Roles of European blocking and tropical extratropical interaction in the 2010 Pakistan flooding

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 38,, doi: /2011gl047583, 2011 Roles of European blocking and tropical extratropical interaction in the 2010 Pakistan flooding Chi Cherng Hong, 1 Huang Hsiung Hsu, 2 Nai Hsin Lin, 2 and Hsun Chiu 1 Received 29 March 2011; revised 5 May 2011; accepted 6 May 2011; published 9 July [1] A sequence of monsoon surges struck Pakistan and Northwestern India during late July early August The unusually heavy monsoon rainfall resulted in recordbreaking floods, which affected 20 million people with a death toll of near Simultaneously, a long lived blocking high appeared over Europe and Russia in middle June and persisted for nearly two months. Extreme flooding occurred when the southward penetration of extratropical potential vorticity in the deep trough east of the European blocking and the tropical monsoon surges arrived concurrently in Pakistan. This study demonstrates that the interaction between the tropical monsoon surges and the extratropical disturbances downstream of the European blocking was the key factor leading to the severe flooding in Pakistan. The 2010 La Niña condition contributed indirectly to the flooding by inducing a lowlevel easterly anomaly in South and Southeast Asia, which weakened eastward moisture transport and helped enhance moisture transport (convergence) to (in) the Northern Arabian Sea and Pakistan. Citation: Hong, C. C., H. H. Hsu, N. H. Lin, and H. Chiu (2011), Roles of European blocking and tropical extratropical interaction in the 2010 Pakistan flooding, Geophys. Res. Lett., 38,, doi: /2011gl Introduction [2] Monsoon rain falls in Pakistan every summer, but the rain starting in late July 2010 was unusually intense. A sequence of monsoon surges struck Pakistan and Northwestern India during late July early August. In early August the flood resulted in widespread landslides along the Indus River basin and the worst natural hazard in Pakistan, which affected 20 million people with a death toll of nearly [3] The Asian summer monsoon varies in intensity from year to year for many reasons. One such factor is the remote influence of the El Niño Southern Oscillation (ENSO, which usually enhances (suppresses) the South Asian summer monsoon during La Niña (El Niño) [e.g., Alexander et al., 2002]. The La Niña condition was already developed in July 2010 and might be a factor in shaping the intense 2010 monsoon. [4] During the Pakistan flood, a long lived blocking high, well established in middle June persisted for nearly two months over Europe and Russia [Dole et al., 2011; Barriopedro et al., 2011]. Previous studies revealed that 1 Department of Science, Taipei Municipal University of Education, Taipei, Taiwan. 2 Department of Atmospheric Science, National Taiwan University, Taipei, Taiwan. Copyright 2011 by the American Geophysical Union /11/2011GL blocking significantly affected the local and downstream weather [e.g., Carrera et al., 2004]. As shown below, strong extratropical wave activity downstream of the European blocking was observed during the flooding period when a sequence of monsoon surges arrived in Pakistan. [5] This study shows from the large scale circulation perspective that the strong tropical extratropical interaction due to the coupling of monsoon surges and extratropical wave activity downstream of the European blocking was the major reason for the severe flooding in Pakistan. The indirect effect of the 2010 La Niña on the large scale circulation and the Pakistan flooding is also discussed. 2. Data and Methodology [6] The daily datasets used in this study include 1) wind, temperature and geopotential NCEP/NCAR Reanalysis I height [Kalnay et al., 1996], 2) NOAA Extended Reconstructed Sea Surface Temperature (SST) [Smith et al., 1996], 3) outgoing longwave radiation (OLR) [Liebmann and Smith, 1996], 4) Tropical Rainfall Measuring Mission Tropical Microwave Imager (TRMM TMI) satellite products [Huffman et al., 2007]. The Indian summer monsoon (IM) index defined as U 850 (5 15 N, E) U 850 (20 30 N, E) [Wang and Fan, 1999] was used to represent the monsoon flow strength in the Indian Ocean (IO). The monsoon surge is defined as: the daily rainfall over the Northern Pakistan (box in Figure 1a) is larger than 9 mm (climatological mean stand deviation) and persists for at least two days. The monsoon surges shown in Figure 1b is consistent with the heavy rainfall events defined by Webster et al. [2011] (Figure 1b). The extreme rainfall exceeding 30 mm on July 12 was not included because it was an one day event (Figure 1c). 3. Extreme Rainfall and Anomalous Large Scale Circulation [7] The daily rainfall time series in Northern Pakistan (box in Figure 1a) shows that four monsoon surges occurred during late July early August (Figure 1b). The first surge occurred on July, the second on July, the third on 6 8 August, and the fourth on August. The daily rainfall in the strongest surge on July exceeded 25 mm day 1 ; 5 times larger than the climatological mean ( 5 mm day 1 ). These events synchronized with the monsoon surges (as indicated by the IM curve) and the southward penetrating extratropical disturbances (as indicated by the 315K isentropic potential vorticity (IPV) curve) (Figure 1c). A wavelet analysis of the IM index revealed the existence of a day monsoon fluctuation during June July (Figure 1d). The day fluctuation started weakening 1of6

2 Figure 1. (a) TRMM TMI rainfall anomaly during 19 July 12 August The time series (averaged over Northern Pakistan, red box in Figure 1a) of daily (b) TRMM TMI rainfall, (c) IM index, isentropic (315K) potential vorticity (unit: 10 7 s 1, gray line), and rainfall (green bar), and (d) power spectrum of the IM index derived from wavelet analysis (basis function Morlet with parameter 6). The cone of influence of wavelet power spectrum is shown in the blue curve and the spectrum exceeding 95% confidence level is hatched. Blue shading in Figures 1c and 1d denotes the period of Pakistan flooding. after the last surge in mid July. The monsoon remained active for about a month and fluctuated with a period of around 10 days (Figures 1c and 1d). It is interesting to note that the heavy rainfall occurred during late July and early August when both the 10 day monsoon surge and extratropical perturbation occurred in synchronization. Such synchronization was not observed in the rest of the period shown in Figure 1c. [8] The Hovmüller diagram of the OLR anomaly averaged over E presented in Figure 2a indicates that the monsoon surges between late July and early August were characterized by a northward propagating convective perturbation originating near the equator and arriving at Northern Pakistan when the heavy rainfall occurred. These tropical origin convection perturbations, which likely played an important role in triggering the heavy rainfall 2of6

3 Figure 2. Hovmüller diagrams of (a) time longitude (averaged over E) OLR anomaly, and (b) time latitude (averaged over 5 S 5 N) 200 hpa velocity potential anomaly. Same as in Figure 2a but for the (c) potential vorticity (unit:10 7 s 1 ) at 315K, and (d) vertical integrated ( hpa) specific humidity (unit: g kg 1 ). The dashed contour (unit: 10 6 s 1 ) in Figure 2c indicates the divergence at 315K and only the divergence less than 10 6 s 1 is plotted. events, can be further traced back to Eastern Africa and the Atlantic, as seen in Figure 2b, following the track of a Madden Julian Oscillation that propagated from the Greenwich line to 120 E between July. [9] The extratropical influence can be seen clearly in the Hovmüller diagram of PV and divergence at 315K (equivalent to 500 hpa at 60 N and 700 hpa at 40 N) and vertically integrated specific humidity presented in Figures 2c and 2d, respectively. There were several pulses of southward penetrating PV anomalies between mid July and mid August. The corresponding pulses seen in Figure 2c indicate that these anomalies were the southward moving descending cold air from the upper troposphere in the north. The arrivals of the extratropical PV to the north of Pakistan corresponded well to the appearance of an anomalous convergence at 315K and the high moisture content over Northern Pakistan (e.g., N, Figure 2d). As presented in Figure S1 of the auxiliary material, the southward and downward penetration of the extra tropical PV often corresponded to an 3of6

4 Figure 3. Anomalies during 19 July 12 August 2010: (a) 500 hpa height (interval 50 m) and 850 hpa temperature anomaly (shading), (b) 200 hpa stream function (shading, unit: 10 7 m 2 s 1 ) and wave activity flux (unit: m 2 s 2 ), and (c) the latitude height cross section (averaged over E) of the moisture flux [unit: (10 3 gkg 1 ms 1, 10 5 gkg 1 hpa s 1 )], equivalent potential temperature (shading), and temperature (contour, interval 1K) anomalies. 4of6

5 enhanced upward motion of moister air. 1 Since the anomalous convergence was basically stationary features, it seemed to be triggered locally by the anomalous PV from the north. The triggering effect of cold dry high PV air on the convection when it penetrated southward to warm moist surface has been reported in previous studies [e.g., Hsu et al., 1990]. In addition to the close correspondence with the PV anomaly, the specific humidity plot (Figure 2d) also reveals two northward propagating events from 20 N to 35 N, which were associated with the northward propagating convective perturbation seen in Figure 2a. This northward propagating convective was found to result in the Pakistan flood in late July and early August (K. Lau, K. M., and K. M. Kim (2011), The 2010 Pakistan flood and Russian heat waves: Teleconnection of hydrometeorologic extreme, J. Hydrometeorology (in revision). Note that the second and third extreme rainfall events (monsoon surges, Figure 1b) occurred when both the tropical and extratropical origin perturbations met near Pakistan. The first event occurred in a similar but slightly different situation when the tropical convection reached 15 N and a southward penetrating PV reached North Pakistan. It follows that the coupling of tropical monsoon surge and extratropical PV outbreaks may be the major reason for the extreme rainfall events in late July and early August. [10] The circulation composites during 19 July 12 August when four rain surges occurred are shown in Figure 3 to illustrate more clearly the circulation structure associated with the extreme rainfall events. The 500 hpa height and 850 hpa temperature anomalies clearly reveal the occurrence of a warm blocking high over Europe and Russia during this period (Figure 3a). The blocking high resulted in an unusual persistent dry and warm condition and a severe heat wave in Europe and wild fires in Russia in July The long lived blocking not only affected local weather but also had a tight telecommunication with the Pakistan flood (K. M. Lau and K. M. Kim, The 2010 Pakistan flood and Russian heat waves: Teleconnection of hydrometeorologic extreme, submitted to Journal of Hydrometeorology). Specifically, the deep trough with cold temperature anomaly east of the blocking extended southward along the western edge of the Tibetan Plateau to Pakistan and the Northern Arabian Sea. The ascending region that usually occurs ahead of the upper level trough would provide an environment favorable for convection development. A comparison with the rainfall anomaly (Figure 1a) indeed demonstrates this spatial relationship: positive rainfall anomaly in Pakistan and the Northeastern Arabian Sea was located to the southeast of the cold trough. [11] During the prevailing blocking period, the extratropical jet was split into two branches (not shown) and two upper level wave like structures were clearly identified downstream of the European blocking (Figure 3c). The southern wave like structure spread through Northern Pakistan, Northwest China and further east to Korea along a path following closely the waveguide identified by Ambrizzi et al. [1995] and resembling the Silk Road pattern mentioned by the previous studies [e.g., Enomoto et al., 2003; Hu et al., 2005; Ding and Wang, 2007]. The downstream impact of the long lived blocking can be further illustrated by the wave activity flux (WAF) [Takaya and Nakamura, 2001] shown in Figure 3c. Two rays of WAF were found: the 1 Auxiliary materials are available in the HTML. doi: / 2011GL northern one arching northward toward the Bering Sea and the southern one arching southward toward East Asia along the climatological wave guide described above. The deepening trough that affected Pakistan heavy rainfall was part of the southern WAF branch and could be at least partially attributed to the downstream energy dispersion originating from the European blocking. [12] The cross section shown in Figure 3c reveals the rising moist air from the south over the southern slope of the high rising topography, the northward moisture transport in the whole troposphere and the upward decreasing equivalent potential temperature south of the steep terrain. It suggests that the heavy rainfall events occurred in a convectively unstable background with abundant moisture supply from the south, which was likely associated with the active tropical convection and the northward propagating monsoon surges. Since the heavy rainfall occurred near the high rising mountains, it is reasonable to suggest that the combined effect of monsoon surges, extratropical disturbances and topography led to the unusual flooding event. 4. Role of La Niña [13] Two thousand ten was a La Niña developing year and the cold SST anomaly (SSTA) in the Equatorial Eastern Pacific was already developed during the Pakistan flooding (Figure S2 of the auxiliary material). The above normal rainfall in the Maritime Continent (not shown) was a typical atmospheric response to the La Niña like SSTA. As expected, a Gill type anticyclone in response to the cold SST anomaly (and weaker convection) in the equatorial Central/ Eastern Pacific was observed over the Western North Pacific (WNP). Associated with the anomalous anticyclone was a strong easterly anomaly over the tropical WNP that is reflected in the anomalous westward moisture transport in Figure S2b. The westward moisture transport anomaly extending further westward to the South China Sea, the Indochina Peninsula and the Bay of Bengal was likely part of the Gill type cyclonic response (e.g., the cyclonic anomaly in the South China Sea) to the anomalously abundant rainfall in the Maritime Continent. Although the westward transport anomaly did not increase the rainfall in Pakistan directly, it enhanced the moisture flux convergence in Pakistan by weakening the eastward moisture transport of the Indian monsoon flow. As a result, more moisture was able to be transported and converged in Northwest India and Pakistan. Figure S2 clearly shows two paths of anomalous moisture flux, the easterly anomaly from the WNP and the southerly anomaly from the tropical Western Indian Ocean, which contributed to the strong moisture convergence in Pakistan and therefore the unusual heavy rainfall. 5. Conclusion [14] The results of this study are summarized in the schematic diagram shown in Figure S3 and described below. [15] 1. A sequence of northward propagating monsoon surges was identified during the unusually heavy flooding in Pakistan. Simultaneously, a long lived blocking high, well established in middle June and persisting for nearly two months, occurred over Russia and Europe. The deepening trough east of the blocking was found to penetrate southward to North Pakistan, which helped enhance the 5of6

6 convection in Pakistan and the monsoon surges reaching higher latitudes than normal. [16] 2. The IPV analysis reveals that the southward penetration of the cold dry high PV air associated with the trough occurred coincidentally with the arrival of the monsoon surges at Pakistan. The extratropical tropical coupling was evident during the Pakistan flooding. The high PV air induced anomalous low level convergence and upward motion when it penetrated southward and downward to Pakistan and provided a favorable environment for the further northward propagation of monsoon surges. [17] 3. The La Niña condition was already developed in July The La Niña remotely forced a significant easterly wind (and moisture transport) anomaly in the Western Pacific and Eastern Indian Ocean, which in turn enhanced the moisture flux convergence in Pakistan through weakening the eastward moisture transported by the Indian monsoon flow. [18] This study demonstrates the important role of tropical extratropical interaction in triggering the 2010 Pakistan extreme rainfall. The observed evidence clearly show that the coupled intra seasonal variation between the Rossby wave activity originating from the Northeastern Atlantic and the summer monsoon surges originating in the western tropical Indian Ocean played an essential role in the occurrences of extreme rainfall events. The large scale circulation anomalies associated with the long lived European blocking and the La Niña both contributed to heavy rainfall by modifying the extratropical wave activity and tropical moisture flux, respectively. [19] Acknowledgments. Comments of two anonymous reviewers are highly appreciated. This study was supported by NSC NSC M and NSC M MY3. [20] The Editor thanks the two anonymous reviewers for their assistance in evaluating this paper. References Alexander, M. A., I. Bladé, M. Newman, J. R. Lanzante, N. C. Lau, and J. D. 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Toma, and H. M. Kim (2011), Were the 2010 Pakistan floods predictable?, Geophys. Res. Lett., 38, L04806, doi: / 2010GL H. Chiu and C. C. Hong, Department of Science, Taipei Municipal University of Education, 1 Ai Kuo West Rd., Taipei 10048, Taiwan. H. H. Hsu and N. H. Lin, Department of Atmospheric Sciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan. (hsu@atmos1.as.ntu.edu.tw) 6of6