Summer Upper-Level Vortex over the North Pacific

Transcription

1 Summer Upper-Level Vortex over the North Pacific Tsing-Chang Chen,* Ming-Cheng Yen, + Gin-Rong Liu, # and Shu-Yu Wang* ABSTRACT The midocean trough in the North Pacific may form a favorable environment for the genesis of some synoptic disturbances. In contrast, the North Pacific anticyclone may hinder the downward penetration of these disturbances into the lower troposphere and prevent the moisture supply to these disturbances from the lower troposphere. Because no thick clouds, rainfall, and destructive surface winds are associated with these disturbances to attract attention, they have not been analyzed or documented. Actually, the upper-level wind speed within these disturbances is sometimes as strong as tropical cyclones and has the possibility of causing air traffic hazards in the western subtropic Pacific. With infrared images of the Japanese Geostationary Meteorological Satellite and the NCEP NCAR reanalysis data, 25 North Pacific disturbances were identified over six summers ( ). Two aspects of these disturbances were explored: spatial structure and basic dynamics. For their structure, the disturbances possess a well-organized vortex in the middle to upper troposphere with a descending dry/cold core encircled by the moist ascending air around the vortex periphery; the secondary circulation of the vortex is opposite to other types of synoptic disturbances. Since vorticity reaches maximum values along the midocean trough line, barotrophic instability is suggested as a likely genesis mechanism of the vortex. After the vortex is formed, the horizontal advection of total vorticity results in its westward propagation, while the secondary circulation hinders this movement. Along its westward moving course, close to East Asia, there is a reduction in vortex size and a tangential speed increase inversely proportional to the vortex size. Diminishing its horizontal convergence/descending motion by the upper-tropospheric East Asian high and the lower-tropospheric monsoon low, the vortex eventually dissipates along the East Asian coast. 1. Introduction During northern warm seasons, the overhead sun moves northward across the equator to the Northern Hemisphere. Because of the decrease in the meridional thermal gradients, particularly in midlatitudes, the Northern Hemisphere summer circulation becomes less rigorous in its intensity. In spite of this *Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa. + Department of Atmospheric Science, National Central University, Chung-Li, Taiwan. # Center for Space and Remote Sensing Research, National Central University, Chung-Li, Taiwan. Corresponding author address: Tsing-Chang (Mike) Chen, Atmospheric Science Program, Department of Geological and Atmospheric Sciences, 3010 Agronomy Hall, Iowa State University, Ames, IA tmchen@iastate.edu In final form 16 March American Meteorological Society seasonal change, certain circulation elements still distinctly stand out. As shown in Fig. 1a, the upper troposphere exhibits the major Tibetan high and the smaller Mexican high, both separated by the strongly tilted oceanic troughs over the North Pacific and the North Atlantic (Krishnamurti 1971). Northwest of these two subtropical midocean troughs are the weak midlatitude troughs along the east coasts of East Asia and North America. Corresponding to these upper midlatitude troughs are the surface lows over the Bering Sea and the Labrador Sea. In contrast, beneath the two subtropical oceanic troughs are oceanic anticyclones divided by the Indian monsoon trough and the North American heat low (White 1982). In the deep Tropics adjacent to these subtropical circulation elements are equatorial troughs, for example, the intertropical convergence zone. As summarized by Krishnamurti (1979) and Nitta (1982), tropical synoptic disturbances embedded in the Asian summer monsoon system consist of equatorial waves in the western tropical Pacific (e.g., Yanai et al. Bulletin of the American Meteorological Society 1991

2 As depicted by Krishnamurti (1971), the Asian monsoon region and the North Pacific are overlaid by an east west circulation cell with its ascending branch over the East Asian continent and the western subtropic Pacific, and its descending branch over the central Pacific. The contrast between the vertical structure of Z E (25 N) anomalies (Fig. 1b) and the environmental vertical motions over the central North Pacific is basically opposite to that over the subtropical Asian monsoon region. Regardless of the vertical development of monsoon depressions limited by the Tibetan high (e.g., Chen and Yoon 2000), the Indian monsoon trough [indicated by negative Z E (25 N) anomalies in Fig. 1b] with its environmental ascending motion, forms a favorable oc- FIG. 1. Summer (JJA) geopotential height fields: (a) Z (200 mb), (b) longitude height cross section of eddy geopotential height at 25 N, Z E (25 N), and (c) Z currence region of these disturbances. (850 mb). The midlatitude storm track (indicated by the rms values of the 2 7-day Likewise, the subtropical midocean filtered v field at the concerned level), and high occurrence frequencies of monsoon depressions over the south Asian monsoon region (Chen and Weng 1999) and equatorial waves over the western tropical Pacific (Chen and Weng 1998) are indicated scending motion may also be a favor- trough with the environmental de- by red areas in (a) and (b). Within these red areas, values of rms (v) are larger than able region for the genesis of synoptic 40 m s 1 at 200 mb and 6 m s 1 at 850 mb, and frequencies of monsoon depression disturbances, in spite of the potential and equatorial waves are one occurrence per 1 1 and one occurrence per 4 5, hindrance of disturbance genesis by respectively. The midocean trough lines are indicated by two thick green lines in (a). the lower-tropospheric North Pacific Values of Z (200 mb) m in (a) and Z (850 mb) 1500 m in (c) are stippled with blue color, while values of Z (850 mb) larger than 1540 m are stippled with darkblue color. Positive values of Z E anticyclone [indicated by positive Z E (25 N) are colored light blue. The topography at (25 N) anomalies in Fig. 1b]. The majority of the water vapor exists in the 25 N in (b) is depicted by the long-term-mean surface pressure at this latitude with dark color. Contour intervals of Z (200 mb), Z E (25 N), and Z (850 mb) are 50, 20, lower troposphere. Without reaching and 20 m, respectively. the lower troposphere, any middle- to upper-tropospheric disturbance may 1968) and monsoon depressions in the Indian monsoon trough (e.g., Krishnamurti et al. 1977; Saha et al. tain cumulus convection. Consequently, these distur- not be able to pump up significant moisture to main- 1981). In midlatitudes, the summer storm tracks are bances bear no noticeable clouds and are not strongly portrayed by White (1982) in terms of departure variances of several meteorological variables. In order to Weather disturbances with destructive wind speed identifiable in satellite imagery. depict the activity of synoptic disturbances, occurrence in the lower troposphere and disastrous rainfall at the surface always attract attention not only from the meteoro- frequencies of equatorial waves compiled by Chen and Weng (1998), monsoon depressions identified by logical community, but also from the public. Perhaps, Chen and Weng (1999), and the midlatitude synoptic due to the lack of these two factors, no such synoptic disturbances indicated by meridional wind variances disturbances over the North Pacific anticyclone have are shown by red areas in Figs. 1a and 1c. It is revealed yet to be documented (at least to our knowledge). Recently, with available imagery from the Japanese Geo- in these figures that synoptic disturbances climatologically generate and develop over the surface low pressure regions whose cyclonic flow and ascending Centers for Environmental Prediction National Censtationary Meteorological Satellite (GMS), National motion of the large-scale environment conceivably ter for Atmospheric Research (NCEP NCAR) reanalysis data (Kalnay et al. 1996), and an island facilitate the genesis of synoptic disturbances. station 1992 Vol. 82, No. 9, September 2001

3 of high elevation ( 4 km) in the western subtropical Pacific, these disturbances were for the first time identified. As will be shown later, the maximum tangential speed of these disturbances may exceed 30 m s 1 when they reach the western subtropical Pacific. This region has one of the busiest flight routes for many major international airlines. These barely visible synoptic disturbances observed by the GMS may constitute an air-traffic hazard. The impact of these disturbances on the air-traffic safety over the western Pacific may be beyond our comprehension. For this very reason, it is highly desirable to understand the spatial structure and basic dynamics of these disturbances. Thus, it is our intent in this paper not only to report the preliminary results of our analysis, but also to call the attention of the meteorological community to these disturbances. This paper is structured in the following fashion. The propagation property and synoptic structure of a Pacific vortex will be illustrated in section 2, wherein the vortex of interest passed through a tall mountain station with an elevation close to 4 km. Surface observations of this case at the elevated station are presented in section 3. Basic dynamics of the disturbance inferred from a simple diagnosis are shown in section 4. Some climatology of the Pacific disturbances derived from the 25 selected cases over the period is summarized in section 5, while some concluding remarks are offered in section The North Pacific summer vortex: A case study Different approaches are used to identify weather disturbances in different climatic regions: midlatitude cyclones (e.g., Zishka and Smith 1980), equatorial waves (e.g., Chen and Weng 1998), and monsoon depressions (e.g., Saha et al. 1981; Chen and Weng 1999). As will be shown later in this section, synoptic disturbances (floating above the North Pacific anticyclone) analyzed in this study differ characteristically from other weather disturbances. We thus identify the summer upper-level vortex over the North Pacific with the following criteria: 1) A vortex with high clouds around its periphery is discernable in the GMS infrared (IR) image during the part of its life cycle over the central-western Pacific. 2) The vortex identified by the GMS IR image is verified by a closed cyclonic center shown in the 200-mb streamline charts constructed with the NCEP NCAR reanalysis winds. 3) The life span of an identified vortex should be 3 days or longer. 4) Any vortex originating east of 170 E (the eastern boundary of the GMS IR observations) with a life span shorter than 3 days (even identified by streamline) and invisible in the GMS IR image is ignored. Based on these criteria, 25 upper-level vortices over the North Pacific are identified during the summers of the period. Climatology of these vortices will be presented in section 5, but an interesting case is used here to illustrate some of their properties. The case selected to serve this purpose is 19 June 4 July 1998, because of its propagation across Taiwan. Thus, observations at a mountain station with an elevation close to 4 km on this island can be used to confirm some properties of the Pacific upper-level vortex depicted by the NCEP NCAR reanalysis data. The Pacific upper-level vortex of 19 June 4 July 1998 was not clearly discernable in the GMS IR imagery until it approached the western North Pacific in late June. For illustration, a sequence of three GMS IR images at 0000 UTC on 29 June 1 July 1998 are shown in Fig. 2. The vortex is well defined by cirrus clouds (white color) surrounding its southern half with some fragmented thin clouds located around its center. The size of the vortex became smaller as it moved westward toward Taiwan. The trajectory of this vortex (indicated by a dashed line connecting centers identified by closed 200-mb streamline) is shown over most of its entire life cycle in Figs. 3b and 3c. Locations of these centers east of 170 E are not shown; even though they were identified in the 200-mb streamline chart but not in the GMS IR image. A clear westward propagation of this vortex is revealed from its center locations against time. The vortex centered in the Philippine Sea (30 Jun 1998; indicated by a stippled circle in Fig. 3b) matched relatively well with its GMS IR image in Fig. 3a. At this stage, the vortex size is about 1500 km. The comparison of streamline charts at 200 (Fig. 3b) and 925 mb (Fig. 3c) is interesting in that such a highly visible upper-tropospheric vortex does not show any signature in the lowest layer of troposphere. This contrast seemingly implies that the downward penetration of this vortex is hindered by the lower-tropospheric subtropical anticyclone in the North Pacific. The propagation property and horizontal scale of the Pacific vortex are similar to equatorial waves and monsoon Bulletin of the American Meteorological Society 1993

4 depressions, except for the fact that cyclonic vortices of these two types of synoptic disturbances exist in the lower half of the troposphere. After 30 June, the vortex movement turned northwestward (Fig. 3b). It does not seem to be advected along this direction by the 200-mb flow in the western North Pacific, which is dominated by a juxtaposition of three circulation cells. In contrast, southeasterlies of the North Pacific anticyclone in the Philippine Sea (Fig. 3c) are likely to advect the vortex when it penetrates sufficiently down to reach the lower troposphere. The potential interaction between the vortex and the lower-tropospheric flow may be inferred with the vertical structure of the vortex. Shown in Fig. 4 are (longitude-vertical at 17.5 N and latitude-vertical at E) cross sections of eddy geopotential height (Z E ), vertical motion (ω), and specific humidity (q), which are characterized by the following features: 1) The vortex center at 200 mb is marked by an open circle in the Z E cross sections. The maximum amplitude appears in the upper troposphere, but the vortex extends down to the middle troposphere as indicated by negative values of Z E anomalies centered at 17.5 N, E. 2) The vortex exhibits a well-organized descending motion core (i.e., positive values of ω) encircled by ascending motion (i.e., negative values of ω) particularly along the west-south periphery of the FIG. 2. GMS IR images of the Pacific vortex at 0000 UTC on 29 Jun (top), 30 Jun (middle), and 1 Jul 1998 (bottom). Clouds are denoted by white color. vortex. The cloud distribution shown in Fig. 3a (a clear region encircled by circular high clouds) coincides with the vertical motion of the vortex. 3) Two interesting features are revealed from the q cross sections. First, the ascending motion around the vortex pumps moisture up to maintain cirrus clouds around the vortex periphery where values of moisture are larger. In contrast, the descending motion within the vortex pushes moisture down so that a clear region forms there. Second, moisture becomes more abundant close to Southeast Asia and the Tropics. The larger moisture gradient (Figs. 4c and 4f) and the stronger vertical motion (Figs. 4b and 4e), facilitate cloud formation along the south periphery of the vortex, especially in the region close to East Asia. Some comments about the quality of the NCEP NCAR reanalysis data and the vortex structure may be drawn from the aforementioned salient features of the vortex vertical structure: 1) The model bias of the data assimilation system has been a serious concern of operational centers (Schubert et al. 1993; Gibson et al. 1994; Kalnay et al. 1996). Locations of the vortex s center identified by the GMS IR image and the NCEP streamline charts show slight differences. Regardless of these small discrepancies in the location of the vortex s center and the small horizontal scale of the vortex over a vast ocean, the consistency between GMS IR imagery (Fig. 3a) and vertical distributions of meteorological variables shown in Fig. 4 suggests that the vortex is reasonably depicted by the NCEP NCAR reanalysis data. 2) Ascending motions associated with midlatitude cyclones (e.g., Bjerknes and Holmboe 1944; Chen et al. 1996), equatorial waves (e.g., Reed and Reeker 1971), and monsoon depressions (e.g., Saha and Saha 1988) always exist within these disturbances to form a direct secondary circulation. The latent heat released by cumulus convection induced by the vertical motions 1994 Vol. 82, No. 9, September 2001

5 within the disturbance is fedback to maintain this disturbance. As indicated by the ω cross sections (Fig. 4b), the secondary circulation of the Pacific summer upper-level vortex is opposite to the aforementioned disturbances. Since the cyclonic flow of the vortex is not maintained by latent heating, what may be the cause to maintain the longevity of the vortex? A heuristic argument is suggested here. It was pointed out previously that the midocean trough in the North Pacific is coupled with the descending branch of the planetary-scale east west circulation. Based upon the conservation of either vortex strength or angular momentum, the coldcore descending motion within the vortex may draw its support from the environmental flow. clouds along the western periphery of the vortex on 3 July (not shown) merged with the cloud band of an east China front. In order to avoid the complication in our analysis caused by the blending of these two cloud systems, we focused our analysis on the synoptic environment of the vortex on 2 July, 1 day earlier, instead. As indicated by the GMS IR image (Fig. 5a) and the 200-mb streamline chart (Fig. 5b) at 0000 UTC of 3. Station observations In section 2, a coherent structure of the Pacific summer upper-level vortex in several variables (Figs. 3 and 4) was obtained from the NCEP NCAR reanalysis data. Because of the small horizontal scale ( km) of the vortex and the lack of sufficient radiosonde observations over the vast North Pacific Ocean, the depiction of the vortex structure with the reanalysis data may not be completely model-bias free. Nevertheless, some verification may be done with the following observations: 1) Using surface and upper-air radiosonde observations in the synoptic analysis, the Japanese Meteorological Agency (JMA) operationally issues weather charts at various mandatory levels. 2) It is shown in Fig. 3 that the vortex (19 Jun 4 Jul 1998) presented in section 2 propagated through Taiwan on 3 July. A mountain station [Yu-Shan; World Meteorological Organization (WMO) station 46755] on this island has an elevation close to 4 km. According to Chen and Yen (1999) and Chen et al. (2000a), the annual-mean surface pressure at this station is about 640 mb [which is about the same as that at Lhasa, Tibet (Saha et al. 1994)]. Because of the station s altitude, direct measurements of some properties may be made of this vortex. Before verifying some properties of the vortex with the two aforementioned data sources, its structure depicted by the reanalysis data on 2 July is shown in Fig. 5 (for reference). The GMS IR image reveals that FIG. 3. The synoptic condition of the Pacific vortex at 0000 UTC on 30 Jun: (a) the GMS IR image of the Pacific vortex, and streamline charts at (b) 200 and (c) 850 mb. Clouds in (a) are represented by white color. Daily locations of the vortex connected by dashed lines are superimposed on the two streamline charts over most of this vortex life cycle. The vortex at 0000 UTC on 30 Jun is marked by a stippled circle. Bulletin of the American Meteorological Society 1995

6 FIG. 4. Vertical structure of the vortex on 30 Jun illustrated by longitude height cross sections of (a) eddy geopotential height Z E (17.5 N), (b) vertical motion ω (17.5 N), and (c) specific humidity q (17.5 N); and latitude height cross sections of (d) Z E (127.5 E), (e) ω (127.5 E), and (f) q (127.5 E). Positive values of Z E and ω, and values of q larger than 4 g kg 1, are stippled. The vortex center at 200 mb is marked by an open circle in (a), (b), (d), and (e); while the location of the vortex center is denoted by a thick arrow in (c) and (f). Contour intervals for Z E, ω, and q are 7.5 m, mb s 1, and 2 g kg 1, respectively. 2 July, the vortex was approaching Taiwan. A slight difference in the center location of the vortex between these two data sources may be attributed to a bias of the NCEP reanalysis data. Compared to its size on 30 June (Fig. 3b), the vortex became smaller ( 1000 km) during its intrusion on the east Asian high. The vortex also vertically penetrated down to 600 mb [inferred from negative anomalies of eddy geopotential height at 22.5 N, Z E (22.5 N), in Fig. 5c]. The evolution of the east southeast Asian monsoon is reflected by the coupling between the Mei-yu front and the South China Sea (SCS) monsoon life cycle (Chen et al. 2000b). Separating the surface weather map of 2 July (Fig. 6c) with the 1008-mb isobar into two portions, the surface synoptic condition is characterized by the East-Asian continental low in the west and the North Pacific anticyclone in the east. The Mei-yu front (indicated by two surface low centers off the east coast of East Asia and the Japan Sea), which was already located north of the Yangtze River, is embedded in the former. On the other hand, Taiwan and the Philippines were covered by the North Pacific anticyclone extending southwestward. According to Chen et al. (2000b), the synoptic condition on 2 July was progressing toward the SCS monsoon break condition. The northwestward-propagating vortex was actually floating aloft over the North Pacific anticyclone. Thus, there is no low pressure perturbation discernable between Taiwan and the Philippines in the surface pressure map (Fig. 6c). Accompanying the continental surface low system (Fig. 6c) was an upperair short-wave trough situated along the east coast of northeast Asia (Fig. 6b). The vortex, which should be located south of this trough, is not discernable in the 500-mb geopotential height. Instead, a thermal vortex straddling the Luzon Strait (between Taiwan and the Philippines) appeared in the middle of the high pressure system over the east Asian western Pacific region. In contrast, a well-defined cellular low emerged from the 300-mb geopotential height (Fig. 6a). As revealed from the 300-mb thermal field, the vortex exhibits a cold core. Strong tropical vortices, like tropical cyclones (e.g., Anthes 1982) and monsoon depressions (e.g., Sikka 1979), usually have a strong ascending warm core. The cold-core structure is another characteristic of the vortex opposite to other strong tropical vortices. Dynamically, the descending cold core of the vortex is a result of the upper-tropospheric convergence. Although this cold-core feature was not presented with the reanalysis data, the vortex identified in the JMA synoptic chart is consistent with the reanalysis. The comparison of surface observations at Yushan (the tallest mountain station, indicated by an open triangle in Fig. 7d) and Chiayi [a low plains station (WMO station 46758), denoted by an open square in Fig. 7d] in Taiwan may shed some light on several properties of the vortex. Time series of surface pressures (p s ), surface relative humidities (RH), and sur Vol. 82, No. 9, September 2001

7 face temperatures (T s ) for the period extending from 24 June to 9 July are shown in Fig. 7. For comparison, the daily mean values (dashed line) are superimposed on every time series. Regardless of the pronounced diurnal/semidiurnal variations in all three variables, no unusual changes in these subdaily variations appeared at Chiayi during the vortex passage through Taiwan on 2 3 July. In contrast, noticeable decreases in p s, RH, and T s occurred at Yushan (indicated by thick arrows) during the vortex passage. Recall that the well-organized descending motion within the vortex (Fig. 4b) is coupled with the cold core (Figs. 6a and b) and the reduction of moisture in the lower troposphere (Fig. 4c). These vortex properties are reflected by surface observations at the Yushan station. Evidently, the vortex penetrated down to the lower half of the troposphere during its passage across Taiwan so that its perturbation of some variables can be detected at this tall mountain station. 4. Dynamics The Pacific summer upper-level vortex is documented for the first time. Thus, based upon some simple diagnoses with the 19 June 4 July case, some basic dynamics of the vortex are suggested with a hope that some future studies will follow up to explore them in further detail. For illustration, we focused on three stages of the vortex s evolution: genesis, mature, and demise. Since only the detailed analysis of a case is presented, the three stages of the vortex s life cycle is relatively easy to determine. The first appearance of the vortex occurred on 19 June, while its demise happened on 4 July. Obviously, the dates of 19 June and 3 July (the vortex merged with an east Asian front on 4 Jul) belong to the initial and decaying phases, respectively. The vortex may be considered to become mature when the vortex s maximum tangential speed is larger than 16.5 m s 1 (mean maximum tangential speed), diameter is smaller than 1600 km (mean diameter), and high clouds along the vortex periphery are clearly discernable. Since the vortex is a strong one like a tropical cyclone, the suggested dynamics for each stage are illustrated only by single-day charts. a. Genesis What may be the genesis mechanism for the vortex? Some highlight of the genesis mechanisms of synoptic disturbances shown in Fig. 1 may be of some help in searching for vortex s genesis mechanism. FIG. 5. Structure of the vortex at 0000 UTC on 2 Jul: (a) GMS IR image of the vortex, (b) 200-mb streamline chart, and (c) longitude height cross section of eddy geopotential height at 22.5 N, Z E (22.5 N). Clouds in (a) are represented by white color. The vortex is marked by a stippled circle in (b), while the vortex center at 200 mb is denoted by an open circle in (c). The contour interval of Z E (22.5 N) is 7.5 m. Midlatitude cyclones, which exhibit a vertically westward tilt during their development, are generated by baroclinic instability (Charney 1947; Eady 1950). Equatorial waves may be induced by the barotropic instability of an easterly flow (Nitta and Yanai 1969). A vertical phase reversal appears in these wave disturbances with a cyclonic motion below 500 mb and Bulletin of the American Meteorological Society 1997

8 an anticyclonic motion above 300 mb (Reed and Recker 1971). The conditional instability of the second kind (Charney 1973) barotropic baroclinic instability was suggested as a possible genesis mechanism of monsoon depressions (Shukla 1978). However, it is a well-known fact that they are the result of the redevelopment of the westward-propagating residual lows across Indochina (Krishnamurti et al. 1977; Saha et al. 1981; Chen and Weng 1999). These depressions are capped by the Tibetan high below 300 mb (Chen and Yoon 2000). Differences of the basic structure and characteristics between the Pacific summer upper-level vortex and other synoptic disturbances are the following: 1) The vortex, which exhibits a cold core, does not seem to be maintained by the latent heat released by cumulus convection within the vortex. 2) The secondary circulation of the vortex (with the strong descending motion in the center and ascending motion around the vortex periphery) is opposite to other synoptic disturbances. 3) The existence of the vortex above the North Pacific anticyclone prevents its possible interaction with the lower troposphere and ocean. 4) The vortex exhibits neither a phase reversal nor a westward tilt in its vertical structure. FIG. 6. Operational weather charts at 0000 UTC on 2 Jul issued by the Japanese Meteorological Agency: (a) 300 mb, (b) 500 mb, and (c) surface. In addition to these basic differences of structure and characteristics, the vortex identified in its first day (19 Jun) was embedded in large-scale environmental flow that possesses a vertical phase reversal [because the lower-tropospheric North Pacific anticyclone (Fig. 6a) is overlaid by the upper-tropospheric oceanic trough (Fig. 8a)]. As revealed from the height or streamfunction cross section (not shown), the vortex in its initial phase (indicated by a faint cloud image) only existed in the upper troposphere. These observations indicate that the genesis mechanisms of other synoptic disturbances may not be applicable to the Pacific summer upper-level vortex. Note that the vortex (denoted by a stippled circle in Fig. 8a) is located at the oceanic trough line where total vorticity of the 1998 Vol. 82, No. 9, September 2001

9 large-scale environmental flow may attain its maximum value. According to barotropic instability theory (Kuo 1949), this large-scale environmental flow may satisfy the necessary condition of barotropical instability, that is, ( ζ F + f )= 0, (1) y where ζ and f are relative vorticity and Coriolis parameter, respectively. The symbol ( ) F represents a combination of zonal and long-wave (waves 1 7) regimes of ( ). Based upon the GMS IR images and 200-mb streamline charts, we found that the most proper way to isolate the vortex from the environmental flow is to introduce a scale separation of the flow field into a long- (waves 1 7) and short- (waves 8 25) wave regime. It is shown clearly in Fig. 8b that at its onset stage, the vortex straddles the isoline of ( ζ F + f )= 0. y This dynamic feature of the large-scale environmental flow strongly suggests that barotropic instability is a possible genesis mechanism of the Pacific vortex. After the vortex embryo is developed, this cyclonic disturbance converges upper-level cold air and results in the descending motion in its center to form a secondary circulation opposite to other types of synoptic disturbances. terms obtained in our analysis, the streamfunction budget equation may be written as ψ 2 ζ 2 2 u + vβ + f z ( ) ( V ) t x.(2) ψ t ψ A1 ψ A2 ψ x1 The notation used in Eq. (2) is conventional; u z, ν, and β are zonally averaged zonal wind, meridional wind, and meridional gradient of planetary vorticity ( f ). As expressed in this equation, streamfunction tendency (ψ t ) is induced primarily by three dynamic processes: zonal advection of relative vorticity (ψ A1 ), meridional advection of planetary vorticity (ψ A2 ), and planetary vortex stretching (ψ x1 ). A simple harmonic analysis will be applied to Eq. (2) to divide it into the long- [waves 1 7, ( ) L ] and short- [waves 8 25, ( ) S ] wave components. For convenience, we define ψ A12 as ψ A12 ψ A1 + ψ A2 2 [ V (ζ + f )]. (3) Because of the data quality of divergence, we focused our budget analyses at 150 mb. Since the long wave b. Mature On 30 June, the vortex became well developed over the Philippine Sea (Figs. 2 4). The upper-tropospheric flow on this date will be analyzed to illustrate the basic dynamics of the vortex during its mature phase. The vorticity budget equation (e.g., Holton 1992) is often used diagnostically to serve this purpose. However, it may be cumbersome, if not impossible, to apply a scale separation to vorticity; it will be easier to apply it to streamfunction (ψ) (which is the inverse Laplace transform of vorticity). Therefore, the streamfunction budget (e.g., Sanders 1984; Kang and Held 1986), instead of the vorticity budget, is adopted in this study. Based on magnitudes of individual FIG. 7. Time series of (a) surface pressure (p s ), (b) surface relative humidity (RH), and (c) surface temperature (T s ) observed at two stations in (d) Taiwan for the period 24 Jun 9 Jul: Yushan (the tallest mountain station) and Chiayi (a low plains station). Locations and altitudes of these two stations are shown in (d). Dark arrows in (a) (c) mark the date 2 Jul 1998 when the vortex was centered at southern Taiwan. The dashed lines represent daily mean values. Bulletin of the American Meteorological Society 1999

10 FIG. 8. The large-scale environmental flow at 0000 UTC on 19 Jun: (a) the filtered 200-mb streamline chart including only the atmospheric flow of zonal-mean and long-wave (waves 1 7) regimes, and (b) the meridional gradient of total vorticity in the filtered 200-mb flow, ζ F + f = y ( ) 0 at 0000 UTC on 19 Jun. The location of the Pacific upper-level vortex is marked by a stippled circle. The contour interval of ζ F + f = y ( ) 0 is s 1 m 1 ; positive values of this quantity are stippled. is not our major concern of the streamfunction function budget analysis, it may suffice to highlight some features of the 150-mb ψ L budget pertaining to this study. As far as the magnitude is concerned, ψ L and A2 ψ L are dominant terms in the ψ L budget. Therefore x1 this budget may be approximated by ψ L t ψ L A2 + ψ L x1. (4) Both ψ L and ψ L are displayed in Fig. 9 superimposed A2 x1 with the vortex (marked by a stippled circle). Negative (positive) values of ψ L (ψ L ) indicate that the meridional advection of planetary vorticity (planetary vor- A2 x1 tex stretching) facilitates (hinders) the development of the vortex. The planetary vortex stretching (ψ χ ) is largely determined by horizontal divergence ( V ),which 1 may be expressed as the Laplacian of velocity potential ( 2 χ). To better understand the basic dynamics of the vortex in the short-wave regime, we introduce the velocity potential in the short-wave regime (χ S ). For the short-wave regime, the ψ S (150 mb) budget of 30 June (Fig. 10) is characterized by the following salient features: 1) The ψ S (Fig. 10a) and χ S (Fig. 10b) of the vortex (marked by a stippled circle) are almost spatially in quadrature. 2) The opposite spacial structure between ψ S S (Fig. 10c) and ψ (Fig. 10d) indicates that these two dynamic processes A12 χl counteract each other. As inferred from the quadrature spatial relationship of ψ S and ψ S and ψ A12 χl S, the vortex should be moved westward by the horizontal advection of total vorticity and eastward by vortex stretching. 1 However, the westward propagation of the vortex indicates that the latter is overpowered by the former. 3) Both ψ S and ψ S are negative (positive) A1 A2 west (east) of the vortex. Apparently the vortex is moved westward coherently by the zonal advection for relative vorticity and meridional advection of planetary vorticity. Thus, the basic dynamics of the vortex during its mature stage may be depicted by the linearized vorticity equation: ζ ζ + uz + vβ = f V. (5) t x In middle latitudes, cyclone waves are propagated eastward by the zonal advection of relative vorticity (e.g., Holton 1992). The westward propagation of 1 The vortex is depicted by a negative ψ S cell (marked by a stippled circle in Fig. 10). In Fig. 10c, ψ S, which exhibits negative (positive) values west (east) of the vortex, enhances negative (positive) A12 values of ψ S west (east) of the vortex center and results in a westward movement of the vortex. Values of ψ χ S 1 (Fig. 10d) with respect to the vortex are opposite to ψ S. Thus, the dynamic process A12 of ψ χ S 1 moves the vortex eastward Vol. 82, No. 9, September 2001

11 monsoon depressions is a result of the vortex stretching caused by the east west secondary circulation of the depression (Saha and Saha 1988; Chen and Yoon 2000). Since the secondary circulation of equatorial waves is opposite to that of the Pacific summer upperlevel vortex, all of the three dynamic processes shown in Eq. (5) may propagate equatorial waves westward (Nitta 1982). Nevertheless, the ψ S budget analysis presented in Fig. 10 clearly shows that the basic dynamics of the vortex differ from those of other synoptic disturbances. c. Demise Two pronounced changes (compared to the central North Pacific) in the large-scale environment close to East Asia are revealed from Figs. 4 and 1: 1) The east west q (17.5 N) cross section (Fig. 4c) shows that the moist lower troposphere becomes deeper near the East Asian continent. Supposedly, more moisture may be pumped up by the ascending motion around the vortex periphery to form thicker clouds aloft. 2) In the upper troposphere, the North Pacific oceanic trough in the east yields to the East Asian high in the west. In the lower troposphere, the North Pacific anticyclone is juxtaposed with the monsoon trough in the west. The North Pacific anticyclone prevents the downward penetration of the vortex, but the monsoon trough west of this anticyclone may hypothetically facilitate the downward development and the westward propagation of the vortex onto the east Asian continent. Contradictory to our argument, the vortex diminished on 4 July 1998, 1 day after reaching the east Asian coast. Apparently, the two aforementioned factors do not constitute a favorable environment for the ensuing development of the vortex. What may cause the demise of this vortex? On 2 July, the vortex was embedded in the east Asian high in the upper troposphere (Figs. 5b and 6a) and floating above the southwestward extension of the North Pacific anticyclone (Fig. 6c). The large-scale environment at this stage is actually not favorable for the further development of the vortex. This argument may also be substantiated by the east west cross section of geopotential height anomalies at 25 N on 3 July (Fig. 11 for the long- and short-wave regimes). Two implications may be drawn from these cross sections: FIG. 9. The 150-mb streamfunction budget of the long-wave regime at 0000 UTC on 30 Jun: (a) ψ L A2 and (b) ψ L χ 1. Positive values of both ψ A2 and ψ χ1 are stippled; the contour interval of these two variables is 50 m 2 s 2. 1) As indicated by Z s (25 N) in Fig. 11b, the vortex E is basically confined to the upper troposphere above the 500-mb level. Thus, the vortex depth on 3 July was not deep enough to pump moisture up from the lower troposphere to form thick clouds around the vortex periphery. 2) The horizontal convergence/descending motion of the vortex is reduced by the upper-tropospheric east Asian high, but the vortex is perhaps not deep enough to be affected by the southeastward extension of the North Pacific anticyclone into the northern part of the South China Sea. Moreover, the ascending motion of the Asian monsoon low may also hinder the downward penetration of the vortex. These factors work in concert to suppress any further development of the vortex and eventually lead to its demise. This argument may be inferred by ψ L (150 mb) shown in Fig. 9b, although we do not show this dynamic process for x1 3 July and for the lower troposphere (where ψ L x1 Bulletin of the American Meteorological Society 2001

12 FIG. 10. The 150-mb streamfunction budget of the short-wave regime at 0000 UTC on 30 June: (a) ψ S, (b) χ S, (c) ψ S, (d) ψ S A12 χ 1, (e) ψ S, and (f) ψ S. Positive values of all variables A1 A2 are stippled: contour intervals of (a), (b), and (c) (f) are 10 6 m 2 s 1, m 2 s 1, and 10 2 s 2, respectively. functions oppositely to this dynamic process in the upper troposphere). 5. Climatology Life spans of the 25 Pacific summer upper-level vortices identified over summers of with the criteria set in section 2 are displayed in Table 1; the occurrence frequency of the vortex is on average about three every summer. Some vortices may survive only 5 days, while others last more than 3 weeks. The wind speed of these vortices is weaker than typhoons in the western Pacific, but their life cycles are comparable. The latent heat released by cumulus convection over the ocean maintains the development of typhoons. The moisture supply to the vortex is not only prevented by the lower-tropospheric anticyclone, but also suppressed by the downward motion within the vortex. Although the ascending motion around the vortex periphery may form thin cirrus clouds, the latent heat released by these clouds does not seem to be an effective energy source in maintaining the vortex. What could be the energy source to maintain the longevity of the vortices? Can they extract energy out of the large-scale environmental flow? Can they be much less dissipated because the vortices do not penetrate into the planetary boundary layer? A heuristic argument was suggested in section 2, but these questions are certainly worth of a future research effort. Trajectories (dashed lines) of all 25 identified vortices are shown in Fig. 12, superimposed the summer- (Jun Jul Aug) mean streamlines at 200 (Fig. 12a) and 850 mb (Fig. 12b). The occurrence frequencies of midlatitude synoptic disturbances and equatorial waves in Fig. 1 are also included for reference. The vortices move westward between midlatitude westerlies and tropical easterlies in the Pacific; most of these trajectories are clustered over the region northwest of the midocean trough in the upper troposphere (Fig. 12a) and along or north of the lower-tropospheric monsoon trough in the western tropical Pacific. It was illustrated in section 4 that the vortex is advected westward by the westerlies of the oceanic trough. However, the propagation direction of the vortex in the western North Pacific may be affected by the southeasterlies along the western rim of the North Pacific anticyclone in the lower troposphere, if the depth of the vortex penetrates down sufficiently. As indicated by their trajectories shown in Fig. 12, vortices usually dissipate when they reach the region between Taiwan and the Philippines. Checking through all selected vortices listed in Table 1, we found that the dissipation mechanism of the vortex suggested in section 4 is applicable to all of them. That is, the vortices may be blocked and dissipated by the ascend Vol. 82, No. 9, September 2001

13 ing branch of the planetary-scale east west circulation formed by the east Asian high (Fig. 12a) in the upper troposphere and the monsoon low in the lower troposphere. It was revealed from Fig. 2 that the size and tangential speed of the vortex change as it propagates westward. Some statistics concerning these properties over the life cycles of the vortices may be of some use to understand its dynamics. Thus, two types of comparison are shown in Fig. 13: 1) diameter versus longitudes and 2) maximum tangential speed around the vortex periphery versus diameter. These contrasts are characterized as follows: 1) The vortex diameters range between 750 and 3000 km larger than typhoons, but smaller than midlatitudes cyclone waves. The average diameter of all vortices over their life cycles is about 1550 km. The vortex size becomes smaller and rarely exceeds this average size west of 130 E. 2) The average maximum tangential speed around the periphery of all vortices is about 16.3 m s 1, which is close to the criterion of a tropical cyclone ( 30 kt). The maximum tangential speed of the vortex increases inversely as the vortex size decreases. This tangential speed can exceed 30 m s 1 when the vortex size is smaller than 1000 km. We speculated previously that the depth growth of the moist layer in the lower troposphere near the East Asian continent may facilitate the cloud formation around the vortex. On the contrary, our observations of the 25 vortices do not seem to indicate that the thickening of the moist layer close to East Asia is an effective means to intensity the vortex. Ignoring the thermodynamic processes of cumulus convection, the conservation of vortex strength, that is, (ζ + f)a = constant (where A is the cross section of the vortex), may FIG. 11. Longitude height cross sections of geopotential height in both the long- and short-wave regimes at 0000 UTC on 3 Jul. Positive values of both Z L (25 N) and Z S (25 N) are stippled; the contour interval of these two variables is 7.5 m. be a plausible dynamic constraint on the vortex development. As the vortex moves toward the western North Pacific, the vortex cross section (A) decreases in size and the relative vorticity (ζ ) of the vortex increases in magnitude. Because planetary vorticity ( f ) may not undergo a significant change, the tangential speed of the vortex consequently increases. A caveat of this argument is the lack of a mechanism for causing the reduction in the vortex size. TABLE 1. Dates and life spans of the summer upper-level vortices over the North Pacific. Year Jun 1 Jul 1 10 Jun 1 14 Jun Jun 26 Jun 5 Jul 1 17 Jun 19 Jun 12 Jul 9 15 Jun 22 Jun 8 Jul Jun 5 14 Jul 9 24 Jun Aug Aug 5 14 Jul Jul 1 19 Jul 19 Jun 3 Jul 25 Jul 5 Aug 11 Jul 5 Aug 29 Jul 2 Aug 19 Jul 3 Aug 4 17 Aug Aug Aug Bulletin of the American Meteorological Society 2003

14 6. Concluding remarks The North Pacific anticyclone in the lower troposphere would prevent the downward penetration of any synoptic disturbance, and block the moisture supply to this disturbance, so that no thick cloud/heavy rainfall associated with synoptic disturbances over the subtropical North Pacific have been analyzed or documented. Nevertheless, the midoceanic trough in the upper troposphere above the oceanic anticyclone is presumably a favorable environment for the genesis of synoptic disturbances. Based upon this presumption, we use streamline charts constructed with the NCEP NCAR reanalysis data and the GMS IR imageries to identify 25 disturbances with well-organized vortices during the summers of These Pacific vortices are characterized by the following salient features revealed from our preliminary analysis. a. Structure 1) The vortex exhibits neither a vertical phase reversal like equatorial waves, nor a vertical tilt like midlatitude cyclones. Because of the hindrance of the downward penetration by the lower-tropospheric North Pacific anticyclone, no signal of the vortex can be detected in the lower troposphere. This situation is opposite to monsoon depressions, whose development above the 300-mb level is suppressed by the Tibetan high. 2) The well-organized descending motion within the vortex is encircled by the ascending motion around the vortex periphery. The secondary circulation of the vortex is opposite to other synoptic disturbances. Therefore, clouds are only formed around the vortex periphery with a clear region within the vortex. 3) In addition to the descending motion within it, the vortex also possesses a cold/dry core that is opposite to other synoptic disturbances. 4) The size and speed of synoptic disturbances may not have a systematic relationship. In contrast, the vortex size and the tangential speed around the periphery of the vortex are inversely proportional; the tangential speed of the vortex increases when the vortex size decreases near the east Asian continent. FIG. 12. Long-term summer streamlines chart at (a) 200 and (b) 850 mb superimposed with trajectories of Pacific upper-level vortices (orange dashed lines), midlatitude storm track (the blue area in midlatitudes), and occurrence area of equatorial waves (lighter blue area in the Tropics). The Pacific oceanic trough line in (a) is marked by a thick line. b. Dynamics Synoptic disturbances (including midlatitude cyclones, monsoon depressions, and equatorial waves) usually have an ascending warm core, embedded in an environmental flow with a surface low system, coupled with ascending motion. On the contrary, the Pacific vortices possess a descending cold core, residing in the upper-tropospheric oceanic trough, coupled with descending motion. The distinction in the large-scale environment may result in differences of basic dynamics between Pacific vortices and other types of synoptic disturbances. Furthermore, the longevity of the vortex is thus likely to draw its support from this large-scale environment. As it propagates close to the east Asian coast, the vortex is dissipated by diminishing its horizontal convergence/descending motion by the east Asian high in the upper 2004 Vol. 82, No. 9, September 2001

15 (Civil Aeronautic Administraiton of Taiwan 1998). The maximum wind speed of the vortex near east Asia can exceed 30 m s 1, close to typhoon intensity. These Pacific vortices, which are like hidden tropical cyclones, may pose some danger to air traffic. The east southeast Asian region is one of major international air-traffic routes for many airlines. The finding of the Pacific vortices not only adds a new type of synopticscale disturbance, but also discloses a new potential air-traffic hazard. The awareness of these Pacific vortices and better understanding of their dynamics, will enable aviation agency forecast centers in this region to provide early warnings to numerous flights through the western subtropic Pacific. To achieve this goal, we urge the use of numerical studies of these vortices with both high-resolution global and regional models. Acknowledgments. This study is supported by the NSF Grants ATM and ATM Comments and suggestions offered by two reviewers are very helpful in improving the paper. We would also like to thank Mr. Jin-ho Yoon for his graphic assistance with finalizing several figures used in this paper, Mrs. Reatha Diedrichs for her typing support, and Ms. Kathryn J. St. Croix for her editing assistance. FIG. 13. Some properties of Pacific upper-level vortices: (a) vortex diameter vs longitudinal location of vortex, and (b) maximum tangential speed along the vortex periphery vs vortex diameter, for all synoptic times of 25 selected vortices over their life cycles. troposphere and the southwestward extension of the North Pacific anticyclone in the lower troposphere. The westward propagation of the Pacific vortex is basically driven by the horizontal advection of total vorticity (including the zonal advection of relative vorticity and the meridional advection of planetary vorticity). The inverse relationship between the size and tangential speed of the vortex almost follows the dynamic constraint with the conservation of vortex strength. As revealed from the operational synoptic charts in Fig. 6, the Pacific vortex is neither a conspicuous feature in the upper troposphere, nor a detectable perturbation at the surface. Five Pacific vortices were identified in this study passing through Taiwan. The GMS IR observations showed high cirrus cloud images of these vortices, but they could not be detected by the Doppler weather radar at the Taipei International Airport References Anthes, R. A., 1982: Tropical Cyclones Their Evolution, Structure and Effects. Meteor. Monogr., No. 41, Amer. Meteor. Soc., 208 pp. Bjerknes, J., and J. Holmboe, 1944: On the theory of cyclones. J. Meteor., 1, Charney, J. G., 1947: The dynamics of long waves in a baroclinic westerly current. J. Meteor., 4, , 1973: Movable CISK. J. Atmos. Sci., 30, Chen, T.-C., and S.-P. Weng, 1998: Interannual variation of the summer synoptic-scale disturbance activity in the western tropical Pacific. Mon. Wea. Rev., 126, , and, 1999: Interannual and intraseasonal variations in monsoon depressions and their westward-propagating predecessors. Mon. Wea. Rev., 127, , and M.-C. Yen, 1999: Annual variation of surface pressure on a high East Asian mountain and its surrounding low areas. J. Climate, 12, , and J.-H. Yoon, 2000: Some remarks on the westward propagation of the monsoon depression. Tellus, 52A, ,, and S. Schubert, 1996: Hydrological processes associated with cyclone system over the United States. Bull. Amer. Meteor. Soc., 77, ,, and J.-D. Tsay, 2000a: Annual and semiannual variations of surface pressure in Taiwan. J. Climate, 13, ,, and S.-P. Weng, 2000b: Interaction between the summer monsoons in East Asia and the South China Sea: Intraseasonal monsoon modes. J. Atmos. Sci., 57, Bulletin of the American Meteorological Society 2005