SUPPLEMENTARY INFORMATION

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Gavin Clark
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1 doi: 1.138/nature877 Background The main sis of this paper is that topography produces a strong South Asian summer monsoon primarily by insulating warm and moist air over India from cold and dry extratropics. Plateau heating in an atmospheric model is shown to have a local effect on Himalayan rainfall but negligible effect on larger-scale, interhemispheric monsoon circulation. The idea that intensity and poleward extent of monsoons may be limited by advection of low entropy air from extratropics exists in literature, but it has been used primarily to explain why monsoon rainfall does not extend as far poleward as peak insolation during boreal summer 1. Most studies examining how horizontal entropy advection might alter monsoon circulations have not examined effect of topography on this advection 2, 3. One study did construct a model having an idealized, rectangular continent off equator with thin vertical walls extending up to 7 hpa on its east and west coasts; se walls were found to enhance continental precipitation by inhibiting zonal advection of cold oceanic air onto continent 4. Anor modeling study found that seasonal onset of regional summer rainfall over India was weakened and delayed more by removing western Himalayan orography than by removing eastern Himalayan orography, and suggested that this was due to suppression of moist static energy advection by orography 5. While se results are all consistent with our findings, none of se studies examined comparative effects of orographic insulation and plateau heating on large-scale, interhemispheric monsoon circulation. The idea that Tibetan Plateau heating drives large-scale South Asian monsoon circulation may have been so persistent at least partly because of identification during boreal summer of planetary-scale upper-tropospheric high pressure system with plateau; this system is frequently called Tibetan high or Tibetan anticyclone 6. However, this upperlevel high is actually centered just south of plateau during boreal summer, as shown in a climatology of 15 hpa geopotential height (Supplementary 1). This is consistent with temperature field displayed in 1a of main paper, and our results suggest that this Tibetan High is produced primarily by heating over continental India that is rmally insulated from extratropics by Himalayas and adjacent mountain ranges. Similar logic holds for upper-tropospheric easterly jet that is in geostrophic balance with this high pressure system, and which has previously been thought to be forced directly by Tibetan Plateau heating 7. 1

2 doi: 1.138/nature x x Supplementary 2 1 Geopotential height at 15 hpa from ERA41.4 for June-August Shading represents geopotential height in m with a contour interval of 2 m, grey lines denote coasts, and thick black contours surround surface pressures lower than 9 hpa and 7 hpa. Supplementary 1 Geopotential height at 15 hpa from ERA4 for June-August. Shading sure system, represents and which geopotential has previously height in been m with thought a contour be interval forced directly of 2 m, by grey Tibetan lines denote Plateau coasts, heatingand 7. thick black contours surround surface pressures lower than 9 hpa and 7 hpa. Additional model results sure system, and which has previously been thought to be forced directly by Tibetan Plateau heating Supplementary 7. 2 presents vertical velocities averaged over s of Tibetan Plateau from ERA4 dataset, from control model, and from model with topography Additional north of model Himalayas results set to zero. The main monsoonal ascent occurs between 1 N and 2 N Supplementary in all three panels, and 2 is presents almost entirely vertical unchanged velocities averaged by removal over of s Tibetan Plateau. of Tibetan PlateauTwo fromadditional ERA4 model dataset, integrations from were control performed model, and to from confirm model that with strength topography of north SouthofAsian Himalayas monsoon set was tohighly zero. The sensitive main monsoonal to zonally ascent elongated occurs between band of 1 mountains N and 2west N in and allsouth threeof panels, plateau, and is almost and notentirely primarily unchanged to some or by removal topographic of feature. Tibetan Plateau. Two In additional first run, all model elevated integrations topography wereover performed Asia was toremoved, confirm that but standard strength topography of South was used Asian elsewhere, monsoonincluding was highly over sensitive East Africa. to The zonally results elongated show that band main of mountains effect ofwest nonand Asian south orography of plateau, on and South notasian primarily monsoon to some is an or enhancement topographicoffeature. cross-equatorial flow in Somali In first jet, consistent run, all elevated with some topography previous over work Asia 8 (Supplementary was removed, but standard 1b). topography Compared was to used run elsewhere, with no topography including over (Supplementary East Africa. The results 1a), re show was that little main change effect in monsoon of non- Asian precipitation orography and on in low-level South Asian monsoon monsoon westerlies. is an enhancement of cross-equatorial flow in Somali jet, consistent with some previous work 8 (Supplementary 1b). Compared to run with no topography (Supplementary 1a), re was little change in monsoon precipitation and in low-level monsoon westerlies. 2

3 doi: 1.138/nature877 1 a) observations (ERA4) b) control model c) model with narrow mountains only.15 pressure (hpa) Supplementary 2 Vertical velocities averaged over s of Tibetan Plateau (75-1 E). a, ERA4 for , same years as model runs. b, Control model. c, Model with surface elevations north of Himalayas set to zero. Surface elevations north of peak in (c) appear nonzero because of retained mountains varies with. Shading represents vertical velocities in Pa s 1, so that blue colors denote upward motion, with a contour interval of.1 Pa s 1. Grid cells with pressures larger than longitudinally averaged surface pressure are masked. In second run, all elevated topography over Asia was removed except for fairly low mountains over continental India and Indochina Peninsula. This was done by finding point at each lying 2 km south of place where surface elevations reached two-thirds of ir maximum value at that, and setting surface elevations to zero north of that point at that particular. Results from this run showed that low mountains of peninsular India and Indochina increased rainfall on windward side of se mountains and over peninsular India, and also produced a slight strengning of low-level monsoon westerlies (Supplementary 1c). These mountains produced little to no increase in precipitation south of Tibet. For comparison, we show results from run with Tibetan Plateau removed that was illustrated in 3c of main paper, but with results plotted here in form of anomalies. When compared to run that included low mountains of peninsular India and Indochina, zonally elongated mountains west and south of Tibetan Plateau produce large increases in southwesterly flow over Arabian Sea and in precipitation south of Tibet. These zonally elongated mountains also produce a substantial increase in precipitation over Arabian Sea and India, although this increase is similar in magnitude to that produced by low mountains of peninsular India and Indochina. 3

4 doi: 1.138/nature877 a) 4 2 b) c) 4 d) Supplementary 3 Changes in precipitation and low-level wind for model runs with modified orography. All panels show anomalies relative to control run for runs with: a, no elevated topography, b, no elevated topography over continental Asia but standard topography elsewhere; c, as in (b) but with addition of low mountains of continental India and Indochina Peninsula; d, as in (c) but with addition of zonally elongated band of mountains west and south of Tibetan Plateau. Shading represents precipitation anomalies in mm/day, and arrows represent anomalous 85 hpa winds. Black contours surround areas with surface pressures lower than 96 hpa, 9 hpa, and 7 hpa, and grey contours denote coastlines. 4

5 doi: 1.138/nature877 References 1. Chou, C., Neelin, J. & Su, H. Ocean-atmosphere-land feedbacks in an idealized monsoon. Q. J. R. Meteorol. Soc. 127, (21). 2. Chou, C. & Neelin, J. Mechanisms limiting southward extent of South American summer monsoon. Geophys. Res. Lett 28, (21). 3. Chou, C. & Neelin, J. Mechanisms Limiting Northward Extent of Norrn Summer Monsoons over North America, Asia, and Africa. J. Climate 16, (23). 4. Privé, N. C. & Plumb, R. A. Monsoon dynamics with interactive forcing. Part II: Impact of eddies and asymmetric geometries. J. Atmos. Sci. 64, (27). 5. Chakraborty, A., Nanjundiah, R. & Srinivasan, J. Theoretical aspects of onset of Indian summer monsoon from perturbed orography simulations in a GCM. Ann. Geophys. 24, (26). 6. Krishnamurti, T. Observational study of tropical upper tropospheric motion field during Norrn Hemisphere summer. J. Appl. Meteor. 1, (1971). 7. Koteswaram, P. The easterly jet stream in tropics. Tellus 1, (1958). 8. Rodwell, M. J. & Hoskins, B. J. A Model of Asian Summer Monsoon.Part II: Cross- Equatorial Flow and PV Behavior. J. Atmos. Sci. 52, (1995). 5

Effect of Orography on Land and Ocean Surface Temperature

Present and Future of Modeling Global Environmental Change: Toward Integrated Modeling, Eds., T. Matsuno and H. Kida, pp. 427 431. by TERRAPUB, 2001. Effect of Orography on Land and Ocean Surface Temperature