Multi-Annual Variations in Winter Westerly Disturbance Activity Affecting the Himalaya

Winter Westerly Disturbances (WWD) are the primary climatic influence within High Mountain Asia (HMA) during the boreal winter. The objective of this research was to investigate the spatiotemporal linkages among regional extreme precipitation and large-scale circulation in High Mountain Asian (HMA). Our analysis relies on remotely sensed (TRMM 3B42) and ground interpolated precipitation (APHRODITE), and on reanalysis data (CFSR). We observed differences in circulation between central Himalaya (CH) and western Himalaya/ Karakoram (KH) significant precipitation events, which evidenced that though systems producing extreme precipitation in the two regions are dynamically similar, they are often spatiotemporally independent.

Wavelet power spectrum analysis of 200hPa geopotential height (H200) anomalies indicated differing trends in synoptic scale variability across HMA. The zonal track of WWD to the west of the Himalaya experienced an increase in the magnitude of individual events since 1979, as well as the frequency of strong events. Specifically, the synoptic power spectrum for the region corresponding to the center of the WWD trough affecting the KH during day-0 of lag-composite analysis experienced an intensification of activity for the period 1979-2010 with the strongest increase occurring from 2003-10. In contrast, the region corresponding to CH WWD activity observed a significant negative trend since 1979. The dipole in WWD activity trends between the respective locations of upper-level troughs that are responsible for extreme precipitation in the KH and CH likely elicited changes in regional precipitation patterns.

This study further examined linkages between synoptic power of the upper level geopotential height field, precipitation, and upper tropospheric circulation as modified by the main modes of climate variability affecting Asia on interannual time-scales. The Siberian High (SH), the Arctic Oscillation (AO), and the El Niņo-Southern Oscillation (ENSO) were investigated with regard to HMA climate. Although the relationships between these modes and WWD propagation are complex, their signatures in the upper-level jet and regional precipitation clearly evidence the importance of their multiannual variability for the observed trends in KH and CH synoptic disturbance activity. Positive phases of the SH and AO are linked to a dichotomous response between the KH and CH, with CH precipitation increasing during positive (negative) AO (SH) months. A strong AO increases the magnitude of westerlies over Asia, thus enhancing divergence aloft and reducing subsidence over Siberia, which is associated with the negative phase of the SH. After 1989, the AO index was strongly positive with decreasing strength into the 2000s. The strong positive phase of the AO appears related to a strengthening of WWD activity in the Himalaya and is associated with increased precipitation. Additionally, composites of precipitation during El Niņo/La Niņa events indicated increased precipitation and the delayed northward migration of the jet during the index's warm-phase. Understanding linkages between HMA climate and the global atmosphere will improve projections of regional WWD variability into future decades and minimize uncertainty surrounding Asia's water resources.

Currently, our group is evaluating CMIP5 model simulations in reproducing the AO and SH, which affect extreme precipitation events in HMA. Using these data, we will establish the influence of the given modes over longer periods and will compare historical with unforced simulations to determine the role of climate change in modified synoptic variability over Asia.