Impacts of intraseasonal oscillation on the onset and interannual variation of the Indian summer monsoon

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1 Chinese Science Bulletin 2009 SCIENCE IN CHINA PRESS Springer Impacts of intraseasonal oscillation on the onset and interannual variation of the Indian summer monsoon QI YanJun 1,2,3, ZHANG RenHe 2, LI Tim 4 & WEN Min 2 1 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China; 2 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing , China; 3 Graduate University of the Chinese Academy of Sciences, Beijing , China; 4 International Pacific Research Center and Department of Meteorology, University of Hawaii at Manoa, Honolulu 96816, HI, USA The role of the intraseasonal oscillation (ISO) on the seasonal and interannual variations of the Indian summer monsoon is investigated based on the analysis of observational data. It is shown that the ISO significantly contributes to the establishment of low-level westerlies during the monsoon onset and developing periods. The effect of the ISO on the annual cycle of the monsoon is through nonlinear eddy momentum transport. On the interannual timescale, the Indian summer monsoon rainfall exhibits a significant out-of-phase relationship with the ISO intensity over the Indian monsoon region. In strong ISO years it appears the weak monsoon when there is an abnormal high over the India subcontinent in the lower troposphere. In weak ISO years there exists an abnormal low and the strong monsoon appears. intraseasonal oscillation (ISO), Indian summer monsoon, interannual variation The activity of intraseasonal oscillations (ISOs) in the tropics is closely associated with the variability of the Asian summer monsoon [1 5]. Especially over India the active and break periods of the summer monsoon are usually associated with the northward propagating ISOs from the equatorial Indian Ocean [1 3]. The subseasonal variability of the Indian monsoon exhibits two distinctive periodicities in days and days respectively [6,7]. Here we refer to the former as the bi-weekly oscillation (BWO) and the latter as ISO. Many previous studies focused on the characteristics of the day ISO due to its close relationship with the annual cycle and interannual variability of the Indian summer monsoon [8,9]. For example, the onset and withdraw of the Asian summer monsoon are often related to the evolution and phase of ISOs [10,11]. Goswami et al. [9,12] found that the low pressure systems which associated with the monsoon are usually modulated by the ISO activity during the onset and developing stage of Indian summer monsoon. These studies depicted the impact of ISO on the variations of the Indian summer monsoon qualitatively by using the composite and statistical methods. Different from the studies above, we analyze the contribution of ISO to the monsoon quantitatively based on the atmospheric motion equations during the onset and evolution of the Indian monsoon, and try to explain the physical mechanism how the ISO affects the monsoon annual cycle through a possible upscale feedback process. Simultaneously, the interannual relationship between the summer ISO intensity and the Indian monsoon strength is investigated. The data used for this analysis include the NCEP/NCAR reanalysis at a resolution of , daily outgoing longwave radiation (OLR) data from NOAA (also at a resolution) and the all-india rainfall index (AIRI, averaged rainfall in June Received May 27, 2008; accepted August 22, 2008; published online November 4, 2008 doi: /s z Corresponding author ( renhe@cams.cma.gov.cn) Supported by National Basic Research Program of China (Grant No. 2006CB403602), National Natural Science Foundation of China (Grant Nos and ) and Chinese COPES Program (Grant No. GYHY ) csb.scichina.com Chinese Science Bulletin March 2009 vol. 54 no

2 September based on rain gauges across India) for the period of The most pronounced circulation feature associated with the Asian summer monsoon is the prevailing westerly at low level in the monsoon region. Figure 1 shows climatological 850-hPa winds in June-September. During the monsoon season, the strong westerlies dominate the region from 5 to 25 N and from the Arabian Sea to the Bay of Bengal. Figure 1 Climatology 850 hpa wind in summer (June September) for To examine whether the ISO perturbation plays a role in setting up the seasonal mean monsoon westerlies, we consider the following zonal momentum equation: u u u 1 p + u + v fv= + Fx, (1) t x y ρ x where f is the Coriolis parameter; u, v represent the zonal and meridional winds, respectively; ρ is the air density; and p is the pressure; F x denotes the frictional term. For an arbitrary variable A (x,y,t), one may express it as a sum of the mean state A and a perturbation component A : A = A+ A, (2) where a bar denotes the mean monsoon quantity (with a period longer than 90 days), and a prime represents the intraseasonal perturbation (at a period of days). Substituting eq. (2) into eq. (1), applying the continuous equation, and considering only the impact of the ISO perturbation on the mean flow, one may derive the following equation: u uv uu. (3) t y x The equation above represents a nonlinear dynamical relationship between the ISO and the seasonal mean monsoon. It states that the tendency of the climatological monthly zonal wind at 850 hpa, u, is determined in part by the nonlinear eddy momentum transport owing to day ISO activity. Since the box in Figure 1 outlines the region of the strongest westerlies in the monsoon region, we choose this box to investigate the nonlinear dynamic impact of the ISO on the establishment of the monsoon westerly. A harmonic analysis is applied to the 850 hpa wind over the box for each year from 1975 to The first 4 harmonics are used to represent the annual cycle of the seasonal mean monsoon, and the tendency ( u / t) is then calculated based on the mean annual cycle. To reveal the nonlinear rectification effect, the calculated uv / y uu / x eddy momentum transport term ( ) are also subject to a time filtering and is projected into the first 4 harmonics. Figure 2 shows the composite time series of the climatological 850-hPa zonal wind ( u ), the zonal wind u / t, and the ISO eddy momentum tendency ( ) transport term ( uv / y uu / x) averaged over the monsoon region (i.e., the box in Figure 1) perturbation during the period of The zonal wind at lower troposphere (dotted line in Figure 2) exhibits a clear annual variation, with easterlies prevailing from October to May and westerlies from the middle May to the early October. The maximum westerly occurs in the middle of July, and the maximum positive tendency of the zonal wind u / t (dashed line in Figure 2) appears in June. This implies that the monsoon westerly rapidly develops during the Indian monsoon onset period (June). The tendency of the zonal wind becomes negative or near zero from the middle of July to the end of the year. Figure 2 shows that the ISO-induced nonlinear eddy momentum transport contributes to 40% of the observed zonal wind tendency during the monsoon onset period (June). This indicates that the ISO has a significant contribution to the establishment of the monsoon westerly BRIEF COMMUNICATION ATMOSPHERIC SCIENCES QI YanJun et al. Chinese Science Bulletin March 2009 vol. 54 no

3 Figure 2 Time series for 850 hpa zonal wind (dotted line; left ordinate, unit: m/s), tendency of zonal wind (dashed line; right ordinate, unit: m/s 2 ) and eddy momentum transport by the ISO perturbation (solid line; right ordinate, unit: m/s 2 ) over the region of ( N, E). during its developing stage. The correlation between the time series of the observed zonal wind tendency (the dashed line in Figure 2) and the eddy momentum transport term (the solid line in Figure 2) throughout the entire annual cycle reaches 0.67, suggesting that the ISO perturbation not only plays an important role in the triggering and establishment of the monsoon westerlies, but also contribute to the wind tendency in other periods. For example, when the westerlies weaken from the middle of July, the ISO contribution also changes to negative. This indicates that the ISO may contribute to the negative wind tendency during the weakening stage of the monsoon westerly. The significant correlation between the two time series points out the overall role of ISO in the annual cycle of the Indian summer monsoon. The impact of the ISO perturbation on the annual variation of the monsoon is mainly through the convergence of the eddy momentum transport flux. Thus, the annual cycle of the Indian monsoon partially depends on the upscale feedback of ISO perturbations. Next, we examine the relationship between the interannual variation of the ISO intensity and the Indian monsoon rainfall during boreal summer. The summer ISO intensity is measured by the standard deviation of day filtered OLR in June September. The linear correlation between the summer mean ISO intensity at each grid and AIRI for the period of exhibits a strong out-of-phase relationship over the Indian monsoon region (Figure 3(a)). This indicates that the enhanced (reduced) ISO activity corresponds to a weakened (strengthened) seasonal mean Indian monsoon. This points out a possible two-way interaction scenario between the ISO and the seasonal mean monsoon. Figure 3(b) shows the time series of AIRI and the ISO intensity averaged over the box (10 20 N, E) shown in Figure 3(a). Both the ISO intensity and the summer mean monsoon rainfall exhibit strong interannual variations. The correlation between the two time series reaches We define the year with the ISO intensity above 1 (less than 1) standard deviation as a strong (weak) ISO year (Figure 3(b)). Based on the criterion, we select 4 strong ISO years (1979, 1986, 1987, 2002) and 4 weak ISO years (1980, 1983, 1985, 1993), respectively. Figure 4 shows the composites of 850-hPa wind anomalies in summer (June September) for the strong and weak ISO years. In the strong ISO composite (Figure 4(a)), pronounced easterly anomalies appear along 15 N from the eastern Bay of Bengal westward to the western Arabian Sea, whereas strong westerlies are seen north of 20 N. As a result, an abnormal high appears over the India subcontinent. In the weak ISO composite (Figure 4(b)), the circulation anomaly at 850-hPa is reverse compared with that of the strong ISO composite. The westerly anomalies dominate the regions along 10 N from the Arabian Sea to the western Bay of Bengal, while 882 QI YanJun et al. Chinese Science Bulletin March 2009 vol. 54 no

4 BRIEF COMMUNICATION Figure 3 (a) Correlation coefficients between the summer (June September) ISO intensity and AIRI for period Shading areas exceed the 95% significance level. (b) Time series of the standard deviation anomaly of the day filtered OLR field averaged over the box in Figure 3(a) (bar) and the all-india monsoon rainfall anomaly (curve). Two thin solid lines correspond to ±1 standard deviation. the easterly anomalies are located north of 20 N. As a result, the India Peninsular is dominated by an abnormal low in the lower troposphere. The low-level abnormal high (low) is associated with a weakened (strengthened) summer mean monsoon over India. The relationship is consistent with the negative correlation between the summer ISO intensity and the seasonal mean monsoon. In this study, the impact of ISO on the establishment and evolution of the monsoon mean westerly over the Indian monsoon region is investigated. Our diagnosis of the NCEP/NCAR reanalysis data reveals that the ISO may impact the annual cycle of the monsoon circulation through the nonlinear eddy momentum transport. Also we reveal the relationship between the interannual variations of the boreal summer ISO intensity and the Indian monsoon strength. A significant negative correlation between them appears over the Indian monsoon region. The enhanced (reduced) ISO intensity over India corresponds to the weakened (strengthened) monsoon anomaly, with the low-level anticyclonic (cyclonic) circulation anomaly appearing in the India Peninsula. Following this study, we intend to further analyze the physical cause of the negative ISO-mean monsoon relationship and extend the current analysis to other monsoon regions such as the East Asian and western North Pacific monsoon regions due to the close linkage between the Indian and East Asian summer monsoon [13]. These results will be reported elsewhere. ATMOSPHERIC SCIENCES QI YanJun et al. Chinese Science Bulletin March 2009 vol. 54 no

5 Figure 4 Composite patterns of the summer (June September) wind anomaly at 850 hpa in strong (a) and weak (b) ISO years. 1 Krishnamurti T N, Subrahmanyam D. The day mode at 850 mb during MONEX. J Atmos Sci, 1982, 39(9): Yasunari T. Cloudiness fluctuations associated with the Northern Hemisphere summer monsoon. J Meteorol Soc Jpn, 1979, 57(3): Sikka D R, Gadgil S. On the maximum cloud zone and the ITCZ over Indian longitudes during the southwest monsoon. Mon Weather Rev, 1980, 108(11): Li C Y, Long Z X, Zhang Q Y. Strong/weak summer monsoon activity over the South China Sea and atmospheric intraseasonal oscillation. Adv Atmos Sci, 2001, 18(6): He J H. Discussion of meridional propagation mechanism of quasi 40-day oscillation. Adv Atmos Sci, 1990, 7(1): Krishnamurti T N, Bhalme H N. Oscillations of monsoon system. Part I. Observational aspects. J Atmos Sci, 1976, 33(10): Wen M, Zhang R. Possible maintaining mechanism of climatological atmospheric quasi-biweekly oscillation around Sumatra. Chin Sci Bull, 2005, 50(10): Wang B, Webster P J, Kikuchi K, et al. Boreal summer quasi-monthly oscillation in the global tropics. Clim Dyn, 2006, 27: Goswami B N, Mohan R S A. Intraseasonal oscillations and interannual variability of the Indian summer monsoon. J Clim, 2001, 14(6): Murakami T, Chen L X, Xie A. Relationship among seasonal cycles, low-frequency oscillations and transient disturbances as revealed from outgoing longwave radiation data. Mon Weather Rev, 1986, 114(8): Wang B, Xu X. Northern hemisphere summer monsoon singularities and climatological intraseasonal oscillation, J Clim, 1997, 10(5): Goswami B N, Ajayamohan R S, Xavier P K, et al. Clustering of synoptic activity by Indian summer monsoon intraseasonal oscillations. Geophys Res Lett, 2003, 30(8), 1431, doi: /2002GL Zhang R. Relations of water vapor transports from Indian monsoon with those over East Asia and the summer rainfall in China. Adv Atmos Sci, 2001, 18(5): QI YanJun et al. Chinese Science Bulletin March 2009 vol. 54 no

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