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Revisiting the Role of Mountains in the Northern Hemisphere Winter

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Revisiting the Role of Mountains in the Northern Hemisphere Winter 1 Revisiting the Role of Mountains in the Northern Hemisphere Winter 2 Atmospheric Circulation ∗ 3 R.H. White 4 Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia 5 J.M. Wallace and D.S. Battisti 6 Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA ∗ 7 Corresponding author address: R.H. White, Department of Earth, Ocean and Atmospheric Sci- 8 ences, University of British Columbia, Vancouver, Canada. 9 E-mail: [email protected] Generated using v4.3.2 of the AMS LATEX template 1 ABSTRACT 2 10 The impact of global orography on Northern Hemisphere wintertime cli- 11 mate is revisited using the Whole Atmosphere Community Climate Model, 12 WACCM6. A suite of experiments explores the roles of both resolved orog- 13 raphy, and the parameterized effects of unresolved orographic drag (hereafter 14 parameterized orography), including gravity waves and boundary layer turbu- 15 lence. Including orography reduces the extra-tropical tropospheric and strato- 16 spheric zonal mean zonal wind, U, by up to 80%; this is substantially greater 17 than previous estimates. Ultimately parameterized orography accounts for 18 60-80% of this reduction; however, away from the surface most of the forc- 19 ing of U by parameterized orography is accomplished by resolved planetary 20 waves. We propose that a catalytic wave-mean-flow positive feedback in the 21 stratosphere makes the stratospheric flow particularly sensitive to parameter- 22 ized orography. Orography and land-sea contrast contribute approximately 23 equally to the strength of the mid-latitude stationary waves in the free tro- 24 posphere, although orography is the dominant cause of the strength of the 25 Siberian high and Aleutian low at the surface, and of the position of the Ice- 26 landic low. We argue that precisely quantifying the role of orography on the 27 observed stationary waves is an almost intractable problem, and in particu- 28 lar should not be approached with linear stationary wave models in which U 29 is prescribed. We show that orography has less impact on stationary waves, 30 and therefore on U, on a backwards rotating Earth. Lastly, we show that at- 31 mospheric meridional heat transport shows remarkable constancy across our 32 simulations, despite vastly different climates and stationary wave strengths. 3 33 1. Introduction 34 Since the middle of the 20th century orography has been known to have a substantial impact on 35 large-scale circulation and stationary waves (Charney and Eliassen 1949; Bolin 1950). Orography 36 subsequently affects regional climates around the world, as reflected in the climatology of surface 37 temperatures (Seager et al. 2002) and precipitation (Broccoli and Manabe 1992; Wills and Schnei- 38 der 2015). Traditionally the North American Cordillera (or Rocky mountains) and the Tibetan 39 plateau and Himalaya have been considered the most important topographic features influencing 40 atmospheric flow (e.g. Held et al. 2002). However, recent work suggests that the smaller Mon- 41 golian plateau to the north of the Himalaya is the most important region of Northern Hemisphere 42 (NH) orography during boreal winter, due to its location within the latitude of strong near-surface 43 zonal winds (White et al. 2017, 2018). 44 Earth’s atmospheric stationary waves are manifest in the planetary scale wave-like structures 45 in climatological sea level pressure or geopotential height. In the NH the dominant features of 46 the wintertime stationary waves are so prominent they have been given names: the Aleutian low, 47 the Icelandic low, and the Siberian high. The forcing of stationary waves is mainly by orography 48 (Charney and Eliassen 1949; Bolin 1950) and zonal asymmetries in atmospheric diabatic heat- 49 ing associated with land-sea temperature contrasts (e.g. Smagorinsky 1953; Manabe and Terpstra 50 1974; Hoskins and Karoly 1981; Valdes and Hoskins 1991). In addition, orography can also influ- 51 ence diabatic heating, affecting the stationary waves (Chang 2009). Surface heat fluxes associated 52 with ocean currents can also play a modifying role (Garfinkel et al. 2020). 53 From the 1980s to 2010 numerous studies examined the relative contributions of ‘orographic’ 54 and ‘thermal’ forcing to the observed stationary waves, attempting to quantify the role of orogra- 55 phy. This research, collectively using linear and non-linear stationary wave models, as well as full 4 56 general circulation models (GCMs), finds that orography produces anywhere between 30% and 57 66% of the observed stationary wave amplitude (Held 1983; Chen and Trenberth 1988a,b; Nigam 58 et al. 1988; Valdes and Hoskins 1989), with recent estimates towards the lower end of this range 59 (Ting et al. 2001; Held et al. 2002; Chang 2009). The large range of results can be at least partially 60 understood from differences in the background flow into which the orography was placed, includ- 61 ing differences in low-level winds (Valdes and Hoskins 1989; Held and Ting 1990; Ringler and 62 Cook 1995; Held et al. 2002), as well as model differences in atmospheric dissipation strength and 63 imposed drag (Valdes and Hoskins 1989). Additionally, as suggested by Held et al. (2002) and 64 shown by Chang (2009), orography impacts the patterns of atmospheric diabatic heating, and thus 65 a separation of the stationary waves into contributions from ‘orography’ and ‘thermal’ forcing is 66 not a well posed problem. Indeed, using an intermediate complexity GCM Garfinkel et al. (2020) 67 show that such strong non-linearities exist between different forcing mechanisms of the stationary 68 waves, that, over the Pacific, the linear addition of the sources gives a response of different sign 69 to the full nonlinear result. Garfinkel et al. (2020) also show that zonal asymmetries in ocean heat 70 fluxes play a small, but not insignificant, role through their influence on the distribution of sea 71 surface temperatures (SSTs). Using a comprehensive GCM Brayshaw et al. (2008, 2009) explored 72 the influence of idealized land masses (e.g. rectangular continents), orography (e.g. gaussian 73 shaped mountains) and SST gradients. In this study we focus only on the influence of present day 74 orography vs land-sea contrast. 75 In comparison with stationary waves, less attention has been paid to the impact of orography on 76 the axisymmetric (zonal mean) flow. Held et al. (2002) notes that the structure of the tropospheric 77 zonal mean zonal wind, U, in particular the sub-tropical tropospheric jets, is broadly similar be- 78 tween the Northern and Southern Hemispheres (NH and SH), suggesting that the orography of the 79 NH has relatively little impact in the troposphere. This is consistent with early modelling results 5 80 (Manabe and Terpstra 1974). Conversely, a strong asymmetry exists between the wintertime zonal 81 mean stratospheric jets in each hemisphere, caused by large differences in planetary wave activ- 82 ity generated by land-sea contrasts and orography (Waugh and Polvani 2010; White et al. 2018). 83 Early modelling results suggested that orography reduces the speed of the NH winter stratosphere 84 jet by approximately 15% (Manabe and Terpstra 1974); however, using a modern GCM White 85 et al. (2018) find that the presence of the ‘Mongolian mountains’ of Asia alone reduces strato- 86 spheric U in winter by up to 40%. The parameterization of the sub-grid scale orography, which 87 has been much improved in recent models, has been shown to be critical for correctly simulating 88 the effects of orography on the zonal mean wind (Palmer et al. 1986; Sandu et al. 2016) as well 89 as the general circulation, including stationary waves (Wallace et al. 1983; Sigmond and Scinocca 90 2010). 91 Multiple mechanisms exist for how orography impacts the stationary waves and axisymmetric 92 flow. Orography acts as a large-scale obstruction to the flow, creating stationary waves (Charney 93 and Eliassen 1949; Bolin 1950); these stationary waves can also influence the axisymmetric flow 94 through wave-mean-flow interactions (Eliassen and Palm 1961; Charney and Drazin 1961; An- 95 drews and McIntyre 1976; Edmon et al. 1980). Other impacts of orography on the circulation that 96 are at least partially resolved in models include orographic pressure torque, created by gradients 97 in pressure between the western and eastern flanks of mountains (e.g. Wahr and Oort 1984), and 98 differential heating (both radiative and latent) caused by orography (e.g. Ringler and Cook 1995; 99 Wills and Schneider 2018). Orography also affects circulation through subgrid-scale impacts that 100 must be parameterized at the resolution of past and current GCMs (e.g. Wallace et al. 1983; Boer 101 et al. 1984). Sandu et al. (2016) give a thorough description of the subgrid-scale effects of orog- 102 raphy and their typical representation in atmospheric models. In short, parameterized physics 103 typically include: the forcing of gravity waves, which break close to the surface as well as in the 6 104 upper atmosphere affecting the circulation (e.g. Boer et al. 1984; McFarlane 1987; Miller et al. 105 1989; Shaw et al. 2009) and the deceleration of the near-surface flow by turbulent or frictional 106 small scale processes, including subgrid-scale blocking of flow (e.g. Webster et al. 2003; Pithan 107 et al. 2016; Sandu et al. 2019). Parameterizing the subgrid-scale effects of orographic affects both 108 atmospheric U (Richter et al. 2010; Sandu et al. 2016; Lindvall et al. 2017; van Niekerk et al. 109 2018) as well as the stationary waves (e.g. Wallace et al. 1983; Sigmond and Scinocca 2010; van 110 Niekerk et al. 2017; Sandu et al. 2019). The representation of such processes within GCMs has 111 improved in recent years, through both increased resolution and improved parameterizations (e.g.
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