Dominant Process for Northward Propagation of Boreal Summer Intraseasonal Oscillation Over the Western North Pacific

Abstract

Northward propagation of boreal summer intraseasonal oscillation (BSISO) over the Western North Pacific (WNP) has significant impacts on extreme events over Asia and Europe. Here we test hypotheses that northward propagation mechanisms over the WNP may differ from those over the Indian Ocean (IO) by performing numerical experiments with changing mean states through lowering the Tibetan Plateau (TP). Our results suggest that air-sea interaction plays a dominant role in the propagation over the WNP, whereas the mean vertical wind shear mechanism is the major driver over the IO. Lowering TP significantly reduces sea surface temperature (SST) anomalies to the north of BSISO center due to the enhanced surface wind and latent heat flux anomalies. This air-sea interaction reduces upward transport of heat and moisture from surface to lower troposphere, weakening the northward propagation over the WNP. This study implies changes in SST patterns under global warming may influence BSISO propagation. Plain Language Summary A heavy rainfall system propagates slowly from the equatorial western Pacific to the south China Sea and north of the Philippine Sea on a 20-70-day time scale. This rainfall system affects the onset of East Asia monsoon and extreme weather including typhoons. Therefore, a better understanding of the dominant processes of the northward propagation of this heavy rainfall system may contribute to reducing its social and economic damage. For this purpose, we performed a set of climate model experiments. We found that lowering the Tibetan Plateau significantly decreases the meridional gradient of the warm SST anomalies associated with the heavy rainfall system, suggesting that the air-sea interaction plays a dominant role in the BSISO northward propagation over the Western North Pacific. In contrast, vertical shear of zonal winds and associated positive vorticity anomalies, the major driver over the Indian Ocean, does not contribute to the northward movement over the Western North Pacific.

Diversity of the Boreal Summer Intraseasonal Oscillation

“…Note that the generation of negative SST anomalies (Figure 7h) can be attributed to the Rossby‐wave zonal winds in the previous pentads (pentad −2, Figure 7b), which induces positive evaporation anomalies (Figure 8) that cools the sea surface (Figure 7). Thus, it suggests that the northward propagation over WP is associated with the air‐sea interaction, consistent with the previous results on the northward propagation mechanism of the ISO over the WNP (Hsu & Weng, 2001; Liu & Wang, 2014; B. Wang & Zhang, 2002; T. Wang, Yang, et al., 2018; Yang et al., 2020).…”

Dominant Processes for Dependence of Boreal Summer Intraseasonal Oscillation on El Niño Phases

“…The enhancement of both warm SST and negative LHF anomaly facilitates creating low‐level cyclonic circulation and convergence. The northward propagation mechanisms in this study have shown to well follow the results revealed in previous studies (Jiang et al., 2004; Yang et al., 2020). All changes in air‐sea interaction processes, as well as dynamical processes, occur more distinctly in the E‐DV than in the E‐DC summers.…”

“…The strong cyclonic circulation of these surface zonal wind anomalies over 0°-25°N are in good agreement with the characteristics of the relative vorticity over 5N°-15°N in Figure 5c. Besides, we can see the regressed pattern of surface zonal wind anomalies that strong easterlies to the north of the convection and westerlies within it in the western Pacific coincide with the development of warm and cold SST anomalies in Figures Many studies (Jiang et al, 2004;Kemball-Cook & Wang, 2001;Yang et al, 2019Yang et al, , 2020 have mentioned that the SST distribution, as well as the mean flow, has a strong effect on the character of the BSISO convection. In Figure 5f, we examine the role of air-sea interaction in the northward propagation of convection in the western Pacific for E-DV and E-DC summers.…”

“…Many studies (Jiang et al., 2004; Kemball‐Cook & Wang, 2001; Yang et al., 2019, 2020) have mentioned that the SST distribution, as well as the mean flow, has a strong effect on the character of the BSISO convection. In Figure 5f, we examine the role of air‐sea interaction in the northward propagation of convection in the western Pacific for E‐DV and E‐DC summers.…”

Assessment of the Ability of CMIP6 GCMS to Simulate the Boreal Summer Intraseasonal Oscillation Over Southeast Asia

“…A body of literature exists on the studies of BSISO simulation (e.g., Waliser et al, 2003b;Li and Mao, 2016;Hu et al, 2017;Yang et al, 2020). In general, most climate models continue to underestimate the intraseasonal variability of the BSISO and associated monsoon variability.…”