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

Abstract

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.

Orbital Forcing Strongly Influences the Poleward Shift of the Spring Himalayan Jet During the Past Millennium

“…When compared over the period of overlap , the HJL variance in CESM1 does not match with tree-ring reconstruction. The CESM1 shows a negative trend in spring HJL over the past four centuries, which is different from the positive trend in tree-ring reconstructed variance in spring jet latitude (Thapa et al, 2020;Figure S4b in Supporting Information S1). The CESM1 also overestimates the magnitude of variance relative to the reconstruction.…”

“…The greenhouse gas forcing tends to cancel the effects of natural forcings, reducing variance during both seasons. Tree‐ring reconstructions show enhanced variance in jet position over Asia during the twentieth century (Thapa et al., 2020; Wright et al., 2015). In other regions, observation‐ and model‐based studies also demonstrate enhanced variability since the mid‐twentieth century, and this trend has been attributed to anthropogenic climate change (Barnes & Polvani, 2013; Coumou et al., 2015; Francis & Vavrus, 2012; Mann et al., 2017).…”

“…In order to evaluate model performance in reproducing STJ properties, we used monthly wind data from several reanalyzes including the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP/NCAR) 40‐year reanalysis project (Kalnay et al., 1996), the ERA‐5 (Bell et al., 2020; Hersbach et al., 2018), the Japanese 55‐year Reanalysis (Kobayashi et al., 2015), and the twentieth Century Reanalysis‐Version3 (Slivinski et al., 2019). Based on its comparatively reliable jet variance and longer calibration period, we chose to use NCEP/NCAR data, which has a 2.5°resolution, as the primary validation data set (see Thapa et al., 2020 for a detailed explanation).…”

“…Model simulations of summer jet over eastern and western Asia also show no long‐term trend in its location (Jiang et al., 2020). However, there has been a significant increase in interannual STJ variance over central and East Asia during the past millennium (Jiang et al., 2020; Thapa et al., 2020; Wright et al., 2015). In addition, a recent tree‐ring reconstruction shows a positive trend in extreme poleward excursions of the spring STJ over the Himalayas during the past four centuries (Thapa et al., 2020).…”

“…However, there has been a significant increase in interannual STJ variance over central and East Asia during the past millennium (Jiang et al., 2020; Thapa et al., 2020; Wright et al., 2015). In addition, a recent tree‐ring reconstruction shows a positive trend in extreme poleward excursions of the spring STJ over the Himalayas during the past four centuries (Thapa et al., 2020). Understanding the dynamics of potential forced trends in the mean and extreme positions of the jet is needed to determine its sensitivity in the face of anthropogenic climate change.…”