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  • View in gallery

    Daily occurrence numbers of TP vortices (bar chart) and the active periods of TP vortices (gray shaded) in 1998 summer.

  • View in gallery

    (a) Locations of Tibetan Plateau vortices (denoted with “C”) in the 1998 summer with 4-month (May–August) averaged 500-hPa wind fields. (b) Locations of Tibetan Plateau vortices that occurred during active periods with its corresponding composite 500-hPa wind fields and TBB (K) in the same periods (shaded). The region west of 80°E is beyond the coverage of the GMS-5 satellite. The dotted line with 2500 m of topography is used to highlight the Tibetan Plateau.

  • View in gallery

    Nine active periods of TP vortices (gray shaded) and ensemble time series of the (top) 10–30-day and (bottom) 30–60-day ISO of 500-hPa relative vorticity (10−5 s−1) over the central and east TP (29°–36°N, 85°–100°E) and standard error (blue shaded) of CFSR, ERA-40, and NCEP.

  • View in gallery

    Temporal evolution of the 10–30-day ISO of 500-hPa relative vorticity and TP vortices (a) in zonal direction (averaged between 29° and 36°N) and (b) in meridional direction (averaged between 85° and 100°E). The shading represents positive vorticity phase of the 10–30-day ISO; TP vortices are denoted as “C.” The black line segments at the right-hand side signify the clustering periods shown in Fig. 2.

  • View in gallery

    Longitudinal–vertical cross sections of composite results for 10–30-day ISO phases averaged between 29° and 36°N: (a) the transition from the negative to the positive phase; (b),(d) the positive phase and the negative phase; and (c) the transition from the positive to the negative phase. The shaded is the filtered vertical velocity (Pa s−1, the negative value means ascending motion). The red contours are the filtered equivalent potential temperature. The gray polygon denotes the topography.

  • View in gallery

    Composite results at 500 hPa for 10–30-day ISO phases: (a) the transition from the negative to the positive phase; (b),(d) the positive phase and the negative phase; and (c) the transition from the positive to the negative phase. The vectors are filtered horizontal wind (m s−1). The shaded is the filtered divergence of moisture flux (10−5 g hPa−1 cm−2 s−1, the negative value means moisture convergence). The gray line with 2500 m of topography is used to highlight the Tibetan Plateau.

  • View in gallery

    Composite results at 300 hPa for 10–30-day ISO phases: (a) the transition from the negative to the positive phase; (b),(d) the positive phase and the negative phase; and (c) the transition from the positive to the negative phase. The vectors are filtered horizontal wind (m s−1). The gray line with 2500 m of topography is used to highlight the Tibetan Plateau.

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Clustering of Tibetan Plateau Vortices by 10–30-Day Intraseasonal Oscillation

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  • 1 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, Beijing, China
  • | 2 College of Atmospheric Sciences, Chengdu University of Information Technology, Chengdu, China
  • | 3 IPRC, SOEST, University of Hawai‘i at Mānoa, Honolulu, Hawaii
  • | 4 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • | 5 Earth and Ocean Sciences, Nicholas School, Duke University, Durham, North Carolina
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Abstract

Tibetan Plateau (TP) vortices and the related 10–30-day intraseasonal oscillation in May–September 1998 are analyzed using the twice-daily 500-hPa synoptic weather maps, multiple reanalysis datasets, and satellite-retrieved brightness temperature. During the analysis period, distinctively active and suppressed periods of TP vortices genesis are noticed. In 1998, nine active periods of TP vortices occurred, which were largely clustered by the cyclonic circulations associated with the intraseasonal oscillation of 500-hPa relative vorticity. In addition to the well-recognized 30–60-day oscillation, the clustering of TP vorticity in the 1998 summer are more likely modulated by the 10–30-day oscillation, because all active periods of TP vortices fall into the positive phase of the 10–30-day oscillation in 1998. Even in the negative (i.e., anticyclonic) phases of the 30–60-day oscillation, the positive (i.e., cyclonic) 500-hPa 10–30-day oscillation can excite the clustering of TP vortices. This result indicates that the 10–30-day oscillation more directly modulates the activities of TP vortices by providing a favorable (unfavorable) cyclonic (anticyclonic) environment. The analysis of the 10–30-day atmospheric oscillation suggests that the westerly trough disturbances, in conjunction with convective instability due to low-level warm advection from the Indian monsoon region, are important in the clustering of TP vortex activities. In particular, the moisture flux from the southwest boundary of TP is essential to the accumulation of convective energy. Thus, a better understanding and prediction of the 10–30-day intraseasonal oscillation is needed to advance the extended-range forecasting of TP vortices and their downstream impacts on the weather and climate over East Asia.

School of Ocean and Earth Science and Technology Contribution Number 9010 and International Pacific Research Center Contribution Number 1015.

Corresponding author address: Pengfei Zhang, Institute of Atmospheric Physics, Beijing 100029, China. E-mail: zhangpf@lasg.iap.ac.cn

Abstract

Tibetan Plateau (TP) vortices and the related 10–30-day intraseasonal oscillation in May–September 1998 are analyzed using the twice-daily 500-hPa synoptic weather maps, multiple reanalysis datasets, and satellite-retrieved brightness temperature. During the analysis period, distinctively active and suppressed periods of TP vortices genesis are noticed. In 1998, nine active periods of TP vortices occurred, which were largely clustered by the cyclonic circulations associated with the intraseasonal oscillation of 500-hPa relative vorticity. In addition to the well-recognized 30–60-day oscillation, the clustering of TP vorticity in the 1998 summer are more likely modulated by the 10–30-day oscillation, because all active periods of TP vortices fall into the positive phase of the 10–30-day oscillation in 1998. Even in the negative (i.e., anticyclonic) phases of the 30–60-day oscillation, the positive (i.e., cyclonic) 500-hPa 10–30-day oscillation can excite the clustering of TP vortices. This result indicates that the 10–30-day oscillation more directly modulates the activities of TP vortices by providing a favorable (unfavorable) cyclonic (anticyclonic) environment. The analysis of the 10–30-day atmospheric oscillation suggests that the westerly trough disturbances, in conjunction with convective instability due to low-level warm advection from the Indian monsoon region, are important in the clustering of TP vortex activities. In particular, the moisture flux from the southwest boundary of TP is essential to the accumulation of convective energy. Thus, a better understanding and prediction of the 10–30-day intraseasonal oscillation is needed to advance the extended-range forecasting of TP vortices and their downstream impacts on the weather and climate over East Asia.

School of Ocean and Earth Science and Technology Contribution Number 9010 and International Pacific Research Center Contribution Number 1015.

Corresponding author address: Pengfei Zhang, Institute of Atmospheric Physics, Beijing 100029, China. E-mail: zhangpf@lasg.iap.ac.cn

1. Introduction

In boreal summer, low-level cyclonic vortices frequently occur over the Tibetan Plateau (TP). They are subsynoptic or meso--scale convective systems with a typical horizontal scale of several hundred kilometers. These mesoscale convective systems are called Tibetan Plateau vortices (Gao et al. 1981; Tao and Ding 1981), whose geneses and development are largely influenced by the unique thermo-dynamical and dynamical environments of the plateau, such as topography, latent heat release, low-level convergence, surface sensible heating, etc. (e.g., Dell’Osso and Chen 1986; Gao 2000; Li et al. 2002; Shen et al. 1986; Shi et al. 2008; Sugimoto and Ueno 2010; Wang 1987). The TP vortices usually form in the western and central plateau and propagate eastward with a life span of several hours to 3 days. As one of the major rain-bearing systems over the adjacent areas of the plateau (Luo 1992; Yamada and Uyeda 2006; Ye and Gao 1979), the TP vortices can cause extreme rainfall events in East Asia, including China, Korea, and Japan (e.g., Rui et al. 1987; Tao and Ding 1981; Wang and Orlanski 1987; Wang et al. 2005). For example, the triggering of the heavy rainfall and severe flooding in 1998 over the Yangtze River basin has been attributed to eastward-propagating TP convective systems (vortices) (Shi et al. 2008; Yasunari and Miwa 2006; Yu 2001).

Previous studies show that, the occurrence of TP vortices exhibits apparently active and suppressed periods (Luo et al. 1994). During the active periods, TP vortices continuously generate within several days, which is also known as the clustering of TP vortices (Sun and Chen 1994). Previous studies have suggested that the clustering of TP vortices is primarily modulated by the 30–60-day intraseasonal oscillation (ISO) over the TP (Sun and Chen 1994; Zhang et al. 1991), analogous to the clustering of monsoon low pressure systems by the northward propagation of the 30–60-day Indian monsoon ISO (e.g., Goswami et al. 2003) or the genesis of tropical cyclones by the Madden–Julian oscillation (e.g., Maloney and Hartmann 2000).

In addition, submonthly (10–30 day) ISO can also modulate convective activities over the TP (Fujinami and Yasunari 2004; Yasunari and Miwa 2006) and its surrounding areas (Sato 2013). Previously, the 10–30-day ISO is thought to be important mainly for the tropics (e.g., Kikuchi and Wang 2009; Goswami and Mohan 2001;Wen et al. 2010; Yang et al. 2010; Chen and Sui 2010). To date, the importance of the 10–30-day ISO has also been identified over the TP in boreal summer (e.g., Fujinami and Yasunari 2009; Yang et al. 2013). Different from the tropical ISO that presents barotropic structures, the 10–30-day ISO over the TP is characterized by a baroclinic structure (Fujinami and Yasunari 2009; Yang et al. 2013). Such a baroclinic structure might be attributed to the distinctive thermal conditions of the TP during summer (e.g., Liu et al. 2007; Fujinami and Yasunari 2004; Yasunari and Miwa 2006). Furthermore, the 10–30-day ISO can trigger TP vortex, whose downstream propagation can cause high-impact weather events over the Yangtze River basin, such as extreme rainfall and flooding.

Despite its important climate implications, the generation and evolution of the 10–30-day ISO over the TP has not been analyzed comprehensively. The relative importance of the 30–60-day oscillation and the 10–30-day oscillation to the high-frequency weather activities over TP and the physical processes remain unclear. Further, these modulating processes are seasonal and location dependent (e.g., Bessafi and Wheeler 2006; Ferreira et al. 1996; Molinari et al. 1997). Untangling the characteristic periodicity of TP vortices clustering and elucidating the underlying physical processes are the major objectives of this study. Using 1998 summer as a sample, the scientific questions are as follows: 1) what are the characteristics of the TP vortices clustering events?, 2) what is the background circulation favorable for the TP vortices clustering?, and 3) how does the atmospheric ISO modulate those clustering events?

The rest of this paper is organized as follow. The data and methods used to identify TP vortices and to analyze the associated large-scale atmospheric circulation are described in section 2. The relationships between the clustering of TP vortices and atmospheric ISO are presented in section 3. Section 4 investigates the possible underlying mechanisms. Section 5 summarizes the major findings and discusses future research topics.

2. Data and analysis of Tibetan Plateau vortices

a. Data and methods

TP vortex is defined as a closed low pressure on 500-hPa (lower troposphere over TP) synoptic weather maps or a cyclonic circulation observed by at least three meteorological stations (Luo 1992). We have surveyed available twice-daily (306 data samples) sounding observations during May–September of 1998, which are provided by Sichuan Meteorological Bureau. The locations and dates of all identified TP vortices are recorded according to the definition in Luo (1992). The brightness temperature (TBB) field from the Geostationary Meteorological Satellite-5 (GMS-5) IR data is used to describe the convective environments of TP vortices. To investigate the atmospheric circulations in which TP vortices developed, we analyze the atmospheric dynamic and thermodynamic fields from the Climate Forecast System Reanalysis (CFSR; Saha et al. 2010), the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40; Uppala et al. 2005), and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis 1 (Kalnay et al. 1996) in the same period. The ensemble technique of the reanalysis datasets is applied to minimize the potential spread in the results obtained using different reanalysis datasets. Furthermore, the comparison between multiple reanalysis datasets also ensures the robustness of the analysis results (Li et al. 2013). The relative vorticity is calculated from wind fields, and the Butterworth filter is applied to obtain the 10–30- and 30–60-day ISO. Temperature and specific humidity are also used in this study to calculate equivalent potential temperature, which is applied to understand the potential development of convective disturbances.

b. Statistics of Tibetan Plateau vortices in 1998 summer

Figure 1 shows the daily occurrence of TP vortices during May–September of 1998. In this year, vortices occurred from early May to early September. There are certain periods during which many TP vortices emerged almost simultaneously while other periods no TP vortices presented at all. The active periods of vortices are defined as the clustering of TP vortices. To investigate the causes of the clustering process, we define an active period of TP vortices when the following three criteria are fulfilled: 1) TP vortices occur and persist for at least three continuous days, 2) the minimum number of vortices in a given period is four, and 3) the temporal interval between two vortices should not be greater than 1 day.

Fig. 1.
Fig. 1.

Daily occurrence numbers of TP vortices (bar chart) and the active periods of TP vortices (gray shaded) in 1998 summer.

Citation: Monthly Weather Review 142, 1; 10.1175/MWR-D-13-00137.1

According to the definition, nine active periods (42 days) are identified for the 1998 summer (Fig. 1, gray shaded). These nine active periods account for about 80% of total TP vortices occurred in the entire summer. The majority of the active periods last about 5 days, with the longest period lasting 7 days. The continuous occurrences of these mesoscale vortices suggest that their genesis might be modulated by low-frequency large-scale background environments (e.g., Sun and Chen 1994; Yasunari and Miwa 2006; Zhang et al. 1991), which will be investigated in the following sections.

3. Clustering of Tibetan Plateau vortices by intraseasonal oscillations

In boreal summer, the TP vortices preferentially occur in the central plateau (e.g., Tao and Ding 1981; Zhu and Chen 2003), and some of them propagate downstream (e.g., Rui et al. 1987; Yasunari and Miwa 2006; Yu 2001). This preference in genesis location of vortices is also reflected in the 1998 summer. As shown in Fig. 2a, most TP vortices are generated in central TP with a few scattered around. The background circulation (500-hPa winds) associated with 1998 TP vortices are documented using CFSR, ERA-40, and NCEP reanalysis. Because anomalous large number of TP vortices occurred in 1998 summer, both the seasonal-mean circulation (Fig. 2a) and the typical circulation pattern in which the active vortices periods occur (Fig. 2b) are analyzed. Figure 2 shows the results based on CFSR data. We also analyze ERA-40 and NCEP–NCAR data, and obtain similar results (not shown).

Fig. 2.
Fig. 2.

(a) Locations of Tibetan Plateau vortices (denoted with “C”) in the 1998 summer with 4-month (May–August) averaged 500-hPa wind fields. (b) Locations of Tibetan Plateau vortices that occurred during active periods with its corresponding composite 500-hPa wind fields and TBB (K) in the same periods (shaded). The region west of 80°E is beyond the coverage of the GMS-5 satellite. The dotted line with 2500 m of topography is used to highlight the Tibetan Plateau.

Citation: Monthly Weather Review 142, 1; 10.1175/MWR-D-13-00137.1

Seasonal-mean circulation in the 1998 summer is characterized by the prevalence of westerly wind over the plateau (Fig. 2a). However, such seasonal-mean circulation pattern is not the exact environments in which most vortices developed, and thus may not fully explain the favored occurrence of TP vortices in the summer of 1998. The circulation pattern composited during the active TP vortices periods (42 days) is more relevant (Fig. 2b), which shows significant differences from the seasonal mean. In Fig. 2b, a large-scale cyclonic circulation emerges over the central plateau and major TP vortices appear on the southeast side of the circulation center, aligning with a significant convergence flow around 32.5°N. This result suggests that the circulation pattern in Fig. 2b rather than that in Fig. 2a is more likely a direct contributor to the active development of TP vortices. Meanwhile, low TBB is observed to the southeast of the cyclonic circulation, collocating with the convergence zone, which is consistent with the enhanced convective activities. It indicates that these convective clouds (low TBB) are mainly produced by convergence associate with low vortices circulation. The results are consistent with the results of on a case study by Wang et al. (2003). This convective-rich region, in turn, favors the development of TP vortices. Inferred from the latitude of these clouds and convergence zone, it is likely that these convective activities are supplied by moisture transported by the southerly wind from the Indian monsoon region.

According to our analysis, the circulation pattern associated with specific TP vortex periods and the large-scale seasonal mean circulation might be different. However, the circulation pattern shares more similarity between specific vorticity clustering events. Thus, caution is necessary when using seasonal-mean wind field to represent background flow. In the following, we will show that the anomalous circulation of ISO contributes significantly to this favored circulation pattern (Fig. 2b) for the active development of TP vortices.

The contribution of the ISO to the occurrence of TP vortices is analyzed by investigating the 500-hPa relative vorticity over the TP. Because the majority of TP vortices develops in the central plateau (Fig. 2), the area-averaged relative vorticity (29°–36°N, 85°–100°E) is used in following analysis. In this region, the 10–30- and 30–60-day oscillations coexist. Figure 3 shows the time series of the 10–30- and 30–60-day oscillations of 500-hPa relative vorticity as an ensemble of the CFSR, ERA-40, and NCEP–NCAR reanalysis. The ensemble time serial is calculated as the mean of individual areal-averaged time series using the three different reanalysis datasets. All three datasets are highly consistent in characterizing these ISO signals, ensuring the robustness of our analysis. This is quite different from the case in the tropical region, where intraseasonal variability in the CFSR reanalysis is much stronger than that in the NCEP–NCAR reanalysis (Fu et al. 2011; Wang et al. 2012).

Fig. 3.
Fig. 3.

Nine active periods of TP vortices (gray shaded) and ensemble time series of the (top) 10–30-day and (bottom) 30–60-day ISO of 500-hPa relative vorticity (10−5 s−1) over the central and east TP (29°–36°N, 85°–100°E) and standard error (blue shaded) of CFSR, ERA-40, and NCEP.

Citation: Monthly Weather Review 142, 1; 10.1175/MWR-D-13-00137.1

For the 30–60-day oscillation, all five peaks modulate the clustering of TP vortices (Fig. 3). There are 60 (30) TP vortices (Fig. 1) occur in its positive (negative) phase. This is generally consistent with Sun and Chen (1994), who pointed out that the majority of TP vortices appears during the westerly phase of 30–60-day oscillation. However, the 30–60-day oscillation alone cannot fully account for the clustering of TP vortices, because six active periods fall into the positive phase while still the other three occur in the negative phase in 1998.

The three clustering events occurred during the negative phase of 30–60-day oscillation can be explained by the 10–30-day oscillation. In the 1998 summer, all nine active TP vortices periods concur with the positive phase of 10–30-day oscillation (Fig. 3), although not all positive peaks of 10–30-day oscillation will induce the clustering of TP vortices. In contrast to the 30–60-day oscillation, no clustering of TP vortices is observed with the negative phase of 10–30-day oscillation. On the other hand, there are 73 (17) TP vortices (Fig. 1) that occur in the positive (negative) phase of the 10–30-day oscillation. Thus, the clustering of the TP vortices better matches the 10–30-day oscillation than the 30–60-day oscillation as noticed before (Sun and Chen 1994). This result highlights the importance of the 10–30-day oscillation in the clustering of TP vortices. The mechanism and physical processes linking the 10–30-day oscillation with the TP vortices thus deserve a more detailed investigation.

4. The mechanism linking the 10–30-day oscillation and the clustering of TP vortices

To further understand the modulation of 10–30-day oscillation on the clustering of TP vortices, the longitudinal–temporal and latitudinal–temporal evolutions of 10–30-day oscillation of 500-hPa relative vorticity and the occurrences of TP vortices are presented in Fig. 4. Since the resolution of CFSR data is considerably higher than ERA-40 and NCEP, and the results from CFSR does not differs significantly from the ensemble of all three, only the results from CFSR are presented and discussed in the following.

Fig. 4.
Fig. 4.

Temporal evolution of the 10–30-day ISO of 500-hPa relative vorticity and TP vortices (a) in zonal direction (averaged between 29° and 36°N) and (b) in meridional direction (averaged between 85° and 100°E). The shading represents positive vorticity phase of the 10–30-day ISO; TP vortices are denoted as “C.” The black line segments at the right-hand side signify the clustering periods shown in Fig. 2.

Citation: Monthly Weather Review 142, 1; 10.1175/MWR-D-13-00137.1

Figure 4a gives the longitudinal–temporal evolution of the 500-hPa relative vorticity. The 10–30-day oscillations exhibit apparent eastward propagation from the western Tibetan Plateau to the Yangtze River basin. On their eastward passages, the positive anomalies of relative vorticity associated with 10–30-day oscillation spawn a series of mesoscale vortices over the Tibetan Plateau. The modulations of the 10–30-day oscillation on TP vortices can also be seen in the latitudinal–temporal plot (Fig. 4b). There are 78 (62) vortices occurring in the positive phases in Fig. 4a (4b). The oscillations from the Indian subcontinent may imply the interaction of monsoon and midlatitude westerly disturbances over the TP at the 10–30-day scale. Similar processes are suggested by Yasunari and Miwa (2006), which will be further elaborated on later. It is noteworthy that, however, not all cross-plateau positive phases of 10–30-day oscillation excite the clustering of TP vortices (e.g., the one in 20–23 May). The positive phase of the 10–30-day oscillation is probably an important factor in initiating the clustering of TP vortices, but not the only factor (also see Fig. 3). The additional factors contributing to the clustering of TP vortices need further investigation.

From the analysis above, the propagation of the 10–30-day intraseasonal oscillation is closely linked to the clustering of TP vortices. How does the evolution of the atmospheric perturbations of the 10–30-day oscillation contribute to the clustering of the TP vortices? To answer this question, thermodynamic (i.e., temperature, moisture) and dynamic (i.e., vertical and horizontal wind) structures of the atmospheric perturbations over the TP associated with the 10–30-day oscillation are analyzed. The life cycle of the 10–30-day oscillation (Fig. 3) is separated into four phases: negative-to-positive transition, positive phase, positive-to-negative transition, and negative phase. Composites with five 10–30-day oscillation cycles in 1998 summer (4–17 May, 2–17 June, 3–15 July, 16–31 July, and 26 August–9 September) for individual phases are then derived.

Figure 5 shows the vertical cross sections of the composite perturbations of upward motion and equivalent potential temperature for the four phases averaged over the 29°–36°N latitudinal bands where most TP vortices present. Figure 6 shows the composite horizontal wind and divergence of moisture flux at 500 hPa (lower troposphere over the TP). Figure 7 is the composite 300-hPa horizontal wind. The first phase (phase 1) corresponds to the transition of the 500-hPa relative vorticity from negative to positive phase (see Fig. 3). In phase 1, the dynamic field is characterized by anomalous ascending motion (Fig. 5a) located at the upstream of the Tibetan Plateau associated with a westerly trough (Fig. 7a). A near-surface convergence zone lies at the west and central TP (Fig. 6a). Meanwhile, the thermodynamic field shows high equivalent potential temperature anomaly at the lower troposphere (from the surface to 400 hPa over the central TP), suggesting the accumulation of moist static energy in phase 1 (Fig. 5a). The equivalent potential temperature decreases with height (), indicating high convective available potential energy (CAPE), and convectively unstable stratification. Furthermore, the presence of the high equivalent potential temperature over the western-central TP (Fig. 5a) is associated with the moist advection in the lower troposphere as shown in Fig. 6a. Warm and moist air parcels are transported into the convergence zone in the 85°–95°E of the TP by the prevailing wind from the southwest of the TP. The accumulation of the warm and moist air parcels over the TP indicate the potential of moist static energy releasing, which favors the intensification of ascending. The atmospheric condition over the TP is convectively unstable in phase 1.

Fig. 5.
Fig. 5.

Longitudinal–vertical cross sections of composite results for 10–30-day ISO phases averaged between 29° and 36°N: (a) the transition from the negative to the positive phase; (b),(d) the positive phase and the negative phase; and (c) the transition from the positive to the negative phase. The shaded is the filtered vertical velocity (Pa s−1, the negative value means ascending motion). The red contours are the filtered equivalent potential temperature. The gray polygon denotes the topography.

Citation: Monthly Weather Review 142, 1; 10.1175/MWR-D-13-00137.1

Fig. 6.
Fig. 6.

Composite results at 500 hPa for 10–30-day ISO phases: (a) the transition from the negative to the positive phase; (b),(d) the positive phase and the negative phase; and (c) the transition from the positive to the negative phase. The vectors are filtered horizontal wind (m s−1). The shaded is the filtered divergence of moisture flux (10−5 g hPa−1 cm−2 s−1, the negative value means moisture convergence). The gray line with 2500 m of topography is used to highlight the Tibetan Plateau.

Citation: Monthly Weather Review 142, 1; 10.1175/MWR-D-13-00137.1

Fig. 7.
Fig. 7.

Composite results at 300 hPa for 10–30-day ISO phases: (a) the transition from the negative to the positive phase; (b),(d) the positive phase and the negative phase; and (c) the transition from the positive to the negative phase. The vectors are filtered horizontal wind (m s−1). The gray line with 2500 m of topography is used to highlight the Tibetan Plateau.

Citation: Monthly Weather Review 142, 1; 10.1175/MWR-D-13-00137.1

This thermodynamic and dynamic setting favors the onset of the positive phase (phase 2) of the 10–30-day oscillation. In phase 2, the anomalous westerly trough in the upper level moves to 83°E (Fig. 7b). The ascending motion is located downstream of the westerly trough where it attains its maximum value over the central and eastern TP (Fig. 5b). In this phase, the positive equivalent potential temperature anomaly in 90°–100°E presents a quasi-vertical structure and is collocated with strong ascending motion, indicating that the high CAPE accumulated in phase 1 is released to intensifies the ascending motion, which in turn excites the low-level positive vorticity. In this phase, an anomalous cyclonic circulation is over the TP with its center around (33°N, 91°E) (Fig. 6b). The significant moisture convergence over the central-eastern TP suggests that moist air parcels are supplied by the southerly wind from Indian monsoon region through the southeast of the TP (Fig. 6b). At the southeast slope of TP, the submonthly oscillation characteristic of moisture is significant in monsoon season (Sato 2013). In this phase, both the large-scale cyclonic vorticity and the convergence anomalies induced by the 10–30-day oscillation over the TP provide a favorable environment for the development of severe synoptic and mesoscale convective systems, such as TP vortices.

When the 10–30-day oscillation evolves from a positive to a negative phase (phase 3), the upward motion over the Tibetan Plateau is replaced with downward motion (Fig. 5c). Meanwhile, the cyclonic circulation and upward motion are significantly weakened and move to the eastern edge of the TP (Figs. 5c and 6c). Associated with the eastward propagation of the cyclonic circulation and the set in of an anticyclonic circulation, the lower troposphere over the TP is dominated by northerly wind (Fig. 6c). The upper troposphere over the western and central TP is occupied by anomalously strong northerly associated with the westerly trough (Fig. 7c). The associated advection of the dry and cold air mass (Figs. 6c and 7c) favors the enhancement of descending (Fig. 5c). These conditions are unfavorable to the development and maintenance of TP vortices. Some TP vortices move downstream to the east of the TP following the westerly, which can cause intensive precipitation and flooding over the Yangtze River basin (Shi et al. 2008; Yu 2001).

In the last phase (phase 4, the negative or dry phase) of the 10–30-day oscillation, strong downward motion associated with a low-level anticyclonic circulation dominates the entire TP region (Figs. 5d and 6d). Cold advection (northerly wind) prevails particularly in the eastern TP at the lower troposphere (Fig. 6d) and tilts westward vertically (Fig. 7d), which brings clouds and dry air into the plateau. Deep convective systems are largely suppressed because of the subsidence and negative instability associated with the decrease of atmospheric temperature and moisture content. Consequently, the activity of the TP vortex is largely suppressed in this phase (Fig. 1). At the same time, a deep moist layer reaching the west TP from upstream indicates the potential onset of the next 10–30-day oscillation cycle (Fig. 5d). During the cycle of the 10–30-day oscillation, 8 of TP vortices occur in phase 1 (18%), 29 occur in phase 2 (64%), 6 occur in phase 3 (13%), and only 2 vortices appear in phase 4 (4%). The occurrence of TP vortices during the active phase of the 10–30-day oscillation is 10 times more than that during the suppressed phase.

5. Conclusions

In this study, we have analyzed the spatiotemporal evolutions of the Tibetan Plateau (TP) vortices and the associated large-scale background circulation in 1998 summer. The year 1998 is selected because of the abnormally large amount of TP vortices and their severe downstream impacts (Shi et al. 2008; Yasunari and Miwa 2006; Yu 2001). In that year, most of the vortices develop over the central-eastern TP. Considering very sparse observational stations in the western Tibetan Plateau, it is possible that some vortices have not been detected from the synoptic maps used in this study. Based on three different reanalysis datasets (CFSR, ERA-40, and NCEP–NCAR), we find that the background circulation favoring the clustering of the TP vortices (Fig. 2b) differs from the seasonal-mean wind fields (Fig. 2a). The former shows a closed cyclonic circulation over the central Tibetan Plateau, which is unclear in the latter. This result suggests that the usage of seasonal-mean circulation as background condition to interpret the occurrence of TP vortices has its limitation (Fig. 2).

The occurrences of TP vortices exhibit apparently active and quiescent periods in the 1998 summer. The active periods can last 4–7 days. Nine active periods are identified in this specific year. All nine active periods fall into the active phase of the 10–30-day oscillation, even though the amplitude of some ISOs is relatively weak. The amount of TP vortices that occurred in the positive phase is 10 times more than that in the negative phase. It thus indicates that the phase of the 10–30-day ISO is important to the clustering of TP vortices. This strong modulation of the 10–30-day oscillation offers an opportunity for extended-range forecasting of the clustering of TP vortices (Figs. 3 and 4). In addition, among all nine clustering events, six (three) of them occur in the positive (negative) phase of the 30–60-day oscillation of 500-hPa relative vorticity. The positive 30–60-day oscillation phase can be another predictor for the clustering of TP vortices (Sun and Chen 1994) because each of the six peaks of this intraseasonal mode corresponds to an active period of TP vortices (Fig. 3). However, the modulation of the 10–30-day oscillation is more dominant than the 30–60-day oscillation in this year.

The 10–30-day oscillation modulates the activity of TP vortices mainly through changing the large-scale dynamical and thermo-dynamical environments over the Tibetan Plateau. During the active and suppressed phases of the 10–30-day oscillation, a large-scale low-level cyclonic circulation (convergence) and anticyclonic circulation (divergence) alternatively dominates over the central-eastern TP, so do the occurrences of TP vortices. In the active phase of the 10–30-day oscillation (Figs. 5b and 6b) the large-scale convergence of moisture advection through the southwest of TP connecting to the Indian monsoon activity leads to the high moist static energy accumulation at the lower troposphere over TP. The low-level convergence excites deep convective systems over the central-eastern TP. The latent heat release of the deep convection can enhance the upward motion through a convection–circulation feedback. The moisture supply through the southeast of TP further ensures the persistence of convective disturbances. The large-scale low-level cyclonic circulation along with rich moisture supply from the Indian monsoon region provides a favorable environment for the clustering of synoptic-mesoscale convective systems such as TP vortices. Since moisture supply from the southwestern boundary of TP and a cyclonic perturbation are two major precursors for the onset of the active phase of the 10–30-day oscillation over the central-eastern TP, the observations of these signals are important for the forecast of the clustering of mesoscale convective systems (TP vortices).

Since the TP vortices are important rain-bearing systems in the adjacent areas of the TP as well as its downstream region such as Yangtze River basin, the capability to forecast the clustering of TP vortices more than one week in advance implies great socioeconomic values. From Fig. 3, it is noted that a positive phase normally evolves into next positive phase after about 7–15 days in the 10–30-day oscillation. However, the magnitude of the next peak and the timing of achieving the peak of the next cycle vary from cycle to cycle. Many methods are proposed to advance the potential predictability. For example, Goswami and Xavier (2003) constructed an empirical model to forecast monsoon break (or active) by predicting the phase transition of monsoon ISO based on 23-yr observation data; Fu et al. (2007) found that the air–sea coupled model appears to extend the predictability of monsoon ISO than atmosphere-only model. Our results document the strong modulations of the 10–30- and 30–60-day oscillations on the clustering of TP vortices, and thus offer an observational basis for the extended-range forecasting of the clustering of TP vortices. However, it is noteworthy that the sample size of the 10–30-day oscillation and clustering periods in this study is relatively small. To ascertain the predictability, more data samples are needed. In addition, not all positive phases of the 10–30-day oscillation (e.g., 20–23 May) cluster the activity of TP vortices. Thus, more statistics of the 10–30-day ISO and TP vortices should be analyzed and further researches are needed to address the following questions: 1) why some 10–30-day ISOs cannot induce the clustering of TP vortices? 2) In addition to 10–30-day ISO, can the TP vortices also be influenced by other factors? 3) To what extend can the 10–30-day oscillation contribute to predictability of the clustering of TP vortices? To further advance our understanding of the intraseasonal variability over the TP and its modulations of the vortices, the following questions should be addressed: what causes the origin of the intraseasonal variability in this region: locally (Liu et al. 2007) or remotely? The present finding is limited for 1998 summer. Future research with more years is needed to understand what determines the interannual variations of the 10–30- and 30–60-day oscillations and their modulations of TP vortices.

Acknowledgments

The authors thank Dr. Yuanfa Gong and Dr. Minyan Wang for providing GMS-5 TBB data, Prof. Guoxiong Wu and two anonymous reviewers for their insightful comments, and Mr. Patrick T. Brown and Miss Yueyue Yu for their editorial assistance. GL is supported by the National Basic Research Program of China (2012CB417202), the NSFC (41175045), and the CMA project (GYHY201206042). PZ and YL are supported by the NBRPC (2010CB950403) and the NSFC (40925015). XF is supported by the IPRC through the JAMSTEC, NASA, and NOAA institutional grants and external grants from the APEC Climate Center, NSF, and the CMA project (GYHY201206016).

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