Local cooling and drying induced by Himalayan glaciers under global warming

Understanding the response of Himalayan glaciers to global warming is vital because of their role as a water source for the Asian subcontinent. However, great uncertainties still exist on the climate drivers of past and present glacier changes across scales. Here, we analyse continuous hourly climate station data from a glacierized elevation (Pyramid station, Mount Everest) since 1994 together with other ground observations and climate reanalysis. We show that a decrease in maximum air temperature and precipitation occurred during the last three decades at Pyramid in response to global warming. Reanalysis data suggest a broader occurrence of this effect in the glacierized areas of the Himalaya. We hypothesize that the counterintuitive cooling is caused by enhanced sensible heat exchange and the associated increase in glacier katabatic wind, which draws cool air downward from higher elevations. The stronger katabatic winds have also lowered the elevation of local wind convergence, thereby diminishing precipitation in glacial areas and negatively affecting glacier mass balance. This local cooling may have partially preserved glaciers from melting and could help protect the periglacial environment. High-elevation meteorological observations and reanalysis data indicate local cooling and drying near Himalayan glaciers due to enhanced katabatic winds in response to global warming.

Understanding the response of Himalayan glaciers to global warming is vital because of their role as a water source for the Asian subcontinent.However, great uncertainties still exist on the climate drivers of past and present glacier changes across scales.Here, we analyse continuous hourly climate station data from a glacierized elevation (Pyramid station, Mount Everest) since 1994 together with other ground observations and climate reanalysis.We show that a decrease in maximum air temperature and precipitation occurred during the last three decades at Pyramid in response to global warming.Reanalysis data suggest a broader occurrence of this effect in the glacierized areas of the Himalaya.We hypothesize that the counterintuitive cooling is caused by enhanced sensible heat exchange and the associated increase in glacier katabatic wind, which draws cool air downward from higher elevations.The stronger katabatic winds have also lowered the elevation of local wind convergence, thereby diminishing precipitation in glacial areas and negatively affecting glacier mass balance.This local cooling may have partially preserved glaciers from melting and could help protect the periglacial environment.
Himalayan glaciers have been losing mass consistently, with an acceleration in the last five decades, including at the iconic Mount Everest [1][2][3] .Whereas the increase in temperature over the last decades is recognized as a first-order control on melting 4 , how glaciers modulate the local climate remains unknown, also because of limited high-elevation stations 5 .This challenges both our understanding of ongoing changes and modelling skills to anticipate future changes 6 .
Here, we investigate the non-stationarity of the Himalayan climate (temperature, precipitation and wind) during the last three decades from the hourly to the seasonal scale.We use all existing daily Article https://doi.org/10.1038/s41561-023-01331-y In agreement with the Pyramid off record, a decrease in T max is evident in the entire region over the glacier surfaces (Fig. 1a).A similar pattern of decreasing trends is observed for T mean (Supplementary Figs. 2,  3, 6 and 7).The decrease in T max is particularly strong in Lahaul-Spiti and in Central-Eastern Himalaya, characterized by the largest ice masses, and is less strong in Western Himalaya, where glaciers are less abundant 8,11 (Fig. 1a).
Whereas an elevation-dependent warming (that is, stronger warming rate at higher elevations) has been suggested at the global level 6 , we find that glaciers are associated with a significant reduction of the local warming.In non-glacial conditions, we also observe a direct elevation-dependent warming, in particular above 4,000 m a.s.l.(up to +0.05 °C yr −1 ) (Fig. 1b).Cooling trends (up to −0.05 °C yr −1 ), however, are evident at all glacierized locations (black points in Fig. 1b) (P < 0.001 for the analysis of variance test on the means).At lower elevations, trends are positive, although less strong (mean +0.023 °C yr −1 , P < 0.1) than those found at higher, non-glacierized elevations.The magnitude of the low-elevation trends in ERA5-Land corresponds well with the trend that we observe at 27 ground stations in Nepal below 2,000 m a.s.l.(mean +0.026 °C yr −1 , P < 0.01, Supplementary Fig. 8).
Negative trends in T max are associated with glacier masses consistently across reanalysis and all available observations (Supplementary Fig. 5a-c).The smaller time window over which a trend is evident (beginning of the warm season) is probably due to a smaller influence of glaciers induced by the reduced extension of glacial masses and the larger minimum distance from the glacier fronts in the station surroundings (Supplementary Table 1 and Supplementary Fig. 9).On the contrary, we find no clear T max trends in the warm season (Supplementary Fig. 5d,e) for those stations without glaciers within a 20 km radius (Supplementary Table 1 and Supplementary Fig. 9).

Conceptual model of cooling and drying
The unexpected finding of cooling at high elevation close to glacier masses requires revisiting our understanding of glacier-atmosphere interactions.In the ablation season, the free atmosphere temperature (T fa ) on midlatitude glaciers is typically higher than the glacier surface temperature (T s ), which is close to 0 °C, since the excess energy at the ice surface is used for melting (Fig. 2a) 12 .
Katabatic winds arise from adiabatic warming due to air subsidence and cooling of the near-surface air by sensible heat exchange with the glacier surface 13 .The interplay of these processes lowers the 2 m on-glacier temperature (T g ) (the so-called glacier cooling effect) 14,15 giving rise to density gradients that drive the air down glacier.
Katabatic winds draw air masses from upper regions 16 and we postulate an air convergence over upper parts of these glaciers, where strong air exchanges with higher atmospheric layers may occur 17 .This process generates a divergence of air masses along the northern and southern Himalayan valleys and causes further drying of the katabatic winds.
The variations of T g along the glacier flowline are complex to model 14,18 .A growing body of studies, however, has shown stronger cooling effects at greater flow distances, until a maximum cooling point (from I 0 to II 0 in Fig. 2a) 16 .This point can be followed by an increase in temperature over the glacier tongues (III 0 in Fig. 2a) 18 due to up-valley afternoon winds 19 counterbalancing the cooling effect of katabatic winds 18 .Recently, ref. 20 postulated that these moist, upward air masses counteract the cool, downwards katabatic winds, creating a local convergence.This process can force the warm and moist monsoonal air masses to uplift, inducing precipitation to occur near to, or below, the glacier fronts.
During non-stationary conditions, as in a global warming phase (Fig. 2b), higher free atmosphere temperature induces higher sensible heat exchange between the glacier surface and the observational climatic time series at high elevation and ERA5-Land reanalysis data.We take advantage of the longest time series of meteorological data at a glacierized elevation in Himalaya, the Pyramid Observatory Laboratory (on the southern slopes of Mount Everest, Nepal, 5,035 m above sea level (a.s.l.), off glacier, hereafter Pyramid off ) (Supplementary Fig. 1), which has continuously recorded hourly meteorological data since 1994.We also exploit the only five existing long-term climatic time series close to the main Himalayan ridge, located from 3,900 to ~4,500 m a.s.l.(Supplementary Table 1).We explore the physical processes behind the observed climate dynamics using three more stations along the southern slopes of Mount Everest, together with ozone data collected at Pyramid off .Building on the agreement between ground observations and ERA5-Land reanalysis data, we extend our findings from Mount Everest to the entire Himalayan range, revealing an unknown picture of high mountain climate in the region.

Cooling trends of diurnal air temperature
In contrast to most regional and global records 7 , we find that the mean annual air temperature (T mean ) has been stationary at Pyramid off during the last three decades (−0.002 ± 0.009 °C yr −1 , P > 0.1, 1994−2020 period, Supplementary Table 2 and Supplementary Fig. 2, black line).This unexpected observation seems in contrast with the attribution of the accelerated glacier mass loss to increasing air temperature 8 .To reconcile this apparent discrepancy, we take advantage of the unique climatic dataset at Pyramid off and analyse its diurnal temperature (T max ) and nocturnal temperature (T min ) separately, partitioning the year into two periods: the cold season from November to April and the warm season from May to October.We find that the T mean trend has been brought to zero by a significant decrease in T max (−0.040 ± 0.020 °C yr −1 , P < 0.01) during all months of the warm season.This decrease in T max started about 15 years ago (Fig. 1c,d, orange line), in contrast to T min , which has increased consistently, mainly during the cold season (+0.046 ± 0.019 °C yr −1 , P < 0.01, Supplementary Fig. 3b, blue line).T max exerts a crucial control on both melting processes and katabatic wind generation, when T max is generally above +0 °C, while T min is mainly negative during the cold season and remains so despite the increase.Therefore, we focus on the non-stationarity of T max , which is the key control of glacier melt and its sensitivity to warming.
We find negative T max trends at all the other existing stations close to the Himalayan glaciers: a similar trend exists at Pheriche off (4,260 m a.s.l.), in the same valley as Pyramid off , which also excludes instrumental errors at single sites (Supplementary Fig. 4a); and negative T max trends occur at all the southernmost stations on the Tibetan Plateau 9 , mainly at the beginning of the warm season (May to July) (Supplementary Fig. 5, location in Fig. 1a).The stations with weaker declines in T max are farther from the glaciers than Pyramid off .The consistency of the cooling trends across all available high-elevation stations lends confidence that this is a process typical of Himalayan glacierized regions beyond the Everest region.
Crucially, we find that the overall decreasing T max trend at Pyramid off , particularly during the warm season, is correctly reproduced by ERA5-Land reanalysis (−0.026 ± 0.014 °C yr −1 , P < 0.05, Fig. 1e,f), which reproduces the non-stationary patterns at the annual and monthly scale (Fig. 1c).

Regional cooling associated with the glacier mass
Given the correspondence between observations and reanalysis at Pyramid off , we explore the warm season T max trend across the entire Himalaya (75° E to 91° E) based on ERA5-Land (Fig. 1a).The region includes glaciers that accumulate snow primarily during winter (predominantly in western Himalaya) and during the summer monsoon (eastern Himalaya) but are homogeneous from a morphometric point of view 10 .To describe the regional trends, we split the transect into four subregions (Lahaul-Spiti, Western, Central and Eastern Himalaya) 3 .

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https://doi.org/10.1038/s41561-023-01331-ynear-surface air.On one hand, the additional heat is used to increase the melting rates, causing enhanced glacier mass losses.On the other hand, a further cooling of the surface air over glaciers generates more intense katabatic winds.
As a key consequence of the increased katabatic winds, the glacier cooling effect intrudes downstream into the periglacial environment, shifting the local convergence between up-and down-valley winds lower (Fig. 2b).Increased dry and cold winds push downwards the

Observations supporting the conceptual model
Measurements at the off-glacier stations downstream of Pyramid off , Pheriche off (4,260 m a.s.l.) and Namche off (3,570 m a.s.l.) and at the on-glacier Changri-Nup (Changri on , 5,700 m a.s.l.) (Supplementary Table 3 and Supplementary Fig. 1; Methods) provide the observational evidence for our model (Fig. 2).
During the warm season, the mean 2 m air temperature recorded on glaciers is above 0 °C from 9:00 to 13:00 (local time) at Changri on (Fig. 3a), a condition favourable for the exchanging of sensible heat, which in turn allows the development of katabatic winds.As a result, from the early morning (9:00) the katabatic winds flow downward reaching the maximum speed during the first hours of the afternoon and decreasing in the evening (Fig. 3b), when the upward winds prevail 19 .
The exchange of air masses between the upper atmospheric layers and the glacier surface can be inferred from ozone (O 3 ) data 17,21 .We find significantly higher ozone concentrations in correspondence to stronger downward winds at Pyramid off (Fig. 3c1).When katabatic winds are stronger, temperatures are lower at Pyramid off (Fig. 3c2), suggesting that air masses are drawn from the surrounding high-elevation reaches of the glaciers.This suggests that, during daytime, stronger katabatic winds are capable of transporting downward O 3 -rich air masses from high elevations to the glacier surfaces and the proglacial domain (Pyramid off ).
The diurnal down-glacier lowering of temperature is evident when analysing how the relationship between temperature and elevation varies during the day (Fig. 3d).We find midday lower temperature gradients until 4,260 m a.s.l.(Changri on /Pyramid off and Pyramid off / Pheriche off show lower temperature gradients between 13:00 and 16:00 and 10:00 and 16:00, respectively), which do not occur downstream at 3,560 m a.s.l.(Pheriche off /Namche off shows higher temperature at midday).Note that the higher temperature during the early morning at the station at higher elevation (Changri on /Pyramid off at 9:00) is caused by the different timing of exposure to solar radiation 22 .An increase of katabatic winds, explaining the observed downstream cooling trend, is evident in the period of record.At Pyramid off , wind speed from the early morning until the first hours of the afternoon increased during 1994-2020, with statistically significant increases from 9:00 to 10:00 (Fig. 3e).As a consequence, air temperature decreased mainly during the warmest hours of the day (that is, T max ), with statistically significant decreases from 12:00 to 17:00.
Evidence of how the glacier cooling effect influences the precipitation amount at high elevation is shown in Fig. 3f.We observe a higher daytime (9:00-17:00) precipitation amount at Pyramid off , than at the stations at lower elevations.The same elevation pattern was reported for Mount Everest and Langtang Valley 20 and for Marsyandi Valley 23 .This is due to the local convergence between (cool, katabatic) down-valley and (warm, monsoonal) up-valley winds described in Fig. 2a.1).A decreasing lapse rate denotes warming at higher elevation or cooling at lower elevation and vice versa.Note that the warming during the early morning at the station at higher elevation (Changri on /Pyramid off at 9:00) is due to the different timing of exposure to solar radiation 22 .e, Trend analysis (1994-2020) of hourly downward wind speed (hollow) and air temperature (solid) at Pyramid off .f, Hourly mean precipitation at the ground stations (ref.20, revised).

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https://doi.org/10.1038/s41561-023-01331-yEvidence of this process is provided by the substantial precipitation decrease (~50% with respect to the 1990s) at Pyramid off for all months of the warm season (−8.9 ± 5.0 mm yr −1 , P < 0.001, Supplementary Fig. 10a,b), during which 90% of the annual precipitation occurs 24 .The same decreasing trend was recorded at Pheriche off (Supplementary Fig. 4b), allowing one to exclude instrumental errors.ERA5-Land is able to reproduce the observed decreasing trends for some summer months (Supplementary Fig. 10c; Methods).Further evidence for the drying in the last decades stem from the evolution of high-elevated lakes; it was previously shown that possible variations in the lakes' evaporation have a decidedly lower relative importance than that of precipitation 25 .Indeed, lakes fed only by precipitation show decreasing trends in area and volume from the mid-1990s both in the Mount Everest region 25 and in the southern Tibetan Plateau 26,27 .
Across the entire Himalayan arc, the decreasing T max trends (Fig. 4a) correspond significantly with decreasing diurnal precipitation trends (P < 0.001).In Lahaul-Spiti, diurnal precipitation is decreasing (Fig. 4b), as it is in Central-Eastern Himalaya, although with a reduced magnitude and crucially only at high elevation (Fig. 4c).In contrast, in West Nepal, where glacier masses are smaller and their effect on the local climate reduced, the pattern is stationary or slightly increasing (Fig. 4d).It is worth noting that, in both Western and Eastern Himalaya (where decreasing trends are found), trends present a lower magnitude or opposite direction at lower elevations.This further confirms that the drying at high elevation is not caused by changes in large-scale atmospheric circulations.

Modelling the cooling and drying
In the recent decades of global warming (Supplementary Fig. 11), ERA5 shows a positive trend in daytime wind speed of katabatic winds (Supplementary Fig. 12b,c) flowing north and south of the Himalayan chain close to the glacier masses.During warmer days, the glacier nodes of ERA5-Land show more negative air temperature gradients (Supplementary Fig. 13b), resulting in enhanced diurnal downslope density winds (Supplementary Fig. 12c), sustained by the steep orographic gradients of the Himalayan chain (Supplementary Fig. 14).The same causal link between increased air temperature and katabatic wind emerged from high-resolution atmospheric simulations (Supplementary Fig. 15b3).As a result, a decrease in T max is evident in the entire Himalayan region in correspondence to the glacier surfaces of ERA5-Land (Fig. 1a).
In stationary conditions, ERA5-Land shows drying below the glaciers in the afternoon hours concomitant to a negative meridional wind gradient (southerly flow) anomaly (Supplementary Fig. 13d).On the contrary, in non-stationary conditions, a key consequence of the cooling (Fig. 1a) induced by increased katabatic winds (Supplementary Fig. 12), is the decreasing of the diurnal precipitation trends across the entire Himalayan arc (Fig. 4a).This is suggested to be caused by a downshift of the local convergence between up-and down-valley winds, which causes drying conditions at glacier elevations.This hypothesis is supported by our high-resolution atmospheric simulations which show this downshift of the convergence front (Supplementary Fig. 15b3).

Implications for climate change impacts in the Himalayas
Our study has provided evidence for a hitherto missed process in glacier-climate interactions at high elevations.We have described a glacier effect on local climate in the Himalaya, associated with global warming, which has lowered the daytime temperature close to the glacier masses.Potentially, glacier effects on local temperature could occur in other mountain chains, where glaciers can develop katabatic winds according to local climate regime, glacier size, slope and debris cover 16,28 .
The proposed conceptual model reconciles the apparent discrepancy between the observed local cooling and the accelerated glacier mass loss in Himalaya.While atmospheric warming is increasing glacier ablation, the lowering of the near-surface air temperature over the glacier surface and consequent enhancement of katabatic winds has shifted the extent of daytime cooling toward the lower reaches of the glaciers and into the proglacial domain.Whereas the local cooling could have partially protected these low glacier reaches from warming, it has further lowered the elevation of the local wind convergence and consequently precipitation has diminished at high elevation, implying a further negative effect on glacier mass balance.This precipitation decrease was proposed by ref. 29, after observing the glacier changes in the Mount Everest region over the last decades, although its relative impact is currently unknown.
In nearby periglacial areas, the effect we describe will have key implications as the decrease in air temperature has affected the elevation band between the mean elevation of glaciers (~5,400 m a.s.l.) (ref. 2) and the periglacial environment even below 4,500 m a.s.l (Fig. 1b).In those elevation bands, areas of permafrost may exist 30 and their reaction to warming may be moderated by local cooling and drying during the summer, which drives permafrost thaw and associated geomorphic effects and hazards 31 through increasingly deep seasonal thawing.Long-term observations for testing this hypothesis, however, are virtually absent 30 and our study provides a compelling motivation to collect more high-elevation, long-term data to prove the new findings and their broader impacts.
Increased cooling during the early warm season in periglacial areas has the potential to reduce snowmelt rates and lengthen the snow season.However, at Pyramid off , snow cover is not affected by this local cooling since during the warm season the snow cover is almost absent, as T max is largely positive at this elevation (Supplementary Fig. 16).This phenomenon we discuss could substantially influence mountain ecosystems.It is well known that mountain plant species have shifted upslope in response to global warming.Unexpectedly, however, ref. 32 observed stable vegetation line elevation, reduction in greening (productivity loss) and decreasing recruitment during the last three decades in three glacierized areas in the Himalayas, providing a surprising, independent validation of our findings of local cooling and drying.
The process we highlighted is potentially of global relevance and may occur on any glacier worldwide where conditions are similar.We limited our analysis to the Himalaya, also considering the scarcity of data in high-elevation areas across the globe but future research should look into its existence in other regions of the world and into the morphological (for example, debris cover, local topography, total glacier area and glacier energy balance) and climatic factors that control it.Some of those elements have a simplified representation in ERA5-Land because of ERA5-Land coarse spatial resolution (or are absent, for example, debris cover).It is promising, however, that the reanalyses are able to represent the first-order controls and main processes leading to cooling and the generation of katabatic winds.Future research should focus on establishing the factors determining the occurrence, magnitude and downward effects of katabatic processes, the cooling they induce and their ability to change glacier mass balance across climates and regions.
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Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material.If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this license, visit http://creativecommons. org/licenses/by/4.0/.© The Author(s) 2023 https://doi.org/10.1038/s41561-023-01331-yERA5 at increased resolution, which is enhanced globally to 9 km (0.1°) compared to 31 km (0.25°) (ERA5) or 80 km (0.70°) (ERA-Interim), whereas the temporal resolution is hourly and the simulations cover from 1950 to the present as in ERA5 (ref.41).High Asia Reanalysis v.2 is a 10 km resolution dynamical downscaling of ERA5 performed using the Weather Research and Forecasting (WRF) model at present only available from 2004 42 .Another dynamical downscaling is a 6.7 km resolution product derived by Climate Forecast System Reanalysis data using the WRF model 36 available until 2014.These last two high-resolution dynamical downscaling products might have potentially improved the spatial representation of mountain precipitation; however, currently, they do not cover the entire period considered in this study.Therefore, we considered T and P data from the state-of-the-art ERA5-Land gridded climate reanalysis.
ERA5-Land is driven by atmospheric forcing derived from ERA5 near-surface meteorology state and flux fields.The meteorological state fields are obtained from the lowest ERA5 model level (level 137) and include air temperature, specific humidity, wind speed and surface pressure.The 2 m air temperature in ERA5-Land is a diagnostic output similar to ERA5 forecasts; that is, it is calculated with 10 m air temperature from ERA5 and the heat flux from ERA5-Land according to the Monin-Obukhov similarity theory.The surface fluxes include downward shortwave and longwave radiation and liquid and solid total precipitation.These fields are interpolated from the ERA5 resolution of about 31 km to ERA5-Land resolution of about 9 km using a linear interpolation method based on a triangular mesh.
In this regard, the Himalayan trends of ERA5 and ERA5-Land for T max were explored here considering the same analysis period (warm season during 1994-2020).We found that the decreasing pattern diagnosed by the two products (Fig. 1a and Supplementary Fig. 19 for ERA5-Land and ERA5, respectively) is the same even if ERA5 is characterized by a lower spatial resolution.
Moreover, we compared T max observed at the Pyramid off and the corresponding ERA5-Land data in terms of correlation (Supplementary Fig. 20a) and bias (Supplementary Fig. 20b).Generally, the uncertainty of this reanalysis product was found to be low for all months of the year.Particularly, the bias results are constant for all considered years.This means that the non-stationarity is not influenced by possible biases during the analysed period.
Finally, ERA5-Land cannot be tested extensively at high elevations because of the lack of ground observations, while this is possible below 2,000 m a.s.l. for the entire area of Nepal.Therefore, we compared ERA5-Land with 157 ground stations (1994-2020 period).The relevant precipitation trends show good agreement (Supplementary Fig. 18).

Treatment of glaciers.
Particularly important in this study is the representation in ERA5-Land of glaciers.As reported in ref. 41, the model formulation (as in ERA5) does not have an independent treatment of glaciers.Grid points with glaciers are assigned with a constant snow mass of 10 m.A threshold of 50% of a grid box covered by ice is used, below which the snow depth keeps the value computed by the snow scheme of the land model.Values above the threshold assign a snow water equivalent value of 10 m.This condition is used to avoid grid points near glaciers with large unrealistic snow depth that result from the interpolation from ERA5 fields to ERA5-Land 41 .
Precipitation and temperature.Currently, ERA5 is the most accurate of the existing reanalysis products for precipitation detection 43 .Others 44 reviewed some recent studies showing the capability of ERA5 (both for temperature and precipitation) in complex terrain such as High Mountain Asia, where in situ meteorological observations are sparse and unevenly distributed.Among these studies, the performance of ERA5 precipitation over High Mountain Asia was analysed by ref. 45 and ref. 46.They found that ERA5 succeeds in reproducing the interannual and decadal variabilities of precipitation and reflecting the spatiotemporal patterns of precipitation over the whole Tibetan Plateau.Considering the temperature trends in mountains, recent studies, devoted to the comparison of different reanalysis products, point out that ERA5 performs well in capturing the non-stationarity of temperature during last decades on the Tibetan Plateau 47 .
In this study, a similar comparison was carried out in Nepal at low elevations (<2,000 m a.s.l.) for T max and precipitation (Supplementary Figs. 8 and 15, respectively).All these results provide robust evidence about the capability of ERA5 to correctly describe the large-scale trends.Regarding the high Himalayan elevations, Pyramid off data have never been assimilated into ERA5 and this has not occurred for other high-elevated stations as well, considering their general absence at these elevations.Thus, any possible problem related to changes in station density or location over time should not have affected the local climatic trends.Therefore, Pyramid off data are used here for validating the capability of ERA5-Land to simulate the non-stationarity of precipitation and temperature at the local level.
Trends analysis: the sequential Mann-Kendall test.In this study, the Mann-Kendall test (MK) was applied at the monthly scale (after daily data aggregation) to analyse the non-stationarity of observational and reanalysis data.This test is widely adopted to assess significant trends in hydrometeorological time series 48 .This test is non-parametric, thus being less sensitive to extreme sample values and is independent from the hypothesis about the nature of the trend, whether linear or not.The MK test verifies the assumption of the stationarity of the investigated series by ensuring that the associated normalized Kendall's tau-b coefficient 48 , µ(τ), is included within the confidence interval for a given significance level (for α = 5%, the µ(τ) is below −1.96 and above 1.96).We used the SS proposed by ref. 49 as a robust linear regression allowing the quantification of the potential trends revealed by the MK.The significance level is established for P < 0.05.We defined a slight significance for P < 0.10.The uncertainty associated with the SS (1994-2013) is estimated through a Monte Carlo uncertainty analysis.Further details on this uncertainty are reported in ref. 24.In the sequential form (seqMK) µ(τ) the test is applied forward starting from the oldest values (progressive trend) and backward starting from the most recent values (retrograde trend).The crossing period allows us to identify the approximate starting point of the trend.

Fig. 1 |
Fig. 1 | Trend analysis for maximum 2 m air temperature in Himalaya during 1994-2020.a, Himalayan trend of ERA5-Land T max referred to the warm season (1994−2020).The map shows the location of the observational air temperature time series and the glacier mask used in ERA5-Land.Cold and warm colours represent decreasing and increasing trends, respectively, with associated Mann-Kendall (MK) test significance.The insert (i) provides a better picture of the ERA5-Land trends in the surrounding of the analysed ground stations.b, Elevation-dependence of T max SS for all cells in a (grey).The black points represent the cells located inside the glacier mask.c,e, Observational data (Pyramid off ) (c) and the correspondent pixel of ERA5-Land (e).The grids display the results of the MK test applied at the monthly scale and calculated from the beginning of the series to the given year.The colour bar represents the normalized Kendall's tau coefficient µ(τ).The colour tones below −1.96 and above 1.96 are significant (α = 5%).On the right, the monthly Sen's Slope (SS) and the significance levels for 1994-2020 ( • P < 0.1, *P < 0.05, ** P < 0.01, *** P < 0.001).d,f, Observational data (Pyramid off ) (d) and the correspondent pixel of ERA5-Land (f).The progressive µ(τ) (solid lines) and retrograde (dotted line) of the seqMK test (that is, calculated from the beginning or from the end, respectively, of the series to the given year) for the cold season (NDJFMA) (blue), the warm season (MJJASO) (orange) and for the entire year (black).For each year, below-zero lines indicate negative trends (calculated from 1994).

TFig. 2 |
Fig. 2 | Schematic diagrams explaining the air cooling observed in the surroundings of Himalayan glaciers.a,b, In both diagrams, an idealized case of wind interactions (vertical, katabatic and up-valley), that potentially dictate the along-flowline structure of on-glacier air temperature, is represented.a, Diagram for stationary conditions (subscript 0).Points I, II and III indicate locations of interest: point I 0 represents the glacier zone, where T g is more similar to T fa and therefore less influenced by the glacier cooling effect; point II 0 is the glacier part, where ideally the cooling is at its maximum; point III 0 is the convergence zone between upward and downward winds, where the upshift of moist air masses induces the occurrence of precipitation.b, Diagram for non-stationary conditions (subscript 1).Currently, global warming increases the flux of sensible heat toward the glacier surface and thus enhances the cooling of near-glaciersurface air.Consequently, katabatic winds become more intense and capable of drawing further cold air masses from the higher elevations.As a result, the glacier cooling effect reaches downstream the periglacial environment (from II 0 to II 1 ), downshifting the convergence zone (from III 0 to III 1 ).T s , temperature of glacier surface; T a , temperature ambient; T fa , temperature of free atmosphere; T g , 2 m on-glacier temperature.
Articlehttps://doi.org/10.1038/s41561-023-01331-ywater vapour transported by the summer monsoon reducing moisture supply for precipitation over the upper parts of glaciers and thus reducing glacier accumulation.

Fig. 3 |
Fig. 3 | Observations during the warm season on the south slopes of Mount Everest.a, Hourly mean of the 2 m air temperature at Changri on (thick line, mean; shadow area, standard deviation of the warm season).b, Wind rose and downward wind speed among hours of northerly (90°-270°) flow at Changri on (thick line, hourly mean; shadow area, standard deviation of the warm season).c, Boxplots of ozone concentration (n = 8,088, n = 646, n = 390) (i) and air temperature (ii) related to three classes of hourly wind speed at Changri on defined on the basis of the following criteria: wind direction between 90° and 270° (southward) and wind speed >1 m s −1 .When one of these criteria was not satisfied, we included the observations in the 'no event' class (n = 7,578, n = 656,

Fig. 4 |
Fig. 4 | Himalayan trend of ERA5-Land diurnal (8:00-20:00) accumulated precipitation in the warm season (from May to October, during 1994-2020).a, The map reports the glacier mask used in ERA5-Land and the −0.01 °C yr −1 T max cooling trend isolines (Fig.1).Cold and warm colours represent decreasing and increasing precipitation trends (% yr −1 ), respectively.The black line indicates their significance (P < 0.05).Three regions of interest have been selected to