Does the formation of polar cells associated with the mid-high latitude double-wave circulation allow for blocking in the atmosphere? Do high-pressure areas, such as plateaus and high altitudes such as Mount Everest, contribute to the formation of high-pressure cells in synoptic currents?

The formation of the Arctic cell associated with the two-wave middle-high latitude circulation, which is a major atmospheric circulation common to the three-cell and four-cell mean meridional circulations in the Northern Hemisphere, is analyzed using a long period of reanalysis data. In the context of the two-wave middle-high latitude circulation, when the high near the Arctic region from 120◦E to 80◦W (AH120E80W) weakens and withdraws eastward and the low near the Arctic region from 80◦W to 120◦E (AL80W120E) strengthens and expands northeastward, the Arctic tends to be controlled by obvious low pressure and associated upward motion, leading to the formation of the Arctic cell. The eastward withdrawal of the AH120E80W is attributed to an eastward retreat of the North Pacific Low, because it promotes the strong anticyclonic wind shear associated with the maintenance of the AH120E80W to migrate eastward. The eastward retreat of the North Pacific Low is induced by the decrease in the width of the East Asian Trough, which results from the response of the high terrain in Central Asia to the weakening of middle-latitude westerly winds caused by a northward shift of the Azores High. On the other hand, the eastward withdrawal of the AH120E80W results in the decay of the Arctic high, causing the winds near the Arctic to change from easterly to westerly. At the same time, the northward shift of the Azores High promotes the strong Icelandic Low to expand poleward. The combination of the Arctic westerly winds and the poleward expansion of the strong Icelandic Low leads to the northeastward expansion of the AL80W120E.The three mean meridional cells, Hadley cell, Ferrel cell and Polar cell, have been widely identified from the perspective of latitude and time average circulation by employing divergent wind and pressure vertical velocity (Huang et al., 2004) since it was proposed by Ferrel in 1860s (Persson, 2006). They occur in each hemisphere, with the Hadley cell ranging between the equator and about 30◦ latitude, the Polar cell covering from about 60◦ latitude to the poles, and the Ferrel cell located between them (Qian et al., 2015b). The Hadley cell and Polar cell are thermally driven cells forced by diabatic heating, whereas the Ferrel cell is an indirectly thermally driven cell principally forced by the transient baroclinic eddy activity (Holton, 1992; Wang, 2002). The three cells act to transfer heat energy, water vapor and momentum between different latitudes, and they are transferred where the cells meet (Wang et al., 2005; Hartmann, 2016). The energy and mass transfer, together with the pressure belts and trade winds associated with the three cells, have a profound impact on global climate and weather. The strengthening trend and poleward expansion of the Hadley cell (Kang and Lu, 2012) result in a poleward shift of the subtropical dry zone and the tropospheric warming in middle latitudes (Mitas and Clement, 2005; Fu et al., 2006; Stachnik and Schumacher, 2011; Su et al., 2014). Furthermore, the extent of the Hadley cell has an important effect on the poleward transportation of trace gases and aerosols (Yang et al., 2019) and stratospheric ozone depletion (Son et al., 2010; Kang et al., 2011). An enhanced and expanded Ferrel cell can induce strong anomalous southerly warm advection at the surface, which caused an abrupt jump in winter surface air temperature in the late 1980s over the Northern Hemisphere (Kim et al., 2015). In the daily, monthly or longer timescale, the variability of the Ferrel cell is in a good correlation with that of the Northern Hemisphere Annular Mode, which has a large impact on the climate system and weather prediction (Li et al., 2014). An abnormal Polar cell can assist to transport the inter-seasonal planetary-scale zonal wind disturbances at the polar tropopause to high latitudes, which promotes the formation of the Arctic and Antarctic Oscillations (Qian and Liang, 2012). With the help of the Polar cell, the trace gases and aerosols, which greatly influence heat balance through direct radiative effects and indirect effects on cloud formation (Garrett and Zhao, 2006; Lubin and Vogelmann, 2006; Coopman et al., 2018), can be transported in a long distance from middle latitudes to the Arctic region, which may make a significant contribution to the rapid surface temperature increase in the Arctic (IPCC, 2013). In recent years, a fourth meridional cell, named the Arctic cell, was proposed and confirmed through analysis of the zonal mean streamline and mass stream function of multiple reanalysis products and a climate model simulation (Qian et al., 2015a, 2015b; Qian et al., 2016a). Analyses of the stationary eddy heat and momentum fluxes and the global precipitation rate also confirmed the existence of this cell (Qian et al., 2016b). The Arctic cell occurs in the troposphere north of 80◦N, with a weaker strength compared to the other three cells. Substantial decline of sea ice in the Arctic over the last decades has been demonstrated by many studies (e.g., Serreze et al., 2007; Screen and Simmonds, 2010; Ding et al., 2014), and the strengthening subsidence flow associated with the Polar cell and Arctic cell was indicated to contribute to the surface air temperature warming and sea ice concentration declining over the Arctic (Qian et al., 2015b, 2016b). Fig. 2. (a and b) Distributions of 500-hPa geopotential height departure (color-shaded; units: gpm) averaged by the events of (a) four cells (4Cs) and (b) three cells (3Cs) of 1 − wsd significance that exhibit a two-wave middle-high latitude circulation. (c and d) Zonal distributions of meridionally (30◦–80◦N) average 500-hPa geopotential height departure for the events of (c) four cells and (d) three cells of 1 − wsd significance exhibiting a two-wave middle-high latitude circulation. The rm and rs in (a) indicate the mean and standard deviation of the correlation coefficients between the average distribution in (a) and the distribution of each event used to calculate the average distribution, and the same for the rm and rs in (b). (a) and (c) are drawn from 38 events, and (b) and (d) from 45 events.are obtained by examining the zonally and vertically average monthly pressure vertical velocity north of 82◦N (the determination of this boundary latitude will be explained in next section), according to the definition of the Arctic cell by Qian et al. (2016a, 2016b). Specifically, a four-cell (or Arctic-cell) event is defined as occurring when the zonal (0–360◦), vertical (1000–200 hPa) and Arctic (north of 82◦N) mean of monthly vertical velocity (shortly referred to as w) is upward, whereas a three-cell event is defined when the w is downward. On the other hand, the mean and standard deviation of the w throughout the period of 1979–2018, referred to as wm and wsd, are calculated. On this basis, the events of three cells and four cells, in which the absolute value of the difference between the w and wm exceeds one and two times wsd, are taken for further analyses. On this basis, a major atmospheric circulation, which is common to the three cells and four cells in the Northern Hemisphere, will be discovered through the investigation of the atmospheric circulations of these two kinds of events, using the monthly ECMWF reanalysis data. Then, the characteristics of this major atmospheric circulation for the events of three cells and four cells will be analyzed comparatively to reveal the key synoptic ingredients that promote the formation of the Arctic cell, using the daily ECMWF reanalysis data. The t function (Chervin and Schneider, 1976) (Eqs. (1)–(3)) will be used to test the significance of the difference in the atmospheric circulation characteristics between the four-cell events and the threecell event. In Eqs. (1)–(3), x and y indicate the variables associated with the atmospheric circulation (such as geopotential height departure and zonal wind speed) for the events of four cells and three cells with sizes m and n, respectively. Eventually, the effects of the major atmospheric circulation on the key synoptic ingredients will be discussed.As mentioned in Section 2, the pressure vertical velocity is applied to identify the events of three cells and four cells in the Northern Hemisphere. Accordingly, the evolution of the zonally (0◦–360◦) and vertically (1000–200 hPa) average monthly vertical velocities in the Northern Hemisphere from 1979 to 2018 are depicted in Fig. 1a. The ascending branches of the Hadley cell and Ferrel cell near the equator and 60◦N, respectively, and the descending branch in between, are clearly shown from the figure. In addition, the region north of 82◦N presents upward and downward motion alternatively during 1979–2018. The downward motion in this region corresponds to the descending branch of the Polar cell (related to the three cells), while the upward motion in this region corresponds to the ascending branch of the Arctic cell (related to the four cells), which is consistent with the definition of the Arctic cell, whose ascending branch is generally located north of 80◦N (Qian et al., 2015b). In terms of vertical velocity, the central positions of the Polar cell and the Arctic cell are near 800 hPa (Figure not shown). In view of this result, the time series of w from 1979 to 2018, as well as the wm and wsd associated with it, are shown in Fig. 1b, in which the positive and negative w indicate the events of four cells and three cells, respectively. In order to better highlight the difference in atmospheric circulation characteristics between the three cells and four cells, the events of 1 − wsd significance (the absolute difference between the w and the wm is larger than one wsd), accounting for 69 three-cell events and 71 four-cell events, are applied for further analysis. Four cells and three cells are clearly seen from the verticallatitude sections of zonally average monthly vertical velocity averaged by the 71 four-cell events and the 69 three-cell events (Fig. 1c and d). These vertical-latitude sections of zonally average monthly vertical velocity are representative, because basically they are highly correlated with those of the four-cell and three-cell events (on average 0.8 and 0.83, respectively). Furthermore, from the perspective of the vertical range of vertical velocity, the depths of the Arctic cell and Polar cell reach up to about 200 hPa for the events of four cells (Fig. 1c), while the depth of the Polar cell can reach higher than 200 hPa for the events of three cells (Fig. 1d). Examination of the 71 four-cell events and 69 three-cell events reveals a major atmospheric circulation characterized by a stationary twowave middle-high latitude (30◦–80◦N) circulation at 500 hPa. This atmospheric circulation presents two high geopotential height departure centers in the northwestern North America and the region from Central Atlantic to Central Asia (the North America High and the Euro-Asia High), and two low geopotential height departure centers in the northeastern North America and the region from Northeast Asia to North Pacific (the Icelandic Low and the North Pacific Low) (Fig. 2a and b), which basically correspond to the semi-permanent geopotential height centers in the middle-high latitudes that are induced by the land-sea thermal contrast and modulated by planetary waves (He and Huang, 2014). It accounts for 38 (about 53%) four-cell events and 45 (65%) three-cell events in the context of 1 − wsd significance, and is representative of these events, with high correlations of 0.81 and 0.84 on average, respectively. The zonal distributions of the geopotential height departure averaged between the middle-high latitudes (30◦–80◦N) for all four-cell and three-cell events (38 and 45 events) also exhibit the characteristics of two-wave circulation (Fig. 2c and d). It indicates that the two-wave middle-high latitude circulation is a major and common atmospheric circulation for the events of three cells and four cells. In view of this, the effects of this atmospheric circulation on the formation of the Arctic cell can be revealed by comparing the two-wave middlehigh latitude circulation characteristics of these two kinds of events.From the perspective of mean meridional circulation, the three cells consisting of the Hadley cell, Ferrel cell and Polar cell, have been widely recognized. Numerous studies have demonstrated that the three cells play a vital role in determining global climate and weather. The Polar cell among them has an important influence on the changes of surface temperature and sea ice in the Arctic. In recent years, the Arctic cell or four cells was proposed and confirmed by the analysis of multiple reanalysis products. The interaction between the Arctic cell and Polar cell determines the strength and position of the subsidence flow located between them. This subsidence flow has a considerable contribution to the surface air temperature warming and sea ice concentration declining in the Arctic. However, the causes for the formation of the Arctic cell are still unclear so far. In view of this, the present study attempts to discuss the causes for the formation of the Arctic cell from the perspective of atmospheric circulation. The discussion addresses a major atmospheric circulation that is common to the four cells and three cells in the Northern Hemisphere, using a long period (1979–2018) of reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF). The major findings are summarized as followed. A major atmospheric circulation that is common to the three-cell events and the four-cell events in the Northern Hemisphere is identified. This atmospheric circulation is characterized by a quasi-stationary two-wave middle-high latitude circulation composed of the Icelandic Low, Euro-Asia High, North Pacific Low and North America High (see “IL”, “AZH-CAH”, “NPL” and “NAH” in Fig. 15). At the same time, a high near the Arctic region from 120◦E to 80◦W (AH120E80W) and a low near the Arctic region from 80◦W to 120◦E (AL80W120E) occur along with the two-wave middle-high latitude circulation (see “AH120E80W”