Impacts of the basin-wide Indian Ocean SSTA on the South China Sea summer monsoon onset

INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: 1579 1587 (2008) Published online 30 January 2008 in Wiley InterScience (www.interscience.wiley.com).1671 Impacts of the basin-wide Indian Ocean SSTA on the South China Sea summer monsoon onset Yuan Yuan, a,c,d Wen Zhou, a,b Johnny C. L. Chan a,b * and Chongyin Li c a City U-IAP Laboratory for Atmospheric Sciences, City University of Hong Kong, Hong Kong, China b Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China c LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China d Graduate School of Chinese Academy of Sciences, Beijing, China ABSTRACT: This article explores the impacts of the Indian Ocean basin-scale sea surface temperature anomaly (SSTA) on the South China Sea (SCS) summer monsoon onset. Basin-wide warming in the tropical Indian Ocean (TIO) is found to occur in the spring following an El Niño event, and the opposite occurs for a La Niña event. Such changes of the Indian Ocean SSTA apparently prolong the El Niño-Southern Oscillation (ENSO) effects on the subsequent Asian summer monsoon, mainly through modifying the strength of the Philippine Sea anti-cyclone. Warming in the TIO induces an anomalous reversed Walker circulation over the tropical Indo Pacific Ocean, which leads to descending motion, and hence suppressed convection in the western Pacific. The intensified Philippine Sea anticyclone in May and June advances more westward and prevents the extension of the Indian Ocean westerly flow into the SCS region, thereby causing a late SCS monsoon onset. The case is opposite for the TIO cooling such that the Philippine Sea anti-cyclone weakens and retreats eastward, thus favouring an early onset of the SCS monsoon. Copyright 2008 Royal Meteorological Society KEY WORDS Indian Ocean basin-wide SSTA; South China Sea summer monsoon onset; Philippine Sea anti-cyclone Received 25 April 2007; Revised 15 November 2007; Accepted 29 November 2007 1. Introduction The El Niño-Southern Oscillation (ENSO) exhibits a great influence on the inter-annual variability of the global climate (Webster et al., 1998), including the Indian monsoon (Khandekar and Neralla, 1984), the summer precipitation over east Asia (Nitta, 1986; Huang and Wu, 1989; Li, 1990a), and the east Asian winter monsoon (Li, 1990b; Zhang et al., 1996). Further, Wang et al. (2000) documented that ENSO effects can persist into the following summer, causing rainfall anomalies over East Asia through the Pacific-East Asian teleconnection, in which an anomalous anti-cyclone over the Philippine Sea is forced by El Niño and maintained by local air-sea interactions. It has also been widely recognized that basin-wide warming in the tropical Indian Ocean (TIO) lags behind a mature phase of an El Niño event by a few months, while a cooling occurs after a La Niña event (Nigam and Shen, 1993; Tourre and White, 1995; Chambers et al., 1999). Based on the atmospheric bridge concept (Klein et al., 1999), recent studies proposed that associated with an El Niño, warm sea surface temperatures (SSTs) in the TIO appear to be primarily driven by surface heat flux * Correspondence to: Johnny C. L. Chan, Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China. E-mail: johnny.chan@cityu.edu.hk anomalies, such as reduced latent heat loss from the ocean and increased solar radiation due to decreased wind speed and suppressed convective activity respectively (Venzke et al., 2000; Lau and Nath, 2003; Shinoda et al., 2004). In addition, Xie et al. (2002) and Huang and Kinter (2002) stated that much of the southwest Indian Ocean SST anomaly (SSTA) is caused by oceanic Rossby waves that propagate from the east. Hence, through both atmospheric and oceanic processes, ENSO may induce SST variations over the TIO. It is, therefore, logical to expect that the basin-wide Indian Ocean SSTA might play a role in prolonging ENSO effects into the following year. One such possible effect is on the South China Sea (SCS) summer monsoon onset that occurs in early summer and marks the beginning of the east and southeast Asian summer monsoon (Tao and Chen, 1987; Lau and Yang, 1997; Ding and Chan, 2005). The objective of this study is, therefore, to investigate the possible impacts of the Indian Ocean SSTA on the SCS monsoon onset. The data and methodology are described in Section 2. A TIO index is then defined in Section 3 to identify the extreme cases of Indian Ocean warming and cooling. Section 4 analyses the composite low-level circulations during the SCS summer monsoon onset for the extreme warming and cooling cases. Physical mechanisms are further investigated in Section 5 and Section 6 gives a summary and discussion. Copyright 2008 Royal Meteorological Society

1580 Y. YUAN ET AL. 2. Data and methodology The data used in this study are mainly from the 55- year (1948 2002) National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, including the monthly and 5- day (or pentad) mean wind and geopotential heights at standard pressure levels, with a horizontal resolution of 2.5 latitude by 2.5 longitude (Kistler et al., 2001). The monthly mean SST data on a 1 latitude longitude grid are derived from the Hadley Centre of the UK Meteorological Office, with a 55-year period of 1948 2002 (Rayner et al., 2003). Inter-annual anomalies are obtained by deviations from the monthly climatology. To isolate the basin-wide SSTA in the TIO, an empirical orthogonal function (EOF) analysis is applied on the spring [(March-April-May MAM)] SSTA in the TIO (40 E 120 E, 20 S 20 N). Composite analyses are performed on the extreme TIO warming and cooling case, with the significance of the results tested using the classical Student s t-test (Chervin and Schneider, 1976). 3. Tropical Indian Ocean basin-wide SSTA As mentioned in the Introduction, mainly through changing the ocean latent heat flux and solar radiation, ENSO could force a basin-wide warming/cooling in the TIO (Venzke et al., 2000; Lau and Nath, 2003), which usually peaks in the boreal spring and lasts till the summer (Pan and Oort, 1983; Nigam and Shen, 1993). The first principal component (PC1) in the EOF analysis of the spring (MAM) SSTA in the TIO shows a basin-scale variation of the same sign, which explains nearly 60% of the total variance (Figure 1(a)). A linear trend is evident in the time series of the PC1 coefficients (Figure 1(b)), the reason for which is beyond the scope of this paper. The normalized detrended coefficient is defined as the TIO index to represent the basinscale SSTA structure over the Indian Ocean in spring (Figure 1(c)). Extreme warming (cooling) cases are chosen to be those during which the TIO index value is above 0.75 (below 0.75) of the standard deviation (Table I). It is clear from Table I that 10 out of 12 warming springs follow an El Niño event, and 8 out of 13 cooling springs follow a La Niña event, further confirming the possible lagged response of the Indian Ocean SSTA to a previous ENSO event. The SCS summer monsoon onset pentads (based on Wang et al., 2004 and Zhou and Chan, 2007) for each warming/cooling case suggest that most warming cases have a monsoon onset after pentad 28 (the climatological mean onset pentad is 28 (May 16 20), Ding, 1994; Ding and Chan, 2005), except 1952 and 1953. On the other hand, in most cooling cases (except 1963, 1965 and 1968), the SCS summer monsoon occurs earlier than the climatological mean. The TIO warming (cooling) thus appears to be related to a late (an early) SCS monsoon onset. Figure 1. (a) The first principal component (PC1) of the spring (MAM) SSTA in the TIO (40 E 120 E, 20 S 20 N). (b) The time series of the PC1 coefficients (open circles), with its linear trend (long-dashed line). (c) The normalized detrended time series of the PC1 coefficients (open circles). The long-dashed lines indicate ±0.75 of the standard deviation of the time series. Recently, Zhou and Chan (2007) pointed out that a warm (cold) ENSO event tends to induce a late (an early) SCS monsoon onset in the following year through the propagation of cold (warm) sub-surface water into the western North Pacific. The current result suggests that TIO warming/cooling may be another contributing factor in prolonging the ENSO effects on the SCS summer monsoon onset.

INDIAN OCEAN SSTA AND THE SOUTH CHINA SEA SUMMER MONSOON ONSET 1581 Table I. Extreme warming and cooling cases in the TIO with corresponding onset pentad of the SCS summer monsoon. The warming/cooling springs are selected based on ±0.75 of standard deviation of the TIO index, which is defined as the normalized detrended time series of the first EOF mode on the spring SSTA over the TIO (40 E 120 E, 20 S 20 N). An asterisk indicates an El Niño (a La Niña) that occurs in the winter prior to a warming (cooling) year, as defined in Trenberth (1997) and Wang and Gong (1999). The shaded years are not included in the composites because of the abnormal onset dates from each group. Warming years SCS monsoon onset pentads Cooling years SCS monsoon onset pentads 1952 27 1951 25 1953 26 1963 30 1958 29 1965 29 1959 30 1968 30 1969 29 1971 25 1970 32 1972 26 1973 33 1974 26 1983 31 1975 23 1987 32 1976 26 1988 29 1984 24 1991 32 1986 27 1998 29 1989 28 2000 26 10/12 8/13 4. South China Sea summer monsoon onset The composite low-level circulations during the SCS summer monsoon onset for the extreme warming/cooling cases show similar features, but with those of the warming cases lagging by about two pentads. In the warming composite, strong easterlies extend from the western Pacific to the southern SCS region, along with an intense anti-cyclone covering the SCS and Southeast China in pentads 27 and 28 (Figure 2(a), (b)). Even though the Asian monsoon circulation has formed over the Indian Ocean in pentad 28, it cannot advance into the SCS but only northward to the Indo China Peninsula (ICP). Then in pentad 29 (Figure 2(c)), the anti-cyclone retreats eastward, followed by the Indian Ocean southwesterly winds extending to the northern SCS. The full establishment of the SCS summer monsoon occurs around pentad 30 31 (not shown), later than the climatological mean. For the cooling composite, the main body of the lowlevel Philippine Sea anti-cyclone has already moved out of the SCS region early in pentads 25 and 26 (Figure 3(a), (b)), with weak westerlies in the southern SCS. The Indian Ocean monsoon flow gets stronger in the next pentad, and most of the ICP, SCS, and Southeast China are occupied by the westerly/southwesterly winds (Figure 3(c)), indicating an earlier monsoon onset. The evolutions for the warming and cooling cases described above suggest that the anti-cyclone over the Philippine Sea may be important in linking the Indian Ocean SSTA and the timing of the SCS summer monsoon onset. This will be examined further in the next section. 5. Physical mechanisms As expected, the composite SSTA in May-June (MJ) for warming cases shows a basin-wide warming over most of the Indian Ocean, along with a dissipating El Niño in the eastern equatorial Pacific (Figure 4(a)). The cooling composite has a nearly opposite pattern, with a decaying La Niña and negative SSTA in the TIO (Figure 4(b)). The warm SSTA structure would likely result in lowlevel winds towards the Indian Ocean, as illustrated by the strong (and statistically significant) 850-hPa easterly anomalies across the Maritime Continent, and westerly anomalies in the equatorial western Indian Ocean (Figure 5(a)). An anomalous low-level cyclone can be identified in the central Indian Ocean around (75 E, 5 N), suggesting the possibility of enhanced convection caused by significant anomalous ascending motion above the warm water (Figure 6(a)). At 150 hpa, anomalous westerly winds are also significant from the eastern Indian Ocean to the western Pacific (Figure 7(a)). An anomalous reversed Walker circulation thus forms over the Indo Pacific Ocean, featured as low-level easterly winds from the western Pacific to the TIO, rising motion over the TIO, upper-level westerlies, and then subsidence in the western Pacific (Figure 6(a)). The Philippine Sea anti-cyclone should therefore be intensified. The condition for the cooling composite is just the opposite, with anomalous low-level westerlies (Figure 5(b)) and upper-level easterlies (Figure 7(b)) overlying the equatorial eastern Indian Ocean and western Pacific, and also subsidence over the TIO cold water (Figure 6(b)). Hence, in the western Pacific, abnormal rising motion, though relatively weak (Figure 6(b)), might weaken the Philippine Sea anti-cyclone to some extent. The 500-hPa anomalous geopotential heights in MJ further confirm the variation of the Philippine Sea anticyclone caused by the Indian Ocean SSTA. In the warming composite (Figure 8(a)), the tropical Indo Pacific Ocean is dominated by positive anomalies of the 500-hPa geopotential height, with maximum values of 10 gpm in the ICP, SCS and the Philippines. The intensified subtropical high exhibits a centre of 5880 gpm over the western Pacific with the 5860-gpm contour extending westward to the west of the ICP. For the cooling composite (Figure 8(b)), negative anomalies are significant in the tropical Indo Pacific. The western Pacific sub-tropical high is relatively weak, with no 5880-gpm contour and the western edge of the 5860-gpm contour only reaching the Philippines, almost 30 longitude eastward compared with that of the warming case. These results suggest that warm SSTs in the TIO could induce an anomalous reversed Walker Cell from the Indian Ocean to the western Pacific, with subsidence and suppressed convection over the latter ocean so as to reinforce the sub-tropical high. The westward-extending anti-cyclone along with the enhanced low-level easterly winds to its south flank would inhibit the extension of the Indian Ocean westerly flow into the SCS region, in turn, causing a late SCS monsoon onset. The situation for cooling cases will just be the opposite.

1582 Y. YUAN ET AL. Figure 2. Composite pentad mean 850-hPa winds for warming cases in pentad (a) 27, (b) 28 and (c) 29. Shaded areas indicate that either zonal or meridional wind is significant above the 95% confidence level. 6. Summary and discussions This article examines the impacts of the TIO basin-wide SSTA on the SCS summer monsoon onset. It is found that warming (cooling) in the TIO apparently leads to a late (early) SCS monsoon onset by enhancing (weakening) the Philippine Sea anti-cyclone. As most warm (cold) cases mature in the spring following an El Niño (a La Niña) event, Indian Ocean SSTA is suggested to prolong ENSO influences on the SCS summer monsoon onset in the subsequent year. We further investigate the possible mechanisms for the variation of the critical Philippine Sea anti-cyclone caused by the Indian Ocean SSTA. After the mature phase of an El Niño event, positive SSTA prevails in the Indian Ocean during the following MJ, which

INDIAN OCEAN SSTA AND THE SOUTH CHINA SEA SUMMER MONSOON ONSET 1583 Figure 3. As in Figure 2, but for cooling cases in pentad (a) 25, (b) 26 and (c) 27. likely triggers upward motion and increased convection there. An anomalous reversed Walker Cell thus forms over the tropical Indo Pacific Ocean. The associated anomalous subsidence and suppressed convection in the western Pacific help strengthen the sub-tropical high. On the contrary, following a previous La Niña event, TIO cooling in the following MJ would weaken the sub-tropical high. Consequently, the westward-advancing (eastward-retreating) sub-tropical high prevents (favours) the extension of the equatorial westerly monsoon flow from the Indian Ocean into the SCS region, leading to a late (an early) SCS summer monsoon onset. Through numerical simulations, Wang et al. (2000) and Lau and Nath (2000) suggested that the persistence of the Philippine Sea anti-cyclone is forced by El Niño and maintained by local air-sea interactions. More recently,

1584 Y. YUAN ET AL. Figure 4. Composite SSTA structure in May-June (MJ) for (a) warming and (b) cooling cases, with solid lines (dots) referring to positive (negative) SSTA. Contour interval: 0.2 C. Figure 5. Composite 850-hPa anomalous winds in MJ for (a) warming and (b) cooling cases, with shaded areas for zonal or meridional wind significant above the 95% confidence level and C ( A ) for anomalous cyclone (anti-cyclone).

INDIAN OCEAN SSTA AND THE SOUTH CHINA SEA SUMMER MONSOON ONSET 1585 Figure 6. Composite anomalous zonal-vertical circulations in MJ averaged in (Equator 10 N) for (a) warming and (b) cooling cases, with shaded areas for vertical motion significant above the 90% confidence level. Zhou and Chan (2007) documented that the strength of the anti-cyclone can be enhanced (suppressed) by the propagation of cold (warm) sub-surface water into the western North Pacific. The current analyses provide an additional mechanism for the strengthening or weakening of the sub-tropical high caused by the Indian Ocean warming or cooling, in agreement with the modeling results of Annamalai et al. (2005). As the Philippine Sea anti-cyclone is an important system in the Pacific East Asian teleconnection (Wang et al., 2000), its maintenance may involve complicated physical processes, with the above three dynamical processes all possibly contributing to its persistence. It should be noted that the current results are derived from extreme warming/cooling springs. Even then, not every TIO warming case has a late SCS monsoon onset, and not all delayed monsoon onsets are associated with positive SSTA over the Indian Ocean. The same can be said for the cooling cases and early monsoon onsets. Several TIO warming/cooling cases are also not associated with previous ENSO events. Therefore, further analyses are still required to investigate the impacts of the Indian Ocean SSTA on the SCS summer monsoon onset, and the teleconnection of the ENSO forcing in the Indian Ocean. Furthermore, our additional research suggests that the anomalous Philippine Sea anti-cyclone resulted from the Indian Ocean SSTA can last till the following summer. As the strength and position of the sub-tropical high are closely related with the summer rainfall in East Asia (Yang and Sun, 2003), we will examine the possible influences of the Indian Ocean SSTA on the summer rainfall pattern over China in a future study.

1586 Y. YUAN ET AL. Figure 7. As in Figure 5, but for the 150-hPa anomalous winds. Figure 8. Composite 500-hPa anomalous geopotential heights averaged in MJ for (a) warming and (b) cooling cases, with the thin solid (long-dashed) lines for positive (negative) anomalies, and the thick lines for the 5860 and 5880-gpm contours as the boundary of the sub-tropical high. Contour interval for anomalies is 5 gpm, and shaded areas indicate significance above 95% confidence level.

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