Forcing mechanism of the Silk Road pattern and the sensitivity of Rossby‐wave source hotspots to mean‐state winds

Abstract

The Silk Road pattern (SRP) is a leading mode of Eurasian atmospheric variability in boreal summer. It remains challenging for many models to predict the temporal phase of the SRP. This study investigates whether the forcing mechanism of the SRP can explain SRP prediction difficulty. Rossby‐wave sources (RWS) associated with the SRP are first identified in a reanalysis. An idealised barotropic model is then used to test wave propagation from all identified RWS, to isolate the relevant forcing locations of the SRP, namely the RWS hotspots. In addition to previous findings, a new hotspot in the central North Pacific is located, which can force the SRP by westward dispersion of a zonally elongated Rossby wave. Furthermore, a new mechanism of the Indian summer monsoon in forcing the SRP is discovered. Results also suggest that Rossby‐wave propagation by itself only explains a small fraction of the SRP amplitude, consistent with baroclinic energy conversion from the mean state, which has previously been shown to be vital for SRP growth—not represented in the idealised model. Seasonal hindcasts from the Pusan National University (PNU) coupled general circulation model are also analysed. The hindcasts can predict the SRP spatially but not temporally. Within a seasonal timescale, the hindcast develops an upper‐level mean‐state wind bias compared with the reanalysis, which is shown by the barotropic model to affect RWS hotspot locations of the SRP. While the reanalysis SRP is associated with North Atlantic sea‐surface temperature (SST) anomalies, the hindcast SRP is associated with tropical Pacific SST. Wind biases in the North Atlantic jet exit, subtropical western Pacific, and tropical Pacific are found to alter wave propagation from the North Atlantic and the North and tropical Pacific, respectively, and can explain the association of the SRP with different SSTs in the reanalysis and hindcast. This sensitivity of hotspots to mean‐state winds is proposed to reduce SRP prediction skill.

Spring Regional Sea Surface Temperatures as a Precursor of European Summer Heatwaves

“…Our findings are only based on the MPI-GE data set, which contains model biases (Giorgetta et al, 2013;Hagemann et al, 2013;Müller et al, 2018) . It has been shown that model wind biases can alter the propagation of Rossby waves induced by North Atlantic SSTAs (Li et al, 2021) , which are relevant for modulating the atmospheric state and triggering heatwaves (Cassou et al, 2005;Kornhuber et al, 2020;Wulff et al, 2017) . Although it has been demonstrated that the low resolution version of the MPI model represents well the connection between spring North Atlantic SSTA and Eurasian surface temperatures via atmospheric wave trains (Chen et al, 2021), the availability of other large ensembles with high temporal frequency and spatial resolution would aid to verify our results.…”

On the Existence of Summer Shamal Wind Induced by the Zagros Mountains in the Middle East

“…However, we hereby show that both mechanisms (i.e., thermal and mechanical forcings) simultaneously contribute to the formation of IA in the ME. On one hand, we see a westward continuous background flow on the east side of the ZAG in summer due to the existence of an anticyclone on the east side of the Caspian Sea so‐called, Turkmenistan anticyclone ( TKA ) (Mofidi & Zarrin, 2012), or Caspian Sea High (Kaskaoutis et al., 2016, 2017; Li et al., 2021; Y. Li et al., 2021) (Figure 3a). On the other hand, considering the diabatic heating equation (Q), we see significant values of negative vertical advection $$\left(-\omega \frac{\partial \theta }{\partial p}\right)$$ (about 2 K/day) from the top of the ZAG to the middle troposphere (Figures 3b and 3c).…”

“…For the proposed formation mechanism, the TKA and AA in the Aral Sea region and the eastern Mediterranean area are known as the Rossby‐wave source region. These two regional pressure systems generate such circulation by producing a waveguide through the Asian jet, which is called the Silk Road pattern (Ding & Wang, 2005; Enomoto, 2004; Enomoto et al., 2003; Li et al., 2021; Y. Li et al., 2021). In addition, TKA is also recognized to generate a quasi‐stationary mid‐tropospheric trough over the Caucasus during the summer by blocking the westerlies (Mofidi, 2007).…”