Abstract
High Mountain Asia (HMA) is dependent upon both the amount and timing of snow and glacier meltwater. Previous model studies and coarse resolution (0.25° × 0.25°, ∼25 km × 25 km) passive microwave assessments of trends in the volume and timing of snowfall, snowmelt, and glacier melt in HMA have identified key spatial and seasonal heterogeneities in the response of snow to changes in regional climate. Here we use recently developed, continuous, internally consistent, and high-resolution passive microwave data (3.125 km × 3.125 km, 1987–2016) from the special sensor microwave imager instrument family to refine and extend previous estimates of changes in the snow regime of HMA. We find an overall decline in snow volume across HMA; however, there exist spatially contiguous regions of increasing snow volume—particularly during the winter season in the Pamir, Karakoram, Hindu Kush, and Kunlun Shan. Detailed analysis of changes in snow-volume trends through time reveal a large step change from negative trends during the period 1987–1997, to much more positive trends across large regions of HMA during the periods 1997–2007 and 2007–2016. We also find that changes in high percentile monthly snow-water volume exhibit steeper trends than changes in low percentile snow-water volume, which suggests a reduction in the frequency of high snow-water volumes in much of HMA. Regions with positive snow-water storage trends generally correspond to regions of positive glacier mass balances.
1. Introduction
Rivers draining from High Mountain Asia (HMA) are relied upon by more than a billion people for agriculture, hydropower, and household use (; ; ). In much of HMA, snow and glacier meltwaters provide key seasonal water buffers that help maintain water availability year-round (; ; ; ; ; ). A large body of research has identified significant changes in HMA’s cryosphere in recent decades, and in particular, the retreat of many regional glaciers (e.g., ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ); however, there exist large spatial heterogeneities in glacier trends (; ; ; ; ; ). Previous work has also identified spatial and seasonal patterns in snow depth and the timing of snowmelt in HMA, which are mostly coherent with changes in glacier mass balances (; ; ; ; ; ). In-depth analyses of changes in HMA’s cryosphere are often limited by lack of in-situ data and rugged terrain which hinders high-resolution data collection (); estimates of climate trends from in-situ, satellite, and modeled data often result in heterogeneous and complex spatial patterns ().
Passive microwave data have long provided the best global dataset for studying snow depth and snow-water storage (). However, they are limited by spatial resolution—data are typically available as 0.25° × 0.25° (∼25 km × 25 km) grid cells which hinders many analyses. Recently, the National Snow and Ice Data Center has re-gridded and re-processed the special sensor microwave imager (SSMI, 1987–2009) and special sensor microwave imager/sounder (SSMI/S, 2003–2016) to a 3.125 km × 3.125 km (∼10 km2) spatial resolution (). In this study, we leverage this high-resolution, cross-calibrated, multi-satellite dataset to consider 1.02 million passive microwave grid cells over 29 complete October-September water years across HMA (25–45°N, 60–110°E, 1987–2016; Figure 1). The enhanced resolution of this dataset allows us to more closely examine spatio-temporal trends in snow-water storage which have previously been shown to have strong impacts on climate and glacier dynamics in the region (; ; ; ).
FIGURE 1
2. Data and Methods
2.1. Study Area and Data Sources
Our study covers the region from 25 to 45°N and from 65 to 105°E, running across some of the most densely populated regions of the world. Several key watersheds, such as the Indus, Syr Darya, Amu Darya, Yangtze, Salween, and Ganges/Brahmaputra drain from HMA (Figure 1A, blue outlines).
Precipitation in HMA is driven by three main weather systems—the Indian Summer Monsoon, the East Asian Summer Monsoon, and the Winter Westerly Disturbances (; ). These major weather systems interact to bring a heterogeneous mix of snow and rain to different regions of HMA. Recent changes in the timing and intensity of precipitation from these major weather systems have been observed (e.g., ; ; ; ; ; ; ; ), and have been shown to impact the timing and volume of snow-water storage and snowmelt (; ; ).
2.2. Satellite Data Preparation
Snow has been extensively studied with passive microwave data—albeit at low spatial resolutions (e.g., 0.25° × 0.25°) (; ; ; ). Recent image processing advances have allowed researchers to take advantage of the elliptical nature of passive microwave footprints to re-process the data onto a much finer spatial grid than previous approaches had allowed. In this study, we use the EASE-grid 2.0 high-resolution passive microwave product (1987–2016) (; ; ; ), which provides the 19 and 37 GHz passive microwave frequencies at spatial resolutions of 6.25 and 3.125 km, respectively. This dataset has been carefully cross-calibrated between the various SSMI and SSMI/S satellite platforms to provide consistent and homogenized data through the entire time series ().
To produce consistent snow-water equivalent (SWE) estimates over the entire study region, we further re-grid the 19 GHz passive microwave data to a 3.125 km spatial resolution. We then remove areas near lakes and areas with shallow or infrequent snow-cover, as these areas are not suitable for long-term SWE trend analysis. Finally, following the methodology of , we use the computationally efficient algorithm proposed by to convert the passive microwave data to snow depth:
We then convert those snow depth estimates to SWE using a constant snow density of 0.24 g/cm3, which has been shown to be a reasonable global average (; ). In short, the algorithm uses the difference between the 19 and 37 GHz passive microwave channels to estimate snow depth based on a comparison with extensive snow survey data collected throughout the Canadian and Russian Arctic (; ). This algorithm is widely used to estimate SWE over diverse terrain, and has served as the basis for further updates to passive microwave SWE retrieval algorithms which take advantage of additional passive microwave channels not carried on SSMI/S to better constrain the impacts of vegetation cover on SWE estimates (; ; ; ; ; ; ). In our low-vegetation study area, we rely on the algorithm to take advantage of the full SSMI/S time series.
2.3. Trend Analysis
For parts of the passive microwave time series, there are multiple overlapping satellite overpasses. For consistency, we aggregate all night-time overpasses (October 1987–September 2016) into an average daily SWE estimate over HMA (number of grid cells = 1,027,847) using the median of all available night-time measurements per day. As the various SSMI satellite platforms have been carefully cross-calibrated (), this step serves simply to homogenize the temporal sampling of the dataset over the entire time period. For computational efficiency, we then further resample each individual daily SWE time series to a temporal frequency of three days before computing trends; in our tests this does not significantly modify computed long-term trends.
Before trend analysis, we first remove the seasonal component of each individual time series via Seasonal Trend Decomposition by Loess (), using a decomposition window of 365 days (Figure 2). This method yields a seasonal signal, long-term signal, and residual short-term signal from a given time series by removing oscillations at the chosen decomposition time frequency. We then test the resulting de-seasoned time series for significant increasing or decreasing trends using the Mann-Kendall test (; ). If there exists a significant trend, we use Sen’s slope method to capture the overall trend at that grid cell (). We thus use a conservative approach by testing for significance both with the Mann-Kendall test and via Sen’s slope method. We only present results from statistically significant (p < 0.05) trends in this study.
FIGURE 2
3. Results
3.1. Long-Term Snow-Water Equivalent Trends
Aggregate trends over the entirety of HMA are slightly negative (sum: −55.5 mm/yr, average: −0.01%) over 3,618 km2× 103 km2, including only trends with p < 0.05 and areas at least 500 m above sea level). While the aggregate trends appear to be small, we emphasize that trends are measured over 9.75 km2 grid cells, and represent a snow-water storage loss of 5.41 m3 × 105 m3 of water per year (Figure 3).
FIGURE 3
It is clear that SWE trends are spatially diverse—positive SWE trends are concentrated in the Karakoram, Pamir, Kunlun Shan, and the high Himalaya (Figures 3 and 4A–D). The most negative trends are concentrated in the south-eastern Tibetan Plateau (Figures 4E,F), at the headwaters of the Yangtze, Salween, and Mekong rivers. There also exist many small-scale features; for example, there are clear alternating positive-negative SWE trend patterns along the front of the Himalaya. While there are multiple possible causes for such small-scale variability, the extreme topography and the microclimates it creates can drastically alter snow-loading and snow-water storage on nearby slopes—particularly when there are large differences in the sun exposure and overall aspect of neighboring slopes (Supplemental Figures S1–S5).
FIGURE 4
3.2. Seasonal Snow-Water Equivalent Trends
When the SWE data are further divided into seasonal components, clear differences in trend direction and magnitude appear (Figure 5). Strong positive winter (December-January-February) trends are visible in most of the highest peaks of HMA—running along the Himalaya, into the Pamir-Karakoram-Kunlun Shan region, as well as through the Tien Shan. These positive trends are visible through the spring (March-April-May) and fall (September-October-November) seasons as well; the only region, however, to maintain positive trends through the full year and in each seasonal slice is the Karakoram-Kunlun Shan region, which has been noted for glacier stability and growth in recent years (; ; ; ; ). These positive trends are offset by large negative SWE trends in lower-elevation regions of HMA and along the eastern edge of the Tibetan Plateau which has seen rapidly decreasing SWE—particularly in the December-January-February and September-October-November periods (Figure 5).
FIGURE 5
3.3. Magnitude Variations in Snow-Water Equivalent Trends
To further explore the dynamics of SWE trends in HMA, we have performed a second set of regressions using monthly SWE percentiles (Figure 6). In short, we calculate the 10th, 25th, 50th, 75th, and 90th percentile SWE value at each pixel over each month using daily-averaged SWE data (October 1987–September 2016), remove the long-term monthly mean value for each given month to reduce the impacts of seasonality, and perform regressions through each SWE percentile separately. This yields a set of SWE trend results based on only the lowest (e.g., 10th percentile) or highest (e.g., 90th percentile) SWE value for each month.
FIGURE 6
When the SWE trend magnitudes at each percentile are compared, differences between high- and low-percentile trends are apparent (Figure 6). In almost all cases, the trends in high-percentile SWE are steeper than those in low-percentile SWE. In positive SWE-trend regions, this indicates that high SWE amounts are becoming relatively more frequent. For example, along the border of India and Pakistan, positive SWE-trend regions (see Figure 3) have a more than six-fold higher trends in 90th percentile monthly SWE than in 10th percentile SWE, indicating a drastic increase in monthly high-SWE day frequency or magnitude. This could be due to the increased strength of the Winter Westerlies, which has been previously reported (). In contrast, positive SWE-trend areas in the Pamir have higher 10th percentile magnitudes than 90th, indicating a that positive SWE trends are driven by increases in low-magnitude SWE rather than in high-magnitude SWE.
In the majority of HMA, however, SWE trends are negative (Figure 3). Thus, the high 90th/10th (75th/25th) percentile trend ratios indicate that declines in SWE have been steeper in the higher end of the monthly SWE distribution, and that high-SWE values are becoming less common overall. This agrees well with previous research, which points to overall increasing temperatures in HMA, particularly on the Tibetan Plateau (e.g., ).
4. Discussion
4.1. Comparison With Previous Work
Previous work by used data at 0.25° × 0.25° spatial resolution from only the SSMI-series of satellites (1987–2009) to establish trends in SWE over HMA. The higher spatial resolution data used in this study yields only slight differences in SWE trend when the same period (e.g., 1987–2009) is considered. However, there are clear differences in the trends presented by and those shown in Figure 3, which are due to the difference in analysis time window. To test the sensitivity of SWE trends to the analysis window, we first break our dataset into three decade-long slices, as seen in Figure 7.
FIGURE 7
It is clear that SWE trends are highly variable in time. The long-term reversal from negative to positive SWE trends seen in Figure 7, however, is supported by analysis of other related climate variables. Previous work has noted changes in regional precipitation and temperature patterns (e.g., ; ; ; ; , ; ; ; ; ) and increases in high-elevation snowcover (; ) in recent years. Furthermore, showed that increasing lake levels on the Tibetan Plateau are strongly correlated with regions of increased precipitation; modeled precipitation data suggest stepwise increases in mean annual precipitation on the Tibetan Plateau between the ∼1980s–1990s and 2000s–onwards (e.g., ).
Recent analysis also indicates that the timing of the snowmelt season has changed over the past decades (). While the long-term trends (1987–2016) were found to be generally negative (e.g., earlier and more rapid snowmelt), recent trends (e.g., 2004–2016) were found to be much more positive (later and slower onset of snowmelt) (). It is therefore possible that there has been a reversal of the long-term losses in SWE storage in HMA; however, it is not clear if this is a temporary or long-term shift in the snow dynamics of HMA.
4.2. Sliding Window and Spatio-Temporal Trend Analysis
The timing and magnitude of SWE trend variability can be further explored by performing the same trend analysis on a set of time windows and start dates. We use time windows of 5, 10, 15, 20, 25, and 29 years, along with each possible combination of start years (e.g., 1987–2011) to examine changes in SWE trends through time (Figure 8).
FIGURE 8
Long-term (>25 years) SWE trends are generally negative; these trends also correspond to a particularly negative period of short-term trends starting in the late 1980s (Figure 8). More recent trends (e.g., 15–20 years) are generally positive starting in the early 1990s. One possible explanation for this phenomena is the previously proposed large-scale changes in regional precipitation over the past decades (e.g., ). However, the impacts of changes in temperature cannot be ruled out—increasing regional temperatures can have highly variable positive and negative impacts on snow-water storage, for example, by enhancing atmospheric water content, snow density, and snowmelt rates.
There also exist strong regional variations in windowed trends (Figures 8B–D), driven by differences in climatic conditions, major weather systems, snow accumulation and ablation regimes, and dust and aerosol melt forcing between regions (; ; ). Generally positive SWE trends in the Kunlun Shan region are contrasted by mixed trends in the Karakoram, and majority negative trends in Eastern Tibet (Figures 3 and 4).
4.3. Relationship to Regional Glacier Changes
Many recent studies have investigated changes in HMA’s glaciers using a range of satellite (; ; ; ; ) and modeling (; ) approaches to derive spatial patterns in glacier gains and losses. Using the Randolph Glacier Inventory (), we can measure the areal extent of glaciers within each passive microwave pixel, and—where glaciers are large enough—derive SWE trends over only glaciated areas, defined here as areas with at least 10% glacier coverage (Figure 9).
FIGURE 9
In general, SWE trends over glaciated terrain are negative, outside of parts of the Tien Shan, Karakoram, and Kunlun Shan. Areas with positive SWE trends agree well with regions of positive glacier mass balance, as presented by and . While there are many factors that influence glacier dynamics, it is likely that changes in snowfall are one of the key drivers of glacier mass gain and loss over HMA (; ; ).
4.4. Data Caveats
It is important to mention caveats to the trend analysis presented in this study. The largest caveat is that passive microwave SWE estimates are often uncertain—especially over large and complex regions such as HMA (; ; ). We also cannot rule out the impacts of both natural seasonality and regional temperature changes on snow densities, which could also modify passive microwave SWE estimates over the course of our time series, and thus are part of the trends that we present as changes in SWE in this study (; ; ). The impact of seasonal oscillations in snow density is somewhat mitigated by removing the seasonal cycle from our data before trend fitting, as some of the seasonality in SWE estimates will be driven by changes in snow density. However, without a more in-depth understanding of snow-density evolution in HMA, we cannot fully constrain this part of our analysis.
Passive microwave signal saturation could bias the presented SWE trends in deep-snow areas, as previous work has suggested that passive microwave SWE estimates saturate around 200 mm of SWE (; ; ). In examining our dataset, we find that the vast majority of HMA is not severely impacted by passive microwave signal saturation (Figure 10); however, it is likely that passive microwave signal saturation still biases some of our trend results. In our percentile regressions, we find that SWE trends generally maintain a consistent direction between high- and low-percentiles, indicating that saturation biases don’t drastically influence SWE trend direction (Figure 6).
FIGURE 10
The third caveat is that trends are somewhat biased by the first and last values of the time series—this could also play a role in the trend reversals seen between previous studies of SWE trends in HMA (e.g., ; ) and this study (Figures 3 and 7). We attempt to minimize this effect by using Sen’s slope estimator, which is less sensitive to the first and last values of the time series (). We further attempt to mitigate the impact of the time window over which the trend is calculated by using multiple overlapping time windows and window lengths (Figure 8). Finally, we compare our results to a percentile-regression approach and find that while the magnitudes of the trends vary between percentiles and between the de-seasoned trend and percentile approaches, the trend directions found are consistent. However, snowfall can have high inter-annual variability, and we do not preclude the possibility that variations in the timing of large snowfall events between years, or shifts in the timing of snowfall and snowmelt () could impact estimated annual and seasonal SWE trends.
Lastly, it is important to emphasize that the trends presented here are relative to the SWE time series as estimated, and are not calibrated by in-situ measurements. While the SWE algorithms used here have been extensively validated throughout the world and have been shown to be generally reliable in low-vegetation areas (; ; ; ; ; ; ; ; ; ; ), the complex topography and inaccessibility of HMA poses unique challenges for in-situ data collection. Unfortunately, calibration data of sufficient spatial and temporal resolution to properly assess our SWE estimates and trend results is not currently available in our study region. Future work could consider other approaches to constraining the estimated SWE trends, for example, by using watershed-level snowmelt runoff measurements across HMA.
5. Conclusions
The increased fidelity and spatio-temporal resolution of newly re-processed passive microwave data allows for important updates to analyses of trends in snow-water storage over HMA. While overall trends are negative, there exist large spatial and seasonal heterogeneities in snow-water storage trends. High variability in year-to-year snowfall means that trends are strongly influenced by the start and end years of any trend analysis. By using multiple overlapping time windows, we show that while long-term snow-water storage trends are majority negative, recent (e.g., past 20 years) trends are more positive. Furthermore, by using a percentile regression approach, we show that trends in high-percentile monthly SWE are generally steeper than those in low-percentile, indicating that there have been spatially diverse changes in the magnitude distributions of SWE across HMA.
Snow-water storage trends over glaciated regions generally align with previous estimates of glacier mass balance—those glaciers that are growing are highly correlated with regions of positive snow-water storage trends. However, snow-water storage trends are distinct between regions and watersheds, and can vary greatly over small distances. As the combined meltwaters from both snow and glaciers are essential to year-round water provision in the densely populated regions surrounding HMA, any changes in water storage must be considered in local and regional water planning. The high resolution and long time series data presented here allows for new and improved estimates of changes in snow-water storage that can be used to inform regional and local analyses of future water availability and watershed-level management plans.
Funding
The State of Brandenburg (Germany) through the Ministry of Science and Education and the NEXUS project supported TS for part of this study (grant to BB).
Statements
Data availability statement
The passive microwave and snow-cover datasets used in this study are available from the NSIDC (; ). Our derived SWE trends are available on Zenodo: https://zenodo.org/record/3898517 ().
Author contributions
TS and BB designed the study, TS prepared and analyzed all data, and BB contributed to the development of the methodology. Both authors wrote the manuscript led by TS.
Acknowledgments
We also acknowledge support from the BMBF ORYCS project.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2020.559175/full#supplementary-material
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Summary
Keywords
snow, glacier, climate change, passive microwave, special sensor microwave imager, special sensor microwave imager/sounder
Citation
Smith T and Bookhagen B (2020) Assessing Multi-Temporal Snow-Volume Trends in High Mountain Asia From 1987 to 2016 Using High-Resolution Passive Microwave Data. Front. Earth Sci. 8:559175. doi: 10.3389/feart.2020.559175
Received
06 May 2020
Accepted
18 August 2020
Published
11 September 2020
Volume
8 - 2020
Edited by
Mohamed Rasmy, National Graduate Institute for Policy Studies, Japan
Reviewed by
Maheswor Shrestha, Water and Energy Commission Secretariat, Nepal
Renguang Wu, Institute of Atmospheric Physics (CAS), China
Jeff Dozier, University of California, Santa Barbara, United States
Updates
Copyright
© 2020 Smith and Bookhagen.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Taylor Smith, tasmith@uni-potsdam.de
This article was submitted to Cryospheric Sciences, a section of the journal Frontiers in Earth Science
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