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  • View in gallery

    (a) Number of strong and weak (multiplied by −1) winter India–Burma Trough (IBT) cases detected by using the IBTI during 1979–2012, and composition of 700-hPa geopotential height (contour, gpm) and vorticity (shading, 10−5 s−1) based on (b) strong and (c) weak IBT cases detected in (a). The dashed box in (b)–(c) indicates the region where the vorticity is averaged as the IBTI.

  • View in gallery

    Period analysis of the time series of daily IBTI in each winter, which is derived from the spectrum analysis. The columns indicate the period derived from the Markov “red noise” spectrum and the vertical error bars indicate the upper and lower periods derived from the 5% and 95% red noise confidence bounds.

  • View in gallery

    Composited differences of daily precipitation (mm day−1) between strong and weak IBT cases over China from Day −2 to Day +3. Only the results significant at the 90% confidence level are shown.

  • View in gallery

    Composited differences of the vertical integral of water vapor flux (vector, kg m−1 s−1) and its divergence (shading, 10−5 kg m−2 s−1) between strong and weak IBT cases from Day −4 to Day +3. Only the composited water vapor fluxes significant at the 90% confidence level are shown.

  • View in gallery

    Evolution of composited 15°–25°N averaged meridional vertical integral of water vapor flux (contour, kg m−1 s−1) and moisture divergence (shading, 10−5 kg m−2 s−1) from Day −6 to Day +6.

  • View in gallery

    Evolution of vertical distribution of composited regional moisture divergence over (a) southwest China (SW, 21°–28°N, 97°–105°E) and (b) southeast China (SE, 21°–28°N, 105°–120°E) from Day −6 to Day +6.

  • View in gallery

    Composited differences of the vertical integral of (a)–(d) synoptic, (e)–(h) intraseasonal, and (i)–(l) interannual water vapor flux (vector, kg m−1 s−1) and its divergence (shading, 10−5 kg m−2 s−1) between strong and weak IBT cases from Day −4 to Day +2.

  • View in gallery

    As in Fig. 5, but for (a) synoptic, (b) intraseasonal, and (c) interannual components.

  • View in gallery

    As in Fig. 6, but for (a),(b) synoptic; (c),(d) intraseasonal; and (e),(f) interannual components.

  • View in gallery

    Composited differences of (a)–(d) synoptic, (e)–(h) intraseasonal, and (i)–(l) interannual 700-hPa horizontal wind (vector, m s−1) and vorticity (shading, 10−6 s−1) between strong and weak IBT cases from Day −4 to Day +2.

  • View in gallery

    As in Fig. 10, but for quasigeostrophic streamfunction (shading, 106 m2 s−1) and horizontal wave activity flux (vector, m2 s−2) proposed by Takaya and Nakamura (2001) at 300 hPa.

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Response of Winter Moisture Circulation to the India–Burma Trough and Its Modulation by the South Asian Waveguide

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  • 1 Department of Geography and Resource Management, Chinese University of Hong Kong, Hong Kong, and Center for Monsoon and Environment Research and School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, China
  • 2 Department of Geography and Resource Management, and Institute of Environment, Energy and Sustainability, Chinese University of Hong Kong, Hong Kong, China
  • 3 Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong, China
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Abstract

The response of moisture circulation to the daily evolution of the India–Burma Trough (IBT) and the modulation of disturbances along the South Asian waveguide are analyzed to seek a potential precursor of winter precipitation over south China. Daily observational precipitation and reanalysis data from ERA-Interim during 1979–2012 are employed. It is found that moisture circulation in response to the IBT is part of the zonally oriented wave trains along the South Asian waveguide, but it persists longer and migrates farther eastward than other lobes. Cyclonic moisture transport enhances the moisture supply to south China as a strong IBT develops, and shifts eastward abruptly after the peak of IBT with enhanced precipitation shifting from southwest to southeast China. This response is a joint effect of synoptic, intraseasonal, and interannual components that show similar wave train structures, whereas slight differences still occur. The synoptic component shows a shorter wavelength, more southerly path, faster phase speed, and group velocity, with the signal from the North Atlantic to the Bay of Bengal (BoB) in 6 days, implying that a disturbance over the North Atlantic is a potential precursor of winter precipitation over south China. The synoptic moisture convergence is more intensive than that at other scales upstream except over Southeast Asia, where all components are comparable. This might result from the constrained moisture source from BoB at the synoptic scale because of a short wavelength, while widespread sources from BoB–western North Pacific (WNP) at other scales as wavelengths are longer.

Corresponding author e-mail: Prof. Yongqin David Chen, ydavidchen@cuhk.edu.hk

Abstract

The response of moisture circulation to the daily evolution of the India–Burma Trough (IBT) and the modulation of disturbances along the South Asian waveguide are analyzed to seek a potential precursor of winter precipitation over south China. Daily observational precipitation and reanalysis data from ERA-Interim during 1979–2012 are employed. It is found that moisture circulation in response to the IBT is part of the zonally oriented wave trains along the South Asian waveguide, but it persists longer and migrates farther eastward than other lobes. Cyclonic moisture transport enhances the moisture supply to south China as a strong IBT develops, and shifts eastward abruptly after the peak of IBT with enhanced precipitation shifting from southwest to southeast China. This response is a joint effect of synoptic, intraseasonal, and interannual components that show similar wave train structures, whereas slight differences still occur. The synoptic component shows a shorter wavelength, more southerly path, faster phase speed, and group velocity, with the signal from the North Atlantic to the Bay of Bengal (BoB) in 6 days, implying that a disturbance over the North Atlantic is a potential precursor of winter precipitation over south China. The synoptic moisture convergence is more intensive than that at other scales upstream except over Southeast Asia, where all components are comparable. This might result from the constrained moisture source from BoB at the synoptic scale because of a short wavelength, while widespread sources from BoB–western North Pacific (WNP) at other scales as wavelengths are longer.

Corresponding author e-mail: Prof. Yongqin David Chen, ydavidchen@cuhk.edu.hk

1. Introduction

Winter precipitation over south China is rare, as this region experiences the dry and cold Southeast Asia monsoon from an inland area. However, its variation can have great impacts, with a deficit easily leading to drought and a surplus bringing freezing rain or heavy snow; both can seriously hamper rapid socioeconomic development. Hence, it is important to study in depth how the underlying factors affect the variation of winter precipitation over south China.

The India–Burma Trough (IBT) is an important system in determining the intensity of winter precipitation over south China, as it supplies most of the moisture from the Bay of Bengal (BoB) when the moisture from the western North Pacific (WNP) is interrupted to a large extent (Duan et al. 2012; Qin et al. 1991; Zhang et al. 2007). It was found by Qin et al. (1991) that associated with the activity of the IBT, the BoB acts as the only moisture source for precipitation over the Yunnan province in winter. Three out of four recent heavy snow events in the Yunnan province resulted from the strong moisture transport from the BoB guided by the IBT (Zhang et al. 2007). When the IBT is strong and persistent, strong precipitation can arise even without a concomitant cold air invasion (Guo et al. 2010). Previous studies on the IBT’s impact on the moisture transport have focused mainly on one or several cases; only a few attempts have been made to discuss these issues systemically (Li and Zhou 2016; T. M. Wang et al. 2011; Wang et al. 2014). Based on the interannual variation of IBT, Li and Zhou (2016) showed that moisture transport in response to the variation of the IBT shows a great baroclinicity over the BoB. When the IBT is strong, southwesterly transport appears ahead of the trough between 850 and 650 hPa, with an anticyclonic moisture circulation underneath and a cyclonic moisture circulation on top. Considering the great importance of the IBT in inducing winter precipitation by modifying the moisture supply, it is worth investigating in detail how IBT activity affects the moisture circulation over south China, including moisture sources and sinks, the horizontal and vertical structures of the transport path, and their propagation features.

In addition to interannual variation, the IBT shows energetic synoptic variation, with its development closely related to disturbances from the midlatitudes. When the disturbances invade the subtropical westerly, the IBT intensifies and migrates from the BoB to the South China Sea (SCS), affecting the precipitation over south China from the west to the east (Suo et al. 2008; Zong et al. 2012; Song et al. 2014), which is termed an IBT process appearing every 5–6 days. If the disturbance originates from the eastern Arabian Sea, it would take around 4 days to migrate to the SCS with a speed of 12.5° day−1, and then disappears (Guo et al. 2010). When the subtropical westerly is strong, disturbances can even originate from northern Africa (Meteorological Observatory of Kwangsi 1977). The evolution of the IBT tends to be highly connected with the signal from upstream, which probably modifies the coupled moisture. Our previous study showed that the moisture circulation coupled with a strong winter IBT is characterized by zonally oriented chains of abnormal anticyclonic/cyclonic moisture circulation from the Mediterranean Sea to the Philippine Sea (Li and Zhou 2016), which is consistent with the Rossby wave train structure found in Branstator (2002) and Watanabe (2004). However, all of these conclusions were drawn based on either monthly or seasonal scales though the activity of IBT is a synoptic process; the influence of the IBT on moisture transport and the possible modulation by upstream signals at a synoptic scale has still not been systemically investigated. In this study, these issues will be examined and the variations will be partioned into different time scales to study the underlying processes carefully, including the phase speed and group velocity of the upstream signals. This can be treated as a continuation of our previous study (Li and Zhou 2016), which was based only on interannual variations.

This study aims to investigate in detail how the daily evolution of the IBT affects the winter precipitation over China by means of modifying the moisture circulation, with the target region not constrained over the BoB, but from the North Atlantic to the WNP. The signals will be partitioned into different time scales to find out whether the synoptic, intraseasonal, or interannual variation dominates the impact of the IBT and what the differences are between them. The rest of this paper is organized as follows. Section 2 describes the observational data and analysis methods. In section 3, the IBT process and its impact on the winter precipitation will be studied briefly. The moisture circulation in response to IBT activity and the modulation by an upstream signal along the South Asian waveguide will be examined in section 4. Their differences and relative importance at synoptic, intraseasonal, and interannual scales will be compared in section 5, followed by a summary and discussion in section 6.

2. Data and methodology

a. Data

Gauge-based daily precipitation data over 756 stations in China, collected and subjected to quality control procedures by the China Meteorological Administration (Bao 2007), were employed. The key reanalysis data applied in this study were the ERA-Interim, produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in preparation for the next-generation reanalysis to replace ERA-40 (Dee et al. 2011). Many improvements have been made in ERA-Interim to deal with the problems with ERA-40 in terms of hydrological balance (e.g., excessive precipitation over the tropical oceans, extravagant total column water vapor, and the global imbalance of precipitation and evaporation) (Dee and Uppala 2008). In this study, the gridded (1.5° × 1.5°) daily vorticity, vertical integral of water vapor fluxes, horizontal wind, specific humidity, and geopotential height were also employed. The time range of the two datasets is the period of 1959–2012.

b. Measuring IBT events

To measure the activity of the IBT in winter, the vorticity-based daily IBT index (IBTI), the normalized 700-hPa vorticity averaged over the northern BoB (15°–25°N, 85°–100°E, indicated by the dashed box in Fig. 1), was used. This is consistent with T. M. Wang et al. (2011), except that the regional western boundary was shifted 5°E eastward so as to highlight the activity center (identified as where maximum vorticity lies in Fig. 1b) at around 92.5°E. Strong (weak) IBT events were identified through the following two steps. First, a strong (weak) IBT event was identified when the regional vorticity over the northern BoB was positive (negative) and the IBTI was larger than 1.0 (less than −1.0). Second, since the IBT tends to appear every 5–6 days (Qin et al. 1991), the interval between two adjacent events is set to be at least 4 days. In this manner, 173 strong and 162 weak IBT events were identified during 1979–2012. For the strong IBT events, the number increases from December to February; there are 54 cases occurring in December, 57 cases in January, and 62 cases in February. This is consistent with previous findings that the IBT tends to enhance in winter with increasing vorticity (Suo and Ding 2009).

Fig. 1.
Fig. 1.

(a) Number of strong and weak (multiplied by −1) winter India–Burma Trough (IBT) cases detected by using the IBTI during 1979–2012, and composition of 700-hPa geopotential height (contour, gpm) and vorticity (shading, 10−5 s−1) based on (b) strong and (c) weak IBT cases detected in (a). The dashed box in (b)–(c) indicates the region where the vorticity is averaged as the IBTI.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

c. Bandpass filtering

The period analysis of the IBTI was calculated based on the power spectrum to illustrate its spectrum characteristics. That is, the time series of TBTI in each winter is executed in the power spectrum analysis; the significant frequency is selected and transferred to the period (Fig. 2). The IBTI shows a wide period range from 3 days to ~30 days in each winter. To study carefully the underlying processes accompanying with the IBT activity in different time scales, the original daily circulation fields will be separated into synoptic (<10 days), intraseasonal (10–90 days), and interannual (>90 days. This last period is termed as an “interannual” instead of “interseasonal” component by considering that only the IBT cases in winter are investigated and thus this low-pass signal mainly represents the interannual variation components. To achieve this, a Fourier method called Lanczos filtering was utilized to filter the variables, including the wind field, geopotential height, water vapor flux, and its divergence. The difference between the sum of the three filtered components and the raw data is very small, with a negligible root-mean-square error of 6.6 × 10−9 in the filtering of the IBTI, indicating that this decomposition method guarantees completeness.

Fig. 2.
Fig. 2.

Period analysis of the time series of daily IBTI in each winter, which is derived from the spectrum analysis. The columns indicate the period derived from the Markov “red noise” spectrum and the vertical error bars indicate the upper and lower periods derived from the 5% and 95% red noise confidence bounds.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

3. IBT processes and their impacts on precipitation

The annual variations of strong and weak IBT cases are shown in Fig. 1a. The IBT was most active in 1991, with 12 strong cases but no weak cases; in contrast, it was weakest in 1983, 1985, and 2011, with fewer than 3 strong cases but more than 8 weak cases. The number of strong IBT cases shows no significant decadal variation during the study period, while the number of weak IBT cases peaked in the early 1980s, then decreased abruptly after 1985, and increased gradually afterward. Based on these IBT cases, the daily evolution of their impacts on precipitation and moisture circulation will be composited. In the ensuing sections, “Day 0” refers to the peak of the IBT events, and Day −n (+n) refers to n days before (after) Day 0.

To depict the structure of strong and weak IBT, the compositions of 700-hPa geopotential height and vorticity are displayed in Figs. 1b,c. A strong IBT case corresponds to the collapse of the subtropical high, with relatively low geopotential height and positive vorticity over the northern BoB. Southwesterly flow ahead of the trough dominates south China. In contrast, a weak IBT corresponds to the domination of the subtropical high, with relatively high geopotential height and negative vorticity over the BoB. South China is dominated by a westerly instead of a southwesterly; the moisture transport from the BoB tends to be interrupted.

To demonstrate the impacts of the IBT, the difference of daily precipitation over China between strong and weak IBT cases from Day −2 to Day +3 are shown in Fig. 3. Generally, the enhanced winter precipitation induced by strong IBTs shifts from the west to the east over south China. Before the peak of IBT, precipitation over south China starts to increase gradually. Enhanced precipitation first appears over southwest China on Day −2, and then extends to southeast China on Day −1. Abnormal positive precipitation enhances and expands to the south of the Yangtze River valley at the peak of the IBT, and shifts eastward to southeast China with its magnitude increasing even more dramatically one day later (Day +1); simultaneously, the impact of the IBT starts to fade out over southwest China. On Day +2, the IBT-induced positive precipitation weakens over southeast China and becomes negative over southwest China. Subsequently, a northward extension of positive precipitation anomalies to the north of the Yangtze River valley is observed, but in a weak manner (Day +3). To summarize, enhanced precipitation appears over south China several days before the peak of a strong IBT case, peaks on Day 0, shifts from the west to the east as the trough migrates eastward, and expands northward as the trough fades out. It is widely accepted that strong moisture transport from the BoB guided by the southwest flow ahead of the trough is a key cause of such winter precipitation enhancement (Qin et al. 1991; Duan et al. 2012; Zhou et al. 2009). In the following sections, this issue will be examined in detail.

Fig. 3.
Fig. 3.

Composited differences of daily precipitation (mm day−1) between strong and weak IBT cases over China from Day −2 to Day +3. Only the results significant at the 90% confidence level are shown.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

4. Moisture circulation in response to the IBT

a. Wave train structure

To demonstrate clearly how moisture is transported by the IBT to south China and how the abnormal moisture transport evolves as the IBT develops, matures, and decays, the daily evolution of the water vapor flux and its divergence (from Day −6 to Day +3) are depicted in Fig. 4. Generally, the evolution of moisture circulation coupled with a strong IBT process is characterized by zonally oriented wave chains along the South Asian waveguide. The signal propagates from the North Atlantic to the WNP with cyclonic lobes over the North Atlantic, the Red Sea, and the northern BoB, and anticyclonic lobes over the southern Europe, the Arabian Sea, and the WNP. Southward moisture transport associated with abnormal moisture divergence lies behind (ahead of) the cyclonic (anticyclonic) lobes, while northward moisture transport associated with abnormal moisture convergence appears ahead of (behind) the cyclonic (anticyclonic) lobes.

Fig. 4.
Fig. 4.

Composited differences of the vertical integral of water vapor flux (vector, kg m−1 s−1) and its divergence (shading, 10−5 kg m−2 s−1) between strong and weak IBT cases from Day −4 to Day +3. Only the composited water vapor fluxes significant at the 90% confidence level are shown.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

Accompanying strong IBT events, the abnormal cyclonic moisture transport over the North Atlantic and the anticyclonic moisture transport over Europe start to build up around one week before the peak of the trough (Fig. 4a). In the following days, such fluctuation of moisture circulation propagates downstream, with the activity centers over the Red Sea and Arabian Sea intensifying gradually. As the IBT develops, the cyclonic moisture transport starts to establish over BoB, with the northerly moisture transport diverging over the Indian subcontinent–BoB and the southerly transport converging over the Indochina Peninsula–SCS. It enhances abruptly from Day −2 to Day 0 (Figs. 4e–g). The strong moisture is imported into south China mainly via the southern boundary, instead of the climatological key input boundary—the western boundary (Li et al. 2013). On Day +1, the IBT-related moisture transport shifts eastward abruptly, with the southwesterly transport ahead of the trough enhancing and extending farther north of the Yangtze River valley and the WNP (Fig. 4h). Such eastward and northward extension could be ascribed to the development of an anticyclone over the WNP (figure not shown) as the signal along the South Asian waveguide propagates downstream to the WNP. Subsequently, the abnormal moisture transport behind the trough decays as the IBT flattens, while the abnormal southwesterly transport over east China and the WNP persists as the anticyclonic moisture circulation over the WNP enhances (Fig. 4i). On Day +3, as the anticyclone over the WNP migrates eastward, and the abnormal southerly transport with moisture convergence shifting to southern Japan; the abnormal moisture convergence over east China vanishes (Fig. 4j). Hence, the moisture transport associated with the evolution of the IBT shows a remarkable wave train structure along the East Asian jet waveguide, with the signal not only propagating but also migrating slightly downstream.

b. Downstream propagation

For a clear depiction of the propagation of moisture transport and its divergence fluctuations along with the evolution of the IBT trough, the variations of the 15°–25°N averaged meridional moisture transport and moisture divergence are displayed in Fig. 5. Corresponding to the zonally oriented chains of moisture circulation, enhanced southerly/northerly moisture transport and moisture convergence/divergence dominate South Asia alternately with the zonal group velocity much faster than the zonal phase speed. The abnormal moisture transport over the northern BoB is part of such wave trains. It shows features similar to those of the upstream signals and also has its own features. It tends to persist longer, for more than seven days, and migrate farther eastward from the northern BoB to the SCS–WNP. After the peak of the IBT, abnormal southerly transport and moisture convergence ahead of the IBT migrate across the Indochina Peninsula abruptly and then enhance over the WNP, although the northerly transport behind the trough decays. Such reintensification results mainly from the superimposition of the southerly flow to the west of the anticyclonic lobe over the WNP. It is noteworthy that the response of such moisture circulation anomalies to the strong IBT is controlled largely by wind instead of moisture anomaly, with the magnitude of the circulation anomaly–induced transport one order of magnitude larger than that of the moisture anomaly–induced transport (figure not shown).

Fig. 5.
Fig. 5.

Evolution of composited 15°–25°N averaged meridional vertical integral of water vapor flux (contour, kg m−1 s−1) and moisture divergence (shading, 10−5 kg m−2 s−1) from Day −6 to Day +6.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

c. Moisture supplied to southwest and southeast China

The IBT shows asynchronous west–east impacts over south China. Figure 6 depicts the evolution of the vertical structure of regional moisture divergence over southwest (21°–28°N, 97°–105°E) and southeast China (21°–28°N, 105°–120°E). Several salient features emerge. First, great baroclinicity appears in the vertical structure of the regional moisture convergence anomalies. At the peak of the IBT, the abnormal moisture divergence shows a dipole structure over southwest China, with strong moisture convergence in a lower thin layer and weak moisture divergence on top. In contrast, it shows a sandwichlike structure over southeast China, with strong moisture convergence over 850–500 hPa and moisture divergence near the surface and on top. The IBT-induced strong moisture convergences peak at around 700–800 hPa over both regions (Li and Zhou 2016). Second, the intensity of the trough-induced moisture convergence over southwest China is much stronger than that over southeast China but constrained in a thinner layer. Third, the impact of the IBT evolves differently over two regions. Over southwest China, it intensifies gradually but vanishes abruptly, while it evolves less remarkably over southeast China. To summarize, the evolution and vertical structure of the IBT-induced moisture supply differ greatly between southwest and southeast China, but the underlying cause is still unclear. It might result from the topographic effect, which needs further investigation in the future.

Fig. 6.
Fig. 6.

Evolution of vertical distribution of composited regional moisture divergence over (a) southwest China (SW, 21°–28°N, 97°–105°E) and (b) southeast China (SE, 21°–28°N, 105°–120°E) from Day −6 to Day +6.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

5. Evolution of the responding moisture circulation at different time scales

In a previous study, the response of moisture circulation to the variation of the IBT at the interannual scale was investigated, and a similar wave train structure was detected along the South Asian jet waveguide when the winter IBT was strong (Li and Zhou 2016). Such a jet waveguide effect was also reported in other studies (Branstator 2002; Watanabe 2004; Song et al. 2014), but all were conducted based on either monthly or seasonal data, in which the synoptic or intraseasonal signal might be smoothed out. Questions may arise about which temporal-scale variation is more important for the moisture circulation in response to the IBT modulated by the wave train signals detected in the previous sections—the synoptic, intraseasonal, or interannual time scale? And do the wave trains at different time scales show different features?

To address these questions, the composition of moisture circulation based on strong/weak IBT events was analyzed based on synoptic, intraseasonal, and interannual components, respectively (Figs. 78). It is evident that the downstream propagation of wave train signals features moisture circulation anomalies at all time scales, whereas the wavelength, path, and phase speed and group velocity show slight differences. For the synoptic moisture circulation, the wave trains are characterized by a shorter wavelength, a southward path, and faster phase speed (around 8° day−1) and group velocity (around 16° day−1). From the North Atlantic to the BoB, there are six activity centers that shift from around 40° to 20°N; the fluctuation of moisture circulation is not detected north of 60°N. The zonal phase speed of the signals is around 8° day−1 at the synoptic scale, around 4° day−1 and 0° day−1 at the intraseasonal and interannual scales, respectively (Fig. 8). For the intraseasonal moisture circulation, the wavelength increases, with five activity centers from the North Atlantic to the BoB; although the signals are still constrained to the south of 60°N, the activity centers tend to shift slightly northward. For the interannual moisture circulation, a great discrepancy appears in the propagation path. The upstream signal splits into two paths to the east of Europe; the southern path is similar to that at the intraseasonal time scale; the northern path is less extended, with only one significant lobe north of the Caspian Sea.

Fig. 7.
Fig. 7.

Composited differences of the vertical integral of (a)–(d) synoptic, (e)–(h) intraseasonal, and (i)–(l) interannual water vapor flux (vector, kg m−1 s−1) and its divergence (shading, 10−5 kg m−2 s−1) between strong and weak IBT cases from Day −4 to Day +2.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

Fig. 8.
Fig. 8.

As in Fig. 5, but for (a) synoptic, (b) intraseasonal, and (c) interannual components.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

The magnitudes of water vapor fluxes at different time scales are comparable, while those of moisture convergence show great diversity. The synoptic moisture convergence tends to be more intensive than the intraseasonal and interannual ones along the wave trains except over Southeast Asia, where all time scales are comparable (Fig. 8). At the synoptic scale, as the wavelength is short, the abnormal southwesterly transport ahead of the IBT affects only the Indochina Peninsula, with the moisture originating from the BoB (Fig. 7c), while at the intraseasonal and interannual scales, the southwesterly transport can expand to the SCS and couple with the downstream anticyclonic lobe over the WNP, guiding abundant moisture from both the BoB and WNP to south China (Figs. 7g and 7k). At the interannual scale, the abnormal moisture diverged over the southern SCS and the Philippine Sea is also advected to south China (Figs. 7i–l). This might be contributed by the Philippine Sea anticyclone, which is remotely forced by El Niño (Li and Zhou 2012, 2016; Li et al. 2014; Zhang et al. 2015). By enhancing the southwesterly flow ahead of the IBT, El Niño tends to intensify the IBT via the Philippine Sea anticyclone. To summarize, the moisture transport anomalies related to abnormal IBT activity are remarkable at different time scales, and all show a wave train structure along the South Asian waveguide, although slight differences exist in their path, phase speed, group velocity, and wavelength. The IBT-induced moisture convergence over Southeast Asia is a result of the joint effects of synoptic, intraseasonal, and interannual time scales.

The impact of IBT activity on the moisture supply to southwest and southeast China at different time scales is shown in Fig. 9. For the vertical structure, the regional moisture divergences at different time scales show a similar structure, that is, a dipole structure over southwest China and a sandwichlike structure over southeast China. The magnitude of synoptic abnormal moisture convergence/divergence tends to be larger; however, this stronger moisture convergence is counteracted by the stronger moisture divergence at other levels, resulting in comparable IBT-induced vertically integrated moisture convergence over Southeast Asia at all time scales. For the persistence, the synoptic moisture convergence appears only near the peak of the IBT, whereas the intraseasonal convergence appears around 4 days before and decays 2 days later, and the interannual convergence lasts even longer. In other words, the interannual signal tends to provide a favorable background for intensive moisture convergence during a strong IBT process. The joint effects of interannual and intraseasonal signals result in a gradual increase in moisture convergence before the peak; whereas the synoptic signal is crucial to the abrupt increase and termination of strong moisture convergence and thus precipitation over southeast and southwest China.

Fig. 9.
Fig. 9.

As in Fig. 6, but for (a),(b) synoptic; (c),(d) intraseasonal; and (e),(f) interannual components.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

6. Conclusions and discussion

a. Conclusions

We have carried out a study of the impact of the daily evolution of the IBT on moisture circulation based on the reanalysis data from ERA-Interim during 1979–2012. We selected abnormal IBT processes based on the daily 700-hPa vorticity averaged over the northern BoB (15°–25°N, 85°–100°E) and composited the water vapor fluxes and divergence in response to the daily evolution of IBT processes. It is found that the responding moisture circulation is characterized by zonally oriented wave trains along the South Asian waveguide from the North Atlantic to the WNP, with abnormal northward transport and divergence behind (ahead of) the cyclonic (anticyclonic) lobes, and abnormal southward transport and convergence ahead of (behind) the cyclonic (anticyclonic) lobes. The abnormal moisture transport by the IBT is part of such wave trains but persists longer and migrates farther eastward from the northern BoB to the WNP as superimposed by the southerly flow to the west of an anticyclonic lobe over the WNP.

As a strong IBT develops, cyclonic moisture transport starts to build up over the Indian subcontinent–Indochina Peninsula, enhancing the moisture transport to south China gradually via the southern boundary. Such a moisture transport shifts eastward abruptly after the peak of the IBT with the IBT-caused winter precipitation anomaly shifting from southwest to southeast China. Such a moisture circulation response is the joint effects of synoptic, intraseasonal, and interannual components, which show a similar wave train structure except for slight differences in wavelength, path, phase speed, and group velocity. The synoptic moisture convergence is more intensive than the other components along the waveguide except over Southeast Asia, where all components are comparable. Both interannual and intraseasonal signals are responsible for the gradual increase of moisture convergence before the peak of the IBT; whereas the synoptic signal is crucial for the abrupt enhancement and termination of strong moisture convergence and thus precipitation over southeast and southwest China. Hence, we concluded that the disturbance in the upstream North Atlantic/Mediterranean Sea is a potential precursor of winter precipitation over south China as it modulates the activity of the IBT and thus the moisture supply to south China via the zonally oriented wave trains along the South Asian waveguide.

b. Discussion

As it is the wind disturbance instead of the moisture disturbance that dominates the aforementioned abnormal moisture circulation (figure not shown), the lower-level atmospheric circulation corresponding to abnormal IBT activity was examined (Fig. 10). It shows a structure and evolution similar to that of the moisture circulation. The propagation of disturbances characterizes vorticity and wind fields along South Asia. At the synoptic scale, the signal propagates from the North Atlantic to the BoB in 5 days, with activity centers peaking day by day. The positive vorticity over the BoB tends to originate over the northern Arabian Sea around 5 days before. This is consistent with the previous study by Fan (1980), in which the IBT originated at around 70°E and moved downstream. At the intraseasonal scale, the activity centers tend to shift slightly northward with a longer wavelength. At the interannual scale, two propagation paths are observed, with the northern path even more intensive than the southern path. On Day 0, the IBT is a joint result of positive vorticity and cyclonic circulation anomalies over the northern BoB at different time scales. As the propagation of upstream disturbances couple well with the IBT activity at synoptic, intraseasonal, and interannual scales, better prediction of IBT activity at short, medium, and long ranges can be expected if more attention is paid to upstream disturbance over the North Atlantic or the Mediterranean Sea.

Fig. 10.
Fig. 10.

Composited differences of (a)–(d) synoptic, (e)–(h) intraseasonal, and (i)–(l) interannual 700-hPa horizontal wind (vector, m s−1) and vorticity (shading, 10−6 s−1) between strong and weak IBT cases from Day −4 to Day +2.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

Wave activity flux is a useful tool for illustrating a “snapshot” of a propagating packet of stationary or migratory quasigeostrophic wave disturbances and thereby for inferring where the packet is emitted and absorbed (Takaya and Nakamura 2001). By analyzing its evolution (Fig. 11), we attempt to explore the source and propagation of energy nurturing the IBT. In Fig. 11, the aforementioned wave trains also appear in the upper level, indicating their barotropic vertical structure, with the upper signals shifting slightly northward, which might result from the absence of a terrain blocking effect. Origin and persistence of the wave are associated with the energy emission from the North Atlantic (Zhu et al. 2011). The propagation of Rossby wave energy along the South Asian jet stream is remarkable at all time scales, verifying its crucial role to the variations of the IBT and the responding moisture circulation. The downstream lobes where energy is absorbed intensify gradually, whereas the upstream lobes where energy is emitted decay. At the synoptic scale, the propagation of energy from the Mediterranean Sea to the BoB starts on Day −4 and takes 5 days. At the intraseasonal scale, although the phase speed of wave trains is slower, the propagation of wave energy is almost synchronous. At the interannual scale, the emission of energy from the anticyclone over the Mediterranean Sea splits into two paths, with the southern one going to the BoB along the South Asian jet stream and the northern one going to northern East Asia along the midlatitude westerlies termed the “East Atlantic/West Russia teleconnection pattern” in previous research (Barnston and Livezey 1987; X. Wang et al. 2011; Wang et al. 2013; Orsolini et al. 2015). Such a northern path is only found in the interannual scale, which might be attributed to its stationary property and the northward extension of upstream signals over Europe/northern Europe in the interannual scale rather than in synoptic and intraseasonal scales, which is crucial to the downstream propagation of the signal along the midlatitude westerlies (Wang et al. 2013). A possible hint for the physical causes of such an interannual teleconnection was proposed by X. Wang et al. (2011) that it might be triggered by the SST-induced heating anomaly over the North Atlantic, which modifies the transient eddy activities in the North Atlantic storm track (Zhu et al. 2011).

Fig. 11.
Fig. 11.

As in Fig. 10, but for quasigeostrophic streamfunction (shading, 106 m2 s−1) and horizontal wave activity flux (vector, m2 s−2) proposed by Takaya and Nakamura (2001) at 300 hPa.

Citation: Journal of Climate 30, 4; 10.1175/JCLI-D-16-0111.1

Acknowledgments

This work is financially supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project CUHK441313) and the National Natural Science Foundation of China (Projects 41405045 and 41375096).

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