Poleward Excursions by the Himalayan Subtropical Jet Over the Past Four Centuries

Since the 1980s, the subtropical jet stream has generally moved poleward, but its behavior varies 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 spring (March‐April‐May) jet is typically anchored immediately south of the Himalayas but has rarely (in 1956, 1971, 1984, and 1999) moved poleward to pass over Kyrgyzstan and northwest China. A tree‐ring reconstruction of the jet's latitude indicates that such poleward excursions may have become more frequent after 1950, but it is not clear whether that behavior is unprecedented within the past four centuries. These insights into the behavior of the Himalayan subtropical jet may improve seasonal weather forecasts for the region and provide a target for climate simulations to test whether the recent spate of excursions is unusual and due to anthropogenic warming. Plain Language Summary The subtropical jet stream (STJ) is a band of high‐speed westerly winds in the upper atmosphere located near the subtropics. Since the 1980s, the STJ has generally moved poleward, but its behavior varies strongly by region and season. We studied the STJ over the Himalayas and found, during 1948–2018, that the spring STJ is typically anchored near the Himalayan foothills. In a few years (1956, 1971, 1984, and 1999); however, it has moved far north to pass over Kyrgyzstan and northwest China. A tree‐ring reconstruction of the jet's north‐south position suggests that this type of poleward tracks has happened before, but the past few decades may be unusual due to the number of such events. Including the potential north‐south range of the Himalayan subtropical jet into forecasting models might help improve the accuracy of spring weather predictions in this region. Our multicentury record can be used to test how well models simulate the Himalayan jet and investigate whether anthropogenic warming is the cause of the recent poleward extremes. Key Points The spring subtropical jet is typically anchored near the Himalayan foothills but did move far poleward in a few cases between 1948 to 2018 We used tree‐ring chronologies to reconstruct the latitudinal position of the spring Himalayan subtropical jet back to 1625 Poleward excursions may be more common after 1950 but are not necessarily captured reliably by the reconstruction