Holocene, the low-latitude temperature is decreased while the high-latitude temperature is increased in three key regions at lower to upper levels (Figure 5), leading to weakened meridional temperature gradient. The weakening of the temperature gradient in the upper-level might be an important factor leading to the northward shift of the upper-level westerly jet stream via pressure gradient. The distribution of meridional pressure gradient indicates that the NH westerly jet is located in the region with the largest meridional pressure gradient (Figure 6a). According to the change of the meridional pressure gradient (Figure 6c), with 50° N as the boundary, the pressure gradient on the south side of the mid-latitude Eurasia and the Central Pacific is characterized by a positive anomaly (i.e., the negative pressure gradient is weakened), and the pressure gradient on the north side is characterized by a negative anomaly (i.e., the negative pressure gradient is strengthened), driving the westerly jet stream over the Eurasia and the Pacific to move northward. The deviation over the Eurasia is larger than over the Pacific. Over North America and North Atlantic, although the negative meridional pressure gradient on the south side is weakened, the enhanced negative meridional pressure gradient on the north side is also conducive to the northward movement of the jet stream. However, because of the smaller negative meridional pressure gradient in the northeastern part of North America and North Atlantic, the northward movement of the pressure gradient is suppressed, thereby reducing the northward movement of the westerly jet stream over these regions (Figure 3a). As a result, the simulated jet stream over the North America has the least northward movement. For the individual models, all of which show consistent northward shift of the jet stream, also show similar meridional pressure gradient changes but with different amplitudes. The different amplitudes have well explained the model difference of position changes. For example, the changes of the meridional pressure gradient over the eastern Eurasian and the Pacific regions show larger amplitudes in MRI-CGCM3 and CCSM4 than those in MPI-ESM-P and CNRM-CM5 (Figures not shown).Atmosphere This2020 is, 11 consistent, 1193 with the changes of the latitudinal position of the westerly jet stream, which11 of 19 show larger northward shift over the Pacific region in MRI-CGCM3 and CCSM4 (Figures not shown).

(a) (b)

FigureFigure 6. 6. TheThe meridional meridional and and zonal zonal pressure pressure gradient gradient at at 200 200 hPa. hPa. (a ()a The) The meridional meridional pressure pressure gradient gradient inin PI PI.. (b (b) )The The zonal zonal pressure pressure gradient gradient in in PI PI.. (c ()c )The The difference difference of of meridional meridional pressure pressure gradient gradient between between MHMH and and PI PI.. (d (d) )The The difference difference of of zonal zonal pressure pressure gradient gradient between between MH MH and and PI PI.. The The stippled stippled areas areas indicateindicate that that the the differenc differencee is is statistically statistically significant significant at at the the confidence confidence level level of of 95%. 95%.

The change of the longitude position of the westerly jet stream shows that it shifts westward in Eurasian and Atlantic regions and shifts eastward in the Pacific region (Figure3b). The jet center over the Eurasia is located on the east side of the negative zonal pressure gradient, whereas the jet center over the Pacific and the Atlantic is located on the west side of the positive zonal pressure gradient (Figure6b). According to the zonal pressure gradient anomaly field (Figure6d), with 110 ◦ E as the boundary, the pressure gradient on the east side of the Eurasia features a positive anomaly (i.e., the negative pressure gradient is weakened), and the pressure gradient on the west side is characterized by a negative anomaly (e.g., the negative pressure gradient is enhanced), which causes the westerly jet stream over the Eurasia to move westward. Based on the anomaly field over the North America and North Atlantic region, with 60◦ W being the boundary, the pressure gradient on the west side is characterized by a positive anomaly (i.e., the negative pressure gradient is weakened), whereas the pressure gradient on the east side features a negative anomaly (i.e., the positive pressure gradient is weakened), thereby causing the westward movement of the jet stream centered over the Atlantic region. Over the Pacific region, the zonal pressure gradient anomaly indicates a strengthening of the positive gradient to the east of the jet stream center, which is conducive to the eastward movement of the jet stream center over the Pacific region. The experiment in this study also proves this point. There are small differences among the four models in simulating the changes of the longitudinal positions of the jet stream centers. The zonal pressure gradient also implies a small difference among the four models. The westerly jet stream over the Pacific region in CCSM4 has the largest eastward shift, which might be caused by the strong simulated negative anomaly at 140◦ W (Figures not shown). In contrast, the other models present small negative anomalies at 140◦ W. Note that the westerly jet stream moves eastward in the Pacific region, which is in contrast with the aforementioned hypothesis, i.e., northward shift might be accompanied with westward shift. This requires further investigation.

4.1.2. Transient Eddy Forcing The intensity of the transient eddy activity is mainly caused by the anomalies of the baroclinic instability that is dynamically related to the westerlies [20]. Changes of the midlatitude baroclinic instability are related to the jet stream in the upper troposphere through the anomalous eddy activity [28]. Atmosphere 2020, 11, 1193 12 of 19

Hence, the transient eddy activity during the mid-Holocene is compared to that during the preindustrial period. Here, the transient eddy kinetic energy (EKE) is calculated to represent the transient eddy activity [73,74]. 2 2 EKE = U0 + V0 , (1) where U and V are respectively the monthly data of zonal and meridional full wind velocities in pressure coordinates. The overbar denotes a time average and the prime a deviation from this time average. Figure7 displays the distribution of EKE at 200 hPa in summer during PI and MH. Two distinct EKE maximum centers are found in the North Pacific and North Atlantic along the westerly jet stream axis during the two periods. The EKE maximum center over the North Pacific is in consistent with the jet center (Figure2f). No distinct EKE center is found over the Eurasian continent. Meanwhile, the intensity of the EKE center over the North Pacific is larger than that over the North Atlantic. These conditions are similar in both of the periods. Xiao et al. [16] studied the relationship between the Atmosphereposition of2020 the, 5, westerly x FOR PEER jet REVIEW axis and the intensity of the eddy energy and found that the transient12 eddy of 18 activity whose southward shift was in conjunction with the displacement of the westerly jet in the consistentsame direction. with Xiao They et also al. [1 found6]. Therefore, that there with was the no combined distinct EKE effectscenter of EKE over forcing the Eurasian and the continent. thermal forcing,In Figure the7, the northward two EKE shiftmaximum of the centers,jet stream corresponding is larger over to Western the jet stream, Pacific are than both over shifted Eurasia. northwards withHowever, a reduced theintensity. EKE forcing It means cannot that the explain impact the of eastEKE-west centers shift on of the the westerly jet stream, jet stream which over needs the furtherNorth Pacific studies and in the North future. Atlantic is northward shifting.

2 2 Figure 7. DDistributionsistributions of of the the transient transient eddy eddy kinetic kinetic ene energyrgy inin summer summer (EKE, (EKE, m m2s−2;s − calculate; calculatedd by Equationby Equation (1)) (1))and andthe westerly the westerly jet axis jet at axis 200 at hPa 200 for hPa (a) forPI and (a) PI(b) and MH (.b The) MH. red The dashed red line dashed and linethe blueand thesolid blue line solid are line the areindicator the indicatorss of the westerly of the westerly axes in axes the inpreindustrial the preindustrial period periodand the and Mid the- Holocene,Mid-Holocene, respectively. respectively.

4.2. WeakenedThe absent IntensityEKE center over Eurasia indicates that over this region the westerly jet stream is much less relevant with the transient eddy activity and is mainly driven by the thermal forcing, which is consistentTo learn with the Xiao mechanism et al. [16s]. of Therefore, the position with and the intensity combined anomalies eﬀectsof ofEKE the westerlyforcing and jet, the thermal andforcing, dynamic the northward factors are shift analyzed of the detailly jet stream in isthis larger section. over Western Paciﬁc than over Eurasia.

4.2.1. Thermal Forcing During the mid-Holocene, the temperature increases in the middle and high latitudes of NH, whereas it decreases in the middle and low latitudes (Figure 4a), leading to a weakened meridional temperature gradient. Previous study showed that the temperature gradient is a critical factor influencing both the intensity and the location of the westerly jet [39]. Hence, the meridional temperature gradient and its changes (Figure 8) are presented in this study. There is positive temperature gradient over the Pacific region near 30° N during the mid-Holocene (i.e., the meridional temperature gradient is weakened). The temperature gradient to the southwest (20° N) and north (40° N) of 30° N are both characterized by negative anomalies. Over the North America regions, the meridional temperature gradient is characterized by a positive anomaly to the south of 50° N, and a negative anomaly to the north of 50° N. Over the North Atlantic, there is positive temperature gradient. Changes in the meridional temperature gradient over the Eurasia are relatively scattered (Figure 8b). The upper-level westerly jet stream is basically formed by the thermal wind, as well as the major atmospheric circulation systems. According to the thermal wind relation, the strongest part of the jet stream should be approximately located in the regions where the temperature gradient is the largest. The meridional temperature gradient anomalies in the Pacific, North America and North

Atmosphere 2020, 11, 1193 13 of 19

However, the EKE forcing cannot explain the east-west shift of the jet stream, which needs further studies in the future.

4.2. Weakened Intensity To learn the mechanisms of the position and intensity anomalies of the westerly jet, the thermal and dynamic factors are analyzed detailly in this section.

4.2.1. Thermal Forcing During the mid-Holocene, the temperature increases in the middle and high latitudes of NH, whereas it decreases in the middle and low latitudes (Figure4a), leading to a weakened meridional temperature gradient. Previous study showed that the temperature gradient is a critical factor influencing both the intensity and the location of the westerly jet [39]. Hence, the meridional temperature gradient and its changes (Figure8) are presented in this study. There is positive temperature gradient over the Pacific region near 30◦ N during the mid-Holocene (i.e., the meridional temperature gradient is weakened). The temperature gradient to the southwest (20◦ N) and north (40◦ N) of 30◦ N are both characterized by negative anomalies. Over the North America regions, the meridional temperature gradient is characterized by a positive anomaly to the south of 50◦ N, and a negative anomaly to the north of 50◦ N. Over the North Atlantic, there is positive temperature gradient. Changes in the meridional temperature gradient over the Eurasia are relatively scattered (Figure8b). The upper-level westerly jet stream is basically formed by the thermal wind, as well as the major atmospheric circulation systems. According to the thermal wind relation, the strongest part ofAtmosphere the jet stream2020, 5, x shouldFOR PEER be REVIEW approximately located in the regions where the temperature gradient13 of is18 the largest. The meridional temperature gradient anomalies in the Pacific, North America and North AtlanticAtlantic regionsregions areare weakenedweakened betweenbetween 3030◦° N andand 4040◦° N,N, leadingleading toto aa decreasedecrease inin thethe meridionalmeridional pressurepressure gradientgradient (Figure(Figure5 5)) and and thus thus a a weakened weakened jet jet stream. stream. However,However, changes changes inin the the meridional meridional temperaturetemperature gradientgradient areare unableunable toto fullyfully explainexplain thethe intensity change of the westerly jet over Eurasia.

(a) (b)

FigureFigure 8. 8. The meridional surface surface temperature temperature gradient. gradient. (aa)) The meridional surface surface climatology climatology temperaturetemperature gradientgradient in in PI. PI. ( b(b)) The The di differencefference between between MH MH and and PI. PI. The The stippled stippled areas areas indicate indicate that that the dithefference difference is statistically is statistically significant significant at the at confidencethe confidence level level of 95%. of 95%. 4.2.2. Dynamic Forcing 4.2.2. Dynamic Forcing Bjerknes [75] stated that the strong Hadley circulation brought the low-latitude westerly angular Bjerknes [75] stated that the strong Hadley circulation brought the low-latitude westerly angular momentum to the mid-latitudes, thereby strengthening the mid-latitude westerlies. Based on the momentum to the mid-latitudes, thereby strengthening the mid-latitude westerlies. Based on the changes of meridional circulation and vertical velocity, the ascending flow near 5–10 N and the changes of meridional circulation and vertical velocity, the ascending flow near 5–10◦° N and the descending flow near 30 N are weakened (Figure9a), indicating a weakening of the Hadley circulation descending flow near ◦ 30° N are weakened (Figure 9a), indicating a weakening of the Hadley in NH during the mid-Holocene. The weakened Hadley cell then leads to the weakened westerly jet circulation in NH during the mid-Holocene. The weakened Hadley cell then leads to the weakened stream. Note that the changes of the Hadley circulation are different in different regions. The Hadley westerly jet stream. Note that the changes of the Hadley circulation are different in different regions. circulation is weakened the greatest in Eurasia (Figure9b), followed by the Pacific region (Figure9c), The Hadley circulation is weakened the greatest in Eurasia (Figure 9b), followed by the Pacific region whereas it is enhanced in the North America and North Atlantic regions (Figure9d). (Figure 9c), whereas it is enhanced in the North America and North Atlantic regions (Figure 9d).

(a) (b)

Atmosphere 2020, 5, x FOR PEER REVIEW 13 of 18

Atlantic regions are weakened between 30° N and 40° N, leading to a decrease in the meridional pressure gradient (Figure 5) and thus a weakened jet stream. However, changes in the meridional temperature gradient are unable to fully explain the intensity change of the westerly jet over Eurasia.

(a) (b)

Figure 8. The meridional surface temperature gradient. (a) The meridional surface climatology temperature gradient in PI. (b) The difference between MH and PI. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.

4.2.2. Dynamic Forcing Bjerknes [75] stated that the strong Hadley circulation brought the low-latitude westerly angular momentum to the mid-latitudes, thereby strengthening the mid-latitude westerlies. Based on the changes of meridional circulation and vertical velocity, the ascending flow near 5–10° N and the descending flow near 30° N are weakened (Figure 9a), indicating a weakening of the Hadley circulation in NH during the mid-Holocene. The weakened Hadley cell then leads to the weakened westerly jet stream. Note that the changes of the Hadley circulation are different in different regions.

TheAtmosphere Hadley2020 circulation, 11, 1193 is weakened the greatest in Eurasia (Figure 9b), followed by the Pacific region14 of 19 (Figure 9c), whereas it is enhanced in the North America and North Atlantic regions (Figure 9d).

Atmosphere 2020, 5, x FOR PEER REVIEW 14 of 18 (a) (b)

FigureFigure 9. CCirculationirculation anomalies anomalies in MH in MH and andPI. (a PI.) M (odela) Model-mean-mean circulation circulation anomalies anomalies showing showing annual- annual-meanmean changes changes in meridional in meridional and verticaland vertical motion motion (vectors; (vectors; the the scale scale for for vertical vertical motion motion has been been increasedincreased byby 100100 timestimes toto aidaid viewing)viewing) inin the northernnorthern h hemisphereemisphere and vertical m motionotion ( (color-shaded;color-shaded; unitsunits in in Pa Pa/s/s).) (.b –(db)– Referd) Refer to the to threethe three key regions—Eurasia, key regions—Eurasia, the North the Pacific, North andPacific, the Northand the Atlantic, North respectively.Atlantic, respectively The stippled. The areas stippled indicate areas that indicate the di ffthaterence the isdifference statistically is statistically significant atsignificant the confidence at the levelconfidence of 95%. level The of vectors 95%. T arehe shownvectors where are shown the meridional where the windmeridional is statistically wind is significant.statistically significant.

ByBy analyzinganalyzing the the changes changes in in the the meridional meridional temperature temperature gradient gradient (thermodynamic (thermodynamic factor) factor) and and the variationthe variation of the of Hadley the Hadley circulation circulation (dynamic (dynamic factor), factor), the intensity the intensity of the westerly of the westerly jet presents jet dipresentsfferent resultsdifferent over results the threeover the key three regions. key regions. The thermodynamic The thermodynamic and dynamic and dynamic effects botheffects contribute both contribute to the weakeningto the weakening of jet stream of jet stream over the over Pacific the Pacific region, region, which makeswhich itmake the strongests it the strongest weakening weakening region among region theamong three the regions. three regions. Over Eurasia, Over theEurasia, dynamic the dynamic factor plays factor a key plays role a in key the role weakening in the weaken of the jeting stream. of the Overjet stream. the North Over America the North and America North Atlantic and North regions, Atlantic the thermodynamic regions, the thermodynamic factor tends to factor weaken tends thejet to whileweaken the the dynamic jet while factor the tendsdynamic to strengthenfactor tends the to jet. strengthen This opposite the jet. e ffThisect thenopposite results effect in the then intensity results ofin thethe jetintensity stream of in the the jet North stream America in the andNorth North America Atlantic and regions North toAtlantic go through region thes to smallest go through change the amongsmallest the change three keyamong regions the three (Figure key3c). regions (Figure 3c). 5. Conclusions 5. Conclusions In this study, four high-resolution models participated in CMIP5/PMIP3 were used to investigate In this study, four high-resolution models participated in CMIP5/PMIP3 were used to the characteristics and mechanisms of the position and intensity changes of the NH upper-level investigate the characteristics and mechanisms of the position and intensity changes of the NH upper- westerly jet in summer during the mid-Holocene. The main conclusions are as follows. level westerly jet in summer during the mid-Holocene. The main conclusions are as follows. Compared with the PI period, the summer NH upper-level westerly jet during the mid-Holocene was generally shifted towards the polar region, with weakened intensity. The largest northward shift was found in the Pacific region, with its intensity decreasing to the greatest extent. The smallest northward shift occurred in the North America and North Atlantic regions, with its intensity weakened to the least extent. Zonally, in the Eurasia and North America and North Atlantic regions, the westerly jet center moved westward, whereas in the Pacific region, the westerly jet center moved eastward. The change of the latitudinal position of the jet stream has been investigated. From the perspective of the thermal factor, the orbital-induced insolation change and the strengthened monsoon-rainfall latent-heat release could lead to the temperature increasing in the mid-high regions while decreasing in the mid-low latitudes. Moreover, the temperature changed more greatly over land than over the adjacent oceans. The temperature changes could result in the pressure changes, and thus affected the meridional and zonal pressure gradients in NH, which could make the westerly jet stream move northward. From the perspective of the dynamic factor, there were two EKE maximum centers over the North Pacific and North Atlantic, they were both shifted northward, which might lead to the northward shift of the westerly jet stream. No distinct EKE center was found over the Eurasian continent, indicating that the westerly jet stream over this region was much less

Atmosphere 2020, 11, 1193 15 of 19

Compared with the PI period, the summer NH upper-level westerly jet during the mid-Holocene was generally shifted towards the polar region, with weakened intensity. The largest northward shift was found in the Pacific region, with its intensity decreasing to the greatest extent. The smallest northward shift occurred in the North America and North Atlantic regions, with its intensity weakened to the least extent. Zonally, in the Eurasia and North America and North Atlantic regions, the westerly jet center moved westward, whereas in the Pacific region, the westerly jet center moved eastward. The change of the latitudinal position of the jet stream has been investigated. From the perspective of the thermal factor, the orbital-induced insolation change and the strengthened monsoon-rainfall latent-heat release could lead to the temperature increasing in the mid-high regions while decreasing in the mid-low latitudes. Moreover, the temperature changed more greatly over land than over the adjacent oceans. The temperature changes could result in the pressure changes, and thus affected the meridional and zonal pressure gradients in NH, which could make the westerly jet stream move northward. From the perspective of the dynamic factor, there were two EKE maximum centers over the North Pacific and North Atlantic, they were both shifted northward, which might lead to the northward shift of the westerly jet stream. No distinct EKE center was found over the Eurasian continent, indicating that the westerly jet stream over this region was much less relevant with transient eddy activity and was mainly driven by the thermal forcing. The mechanisms of the east-west shift of the westerly jet stream remains unknown and require further studies. The changes of the jet intensity were related to the thermodynamic factor (meridional temperature gradient) and the dynamic factor (Hadley circulation). The greatest weakening of the upper-level jet in the Pacific region was caused by the joint effects of thermodynamic and dynamic factors, and the smallest weakening in the North America and North Atlantic regions was induced by the offset effects of thermodynamic and dynamic factors. Dominated by the dynamic factors, the westerly jet stream over Eurasia was moderately weakened among the three key regions. Our study has revealed that during the warm mid-Holocene, the summer upper-level westerly jet weakened and shifted northward, which was to some extent opposite to the changes during the cold LGM. This indicates that the responses of atmospheric circulation to different climate backgrounds are quite complex, and we should be very careful when investigating the future change under global warming.

Author Contributions: Conceptualization, C.X., M.Y.; methodology, C.X., M.Y.; software, C.X., M.Y., L.N. and J.L.; validation, C.X., M.Y. and L.N.; formal analysis, C.X., M.Y.; resources, C.X., M.Y., L.N. and J.L.; writing—original draft preparation, C.X., M.Y.; writing—review and editing, C.X., M.Y. and L.N.; visualization, C.X., M.Y.; supervision, M.Y., L.N. and J.L.; project administration, M.Y., L.N. and J.L.; funding acquisition, M.Y., L.N. and J.L. All authors have read and agreed to the published version of the manuscript. Funding: The simulation data was provided by the Paleoclimate Modelling Intercomparison Project Phase 3 (https://esgf-node.llnl.gov/search/cmip5/). This research was jointly supported by Strategic Priority Research Program of Chinese Academy of Sciences (Grant XDB40000000), the National Key Research and Development Program of China (grant No. 2016YFA0600401), the National Natural Science Foundation of China (grant nos. 41671197, 42075049, 41971021 and 41971108), Open Funds of State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS (SKLLQG1930, SKLLQG1820), and the Priority Academic Development Program of Jiangsu Higher Education Institutions (PAPD, grant No. 164320H116). Acknowledgments: Thanks for the two anonymous reviewers and Academic Editor for their great help to the manuscript. And thank Nanjing Hurricane Translation for reviewing the English language quality of this paper. Conﬂicts of Interest: We declare no conﬂict of interest.

References

1. Tao, S.Y.; Chen, L.X. A Review of Recent Research on the East Asian Summer Monsoon (in China); Oxford University Press: Oxford, UK, 1987; pp. 60–92. 2. Tao, S.Y.; Wei, J. The westward, northward advance of the subtropical high over the West Paciﬁc in summer. J. Appl. Meteorol. Sci. 2006, 17, 513–525. Atmosphere 2020, 11, 1193 16 of 19

3. Sun, J.; Zhang, M.; Liu, T. Spatial and temporal characteristics of dust storms in China and its surrounding regions, 1960–1999: Relations to source area and climate. J. Geophys. Res. Atmos. 2001, 106, 10325–10333. [CrossRef] 4. Nagashima, K.; Tada, R.; Tani, A.; Sun, Y.; Isozaki, Y.; Toyoda, S.; Hasegawa, H. Millennial-scale oscillations of the westerly jet path during the last glacial period. J. Asian Earth Sci. 2011, 40, 1214–1220. [CrossRef] 5. Lu, R.Y. Associations among the components of the East Asian summer monsoon system in the meridional direction. J. Meteorol. Soc. Jpn. 2004, 82, 155–165. [CrossRef] 6. Kuang, X.Y.; Zhang, Y.C. Impact of the position abnormalities of East Asian subtropical westerly jet on summer precipitation in middle-lower reaches of Yangtze River. Plateau Meteorol. 2006, 25, 382–389. (In Chinese) 7. Chiang, J.C.H.; Swenson, L.M.; Kong, W. Role of seasonal transitions and the westerlies in the interannual variability of the East Asian summer monsoon precipitation. Geophys. Res. Lett. 2017, 44, 3788–3795. [CrossRef] 8. Zhang, Y.C.; Kuang, X.Y. The Relationship between the Location Change of the East Asian Subtropical Westerly Jet and Asian Summer Monsoon Onset. Torrential Rain Disasters 2008, 27, 97–103. 9. Zhang, Y.C.; Guo, L.L. Multi-model ensemble simulated changes in the subtropical westerly jet over East Asia under the global warming condition. Sci. Meteorol. Sin. 2010, 30, 694–700. 10. Zhang, Y.-C.; Kuang, X.; Guo, W.; Zhou, T. Seasonal evolution of the upper-tropospheric westerly jet core over East Asia. Geophys. Res. Lett. 2006, 33.[CrossRef] 11. Shao, P.C.; Li, D.L.; Chunxue, W. Spatial and Temporal Changes of Summer Rain in the Yellow River Basin and Its Relation to the East Asia Subtropical Westerly Jet in Last 50 Years. Plateau Meteorol. 2015, 34, 347–356. 12. Woollings, T.; Hannachi, A.; Hoskins, B. Variability of the North Atlantic eddy-driven jet stream. Q. J. R. Meteorol. Soc. 2010, 136, 856–868. [CrossRef] 13. Limpasuvan, V.; Hartmann, D.L. Eddies and the annular modes of climate variability. Geophys. Res. Lett. 1999, 26, 3133–3136. [CrossRef] 14. Yang, S.; Lau, K.M.; Kim, K.M. Variation of the East Asian jet stream and Asian Pacific American winter climate Anomalies. J. Clim. 2002, 15, 306–325. [CrossRef] 15. Liao, Q.H.; Gao, S.T.; Wang, H.J. Anomalies of the extratropical westerly jet in the north hemisphere and their impacts on east Asian summer climate anomalies. Chin. J. Geophys. 2004, 47, 11–18. (In Chinese) [CrossRef] 16. Xiao, C.; Zhang, Y. Simulation of the westerly jet axis in boreal winter by the climate system model FGOALS-g2. Adv. Atmos. Sci. 2013, 30, 754–765. [CrossRef] 17. Liang, X.Z.; Wang, W.C. Association between China monsoon rainfall and troposphere jets. Q. J. R. Meteorol. Soc. 1985, 124, 2597–2623. [CrossRef] 18. Chiang, J.C.H.; Lee, S.-Y.; Putnam, A.E.; Wang, X. South Pacific Split Jet, ITCZ shifts, and atmospheric North–South linkages during abrupt climate changes of the last glacial period. Earth Planet. Sci. Lett. 2014, 406, 233–246. [CrossRef] 19. Lucas, C.; Nguyen, H.; Timbal, B. An observational analysis of Southern Hemisphere tropical expansion. J. Geophys. Res. Atmos. 2012, 117.[CrossRef] 20. Park, H.; Kim, S.-J.; Seo, K.-H.; Stewart, A.L.; Kim, S.-Y.; Son, S.-W. The impact of Arctic sea ice loss on mid-Holocene climate. Nat. Commun. 2018, 9, 4571. [CrossRef] 21. Montaggioni, L.F.; Le Cornec, F.; Corrège, T.; Cabioch, G. Coral barium/calcium record of mid-Holocene upwelling activity in New Caledonia, South-West Pacific. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2006, 237, 436–455. [CrossRef] 22. Xu, H.; Zhou, K.; Lan, J.; Zhang, G.; Zhou, X. Arid Central Asia saw mid-Holocene drought. Geology 2019, 47, 255–258. [CrossRef] 23. Lauterbach, S.; Witt, R.; Plessen, B.; Dulski, P.; Prasad, S.; Mingram, J.; Gleixner, G.; Hettler-Riedel, S.; Stebich, M.; Schnetger, B.; et al. Climatic imprint of the mid-latitude Westerlies in the Central Tian Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years. Holocene 2014, 24, 970–984. [CrossRef] 24. Baker, J.L.; Lachniet, J.L.B.M.S.; Chervyatsova, O.; Asmerom, Y.; Polyak, Y.A.V.J. Holocene warming in western continental Eurasia driven by glacial retreat and greenhouse forcing. Nat. Geosci. 2017, 10, 430–435. [CrossRef] 25. Zhang, R.; Liu, X. An Analogy Analysis of Summer Precipitation Change Patterns between Mid-Holocene and Future Climatic Warming Scenarios over East Asia. Sci. Geogr. Sin. 2009, 29, 679–683. Atmosphere 2020, 11, 1193 17 of 19

26. Joussaume, S.; Taylor, K.E. Status of the paleoclimate modeling intercomparison project (PMIP). In Proceedings of the First International AMIP Scientific Conference, Monterey, CA, USA, 15–19 May 1995; WCRP Report; pp. 425–430. 27. Held, I.M.; Hou, A.Y. Zonal jet structure and the leading mode of variability. J. Atmos. Sci. 1980, 37, 515–533. [CrossRef] 28. Held, I.M. Momentum transport by quasi geostrophic eddies. J. Atmos. Sci. 1975, 32, 1494–1497. [CrossRef] 29. Lin, C. The Relationship between East Asian Summer Monsoon Activity and Northward Jump of the Upper Westerly Jet Location. Chin. J. Atmos. Sci. 2004, 28, 641–658. 30. Huang, X.; Jing, J. The diagnostic analysis of the impact of ENSO events on East Asia subtropical westerly jet. Sci. Meterol. Sin. 2008, 28, 15–20. 31. Routson, C.C.; McKay, N.P.; Kaufman, D.S.; Erb, M.P.; Goosse, H.; Shuman, B.N.; Rodysill, J.R.; Ault, T. Mid-latitude net precipitation decreased with Arctic warming during the Holocene. Nature 2019, 568, 83–86. [CrossRef] 32. Lamy, F.; Kilian, R.; Arz, H.W.; Francois, J.-P.; Kaiser, J.; Prange, M.; Steinke, T. Holocene changes in the position and intensity of the southern westerly wind belt. Nat. Geosci. 2010, 3, 695–699. [CrossRef] 33. Williams, G.P.; Bryan, K. Ice Age Winds. An Aquaplanet Model. J. Clim. 2006, 19, 1706–1715. [CrossRef] 34. Cai, Q.Q.; Zhou, T.J.; Wu, B.; Li, B.; Zhang, L. The East Asian subtropical westerly jet and its interannual variability simulated b y a climate system model FGOALSg-l. Acta Oceanol. Sin. 2011, 33, 38–48. 35. Kuang, X.; Zhan, Y. The Characteristics of the Coupling Variation between the Subtropical Westerly Jet and the Surface Heating Fields over East Asia. Chin. J. Atmos. Sci. 2007, 31, 77–88. 36. Dong, M.; Yu, J. Relationship between the changes of the westerly jet and the tropical convective heating in East Asia. Chin. J. Atmos. Sci. 1999, 23, 62–70. 37. Huang, R.H.; Sun, F.Y. Interannual variation of the summer teleconnection pattern over the Northern Hemisphere and its numerical simulation. Chin. J. Atmos. Sci. 1992, 16, 52–61. (In Chinese) 38. Yang, L.; Zhang, Q. Climate Features of Summer Asia Subtropical Westerly Jet Stream. Clim. Environ. Res. 2008, 13, 10–20. 39. Lorenz, D.J.; Hartmann, D.L. Eddy–zonal flow feedback in the Southern Hemisphere. J. Atmos. Sci. 2001, 58, 3312–3327. [CrossRef] 40. Yang, L.; Zhang, Q. Anomalous Perturbation Kinetic Energy of Rossby Wave along East Asian Westerly Jet and Its Association with Summer Rainfall in China. Chin. J. Atmos. Sci. 2007, 31, 586–595. 41. Enomoto, T.; Hoskins, B.J.; Matsuda, Y. The formation mechanism of the Bonin high in August. Q. J. R. Meteorol. Soc. 2003, 129, 157–178. [CrossRef] 42. Herzschuh, U.; Cao, X.; Laepple, T.; Dallmeyer, A.; Telford, R.J.; Ni, J.; Zheng, Z. Position and orientation of the westerly jet determined Holocene rainfall patterns in China. Nat. Commun. 2019, 10, 1–8. [CrossRef] 43. Fu, Q.; Lin, P. Poleward shift of subtropical jets inferred from satellite-observed lower-stratospheric temperatures. J. Clim. 2011, 24, 5597–5603. [CrossRef] 44. Yin, J.H. A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophys. Res. Lett. 2005, 32, L18701. [CrossRef] 45. Archer, C.L.; Caldeira, K. Historical trends in the jet streams. Geophys. Res. Lett. 2008, 35, L08803. [CrossRef] 46. Norris, J.R.; Allen, R.J.; Evan, A.T.; Zelinka, M.D.; O’Dell, C.W.; Klein, S.A. Evidence for climate change in the satellite cloud record. Nature 2016, 536, 72–75. [CrossRef] 47. Scheff, J.; Frierson, D. Twenty-first-century multimodel subtropical precipitation declines are mostly midlatitude shifts. J. Clim. 2012, 25, 4330–4347. [CrossRef] 48. Yang, H.; Lohmann, G.; Lu, J.; Gowan, E.J.; Shi, X.; Liu, J.; Wang, Q. Tropical expansion driven by poleward advancing mid-latitude meridional temperature gradients. J. Geophys. Res. Atmos. 2020, 125, e2020JD033158. [CrossRef] 49. Yang, H.; Lohmann, G.; Krebs-Kanzow, U.; Ionita, M.; Shi, X.; Sidorenko, D.; Gong, X.; Chen, X.; Gowan, E.J. Poleward shift of the major ocean gyres detected in a warming climate. Geophys. Res. Lett. 2020, 47, e2019GL085868. [CrossRef] 50. Chen, G.; Lu, J.; Frierson, D.M.W. Phase Speed Spectra and the Latitude of Surface Westerlies: Interannual Variability and Global Warming Trend. J. Clim. 2008, 21, 5942–5959. [CrossRef] Atmosphere 2020, 11, 1193 18 of 19

51. Barnes, E.A.; Polvani, L. Response of the midlatitude jets and of their variability to increased greenhouse gases in the CMIP5 models. J. Clim. 2013, 26, 7117–7135. [CrossRef] 52. Chen, F.; Chen, J.; Huang, W.; Chen, S.; Huang, X.; Jin, L.; Jia, J.; Zhang, X.; An, C.; Zhang, J.; et al. Westerlies Asia and monsoonal Asia: Spatiotemporal differences in climate change and possible mechanisms on decadal to sub-orbital timescales. Earth-Sci. Rev. 2019, 192, 337–354. [CrossRef] 53. Oster, J.L.; Ibarra, D.E.; Winnick, M.J.; Maher, K. Steering of westerly storms over western North America at the Last Glacial Maximum. Nat. Geosci. 2015, 8, 201–205. [CrossRef] 54. Rojas, M. Sensitivity of Southern Hemisphere circulation to LGM and 4 CO climates. Geophys. Res. Lett. × 2 2013, 40, 965–970. [CrossRef] 55. Chavaillaz, Y.; Codron, F.; Kageyama, M. Southern westerlies in LGM and future (RCP4.5) climates. Clim. Past 2013, 9, 517–524. [CrossRef] 56. Sime, L.C.; Hodgson, D.; Bracegirdle, T.J.; Allen, C.; Perren, B.; Roberts, S.; De Boer, A.M. Sea ice led to poleward shifted winds at the Last Glacial Maximum: The influence of state dependency on CMIP5 and PMIP3 models. Clim. Past 2016, 12, 2241–2253. [CrossRef] 57. Kaspar, F.; Spangehl, T.; Cubasch, U. Northern hemisphere winter storm tracks of the Eemian interglacial and the last glacial inception. Clim. Past 2007, 3, 181–192. [CrossRef] 58. Kim, S.-J.; Jun, S.-Y.; Kim, B.-M. Sensitivity of southern hemisphere westerly wind to boundary conditions for the last glacial maximum. Quat. Int. 2017, 459, 165–174. [CrossRef] 59. Wang, N.; Jiang, D.; Lang, X. Northern Westerlies during the Last Glacial Maximum: Results from CMIP5 Simulations. J. Clim. 2018, 31, 1135–1153. [CrossRef] 60. Schenk, F.; Zorita, E. Reconstruction of high-resolution atmospheric fields for Northern Europe using analog-upscaling. Clim. Past 2012, 8, 1681–1703. [CrossRef] 61. Taylor, K.E.; Ronald, S.; Meehl, G. A Summary of the CMIP5 experiment design. PCDMI Rep. 2007, 4. 62. Braconnot, P.; Harrison, S.P.; Otto-Bliesner, B.; Abe-Ouchi, A. The Paleoclimate Modeling Intercomparison Project contribution to CMIP5. Clivar Exch. 2011, 56, 15–19. 63. Krinner, G.; Lézine, A.-M.; Braconnot, P.; Sepulchre, P.; Ramstein, G.; Grenier, C.; Gouttevin, I. A reassessment of lake and wetland feedbacks on the North African Holocene climate. Geophys. Res. Lett. 2012, 39, 78–87. [CrossRef] 64. Braconnot, P.; Harrison, S.P.; Kageyama, M.; Bartlein, P.J.; Masson-Delmotte, V.; Abe-Ouchi, A.; Otto-Bliesner, B.; Zhao, Y. Evaluation of climate models using palaeoclimatic data. Nat. Clim. Chang. 2016, 2, 417–424. [CrossRef] 65. Rojas, M.; Moreno, P.; Kageyama, M.; Crucifix, M.; Hewitt, C.; Abe-Ouchi, A.; Ohgaito, R.; Brady, E.C.; Hope, P. The southern westerlies during the Last Glacial Maximum in PMIP2 simulations. Clim. Dyn. 2009, 32, 525–548. [CrossRef] 66. Dong, M.; Zhu, W.; Xu, X. Variation of surface heat flux on the atmosphere over East Asia in early summer. Q. J. Appl. Meteorol. 2001, 12, 458–468. (In Chinese) 67. Huang, R.H.; Gambo, K. The response of a hemispheric multilevel model atomosphere to forcing by topography and stationary heat sources. J. Meterorol. Soc. Jpn. 1982, 60, 78–108. [CrossRef] 68. Wu, J.; Liu, Q.; Cui, Q.Y.; Xu, D.K.; Wang, L.; Shen, C.M.; Chu, G.Q.; Liu, J.Q. Shrinkage of East Asia winter monsoon associated with increased ENSO events since the Mid-Holocene. J. Geophys. Res. Atmos. 2019, 124, 3839–3848. [CrossRef] 69. Wu, P.; Liu, Z. A simulation study on spatio-temporal asynchronism of East Asian summer’s precipitation variation since the mid-Holocene. Quat. Sci. 2013, 33, 1138–1147. 70. Sun, W.; Wang, B.; Zhang, Q.; Pausata, F.S.; Chen, D.; Lu, G.; Yan, M.; Ning, L.; Liu, J. Northern Hemisphere land monsoon precipitation increased by the Green Sahara during mid-Holocene. Geophys. Res. Lett. 2019, 46, 9870–9879. [CrossRef] 71. Liu, Z.; Harrison, S.P.; Kutzbach, J.; Otto-Bliesner, B. Global monsoon in the Mid-Holocene and oceanic feedback. Clim. Dyn. 2004, 22, 157–182. [CrossRef] 72. Ren, X.J.; Zhang, Y.C.; Xiang, Y. Connections between wintertime jet stream variability, oceanic surface heating, and transient eddy activity in the North Pacific. J. Geophys. Res. 2008, 113, D21119. [CrossRef] 73. Hoskins, B.J.; Hodges, K.I. New perspectives on the Northern Hemisphere winter storm tracks. J. Atmos. Sci. 2002, 59, 1041–1061. [CrossRef] Atmosphere 2020, 11, 1193 19 of 19

74. Lau, N.; Oort, A.H. A Comparative Study of Observed Northern Hemisphere Circulation Statistics Based on GFDL and NMC Analyses. Part II: Transient Eddy Statistics and the Energy Cycle. Mon. Weather Rev. 1982, 110, 889–906. [CrossRef] 75. Bjerknes, J. A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature. Tellus 1960, 18, 802–829.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).