1. Introduction
Subtropical upper-troposphere westerly jet streams, driven directly by the equator-to-pole temperature gradient and strongly varying in strength, position, and shape throughout the year, are the prominent feature of atmospheric circulations at midlatitudes (Lee et al. 2019). They can be single or multiple, strong or weak, and meandering or straight in the planetary-scale atmosphere, and they strongly affect the type of atmospheric circulation and many weather phenomena, such as cyclogenesis, frontogenesis, blocking, and storm track activity (Chowdary et al. 2019). The branch of upper-troposphere westerly jets over East Asia and the North Pacific, hereafter referred to as the EAWJ, acts as a bridge for the eastward propagation of upstream signals and has a remarkable annual cycle in terms of intensity and meridional displacement (Ding and Wang 2005; Yasui and Watanabe 2010; Xie et al. 2015). The interactions of the EAWJ with the East Asian monsoon notably affect the spatiotemporal pattern and seasonal evolution of rainfall over East Asia (Sampe and Xie 2010; Herzschuh et al. 2019). The EAWJ anchors the northern border of rain belts in the primary rainy season from May to September (Xuan et al. 2011; Lin 2013; Li and Zhang 2014), and its seasonal meridional displacements thus provide valuable guidance for predictions of the weather and climate over East Asia (Huang et al. 2015; Wei et al. 2017).
A growing number of studies have shown that zonal variations of the EAWJ might have profound effects on the spatiotemporal rainfall pattern over East Asia (Zhang et al. 2006; Kuang et al. 2014; Xie et al. 2015; Du et al. 2009, 2016) and the storm track intensities over North Pacific (Jaffe et al. 2011). In winter, an intensified EAWJ usually presents a prominent wave pattern and is associated with colder and drier conditions in East Asia (Yang et al. 2002). In summer, upper-level disturbances propagating along the EAWJ induce upwelling and affect low-level horizontal moisture transport, both of which contribute to the formation of rain belts to the south of westerly jet streams (Horinouchi 2014). The EAWJ covaries with the location of mei-yu–baiu rain belts over East Asia and anchors the zonally prolonged rain belt through the advection of warm air, organizing ascending motions and directing transient weather disturbances from the upstream (Sampe and Xie 2010). Therefore, zonal extensions and retractions of the EAWJ can affect the seasonal evolution of rain belts over eastern China (Xie et al. 2015). Particularly, meandering EAWJs are primarily responsible for the change in rain belt orientations over East Asia and the northwestern Pacific (Horinouchi and Hayashi 2017). Several studies further attributed the occurrence of high-impact weather extremes at midlatitudes to an amplified waviness of the jet streams. Anomalous, persistent meandering of the jet streams over Eurasia favors the occurrence of heat waves and severe flooding in summer (Petoukhov et al. 2013; Coumou et al. 2014; Screen and Simmonds 2014; Nakamura and Huang 2018) and affects the wind, precipitation, and cold extremes over the North Atlantic and North Pacific in winter (Francis and Vavrus 2012, 2015). Notably, the link between jet waviness and the occurrence of weather extremes is stronger at the regional scale than the hemispheric scale and needs to be identified regionally by the sector, season, and type of extreme event (Francis and Vavrus 2015; Röthlisberger et al. 2016).
The physical mechanisms of the meandering jet streams involve many atmospheric processes and complex dynamic and thermodynamic interactions, such as those of the Hadley cell, El Niño–Southern Oscillation events, blocking weather systems, teleconnection patterns, and Rossby waveguide effects (Du et al. 2016; Barnes and Simpson 2017; Xue and Zhang 2017; Zhang et al. 2017; Hong and Lu 2016; Li et al. 2020). The waveguide effect of jet streams is particularly important to midlatitude atmospheric circulation and a stagnant weather pattern over East Asia and North Pacific (Branstator 2002; Ding and Wang 2007). Rossby wave trains propagating along jet streams can maintain a particular zonal pattern through their efficient extraction of kinetic and available potential energy from the background flow (Polvani and Waugh 2004). Jet streams are more zonal or small-amplitude fluctuating in summer than in winter and waveguides are thus oriented west to east and have the potential to become circumglobal wave trains in summer (Coumou et al. 2018). The circumglobal wave trains present several large trough–ridge systems that generate heatwaves and severe flooding events at midlatitudes (Schubert et al. 2011; Wang et al. 2013; Stadtherr et al. 2016; Kornhuber et al. 2017a). Wave resonance resulting from synoptic-scale waves trapped in the circumglobal waveguide in turn greatly increases jet stream waviness (Petoukhov et al. 2013, 2016; Coumou et al. 2014; Kornhuber et al. 2017b).
When there is a solid theoretical basis that variations in the EAWJ intimately covary with the location, orientation, and spatial pattern of rain belts over East Asia, understanding the relationships between the EAWJ’s zonal variations and the spatial features of rain belts in terms of the position, shape, orientation, and spatial scales is conducive to weather and climate predictions in East Asia. However, interactions of the South Asia high (SAH) at the 100-hPa level, the EAWJ at the 200-hPa level, the western Pacific subtropical high (WNPSH) at the 500-hPa level, and moisture transport in the lower atmosphere can produce notably different spatial configurations and rain belts within daily, weekly, monthly, and seasonal time scales (Sampe and Xie 2010; Horinouchi and Hayashi 2017). The spatial structures and scales of both the EAWJ and rain belts are thus difficult to measure on a rational spatiotemporal scale for different types of heavy precipitation event (Du et al. 2020). A low understanding of the link between zonal variations of the EAWJ and the spatial distribution of rain belts in East Asia can therefore be partially attributed to the great difficulty of quantifying the spatial structure of jet streams and rain belts for each heavy rainfall event (HRE). Applying a more quantitative method, effects of the EAWJ zonal variations on the position, orientation, and scale of rain belts were explored in this study. We first identified all heavy rainfall events lasting 3 days (3-day-HREs) and quantified their spatial scales using an improved rotating calipers algorithm developed by Du et al. (2020). A jet axis tracking solution was then proposed to track the EAWJ axes related to all 3-day-HREs in the period 1983–2020, and all the EAWJ axes were classified using a K-means clustering algorithm (Pham et al. 2005). Finally, a composite analysis was conducted to elucidate the effect of different types of the EAWJ on spatial scales and orientations of the 3-day-HRE rain belts, and possible physical mechanisms were discussed.
2. Data
A precipitation dataset estimated from remotely sensed information using artificial neural networks (PERSIANN) for 1983–2020 (Ashouri et al. 2015) was used to identify all 3-day-HREs and their spatial scales. Meteorological parameters of the wind speed, temperature, height, and specific humidity were obtained from the ERA5 global reanalysis project of the European Centre for Medium-Range Weather Forecasts (Hersbach et al. 2020) and adopted as primary data for diagnosing the mechanism of zonal variations of the EAWJ. All these long-term datasets with a fine resolution of approximately 0.25° × 0.25° were especially useful in exploring quantitatively the link between zonal variations of the EAWJ and the spatial scale of rain belts over East Asia. A dataset of the satellite-derived lower tropospheric temperature (Christy et al. 2011) was used to validate the mechanism of zonal variations of the EAWJ.
3. Methods
a. Quantifying spatial features of each HRE case
Under the atmospheric condition of a stationary front zone, intense moisture transport, and a strong WNPSH and EAWJ, the HREs generally last about 2–3 days and form continuous rain belts in East Asia (Guan et al. 2020), in which the rainfall area and volumetric rainfall amount enclosed by daily 50- or 100-mm isorainfall lines and the maximum daily precipitation are commonly used to assess the flood risks (Chen et al. 2017). Flood-related HREs are thus defined as the events lasting more than 3 days and having areas in which total rainfall amount is at least 50 mm exceeding 105 km2 over eastern China (Wang and Hu 1993). Notably, daily HREs resulting from mesoscale weather systems generally produce several scattered rainfall belts with an extent of about 150–400 km (Chen et al. 2013; Touma et al. 2018). Long-lived HREs can form large rain belts over East Asia but also experience notable atmospheric circulation changes during their life cycles, leading to strong differences in the spatial scale and pattern of rain belts (Ninomiya and Shibagaki 2007; Du et al. 2020). As an example, rain belts of a persistent HRE case on 4–12 July 2020 had notable differences in the extent, position, length, width, and orientation on 5–7 and 10–12 July (Fig. 1c). We further investigated the typical duration of HREs that had relatively stable rain belts with the continuous rainfall area exceeding 105 km2 in the East Asia monsoon region of 20°–50°N, 90°–125°E during 1983–2020. Each rainfall day belonging to persistent HREs in 1983–2020 was first identified by applying the 1-, 3-, and 5-day moving-window methods with a 1-day increment and a respective criterion of rainfall areas in which total rainfall in the 1-day and 3-day moving windows is at least 50 mm and total rainfall in the 5-day moving window is at least 100 mm exceeding 105 km2. Three datasets of persistent HREs were obtained after separately merging those continuous rainfall days for the 1-, 3-, and 5-day moving window methods. Results revealed that about more than 70% of persistent HREs had a duration of about 1–6 days (Fig. 1a). The sample distributions of HREs identified by the 3- and 5-day moving windows showed a clear peak at 3- and 4-day durations. Daily HREs were dominant for the 1-day moving method and their scattered rain belts led to great difficulty in quantifying the spatial structure of rain belts. The 3-day moving window method was thus applied to identify all 3-day-HREs in 1983–2020.
The spatial scales and orientation of each 3-day-HRE rain belt were quantified using a rotating calipers algorithm (Toussaint 1983; Du et al. 2020). A contour tracing algorithm developed by Wang (2012) was first applied to find all precipitation centers having a total rainfall amount exceeding 50 mm for each 3-day-HRE and their corresponding convex polygons were extracted. The rotating calipers algorithm was then applied to find the “minimum-area enclosing rectangle” of each convex polygon and its minimum rotating angle from the horizontal axis. Finally, the long and short sides, rotating angle, and 50-mm rainfall area in the minimum-area enclosing rectangle were used to represent the length, width, orientation, and extent of the rain belt of a 3-day-HRE. Two examples are shown in Fig. 1c. Considering large spatial differences in the rainfall intensity, a relatively smooth 50-mm rain belt axis was obtained in the minimum-area enclosing rectangle by averaging the latitudes or longitudes having rainfall amount above 50 mm along the rectangle long side.
b. Tracking the EAWJ axes
Jet streams are defined as the zonal wind maxima; a continuous jet core generally has one main jet axis and sometimes multiple branches. Here, a jet axis tracking solution was proposed to track the main jet axes and dominant branches for each EAWJ case related to the 3-day-HREs in 1983–2020. A contour tracing algorithm (Wang 2012) was first applied to find all jet cores with the wind speed increasing in intervals of 5 m s−1 from the minimum threshold of jet streams (25 m s−1) to the maximum wind speed over the Northern Hemisphere for a 3-day-HRE case (contours in Figs. 1b and 1d). Applying a rotating calipers algorithm (Toussaint 1983; Du et al. 2020), all minimum-area enclosing rectangles were then generated for each jet core at different wind speed thresholds (Figs. 1b and 1d). The main jet axis and dominant jet branches in a jet core was identified by the following algorithm.
The point at which the wind speed was a maximum in a minimum-area enclosing rectangle was identified as the starting point of a main jet axis. This axis was obtained by tracing the next maximum wind speed point at the adjacent grid points where the wind speed exceeded the wind speed threshold, until the last maximum wind speed point in the minimum-area enclosing rectangle reaching the jet core boundary (Fig. 1b) or the data domain boundaries (Fig. 1d). Multiple wind speed thresholds were used to identify all possible jet cores and might produce some overlapped jet axes in a jet core. The main jet axes (red lines in Figs. 1b and 1d) for an EAWJ case were thus obtained by separately merging those overlapped parts into one axis in each minimum-area enclosing rectangle.
The dominant jet branch was defined as an axis having one point on the minimum-area enclosing rectangle edges but not on the main jet axes. All intersections of a jet core and its minimum-area enclosing rectangle edges at a wind speed threshold—not on the main jet axes—were thus identified as the starting points of jet branches. Each jet branch was obtained by tracing the next maximum wind speed point at the adjacent grid points surrounding the starting point until the last maximum wind speed point having the wind speed below the minimum threshold (25 m s−1) or reaching the data domain boundaries. Jet branches were ultimately obtained by merging their overlapped parts into one axis and removing the overlapped parts with the main jet axes (light blue lines in Fig. 1d).
Figures 1b and 1d show two examples of tracking the EAWJ axes. The 3-day-HRE case on 5–7 July 2020 had a single jet core over East Asia and North Pacific (Fig. 1b), and four overlapping jet axes (green, orange, blue, and red lines) were identified from four minimum-area enclosing rectangles with wind speed thresholds of 30, 35, 40, and 45 m s−1. Ultimately, the red line having the longest length was identified as the EAWJ axis on 5–7 July 2020. Applying this algorithm, three main jet axes (red lines) and two dominant branches (light blue lines) were also successfully identified for the EAWJ case having three discontinuous jet cores on 3–5 September 2020 (Fig. 1d). The positions and separations of rain belts and the EAWJ were measured by using the average positions of each rain belt and its nearest axis among the main jet axes and branches for each 3-day-HRE case in 1983–2020.
c. K-means clustering of all EAWJ axes related to the 3-day-HREs
The main jet axes can well represent the zonal spatial structures of EAWJ without regard to the jet branches. Monthly composited main jet axes related to all 3-day-HREs in 1983–2020 showed a flat or longwave pattern in the zonal structure during April–September (Fig. 2a). A wavelet analyzing method developed by Torrence and Compo (1998) was used to analyze the spatial structure of each EAWJ axis, in which a jet axis having a statistically significant wavelet scale (0.01 significance level) within local longitudes was counted one case over these longitudes. The results revealed that approximately 90% of all main EAWJ axes related to the 3-day-HREs showed a statistically significant longwave structure with a wavelength about 6000–12 000 km (Fig. 2b). They had continuous jet cores at 90°E–180° and were conducive to quantifying the relationship between the EAWJ’s zonal variations and rainfall spatial distribution by using the positions and separations of rain belts and the EAWJ axes.
(a) Sample distributions of HREs identified by the 1-, 3-, and 5-day moving-window algorithms, respectively, examples of tracing the (b) continuous and (d) discontinuous jet axes, and (c) characterizing the orientation, position, and spatial scales of rain belts for two 3-day-HRE cases on 5–7 and 10–12 July 2020. The numbers in the parentheses on the y axis of (a) indicate the sample size of HREs identified by the 1-day moving window.
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
(a) Monthly composited jet axes related to 3-day-HREs from April to September, (b) the sample proportion having a 0.01 significance level wave structure at different longitudes and wavelength scales in all EAWJ axes, (c) the similarity of JetAxis1 on 15–17 July 2014 and JetAxis2 on 6–8 August 2020 over 90°E–180°, and (d) estimation of the optimal K value for the EAWJ’s K-means clustering.
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
4. Influence of jet zonal variation on rainfall patterns and frequency
a. Classifications of the EAWJ’s zonal structure
A K-means clustering analysis on the EAWJ’s axes related to 3-day-HREs indicated that they showed a tilted pattern (Fig. 3a), wavy pattern (Fig. 3b), and relatively flat pattern (Fig. 3c) over eastern China and the North Pacific during April–May, June–July, and August–September, respectively. The F-test variance analysis for meridional locations of the composited jet axes further confirmed the statistically significant difference in the zonal structure of three EAWJ patterns for April–May, June–July, and August–September (0.05 significance level). The tilted pattern had a largely similar zonal structure with a northeast–southwest orientation over the western North Pacific and a relatively flat structure with an east–west orientation over the Asian continent throughout April–September. A shallow trough was located on the East Asian coastline around 120°E. The relatively flat pattern featured a straight zonal structure over the East Asian continent and a small-amplitude wave over the North Pacific. The wavy pattern had a largely constant wavy structure over from East China to the western North Pacific in June–July and August–September, with there being two troughs on the eastern coastline of China and areas near 170°E. The wavy pattern in April–May had a zonal structure that was the reverse of that in June–July and August–September.
The composited jet axis for (a) tilted, (b) flat, and (c) wavy EAWJs related to the 3-day-HREs in April–May, June–July, and August–September, respectively, and (d)–(l) all jet axes of each EAWJ type (brown lines), the composited rain belt axis (thick black lines), and the precipitation proportion (shading) by longitude accounting for regional total rainfall amount of 3-day-HREs in April–May, June–July, and August–September.
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
More importantly, the tilted and wavy EAWJs had a large fluctuating amplitude about more than 1000 km in the zonal structure over from the East Asia to the western North Pacific (Figs. 3a–c) and strongly affected the precipitation distribution (shading in Figs. 3d–l) and the orientation of rain belts (thick black lines in Figs. 3d–l). The average position of rain belts had an orientation or a wavy structure similar to the composited jet axis at 125°E–180° for all three types of the EAWJ. Enhanced precipitation was steadily located over southern Japan at 125°–135°E during April–September, where rainfall amount in three days exceeding 50 mm at each longitude accounted for more than 3% of the regional total rainfall amount of all 50-mm rain belts in the region of 100°E–180°. The precipitation proportion by longitude accounting for the regional total rainfall amount had a strong seasonal change in the range of 1%–3% at for all three types of the EAWJ over eastern China at 100°–125°E. Relatively weak precipitation was located over the central North Pacific at 135°E–180°.
Although three types of the EAWJ had comparable occurrences accounting for 35.6%, 27.3%, and 37.1% of all cases throughout April–September, they had strongly varying occurrences within April–May, June–July, and August–September separately. Tilted and flat patterns were dominant in April–May and June–July, and flat and wavy patterns were dominant in August–September (Figs. 3a–c), mainly because of the strong seasonal evolution of the EAWJ intensity, extent, and shape during April–September. The largest difference occurred in June–July, reaching about 228 cases. Among all cases of each EAWJ type, jet axes had a similar zonal structure but varying meridional locations due to the seasonal evolution of the EAWJ’s meridional locations in East Asia and North Pacific (brown lines in Figs. 3d–l). It is necessary to evaluate the impact of the EAWJ’s zonal spatial structures and meridional displacements on the spatial scales, position, and orientation of rain belts. In addition, three types of the EAWJ alternately occurred in both April–May and June–July for 1983–2020 and were long-lived in August–September (figure omitted). Each type of the EAWJ had a short duration of approximately 3 days in April–May and approximately 5 days in June–July and a long duration of approximately 9 days in August–September. Therefore, comparing with those in April–May and June–July, the EAWJ’s zonal spatial structure persistently remained a particular type during August–September.
b. Spatial scales of the 3-day-HRE rain belts
Applying the methods described in section 3a, we obtained 4285 cases of 3-day-HREs for the period 1983–2020, among which there were 12 952 samples for 50-mm rain belts. Most of the 3-day-HREs were concentrated in the primary rainy season from June to September, accounting for 72.2% of all cases (Table 1). Occurrences of the 3-day-HREs classified by the rain belt orientation showed a quasi-normal distribution (Fig. 4a), and the mean length, width, and extent of the 50-mm rain belts of those subsamples are shown in Fig. 4b. Sample distributions of the 3-day-HREs had a clear peak at a rain belt orientation of approximately 15.0° (Fig. 4a), where the occurrences, length, width, and extent of rain belts were approximately 1233 occurrences, 1628.0 km, 578.5 km, and 58.7 × 104 km2, respectively (Fig. 4b). The length, width, and extent of all 50-mm rain belts in 1983–2020 respectively averaged 1250 km, 540 km, and 41.9 × 104 km2 (Table 1), which were less than those for samples having a peak occurrence (Figs. 4a,b). The 3-day-HREs had a strongly varying length scale of approximately 1000–1800 km and a stable width of approximately 540 km during the annual cycle, with there being notably different length scales in the orientations from −15° to 15°, from ±15° to ±45°, and from ±45° to ±90° (Figs. 4b and 4d).
(a),(c) Occurrences, and (b),(d) the length and width (km) and the rain extent (104 km2) of rain belts for all 3-day-HREs varying with (top) the rain belt orientation and (bottom) the seasons.
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
Statistical characteristics of spatial scales of different types of 3-day-HREs in 1983–2020.
We classified all 3-day-HREs as having a zonal pattern (from −15° to 15°), tilted pattern (from ±15° to ±45°), or meridional pattern (from ±45° to ±90°) (Figs. 4a,b). Overall, 65% of the 3-day-HREs had tilted and meridional rain belts. Approximately 82.8% of zonal 3-day-HREs and approximately 83.6% of tilted 3-day-HREs occurred in the period 91–270 days (May–September), with the largest occurrence being at least 300 in the period 160–270 days for zonal 3-day-HREs and in the period 160–250 days (June–July) for tilted 3-day-HREs (Fig. 4c). Most of the meridional patterns were concentrated in the period 191–250 days (August–September) with a slight right skew and had a largest occurrence of approximately 250 in the period 210–220 days. The left-skewed distribution of the zonal 3-day-HREs, right-skewed distribution of the meridional 3-day-HREs, and normal distribution of the tilted 3-day-HREs formed a bimodal pattern for all 3-day-HREs (Fig. 4c). Zonal and tilted 3-day-HREs had a compatible mean length of approximately 1300 km in April–May, June–July, and August–September; this length is longer than that for meridional 3-day-HREs by approximately 300–500 km (Table 1). Three 3-day-HRE types divided by the orientation still had a stable width of ∼540 km.
c. Relationship between rain belts and different types of the EAWJ
The relationship of rain belts and the EAWJ was measured by the rain belt axes and their nearest jet axes. Although the EAWJ had a notable difference in the meridional position among all cases of each type (Figs. 3d–l), Fig. 5 shows that the change in the average distance between rain belts and jet axes with longitude was stable for all types of the EAWJ in April–May, June–July, and August–September, with the exception of the tilted EAWJ in April–May. Statistical methods of the mean t test and the variance F test for the distances between rain belts and jet axes varying with the longitudes also revealed the statistically significant relationship between rain belts and jet axes at 100°–150°E (0.05 significance level); that, is the average distance between jet axes and rain belts decreased gradually from 1500 to 500 km at 105°–125°E and remained steady at approximately 500 km at 125°E–180° for all types of the EAWJ (Figs. 5a–c). The rain belts thus covaried with the spatial displacements and orientation of the EAWJ and mainly located about 500 km to the south of the jet axis over eastern China and the North Pacific. Both the similar orientation and small separation of jet axes and rain belts indicate that the EAWJ directly affects the position and orientation of rain belts at 125°E–180°. Rain belts can occasionally extend northward into the jet stream at 150°E–180° for the wavy EAWJs in April–May and August–September.
(a)–(c) Variation in the distance between 50-mm rain belts and the EAWJ axes and (d)–(f) the occurrence of 50-mm rain belts with the longitude in April–May, June–July, and August–September.
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
Particularly, the F-test variance analysis for the rainfall frequency of 3-day-HREs varying with the longitudes showed a significant difference among three EAWJ types in April–May and June–July and little difference in August–September. Comparing with the wavy EAWJ pattern, the tilted and flat EAWJ patterns having a larger sample size in April–May and June–July also were associated with more occurrences of the 50-mm rain belt at 100°–145°E (Figs. 5d,e). Rain belts had a strong varying maximum occurrence of approximately 152–543 in April–May, June–July, and August–September and were mainly located at 100°–145°E (Figs. 5d–f). The two regions of 110°–125°E and 125°–135°E received the most rainfall, with rain belts maintaining a high occurrence exceeding 300 in April–May and June–July (Figs. 5d,e). Correspondingly, the occurrence of rain belts had two peaks at 110°–125°E and 125°–135°E in April–May and June–July and a single peak at 125°–135°E in August–September. Over the East Asian continent around 110°–125°E, the rain belts had a lower peak of occurrence of approximately 136–313 in April–May, a higher peak of approximately 268–510 in June–July, and no peak in August–September. However, there was a steady peak at approximately 125°–135°E for the three types of EAWJ in April–May, June–July, and August–September. In short, the 3-day-HREs mainly occurred in June–July with the rain belts mainly locating over southern Japan and eastern China, and were associated with tilted and flat patterns of the EAWJ.
5. Possible mechanism of the EAWJ affecting rainfall spatial patterns
a. Linkage among the EAWJ’s zonal variation, rainfall distribution, and the WNPSH
A composite analysis revealed that the tilted, wavy, and relatively flat EAWJs had a continuous jet core over East Asia and North Pacific and showed a spatial structure similar to that of the WNPSH north flank over the North Pacific at 110°–200°E, with a 1000-km-width intersection zone (Fig. 6). The composited rain belt having a rainfall amount exceeding 25 mm for each EAWJ type was sandwiched by the jet axis and the WNPSH and therefore had a similar orientation to the EAWJ and the WNPSH in April–May, June–July, and August–September. Interactions between the EAWJ and WNPSH in each 3-day-HRE might contribute to the spatiotemporal distribution of rainfall over East Asia and the North Pacific.
A composite analysis of the zonal wind speed (solid black lines) and jet axis (thick black line) at the 200-hPa level, geopotential height (dashed black lines with the interval of 20 gpm), vertical velocity ω (shading), and Q vector (vectors) at the 500-hPa level, and the averaged rain belts of rainfall amount being more than 25 mm (green dots and lines) in (a)–(c) April–May, (d)–(f) June–July, and (g)–(i) August–September.
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
Since the vertical velocity is generally close to zero near the tropopause, upper-level horizontal divergence is well associated with the patterns of vertical motion in the middle troposphere. As shown in Fig. 6, coupling between the EAWJ and WNPSH provided a synoptic-scale atmospheric condition conducive to development of the rising motion between the jet stream equatorward and the WNPSH poleward, with strong 500-hPa vertical velocity (ω) below −0.2 Pa s−1. Strong rising motions and rain belts around the western WNPSH were anchored by the EAWJ over from the Bay of Bengal and South China Sea to the North Pacific, particularly being closely related to the EAWJ zonal patterns over the North Pacific. As a result, from over eastern China to the North Pacific, both rainfall and strong rising motions were sandwiched by the EAWJ and WNPSH, spatially limited in the dynamic uplift regions near the jet stream and parallel with the jet axis, meanwhile without rainfall and strong rising motion over the downstream of jet cores. Hence, the dynamical forcing of EAWJ can be attributed to forming a wavy or straight rain belt over from eastern China to the North Pacific through anchoring and shaping the northern border of rain belts and strong rising motions. In addition, it was difficult to quantify the WNPSH boundaries due to strong changes in the intensity and shape of geopotential heights among different EAWJ types, whereas the EAWJ showed a clear jet axis that was well represented the spatial structure of jet streams (Fig. 6). It was easier to quantify the spatial relationship between jet streams and rain belts than that between the WNPSH and rain belts.
Strong rising motions were also observed north of the EAWJ over the East Asian continent but without notable precipitation (Fig. 6). Therefore, intense vapor transport from ocean was important to the large-scale rainfall. We further explored the physical mechanisms of the EAWJ affecting the vapor transport. Three long-lived cases, namely a flat pattern on 3–25 May 2014, a wavy pattern on 13–24 July 2015, and a tilted pattern on 7–21 September 2014, had zonal structures most similar to those of their clustering centers and were used to explore interactions of the EAWJ and the WNPSH and their effects on both the vapor transporting trajectory and the rainfall spatial distribution.
Consistent with the geostrophic balance, jet streams were stronger where the horizontal gradient of the geopotential height was stronger at midlatitudes (Figs. 7a–c). A narrow and elongated horizontal corridor of intense vapor transport surrounding the WNPSH played a crucial role in determining the 3-day-HRE spatial distribution and scale and had a spatial structure similar to that of the jet stream over East Asia and the North Pacific. A high-amplitude trough–ridge system along the upper-level jet stream appeared over the western North Pacific (Figs. 7a and 7c) and East Asian continent (Fig. 7b) and obviously affected the trajectory of intense vapor transport from East Asia to the North Pacific. We followed Ralph et al. (2019) and calculated the integrated vapor transport (IVT) as
(a)–(c) Wind speed exceeding 25 m s−1 at the 200-hPa level (purple contours), 500-hPa geopotential height (blue contours; gpm), integrated vapor transport vectors, and their total amounts (shading) and (d)–(f) the pressure–longitude section of the anomalous geopotential height (black contours), vertical velocity (blue contours; 10−2 Pa s−1), and vapor fluxes (green contours) for the flat, wavy, and tilted patterns of the EAWJ: 25°–35°N in (d), 30°–60°N in (e), and 30°–50°N in (f).
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
The climate of the Northern Hemisphere generally has pronounced zonal asymmetry in the spatial pattern of planetary-scale Rossby waves arising from the land–sea thermal contrast, atmospheric diabatic heating, and topography (Wills et al. 2019). Interactions between the EAWJ and low-level circulation strongly affected the vapor transport through a Rossby wave train propagating along the jet stream at midlatitudes (Figs. 7d–f). Geopotential height anomalies occurred coherently in the middle-to-upper troposphere over Eurasia, and a large-amplitude negative anomaly extended downward to the lower troposphere over the western North Pacific. The Rossby wave train, depicted by the geopotential height anomalies in Figs. 7d–f, is an important external driver of the EAWJ and the East Asian monsoon embedded in the jet streams at 30°–60°N because teleconnections and jet variability are physically related. A strong geopotential height anomaly along the strong westerly winds (above 20 m s−1) appeared in the middle and upper troposphere (500–50 hPa) over the Eurasian continent and reached the lower troposphere over the North Pacific (Figs. 7d–f). The wave train had an equivalent barotropic structure with circulation anomalies reaching the lower troposphere and affected the spatial structure of the WNPSH and the vapor transport over the North Pacific through the strong waveguide effect of jet streams. High-amplitude negative anomalies of the geopotential height at the EAWJ troughs weakened or blocked vapor transport along the northern flank of the WNPSH (shading in Figs. 7a–c and green contours in Figs. 7d and 7e) and ultimately affected the spatial scale and orientation of rain belts. In addition, strong rising motions were dominant under the EAWJ and showed a length scale similar to the vapor transporting trajectory.
b. Thermal and dynamical forcing of zonal variations of the EAWJ
The zonal spatial pattern of jet streams is most likely affected by large-scale planetary waves forced by the orography, land–sea thermal contrast, and atmospheric activities at middle and high latitudes. The composite analysis of tilted, wavy, and relatively flat EAWJs suggests a complex interaction between the EAWJ and WNPSH. A strong southerly component along the WNPSH’s northwestern flank favors the strengthening of upper-level jet streams. When the subtropical high belt abruptly shifted northward around 50°N over the Middle East, the upper-level jet stream also shifted northward but remained above the northern flank of the subtropical high pressure (Fig. 7b). Spatial patterns of the EAWJ thus might covary with the zonal variations of the subtropical high pressure belt over the Eurasian continent and North Pacific. A better characterization of the upper-level jet streams is needed to understand the connection of the meandering EAWJ with the local circulation that responds to the zonal variation in atmospheric circulations at low and high latitudes. The East Asian monsoon has been considered to be a giant sea breeze driven by the land–ocean heating contrast (Kucharski et al. 2011). The distribution of the zonal temperature anomalies (shading in Figs. 8a–c) had a much warmer tropospheric atmosphere over the Eurasian continent than over the North Pacific. Corresponding to the vertical thermal structure, the zonal geopotential height anomalies were characterized by a baroclinic vertical structure with an upper-level low and a lower-level high over ocean and an upper-level high and a lower-level low over the Eurasian continent. The lower-level high over the North Pacific reflected the WNPSH whereas the upper-level high indicated the SAH. The land–ocean heating contrast produced two opposite vertical circulations over the Eurasian continent and North Pacific (Figs. 8a–c), which excited a wavenumber-1 planetary-scale stationary wave at lower latitudes (10°–30°N). Small geopotential height anomalies show that the effect of dynamics was small compared with the thermodynamic effect of meridional temperature gradients at lower latitudes (10°–30°N). The largest-scale waves throughout the troposphere with zonal wavenumber 1 primarily arising from the land–ocean heating contrast provided an ultralong wave background for upper-level jet streams at the midlatitudes.
(a)–(f) Pressure–longitude section of the anomalous geopotential height (black contours), vertical velocity (blue contours; 10−2 Pa s−1), vapor fluxes (green contours), temperature anomaly (red-blue shading), and wind speed (red shading) for three typical cases of the EAWJ: 10°–20°N in (a), 20°–30°N in (b) and (c), 40°–60°N in (d), and 45°–65°N in (e) and (f).
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
The amplitude of the dynamically forced Rossby waves at mid- to high latitudes was notably stronger than that of thermally forced Rossby waves at lower latitudes (Figs. 8d–f). Large geopotential height anomalies throughout the troposphere showed a strong Rossby wave train with wavenumber K = 2–3. This Rossby wave train had a phase structure different from that at lower latitudes. The spatial structure of upper-level jet streams depended on the zonal phase structure of the stationary waves over their southern and northern sides. More different or similar phase structures of planetary waves between lower and middle latitudes produced meandering or relatively flat jet streams at midlatitudes. Figures 7 and 8 show that relatively flat jet streams were consistent with a similar phase of Rossby waves at lower and middle latitudes. Zonal asymmetry in the zonal phase structure of Rossby waves at lower and high latitudes (Figs. 8b and 8e) contributed to a strong meandering jet stream (Fig. 7e). It is therefore possible to characterize the momentum fluxes from high latitudes and the thermodynamic influence from lower latitudes as being responsible for the zonally spatial pattern of upper-level jet streams. In addition, both dynamical forcing and thermodynamic forcing on the Tibetan Plateau and Mongolian Plateau play important roles in strengthening or inducing a meandering EAWJ (Shi et al. 2015).
c. Contributions of the vertical thermal anomalies
Atmospheric thermal anomalies play an important role in forming the meandering jet streams at midlatitudes because the jet streams are driven directly by the meridional temperature gradient. In terms of the pressure vertical coordinate P, the vertical shear in the zonal wind,
Compared with the situation at high latitudes, the weak and small-scale thermal anomalies at midlatitudes were associated with a flat jet stream having a small wavy amplitude over East Asia (Fig. 9a). Particularly, jet streams were prone to meandering when embedded with strong cold and warm anomalies (Figs. 9b and 9c), with zonal wavenumbers varying from 2 to 4 over the Middle East and North Pacific. Strong and large-scale thermal anomalies exceeding ±2.0 × 107 J m−2 at midlatitudes amplified the two large-scale trough–ridge systems over the Tibetan Plateau, Mongolian Plateau, and western North Pacific (Figs. 9b and 9c). There was a persistent large-amplitude trough–ridge system over the western North Pacific owing to the dynamic blocking of the WNPSH. Reverse signs of the atmospheric thermal anomalies corresponded to the reverse phase structures of jet streams (Figs. 9c,d and 9e,f). Additionally, the relationship between jet streams and atmospheric thermal anomalies were shown by the satellite-retrieved lower tropospheric temperature anomalies, as presented by purple lines in Figs. 9a–c.
(a)–(c) Spatial distribution of the total thermal anomaly (red–blue shading), satellite-retrieved lower-tropospheric temperature anomaly (purple contours), and total rainfall amount (green shading), and (d)–(f) 500-hPa geopotential height (blue contours; gpm), maximum integrated vapor transport vectors and their total amounts (red shading). Black lines in all panels represented the jet axes.
Citation: Journal of Climate 35, 20; 10.1175/JCLI-D-21-0872.1
When the upper-level jet streams were situated directly above the WNPSH over the western North Pacific, the wave trough or ridge of jet streams combined with the WNPSH directly affected the position, shape, and orientation of intense vapor transport over East Asia and the western North Pacific (Figs. 9d–f). The upper-level jet streams were generally far from the WNPSH over the East Asian continent, where the western WNPSH dominantly steered the position and orientation of the vapor transporting trajectory. When the large trough of jet streams was located over the North Pacific around 140°E and the WNPSH extended westward to southern China, two strong vapor transporting cores appeared over the East Asian continent and the North Pacific along the western and northern flanks of the WNPSH (Figs. 9d,e). Therefore, the EAWJ affects the position and orientation of rain belts over the East Asian continent when the deep trough of jet streams is located there. Relatively flat jet streams and the northern flank of the WNPSH on 27–30 April 1991 were accompanied by relatively continuous vapor transporting trajectory over East Asia and the western North Pacific (Fig. 9d).
6. Conclusions
Analysis of the duration of HREs using a moving window algorithm revealed that the HREs typically last 3 days and can produce relatively continuous rain belts with the 50-mm total rainfall area exceeding 105 km2 over the East Asian monsoon region in April–September. Coupling between the EAWJ and WNPSH represents a synoptic-scale atmospheric condition conducive to development of the rising motion over between the jet stream equatorward and the WNPSH poleward, which is mainly responsible for the strongly varying spatial scale, shape, and orientation of rain belts of the 3-day-HREs over from eastern China to the North Pacific during 1983–2020. A similar banded structure of both the rising motions and the rain belt having rainfall amount exceeding 25 mm was sandwiched by the EAWJ and the WNPSH, and was paralleled with the jet axis over from eastern China to the North Pacific in April–May, June–July, and August–September. We proposed a method of tracking jet axes that had good adaptability to the continuous, discontinuous, and multibranch jet streams. Combined with a rotating calipers algorithm, wavelet analysis, and K-means clustering algorithm, spatial structures of both the EAWJ and rain belts were further quantified, and the influence of zonal variation of the EAWJ on rainfall patterns and frequency of the 3-day-HREs over East Asia was also investigated. The results revealed that more than approximately 90% of the EAWJ axes had a statistically significant zonal structure with a wavelength of approximately 6000–12 000 km over the midlatitudes in 1983–2020 and featured a tilted, wavy, or relatively flat pattern over East Asia and the North Pacific. Three patterns of the EAWJ alternately appeared in April–May and June–July and lasted a short duration of approximately 3–5 days, and had a longer duration of approximately 9 days in August–September. Correspondingly, the 50-mm rain belt of 3-day-HREs also had a zonal pattern, tilted pattern, and meridional pattern during April–September, with a strongly varying length scale of approximately 1310, 1290, and 1030 km, respectively, and a relatively stable width of approximately 540 km. Rain belts of the 3-day-HREs frequently appeared over eastern China and southern Japan and stably located at 500 km to the south of the jet axes although all EAWJ types have strong seasonal variations in spatial position and orientation. When the rainfall and associated latent heating uplift were spatially limited in the dynamic uplift regions near the jet stream, the dynamical forcing of EAWJ can be attributed to forming a wavy or straight rain belt over East Asia and the North Pacific.
The intense vapor transporting trajectory with the IVT exceeding 250 kg m−1 s−1 had a length scale comparable to the rain belt of 3-day-HREs, and was intimately linked to the EAWJ’s zonally variations in the orientation and spatial scales over East Asia and the North Pacific. Pronounced zonal asymmetry in spatial patterns of planetary-scale Rossby waves arising from the land–sea thermal contrast, atmospheric diabatic heating, and topography dominantly contributed to the meandering spatial structure of the EAWJ. The largest-scale waves throughout the troposphere primarily arising from the land–ocean heating contrast provided an ultralong wave background for upper-level jet streams over the Eurasian continent and North Pacific. Large geopotential height anomalies throughout the troposphere showed a strong Rossby wave train on the northern side of jet streams at midlatitudes. The EAWJ spatial structure thus depended on the propagation pathway and phase structure of Rossby waves over both southern and northern sides of jet streams. More different or similar phase structures of planetary waves between lower and middle latitudes produced meandering or relatively flat jet streams at midlatitudes. Atmospheric thermal anomalies were ultimately responsible for the baroclinic Rossby waves that modified the spatial structure of jet streams. A zonally oscillating trough–ridge system, featuring an equivalent barotropic structure with strong negative anomalies of geopotential height reaching the lower troposphere, can weaken or block vapor transport along the WNPSH’s northern flank, and is ultimately responsible for the strongly varying spatial scale, shape, and orientation of rain belts over East Asia and the North Pacific.
Acknowledgments.
This work was supported by the National Natural Science Foundation of China (Grants 42075027 and 41930969), National Key Research and Development Program (2020YFA0608901), and National Basic Research Program of China (973 Program) (Grant 2015CB453200).
Data availability statement.
The climate data record of the precipitation (PERSIANN), daily outgoing longwave radiation, and satellite-derived lower tropospheric temperatures are freely available from the National Centers for Environmental Information (https://www.ncei.noaa.gov/products/climate-data-records). The ERA5 global reanalysis data are available from the European Centre for Medium Range Weather Forecasts (https://cds.climate.copernicus.eu/).
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