1. Introduction
The downstream effect of East Asian and western Pacific disturbances on the weather and climate in eastern North Pacific and western North America has been recognized as one of the important issues for further understanding and improvement of the extratropical predictability, which is also one of the research goals of The Observing System Research and Predictability Experiment (THORPEX; Shapiro and Thorpe 2004). It has long been known that the tropical convective activity in the western North Pacific initiates Rossby wave–like perturbations propagating northward toward northeast Asia and eastward toward the extratropical North Pacific and North America (e.g., Nitta 1987; Huang and Sun 1992). Recent studies (e.g., Lau and Weng 2002; Lau et al. 2004) reported the recurrent teleconnection patterns linking the perturbations in East Asia and North America. Similar downstream-propagating perturbations were also observed following the occurrence of intraseasonal fluctuation and tropical cyclones (TCs) in the tropical western North Pacific (e.g., Fukutomi and Yasunari 1999; Hsu and Weng 2001, Shapiro and Thorpe 2004; Tsou et al. 2005; Kawamura et al. 1996; Kawamura and Ogasawara 2006; Jiang and Lau 2008). Downstream development of these extratropical perturbations often occurs along the extratropical jet stream, which acts like a Rossby waveguide (e.g., Branstator 1983; Karoly 1983; Hsu and Lin 1992; Hoskins and Ambrizzi 1993; Ambrizzi et al. 1995). This waveguide forms an important part of the circumglobal wave pattern reported in recent studies (Branstator 2002; Ding and Wang 2005).
Recent studies (Ko and Hsu 2006, hereafter KH06; Ko and Hsu 2009, hereafter KH09) discovered a 7–30-day wave pattern propagating north-northwestward from the northeast of Papua New Guinea to the area between Taiwan and Japan during mid to late summer. Most of the identified wave patterns were associated with at least one tropical cyclone. The composite 7–30-day filtered 850-hPa streamfunction at peak phase (day 0) and associated TC tracks, based on KH09, are reproduced in Fig. 1. A TC/submonthly wave pattern was clearly seen in the lower troposphere in the western North Pacific. TCs embedded in the cyclonic circulation of the wave pattern moved north-northwestward together with the wave pattern toward the East China Sea (KH06; KH09). Most of the TCs associated with the wave pattern followed the recurving path and gradually turned northeastward toward the midlatitudes after day 0. A wave-like pattern across the Pacific basin is also evident in the extratropical North Pacific.
Following KH06 and KH09, the present study analyzes and compares the wave activity in the extratropical North Pacific before and after the TC/submonthly wave pattern reaches the recurving region. The downstream perturbation development toward the eastern North Pacific and western North America following the TC/submonthly wave pattern is identified and presented here.
2. Data and procedures
The data used in this study were retrieved from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40; Uppala et al. 2005). The 6-hourly (0000, 0600, 1200, and 1800 UTC) horizontal winds, vertical velocity, geopotential, and temperature on a 2.5° × 2.5° latitude–longitude grid were used for diagnostics. The data period for the present study is July–September of 1979–2001. This study adopts similar case selection criteria as in KH09. The readers are referred to KH09 for details. The procedure is briefly described as follows: The submonthly cases were selected based on the 7–30-day filtered 850-hPa wind speed averaged over the base region (20°–35°N, 125°–140°E) where the maximum variance of 850-hPa wind speed is found in the western North Pacific. The submonthly cases were selected when the positive wind speed anomaly was greater than 2 m s−1, which is equivalent to 1.5 standard deviation. This threshold value (2 m s−1) is larger than that (1 m s−1) adopted in KH09 to retain only stronger cases that exhibit more significant downstream impact. A TC/submonthly case was further selected when at least a TC appeared in the base region (with a 1.5° latitude–longitude buffer zone just outside the edge) between days −1 and +1 of a selected submonthly case. The resultant number of cases for the composite is 71, and the averaged case number each year is about 3.
The wave activity flux (WAF) developed by Takaya and Nakamura (2001) is calculated for each case based on 7–30-day filtered streamfunction anomaly and composited to examine the energy propagation of the documented wave-like pattern in the extratropical North Pacific. To investigate the source of the wave-like pattern, the Rossby wave source (RWS) developed by Sardeshmukh and Hoskins (1988) was calculated to illustrate the source of vorticity perturbation. In this study, a 15-day average centered at the peak time of each selected case was used as the mean state in the calculation of WAF and RWS. The 7–30-day filtered fields are thus defined as the perturbations to compute the RWS and WAF. This approach was applied in previous studies (e.g., Hong et al. 2008) and has been proved useful in analyzing the wave dynamics.
3. Results
The Hovmöller diagram of 200-hPa meridional wind anomaly (7–30 day) averaged between 45° and 55°N, where the largest variance is located, shown in Fig. 2 presents the temporal evolution of the extratropical perturbation over the North Pacific before and after day 0. It is readily seen that the perturbations were weak before day 0 and loosely organized. Conversely, the perturbations strengthened significantly after day 0 and became well organized. While the individual anomaly moved slowly eastward (approximately 4° day−1), a wave-like pattern developed quickly across the extratropical North Pacific toward North America. The fact that the largest amplitude region of the wave pattern continues shifting eastward suggests eastward energy dispersion. Results of a statistical significance test also indicate the significant change after the arrival of the TC/submonthly wave pattern at the recurving region. Before day 0, only a small portion of perturbation in the western Pacific was statistically significant. On the contrary, most of the perturbations in the wave-like pattern were statistically significant after day 0, when the pattern propagated through the extratropical North Pacific.
Figure 3 presents the vertical cross section of meridional wind anomaly and anomalous WAF averaged between 45° and 55°N at days −1, +2, and +5. The anomalous WAF was obtained by subtracting the 15-day mean WAF. The wave pattern generally exhibited an equivalent barotropic vertical structure, and the WAF was the strongest in the upper troposphere. At day −1, the meridional wind perturbations in the extratropical North Pacific were generally weak and not statistically significant. Additionally, an area of westward-pointing WAF anomalies was located over the central Pacific. By day +2, a significant wave pattern was clearly well developed from 120°E to the date line, with the largest amplitude to the west of the date line and smaller amplitude to the east of the date line. Moreover, an eastward-pointing WAF anomaly area emerged in the western North Pacific, while the westward-pointing WAF anomalies shifted to 170°–140°W. After day +2, this wave pattern moved eastward along with the eastward-pointing WAF anomalies, when the perturbations in the western North Pacific weakened slightly and those in the eastern North Pacific strengthened.
Interestingly, the eastward movement of the WAF is faster than the movement of the wave-like pattern. The eastward-pointing WAF anomalies developed in the western North Pacific (e.g., 120°E) at day −1, propagated eastward, and reached North America (e.g., 120°W) at day +5. During the same period, the southerly anomaly originally in 170°E at day −1 moved to 160°W by day +5. This contrast between the speeds of energy propagation and wave pattern reflects the characteristics of a quasi-stationary Rossby wave.
The evolution of the upper-level circulation is further illustrated by the composites of the 7–30-day streamfunction at 200 hPa, as shown in Fig. 4. The anomalous WAF is also displayed. At day −3, perturbations were clearly present in the western North Pacific, while there is no well-organized wave structure in the extratropical North Pacific. Because of the prevailing mean westerly flow in the extratropical North Pacific, the WAF generally points eastward (Takaya and Nakamura 2001). Positive and negative anomalous WAF indicates the strengthened and weakened downstream energy dispersion and development, respectively, relative to the advection effect of the prevailing mean westerly flow. The westward-pointing WAF anomaly in the extratropical North Pacific at day −3 means weaker eastward energy propagation than usual. The TC/submonthly wave pattern was well established at day 0 in the tropical western North Pacific. It resembles the wave pattern in the lower troposphere shown in Fig. 1 but in reversed signs, with a vertical structure similar to that of the first baroclinic mode. Additionally, a better organized wave pattern appeared in the extratropical western North Pacific, indicating a possible downstream development of perturbation initiated in the western North Pacific. The extratropical trans-Pacific wave train was well established at day +3 when an anomalous anticyclonic circulation emerged over Japan. By day +6, the anomalies in the western North Pacific weakened, while positive and negative anomalies were well developed in the Gulf of Alaska and western North America, respectively. Continuously eastward amplification of perturbation with time was clearly seen by comparing these figures.
At day −3, the anomalous WAF vectors mostly pointed westward to the east of 140°E, whereas the eastward-pointing WAF anomalies were confined in a limited region west of 140°E. This indicates a situation of weak wave activity in the extratropical North Pacific. As the extratropical wave pattern well developed at day 0 (Fig. 4), the eastward-pointing anomalous WAF gradually expanded to the whole extratropical western North Pacific, indicating the enhancement of downstream development of wave activity. The eastward-pointing WAF anomalies were further enhanced and expanded to the date line at day +3, and they propagated eastward across the extratropical North Pacific by day +6 (Fig. 4). The change of direction in WAF anomalies indicated that the enhancement of eastward transport of energy after day 0. The aforementioned features resemble the results of Jiang and Lau (2008), except that they used southwestern North America as the index and examined backward to yield the trans-Pacific energy transport from the western North Pacific through the barotropic Rossby wave trains.
Results shown here reveal the important role of the TC/submonthly wave pattern, after its arrival in the extratropical western North Pacific, in enhancing or even initiating the extratropical trans-Pacific wave activity. Figure 5 shows the meridional–vertical cross section of the 7–30-day diabatic heating and WAF averaged over 120°–130°E. The positive diabatic heating located near 20°N at day −2 reflected the deep convection associated with the TC/submonthly wave pattern. By day 0, the positive diabatic heating moved northward to 25°–30°N and strengthened. The northward propagation and strengthening of the diabatic heating from day −2 to day 0 reflected the recurving movement of the wave pattern along with TCs. The northward-pointing WAF, which was seen to the north of the positive diabatic heating and almost in the whole troposphere at both days −2 and 0, indicated the northward energy emanation from the anomalous deep convection. The WAF also strengthened when the strengthening of diabatic heating occurred. The anomalous WAF in the upper troposphere north of 30°N was mostly southward pointing, and the zonal component of WAF was weaker and not well organized at days −2 and 0.
At day 0, the negative diabatic heating (i.e., cooling) was well developed to the north of the heating in 35°–50°N. Unlike the deep heating structure, the cooling prevailed mostly in the upper troposphere. In the next few days (e.g., day +2), the cooling stretched northward to 70°N and covered essentially the whole region north of the diabatic heating. This cooling occurred in the region of the northward and downward outflow associated with the deep convection in the TC/wave pattern (not shown). At the same time, the positive zonal WAF anomaly appeared in the upper troposphere in the cooling region, while the northward WAF to the north of the deep convection stayed to the south of 40°N and started weakening. The collocation of the zonal WAF anomaly and cooling seems to imply the enhancement of zonal WAF by the diabatic cooling, instead of being triggered by the anomalous northward WAF associated with the deep heating. In addition, Ko and Vincent (1996) found that the upper-level outflow generated by tropical convection tended to enhance the zonal wind poleward of the convection through the Coriolis force. Because the WAF in the extratropical North Pacific was dominated by the westerly flow, the upper-level outflow generated by the TC/wave pattern likely enhanced the zonal flow and in turn resulted in stronger downstream energy propagation toward the east.
To verify the role of TC/wave pattern in triggering the extratropical wave activity in the North Pacific, the RWS that represents the vorticity generation by the divergent wind was examined in the present study. Figure 6 shows that, at day −2, a north–south-oriented RWS pattern existed in the western North Pacific where the TC/wave pattern was located. The RWS pattern then expanded in the east–west direction (day 0); finally, at day +2, the pattern became oriented east–west when the meridional pattern disappeared. This result indicates that the continuous generation of rotational flow in the western North Pacific before day 0 was associated with the north-northwestward moving TC/wave pattern. The arrival and recurving of the TC/wave pattern in turn enhanced the vorticity generation in the extratropical Pacific waveguide and further downstream development of wave activity toward the eastern North Pacific. Although it was not clear that the TC/submonthly wave pattern could really excite a totally new wave train in the extratropical North Pacific, the tropical wave pattern along with TCs could at least enhance the wave activity in the extratropical western North Pacific and the following downstream propagation through the extratropical North Pacific.
4. Summary
Downstream development of upper-level wave activity in the extratropical North Pacific, following the recurving TC/submonthly wave pattern in the tropical western North Pacific, is demonstrated here. It is suggested that the TC/submonthly wave pattern in the tropical western North Pacific initiated and/or enhanced the perturbations in East Asia and the extratropical western North Pacific, which propagate and disperse energy eastward through the Rossby waveguide near the Pacific jet stream (e.g., Ambrizzi et al. 1995). The barotropic and/or baroclinic energy conversion may also have contributed to the amplification of the perturbations in the extratropical North Pacific. This mechanism is not examined in this study and will be explored in a later study.
The downstream development of submonthly perturbation, which was initiated or enhanced in the western Pacific, might have significant impacts on the eastern North Pacific and North American weather. Better understanding and reasonable prediction of such phenomenon is likely to improve the long-range weather predictability (Orlanski and Sheldon 1993; Chen and Newman 1998; Shapiro and Thorpe 2004; Jiang and Lau 2008) in these extratropical regions.
Acknowledgments
The authors thank Mr. Yen-Min Lee for computing the streamfunction data. The authors also thank two anonymous reviewers for pointing out several items that led to better quality of the manuscript. This study was supported by the National Science Council, Taiwan under Grants NSC 96-2111-M-017-001-AP4 and NSC 97-2111-M-002-004-AP4.
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Composite 7–30-day streamfunction at 850 hPa (day 0). The interval is 5 (×105 m2 s−1) for the streamfunction. Also shown are the tropical cyclone tracks associated with the cases (black thin lines).
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composite 7–30-day streamfunction at 850 hPa (day 0). The interval is 5 (×105 m2 s−1) for the streamfunction. Also shown are the tropical cyclone tracks associated with the cases (black thin lines).
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composite 7–30-day streamfunction at 850 hPa (day 0). The interval is 5 (×105 m2 s−1) for the streamfunction. Also shown are the tropical cyclone tracks associated with the cases (black thin lines).
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Hovmöller diagram of the 7–30-day filtered meridional wind averaged over 45°–55°N at 200 hPa. Shaded areas represent areas >95% confidence level: shaded in blue (positive) and in yellow (negative).
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Hovmöller diagram of the 7–30-day filtered meridional wind averaged over 45°–55°N at 200 hPa. Shaded areas represent areas >95% confidence level: shaded in blue (positive) and in yellow (negative).
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Hovmöller diagram of the 7–30-day filtered meridional wind averaged over 45°–55°N at 200 hPa. Shaded areas represent areas >95% confidence level: shaded in blue (positive) and in yellow (negative).
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composite cross section of 7–30-day meridional wind averaged over 45°–55°N at 200 hPa. The interval is 1 (m s−1). Areas >95% confidence level are shaded in blue (positive) and in yellow (negative). Also shown are the WAFs (vectors) with the 15-day means removed and vectors <5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composite cross section of 7–30-day meridional wind averaged over 45°–55°N at 200 hPa. The interval is 1 (m s−1). Areas >95% confidence level are shaded in blue (positive) and in yellow (negative). Also shown are the WAFs (vectors) with the 15-day means removed and vectors <5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composite cross section of 7–30-day meridional wind averaged over 45°–55°N at 200 hPa. The interval is 1 (m s−1). Areas >95% confidence level are shaded in blue (positive) and in yellow (negative). Also shown are the WAFs (vectors) with the 15-day means removed and vectors <5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of 7–30-day streamfunction and anomalous WAF at 200 hPa. The interval is 5 (105 m2 s−1) for the streamfunction. Vectors with length <5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of 7–30-day streamfunction and anomalous WAF at 200 hPa. The interval is 5 (105 m2 s−1) for the streamfunction. Vectors with length <5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of 7–30-day streamfunction and anomalous WAF at 200 hPa. The interval is 5 (105 m2 s−1) for the streamfunction. Vectors with length <5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of the cross section for the 7–30-day diabatic heating (shading), zonal component of WAF (contour), and WAF (vector) averaged over 120°–130°E. The interval is 3 (×10−5 m2 s−2) for the zonal component and is 5 (J kg−1 s−1) for diabatic heating. Areas ≥5 are shaded in red (yellow) and areas ≤−5 are shaded in blue. The 15-day mean has been removed for the zonal component. Vectors with length ≤5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of the cross section for the 7–30-day diabatic heating (shading), zonal component of WAF (contour), and WAF (vector) averaged over 120°–130°E. The interval is 3 (×10−5 m2 s−2) for the zonal component and is 5 (J kg−1 s−1) for diabatic heating. Areas ≥5 are shaded in red (yellow) and areas ≤−5 are shaded in blue. The 15-day mean has been removed for the zonal component. Vectors with length ≤5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of the cross section for the 7–30-day diabatic heating (shading), zonal component of WAF (contour), and WAF (vector) averaged over 120°–130°E. The interval is 3 (×10−5 m2 s−2) for the zonal component and is 5 (J kg−1 s−1) for diabatic heating. Areas ≥5 are shaded in red (yellow) and areas ≤−5 are shaded in blue. The 15-day mean has been removed for the zonal component. Vectors with length ≤5 (×10−5 m2 s−2) are omitted.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of Rossby wave source. The contour interval is 30 (×10−12 s−2) and zero contours are omitted. Areas ≥30 are shaded in blue, and areas ≤−30 are shaded in red.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of Rossby wave source. The contour interval is 30 (×10−12 s−2) and zero contours are omitted. Areas ≥30 are shaded in blue, and areas ≤−30 are shaded in red.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
Composites of Rossby wave source. The contour interval is 30 (×10−12 s−2) and zero contours are omitted. Areas ≥30 are shaded in blue, and areas ≤−30 are shaded in red.
Citation: Journal of Climate 23, 8; 10.1175/2009JCLI3248.1
