Remote forcing of Indian Ocean warming on Northwest Pacific during ...

Chinese Journal of Oceanology and Limnology Vol. 31 No. 6, P. 1363-1371, 2013 http://dx.doi.org/10.1007/s00343-013-3075-1

Remote forcing of Indian Ocean warming on Northwest Pacific during El Niño decaying years: a FOAM model approach* HU Haibo (胡海波)1, **, HONG Xiaoyuan (洪晓媛)1, 2, 3, ZHANG Yuan (张媛)4, YANG Xiuqun (杨修群)1, LIU Wei (刘伟)5, LU Huaguo (卢华国)6, YANG Jianling (杨建玲)7 1

School of Atmospheric Sciences, Nanjing University, Nanjing 210093, China

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Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

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University of Chinese Academy of Sciences, Beijing 100049, China

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School of Marine Sciences, Nanjing University of Information Science and Technology, Nanjing 210044, China

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Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 95101, USA

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School of Languages and Cultures, Nanjing University of Information Science and Technology, Nanjing 210044, China

7

Key Laboratory of Meteorological Disaster Preventing and Reducing in Ningxia, Ningxia Meteorological Bureau, Yinchuan 750002, China

Received Mar. 22, 2013; accepted in principle Apr. 23, 2013; accepted for publication May 6, 2013 © Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2013

Abstract This paper attempts to analyze in detail the remote influence of the Indian Ocean Basin warming on the Northwest Pacific (NWP) during the year of decaying El Niño. Observation data and the Fast OceanAtmosphere coupled Model 1.5 were used to investigate the triggering conditions under which the remote influence is formed between the positive sea surface temperature (SST) anomaly in the North Indian Ocean and the Anomalous Northwest Pacific anticyclone (ANWPA). Our research show that it is only when there is a contributory background wind field over the Indian Ocean, i.e., when the Indian Summer Monsoon (ISM) reaches its peak, that the warmer SST anomaly in the North Indian Ocean incites significant easterly wind anomalies in the lower atmosphere of the Indo-West tropical Pacific. This then produces the remote influence on the ANWPA. Therefore, the SST anomaly in the North Indian Ocean might interfere with the prediction of the East Asia Summer Monsoon in the year of decaying El Niño. Both the sustaining effect of local negative SST anomalies in the NWP, and the remote effect of positive SST anomalies in the North Indian Ocean on the ANWPA, should be considered in further research. Keyword: El Niño decaying year; Indian Ocean Basin warming (IOB warming); Indian Summer Monsoon (ISM); Fast Ocean-Atmosphere Model 1.5 (FOAM1.5); anomalous Northwest Pacific (NWP) anticyclone

1 INTRODUCTION An obvious teleconnection is recognized between the East Asia Monsoon and the Tropical Pacific during the year of decaying El Niño (the second year of the El Niño 2-year period), which intensifies the East Asian Summer Monsoon precipitation (Fu and Teng, 1988; Zhang et al., 1996; Weisberg and Wang, 1997; Kawamura, 1998, Tao and Zhang, 1998; Wang et al., 2000). The Anomalous Northwest Pacific anticyclone (ANWPA), a crucial phenomenon of the system, bridges the sea surface temperature (SST) anomalies in the Equatorial Pacific and the Summer Monsoons

over East Asia (Wang et al., 2000). The lowertropospheric ANWPA develops rapidly during late fall when El Niño reaches a positive maximum phase and it remains until the spring or summer of the following year. At least two mechanisms have been recognized for

* Supported by the National Basic Research Program of China (973 Program) (Nos. 2010CB428504, 2012CB956002), the National Natural Science Foundation of China (Nos. 40906005, 41105059, 41065005, GYHY201106017, GYHY201306027), and the National Key Technology Research and Development Program (No. 2009BAC51B01) ** Corresponding author: [email protected]

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the persistence of the ANWPA in the year of decaying El Niño. The first emphasizes the local positive oceanatmosphere feedback between negative Northwest Pacific (NWP) SST anomalies and a local negative lower-tropospheric vorticity anomaly (Weisberg and Wang, 1997; Wang et al., 2000). Against the positive anomaly in SST in the eastern-central equatorial Pacific in a year of a developing El Niño (the first year of the El Niño 2-year period), a lower-tropospheric cyclonic anomaly is generated to the west of this warming anomaly in Rossby-wave mode, which then spreads westward. To the west of the cyclonic anomaly, the northeast trade winds are strengthened, generating increased evaporation and a negative SST anomaly in the NWP. The negative SST anomaly then causes an anticyclonic anomaly and generates a northeast wind anomaly, which cools the SST further through Wind-Evaporation-SST (WES) feedback (Weisberg and Wang, 1997; Wang et al, 2000, 2003; Xie et al., 2009). Especially in August, the atmosphere over the NWP becomes increasingly sensitive to the local SST forcing because of the background mean precipitation (Xiang et al., 2013). However, the mature phase of El Niño can only maintain the prevailing trade winds to the following spring, by which time, during the Asian Summer Monsoon season, the southwesterly trade winds dominate, especially over the western part of the NWP (Hu et al., 2011). However, Xiang et al. (2013) mention that the WES feedback could be viable over the NWP despite the weakened easterly winds during summer. As a result, the positive WES feedback might weaken in the eastern part of the NWP (Xiang et al., 2013) and possibly even turn into negative feedback over the western part (Wu et al., 2009; Hu et al., 2011). The second mechanism for the persistence of the ANWPA is the remote forcing responsible for the Indian Ocean Basin (IOB) warming. The Indian Ocean preserves the SST anomaly signal of El Niño events in the eastern-central Pacific Ocean like a capacitor (Du and Xie, 2008; Du et al., 2009; Xie et al., 2009). With the decay of El Niño SST anomalies in spring, a positive SST anomaly might persist over the Indian Ocean (Watanabe and Jin, 2002, 2003; Krishnamurthy and Kirtman, 2003; Misra, 2004; Kug et al., 2005; Yang et al., 2007). As the Indian Ocean warms, the tropospheric temperature increases owing to the moist adiabatic adjustment, which might generate a Kelvin wave traveling through the IndoWest Pacific (Xie et al., 2009; Du et al., 2011). Over the NWP, the equatorial Kelvin wave promotes the

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easterly anomalies. To the north of these easterly anomalies, resultant divergence depresses convection and forms the lower-tropospheric anomalous anticyclone over a negative SST anomaly in the NWP (Xie et al., 2009). It remains difficult to accurately distinguish the leading role between the IOW and ANWPA, because Wang et al. (2013) and Xiang et al. (2013) have proven that the IOW is not only a forcing to the ANWPA, but that it might also be a result of air-sea interaction associated with the ANWPA over the NWP. In the remote forcing mechanism, it is not clear whether the tropical Kelvin wave could exist persistently during the IOB warming period, because the Indian Ocean is the most significant tropical monsoon basin. As shown in Hu et al. (2011), the outbreak of the East Asia Summer Monsoon would undermine the WES feedback in the NWP. Therefore, a question is raised; how to understand the detail of the Indian Summer Monsoon (ISM) transition through Ekman divergence with a Kelvin wave mechanism in the Indo-West Pacific? Considering this problem, we studied the remote forcing of the IOB warming and its climatological implication when the ISM breaks out, to reveal the role of the Indian Ocean seasonal transition in the Kelvin-wave-induced Ekman divergence. In Section 2, we describe the datasets and the Fast OceanAtmosphere Model 1.5 (FOAM1.5), as well as the design of the model experiments. In Section 3, we show observational diagnoses and atmospheric responses to different scenarios of North Indian Ocean warming in FOAM1.5. A discussion of the issue is presented in Section 4 together with a summary.

2 DATASET AND METHOD 2.1 Data and methods The capacitance effect of the tropical Indian Ocean has been increasingly recognized since 1979 (Huang et al., 2010). Therefore, the data used in our study include the global monthly reanalysis dataset NCEPDOE R2 (hereafter referred to as NCEP-2) from the National Centers for Environment Prediction (NCEP) covering the period 1982–2009. The NCEP-2 dataset is a continuation of the NCEP-1 (Kanamitsu et al., 2002) reanalysis data plan. It has corrected the known artificial error mistaken for the improved edition of NCEP-1, involving sea surface wind speed, surface latent flux and surface sensible flux (http://nomads. ncdc.noaa.gov). Reynolds SST data are used for the ocean sea surface temperature diagnosis. The spatial

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2 Vorticity Index Niño Index IOB Index Precip Index v Index

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Fig.1 Evolutions of monthly mean anomaly fields of Niño-3.4 SST (170°–120°W, 5°S–5°N), Indian Ocean SST (30°–120°E, 30°S–26°N), NWP (125°–147.5°E, 10°–25°N) vorticity (4×105/s), precipitation and meridional component of winds in the North Indian Ocean (40°–120°E, 0°–26°N) of (EI) and (E) from Dec. to the following Nov. All data have been normalized, “(0)”means months in El Niño developing year, “(1)” means months in El Niño decaying year.

resolution of the Reynolds SST is 1°×1° and the temporal resolution is a month. The period covered by this SST data extends from January 1982 to December 2009 (http://www.nhc.noaa.gov/aboutsst.shtml). The CMAP data from 1982.1–2009.9 are used for the precipitation analysis. In all fields, the climatological mean and the linear trends are removed to eliminate the interference of long-term climate change. According to the development of the IOB warming and the consequent climatic effects in an El Niño cycle, two categories of El Niño event are classified by the presence or absence of the IOB warming in the following year, namely: (EI) and (E) (Hu et al., 2011). Furthermore, we define the El Niño index as the SST anomalies in the tropical Pacific Niño3.4 (5°S–5°N, 170°W–120°W), and the IOB warming index as the SST anomalies in the Indian Ocean (30°S–26°N, 30°E–120°E). The beginning of an IOB (El Niño) event is set as the IOB (El Niño) index that exceeds its 0.75 standard deviation for at least 5 months consecutively. In accordance with this criterion, we obtain three (EI) cases (1982/1983, 1987/1988, 1997/1998) and three (E) cases (1991/1992, 1994/1995, 2006/2007), and there are further discernible cases using ERSST data and NCEP-1 atmosphere data (Kug and Kang, 2006; Hu et al., 2011). El Niño events mature in December, then begin decaying and disappear completely by the

following May. However, the IOB warming events occur approximately one season later. A composite analysis is made and the three (EI) events and three (E) events are hereafter named as the composite of (EI) events and (E) events (Fig.1), respectively. 2.2 Model and experiments We used FOAM1.5 to study the influence of the IOB warming on the Northwest Pacific. FOAM1.5 is an atmosphere-ocean interaction model, which has been developed by the Department of Mathematics and Computer Science, of Argonne National Laboratory and Space Science Technology Center, of the Wisconsin-Madison University (Liu et al., 2000; Wu and Liu, 2003). It has a fast calculation speed and is fit for researching natural variability. In FOAM1.5, the combination of a low-resolution (R15) atmosphere model with 18 vertical levels in sigma coordinates and a highly efficient medium-resolution ocean model with 32 vertical levels are used with no flux corrections. It is comparable with higher resolution models of climate simulation. The design of the experiment is inspired by Du et al. (2011), who found that during an El Niño cycle, positive SST anomalies appear in the IOB, especially in the North Indian Ocean, and a baroclinic Kelvin wave is induced in the lower atmosphere of the subtropics in the northern hemisphere. Therefore, the

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positive SST anomalies in the North Indian Ocean are very important in heating the atmosphere. It is still not clear whether the remote effect of the IOB warming continues throughout the entire year, or how such a warming brings about an eastward-spreading baroclinic Kelvin wave over the Northwest Pacific. To address these questions, a 200-year control run (CTRL) and three sets of sensitivity experiments are carried out using FOAM1.5. Additionally, 50 ensemble members are included in each sensitivity experiment, which are initialized on the same particular date of the CTRL in different years, with the same positive temperature anomalies in the mixed layer added to the upper North Indian Ocean (40.781 2°–108.281°E, 0.703 12°–30.234 4°N) in different months. These are referred to as IOB_W3 (initial mixed layer temperature anomaly is added on March 1, at a time long before the ISM breaks out in CTRL), IOB_W5 (same mixed layer temperature anomaly added on May 1, at around the time of the outbreak of the ISM in CTRL) and IOB_W8 (same mixed layer temperature anomaly added on August 1, when the ISM breaks out completely in CTRL). For the initial conditions, mixed layer temperature anomalies are introduced in the upper 40 m (the model mixed layer in the Indian Ocean); the temperature anomalies at the lateral and meridional boundaries are 0°C, and from the border towards the middle, the temperature rises as a sinusoidal function up to 1.5°C. Then the coupled model is freely integrated for 1 year. In each experiment, we use the ensemble mean to examine the coupled response to an initial mixed layer warming over the North Indian Ocean.

3 RELATIONSHIP BETWEEN IOB WARMING AND ANWPA IN OBSERVATION AND FOAM 3.1 Analysis of observation data As mentioned above, the IOB warming usually develops during the El Niño cycle (in EI events) and matures about one season after the peak of El Niño. According to Fig.1, an anticyclonic vorticity anomaly always exists over the NWP in both (EI) and (E) events before El Niño dissipates. However, in the (EI) events, the anticyclonic vorticity remains after the El Niño begins to decay, whereas in the (E) events, the anticyclonic vorticity anomaly disappears or even converts to a cyclonic anomaly in the summer of the decaying El Niño. In addition, the anticyclonic

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anomaly in the (EI) events occurs with a positive IOB warming index value throughout the entire summer of the decaying El Niño (Fig.1), which suggests that the IOB warming is conducive to the persistence of the ANWPA. Notably, from the spring to the late summer of the decaying El Niño, the index of the anticyclonic anomaly intensifies distinctly around August, although the IOB warming index remains nearly constant. It appears that the IOB warming has different effects on the anomalous anticyclone over the NWP during the summer of the decaying El Niño. Figure 2 shows the differences in SST, precipitation and sea surface wind between (EI) and (E) events in the summer of the decaying El Niño. In the (EI) events, accompanying substantially positive SST anomalies in the North Indian Ocean, the local precipitation anomaly intensifies and lasts from June until August. In August especially, the intensity and range of the precipitation anomalies over the North Indian Ocean reach their maximum. However, negative precipitation anomalies and weak SST anomalies in the (E) event appear in the North Indian Ocean in summer. Wind anomalies emerge in distinct patterns corresponding to the (EI) and (E) events. For the (EI) events, an obvious anticyclonic anomaly over the NWP, centered at 10°N and 140°E, persists throughout the entire summer and becomes enhanced in August. In contrast, the (E) events display an anticyclonic anomaly over the NWP, which is weaker and disappears completely around August. Moreover, as El Niño decays in June, the positive SST anomaly over the tropical Pacific turns rapidly into a negative anomaly in the (EI) events, but exhibits little change in the (E) events. Furthermore, the anticyclonic anomaly over the NWP is more obvious with significant anomalies of positive SST and precipitation over the North Indian Ocean in the summer of the decaying El Niño (Fig.2). Further analyses show that although the positive North Indian Ocean SST anomaly in the (EI) events persists invariably, or even abates in spring and summer, the anticyclonic anomaly over NWP strengthens abruptly in August (Figs.1 and 2). In addition, obvious cooling SST anomalies exist in the west of the tropical Pacific from early summer in the year of the decaying El Niño (Fig.2). Wang et al. (2013) proved that this cooling SST could also induce an anticyclone over the NWP. Correspondingly, the precipitation anomaly and seasonal meridional winds over the North Indian Ocean also have maximum values in August (Fig.1).

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Fig.2 Spatial distributions of anomalous fields of SST (shaded, °C), winds (vector, m/s) and precipitation (contour, mm/day) over the Indian-west Pacific in (EI) and (E) events The contour intervals are 2 mm/day, starting from -5 mm/day. All variables are significant at the 90% confidence level according to a t-test.

We argue that the IOB warming effect is directly responsible for the anticyclonic anomaly over the NWP. According to Xie et al. (2009), the IOB warming can induce a Matsuno-Gill pattern response over the Indo-Pacific in the summer of the decaying El Niño, with a Kelvin wave generated in the east, which leads to an anticyclonic anomaly over the NWP (ANWPA) through Ekman divergence. However, our results show that the anticyclonic anomaly over the NWP increases sharply around August, but with no change in the IOB warming index. Meanwhile, the precipitation over the North Indian Ocean increases suddenly around August during the summer of the decaying El Niño (Fig.1). Therefore, a series of questions arise: why does the ANWPA and the precipitation anomaly over the North Indian Ocean increase suddenly around August during the summer of the decaying El Niño? Does the same IOB warming pattern evoke different responses in the ANWPA during the spring and summer of the decaying El Niño? To answer these questions, we carry out three

experiments with identical mixed layer temperature anomalies added in the upper North Indian Ocean in different months. Details of these experiments are described in Section 2.2. 3.2 Modeling results To test the response of atmospheric circulation and precipitation to the same IOB warming pattern at different times, three experiments are actualized as IOB_W3, IOB_W5 and IOB_W8. Figure 3 shows the monthly evolution of precipitation, sea surface wind and SST anomalies of IOB_W3. In this experiment, the positive temperature anomaly in the mixed layer is added to the North Indian Ocean on March 1, which keeps weakening and almost disappears around July. In the first 3 months (March to May), the warm SST anomalies decay slowly with almost the same atmospheric circulation and precipitation responses. From March to May, significant convergence of wind anomalies at the sea surface exist over the warm SST anomalies of the North Indian Ocean, accompanied

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Fig.3 Spatial distributions of SST anomalies (shaded, °C), surface wind anomalies (vector, m/s), precipitation anomalies (contour, mm/day) of IOB_W3 in FOAM model, from March to August Dashed rectangle is the area where the anomalous SST forcing is added. The contour intervals are 1 mm/day. SSTA, precipitation and wind anomalies have passed the t-test.

by obvious positive precipitation anomalies. However, there is little response over the NWP. Subsequently, in June, the precipitation and surface wind anomalies of the North Indian Ocean weaken gradually with a decay of positive SST anomalies. A weak easterly wind anomaly continues over the NWP until August, although the North Indian Ocean SST anomaly almost disappears. Meanwhile, an anticyclonic anomaly appears over the Indo-West Pacific with negative precipitation anomalies over the NWP. Therefore, it seems that the IOB warming might weaken the ongoing El Niño and even lead to a La Niña event with obvious divergent surface wind anomalies over the tropical Pacific (Fig.3), which is similar to the findings of Annamalai et al. (2005). Two experiments are carried out with the same mixed layer temperature anomalies added to the North Indian Ocean on May 1 (IOB_W5) and August 1 (IOB_W8), respectively. In IOB_W5, the atmosphere response resembles that in IOB_W3

during the May–July period, which is limited to the North Indian Ocean with surface wind convergence and little rainfall (not shown). The strength of the local SST anomalies continues to weaken until August, and a slightly stronger easterly wind with an anticyclonic anomaly exists over the NWP, which exhibits greater amplitude than that in IOB_W3. However, the same is not true of IOB_W8. Beyond the local surface wind convergence response over the North Indian Ocean, significant easterly wind anomalies with suppressed precipitation are generated over the NWP as soon as the temperature anomalies in the mixed layer are added to the North Indian Ocean (Fig.4i). Therefore, we conclude that the same IOB warming pattern can provoke different atmospheric responses in terms of wind and precipitation. However, the response of the model is very sensitive to the timing of this mixed layer temperature anomaly in the North Indian Ocean. To explore the mechanism for the different

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Fig.4 SST anomalies (shaded, °C) a. Wind anomalies (vector, m/s); b. Latent heat flux anomalies (contour, interval 10 w/m2); c. Sensible heat flux anomalies (contour levels -15, -10, -5, -1, 1, 5, 10, 15, unit w/m2) in Mar of IOB_W3; d. Precipitation anomalies (shaded) and wind (vector) in Mar of IOB_W3; e–h. Same as a–d but in May for IOB_W5; i–l. in Aug. for IOB_W8, respectively. All variables have passed the t-test.

atmospheric responses, we resort to a heat flux analysis. According to Xie (2009), a strong tropical heating can generate an eastward transport Kelvin wave and then form an anticyclonic anomaly by Ekman divergence; hence, the atmospheric heating budget over the Indian Ocean is the main cause for the induced Kelvin wave, especially the atmospheric heat flux anomalies induced by the SST anomalies in the North Indian Ocean. For the sake of simplicity, we will use sensible and latent heat flux anomalies as typical cases. Over the North Indian Ocean, the largest latent and sensible heat flux anomalies exist in all three experiments (Fig.4). The latent heat flux anomalies are generally greater than the sensible heat flux anomalies ranging from ±14–±1 w/m2. Positive (upward) sensible flux anomalies only exist over the positive SST anomalies in the North Indian Ocean, while strong positive (upward) latent flux anomalies

exist over the Indian Peninsula and North Indian Ocean. Among the three experiments, there are significant differences for the latent heat flux anomaly, precipitation anomaly and sea surface wind anomaly fields (Fig.4). In particular, the largest latent heat flux anomaly appears in August in IOB_W8 (Fig.4j), which is about three times larger than that in both IOB_W3 (Fig.4b) and IOB_W5 (Fig.4f). Concurrent with the largest latent heat flux anomaly, the largest precipitation anomalies occur over the Indian Peninsula and North Indian Ocean in August in IOB_ W8 (Fig.4l). The most significant easterly surface wind anomalies with local surface convergence wind anomalies exist over the NWP (Fig.4i). The reason for the differences in latent heat flux anomalies among the three experiments is that the atmospheric circulation over the IOB is quite different from spring to summer. In March of IOB_W3, prior

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to the development of the ISM, a weak northeasterly winter monsoon prevails over the North Indian Ocean (Fig.4d), despite the pre-setting of a North Indian Ocean positive mixed layer temperature anomaly in this experiment. Furthermore, sea surface evaporation is weak and the latent heat release to atmosphere is limited. Therefore, the positive SST anomaly can be maintained in the ocean during the entire spring in IOB_W3 (Fig.3). In May of IOB_W5, the ISM just begins to set in the model. The southeasterly trade winds in the southern hemisphere cross the equator and turn into southwesterly trade winds, replacing the northeasterly winds as the dominant winds over the North Indian Ocean (Fig.4h). With this southwesterly wind, both the evaporation and the latent heat released to atmosphere are enhanced. In August of IOB_W8, the ISM reaches its peak with the strongest southwesterly surface winds (Fig.4l) and the largest latent heat flux anomalies over the Indian Peninsula and North Indian Ocean (Fig.4j). The precipitation anomalies of three experiments also reinforce this point (Fig.4d, 4h, 4l). Furthermore, the strong positive precipitation anomalies release a mass of latent heat into the atmosphere, making it liable to produce the Matsuno-Gill pattern and the easterly sea surface wind anomalies over the Indo-West Pacific. In terms of the surface wind anomalies, among the three experiments, the most significant anticyclonic anomaly over the NWP is found in IOB_W8.

4 SUMMARY AND DISCUSSION Teleconnections between the equatorial Pacific SST anomalies and the East Asian climate have impacts on the intensity and extent of the East Asian Summer Monsoon. Therefore, as an important bridging system, research on the anomalous ANPWA is important for the prediction of the East Asian Summer Monsoon. The persistence of the NWP anticyclonic anomaly in (EI) events endures longer than that in (E) events. In the decaying year of El Niño, the ANWPA persists from summer to autumn with the IOB warming. From the variance of the ANWPA vorticity index, an abrupt strengthening appears in August in (EI) events. For the North Indian Ocean precipitation index, the same abrupt strengthening also happens, whereas in the IOB warming index, this strengthening is not found. Therefore, the IOB warming effect on the ANWPA might be different in August in the decaying year of El Niño. In addition, the ANWPA vorticity index changes synchronously with the precipitation index

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and the surface northward wind index over the North Indian Ocean. Considering the seasonal transition of the Indian Ocean Basin, the seasonal variation of the ISM over the North Indian Ocean might be important in the abrupt increase of the ANWPA. Easterly wind anomalies exist over the Indo-West Pacific when the ISM breaks out in August in all three sensitivity experiments. The most significant easterly winds with the anticyclonic anomaly appear over the NWP as soon as the positive temperature anomalies in the mixed layer are added into the North Indian Ocean in August. The precipitation with the latent heat flux anomalies reaches a maximum over the Indian Peninsula and North Indian Ocean. Apart from the seasonal transition of the ISM, Du et al. (2011) found that variations in the location of the warming of the Indian Ocean in spring and summer might be important for the strengthening of the ANWPA in summer. As mentioned by Xiang et al. (2013), the atmospheric response to the local negative SST anomaly is more sensitive over the NWP in August, which means that the local WES feedback is still robust, especially over the western part of the NWP. We found that both the intensity and duration of the cooling SST anomalies in the tropical Pacific are quite different in the summer of the decaying (EI) and (E) cases and that the different cold SST anomalies might lead to different vorticity anomalies in the atmosphere over the NWP. Variations in the precipitation anomaly over the North Indian Ocean during summer are more obvious in FOAM than from observation. Huang et al. (2010) indicated that the obvious warming anomalies could be observed in the troposphere, even with small precipitation differences in the summer of the decaying El Niño. All this indicates that the latent heat flux anomalies caused by the seasonal transition of the ISM are important to the enhancement of the ANWPA in August, but that they are not the only factor.

5 ACKNOWLEDGMENT We appreciate http://nomads.ncdc.noaa.gov for providing global monthly mean NCEP-2 data. We thank Professor WU Lixin for the guidance in FOAM1.5 used in this article. References Annamalai H, Liu P, Xie S P. 2005. Southwest Indian Ocean SST variability: its local effect and remote influence on asian monsoons. J. Clim., 18: 4 150-4 167.

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