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PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE 10.1002/2014JC010429 Key Points:  Basin mode in EP El Nino, while semibasin mode in CP El Ni~ no  The SST oscillation periods are different in these two types of El Ni~ no

Correspondence to: D. Wang, [email protected]

Citation: Liu, Q.-Y., D. Wang, X. Wang, Y. Shu, Q. Xie, and J. Chen (2014), Thermal variations in the South China Sea associated with the eastern and central Pacific El Ni~ no events and their mechanisms, J. Geophys. Res. Oceans, 119, 8955–8972, doi:10.1002/ 2014JC010429. Received 15 SEP 2014 Accepted 3 DEC 2014 Accepted article online 8 DEC 2014 Published online 29 DEC 2014

Thermal variations in the South China Sea associated with the ~ o events and their eastern and central Pacific El Nin mechanisms Qin-Yan Liu1, Dongxiao Wang1, Xin Wang1, Yeqiang Shu1, Qiang Xie2, and Ju Chen1 1

State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, People’s Republic of China, 2Sanya Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, Sanya, People’s Republic of China

Abstract In this study, we investigate the interannual variability of the sea surface temperature (SST) in the ~o, namely, the eastern Pacific (EP) El Nin ~o and the South China Sea (SCS) associated with two types of El Nin ~o. First, double warm peaks can occur during both types of El Nin ~o events in the SCS. central Pacific (CP) El Nin ~o, while the warm semibasin mode However, the strong warm basin mode can only develop in the EP El Nin ~o. Associated with an anomalous positive (negative) net surface heat flux in the EP exists during the CP El Nin ~o, along with a shallower thermocline with weaker (stronger) northeasterly wind anomalies, the SST (CP) El Nin anomalies become warmer (cooler) in the developing autumn. Over the background of cooling SST in autumn ~o, therefore, only a weak warming can occur in the subsequent years, which is limited in the westof CP El Nin ern boundary area under the forcing of warm ocean advection. Second, the SST oscillation periods are differ~o. The SST evolution in the EP El Nin ~o is negative-positive with a quasi-biennial ent in these two types of El Nin ~o is positive-negative-positive-negative with an annual oscillation. It seems oscillation, but that in the CP El Nin ~o is phase-locked to the late autumn season. that the double cooling in the CP El Nin

1. Introduction ~o-Southern Oscillation (ENSO) is a dominant mode of climate variability. As the largest semienThe El Nin closed marginal sea, the South China Sea (SCS) is influenced by the ENSO signals, especially the sea surface temperature (SST) [e.g., He and Guan, 1997; Wang et al., 2002; C. Wang et al., 2006; Xie et al., 2003; Liu et al., 2004; Zhang et al., 2010]. The interannual SST anomalies over the SCS show a double-peak feature, following ~o event [C. Wang et al., 2006]. The Vietnam cold filament can disrupt the summer warming in the SCS El Nin [Xie et al., 2003], and a similar cold filament also exists in the boreal winter [Liu et al., 2004], both displaying considerable interannual variability associated with the ENSO. Besides the SST, the sea surface height (SSH) and ocean circulation in the SCS also have strong ENSO signals [e.g., Wu and Chang, 2005; Fang et al., 2006]. The SCS SST can influence the summer monsoon onset [Ding et al., 2004] and rainfall by enhancing the convective instability [Zhou et al., 2010]. The impact of the ENSO can also reach the SCS through the Luzon Strait [Qu et al., 2004; Tian et al., 2006], where eddy activities are significant [e.g., Li et al., 1998]. A branch of the Kuroshio can intrude steadily and persistently into the SCS [Liang et al., 2003]. The Mindoro Strait is another important pathway of water exchange for the SCS [Metzger and Hurlburt, 1996; Sprintall et al., 2012]. The coastal Kelvin wave can transfer the Pacific signals into the SCS via the Mindoro Strait [Liu et al., 2011]. The ENSO events occur more frequently after the late 1990s [Timmermann et al., 1999], which is attributed ~o occurs, to global warming [Yeh et al., 2009; Collins et al., 2010]. In this condition, a new type of El Nin ~o events [e.g., Yu and Kao, 2007]; they have other names, such as named as the central Pacific (CP) El Nin ~o, warm-pool El Nin ~o or El Nin ~o Modoki [Larkin and Harrison, 2005; Ashok et al., 2007; Kug and dateline El Nin ~o for simplicity. The warming center of CP El Nin ~o is in Jin, 2009]. In this paper, we use the name of CP El Nin the equatorial central Pacific and is flanked by colder SST anomalies on both sides of the equatorial Pacific. ~o and its climate impacts are different from those of the eastern Pacific (EP) El The pattern of the CP El Nin ~ ~o, the western North Pacific Nino [e.g., Trenberth and Hoar, 1996; Ashok et al., 2007]. During the CP El Nin summer monsoon is enhanced, while the East Asian summer monsoon is weakened. The impact of CP El

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~o on East Asian climate is more significant than that of EP El Nin ~o during the developing summer [Yuan Nin ~o on typhoons are discussed in some recent studies [e.g., Kim and Yang, 2012]. The influences of CP El Nin et al., 2011; Zhang et al., 2012; Wang et al., 2013; Wang and Wang, 2013]. ~o on the SCS SST, but their conclusions are There are many studies that discuss the influences of the El Nin ~o. Since the El Nin ~o is now classified into two different types, it is necesmostly based on one type of El Nin sary to investigate whether or not there is any significant difference in the SCS under these two types of El ~o. In this study, the significant differences between two types of El Nin ~o events occur in the developing Nin ~o are also autumn, and some new findings, semibasin mode with annual oscillation periods in CP El Nin revealed for the first time. The rest of the paper is organized as follows. Section 2 gives a brief description of the data sets, and defines ~o. The SCS SST variability associated with wind stress, net surface heat flux and therthe two types of El Nin mocline adjustment is presented in section 3. The mixed-layer heat budget is examined in section 4. The conclusion and discussion are given in section 5.

2. Data and Methods The Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) analysis data set from 1958 to 2012 is used in this study [Rayner et al., 2003]. The monthly net surface heat flux is calculated by the longwave radiation, shortwave radiation, sensible heat net flux, and latent heat flux from the NCEP/NCAR reanalysis. The positive flux value represents the heat gained by the ocean, and vice versa. The monthly wind stress and ocean currents data sets are derived from the Simple Ocean Data Assimilation (SODA) product [Carton and Giese, 2008], covering the period from 1958 to 2010. The monthly objectively analyzed subsurface temperature in the upper 700 m from 1945 to 2012 prepared by Ishii et al. [2006] is also used. ~o Modoki index (EMI) is defined as follows: Following Ashok et al. [2007], the El Nin EMI5jSSTAjA 20:5jSSTAjB 20:5jSSTAjC

(1)

The brackets in equation (1) represent the area-averaged SST anomaly over each of the regions: Region A (165 E–140 W, 10 S–10 N), Region B (110 –60 W, 15 S–5 N) and Region C (125 –145 E, 10 S–20 N). The ~o3 index is estimated in the region of (150 –90 W, 5 N–5 S) using the HadISST data set. Nin ~o [e.g., Kug and Jin, 2009; Yeh et al., 2009; Ren and There are many methods to identify the two types of El Nin Jin, 2011]. Ren and Jin [2011] devised two new indices (NCT, NWP) that separately identify the two types of El ~o events. The Nin ~o3 and EMI indices used here are significantly correlated with the two new indices (not Nin ~o3 and NCT at 0.98, and EMI and NWP at 0.95. In this study, the seasonal cycle is removed before shown), with Nin ~o3 and EMI indices (not shown), 5 EP El obtaining the monthly anomalies. According to the time series of Nin ~o (1965/1966, 1972/1973; 1982/1983; 1987/1988; 1997/1998) and 8 CP Nin ~o (1977/1978; 1979/1980; 1990/ Nin 1991; 1992/1993; 1994/1995; 2002/2003; 2004/2005; 2009/2010) were chosen in the following analyses.

3. Results ~ o Events 3.1. SST Anomalies During Different Types of El Nin 3.1.1. Variability of the SCS SST Anomalies Figure 1 displays the 9-point running mean of the SST anomalies in the SCS (100.5 E–125.5 E, 5.5 N– ~o events after the linear trend is removed. During EP El Nin ~o, the SCS 25.5 N) during the two types of El Nin SST anomalies have a dominant double-peak feature around February and August in the subsequent year ~o (Figure 1a), which was first noted by C. Wang et al. [2006]. According to the mean of SCS SST of the El Nin ~o was found, and that is anomalies (Figure 1b), a noticeable warming-cooling annual cycle during CP El Nin not sensitive to the domain chosen. ~o events show that The composite fields of SCS SST anomalies averaged over latitudes for EP and CP El Nin ~o year in both cases, but the warming amplitude the double-peak warming occurs in the subsequent El Nin ~o is weaker than that in El Nin ~o events with different oscillation period and extent (Figure 2). in CP El Nin Next, some common features and differences are discussed in more detail. ~o events, the first peak occurs in First, there are differences in terms of timing and duration. During EP El Nin February (11) or March (11), and the second peak occurs in August (11). The anomalous warming signals

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Figure 1. The nine-point running mean of the HadISST anomalies averaged in the South China Sea (100.5 E–125.5 E, 5.5 N–25.5 N) during (a) eastern Pacific (EP) El Ni~ no and (b) central Pacific (CP) El Ni~ no after removing the linear trend. The amplitude of Ni~ no3 has been divided by 4 before plotting.

Figure 2. Composites of the SST anomaly ( C) in (a) EP El Ni~ no and (b) CP El Ni~ no events (enlarged three times) averaged over the entire SCS after removing the linear trend. The contour interval is 0.1 C, and the thick contour is the zero contour. The red curves indicate the areas with above 95% (90%) confidence level for (EP) CP El Ni~ no events, respectively.

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can last more than 1 year, which begin in October (0) and end in December (11) (Figure 2a). During CP El ~o events, the anomalous warming signals begin in December (0) and end in July (11) (Figure 2b), which Nin ~o events. The first peak occurs in January (11), and is about half a year shorter than that during EP El Nin the second peak occurs in May (11). ~o events, a fast basin-scale warming (Figure Second, the locations of the warming are different. For EP El Nin 2a) can develop in the whole SCS. The maximum warming center of the first peak is located in the western ~o events, the first SCS, and the second maximum warming center is located in the eastern SCS. For CP El Nin significant warming peak is confined to the domain between 105 E and 115 E. For the second warming peak, a basin-scale warming can occur, but with a shorter duration and a maximum center located west~o (Figure 2b). ward than EP El Nin Besides the differences described above, other different characters exist. The anomalous SST pattern during ~o events is negative-positive, that is, negative followed by positive SST anomalies (Figure 2a), while EP El Nin ~o events (Figure 2b). It seems that the influence it is positive-negative-positive-negative during CP El Nin ~ time scales in the two types of El Nino events are different, and the anomalous cooling SSTs in the SCS are ~o are also different in phase-locked to the late autumn season (Figure 2b). The SST responses to EP El Nin ~o events, the peak occurring in August (11) is significant in the northern and southern SCS. During EP El Nin northern SCS, but the warm peak occurring in February (11) is evident in southern SCS (not shown). 3.1.2. The Horizontal SST Anomaly Structures ~o were discussed by The SST anomaly developments in the SCS during different stages of (EP) El Nin ~o are unclear. The SCS C. Wang et al. [2006], but the developments of the SST anomalies during CP El Nin ~o events, but it is positive (negative) during EP (CP) El SST in July (0) is negative during both types of El Nin ~o in October (0) (not shown). Nin ~o is basically consistent with the results of C. Wang The evolution of the SCS SST anomalies during EP El Nin et al. [2006]. The basin-scale warming is evident in all four seasons (Figures 3a1–3d1). The first warming peak occurs in February (11), and the maximum warming center is located along the Vietnam coast (Figure 3b1). The warming signals decay in summer (Figure 3c1), and the second peak occurs in August (11). In JJA (11), the maximum warming center is located at 113 E, 13 N (Figure 3d1), consistent with the location of warm eddy off southern Vietnam [Li et al., 2003], which is difficult to identify from the climatological mean. The strongest warming signal in this region can last for several months. ~o is different from that in EP El Nin ~o (Figures 3a2–3d2). The SST anomaly evolution in the SCS in CP El Nin ~ During CP El Nino, the entire SCS is colder before and during the developing autumn (Figure 3a2) until November (not shown). The weak warming is originated from the western half of the SCS in January (11) and February (11) (Figure 3b2). A larger difference is significant in the whole SCS (Figures 3a3–3b3). After the decay in April (11), the warming signal occurs in June (11) again (Figure 2b). The basin mode in EP CP ~o (Figures 3a1–3d1) is difficult to sustain in CP El Nin ~o, so a semibasin mode occurs in CP El Nin ~o (FigEl Nin  ~ ures 3b2–3c2), with a shorter duration and weaker amplitude (0.2–0.3 C) than those in EP El Nino. The weakening of the tropical troposphere is one direct factor for this weaker SST anomaly [Zubiaurre and Calvo, ~o, the basin-scale warming signals in EP El Nin ~o are very significant (Fig2012]. Compared with the CP El Nin ures 3a3–3d3). 3.2. The Atmospheric Forcing: Wind Stress and Net Surface Heat Flux The monsoon forcing is a major factor controlling the SCS climate [Wyrtki, 1961; Yang et al., 2002]. The com~o events are posite fields of surface wind stress and net surface heat flux during different types of El Nin shown in Figures 4 and 5. The patterns of wind stress in Figure 4 are similar to those in Yuan and Yang [2012, Figure 5]. ~o 3.2.1. For the EP El Nin In autumn and winter, the climatological mean wind is northeasterly (not shown). The ocean releases heat into the atmosphere (so the atmosphere gains heat) in the northern SCS (north of 12 N), and the ocean ~o, the northgains heat in the southern SCS (Figures 5a and 5b). During the developing autumn of EP El Nin easterly wind is weaker (Figure 4a1), the net surface heat flux released (gained) by the ocean into (from) the atmosphere is reduced (increased) north (south) of 12 N (Figure 5a1); so, the SST begins to warm in the whole SCS (Figure 3a1).

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Figure 3. Composites of the South China Sea SST anomalies ( C) in SON(0), D(0)JF(11), MAM(11), and JJA(11) for the EP El Ni~ no (left: a1–d1), CP El Ni~ no (middle: a2–d2, enlarged three times), and differences between EP and CP El Ni~ no (right: a3–d3) respectively. Here (0) and (11) refer to the El Ni~ no year and the subsequent year, respectively. The contour interval is 0.1 C and the differences with 95% confidence level are marked by 1.

~o event becomes mature, the winter monsoon decreases further (Figure 4b1). The SCS warmWhen an EP El Nin ing is sustained, and the first peak is located in the southwestern SCS (center location: 8 N, 107 E; Figure 3b1). The net surface heat flux released by the ocean is further suppressed, which is similar to that in the autumn season, except for the maximum warming center (Figure 5b1). The maximum warming center (Figure 3b1) cannot be explained by the net surface heat flux, because the heat gained by the ocean is reduced in this region (105 – 114 E, 5 –12 N; Figure 5b1). Here the geostrophic heat advection can warm the western boundary region of the SCS [C. Wang et al., 2006]. The heat budget analyses results in section 4 will further confirm this point. In spring and summer, the SCS gains heat from the atmosphere (Figures 5c and 5d). There is still weak winter monsoon in spring, and there is strong summer monsoon in summer (not shown). The anomalous ~o (Figure 4c1), and the net surface heat southwesterly wind is sustained in the decaying spring of EP El Nin flux gained by the ocean is reduced in the box of (111 –120 E, 10 –18 N) in Figure 5c1. Then, the ocean ~o (Figure 3c1). surface warming is weakened in the decaying spring of EP El Nin ~o, the summer monsoon is weakened (Figure 4d1) [Wang et al., During the decaying summer of EP El Nin 2002; Yuan and Yang, 2012], and the SCS becomes warmer again in August (11) (Figure 1a), which is not

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Figure 4. Composites of surface wind stress (arrow; units: N m22) and wind stress curl (shading; units: 1027N m23) in SON(0), D(0)JF(11), MAM(11), and JJA(11) for the EP El Ni~ no (left: a1–d1) and CP El Ni~ no (right: a2–d2), respectively. The contour interval is 0.1 3 1027 N m23.

evident averaged in JJA (11) (Figure 3d1), due to the increasing net surface heat flux gained in the southern and eastern SCS (Figure 5d1). The net surface heat flux play an important role on the surface warming, besides the northwestern SCS (110 –115 E, 10 –15 N), where the mainly contribution is the anomalous entrainment heat flux that results from a downward anomalous Ekman pumping velocity in July (11) [C. Wang et al., 2006]. The surface warming in the SCS that cannot be explained by the net surface heat flux is related to ocean dynamics, such as anomalous entrainment heat flux, or geostrophic heat advection [C. Wang et al., 2006], or the capacitor effect of the long-lasting warming in the tropical Indian Ocean [Yang et al., 2007; Xie et al., ~o, the net surface heat flux is mainly controlled by the latent heat flux (Figure 6a, purple 2009]. For EP El Nin line), and the shortwave radiation flux (Figure 6a, green line) is secondary. ~o 3.2.2. For the CP El Nin ~o is different from that during EP El Nin ~o. Although the impact of CP El Nin ~o The situation during CP El Nin ~o during the developing summer [Weng on the East Asian climate is more significant than that of EP El Nin et al., 2007], the significant differences also exist in the developing autumn, such as the wind anomalies (Figures 4a1 and 4a2), net surface heat flux anomalies (Figures 5a1 and 5a2), and the ocean temperature anomalies (Figures 3a1 and 3a2). Associated with the northeasterly wind anomalies in the autumn of CP El

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Figure 5. Seasonal mean of (top) net surface heat flux, and the composite fields of the net surface heat flux anomaly during the (middle) EP El Ni~ no and (bottom) CP El Ni~ no. Positive value denotes the ocean gains heat. Units: W m22.

~o (Figure 4a2), the net surface heat flux released from the SCS is increased (Figure 5a2); so, the SCS Nin shows a cooling pattern (Figure 3a2). ~o are similar to those during EP El In winter, the variations of net surface heat flux and wind during CP El Nin ~ Nino (Figures 4b1 and 4b2, Figures 5b1 and 5b2). The northeasterly wind is weaker than that in a normal ~o winter (Figure 4b1). The heat gained by the atmosphere year (Figure 4b2), but is stronger than EP El Nin decreases in the northern SCS (Figure 5b2), and the ocean begins to warm in the western SCS (Figure 3b2), starting from February (11) (not shown). The SST warming in the southern SCS (105 –114 E, 5 –12 N; Figure 3b2) cannot be explained by the net surface heat flux (Figure 5b2, the heat gained by the ocean is reduced), so the ocean dynamic process, such as geostrophic heat advection must be important in this box, which will be further discussed through the heat budget analysis in section 4. ~o (Figure 3b2), which is perhaps The cool SST is also sustained in the eastern SCS in the winter of CP El Nin ~o (Figure 3b1). The SST connected with the long memory stored in the ocean in the autumn of CP El Nin ~o [Yuan and over the SCS is colder than normal from the developing autumn to the next spring of CP El Nin Yang, 2012, Figures 4g–4i], and the processes responsible for the differences are complex, which cannot be simply attributed to the atmosphere forcing. In the southwestern SCS, the warm advection favors the surface warming in the western boundary region (Figure 3b2), and the warm advection is induced by the abnormal anticyclonic geostrophic flow. ~o are relatively weaker (Figures 4c2– The wind anomalies in the decaying spring and summer of CP El Nin d2). The net surface heat flux and the SST warming pattern during the decaying spring (Figure 5c2 and 3c2) are similar to those in the developing winter (Figures 5b2 and 3b2). In the northern SCS, the net surface heat flux gained by the ocean is more than that in a normal year (Figure 5c2), but is smaller than that in the

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Figure 6. The net surface heat flux (Qnet), shortwave radiation flux (SWR), longwave radiation flux (LWR), sensible heat flux (SHF), and latent heat flux (LHF) averaged in the SCS during the EP El Ni~ no (a) and the CP El Ni~ no (b). Units: W m22.

~o (not developing winter (Figure 5b2). The second warming peak occurs in May (11) during CP El Nin shown), with its averaged amplitude in spring being weaker than that in winter (Figure 3c2), totally due to reduced net surface heat flux gained by the ocean in the SCS (Figure 5c2). ~o, associated with the increased net surface heat flux gained by the In the decaying summer of CP El Nin ocean in the eastern SCS (Figure 5d2), the SST in the west of the Philippine coast begins to warm in August (11) (Figure 3d2). The west of the Philippine coast is influenced by the anomalous Philippine Sea anticyclone (PSAC) [Wang et al., 2000] and the oceanic coastal Kelvin wave [Liu et al., 2011]; so, the dynamic proc~o, only the contribution of the latent heat flux is significant esses are complex in this region. In the CP El Nin to the net surface heat flux (Figure 6b). In the next section, the thermocline adjustment between two types ~o and the domain hardly explained by the net surface heat flux will further be illustrated. of El Nin 3.3. Thermocline Adjustment ~o is weaker with a short duration, and is almost As described above, the SST anomaly during the CP El Nin ~o can persist into the next year. Previous limited to the half basin (Figures 3b2–c2), but that during EP El Nin studies showed that the central Pacific and East Asia can be linked by anomalous PSAC during the extreme phases of ENSO cycles, which can weaken the winter monsoon and induce warming in the East Asia [Wang ~o is located more westward et al., 2000]. The anomalous low-level anticyclone over the SCS during CP El Nin ~o [Yuan and Yang, 2012]. Besides the influence of anomalous westward shift than the PSAC during EP El Nin ~o, it is necessary to investigate the thermocline variation during of the low-level anticyclone during CP El Nin ~o events. The seasonal cycles of 20 C isotherm depth using different products are compared the two El Nin (not shown), and the Ishii product is used to discuss the interannual variability here. ~o and correlaTo illustrate the variability of the 20 C isotherm depth, the composite fields for EP (CP) El Nin ~o3 (EMI) index are given in Figures 7 and 8. In general, the 20 C isotherm depth in tion coefficients with Nin the southern SCS is more easily affected by the remote forcing of the Pacific, with a correlation coefficient ~o. The differences between the EP and above 20.4 (Figures 7a1–7b1 and 7a2–7b2), especially for CP El Nin ~o are significant. The CP El Nin ~o can change the thermocline depth mostly from 9 N to 13 N durCP El Nin ~o can change the thermocline from 9 N to ing the decaying phase (Figures 7a2–7b2), while the EP El Nin

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17 N during the developing and decaying phases (Figures 7a1–7c1). It seems like that the thermocline depth in the northern SCS is more easily ~o influenced before EP El Nin (Figure 7c1), while that in the southern SCS is more easily ~o influenced after CP El Nin (Figure 7a2). ~o 3.3.1. For the EP El Nin For the developing autumn ~o, the and winter of EP El Nin winter monsoon is weaker than that in a normal year (Figures 4a1 and 4a2), so the windinduced vertical mixing becomes weaker (Figures 9a1 and 9b1, bold pink lines). Chao et al. [1995] pointed out the weak vertical mixing caused by wind relaxation perhaps causes an SST warming in the SCS. Figure 7. Correlation coefficients between the Ni~ no3/EMI index and the 20 C isotherm depth along 9 N, 13 N and 17 N. When estimating the correlation coefficients, the 15-point During the development of an running mean was performed first. ‘‘Before or after EP (CP) El Ni~ no’’ denotes that the Ni~ no3/ ~o event, when the surEl Nin  EMI index lags or leads the 20 C isotherm depth by n months. The shading with r-0.301 is face water is warmer in the above the 95% confidence level. eastern Pacific, the shallower thermocline is seen in the western Pacific. When the net surface heat flux released from the ocean is reduced (Figures 5a1 and 5b1), a shallow thermocline (Figures 8a1 and 8b1) does not necessarily facilitate the cooling of the mixed layer; instead, it can foster a warming of the sea surface [Yu et al., 2006]. In winter, the geostrophic current shows a basin-wide cyclonic circulation in the SCS. During the developing ~o, the basin-wide cyclonic circulation is weakened (Figures 10a1 and 10b1), so the cool winter of EP El Nin advection induced by the winter monsoon decreased by the anomalous anticyclonic geostrophic flow [C. Wang et al., 2006]. Associated with the warm advection by the anticyclonic geostrophic flow, the maximum warming center occurs in the western boundary region of the SCS (105 –114 E, 5 –12 N, Figure 3b1), although the net surface heat flux gained by the ocean is reduced there. In the decaying spring, the SCS also sustains a basin-scale warming (Figure 3c1), while the warming signals are weaker than those in the developing winter (Figure 3b1), due to reduced net surface heat flux gained by the ocean (Figure 5c1). In this season, the thermocline depth is deepened in south of 16 N (Figure 8c1), and the warm temperature signal transfers from surface to subsurface; so, the maximum warming occurs in the depth range of 50–100 m (Figure 9c1). In the decaying summer, associated with the anomalous Ekman convergence (Figure 4d1), the thermocline depth deepens further in the central SCS (Figure 8d1), and the subsurface warming can reach above 1.0 C (Figure 9d1), which is located to the right of the westward wind anomaly. Wang et al. [2002] pointed out that the upwelling at the thermocline is restrained by the Ekman entrainment, and so the warmthermocline memory for thermal signals can be maintained for a long time. ~o 3.3.2. For the CP El Nin ~o, the winter monsoon is stronger than that in a normal year (Figure In the developing autumn of CP El Nin 4a2). Associated with a shallower thermocline (Figures 8a2 and 9a2), the SST cooling anomaly occurs easily (Figure 3a2), when the net surface heat flux released by the ocean is increased (Figure 5a2). The maximum

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subsurface cooling occurs at the depth of 100 m in the central SCS (Figure 9a2), which is different from that during the ~o (Figure 9a1). EP El Nin The winter monsoon is weaker in the developing winter of CP ~o than in a normal year El Nin (Figure 4b2). In the northwestern SCS, the net surface heat flux gained by the atmosphere is suppressed (Figure 5b2), and the sea surface begins to warm (Figure 3b2). In the southwestern SCS (105 –114 E, 5 –12 N), ~o, the similar to that of EP El Nin warm advection induced by the abnormal anticyclonic geostrophic flow (Figures 10a2–10b2) favors the surface warming in the western boundary region (Figure 3b2), although the ocean loses heat to the atmosphere (Figure 5b2). A shallow thermocline (Figure 8b2) can foster an SST warming at the same time [Yu et al., 2006]. The anomalous negative wind stress curl sustains from the developing winter into the decaying spring (Figure 4c2), and the thermocline depth begins to deepen off the Vietnam coast (109–114 E, 10– 16 N) associated Ekman convergence (Figure 8c2). In the southwestern SCS, the warm SST is  weakened compared with that Figure 8. Same as Figure 3, except for the 20 C isotherm depth anomaly for the (left: a1–d1) EP El Ni~ no and (right: a2–d2) CP El Ni~ no. in the developing winter (Figure 3c2), due to reduced net surface heat flux gained by the ocean (Figure 5c2). Under the wind stress forcing, however, the warm SST in the southwestern SCS is subducted into the subsurface (50–100 m) in the central SCS (Figure 9c2). ~o, the thermocline depth in the central SCS deepens further in the decaying summer of Similar to EP El Nin ~o (Figure 8d2). The SST in the west coast of the Philippine Sea begins to warm in August (11) CP El Nin (Figure 3d2), due to increased net heat flux gained by the ocean (Figure 5d2). In the whole SCS, associated with the positive net surface heat flux (Figure 5d2), the SST is warmed up under the Ekman downwelling (Figure 3d2), which perhaps transfer the heat flux from the surface in the southern SCS to the subsurface in the western SCS (Figures 9c2–9d2).

~o 4. Heat Budget Analysis for the CP El Nin The atmosphere influences SST directly through surface heat flux and indirectly via momentum and freshwater fluxes, which subsequently affect ocean currents and turbulent mixing [e. g., Trenberth et al., 1998; Klein et al., 1999].

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Figure 9. Latitude-depth sections showing temperature anomalies ( C) for (left: a1–d1) the EP El Ni~ no composite and (right: a2–d2) the CP El Ni~ no composite. The temperature anomalies are zonally averaged over 105 E–115 E. Also plotted are the mixed layer depth (pink lines) no (solid lines) and CP El Ni~ no (dashed lines). and 20 C isotherm depth (red lines). Bold lines are for the seasonal means of the EP El Ni~

The heat budget equation for the upper-ocean mixed layer may be written as in Qiu [2000]: @Tm Qnet we ðTm 2Td Þ ! ! ! 2! u e :r Tm 2 2 u g :r Tm 5 @t qCp hm hm

(2)

where Tm is the mixed-layer temperature, and is set to SST in this study for simplicity. On the right hand side of (2), Qnet is the net surface heat flux, q is the density of seawater, CP is the specific heat of seawater,

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Figure 10. (a) Depth-averaged surface-current anomaly in the top 46.6 m estimated using the SODA product, and (b) averaged geostrophic current anomaly based on the TOPEX/Poseidon data for the EP El Ni~ no winter (Figures 10a1 and 10b1) and CP El Ni~ no winter (Figures 10a2 and 10b2).

hm is mixed layer depth, ! u e is the Ekman velocity and is related to the surface wind stress vector ! s by ! ! ! u e 5ð s 3 k Þ=ðqfhm Þ, we is vertical entrainment rate, Tb is the water temperature below the base of the ! mixed layer, and ! u g is the surface geostrophic velocity and is related to SSH by ! u g 52grh3 k =f . After linearizing the variables, the perturbation equation can be written as: 0

0

0

@Tm Q ðT 2T Þ 0 ðT 2T Þ ! ! 0 !0 !  ! ! 0 !0 !   g :r Tm 1 u g :r T m Þ2ð u  e :r Tm 1 u e :r T m Þ2w  e m d 2we m d 5 net 2ð u @t qCp hm hm hm

(3)

This equation is similar to that used by C. Wang et al. [2006], except the last two terms in equation (3) are 0 0  e @T =@z and 2we @ T =@z, respectively. Here we is the Ekman pumping (entrainment) velocreplaced by –w ity and is related to the wind stress vector by we 5curlð! s =qf Þ, with positive value for Ekman upwelling, Ekman pumping or divergence and negative value for Ekman downwelling or convergence. In the following, the eight terms in equation (3) are referred to as temperature anomaly tendency, surface heat flux anomaly forcing, mean geostrophic heat advection, anomalous geostrophic heat advection, mean Ekman heat advection, anomalous Ekman heat advection, mean entrainment heat flux, and anomalous entrainment heat flux, respectively. Specifically, the Tm field used is the monthly SST of HadISST data, and the tendency is the difference between two neighboring months (next month minus this month), q51024 kg:m23 , Cp 54007 J:kg21 :K 21 , and hm is the climatological mean mixed-layer depth, which is diagnosed to be 41 m; it is defined as the surface layer whose depth-averaged temperature is 0.8 C higher than the water temperature below. The last two terms, the mean and anomalous vertical temperature gradients, will not be discussed in this paper,

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Figure 11. Same as Figure 2b, except for the heat budget terms with five-point moving mean: (a) net surface heat flux, (b) mean geostrophic heat advection, (c) anomalous geostrophic heat advection, (d) mean Ekman heat advection, and (d) anomalous Ekman heat advection. Figures 11a–11c use the same color scheme, and Figures 11d and 11e use the same color scheme. Units: 1027 C/s. The red curves in Figures 11a, 11d, and 11e indicate the areas with above 90% confidence level. In Figures 11b and 11c, only four events are included when the satellite geostrophic data were available.

because we do not have reliable data sets to calculate them and because the amplitude of the anomalous vertical entrainment heat flux is smaller than the other terms in the SST equation. The geostrophic current (! u g ) of the TOPEX/Poseidon satellite data from 1993 is used to diagnose the mean and anomalous geostrophic heat advection terms. C. Wang et al. [2006] pointed out that the net surface heat flux is a significant factor for the first warming ~o, the ocean loses net heat peak in the SCS, and the geostrophic advection term is secondary. For CP El Nin flux in the developing winter (Figure 5a2), so the cooling signal is significant (Figures 2b and 3a2). For the SST evolution in the SCS (Figure 2b), the net surface heat flux does not play any role for the first warming

Figure 12. The seasonal mean of the change of SST anomalies, net surface heat flux anomalies, mean and abnormal geostrophic heat no advection, and mean and abnormal Ekman heat advection averaged in the domain (105 –114 E, 5 –12 N) during 1997/1998 (a) El Ni~ and (b) CP El Ni~ no events. Units: 1027 C/s.

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Figure 13. Flow charts of the SST evolution for the two different types of El Ni~ no.

peak in January (11), while it begins to increase the SST anomalies in December (0) (Figure 11a), especially in the northern SCS (Figure 5b2). Associated with the mean and anomalous geostrophic advection terms, only weak warming signals occur in the western boundary region (Figures 11b and 11c). This is different ~o [C. Wang et al., 2006]. From the evolution of anomalous geostrophic from the situation during EP El Nin advection (Figure 11c), the warming during January–April (11) is limited to the west of 115 E. That is why ~o (Figure 3b1), but only semibasin mode occurs in CP El Nin ~o (Figure the basin mode can occur in EP El Nin 3b2); in other word, the net surface heat flux warming is absent and the cooling signals are significant in ~o (Figures 5a2 and 13). prior season in CP El Nin ~o, the geostrophic advection is significant and Ekman advection is secondary for the second For EP El Nin warming peak [C. Wang et al., 2006]. Consistent with this notion, the net effect of the geostrophic advection

Figure 14. The composite time series of (a) SST anomalies; (b) the change of SST anomalies; (c) net surface heat flux anomalies; and (d) mean Ekman advection anomalies averaged in the SCS (100.5 E–125.5 E, 5.5 N–25.5 N) during EP (Figures 14a1–14d1) and CP (Figures 14a2–d2) El Ni~ no events. The unit for SST anomalies is  C, and others are 1027 C/s.

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is warming in the central and eastern SCS (Figure 11b), associated with mean northeastward meridional geostrophic flow in July ~o, the situation for the sec(11). For CP El Nin ~o. In ond peak is similar to that for EP El Nin May–June (11), the anomalous geostrophic advection becomes an important contributor for the second warming peak (Figure 11c). Besides the geostrophic current, the Ekman advection is also important for the warming in the SCS (Figure 11d). In the eastern SCS, the net surface heat flux gained by the ocean increases (Figures 11a and 5d2); at the same time, under the Ekman convergence, the second warming signal occurs. For the double cooling signals occurring in October (0) and ~o (Figure 2b), September (11) for CP El Nin the net surface heat flux plays a decisive role (Figure 11a).

Figure 15. The wind stress difference between EP and CP El Ni~ no events. The red vectors denote the difference is significant with 95% confidence level.

Wang et al. [2002] pointed out that the warm advection caused by the southerly anomalies (Figure 4b1) is the major factor during the development phase, while the adjustment of the thermocline in the SCS is not obvious. We want to note that the shallower thermocline depth anomaly in the developing phase is favorable for the warming during EP El ~o (Figure 8a1) and for the cooling during Nin ~o (Figure 8a2). The ocean is more CP El Nin sensitive to the net surface heat forcing under a shallow mixed layer [Yu et al., 2006]. For the second peak, the thermocline becomes deeper than normal (Figures 8c1– 8d1 and 8c2–8d2), so the downwelling mode associated with Ekman convergence can be sustained during this phase.

Additionally, the heat budget analysis is done in the domain (105 –114 E, 5 –12 N) pointed in section 3.2, where the warming cannot be explained by the ~o was chosen for a case study as EP El Nin ~o, and the cases of net surface heat flux. Here the 1997/1998 El Nin ~o are as same as Figure 11. The results show that the change of the SST is increased during October CP El Nin ~o (Figure 12a), but the net surface heat flux is negative. The mean and (0) to March (11) of 1997/1998 El Nin abnormal geostrophic heat advection play important roles on the sea surface warming. While, the mean and abnormal Ekman heat advection are the main contribution to the sea surface warming in JAS (11), consistent ~o (Figure 12b), the first warming is mainly induced by the mean geowith the previous results. For CP El Nin ~o is also attributed to the strophic heat advection, and the second warming occurring earlier than EP El Nin mean and abnormal Ekman heat advection. 







5. Summary and Discussion ~o (EP and CP) and possible In this study, the ocean thermal variability during two different types of El Nin ~o events, the first warming peak of SCS SST occurs in Februmechanisms are investigated. During EP El Nin ~o events, the first peak ary (11) or March (11), and the second peak occurs in August (11). During CP El Nin occurs in January (11), and second peak occurs in May (11), giving a much shorter duration between the ~o events. two peaks than that of EP El Nin

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 in the upper 317.65 m of the SODA products along 120.75 E from 18.75 N to Figure 16. (a) The depth-averaged current velocity u 22.25 N (similar to Liang et al. [2008, Figure 13a]). Shaded areas show positive values, and the contour intervals are 10 cm/s. (b) the time series of the zonal volume transport along 120.75 E between 18.75 N and 22.25 N (similar to Liang et al. [2008, Figure 14]), unit: Sv (1Sv5106 m3 s21).

Although both double peaks (with shorter and longer durations) can occur during two different types of El ~o, the strong warm basin mode can only develop during EP El Nin ~o under the net surface heat flux forcNin ~o, only weak warming ing. Since the atmosphere’s role is absent during the developing autumn of CP El Nin occurs in the western boundary region. The significant difference of the net surface heat flux in SON (0) is ~o events. The first warming is one important reason to the mode differences between two types of El Nin ~o events, which is caused by the geostrophic warm advection. limited to the west of 115 E for CP El Nin ~o, but only semibasin mode can occur during CP El That is why the basin mode can occur during EP El Nin ~o (Figure 13). The net surface heat flux anomalies are reduced, favoring the decay of SST anomalies folNin ~o events. lowing the first peak for both types of El Nin Another difference is the oscillation period of SCS SST anomalies. It has a quasi-biennial oscillation in EP El ~o (2/1), but an annual oscillation in CP El Nin ~o (1/2/1/2). The double cooling signals occur in October Nin ~ (0) and September (11) for CP El Nino, which is phase-locked to the late autumn season, consistent with the net surface heat flux cooling (Figure 11a). Why the oscillation periods are different in two types of El ~o events? From Figure 11, the results show that the oscillation period of SCS SST for CP El Nin ~o seems Nin like to be consistent with the net surface heat flux (Figure 11a) and mean Ekman heat advection (Figure 11d). Totally, the net surface heat flux controls the oscillation periods of the temperature averaged in the ~o events (Figures 14c1 and 14c2). The mean Ekman advection during EP and CP SCS in both types of El Nin ~ ~o is consistent with the SST El Nino are similar (Figures 14d1 and 14d2), but its variability during CP El Nin anomaly (Figure 14a2), especially during the developing phase (Figure 14d2). ~o on the East Asia climate is more significant during the developing summer than The impact of CP El Nin ~o. The western North Pacific summer monsoon is enhanced, while the East Asian summer that of EP El Nin ~o with a significant northeastward wind stress difference in the SCS monsoon is weakened during CP El Nin (Figures 4a2 and Figure 15a). That is the reason why the cool SST anomalies exist in the SCS in the summer ~o will exert a stronger impact on the East Asia climate during the decayand autumn seasons. The EP El Nin ing summer [Yuan and Yang, 2012], probably due to the long lasting anomalous warming over the tropical Indian Ocean [Yang et al., 2007]. ~o, the Philippine Sea anticyclone (PSAC) reduces the normal winter monsoon and causes a During El Nin ~o is built up in the warmer winter along the East Asian coast [Wang et al., 2000]. The PSAC during EP El Nin ~o (Figure 4). In developing phase of El Nin ~o developing summer, which is two seasons earlier than in CP El Nin

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events, the abnormal significant anticyclonic wind stress differences are also significant (Figures 15a and 15b). Associated with an anomalous ascending motion over the equatorial central Pacific and anomalous sinking ~o, the PSAC in CP El Nin ~o extends more northwestward than that in EP El branch to its west during CP El Nin ~o (Figures 4b1–b2 and 15b), which is consistent with the previous results [Yuan and Yang, 2012]. Nin Zhang et al. [2010] noted that the warming trend of basin mode over the coastal China seas is influenced by the advection associated with the Kuroshio. The thermal variability in the SCS is tightly connected with the Pacific ENSO signals [Liu et al., 2011], since the Pacific ENSO signals can be transmitted into the SCS via the Luzon Strait [Qu et al., 2004; Tian et al., 2006]. The SCS throughflow (SCSTF) acts as a heat and freshwater conveyor and is believed to play an important role in regulating the SST in the SCS and in the adjoining tropical seas [e.g., Qu et al., 2006; D. Wang et al., 2006; Yu et al., 2007]. There is a strong cyclonic circulation ~o (Figures 10a1 and 10b1), but there is a weak circulation duranomaly in the Luzon Strait during EP El Nin ~o associated with an anomalous southward current near the Philippine coast (Figures 10a2 and ing CP El Nin 10b2). Although some fine structures cannot be well resolved in SODA products, it is reasonably good to obtain the main feature of the flow in the Luzon Strait given in Liang et al. [2008] (Figure 16). The distribu in the Luzon Strait could be separated into three parts, with eastward at the northern and southern tion of u  in the central Luzon Strait varied seasonally as well as interannual variability [Liang et al., parts, and the u 2008]. Besides, the seasonal variation and deceasing trend of zonal transport in the upper ocean are both similar with the results of Liang et al. [2008]. Of course, we should be aware of the satellite products applied on the shallow seas, and the heat budget analysis with multiple products. But the coastal area is not our focus domain, and so the problem caused by satellite products can be avoided. The connection of the Pacific circulation with the SCS and its influence on the SCS interior thermal structure are still unclear, and need further investigation.

Acknowledgments The authors thank the two anonymous reviewers and Ming Feng for providing constructive comments on an early version of the manuscript. This work is supported by the ‘‘Strategic Priority Research Program’’ of the Chinese Academy of Sciences (XDA11010301, XDA11010302), the National Basic Research Program of China (2011CB403504) and the First Institute of Oceanography, the State Oceanic Administration (LDAA-2012-03). Xin Wang is supported by National Science Foundation of China (41422601, 41376025). The data used in this study are also appreciated, which are downloaded from http://www. metoffice.gov.uk/hadobs/hadisst/data/ download.html; http://www.esrl.noaa. gov/psd/data/gridded/data.ncep. reanalysis.derived.surfaceflux.html; http://apdrc.soest.hawaii.edu/dods/ public_data/SODA and http://rda.ucar. edu/datasets/ds285.3, respectively. This research is also supported by the CAS/SAFEA International Partnership Program for Creative Research Teams.

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