It has been known that the Eastern Asian (EA) topographic forcing has a dominant weakening effect on the stratospheric polar vortex, while the North American (NA) topography has a trivial or an even opposite effect.

“However, when we tried to distinguish the “independent” forcing of the EA and NA from their “dependent” and “joint” topographic forcing in winter stratosphere in their modeling studies using the stratosphere-resolved WACCM, we found a significant mutual interference between the EA and NA.”  introduced Dr. REN  Rong Cai at the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS)

Dr. REN and her team did a series of study concerning the processes via which the mutual interference between the EA and NA topographic forcing occurs from the troposphere to stratosphere. They also emphasized the dominant role of the EA topography in its interference with the NA and in the ‘joint” forcing of EA&NA.

Specifically, without EA, an independent NA can also induce a severe polar warming and weakening of the stratospheric polar vortex like EA;  The independent-EA-forced weakening effect on the polar vortex can be largely decreased and changes to a location displacement when NA exists, while the interference the other way around is such critical that it can completely offset the independent-NA-forced effect, hence explaining the previously known weaker NA’s stratospheric effect (Fig. 1); The critical interference of EA with NA’s effect is due to EA’s much stronger downstream weakening effect on the zonal flow that impinges on NA, which can lead to the NA-forced wave pattern being substantially confined within the North American to North Atlantic region (Fig. 2); The joint effects of EA and NA, rather than being a linear superimposition of their independent effects, are largely dominated by the effects of EA.

The study was published in Journal of Climate and Atmospheric and Oceanic Science Letters.

Fig. 1 Winter zonal-mean zonal wind (contour) and temperature (shading) anomalies forced by (a) an independent NA, (b) an independent EA, (c) a dependent NA when EA exists, and (d) a dependent EA when NA exists. (Image by XIA Xin)

Fig.2  Anomalies of 200-hPa wind flow (m s-1, stream vectors) and geopotential height (gpm; shading) forced by (a) an independent NA and (b) a dependent NA when EA exists. (Image by XIA Xin)

Reference:

Ren*, Rongcai, X. Xia, J. Rao, 2019: Topographic forcing from East Asia and North America in the northern winter stratosphere and their mutual interference, Journal of Climate, 32: 8639-8658, DOI: 10.1175/JCLI-D-19-0107.1
Link: https://doi.org/10.1175/JCLI-D-19-0107.1

Xia xin, Rongcai Ren*, Yu yueyue，2019: Dynamical role of the Rocky Mountain controlled by East Asian topographies in modulating the tropospheric westerly jet in northern winter, Atmospheric and Oceanic Science Letters, 12: 1, 66-72, https://doi.org/10.1080/16742834.2018.1548247

Link: http://159.226.119.58/aosl/EN/Y2019/V12/I1/66

Contact: REN Rongcai, rrc@lasg.iap.ac.cn