Analysis on mechanisms of anomalous variations of tropopause pressure over the Tibetan Plateau during summer in Northern Hemisphere

BaoLin Zhu , ChuHan Lu , ZhaoYong Guan , Zhi Cheng , WenJun Liu

1. Meteorological Service Center of Yunnan, Kunming 650034, China

2. Key Laboratory of Meteorological Disaster of Ministry of Education, Institute of Atmospheric Sciences, Nanjing University of Information Sciences & Technology, Nanjing 210044, China

3. Anhui Climate Center, Hefei 230031, China

4. Haidian District Meteorological Bureau, Beijing 100080, China

Analysis on mechanisms of anomalous variations of tropopause pressure over the Tibetan Plateau during summer in Northern Hemisphere

BaoLin Zhu1*, ChuHan Lu2, ZhaoYong Guan2, Zhi Cheng3, WenJun Liu4

1. Meteorological Service Center of Yunnan, Kunming 650034, China

2. Key Laboratory of Meteorological Disaster of Ministry of Education, Institute of Atmospheric Sciences, Nanjing University of Information Sciences & Technology, Nanjing 210044, China

3. Anhui Climate Center, Hefei 230031, China

4. Haidian District Meteorological Bureau, Beijing 100080, China

Using the monthly mean data from NCEP-NCAR reanalysis, through building tropopause pressure index, we investigated the mechanisms of anomalous variations of tropopause pressure over the Tibetan Plateau during summer in Northern Hemisphere. For comparative analysis we selected representative years of 1992 and 1998 to study, and they were respectively the highest and the lowest year of tropopause pressure anomaly over the Tibetan Plateau. The results are summarized as follows: (1) Over the Tibetan Plateau, the variations of tropopause pressure are well correlated respectively with anomalous temperature and geopotential height in both troposphere and stratosphere. Besides, the anomalous tropopause pressure has also close relation with anomalous surface temperature in the Tibetan Plateau. In 1992, the surface temperature was anomalously low, correspondingly, the tropopause pressure over the Tibetan Plateau was anomalously high; but in 1998, the opposite was the case. (2) Over the Tibetan Plateau, the correlation of tropopause pressure and OLR (Outgoing Longwave Radiation) is found to be positive. Furthermore, by further diagnosing the circulation fields between 850 hPa and 200 hPa levels and the whole troposphere vapour field, we found out that the anomalously high tropopause pressure in 1992 corresponded to the anticyclonic divergence of low level wind fields and the cyclonic convergence of high level wind fields, and coupled with divergence of the whole troposphere vapour fields along with the South Asian High weakened at the same time. While in 1998, the case was opposite to that in 1992. These facts indicated that the anomalous convection resulted in the significant difference of tropopause pressure in 1992 and 1998 over the Tibetan Plateau. (3)The vertically integrated heat budget anomalies were responsible for explaining tropopause pressure anomalies in 1992 and 1998 over the Tibetan Plateau.

Tibetan Plateau; tropopause pressure; anomalous variations; mechanisms; convection anomaly

1. Introduction

The tropopause represents the boundary between the troposphere and stratosphere, and is marked by large changes in convective activity, thermal, dynamic, and chemical structure of the atmosphere. The vertical structure and its change of the tropopause bring about important impacts on stratosphere-troposphere exchange and chemical balance,and also act as a sensitive indicator of anthropogenic forcing climate change (Santeret al., 2003; Bian, 2009). Increased attention has been paid on the large-scale and regional changes of the tropopause, including associated environmental and climatic changes (Thuburn and Craig, 2000,2002). The climatic feature of the tropopause is influenced by the interaction of the Earth system (Maxobep, 1988),whose inner dynamics, external forcing, feedback mechanism and instability lead to its variability. Comprehensively and precisely understanding regional variations of the tropopause is important for understanding the coupling between the stratosphere and the troposphere (Holtonet al.,1995; Shepherd, 2000).

The Tibetan Plateau (hereafter noted as TP), with its large extent and height, greatly impacts the general circulation, climate and severe weather of eastern Asia, with both dynamic and heating effects (Liu, 1999; Daoet al., 2006; Li and Liu, 2006). Holtonet al. (1995) pointed out that the TP is the key region of troposphere-stratosphere mass and energy exchange, and these exchanges are mainly induced by synoptic scale processes like tropopause folding. Linked with dynamic and heating processes of the TP, notable low ozone valley formed in this region (Zhouet al., 1995; Wanget al., 2008), which may be indicated by height of the tropopause (Hoinkaet al., 1996; Wanget al., 2010). Therefore,it is worth investigating the pressure variation and its mechanism of the TP.

The data and index definition representing the strength of pressure over the TP tropopause are described more fully in section 2. The main analyses of the linkage between the pressure variation of the TP tropopause and the stratospheric temperature and geopotential height anomalies are described in section 3. In section 4, we discuss the convective-activity induced mechanism to tropopause-pressure variation over the TP. The results are summarized and discussed in section 5.

2. Data and intensity index of TP tropopause pressure

The observed data used in this study are obtained from the monthly mean reanalysis dataset of the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) for the 22-summer(June-July-August mean) period from 1979 to 2000 (Kistleret al., 2001). The resolution of the NCEP-NCAR reanalysis data is 2.5° in latitude and longitude along with the 17 pressure levels from 1,000 hPa to 10 hPa. The speci fi c variables we used from the NCEP reanalysis are, zonal and meridional wind, air temperature, geopotential height from all the 17 pressure levels, specific humidity (top level: 300 hPa), pressure velocity (top level: 100 hPa) and surface air temperature. Tropopause pressure values are especially relevant to this study. Outgoing Longwave Radiation (OLR) data from NOAA are also used to investigate convective activity near the TP.

As seen in Figure 1, the multi-year mean of tropopause pressure over the TP and its surrounding area exhibit the lowest value in the world. Based on spatially coherent pressure variation over the TP, intensity index of TP tropopause pressure (hereafter shown asITT) is defined as standardized box-average (40°-150°E, 20°-35°N) pressure, which can be written as:

where∑ is the region area within 20°-35°N and 40°-150°E,Mis the multi-year mean ofMt.

As shown in Figure 2, notable inter-annual variability ofITTis evident. In summer of 1992,ITTwas up to a 3.6 standard deviation anomaly, indicating remarkably high pressure over the TP. A distinct opposite situation occurred in 1998,whoseITTwas about -1.5 standard deviation. In fact, the geographic distribution of tropopause pressure anomalies over the TP in 1992 was 9 hPa higher than that in 1998,demonstrating thatITTcan be a proxy of variability of tropopause pressure over the TP. To assess what factors contribute to tropopause pressure over the TP variation, further comparison in 1992 and 1998 will be taken.

Figure 1 Patterns of multi-year mean global tropopause pressure (unit: hPa)

Figure 2 Histogram of summer tropopause pressure index changing with time over the Tibetan Plateau

3. Relationship between tropopause pressure and circulation over the TP

The opposition of vertical structures of air temperature and geopotential height over the TP in 1992 and 1998 is striking (Figure 3). As shown in Figures 3a and 3c, in summer of 1992, the air temperature throughout the depth of troposphere (stratosphere) emerged as notable cooling(heating), leading to air column shrinking (expanding).Hence the associated tropopause pressure descended sharply.An opposite scenario occurred in summer of 1998 (Figures 3b and 3d), which exhibited pronounced lower pressure over the TP tropopause, with positive (negative) anomalous values of air temperature and geopotential height in the troposphere (stratosphere). It is interesting that the vertical distributions of air temperature and geopotential height fields throughout the entire air column were nearly out of phase in 1992 comparing with in 1998. In 1992, the air temperature anomaly change from the bottom up showed as a "- + -"pattern, and the geopotential height anomaly was "-" in troposphere and "+" in stratosphere. In 1998, the distributions were in opposite case.

Figure 3 Longitude-pressure cross-sections of summer temperature anomaly (a: 1992, b: 1998) and geopotential height anomaly (c: 1992, d: 1998) along 20°N-35°N over the Tibetan Plateau

To further explore the difference of air temperature anomaly field between in low-pressure and in high-pressure tropopause, we show in Figure 4 the spatial distributions of anomalous surface air temperature in 1992 and 1998. Consistent with demonstrations in Figures 3a and 3b, the surface air temperature anomaly over the TP in 1992 was marked by big negative values, reflecting very cold weather (Figure 4a), and the case was opposite in 1998 (Figure 4b). Opposite features between in 1992 and in 1998 were also evident on zonal variation of the surface air temperature anomaly over the TP (Figure 4c), generally showing negative anomaly in 1992 and positive anomaly in 1998. Sausen and Senter (2003) pointed out that the mean height of the Global mean tropopause has been rising since 1979, and considered its change consistent with variations of surface temperature, vertical temperature profile and ocean heat storage. The tropopause height over the TP has positive correlation with surface air temperature anomaly and the tropopause pressure has negative correlation with surface air temperature anomaly. The anomalous low surface air temperature on TP in 1992 corresponded to the anomalous high tropopause pressure; the anomalous high surface air temperature in 1992 corresponded to the anomalous low tropopause pressure in 1998.

Figure 4 Ground surface temperature anomaly over the Tibetan Plateau in summer (a: 1992, b: 1998) and its longitudinal cross-section along 20°N-35°N (c)

4. Mechanism of pressure variation over the TP tropopause

4.1. Convective activity influence

To explore the connection of convective activity between circulations in lower and upper troposphere, we examine the correlation between intensity index of TP tropopause pressure (ITT) and Outgoing Longwave Radiation(OLR) over Asian summer monsoon region. As shown in Figure 5a, a positive correlation is identified over the TP,especially significance test proves that the positive correlation in southeastern India and southwestern China surpasses 90% confidence level. The convection intensity between circulations in lower and upper troposphere becomes weak when the pressure over the TP tropopause rises and the OLR increases, in contrary, the convection intensity becomes strengthened when TP tropopause pressure drops and the OLR decreases. Furthermore, the spatial distributions of OLR anomaly in 1992 and 1998 all show agreement to the above-mentioned positive correlation (Figures 5b and 5c). In 1992, the distribution of OLR anomaly differentiated over the TP and its surrounding areas: OLR in north of the Bengal Bay and southwest of China are up to 3 W/m2higher than the multi-year mean, and slightly lower than the mean over other regions (Figure 5b), indicating more longwave radiation loss from ground surface of TP as a whole, consequently resulting in low air temperature in TP. In comparison,the overall spatial pattern of OLR anomaly in 1998 displayed an opposite layout (Figure 5c), which implied stronger convection and more cloud cover over the TP. These results indicate that convection activity over the TP plays an important role in tropopause-pressure variation. In the followed section, we will further discuss the relationship of anomalous convection with circulation anomaly in lower and upper air, water vapor flux convergence, and the anomaly of the South Asian High (SAH).

Figure 5 Correlation between tropopause pressure index and OLR anomaly (a) over the Tibetan Plateau (areas where the confidence level is higher than 90% are shaded) and spatial patterns of OLR anomaly (b: 1992, c: 1998) (unit: W/m2)

4.1.1 Anomalous circulation features in the troposphere

Figures 6 and 7 demonstrated circulations in lower and upper troposphere over Asian summer monsoon region in 1992 and 1998. In 1992, anticyclone-like and divergent circulation was identified over the TP and surrounding areas at lower troposphere of 850 hPa (Figures 6a and 6c);while cyclone-like and convergent circulation occurred at upper troposphere of 200 hPa (Figures 6b and 6d). This pattern favored air descending in the troposphere and restrained the development of convection over the TP, contributing to lower geopotential height and tropopause higher pressure. In contrast, the circulation pattern was the opposite case in 1998 (Figure 7). This pattern was helpful for air evidently ascending in the trpoposphere and favored the development of convection over the TP, contributing to higher geopotential height and tropopause lower pressure.It is worthwhile to note that the anomalous variations of troposphere pressure over the TP correspond to the anomalous wind fields occurrence at east and south sides of TP,indicating that TP high pressure, as an important component of Asian Monsoon System, has close relation with the eastern Asian summer monsoon (EASM) activities in tropopause pressure variations over the TP.

Figure 6 Tropospheric stream function anomaly (unit: ×106 m2/s) and its rotational wind field anomaly (unit: m/s) as well as tropospheric velocity potential function anomaly (unit: ×106 m2/s) and its divergent wind field anomaly (unit: m/s)over Aisan monsoon region in summer of 1992 (a, c: 850 hPa; b, d: 200 hPa)

Figure 7 Tropospheric stream function anomaly (unit: ×106 m2/s) and its rotational wind field anomaly (unit: m/s) as well as tropospheric velocity potential function anomaly (unit: ×106 m2/s) and its divergent wind field anomaly (unit: m/s)over Aisan monsoon region in summer of 1998 (a, c: 850 hPa; b, d: 200 hPa)

4.1.2 Water vapor flux

The features of total column water vapor flux over the TP and surrounding areas accord with anomalous tropospheric wind field (Figure 8). Specially, negative vapor velocity potential anomalies dominated over the Asian region in 1992, with a pronounced divergence center in the southeast of TP, suggesting drier conditions in this region (Figure 8a). Correspondingly, the convection was weak, resulting in positive pressure anomalies over the TP tropopause. An opposite scenario occurred in 1998, during which positive anomalies prevailed over most of the Asian region, with notable convergence near TP (Figure 8b). These facts proved that the convergence or divergence of total water vapor flux might cause anomalous convection over the TP, resulting in troposphere pressure anomaly over the TP.

Figure 8 Patterns of moisture flux potential function anomaly (unit: kg/s) and divergent wind field anomaly (unit: kg/(m·s))integrated from 1,000 hPa to 300 hPa over Asia area (a: 1992, b: 1998)

4.1.3 200 hPa South Asian High

The South Asian High (SAH) as the strongest and steadiest pressure system in the upper troposphere occurs near TP and its surrounding area in summer (Ye and Gao, 1979; Zhuet al., 1980; Tanget al., 1982; Kang and Wu, 1990). The SAH clearly shows close connection with tropopause pressure variability over the TP (Figure 9a). The significant negative correlations display predominantly over the TP and its surrounding area, with the maximum negative coefficient of-0.6 and the mean of -0.49 at 22 samples, and the confidence levels exceed 95%. This negative relation clearly suggests that the TP tropopause pressure will reduce as the SAH strengthens, andvice versa.

Ma (2003) reported that there is positive correlation between the SAH geopotential high and the summer monsoon intensity over the TP. In summer of 1992, negative geopotential high values prevailed in the Asian regions at 200 hPa, indicating weak summer monsoon intensity and weak convection activity over the TP, thereby contributing to positive abnormal tropopause pressure (Figure 9b). In contrast, positive geopotential high values dominated over the Asian regions at 200 hPa in summer of 1998, suggesting vigorous summer monsoon intensity and strong convection over the TP, leading to negative abnormal tropopause pressure (Figure 9c).

Figure 9 Distribution of correlation coefficient between summer 200-hPa geopotential height anomaly over Northern Hemisphere and tropopause pressure index over Tibetan Plateau (a) (areas where the confidence level is higher than 0.05 are shaded),and patterns of geopotential height anomaly over Northern Hemisphere at 200 hPa in 1992 (b) and 1998 (c).

4.2. Heat budget anomalies

To further explore the mechanism of tropopause pressure variation, we referred the research results of Rodwell and Hoskins (1996) and Guan and Yamagata (2003) to examine the vertical integration of heating budget. The thermodynamic equation is expressed as:

The non-adiabatic heating over the TP and its surrounding area in 1992 and 1998 were identified as two opposite scenarios (Figure 10). Particularly, in summer of 1992,anomalous non-adiabatic cooling displayed over the TP with center in its eastern part, leading to temperature dropping in entire troposphere and air condensing; more air moved from the stratosphere to the troposphere, resulting in tropopause height descended (Figure 10a). In summer of 1998, anomalous non-adiabatic heating occurred predominantly over the TP with center located in its middle-west part, contributing to air expansion and tropopause height uplifting over the TP(Figure 10b). Therefore, the significant difference of total air column non-adiabatic heating and its corresponding anomalous convection are the main causes resulting in important difference of tropopause pressure anomaly over the TP.

Figure 10 Total air column anomalous non-adiabatic heating rate (unit: °C/d) averaged from surface to 100 hPa over Tibetian Plateau in summer (a: 1992, b: 1998)

5. Conclusion and discussions

The monthly mean data from NCEP-NCAR reanalysis were used to investigate the mechanisms of anomalous variations of tropopause pressure over the TP during summer of Northern Hemisphere. The highest tropopause pressure anomaly occurred in year 1992, and the lowest tropopause pressure anomaly happened in 1998 over the TP. Some comparative research results have been obtained:

(1) The variations of tropopause pressure are well correlated respectively with anomalous temperature and geopotential height in both troposphere and stratosphere. Anomalous tropopause pressure also correlates well with anomalous surface temperature in TP. In 1992, the surface temperature was anomalously low, correspondingly, the tropopause pressure over the Tibetan Plateau was anomalously high; but in 1998, the opposite was the case.

(2) The correlation between tropopause pressure and OLR is found to be positive. Further diagnosing the circulation fields between 850 hPa and 200 hPa levels and the whole troposphere vapour field, we found that the anomalously high tropopause pressure in 1992 corresponds to the anticyclonic divergence of low level wind fields and the cyclonic convergence of high level wind fields, and coupled with divergence of the whole troposphere vapour fields as the South Asian High weakened at the same time. While in 1998, the case was opposite to 1992. The anomalous convection has resulted in the significant difference of tropopause pressure in 1992 and 1998.

(3) The vertically integrated heat budget anomalies are responsible for explaining tropopause pressure anomalies in 1992 and 1998.

The relationship between tropopause pressure variability over the TP and circulation anomalies has been principally disclosed in this paper, and it is found out that non-adiabatic heating and the corresponding anomalous convection are the possible physical mechanism of anomalous tropopause pressure variability. However, our diagnostic results still need further reanalysis due to TP’s unique geography and complicated climatic influencing factors. Moreover, we need to further study how tropopause pressure variation over the TP be an indicator for regional climate change in China, and try to use it in short term climate prediction.

This work is supported jointly by the National Basic Research Program of China (2010CB428602) and the National Natural Science Foundation of China (41005046,40675025).

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10.3724/SP.J.1226.2012.00154

*Correspondence to: BaoLin Zhu, Meteorological Service Center of Yunnan. No. 77, Xichang Road, Kunming, Yunnan 650034,China. Email: zhubaolin2004@163.com

September 21, 2011 Accepted: December 5, 2011