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  • Author or Editor: Cholaw Bueh x
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Zuowei Xie
and
Cholaw Bueh

Abstract

A deep and cold vortex circulation often occurs over northeast China. Known as the northeast China cold vortex (NCCV), the phenomenon is most active from May to mid-June and can lead to extremely cold local temperatures. This study used rotated principle component analysis to categorize NCCV events into four types, which were characterized by ridges (or blocks) over the following regions: Lake Baikal (BKL), the Yenisei River valley (YNS), the Ural Mountains (UR), and the Yakutsk–Okhotsk region (YO).

On the intraseasonal time scale, it was found that BKL- and YNS-type NCCVs formed when the wave train height anomalies originating from the North Atlantic and Europe propagated to East Asia. In contrast, YO- and UR-type NCCVs formed in conjunction with the development of a meridional dipole pattern over northeast Asia. The existence of a blocking-type circulation over the Yakutsk–Okhotsk region favored maintenance of the NCCV circulation for the long-lived (more than 5 days) NCCV events of the four types. The typical circulation over northeast Asia for the long-lived NCCV event was closely associated with wave breaking, whereas the short-lived (3–5 days) event showed only wave propagation.

The YNS-type NCCV caused cold surface air temperatures (SAT) not only over northeast China, but also over central and south China, whereas the other three types led only to regional cold SAT anomalies over northeast China. All four types of NCCVs caused a precipitation increase over northeast China, and this effect was broader for the UR- and YO-type NCCVs than that for BKL- and YNS-type NCCVs.

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Cholaw Bueh
and
Zuowei Xie

Abstract

The present study identified the large-scale tilted ridge and trough (LSTR and LSTT, respectively) axes of the midtropospheric circulation over mid- and high-latitude regions using an objective identification technique that has not previously been applied. In this method, the identification procedure classified contours of 500-hPa height (Z500) fields into three types: the circumpolar wavy contour, the circumpolar contour containing the meridionally overturned (or wave breaking) part, and the locally closed contour. The ridge and trough points were defined on these three types of contours and, subsequently, the ridge or trough axes were identified after connecting successively the nearest ridge or trough points of the neighboring contours under a minimum distance constraint. The performance of the identification method was tested in the daily Z500 fields during 15 November 2011–15 March 2012. The hit rate, false-alarm ratio, and threat score of the method in test reveal that it performs well with a reasonably good skill. An examination of the wave-breaking features during the same period also suggests that the method performs well in the identification of LSTRs and LSTTs for the meridionally overturned parts of the Z500 contours. This objective technique was also applied to an extensive and persistent cold event over East Asia. Results show that the horizontal extent of the Siberian high corresponds well with the zonal extents of the identified LSTR and LSTT. The identification method in the present study might be useful for identifying the key circulation systems associated with extensive and persistent cold air outbreaks during winter.

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Anran Zhuge
,
Benkui Tan
,
Cholaw Bueh
, and
Zuowei Xie

Abstract

Based on daily data from the Japanese 55-year Reanalysis (JRA-55) covering the winters (NDJFM) from 1958 to 2018, this study examines the growth and decay mechanisms of the baroclinic wave packet (BWP) inferred from regression analysis over the North Pacific. Day-to-day kinetic energy (KE) and available potential energy (APE) budget analysis suggest that BWP is driven mainly by baroclinic energy conversion (CPB), barotropic energy conversion (CKB), and the nonlinear term (CKE). CPB acts as a predominant APE source for BWP. Part of CPB acts to overcome the APE loss caused by transient eddy flux and most of it acts as a dominant KE source to drive BWP throughout its lifespan. CKB acts as a KE source before day −1, and as a major KE sink to damp BWP afterward, in which the north–south gradient of the climatological meridional flow plays a key role. Similarly, CKE acts as a KE source before day 0 and as a major KE sink afterward. The damping effect of CKE comes mainly from the scale interaction through the advection of high-frequency meridional momentum by the low-frequency zonal flow. It turns out that the vertical geopotential flux divergence also plays an active role in the dynamical coupling of different vertical BWP parts. There is persistent geopotential flux transfer from the middle-tropospheric layer into the lower- and upper-tropospheric layers, which serves as a major KE source to drive the BWP anomalies for the two layers and a major KE sink for the middle-tropospheric layer where the baroclinic energy conversion is the strongest.

Significance Statement

Previous studies indicate that baroclinic waves tend to organize into wave packets and are driven by the baroclinic instability. This study finds that the baroclinic waves decay mainly through the barotropic energy conversion and nonlinear processes. The results also indicate that the vertical geopotential flux divergence plays an active role in the dynamical coupling of the BWP anomalies in different layers of the troposphere. It is therefore very important to improve the representations of the climatological-mean flow, wave–mean flow interaction, and wave–wave interaction between high- and low-frequency waves in the midlatitudes, as well as the process of vertical transfer of the geopotential flux in numerical models for a better prediction of weather and climate variability in the extratropic regions.

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