Intra-seasonal Features of an Extreme High Temperature Event in 2011 in Eastern China and Its Atmospheric Circulation

Under the background of global warming, extreme high temperature events (EHTEs) occur more frequently than before, and pose widespread risks as they not only affect public health and ecosystems directly but also damage infrastructure and other societal services. The EHTEs have led to dramatic impacts on human life, agriculture, and water resources (Sun et al. ^[1-2]), and thus receive extensive attention.

In recent decades, studies on the changes and causes of extreme temperature events have been carried out (Ding et al. ^[3]; Sun et al. ^[4]). However, many studies concentrate on the relation between large-scale circulation and EHETs, including South Asia high, Western Pacific subtropical high (WPSH) as well as its ridge line (Liu et al. ^[5]; Li et al. ^[6]). The teleconnection wave train in the westerly jet also has an important effect on extreme high temperature (Li et al. ^[7]; Gao and You ^[8]). However, most of previous studies focused on interdecadal and interannual variation of EHTEs, while the research on intra-seasonal scale and extended period forecast was relatively meager.

The Intra-seasonal Oscillation (ISO) is closely related to weather and climate. It was first discovered in the tropics by Madden and Julian with an analysis of wind fields which displayed a slow eastward propagation with a significant 40-50-day cycle; this eastward propagating fluctuation usually was named as Madden-Julian Oscillation (MJO) ^[9-10]. Subsequently, a large number of studies further revealed its propagation characteristics (Wang and Rui ^[11]; Matthews ^[12]) and initiation mechanisms (Jiang and Li ^[13]; Feng et al. ^[14]). It also plays a profound role in the formation and movement of tropical cyclones (Liebmann et al. ^[15]; Zhu et al. ^[16]), ENSO activities (Chen et al. ^[17]; Yan et al. ^[18]), and the onsets of summer monsoon (Pai et al. ^[19]; Wang et al. ^[20]). Anderson and Rosen have further demonstrated that the ISO exists not only in the tropics but also in the mid-high latitudes ^[21]. Different from the tropical ISO, intra-seasonal signals at mid-high latitudes are usually called Intra-seasonal Variability (ISV). The tropical and mid-latitude forcing on the ISV of eastern Asia and their seasonal evolution have been well studied in recent works (Yang et al. ^[22]; Liu et al. ^[23]).

A major predictability source for extended-range forecast is ISO/ISV. Based on the relationship between weather systems (i.e., temperature and precipitation) and intra-seasonal signals, many studies have established dynamic and statistical models suitable for different predictands, which have certain indications for the extended-range forecast (Yang ^[24-25]). Hsu et al. proposed an extended-range forecast model based on the spatial-temporal projection method ^[26]. Unlike traditional statistical forecasting methods, the spatialtemporal projection model (STPM) adopts the spatialtemporal-coupled evolving pattern of predictors and predictand for modeling, effectively improving the subseasonal forecasting skills of precipitation (Zhu and Li ^[27]; Ma et al. ^[28]), and extreme temperature events (Zhu and Li ^[29-30]).

In recent years, heat waves have occurred frequently in eastern China, most of which are related to the ISO/ISV. Low-frequency anticyclonic anomaly in the lower troposphere and local strong subsidence are main reasons for high temperature events in the MYR (Hsu et al. ^[31]; Gao et al. ^[32]). In general, few studies have focused on the influences of the ISO/ISV on high temperature events, especially the persistent EHTEs in China. Therefore, this paper intends to select typical events to analyze the intra-seasonal characteristics of EHTEs. On the basis of clarifying the relationship between ISO/ISV and EHTEs, the present study strives to understand the mechanism in which the ISO/ISV modulates the EHTEs in summer and thus better apply it to extended-range forecast.

The rest of this paper is organized as follows. Section 2 describes the data, definitions, and methods used in this paper. Section 3 reveals the low-frequency characteristics of the temperature in the MYR of 2011. Section 4 analyzes the ISO characteristics and the evolution of circulation during the EHTE. The thermodynamic diagnosis of high temperature process is discussed in Section 5. Section 6 further discusses the influence of diabatic heating on intra-seasonal variation of subtropical high position. The final section presents the discussion and conclusions.

The daily T_max dataset with a horizonal resolution of 0.5° × 0.5° provided by the China Meteorological Data Network are used in the present study, which are obtained from gauge observations recorded at 2472 stations in China. The reanalysis data used for this study consist of daily interpolated outgoing longwave radiation (OLR) from the National Oceanic and Atmospheric Administration (NOAA) (Liebmann and Smith ^[33]) and several meteorological fields generated by National Centers for Environment Prediction/Department of Energy (NCEP/DOE) (Kanamitsu et al. ^[34]), both of which have a spatial resolution of 2.5° × 2.5°. Three-dimensional variables include temperature, geopotential height, zonal wind, meridional wind, and vertical p-velocity at 17 pressure levels (1000-10hPa). The analysis focuses on extended summer season (i. e., from May to September) for the period of 1980-2018.

The relative threshold is applied to identify the EHTE. The T_max from 1980 to 2018 is arranged in ascending order each year separately and then the 85th percentile of the T_max is obtained as the extreme high temperature threshold value of the grid point in the year. Two criteria are used to select the EHTEs that are related to the ISO:

(1) Apply the standard deviation (STD) to the areaaveraged band-pass-filtered T_max, and the years with standardized STD exceeding + 1 is defined as lowfrequency high temperature year;

(2) In the year that meets (1), a persistent high temperature event is defined when area-averaged daily T_max is greater than area-averaged threshold for five consecutive days or more (with a maximum interval of one day). The persistent high temperature event with the maximum amplitude and both the minimum amplitudes exceeding 0.5 standard deviation in the time series of area-averaged band-pass-filtered T_max is recorded as a high temperature event related to the intra-seasonal oscillation.

To identify the dominant ISO periods, power spectrum analysis is applied to the time series of T_max over the core region from May to September each year. Based on the result of spectrum analysis, two steps are used to derive the intra-seasonal signals. Firstly, the first three harmonics of daily climatology and synoptic fluctuations (by taking a 5-day running mean) are removed from the raw data. Secondly, a 10-25-day Lanczos band-pass filtering is applied on the so-derived anomaly fields.

The thermodynamic equation is used to investigate the factors that cause the local change of temperature, and the equation can be written as follows:

where all symbols follow convention in meteorology. In Eq. (1), each variable can be decomposed into the following components:

where an overbar represents the background field component (> 25d), a single prime represents the low frequency component (10-25d), and a double prime represents the synoptic scale component (< 10d). Therefore, adiabatic change term can be decomposed into nine terms.

Finally, to elucidate the influence of diabatic heating on the position change of the WPSH in the case of EHTE, the complete form of vertical vorticity tendency equation is analyzed.

In this paper, the middle reaches of the Yangtze River (MYR; 26°-33°N, 106°-118°E) is selected as the key area to study EHTEs. The STD of non-filtered T_max in the extended summer season of 1980-2018 is mainly located in the area to the north of 45° N and the MYR area (not shown). The three of China's four "furnaces cities" (cities with especially hot and oppressively humid summer) are located in the MYR. An areaaveraged daily T_max is then subject to a power spectrum analysis. Two statistically significant spectrum peaks at the 10-25-day and 25-40-day intraseasonal time scale are shown in Fig. 1a. Fig. 1b illustrates the STD of 10-25-day filtered T_max; note that the low-frequency STD in most areas of the MYR is above 2℃. It is worth mentioning that the regional averaged intraseasonal temperature variability explains about 44.3% of total variability, which indicates that the extreme high temperature in this area is largely regulated by the 10-25- day intra-seasonal signal.

Figure 1.  (a) Summer mean power spectra of daily T_max over the MYR (solid black line) during 1980-2018. The dashed red line represents the 95% significant level of red noise spectrum. (b) Standard deviation of 10-25-day filtered T_max (units: ℃) in summer (May to September). The box denotes the key region MYR (26°-33°N, 106°-118°E).

According to the method in Section 2, 1988, 2011 and 2013 are obtained as low-frequency high temperature years. Note that the largest positive anomaly occurs in 2011. To clearly investigate the intra-seasonal characteristics of extreme high temperature, 2011 is selected for analysis.

Figure 2 shows the T_max minus threshold and standardized 10-25-day filtered T_max averaged over the MYR in the summer in 2011, it can be found that the daily T_max has obvious intra-seasonal variation features. There is a good consistency between hot days and the rise of the low-frequency temperature series. The heating process occurs in the transition stage of the lowfrequency sequence from negative to positive. Given the influence of ISO/ISV, the obvious high temperature processes include 20-31 July and 9-21 August. This paper will mainly focus on the EHTE in August.

Figure 2.  Time series of T_max minus threshold (blue bar, units: ℃) with values in the left y axis and standardized 10-25-day filtered T_max (red line) with values in the right y axis averaged over the MYR during the summer in 2011. Dashed line represents 0.5 standard deviation.

Day 0 represents the day when the T_max reaches its maximum over the MYR, and day-n (n) represents the day before (after) day 0. The evolution of low frequency circulation at different levels during the EHTE of August 9 to 21 is further analyzed.

Figure 3 (a-f) displays the evolution of the low-frequency geopotential height fields at 200hPa and temperature fields at 925hPa. It can be seen that there is a good correspondence between the temperature anomalies in the lower troposphere and geopotential height anomalies over Eurasia; the heating (cooling) in the lower troposphere could induce an expansion (contraction) of the air column between two constant pressure levels, which is favorable for the positive (negative) geopotential height anomaly in the upper-level.

Figure 3.  Temporal evolution of 10-25-day filtered (a-f) 925hPa temperature (shading; units: ℃) and 200hPa geopotential height (contour; units: gpm), and (g-l) 500hPa winds (vector; units: m s^-1). Shadings in the right panel indicate 500hPa apparent heat source Q₁ (units: W kg^-1). Yellow contours in the right panel are 500hPa geopotential height by 5880 gpm that represent the WPSH location.

On day-9 (Fig. 3a), a wave train is clearly seen over mid-latitudes at 200hPa, with an alternative positive and negative geopotential height anomalies pattern, extending from the Mediterranean Sea to central China with a negative geopotential height anomaly locating in the Okhotsk Sea, and a cold anomaly and negative geopotential height anomaly in the core region. On day-6 (Fig. 3b), there is a wave train in the highlatitudes to the north of 70°N, which has a zonal wave number of 1, and the distribution can be maintained until day-3, which shows an inversely distribution during day 3-6. With the southeastern propagation of wave train in the mid-latitudes, the positive geopotential height anomaly appears in the north of the Black Sea due to the warm advection in the lower troposphere (Fig. 4a). The positive geopotential height anomaly located in the east of the Baikal Lake and the negative anomaly in the Okhotsk Sea on day-9 move eastward and westward, respectively. Meanwhile, the negative geopotential height anomaly and cold anomaly are still over the core region, but all of them are weakened apparently. With the southeastward propagation of this wave train, the positive geopotential height anomaly and warm anomaly gradually control the MYR on day-3 (Fig. 3c).

Figure 4.  Temporal evolution of 10-25-day filtered 850hPa temperature (shading; units: ℃) and wind (vector; units: m s^-1). Gray area indicates the Tibetan Plateau.

The intensity of the positive geopotential height anomaly and warm anomaly to the north of the Black Sea increase significantly, corresponding to weak convective activity, and the warming in the lower troposphere may be caused by local diabatic heating. Meanwhile, the negative geopotential height anomalies moving eastward and westward are merged in Baikal Lake, and expanded meridionally. On day 0 (Fig. 3d), positive geopotential height anomaly and warm anomaly become the dominant system controlling the core region, and the surface temperature reaches the peak point. After that, opposite structures during the cooling process are found in Fig. 3e-f.

Figure 3 (g-l) depicts the evolution of 10-25-day filtered wind fields and the position of the WPSH. On day-9 (Fig. 3g), a mid-latitudes wave train is clearly seen extending from Eastern Europe to southeastern China. There is a cyclonic anomaly over the MYR, and a weak anticyclonic anomaly over the Philippine Sea. As the low-frequency wave train migrates southeastward, the core region gradually transforms into anticyclonic anomaly control, and merges with the anticyclonic anomaly moving northwestward from the Philippine Sea (Fig. 3i). The anticyclonic anomaly is located in the MYR on day 0 (Fig. 3j), and then gradually weakens, with the anticyclonic anomaly in Taiwan moving eastward and the high temperature process ending (Fig. 3k-l). During the EHTE, we also notice an evident consistent movement of the WPSH and low-frequency anticyclonic anomaly, i. e., anticyclonic anomaly moves westward while the WPSH extends westward, which is favorable for heating in the lower troposphere.

During this EHTE, the low-frequency temperature and wind fields in the lower troposphere also show obvious anomalies (Fig. 4). On day-9 (Fig. 4a), an eastwest-oriented cyclonic anomaly is located from southeastern China to the western Pacific. The northern part of the MYR is characterized by a significant divergent flow, inducing anomalous descent motion which is favorable for adiabatic heating. After that, the temperature anomaly over the MYR gradually changes from negative to positive. The warm advection in the domain of 0° - 30° E, 30° - 70° N makes a strong warm anomaly in the northern Caspian Sea on day-6 (Fig. 4b). Lower troposphere heating could induce an expansion of the air column, and the positive geopotential height anomaly is clearly seen in Fig. 3b. On day-6 (Fig. 4b), the cyclonic anomaly in the core region weakens significantly. The anticyclonic anomaly in the east of Taiwan gradually moves westward and becomes the dominant system controlling the area from southeastern China to the western Pacific (Fig. 4c-d), corresponding to the peaking of warm anomaly and surface temperature. After that, the anticyclonic anomaly and warm anomaly gradually weaken, and the high temperature process tends to end. In addition, there is a weak negative meridional advection in the southern MYR from day-3 to day 0, which contributes negatively to the heating process.

In short, the EHTE over the MYR is largely modulated by ISO. According to evolution of circulation anomalies, it can be seen clearly that the circulation structure in the upper troposphere is characterized by the southeastward propagation of mid-latitude low-frequency wave-train. In the mid-lower troposphere, the most remarkable characteristic is a northwestward movement of anticyclonic anomaly in the east of Taiwan, which leads to the western extension of the WPSH. Therefore, what causes the westward movement of anticyclonic anomaly and the WPSH?

To investigate the high temperature process more clearly, we depicted meridional-vertical cross sections of 10-25-day filtered wind, temperature, and geopotential height fields, which are shown in Fig. 5. It can be seen that the structure of temperature anomalies and geopotential height anomalies to the north of 30° N is similar to that discussed by Qian et al. ^[35]. On day-9 (Fig. 5a), a positive geopotential height anomaly center at 200hPa is located around 50°N, and there is a warm (cold) anomaly below (above) this center. An opposite structure is seen around 35°N. The result above indicates that according to the hydrostatic relationship, there is a good correspondence between the low-level temperature anomalies and geopotential height anomalies in the upper troposphere.

Figure 5.  The vertical-meridional cross section of 10-25-day filtered temperature (shading; units: ℃), geopotential height (contour; units: gpm) and wind (vector; meridional wind units: m s^-1, vertical velocity units: -10²Pa s^-1) averaged along the longitudes 106°-118°E during the EHTE. Yellow lines indicate the latitude of the MYR.

It can be seen that the warm (cold) anomaly in the middle troposphere near the core region leads to the expansion (contraction) of the air column, resulting in vertical motion and further forming a direct circulation cell (Fig. 5 a-b). On the contrary, the ascending motion accompanied by the warm anomaly will convert to cool the air column, and the warm anomaly in the middle layer further turn into the cold anomaly, leading to descending motion. Similarly, the cold anomaly in the middle layer gradually turns to the warm anomaly due to the heating effect of descending motion, then forming an indirect circulation (Fig. 5 c-d). To sum up, the beginning and the ending of the high temperature process are accompanied by the transformation of the circulation cell. The increase of temperature in the core region is mainly caused by the combined effect of air column heating induced by descending airflow and southward movement of the warm anomaly in the north of core region. With the southward movement of the warm anomaly, the ascending motion generated under the control of the warm anomaly will gradually reduce the temperature in the core region, and the high temperature process will gradually end.

Through the above analysis, it can be found that vertical motion and temperature advection have an important contribution to the high temperature process. The thermodynamic equation at 925hPa over the MYR (Fig. 6a) are analyzed to elucidate the physical processes contributing to the EHTE occurrence. As shown in Fig. 6a, the diabatic process is always positive during the whole heating process, while the meridional temperature advection always contributes negatively. It is clear that the adiabatic process and zonal temperature advection play the key role in heating the near-surface atmosphere from day-4 to day 0. The thermodynamic equation in 925hPa on the intra-seasonal timescale over the core region is also diagnosed (Fig. 6b), which indicates that 10-25-day intra-seasonal signals play a predominant role in the EHTE. The budget analysis indicates that both the adiabatic process and diabatic process associated with 10-25-day variability contribute to positive temperature tendency during day-7 to day-4, where zonal temperature advection and adiabatic process play the key role from day-3 to day 0. The meridional temperature advection related to 10-25-day variability is always negative during the heating process, which is not conducive to the local heating, but it is the opposite during the cooling process. Meantime, precipitation occurs in the core region (not shown) and releases the latent heat in the early stage of heating process; the diabatic process plays the key role. In the later stage, it is mainly due to the change of circulation fields.

Figure 6.  Thermodynamic equation budget (units: K d^-1) for (a) original and (b) 10-25d filtered over the MYR at 925hPa during the EHTE. Terms include temperature tendency (gray bar), zonal temperature advection (orange line), meridional temperature advection (red line), adiabatic process (dark blue line) and diabatic process (blue line).

Furthermore, we examine the nine terms to determine which contribute to the adiabatic process associated with intra-seasonal variability. The diagnosis result (not shown) shows clearly that the maximum contribution comes from the mean static stability by the vertical velocity related to 10-25-day variability.

According to the analysis of circulation characteristics in the EHTE, we can notice that the westward movement of anomalous anticyclone and the westward extension of the WPSH. Previous studies have pointed out that spatially nonuniform heating will affect the WPSH (Wang et al. ^[36]). To track how does diabatic heating affect the position of the WPSH during this case, the apparent heat source Q₁ at 500hPa and the position of the WPSH (lines 5880gpm) are showed in Fig. 3(g-l), where Q₁ is obtained from thermodynamic equation. Before the peak day, there is a large value center of Q₁ in the northern Bay of Bengal, which has been lasted for a long time and is consistent with the distribution of convective activity center (not shown). With the gradual weakening and disappearance of Q₁ over the Huanghai Sea and Hainan Province, the WPSH gradually moves westward and controls the core region.

The above analysis indicated that the position variation of WPSH is closely related to the diabatic heating. Based on the complete form of vertical vorticity tendency equation proposed by Wu and Liu ^[37], the position variation of the WPSH is explored, and the result shows that in the case of only considering the latent heating, the equation can be simplified according to scale analysis as follows (Liu et al. ^[38]; Lin et al. ^[39]):

where the left-hand side of Eq. (3) denotes the β-effect, and the right-hand side denotes the diabatic heating term. In the northern hemisphere, the geostrophic parameter f increases with latitude and is always greater than zero. f + ξ ≥ 0, and θ_z is always positive.

Figure 7 depicts the vertical distribution of Q₁ in the northern Bay of Bengal (20° - 28° N, 83° - 95° E). From day-9 to day-6, Q₁ increases significantly with height and reaches the maximum in the middle and upper troposphere, which indicates that the diabatic heating in this area is dominated by convective latent heating, corresponding to convective activity at this time (not shown). Thus, on day-6, under the term of β, the southerly wind at 500hPa below the heat source is in favor of the increase of anticyclonic vorticity to the east of heat source, which obviously induces the westward extension of the WPSH. Thereby, heat source in the northern Bay of Bengal in the early stage of extreme high temperature event has certain indicative significance for the western extension of the WPSH.

Figure 7.  Vertical distribution of apparent heat source Q₁ (units: W kg^-1) in the northern Bay of Bengal (20°-28°N, 83°-95°E) from day -9 to day 0.

In this paper, the structure and evolution of lowfrequency circulation in the intra-seasonal scale of an EHTE in the MYR from August 9 to 21, 2011 were explored, and the cause of the EHTE was diagnosed by using thermodynamic equation. The influence of diabatic heating on the position change of the WPSH was also discussed by using the complete form of vertical vorticity tendency equation. The conclusions are as follows:

(1) The T_max in the MYR has a low-frequency oscillation with a period of 10-25 days, which is largely controlled by strong ISO;

(2) In this EHTE, the low-frequency circulation is mainly represented by the southeastward propagation of the low-frequency wave train in the mid-high latitude in the upper level, and the northwestward movement of the low-latitude anticyclonic anomaly in the mid-lower levels, which is conducive to the westward extension of the subtropical high. Under the combined influence of the systems in high- and low-latitudes, as well as the warm anomaly to the north of the core region moving southward and the descending motion in the core region, the T_max gradually changes from negative anomaly to positive anomaly.

(3) The diagnosis of 925hPa thermodynamic equation indicates that the dominant term in the early stage of heating process is diabatic process, and positive temperature tendency in the later stage is primarily attributed to the adiabatic process and zonal temperature advection, while the effect of meridional temperature advection is the opposite.

(4) There is a good correspondence between the changes of the WPSH and the distribution of apparent heat source. In the early stage of the EHTE, the apparent heat source in the northern Bay of Bengal increases significantly with height and reaches the maximum in the mid-upper troposphere. According to the complete form of vertical vorticity tendency equation, negative vorticity appears on the east side of the 500hPa heat source, which is conducive to the westward extension of the WPSH.

It is worth noting that this paper only analyzes a typical EHTE, but not all EHTEs have obvious characteristics of westward extension of the WPSH. Therefore, the high temperature processes that meet certain conditions from 1980 to 2018 are further classified, and the cases with obvious westward extension of the subtropical high are synthetically analyzed. The results show that there are similar distribution characteristics of apparent heat sources, diabatic heating has an impact on the westward extension of the subtropical high, and other characteristics need to be further analyzed.