Journal of Geophysical Research: Atmospheres
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Effects of a semienclosed ocean on extratropical cyclogenesis: The dynamical processes around the Japan Sea on 23–25 January 2008

Authors

  • Masaru Yamamoto

    Corresponding author
    1. Research Institute for Applied Mechanics, Kyushu University, Kasuga, Japan
    • Corresponding author: M. Yamamoto, Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-kouen, Kasuga 816-8580, Japan. (yamakatu@kyudai.jp)

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Abstract

[1] Based on a case study of an extratropical cyclone that developed rapidly near Japan on 23−25 January 2008, this paper proposes a new type of atmospheric response to a surface warming/cooling anomaly in the semienclosed Japan Sea, which is approximately half a synoptic-scale baroclinic wavelength in size. The thermal effects of the Japan Sea during early cyclogenesis spread to the upper levels via an upward wind, when the trough of the baroclinic wave is located west of the Japan Sea. In the case of a relatively cold Japan Sea, the lower level air temperature decreases over the sea, and the baroclinic wave becomes weak around Japan in the early stage. As the cyclone progressively develops and travels east, the positive and negative geopotential height anomalies induced by the sea surface cooling increase over time in the lower level trough and the upper level ridge outside the Japan Sea, respectively. The temperature anomaly spreads to upper levels through the development of a warm conveyer belt. As the cyclone becomes fully developed, a geopotential height anomaly pattern similar to the west Pacific teleconnection forms as a result of the S-shaped trough-ridge structure in the middle and upper troposphere. The cooling/warming effect of the sea influences surface heat fluxes and precipitation in the Pacific region outside the Japan Sea via a synoptic-scale atmospheric bridge caused by the traveling and developing cyclone and cold air outbreak.

1 Introduction

[2] The significance of sea surface temperature (SST) fronts is widely recognized in extratropical cyclogenesis and climatology [Hanson and Long, 1985; Xie et al., 2002; Minobe et al., 2008; Small et al., 2008]. Recently, Booth et al. [2012] have shown that the storm strength increases with the magnitude of the SST perturbation over the entire model domain including the Gulf Stream region, even when the perturbation weakens the SST front. In addition to the sea surface conditions, coupling of the higher-level potential vorticity (PV) with the lower level cyclone is also one of the important factors controlling extratropical cyclogenesis [Takayabu, 1991]. Advection and diabatic production of the PV affect the cyclone development [Grams et al., 2011].

[3] In winter, surface cyclogenesis frequently occurs over the East Asia marginal seas (Figure 1a). The Japanese main islands are located between the Japan Sea and the Pacific, and strong SST gradients develop through the East China Sea, Japan Sea, and northern Pacific. The Japan Sea is a semienclosed ocean (Figure 1b), in which the SST distribution is strongly controlled by the Tsushima Warm Current that transports heat from the Tsushima Strait (34°N, 130°E) to the southern part of the sea. The semienclosed Japan Sea is roughly half a synoptic-scale baroclinic wavelength (scale of an extratropical cyclone, 2000−5000 km) in size, and we do not yet fully understand whether baroclinic waves are modified by a short SST front over this small sea or by cooling/warming over the entire sea area.

Figure 1.

Terrain height distributions (m, color shading) of (a) outer and (b) inner model domains. YS, EC, JP, and OK represent the Yellow, East China, Japan, and Okhotsk Seas, respectively, and the distributions of sea surface temperature anomalies (K, color shading) in (c) COOL and (d) WARM.

[4] Yamamoto and Hirose [2007] reported that extratropical cyclones are enhanced over the sharp SST front in the small semienclosed Japan Sea. However, because the SST is high south of the front, cyclone deepening may be caused not only by the near-surface baroclinicity over the front but also by heat and moisture supplied from the area of high SST. In addition to the SST front, the surface temperature warming/cooling over the relatively small Japan Sea, which is largely or partially controlled by the volume transport of the Tsushima Warm Current [Hirose and Fukudome, 2006], might influence the weather over the northwestern Pacific, probably via active cyclogenesis [Yamamoto and Hirose, 2011]. Although the sea surface warming/cooling effect may be also important for weather and climate, it is not clear why the cold Japan Sea causes the negative geopotential height anomaly over the Okhotsk Sea (around 50°N, 150°E) reported by Yamamoto and Hirose [2011].

[5] The change of the sea surface condition influences surface sensible and latent heat fluxes. The surface heat fluxes modify the surface baroclinicity zone [Horvath et al., 2006] and the diabatic process and influence intensification of the extratropical cyclone in the early developing stage [Kuo et al., 1991]. The air mass modified via the surface fluxes spreads to upper levels via the WCB (warm conveyer belt) (i.e., advection of warm air northward and upward). According to Booth et al. [2012], the storm response to the SST perturbations is driven by the latent heat release in the WCB. When the cyclone passes over the small semienclosed Japan Sea, the WCB plays an important role in the upward transport of the air mass. In contrast, after the cyclone passes, the CAO (cold air outbreak) (i.e., advection of cold air southward) controls the air mass transport over the sea area. The modified air is advected southward in the lower level troposphere by the CAO [Yamamoto, 2012]. Differently from the WCB, the CAO does not spread the air mass to upper levels in the downward motion region west of the cyclone.

[6] Takano et al. [2008] investigated the influence of large-scale atmospheric circulation and local SST on convective activity over the Japan Sea in winter and found a Japan-Siberia pattern similar to the west Pacific (WP) teleconnection [Wallace and Gutzler, 1981]. The teleconnection patterns have one signal around Japan and another signal, with the inverse sign, over Siberia and the Okhotsk Sea. Hirose et al. [2009] showed that the winter WP pattern and precipitation are highly correlated with the autumn volume transport through the Tsushima Strait and suggested that the Tsushima Warm Current could lead to the winter teleconnection via the sea surface conditions. Although the WP index is one of the important climatological indexes that affect Japan, its formation mechanism remains unclear compared with those of other Pacific teleconnections (e.g., the Pacific/North American teleconnection pattern is strongly influenced by the El Niño–Southern Oscillation).

[7] The equatorial ocean process forces the planetary-scale Rossby wave, which modifies the surface condition of the midlatitude ocean. Thus, the teleconnections enable the atmosphere to act as a bridge between different remote oceanic areas [Alexander et al., 2002; Liu and Alexander, 2007]. This process is recognized as an atmospheric bridge in global-scale climatology. In synoptic-scale dynamics, traveling and developing cyclones may act as such an atmospheric bridge between the Japan Sea and the Pacific across the main islands of Japan. However, synoptic-scale remote effects of this type, between a marginal sea and the open ocean via an extratropical cyclogenesis process (termed a synoptic-scale atmospheric bridge in this article), have not been previously investigated.

[8] The aim of this study is to elucidate atmospheric responses to a surface warming/cooling anomaly in the semi-enclosed Japan Sea. In investigating the atmospheric responses, the WP teleconnection and atmospheric bridge are extended from global- to synoptic-scale dynamics. The monthly teleconnection indices of the U.S. Climate Prediction Center show that the WP index approached zero (0.37 in December, 0.92 in January, and 0.01 in February) during the winter of 2007–2008. Assuming that the background WP signal is weak, it is possible to investigate whether the Japan Sea influences the WP (or WP-like) pattern via cyclogenesis. During this period, an extratropical cyclone rapidly developed near Japan on 23−25 January 2008, and this study examines the atmospheric responses to the Japan Sea during the cyclogenesis process.

2 Model

[9] A cyclone that developed rapidly between 0000 UTC on 22 January and 0000 UTC on 25 January 2008 was investigated using the Advanced Research Weather Research Forecasting (WRF) model, ver. 3.2.1 [Skamarock and Klemp, 2008]. Three experiments were run: the control experiment, with no SST anomaly in the Japan Sea (CNTL), and experiments with positive and negative SST anomalies (WARM and COOL, respectively). The model atmosphere from the surface to 50 hPa has 28 levels, and the mother domain shown in Figure 1a covers the area around Japan with a horizontal resolution of 30 km (181 × 145 grid points). To simulate the developing stage of the small cyclone over the Japan Sea, a two-way nested domain with a resolution of 10 km (160 × 181 grid points) was set over the Japan Sea (Figure 1b). The following model options were used: the WRF single-moment six-class graupel microphysics scheme, the rapid radiative transfer model for long-wave radiation, the Dudhia short-wave radiation scheme, the Monin-Obukhov surface scheme, the Yonsei University planetary boundary layer scheme, the Kain-Fritsch (new Eta) cumulus scheme, and the unified Noah land-surface model [Skamarock et al., 2008].

[10] The National Centers for Environmental Prediction/Final Analyses data sets were used as initial and boundary conditions. The Real-Time Global SST data set [Thiebaux et al., 2003] was used for the SST in CNTL, and the Japan Sea SST was reduced and increased by 1 K in COOL and WARM, respectively. The SST anomalies of COOL and WARM are shown in Figures 1c and 1d, respectively. The 1 K anomaly is roughly equal to the interannual temperature amplitude of the Japan Sea. Although SST cooling decreases the temperature difference between land and sea, the change in the coastal temperature gradient does not significantly influence synoptic-scale dynamics because the 1 K anomaly is small in comparison with the preexisting land-sea temperature contrast. Thus, the influence of the coastal changes might be locally limited.

[11] Five ensemble simulations were completed with start times from 1200 UTC on 20 January to 1200 UTC on 21 January 2008. The frequency of the model output was 3 h, and the vertical interval was 50 hPa. The ensemble mean results are discussed in the following sections.

3 Overview of a Rapidly Developing Cyclone

[12] According to the Japan Meteorological Agency (JMA) weather charts, a trough of sea level pressure opened to the south over the Yellow and East China Seas at 0000 UTC on 21 January 2008 and subsequently developed. At 0000 UTC on 23 January (Figure 2a), weak cyclones with central sea level pressures of 1016 and 1012 hPa appeared around the Tsushima Strait (around 35°N, 130°E) and the south coast of Japan, respectively. The cyclone developed rapidly, traveling east over the Japan Sea and the northwestern Pacific for 2 days (23 and 24 January). The central sea level pressure dropped to 986 hPa around 40°N, 140°E at 0000 UTC on 24 January (Figure 2b) and fell rapidly to 968 hPa around 43°N, 155°E at 0000 UTC on 25 January (Figure 2c). They formed one cyclone (not two cyclones) at and above the 700 hPa level around Japan.

Figure 2.

The JMA weather charts at 0000 UTC on (a) 23, (b) 24, and (c) 25 January 2008.

[13] The cyclone was well simulated by the CNTL run, as shown by the contours in Figure 3a. The low, with a sea level pressure below 1000 hPa, formed an elongated oval across Japan. The two surface minima appeared in the oval low at 0000 UTC on 23 January (in the early cyclogenesis stage) and merged at 0000 UTC on 25 January (in the fully developed cyclogenesis stage). However, they had the structure of a single cyclone above the 850 hPa level. Thus, in the synoptic view, the cyclone was identified as a single cyclone system with an elongated oval form. At the surface, the northern minimum of the oval low developed rapidly over the Japan Sea during the early stage. As the southern minimum of the oval low traveled northeast, it deepened and merged with the northern minimum. Finally, the elongated oval developed into the circular form in the fully developed stage.

Figure 3.

Horizontal distributions of anomalies (COOL − CNTL, color shading) of (a) sea level pressure and geopotential height at (b) 850 hPa, (c) 500 hPa, and (d) 300 hPa at 0000 UTC on (left) 23, (middle) 24, and (right) 25 January 2008. The contours indicate the values from CNTL. Shading shows confidence levels of 95% or greater. The gray vectors indicate the anomalous horizontal velocities (m s−1).

[14] When the surface low appeared at 0000 UTC on 23 January, a trough was apparent at 850 hPa and a weak ridge at 300 hPa over the Japan Sea. The trough developed further with time in the lower troposphere (850 hPa), and the cyclone deepened strongly over the Japan Sea at 0000 UTC on 24 January. In the fully developed stage (0000 UTC on 25 January), an S-shaped height pattern was formed by the southern trough and the northern ridge around 40°N−55°N in the middle and upper troposphere (500 and 300 hPa, respectively).

4 Dynamical Effects of a Semienclosed Sea Surface Temperature Anomaly

[15] The cold Japan Sea temperature anomaly (Figure 1c) weakens the low at the surface, as shown by the positive anomalies of sea level pressure (COOL − CNTL) in Figure 3a. The northern portion of the large elongated oval low passes over the Japan Sea and so is affected by the modified SST, while in contrast, the southern portion is not directly affected by the Japan Sea. During the fully developed stage, a positive anomaly in sea level pressure develops in the northern area of the low. In other words, the Japan Sea temperature anomaly directly influences the northern low during the early stage and indirectly affects the southern low, via the merger with the northern low, during the fully developed stage. A positive geopotential height anomaly appears around the low at 850 hPa (Figure 3b), while a negative anomaly is predominant in the northern area of the cyclone at 300 hPa (Figure 3d). This indicates that the temperature anomaly in the Japan Sea weakens both the trough in the lower troposphere and the ridge in the upper troposphere. The upper level ridge develops over time and migrates to the Okhotsk Sea area with the counterclockwise flow of the developing cyclone. At the same time, a negative anomaly develops in the upper level ridge and moves to the north of the cyclone. As the lower level positive and upper level negative anomalies continue to increase, an anomaly pattern similar to the WP teleconnection is predominant at the intermediate level (500 hPa). At the fully developed cyclogenesis stage (0000 UTC 25 January), as a result of the relatively cold Japan Sea weakening the ridge and trough at the 300 hPa level in COOL, a WP-like anomaly pattern forms over the northwestern Pacific.

[16] Figure 4 shows horizontal distributions of potential temperature and its anomalies (COOL − CNTL, K) at 0000 UTC on 23–25 January 2008. The negative temperature anomaly patch appears around the warm tongue traveling northward at the 850 hPa level over the Japan Sea at 0000 UTC on 23 January and spreads to the 500 hPa level at 0000 UTC on 24 January and to the 300 hPa level at 0000 UTC on 25 January. The negative temperature anomaly forms north of the WCB at 500 hPa in the fully developed stage. Thus, as the warm tongue in the early stage develops and becomes the WCB, the influence of the Japan Sea on temperature propagates upward. The positive temperature anomaly at 300 hPa forms in a cold trough at 0000 UTC on 24 January, by weakening the cold trough of the baroclinic wave. The upper level positive anomaly is advected northward and develops into the arc form by 0000 UTC on 25 January.

Figure 4.

Horizontal distributions of potential temperature anomalies (K, COOL − CNTL, color shading) at (a) 2 m, (b) 850 hPa, (c) 500 hPa, and (d) 300 hPa at 0000 UTC on (left) 23, (middle) 24, and (right) 25 January 2008. The contours indicate the potential temperatures in CNTL. Shading shows confidence levels of 95% or greater. The vectors indicate the horizontal velocities (m s−1).

[17] Figure 5 shows horizontal distributions of PV and its anomalies. The 300 hPa PV patch around 45°N, 120°E at 0000 UTC on 23 January deforms to the inverse C shape at 0000 UTC on 24 January and the elongated streamer at 0000 UTC on 25 January. The high PV (>3 PVU) and low PV (<1 PVU) form in the trough and ridge of geopotential height, respectively. The positive PV anomaly is associated with the positive temperature anomaly (Figure 4d) and cyclonic anomalous wind (Figure 3d) at 0000 UTC 24 January. After 24 h, the positive PV anomaly is elongated at the northern tip of the low-PV area. The upper level PV deepens to the 500 hPa level over the surface cyclones in the Japan Sea area (0000 UTC on 24 January) and the Pacific area (0000 UTC on 25 January). At 0000 UTC on 24 January, the Japan Sea cooling induces the locally negative PV anomalies at 850 and 500 hPa (though it is difficult to see the anomalies in the middle panels in Figures 5a and 5b) and the positive PV anomaly around 45°N, 145°E at 300 hPa (in the middle panel in Figure 5c). However, it is unclear that the geopotential height anomalies are induced by the PV reduction due to the weakened diabatic heating in the early developing stage, because the PV patches of the cyclone are local and small near the surface. During the fully developed stage (0000 UTC 25 January), the arc-shaped high PV and its anomalies form around the front at 850 hPa. Both the positive and negative PV anomalies coexist at the same pressure level. The positive and negative PV anomalies at 850 hPa are associated with modifications of the front resulting from the Japan Sea cooling, i.e., a slightly southward shift of the front location via the weakened cyclone, and local changes of the temperature and vorticity around the front. In addition, the positive PV anomaly at 850 hPa may be partly influenced by the lower level PV production [Grams et al., 2011] via the diabatic heating over the area of the positive anomaly of surface heat fluxes (see Figure 11). Although it is difficult to quantitatively separate the PV anomaly into the two possible factors—(i) the slight shift of the front location and (ii) the diabatic PV production—the lower level PV anomaly represents a remote effect of the Japan Sea on frontgenesis over the Pacific.

Figure 5.

Horizontal distributions of potential vorticity (PVU, color shading) in CNTL at (a) 850 hPa, (b) 500 hPa, and (c) 300 hPa at 0000 UTC on (left) 23, (middle) 24, and (right) 25 January 2008. The black and white contours indicate the positive and negative anomalies of potential vorticity (COOL − CNTL), respectively. The contour interval is 1 PUV.

[18] The negative SST anomalies lead to a decrease in air temperature over the Japan Sea, which contributes to the regional stabilization of the atmosphere. The potential temperature decreases below the 600 hPa level over the Japan Sea during the early stage of cyclogenesis (0000 UTC on 23 January, left panel in Figure 6a). The negative anomaly in the vertical wind spreads to the upper troposphere over the Japan Sea in the ascent phase of the baroclinic wave during the early stage (left panel in Figure 6b). This indicates that the upward motion associated with the baroclinic wave is weakened by the cold Japan Sea anomaly. Then, the wave amplitude of the geopotential height weakens in both the upper level ridge and the lower level trough around the Japan Sea (left panel in Figure 6c). Thus, the effect of the Japan Sea rapidly spreads to the upper levels during the early stage, and the amplitude of the baroclinic wave weakens between the surface and 300 hPa around the Japan Sea because of the atmospheric stabilization induced by the cold sea surface.

Figure 6.

Longitude-pressure distributions of (a) potential temperature (θ, K), (b) vertical velocity (w, mm s−1), and (c) geopotential height (Z, m) anomalies (COOL − CNTL, color shading), averaged over 35°N–45°N at 0000 UTC on (left) 23 and (right) 24 January 2008. The contours indicate the geopotential height deviations from the longitudinal average in CNTL.

[19] As the cyclone develops, the positive and negative height anomalies grow in the trough and ridge, respectively. At 0000 UTC on 24 January, the developing low travels to the western edge of the Japan Sea (140°E). The temperature anomaly spreads to the upper level and travels to the 140°E−150°E region (right panel in Figure 6a). The negative anomaly in the vertical velocity around 145°E is enhanced by migrating eastward (right panel in Figure 6b). The positive vertical wind anomaly is also amplified around the eastern coast. For geopotential height, the positive and negative anomalies are amplified and spread vertically in the lower level trough around 140°E and the upper level ridge around 150°E (right panel in Figure 6c). At this time, the cold temperature anomaly is confined below the 750 hPa level, and the vertical wind anomaly is not large over the western Japan Sea around 130°E, because the descent phase of the baroclinic wave is located over the sea.

[20] The Japan Sea influences atmospheric diabatic heating via the sea surface turbulent heat flux, which induces the anomalies of the meteorological elements. The diabatic heating rate (Qd) is estimated from the following heat budget equation:

display math(1)

[21] Figure 7 shows longitude-pressure distributions of the Qd in CNTL and its anomaly, averaged over 35°N–45°N at 0000 UTC on 23 and 24 January. The Qd anomaly is large in the region where the deep heating occurs around 130°E at 0000 UTC on 23 January. The Qd anomaly is accumulated with time and results in the large temperature anomaly over the Japan Sea (left panel in Figure 6a). The negative heating anomaly could induce the negative anomaly of the vertical velocity below the 500 hPa level. However, the production of the PV anomaly due to Qd is not clear over the Japan Sea, because the anomalies of the PV and its production rate are small over this sea area in the early developing stage (not shown). The diabatic heating increases with time and travels east. The positive and negative Qd anomalies are seen over the Japanese main islands at 0000 UTC on 24 January. This is caused by the difference between the locations of the heating maxima in CNTL and COOL.

Figure 7.

Longitude-pressure distributions of the diabatic heating rate Qd in CNTL (K h−1, contour) and its anomaly (COOL − CNTL, color shading), averaged over 35°N–45°N at 0000 UTC on (left) 23 and (right) 24 January 2008.

[22] The amplified negative height anomaly moves to the north of the positive anomaly with the cyclonic migration of the ridge at the 300 hPa level during the fully developed stage (0000 UTC on 25 Januaryin Figure 3d). Figure 8 shows the latitude-pressure cross section of the geopotential height anomaly. The lower level positive height anomaly spreads upward to the upper level around 43°N, while the upper level negative anomaly spreads downward to the 500 hPa level around 53°N. The WP-like pattern, with the northern negative and southern positive anomalies (Figures 3c and 3d), lies between 600 and 200 hPa (color shades in Figure 8), when the cyclone is fully developed. The negative anomaly of the vertical wind is predominant between the positive and negative anomalies of the geopotential heights (dashed contour around 48°N in Figure 8). This implies that the anomaly patterns of the geopotential and vertical wind over the Japan Sea during the early stage develop with time (Figure 6), propagate toward the northern Pacific area, and form the quasi-standing structure (Figure 8) of the WP-like pattern via the rapidly developing cyclogenesis.

Figure 8.

Latitude-pressure distribution of the anomaly of geopotential height (color shading, m) and vertical wind velocity (contour, mm s−1), averaged over 145°E–155°E at 0000 UTC on 25 January 2008.

[23] Figure 9 shows horizontal distributions of the anomalies of daily mean temperature at 2 m, water vapor mixing ratio at 2 m, and horizontal wind velocity at 10 m. The negative anomalies of the temperature (T2) and water vapor mixing rate (q2) are predominant, and the anomalous vector of the wind (u10) is clockwise around the Japan Sea on 23 January. As the northern portion of the cyclone travels east, the anomalies propagate to the Okhotsk Sea and Pacific regions. The negative anomalies of T2 and q2 are found along the southern coast of Japan on 24 January, because the CAO (strong northerly over the Japan in the middle and right panels in Figure 4a) advects the cool and dry air mass modified by the Japan Sea cooling toward the Pacific region.

Figure 9.

Horizontal distributions of anomalies (COOL − CNTL, color shading) of daily mean (a) temperature (K) and (b) water vapor mixing ratio (g kg−1) at 2 m on (left) 24 and (right) 25 January 2008. The vector indicates anomalies of horizontal wind velocities at 10 m (m s−1).

[24] The upward sensible and latent heat fluxes (SHF and LHF) from the sea are defined as

display math(2)
display math(3)

where ρ is the atmospheric density, cP is the specific heat at constant pressure (1004.6 J K−1 kg−1), ι is the latent heat of evaporation (2.5 × 106 J kg−1), CS is the sensible heat transfer coefficient, CL is the latent heat transfer coefficient, and qSAT is the water vapor mixing ratio at saturation. The numerical values for CS, CL, and ρ are calculated from the surface atmospheric condition simulated in the model. The surface fluxes are determined by (SST − T2), (qSAT − q2), and |u10|. Figure 10 shows the horizontal distributions of anomalies of daily mean (SST − T2m) and (qSAT − q2m). For the surface differences of temperature and water vapor, the anomalies are negative over the Japan Sea but positive around the eastern coast of Japan on 23 January. After 1 day, the negative anomalies have spread into the Japan Sea area, while the positive anomalies are located around the center of the cyclone (45°N, 150°E) and the southern coast of Japan (approximately 34°N). These differences in T2 and q2 from the surface are enhanced outside the Japan Sea, because the cool and dry air mass modified by the Japan Sea cooling moves to the Okhotsk Sea and Pacific regions via the migration of the developing cyclone and to the southern coast of Japan via the CAO.

Figure 10.

Horizontal distributions of anomalies (COOL − CNTL, color shading) of daily mean (a) SST − T2m (K), (b) qSAT − q2m (g kg−1), and (c) |u10| (m s−1) on (left) 24 and (right) 25 January 2008.

[25] The strength of the surface wind falls because the surface cyclone is weakened by the Japan Sea cooling on 23 January, and the negative anomalies around the Japan Sea move to the Okhotsk Sea and Pacific regions with the cyclone migration on 24 January. The positive anomaly forms due to the locally strong clockwise anomalous wind at 42°N, 145°E–155°E (Figure 9).

[26] Figures 11a and 11b show the daily mean surface sensible and latent heat fluxes, respectively. The Japan Sea temperature anomaly (Figure 1c) influences not only cyclogenesis via the surface heat fluxes over the Japan Sea but also the surface conditions over the northwestern Pacific outside the Japan Sea. The negative anomalies of the surface sensible and latent heat fluxes are predominant in and around the Japan Sea during the early stage on 23 January. These heat flux anomalies produce a cold air temperature anomaly that weakens the baroclinic wave. After 1 day, while these heat flux anomalies are negative over the Japan Sea, they are positive around the south coast of Japan during the fully developed stage on 24 January. The positive anomalies arise because the relatively cooler and drier northerly wind from the Japan Sea (Figure 9) amplifies the surface heat supply from the warm Pacific along the south coast of Japan in COOL via the enhanced air-sea differences in temperature, moisture, and surface wind (Figure 10). Around 42°N, 150°E, the positive anomalies of these surface heat fluxes are produced by the positive anomalies of (SST − T2), (qSAT − q2), and |u10|. The negative anomalies around 37°N, 150°E are caused by the negative anomalies of |u10|.

Figure 11.

Horizontal distributions of anomalies (COOL − CNTL, color shading) of daily mean (a) surface sensible heat flux (W m−2), (b) surface latent heat flux (W m−2), and (c) precipitation (mm) on 24 (left) and 25 January 2008 (right). The contours indicate the values from CNTL. Shading shows confidence levels of 95% or greater.

[27] As the front evolves, it causes precipitation (contour, Figure 11c). The negative daily precipitation anomaly (blue shading) along 44°N corresponds to the negative anomaly of the vertical velocity (contour, Figure 8), which forms over the Japan Sea in the early stage and propagates to the Pacific with time. Thus, the rainfall is reduced by the weakness of the upward flow due to the Japan Sea cooling. In contrast, the positive precipitation anomaly is caused by the positive anomaly of the vertical velocity, and/or is associated with the slight shift of the precipitation area. In this case, the surface flux over the Pacific does not greatly influence the precipitation, because the positive anomalous surface moisture flux does not produce a positive precipitation anomaly. As mentioned above, the Japan Sea anomaly influences the sea surface fluxes over the Pacific by modifying cyclogenesis. This finding implies that the developing cyclone plays a dynamical role as an atmospheric bridge from the Japan Sea to the Pacific in the regional atmosphere-ocean interaction.

[28] The atmospheric anomaly patterns induced by the 1 K decrease/increase over the Japan Sea are reversible. Figures 12 and 13 represent the horizontal distributions of the anomalies (WARM − CNTL) for sea level pressure, geopotential heights, surface fluxes, and precipitation. The anomaly patterns of the geopotential in WARM are similar to the patterns (but with the reverse sign) in COOL, although the magnitudes of the anomalies are somewhat weaker than those in COOL. As the difference between the anomaly amplitudes in COOL and WARM is not large and the two anomaly areas are almost the same in the two experiments, the WP-like teleconnection may be potentially modified via the cyclogenesis under the Japan Sea condition with a cooling/warming anomaly. The sea level pressure and 500 hPa height anomalies have approximately the same amplitudes as climatological North Pacific Oscillation (NPO)/WP patterns (5 hPa and 50 m, respectively) [Linkin and Nigam, 2008]. However, the simulated spatial patterns are formed in the limited area around the Japan and Okhotsk Seas, which differ from the global-scale NPO/WP patterns. Thus, the frequent appearance of the synoptic-scale WP-like anomaly may modify the climatological WP pattern.

Figure 12.

Horizontal distributions of anomalies (WARM − CNTL, color shading) of (a) sea level pressure and geopotential height at (b) 850 hPa, (c) 500 hPa, and (d) 300 hPa at 0000 UTC on 25 January 2008. The contours indicate the values from CNTL. Shading shows confidence levels of 95% or greater.

Figure 13.

Horizontal distributions of anomalies (WARM − CNTL, color shading) of daily mean (a) surface sensible heat flux (W m−2), (b) surface latent heat flux (W m−2), and (c) precipitation (mm; color shading) on 25 January 2008. The contours indicate the values from CNTL. Shading shows confidence levels of 95% or greater.

[29] For the surface fluxes and precipitation over the sea (Figure 13), the anomalies for WARM are also very similar to those with the reverse sign in COOL. The surface cooling/warming of the Japan Sea affects the remote oceans across the Japanese main islands via the propagating and developing cyclone and the CAO. This indicates that the cyclogenesis acts as a synoptic-scale atmospheric bridge between the Japan Sea and the Pacific. Thus, the Japan Sea warming/cooling induces the WP-like anomaly pattern and atmospheric bridge via the cyclogenesis.

5 Discussion and Concluding Remarks

[30] The semienclosed Japan Sea influences extratropical cyclogenesis and forms a short-term WP-like pattern via SST warming/cooling, with a length scale approximately half the baroclinic wavelength, on 22–25 January 2008. The new dynamical processes can be summarized as follows. In the early stage of the baroclinic wave, the phase of the geopotential height tilts westward with height, and the ascent phase is located between the trough and ridge at 500 hPa, as shown in the left panels in Figure 14a [Holton, 2004]. The temperature anomaly induced by the Japan Sea cooling spreads to the middle level via the upward wind when the trough is located west of the Japan Sea (in the ascent phase of the baroclinic wave), and the amplitude of the baroclinic wave weakens in the height range between the surface and 300 hPa (i.e., the positive height anomaly appears in the lower level trough, and the negative anomaly appears in the upper level ridge) around the Japan Sea. Note that the SST anomaly does not influence the cyclogenesis during the descent phase over the Japan Sea, because the downward wind prevents the upward propagation of the influence of the SST anomaly during the fully developed stage [Ueda et al., 2011]. Accordingly, the early-stage atmospheric influences of the Japan Sea during the ascent phase (anomalies of temperature, vertical flow, and geopotential height induced by the Japan Sea in the left panels in Figure 6) are favorably spread over time toward the northwestern Pacific via the propagating cyclone.

Figure 14.

Schematic diagrams of (a) cross sections of the baroclinic wave system during the early and fully developed stages of cyclogenesis and (b) the atmospheric bridge between the Japan Sea and the Pacific. In Figure 14a, red and blue indicate the trough and ridge of the wave, respectively. The dotted curve represents the anomaly of the geopotential height. The cold anomaly of the air temperature is indicated by “C” above the Japan Sea.

[31] The geopotential height anomalies in the trough and ridge grow as the cyclone develops, and the temperature anomaly over the Japan Sea spreads to the upper levels as the WCB develops. The ridge in the geopotential height moves to the north of the center of the low during the fully developed stage (right panels in Figure 14a), forming the S-shaped structure in geopotential height. The southern trough and the northern ridge in the S-shaped structure accompany the positive and negative anomalies, respectively. As a result, a WP-like height anomaly pattern forms during the fully developed stage. However, the simulated pattern is not the WP teleconnection seen in the climatology [Wallace and Gutzler, 1981], though they are similar. If the atmospheric anomalies form frequently over one season via this cyclogenesis mechanism, the negative Japan Sea temperature anomaly enhances the negative geopotential height anomaly over the Okhotsk Sea reported by Yamamoto and Hirose [2011] and may partly contribute to the formations of the seasonal mean WP pattern described by Hirose et al. [2009]. This scenario should be demonstrated in a large-scale climate model, but this is beyond the scope of this regional case study, which focuses on the dynamical process for one life cycle of a cyclone. Furthermore, the maintenance and frequent formations of the S-shaped trough-ridge structure in the absence of the SST anomaly may also influence the climatological WP pattern. Consequently, additional work using the climate model is required to determine whether or not the climatological WP pattern is really induced by cyclogenesis.

[32] In addition, the Japan Sea cooling/warming influences the surface heat fluxes and precipitation in the Pacific region outside the Japan Sea via two processes (arrows in the right panel in Figure 14b): (i) the eastward propagation of the cyclone modified by the Japan Sea changes the surface fluxes in the southern Okhotsk Sea and the northwestern Pacific and (ii) the modified CAO changes the fluxes along the south coast of Japan. Thus, the cyclogenesis modified by the Japan Sea plays the role of a synoptic-scale atmospheric bridge that connects the Japan Sea to the Pacific across the main islands of Japan (arrow in the left panel in Figure 14b). This is distinguished from the climatological atmospheric bridge of a global-scale teleconnection [Alexander et al., 2002; Liu and Alexander, 2007].

[33] If the cyclone was located over the Pacific area far from the continent, the atmospheric responses to the Pacific SST warming/cooling should be influenced by both the WCB with the ascent motion and the CAO with the descent motion at the same time. In contrast, in the case of the relatively small Japan Sea, which is half the size of the baroclinic wavelength, only the ascent (descent) motion appears over the sea during the early (fully developed) stage. Thus, the atmospheric response to the Japan Sea can be clearly separated into the ascent phase at 0000 UTC on 23 January and the descent phase at 0000 UTC on 25 January. Such a separation of the atmospheric response is caused by the small ocean size. In addition, the geographical location of the Japan Sea is also important because it is located west of the Pacific, and so the impact of the Japan Sea is likely to spread toward the Pacific region as the cyclone travels east.

[34] For a small semienclosed ocean, it is necessary to consider not only the near-surface baroclinicity forced by the short SST front but also the dynamical effect of the warming/cooling over the entire sea. The present work proposes two new types of response to the semienclosed sea surface warming/cooling during rapid cyclogenesis, i.e., the synoptic-scale WP-like height anomaly pattern (Figure 14a) and the atmospheric bridge (Figure 14b). As the influence of the Japan Sea on each episode of cyclogenesis may differ in general terms, further work is required to investigate whether or not the dynamical processes described in the present study are common.

Acknowledgments

[35] The author would like to thank N. Hirose, S. Miller, and anonymous reviewers for fruitful discussions and constructive comments. This study is part of the Kyushu University project titled “Understanding Influences of Global Warming and Rapid Economical Development on the East Asia Marine and Atmospheric Environment” and was supported by MEXT KAKENHI grant 22106003. Data were sourced from the Japan Meteorological Agency, the U.S. Climate Prediction Center, and the National Centers for Atmospheric Research and for Environmental Prediction.

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