Since the 1980s, the subtropical jet stream has generally moved poleward, but its behavior varies strongly by region and season. Here we examine the interannual variability and trends in the latitudinal position of the spring subtropical jet over the Himalayas. During the modern period (1948 to 2018), the jet is typically anchored immediately south of the Himalayas, but in four springs (1956, 1971, 1984 and 1999), it moved poleward to pass over Kyrgyzstan and north-west China. A tree-ring-based reconstruction of the interannual variability in spring Himalayan jet latitude indicates that, relative to the past four centuries, such poleward excursions have been more common in the latter half of the 20 century. These new insights into the behavior of the Himalayan subtropical jet can improve spring weather forecasts for the region and provide a real-world target for climate simulations to test whether recent excursions can be attributed to anthropogenic warming.

The latitudinal position of the subtropical jet over the Himalayas (Himalayan jet latitude or HJL) controls the region’s climate during winter and spring by guiding moisture-delivering storms. Here we use the Community Earth System Model-Last Millennium Ensemble to diagnose forced trends in HJL during the past millennium. During 850-1849, there is a weak equatorward trend in winter HJL. In contrast, the spring HJL has a relatively larger poleward trend, and increases in both variance and frequency of poleward/equatorward excursions. We demonstrate changes in orbital precession reduced the thermal gradient between tropical and subtropical Asia, shifting the spring HJL poleward. During 1850-2005, the spring HJL exhibits no trend due to compensating influences from orbital and anthropogenic greenhouse gas forcings. These findings suggest it is essential climate models properly simulate the effects of and potential interactions between orbital forcing and anthropogenic factors to accurately project Himalayan jet variability and associated storm tracks.