Linking the North Atlantic Oscillation to winter precipitation over the Western Himalaya through disturbances of the subtropical jet

Abstract

Winter (December to March) precipitation is vital to the agriculture and water security of the Western Himalaya. This precipitation is largely brought to the region by extratropical systems, known as western disturbances (WDs), which are embedded in the subtropical jet. In this study, using seventy years of data, it is shown that during positive phases of the North Atlantic Oscillation (NAO+) the subtropical jet is significantly more intense than during negative phases (NAO−). Accordingly, it is shown that the NAO significantly affects WD behaviour on interannual timescales: during NAO+ periods, WDs are on average 20% more common and 7% more intense than during NAO− periods. This results in 40% more moisture flux entering the region and impinging on the Western Himalaya and an average increase in winter precipitation of 45% in NAO+ compared to NAO−. Using empirical orthogonal function (EOF) analysis, North Atlantic variability is causally linked to precipitation over North India—latitudinal variation in the jet over the North Atlantic is linked to waviness downstream, whereas variation in its tilt over the North Atlantic is linked to its strength and shear downstream. These results are used to construct a simple linear model that can skilfully predict winter precipitation over north India at a lead time of one month.

Weakening of western disturbances in response to polar sea ice melt in climate model simulations

“…This analysis reveals that the weakening of WDs happen across all intensity The baroclinic instability associated with the subtropical jetstream (STJ) creates a favourable condition for the genesis and development of WDs (Dimri, 2013;Hunt et al, 2018b;Hunt and Zaz, 2022). The interannual variability in WD activity and winter precipitation over the western Himalayan and Hindu Kush region have been linked to the variability in the STJ (Yadav et al, 2009;Filippi et al, 2014;Hunt and Zaz, 2022). Therefore, the response of STJ to the polar sea ice melt needs to be carefully examined.…”

“…The observations suggest that the STJ has become more wavy in the recent decades in response to an overall weakening of the meridional temperature gradient (Francis and Vavrus, 2015). Baroclinic instability associated with the STJ creates favourable conditions for the growth of synoptic-scale baroclinic disturbances known as western disturbances (WDs; Dimri et al, 2016;Hunt et al, 2018a;Hunt and Zaz, 2022).…”

“…The relative strengths of these teleconnections has varied over the last century, with Yadav et al (2009) noting an increasing influence of ENSO and decreasing influence of the NAO on winter precipitation over northwestern India since 1950. The STJ acts as a medium for these remote climate forcings to influence WD activity (Hunt and Zaz, 2022). In this context, we may anticipate that the response of STJ to the polar sea ice melting can influence the formation and intensification of WDs, subject to the caveat that the strength of this relationship may not be constant in time.…”

Propagation Dynamics from Meteorological Drought to GRACE-Based Hydrological Drought and Its Influencing Factors

“…The ENSO is the key driving factor for drought propagation in most regions around the Pacific Ocean by influencing precipitation and actual evaporation [98][99][100]. The NAO is another important driver with profound impacts on drought propagation by explaining the enhanced or reduced precipitation across Europe, Africa, Australia, and Southern Asia in different seasons [99,[101][102][103][104][105]. The DMI also strongly influences global drought propagation because it affects hydrologic variables around the Indian Ocean and then influences other regions through complex interactions with regional climate systems [106,107].…”

Atmospheric turbulent structures and fire sweeps during shrub fires and implications for flaming zone behaviour

“…This study used the python package PYCWT to calculate the wavelet transform for time frequency analysis. The package is based on Torrence and Compo (1998) and Grinsted et al (2004) and has been used in various studies (Norel et al 2021;Hunt and Zaz 2022). Unlike Fourier transform, wavelet transform reserves the temporal information alongside the frequency information and is more suitable to look at the temporal variations of coherent structures in the turbulence time series from the streamwise and vertical wind velocity.…”