Warming and drying over the central Himalaya caused by an amplification of local mountain circulation

The central Himalayas are experiencing significant climate change, with rising temperatures and reduced monsoon precipitation in recent decades. These changes threaten the region's glaciers, vital water sources for millions downstream. Reanalyses reveal a large-scale upper-tropospheric circulation trend over Asia, with increasing westerly influence at low latitudes, potentially linked to the 'Silk Road Pattern' teleconnection. To understand the impact on the Himalayas, a 36-year dynamical downscaling (1979-2015) using the Weather Research and Forecasting (WRF) model and Climate Forecast System Reanalyses (CFSR) was performed. The downscaling, at 6.7-km grid spacing, covers the Tibetan Plateau, Himalayas, and Karakoram. Previous analyses of this downscaling showed surface warming and drying in the central Himalayas. This study focuses on a subregion (86–92°E and 26–29°N) during the summer monsoon (JJA), where significant warming and drying trends coincide, to examine trends in the diurnal cycle of precipitation, temperature, and winds. The pronounced diurnal cycle, characterized by daytime upslope winds and nighttime downslope winds, significantly impacts precipitation distribution. The hypothesis is that the observed warming and drying are linked to a complex response of the diurnal mountain-valley circulation to the regional summer trend.

Existing literature demonstrates that the central Himalayan region has experienced significant warming and drying in recent decades. Studies highlighting the rapid retreat of glaciers in the region, driven by these meteorological changes, are numerous. The dependence of downstream regions on these glaciers for water resources is also well-established. Reanalysis data indicates a trend towards increased westerly influence at lower latitudes in the upper troposphere over Asia, potentially linked to the Silk Road Pattern, a major teleconnection pattern over Eurasia. Previous research using the same WRF downscaling dataset examined the seasonally averaged variables, but this study focuses on analyzing the diurnal cycle, a key element of the Himalayan monsoon climate.

A 36-year (1979-2015) high-resolution dynamical downscaling was conducted using the Weather Research and Forecasting (WRF) model (version 3.7). The model utilized two nested grids (20 km and 6.7 km) and was driven by the Climate Forecast System Reanalysis (CFSR) data for initial and boundary conditions. The inner domain encompassed the Tibetan Plateau, Himalayas, and Karakoram ranges. The downscaling consisted of 36 individual 13-month simulations (March to April) for each year, with the first three months discarded as spin-up. The Noah multiparameterization land surface model was employed. No convective parameterization was used in the inner domain. The analysis focuses on a smaller subregion (86–92°E, 26–29°N) during the June-July-August (JJA) period. Three-hourly data were archived, allowing for an examination of the diurnal cycle. Trends were calculated for precipitation, temperature (2m), and 10m winds, using a two-tailed Monte Carlo significance test. Climatological values were also computed for comparison. For 3-D variables, data were first interpolated to pressure levels. The analysis employed various methods to examine trends at different times of day and elevations, including averaging over large numbers of grid points and examining individual grid point trends. Comparisons were made with other reanalysis data (ERA-Interim, MERRA-2, JRA-55) to assess the robustness of the findings.

The study revealed contrasting trends in the diurnal cycle. Daytime warming at high elevations is primarily during the day, enhancing anabatic (upslope) winds. This daytime upslope wind trend prevented drying at high elevations. Conversely, significant nocturnal drying is observed at high elevations, associated with enhanced katabatic (downslope) winds. Clearer skies at night led to increased radiative cooling and amplified downslope winds. The analysis of trends in 2-meter temperature, 10-meter winds, and precipitation at various times of day (1200-1500 and 2100-0000 UTC+6) highlighted the non-uniform nature of these changes across the diurnal cycle. The downslope wind trend was more substantial than the upslope trend, both in magnitude and spatial extent. Nocturnal precipitation decrease was primarily between 1800 and 0300 local time. Analysis of cross-sections at 90°E illustrated the amplification of both daytime upslope and nighttime downslope wind accelerations. Increased daytime warming of slopes relative to the free troposphere enhanced upslope winds. At night, reduced warming of slopes compared to the free troposphere strengthened downslope winds. The daytime warming is explained by global warming effects feeding into the WRF simulations through reanalysis data. No significant radiative contribution to warming was observed. However, decreasing snow cover, due to warming and reduced pre-monsoon snowfall, reduced albedo, leading to more incoming shortwave radiation being absorbed. At night, significant negative trends in upper- and mid-tropospheric cloud fraction increased outgoing longwave radiation (OLR). This increased radiative cooling amplified the downslope winds and contributed to the nocturnal drying, particularly at elevations above 3 km. The enhanced downslope winds converged in valleys, preventing drying at lower elevations. This explains the discrepancy between the downscaling results and observational data showing no overall summer precipitation trend at low-elevation stations.

The findings demonstrate that the warming and drying trends in the central Himalayas are not uniformly distributed throughout the day. The amplified diurnal cycle of mountain-valley winds plays a crucial role in these trends. The daytime upslope winds prevent drying, while nighttime downslope winds are primarily responsible for the high-elevation drying. The study's results are consistent with observed glacier retreat and suggest that the amplified diurnal cycle has significantly impacted water resources in the region. The findings highlight the limitations of global and regional climate models in representing these complex orographic processes. Global models tend to project widespread wetting trends, overlooking the potential for drying at high elevations due to the amplification of downslope winds. The use of high-resolution modeling is critical for accurately representing the intricate interactions between topography, radiation, and atmospheric circulation in mountainous regions. This is essential for improving projections of future water resources in the region.

This study reveals a crucial mechanism driving warming and drying in the central Himalayas: an amplification of the diurnal mountain circulation. Enhanced daytime upslope winds prevent daytime drying, while strengthened nighttime downslope winds cause significant high-elevation nocturnal drying. These results underscore the importance of high-resolution modeling to capture complex orographic effects and improve predictions of future hydroclimate and water resource availability. Future research should explore the influence of larger-scale climate patterns on this amplified diurnal cycle and further refine high-resolution climate projections in mountainous regions.

The study's use of a 6.7 km grid spacing represents a limitation, although testing with a finer resolution (2.2 km) indicated no significant changes in the diurnal cycle patterns. The lack of convective parameterization in the model might have impacted the representation of some convective-orographic systems, potentially influencing some of the trends, particularly during the day. The initialization of surface properties from the reanalysis data could also introduce uncertainties, particularly concerning albedo trends and snow cover. The balance between incoming shortwave radiation and surface albedo is also model-dependent and might vary with different model configurations.