1. Introduction
The east Asian cold surge is one of the most conspicuous features of the Asian winter monsoon. The surge brings not only hazardous weather to east Asia (e.g., Chen et al. 2002), but also monsoon rainfall to Southeast Asia (Cheang 1987). Numerous efforts have already been made to explore the possible impact of the El Niño–Southern Oscillation (ENSO) cycle on the Asian winter monsoon [which is a component of the Climate Variability and Predictability program (CLIVAR) Asian–Australian monsoon]. This impact may be reflected by the interannual variation of the east Asian cold surge activity, as suggested by Boyle (1986b), Lau and Chang (1987), and Ding (1990). Actually, extensive efforts along this line have not been made thus far. Defining the cold surge occurrence in terms of the surface pressure at the Siberian high center and temperature changes over three east Asian regions, Zhang et al. (1997) found that the relationship between the east Asian cold surge activity and the Southern Oscillation (SO) is not particularly correlated. On the other hand, the cold surge occurrence frequency determined by Zhang et al. (1997), with the meridional speed in the northern part of the South China Sea is highly coherent with the SO index. By counting only the cold surges reaching Southeast Asia, Chen (2002) showed that the interannual variation of the surge activity is in concert with the ENSO cycle, namely, the cold surge frequency varies interannually closely with the Niño-3.4 sea surface temperature. Regardless of the disparity between findings of Zhang et al. (1997) and Chen (2002), the mechanism connecting the east Asian cold surge activity and the ENSO cycle is unclear.
It was revealed from previous studies (e.g., Joung and Hitchman 1982; Lau and Lau 1984; Chen et al. 2002; Yen and Chen 2002) that an upper-level short-wave train across the eastern seaboard of east Asia and the northwestern Pacific is coupled with a cold surge high–low dipole close to the surface. It is therefore hypothesized that any mechanism facilitating (hindering) the development of the cold surge short-wave train may in turn affect the occurrence frequency of east Asian cold surges. Recently, Chen (2002) showed that the ENSO teleconnection pattern is formed by two wave regimes (separated by a spatial Fourier decomposition along latitudinal circles): long (waves 1–3) and short (waves 4–15). According to the teleconnection wave theory of Hoskins and Karoly (1981), the long-wave regime can penetrate into high latitudes, and relate the ENSO anomalous circulation pattern over North America to the central tropical Pacific sea surface temperature (SST) anomalies. In contrast, the short-wave regime is trapped by the east Asian jet stream and capable of linking the climate systems of east Asia and North America following the ENSO cycle (Chen 2002). The horizontal scale of this North Pacific short-wave train is close to that of the cold surge short-wave train. The development of the latter wave train across east Asia is likely modulated by the former wave train. This modulation process is hypothesized as a possible mechanism to relate the east Asian cold surge to the ENSO cycle.
In order to substantiate the hypothesized interaction between the cold surge short-wave train and the ENSO North Pacific short-wave train, the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data (Kalnay et al. 1996) for the past 21 winters (1979/80–1999/2000) was used to define cold surges and to demonstrate the possible interaction between the two wave trains in terms of a simplified streamfunction budget. The cold surge definition scheme and the streamfunction budget are described in section 2, while the analysis results are presented in section 3. Some concluding remarks are offered in section 4.
2. Definition of cold surge and streamfunction budget
a. Cold surges
The onset time of a cold surge was defined by numerous prior/post Winter Monsoon Experiment (WMONEX) studies in terms of the following changes of surface meteorological conditions within a 24–48-h period (Lau and Chang 1987):
a surface temperature drop in Hong Kong: ΔTS ≥ 5°C;
a surface pressure difference between coastal and central China: Δps ≥ 5 mb; and
υs (prevailing surface northerly in the northern part of the South China Sea) ≥ 5 m s−1.
In order to explore the possible relationship between the Southern Oscillation and the east Asian cold surge activity, Zhang et al. (1997) introduced two different approaches to define cold surge. For the first one, the onset of a cold surge is defined by the following criteria different from those used in the prior/post WMONEX research:
The surface anticyclone center in southern Siberia has a pressure: ps ≥ 1035 mb.
Surface temperature drops within a 24–48-h period: ΔTs ≥ 9°C in central China and ΔTs ≥ 6°C in southern China.
The North Pacific ENSO short-wave train (Fig. 3 of Chen 2002) depicted with the 200-mb streamfunction exhibits a negative (positive) cell centered over South Manchuria and a positive (negative) cell over the southwest of Lake Baikal during warm (cold) ENSO events. How does this short-wave train affect the development of the cold surge short-wave train? The prior/post WMONEX cold surge research was basically developed from the scenario of a single cold surge dipole structure. Actually, a large occurrence rate (∼56%) of east Asian cold surges exhibits a double cold surge dipole structure (Chen et al. 2002), like the case shown in Fig. 1. The cold surge northerlies in the northern port of the South China Sea may be formed by the merger of anticyclonic flows from a newly developed continental and an aging oceanic cold surge dipole. To consider the effect of the North Pacific ENSO short-wave train on the development of the cold surge short waves and to reduce/avoid the possible contamination of the South China Sea northerlies by the aging oceanic cold surge dipole, different definition criteria of cold surges were adopted in terms of the surface weather condition at Pengchiayu (25.6°N, 122.1°E), an island station northeast of Taiwan:
Ts drops at least 4°C within 24–48 h;
ps increases at least 4 mb within 24–48 h;
Northerly wind speed exceeds 6 m s−1;
A cold surge outflow is embedded in a high–low dipole structure in the low-level circulation (e.g., Fig. 1a). A well-defined negative cell of ψs (200 mb; Fig. 1c; ψs is the streamfunction anomalies in the short-wave regime), which belongs to the east coast trough located along the eastern seaboard of northeast Asia.
A cold surge case defined by these criteria is shown in Fig. 1 as an example. The upper-air synoptic condition on 20 January 1980 (Fig. 1b) is dominated by a basin-scale trough aligned with a ridge over central Asia to the west and a ridge along the west coast of North America to the east. The cold surge short-wave train embedded in this large-scale circulation setting is well depicted by the ψs (200 mb) anomalies (Fig. 1a). Established by a high (H2)–low (L2) dipole, a surface cold surge outflow over the eastern seaboard of east Asia is shown in Fig. 1d. This continental cold surge dipole is juxtaposed with an aging oceanic cold surge dipole (H1–L1) in the North Pacific. The double cold surge synoptic system (as presented by Chen et al. 2002) formed by these continental and oceanic surface dipoles is coupled with the upper-level continental ridge (R2)–trough (T2) and oceanic ridge (R1)–trough (T1) structure, respectively. This coupling can be illustrated by the superimposition of east–west circulation (ud, −ω; 35°N) on the longitude–height cross section of ψs (35°N) anomalies (Fig. 1c). Just like the classic cyclones (e.g., Holton 1992; Chen et al. 1996), the upward (downward) branch of the east–west circulation is located ahead of a trough (ridge). A coherent eastward propagation between the cold surge trough T2 and the cold surge front in the downstream of a cold surge outflow established through this coupling was demonstrated by Chen et al. (2002, their Fig. 10). Evidently, the upper-level cold surge short waves and the surface cold surge dipoles presented in Fig. 1 belong to the same cold surge synoptic system.
It was pointed out by Compo et al. (1999) that “absolute agreement on a definition for surges is difficult to find, probably because east Asia is an extensive area and individual researchers construct definitions based on the local response.” However, it is necessary for us to verify whether cold surges defined with our criteria are in line with those identified by other definitions. Since both Lau and Lau (1984) and Boyle (1986a,b) analyzed cold surges of the 1978/79 (WMONEX) winter, it would be convenient for us to verify cold surges defined by our definition with these studies (see Table 1). Using a time series of surface temperature and pressure difference between central China and the east coast against the three criteria (listed above), Lau and Lau (1984, their Fig. 2) defined 11 cold surges during the 1978/79 winter (the central column of Table 1). A careful check of these time series and the three criteria reveals that each surge defined by Lau and Lau (1984) satisfies the third (surface northerly) criterion. Following this approach, four more cases [2 and 23 December 1978, defined by Bolye (1986a,b); 5 and 23 January 1979] should be included. If this approach is adopted with Pengchiayu indices (Fig. 2), two more surges (16 January and 11 February 1979) can be defined (in the left column of Table 1). Using the Hong Kong temperature of December 1978 generated by the European Centre of Medium-Range Weather Forecasts, Boyle (1986a,b) defined six cold surges [listed in the right column of Table 1; one more than those defined by Lau and Lau (1984)]. The occurrence frequency of cold surges identified by the present study may be somewhat more often than in other studies, but not excessive.
b. Streamfunction budget
3. The cold surge activity
a. Interannual variation
Shown in Fig. 3 is a histogram of the cold surges defined in every winter [December–January–February (DJF)] superimposed with departures of the sea surface temperatures (SSTs) in the Niño-3.4 region compiled by the Climate Prediction Center of Washington, D.C. (Smith and Reynolds 1998) from their long-term winter mean values, ΔSST (Niño-3.4). Regardless of their life spans, 364 cases were totally identified: about 17 cases occur on average every winter. Apparently, more events of every season are defined in this study than Zhang et al.'s (1997) observation. This difference may be attributed to the difference of definition criteria used between the two studies. Zhang et al. (1997) defined cold surges with the east Asian continental condition or the surface northerlies in the northern part of the South China Sea. The current study defined all cold surges with not only the east Asian maritime condition, but also the coupling with the ocean-bounded propagating cold surge short-wave trains.
As indicated by ΔSST (Niño-3.4), extreme warm and cold ENSO events are marked by W [Δ SST (Niño-3.4) ≥ 0.5°C] and C [Δ SST (Niño-3.4) ≤ −0.5°C], respectively, in Fig. 3. An interesting feature emerging immediately from the comparison between ΔSST (Niño-3.4) and cold surge occurrence frequency (NC) is the fact that cold surge occurs more frequently during warm than cold ENSO events. The correlation coefficient between Δ SST (Niño-3.4) and NC reaches 0.86. Since the cold surge is the most conspicuous component of the east Asian winter weather system, coherent interannual variations of NC and ΔSST (Niño-3.4) clearly indicate that the ENSO impact on the east Asian winter weather is reflected by the interannual variation of the cold surge activity. Counting cold surges across the Southeast Asia–western tropical Pacific region, Chen (2002) obtained the same relationship between interannual variations in NC and ΔSST (Niño-3.4). Based on their continental definition criteria briefly described in section 2a, Zhang et al. (1997) argued that their NC minimum is 1 yr behind the El Niño event. Using the South China Sea as a northerly index, Zhang et al. (1997) showed that high (low) NC values occur during La Niña (El Niño). It is surprising to see such a disparity of the NC–ΔSST (Niño-3.4) relationship between Zhang et al. (1997) and the current study. Because a concrete link between the interannual variations of NC and the large-scale circulations was not provided by Zhang et al. (1997), can it be possible for us to do it in the present study? Our answer to this question will be presented in the next section.
b. Possible mechanism
It was hypothesized that the interannual variation of the cold surge short-wave train and the cold surge activity is possibly caused by the modulation of the ENSO short-wave train across the North Pacific. In order to substantiate this hypothesis, we shall illustrate this modulation through the dynamical processes maintaining the cold surge short-wave train. This task will be pursued in two steps. First, we illustrate the maintenance mechanism of the cold surge wave train. Then, we show the possible modulation of the cold surge short-wave maintenance by the ENSO short-wave train.
1) Maintenance of the cold surge short-wave train
Shown in Fig. 4 are composites of the day-0 (the date when the criteria set in section 2a are met by a surge) 200-mb streamfunction (
According to Eq. (1), the
Can the evolution and eastward propagation of the cold surge short-wave train be maintained over the short-wave train's life cycle by the counteraction between
2) Effect of the ENSO short-wave train
Based on the linear Rossby wave theory, Hoskins and Karoly (1981) showed that only ultralong waves with zonal wavenumber ≤ 3 can penetrate into the region north of 60°N and form the great arching ENSO teleconnection pattern emanating from the tropical Pacific. On the other hand, Chen (2002) found that a teleconnection pattern formed with zonal wavenumber ≥ 4 links the climate systems on both sides of the North Pacific. Because of the nonlinearity in the atmospheric response to the tropical forcing, the El Niño and La Niña teleconnection patterns may not exactly mirror each other (Hoerling et al. 1997). However, for simplicity in our discussion the possible ENSO impact on the east Asian cold surge activity will be illustrated in terms of differences in the teleconnection patterns in each wave regime between two extreme ENSO phases.
(i) Long-wave regime
Displayed in Fig. 6 is the difference between the composite 200-mb streamfunction anomalies of both warm and cold ENSO winter in the long-wave regime ΔψL(W − C). A statistical significance of ΔψL(W − C) larger than 95% (99%) is lightly (heavily) stippled. The ENSO teleconnection is basically formed by ΔψL(W − C) anomalies east of 150°E, while the east Asian anomalous circulation is reflected by a possible seesaw of the north–south ΔψL(W − C) dipole. Wintertime stationary waves south of 30°N are characterized by a vertical phase reversal in the midtroposphere (Lau 1979). The east–Southeast Asian ΔψL(W − C) cell centered over Taiwan also undergoes this vertical phase change (not shown). In contrast, the northeast Asian ΔψL(W − C) cell (Fig. 6) exhibits a vertically uniform structure. In fact, the lower-tropospheric ψL anomalies underneath the east Asian north–south dipole exhibit only positive (negative) values during warm (cold) ENSO events. It seems likely that the low-level east Asian circulation change may facilitate (hinder) the occurrence of a cold surge during the warm (cold) ENSO winter. Nevertheless, the smaller amplitude and lower statistical significance of the northern ΔψL(W − C) cell in east Asia suggests that this anomaly cell is not as stable as the southern one. This unstable condition in the northern ΔψL(W − C) cell may not warrant its certain effect on the development of the Siberian high, and the ensuing cold surges for every warm/cold ENSO winter. Over the northeast Asia–northwest Pacific region, the contrast between the day-0 Δ
(ii) Short-wave regime
For both the warm and cold ENSO phases, Chen (2002, his Fig. 3) depicted the North Pacific ENSO short-wave train in terms of a composite 200-mb streamfunction departure from the long-term winter-mean values. Being uniform in their vertical structure, the warm (cold) ENSO short-wave train is almost spatially in phase (out of phase) with the day-0 composite cold surge
The impact of the North Pacific ENSO short-wave train on the development of the cold surge short-wave train may not be clearly revealed from a simple comparison between the composite streamfunction anomalies of these two short-wave trains. In order to make this impact more discernable, the Δ
The day-0 Δ
ψSC ψSC Can the North Pacific ENSO short-wave train affect the occurrence/development of the cold surge short-wave train? The spatial coincidence between the day-0 Δ
ψSC The cold surge
ψSC
How does the North Pacific ENSO short-wave train dynamically modulate the cold surge short-wave train? It was shown previously by Figs. 4 and 5 that the cold surge
The mathematical linearity of vorticity advection and vortex stretching in Eq. (1) makes it dynamically possible for the North Pacific short-wave train to modulate the cold surge short-wave train through a simple way. This modulation may be revealed by means of the spatial structure resemblance between Δ
It may be argued that the modulation of the cold surge
Criterion d of our cold surge definition clearly states in section 2a that every identified cold surge dipole is coupled with the upper-level cold surge short-wave train through the east–west circulation, as is the case shown in Fig. 1. If the cold surge short-wave activity across the eastern seaboard can be modulated by the North Pacific ENSO short-wave train, it is likely that across the eastern seaboard both synoptic waves (NSW) and those coupled with cold surge dipoles (NCW) should undergo an interannual variation coherent with the ΔSST (Niño-3.4) and the occurrence frequency of cold surges (NC). Based on the daily streamline charts at 200 and 925 mb, the Japan Meteorological Agency (JMA) daily upper-air and surface analysis maps and the longitude–time diagrams of ψ (200mb) of every winter, total numbers of synoptic waves and cold surge short waves identified are displayed in Fig. 10. Compared to NC and ΔSST (Niño-3.4) shown in Fig. 3, one easily finds that NCW = NC and both NCW and NSW (regardless of their difference) vary interannually in concert with NC and ΔSST (Niño-3.4) as expected. Evidently, the synoptic wave activity is modulated by the North Pacific ENSO short-wave train. This modulation of the upper-air synoptic activity is extremely consistent with some recent studies concerning ENSO effects on synoptic variability (e.g., Straus and Shukla 1997; Matthews and Kiladis 1999). However, not every synoptic wave is coupled with a cold surge, but the occurrence frequency of those coupled with cold surges is modulated by the North Pacific ENSO short-wave train, like all synoptic waves. As stressed in Fig. 1, the upper-level cold surge wave and the surface cold surge dipole are not only coupled together, but also parts of the same synoptic system. Therefore, the modulation of the North Pacific ENSO short-wave train on the cold surge short-wave activity is also reflected by the cold surge activity.
4. Concluding remarks
How is the winter weather/climate system of east Asia affected by the ENSO? This is a long-standing challenge to the east Asian meteorological community. Recently, Chen (2002) demonstrated the existence of an ENSO short-wave train teleconecting the climate systems of east Asia and North America. The horizontal scale of this wave train is comparable to that of the cold surge short-wave train. Because the cold surge is one of the most important winter weather phenomena in east Asia, an effort is made in this study to explore the possible impact of the ENSO on the east Asian weather/climate system through the effect of the North Pacific ENSO short-wave train on the cold surge activity. The major finding of this effort is summarized as follows:
The east Asian cold surge occurrence undergoes a distinct interannual variation in concert with the SST (Niño-3.4) anomalies; cold surge occurs more (less) frequently during warm (cold) ENSO winters.
The streamfunction budget (Laplace inverse transform of the vorticity budget equation) of the North Pacific ENSO short-wave train over east Asia and the northwest Pacific is coincident (opposite) to that of the cold surge short-wave train over the same region during warm (cold) ENSO winters. It is strongly suggested that the North Pacific ENSO short-wave train modulates the cold surge short-wave train in such a way that the cold surge activity is intensified (weakened) during warm (cold) ENSO winters.
The possible impact of ENSO on the east Asian winter weather/climate system through the modulation of the cold surge activity by the North Pacific ENSO short-wave train was drawn from the diagnosis of the streamfunction budget. Some further confirmation of this finding is needed. As shown by Chen (2002), the North Pacific ENSO short-wave train is possibly simulated by the NCAR Community Climate Model (CCM), as was done by Slingo (1998) with the general circulation model of the U.K. University Global Atmospheric Modeling programme. The impact of the North Pacific ENSO short-wave train on the east Asian cold surge activity may be analyzed by using NCAR CCM simulations with/without SST (Niño-3.4) interannual variations. This effort is currently under study and results will be reported in a future study.
Acknowledgments
This study is supported by NSF Grant ATM-9906454 and NASA NSIPP Grant NAG-58293. Comments and suggestions made by two reviewers are very helpful in improving the presentation of this paper. The editorial assistance provided by Brad Temeyer and Michael Falk is highly appreciated.
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Example of an east Asian cold surge (20 Jan 1980): (a) the 200-mb streamfunction in the short-wave regime (waves 4–15) ψS (200 mb); (b) the 200-mb streamline chart superimposed with the east Asian jet (stippled region); (c) the longitude–height cross section of ψS (35°N) with the east–west circulation (uD, −ω); and (d) the 925-mb streamline chart with high speed flow stippled. The contour interval of ψS (200 mb) is 2 × 106 m2 s−1, while scales of uD and −ω are displayed on the right side of (c)
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
Cold surge indices at Pengchiayu (25.6°N, 122.1°E, a small island northeast of Taipei, Taiwan, as indicated on the right) in the 1978/79 (WMONEX) winter: (top) PS (surface pressure; solid line) and Ts (surface temperature; dashed line); and (bottom) |VS| (solid line; isotach) and wind direction (histogram). The defined cold surges are marked by Sg
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
Histogram of east Asian cold surge occurrence frequency (NC) superimposed with the Niño-3.4 SST departures from its long-term mean value. The warm and cold ENSO winters marked by W and C, respectively, on the ΔSST (Niño-3.4) time series are determined by a threshold of ±0.5°C
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
Composite [( )C] of the 200-mb streamfunction budget in the short-wave regime [( )S]: (a) streamfunction
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
The longitude–time diagrams of all quantities in the composite cold surge
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
Difference of composite 200-mb streamfunction anomalies in the long-wave regime (waves 1–3) ΔψL(W − C) between warm (W: 1982/83, 1986/87, 1991/92, 1994/95, 1997/98) and cold (C: 1984/85, 1988/89, 1995/96, 1998/99, 1999/2000) ENSO winters. Positive (negative) values ΔψL(200 mb) are denoted by solid (dashed) lines, while their contour interval is 106 m2 s−1. As indicated by ΔSST (Niño-3.4) five warm ENSO events, five cold ENSO events, and 11 normal winters occurred during the period of 1979–2000. The statistical significance of ΔψL(W − C) is estimated by the same approach as the cold surge
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
Difference of the day-0 composite 200-mb cold surge streamfunction budget in the short-wave regime (waves 4–15) Δ
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
Difference of the 200-mb short-wave streamfunction ΔψS (200 mb) budget between warm and cold ENSO winters: (a) ψS(W − C), (b)
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
The longitude–time diagrams of the composite 200-mb Δ
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
Passage frequency of the upper-level synoptic waves (NSW; solid line with dots) and cold surge short waves (NCW; solid line with open circles) across 120°E every winter (DJF)
Citation: Journal of Climate 17, 2; 10.1175/1520-0442(2004)017<0401:IVOTEA>2.0.CO;2
East Asian cold surges defined in the 1978/79 (WMONEX) winter by Boyle (1986a,b), Lau and Lau (1984), and the current study
