SCIENCE CHINA Earth Sciences

SCIENCE CHINA Earth Sciences RESEARCH PAPER September 2014 Vol.57 No.9: 2269 2280 doi: 10.1007/s11430-014-4893-5 Annual and interannual variations of the Western Pacific Warm Pool volume and sources of warm water revealed by Argo data WU XiaoFen 1*, ZHANG QiLong 3 & LIU ZengHong 1,2 1 Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China; 2 State Key Laboratory of Satellite Ocean Environment Dynamics, State Oceanic Administration, Hangzhou 310012, China; 3 Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Received January 25, 2014; accepted March 24, 2014; published online May 21, 2014 Based on gridded Argo profile data from January 2004 to December 2010, together with the P-vector inverse method, the three-dimensional structure, annual and inter-annual variations in volume of the Western Pacific Warm Pool (WPWP) are studied. The variations of latitudinal and longitudinal warm water flowing into and out of the WPWP and the probable mechanism of warm water maintenance are also discussed. From the surface to the bottom, climatic WPWP tilts southward and its area decreases. The maximum depth could extend to 120 m, such that its volume could attain 1.86 10 15 m 3. Annual variation of the WPWP volume shows two obvious peaks that occur in June and October, whereas its inter-annual variations are related to ENSO events. Based on a climatic perspective, the warm water flowing latitudinally into the pool is about 52 Sv, which is mainly through upper layers and via the eastern boundary. Latitudinally, warm water flowing outward is about 49 Sv, and this is mainly through lower layers and via the western boundary. In contrast, along the latitude, warm water flowing into and out of the pool is about 28 Sv and 23 Sv, respectively. Annual and inter-annual variations of the net transportation of the warm water demonstrate that the WPWP mainly loses warm water in the west-east direction, whereas it receives warm water from the north-south direction. The annual variation of the volume of WPWP is highly related to the annual variation of the net warm water transportation, however, they are not closely related on inter-annual time scale. On the inter-annual time scale, influences of ENSO events on the net warm water transportation in the north-south direction are much more than that in the west-east direction. Although there are some limitations and simplifications when using the P-vector method, it could still help improve our understanding of the WPWP, especially regarding the sources of the warm water. Western Pacific Warm Pool, volume variability, latitudinal flow, longitudinal flow, Argo data, P-vector method Citation: Wu X F, Zhang Q L, Liu Z H. 2014. Annual and interannual variations of the Western Pacific Warm Pool volume and sources of warm water revealed by Argo data. Science China: Earth Sciences, 57: 2269 2280, doi: 10.1007/s11430-014-4893-5 The Western Pacific Warm Pool (WPWP) refers to the body of warm water in central and western Pacific Ocean. It has the highest sea surface temperature (SST) and very strong convective activities, and is also a very significant heat source for global atmospheric motion. The WPWP releases energy to the atmosphere in the forms of latent and sensible heat and thus, it has an impact on the general circulation *Corresponding author (email: wuxiaofen83@163.com) system of the atmosphere. Many studies have shown that WPWP variations would affect the climatic status of tropical and sub-tropical regions by influencing the Walker Circulation and Hadley Circulation, as well as by causing variations of the subtropical high (Huang et al., 1994; Li et al., 1998; Zhang et al., 1999; Zhao et al., 2002; Hu et al., 2008). In addition, latitudinal variations of the WPWP play a significant role in the formation and development of ENSO event (Gill et al., 1983a,1983b; Fu et al., 1986; Picaut et Science China Press and Springer-Verlag Berlin Heidelberg 2014 earth.scichina.com link.springer.com

2270 Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 al.,1995; Picaut et al., 1996), which is a well-known interannual signal of climate change and would bring droughts in Australia, India, and Africa, strong winter storms in the US, and a series of climatic anomalies in East Asia (Yu et al., 1994). Thus, for its importance on global climate changes, as well as on the occurrence of global major disasters, the WPWP has been attracting more and more attention from all over the world. In 1985, the Tropical Ocean and Global Atmosphere (TOGA) program was implemented in the equatorial Pacific, initiating a new era regarding the understanding of the tropical Pacific. In order to get more understanding of the major processes of atmosphere-ocean coupling in the WPWP area, scientists further proposed and implemented the TOGA Coupled Ocean Atmosphere Response Experiment (COARE) program. Wyrtki (1989) originally proposed the standard definition of the warm pool in the report on the results of TOGA-COARE, i.e., a water body with a temperature no less than 28 C, and described the basic characteristics of the warm pool. Subsequently, more studies addressed various aspects of the WPWP, such as its position, volume, and intensity. Further studies (Schneider et al., 1996; Kim et al., 2012) have shown that solar radiation has significant influence on the vertical structure and seasonal variation of the warm pool. Some scholars used both satellite and in-situ observational data to discuss the longitudinal and latitudinal variations of the warm pool center (Ho et al., 1995) (the socalled centroid) and warm pool heat center (Hu et al., 2012). Picaut et al. (1996) indicated the close relationship between the latitudinal variation of the warm pool (or the variation of its eastern boundary) and the development of ENSO events. In addition, Kim (2012) identified that ENSO events are principal factors in the inter-annual variation of the warm pool. Moreover, the WPWP volume could serve as an index for describing the intensity of WPWP and thus, its annual and inter-annual variations become important research topics (Hu S J, 2013). However, restricted by limited information, studies on the variation of warm pool volume are far from adequate. Wyrtki (1989) highlighted that, because of limitations in the spatial resolution of observational data, it had always been hard to determine the horizontal and vertical boundaries of the warm pool and consequently, estimations of the warm pool volume had been difficult. Despite this, he still evaluated that (given the average depth of 80 m and horizontal area of 25 10 12 m 2 ) the volume of the WPWP was about 20 10 14 m 3. Yang (2007) adopted the 28.5 C isotherm and SODA data to analyze the WPWP volume changes, and indicated that its annual variation of WPWP volume was quite weak. A double-peak structure and inter-annual oscillations with periods of 3 6 a and 1.5 a had also been found, however, he did not discuss the reasons for the variations of the WPWP volume. Furthermore, other studies (Clement et al., 2005) argued that if only the atmospheric course was considered, the warm pool would not exist. Although the atmospheric course has significant impact on the SST of the WPWP, other factors such as subtropical severe convection (Ramanathan et al., 1991), ocean surface evaporation (Harmann et al., 1993), humidity of the free tropospheric layer (Pierrehumbert R T, 1995), and lower layer cloud coverage (Miller R L, 1997) could also affect the distribution of the WPWP s SST. Among these, ocean dynamic processes are the most important factor for the formation of the warm pool. The western boundary of the ocean would stop the trade wind drift and cause warm water stacking (Bjerknes J A, 1966). The depth of the thermocline in the western tropical Pacific Ocean is relatively deeper than that in the east. Thus, through the equatorial upwelling, the surface Ekman transport which transfers poleward is connected with the geostrophic currents which flow equatorward beneath the surface water, and further leads to the occurrences of a cold tongue in the eastern Pacific Ocean and a warm pool in the west. Meanwhile, with regard to the influence of ocean dynamics on the WPWP, many papers pointed out that the subtropical gyre in each hemisphere would go through the WPWP, and warm water would flow out of the WPWP via the Northern Equatorial Counter Current (NECC) and Indonesian Through Flow (ITF). When adopting historical annual-averaged temperature-salinity data to analyze the transportation (integrated from the ocean surface to the 12 C isotherm) of the geostrophic currents in the tropical western Pacific Ocean (5 S 20 N, 120 170 E), Toole (1988) argued that the Northern Equatorial Current (NEC) has a transportation of about 43 Sv, which is distributed between the Kuroshio Current (~25 Sv) and Mindanao Ocean Current (MOC, ~18 Sv). The MOC and part of the Southern Equatorial Current (SEC) flow together into the NECC (~33 Sv) and thus transfer the water in the WPWP eastward. Furthermore, the ITF also brings some water (~0.7 Sv) out of the WPWP. However, the authors also noted that deficiencies in data resolution would hindered the explicit definition of each current and consequently, the calculation was approximate, especially regarding the estimation of the SEC. Wyrtki and Kilonsky (Wyrtki et al., 1984) used crusing observational (150 W section, 0 400 m) data between the islands of Hawaii and Tahiti to determine the flows flowing from the eastern side of the WPWP. They found that the transportations of the NEC and NECC were 24 Sv and 20 Sv, respectively, and that the transportation of the southern and northern branches of the SEC were 15 Sv and 26 Sv, respectively. However, not all the currents provide warm water for the WPWP. Wyrtki (1989) indicated that only part of (~50%, ~12 Sv) the NEC provides warm water to the WPWP because of its great depth. However, the NECC is relatively shallower and most of the seawater temperatures are over 27 C, it thus constantly takes warm water out of the WPWP (~15 Sv). He also figured out that the SEC has a cross-equatorial motion. Its northern part is relatively shallower and mainly consists of warm water (~12 Sv), whereas the southern part is deep

Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2271 enough to reach the interior of the thermocline, providing warm water of about 20 Sv (0 8 S). At the same time, currents to the south of 8 S are very complicated and quite weak. Transportation of the ITF is evaluated to be between 5 and 14 Sv, and it is mostly originated from the SEC (Gordon A, 1986). Wyrtki (1989) considered that it would be more rational to the hypothesis that the ITF takes about 8 Sv of warm water out of the WPWP. According to his opinion, 8 Sv warm water was brought out by the ITF, and 15 Sv warm water flowed out of the WPWP through the eastern boundary. Additionally, among the 21 Sv warm water flowed into the WPWP, about 10 Sv flowed out of the WPWP and entered the Southern Hemisphere Subtropical Circulation, and the remaining 11 Sv entered the Southern Hemisphere Subtropical Circulation, thus, the transportation cycled into and out of the WPWP can be balanced. To sum up, restricted by limited data, previous WPWPrelated studies have focused mostly on its surface (e.g., surface temperature, area) or just part of it (e.g., near the equator, western part), and the flow transportation was estimated through section analysis rather than being deemed a whole structure. Moreover, studies on the variation mechanisms and dynamic processes of the WPWP are rare. Therefore, the dynamic factors influencing the variation of the WPWP volume are still unknown and urgently require further study. Argo global ocean observation network has been developed over ten years and has provided over 100000 in-situ temperature and salinity profiles with a depth range of 0 2000 m (Xu et al., 2007). However, there have been few cases using Argo float data to study the WPWP. Therefore, this paper plans to use these high-precision, high-resolution, in-situ observational data to study the essential features and volume variations of the WPWP, and to explore the sources of warm water, as well as its possible maintaining mechanisms, to provide a scientific basis for the study of warm pool variation and its role in climate change. 1 Datasets and computing methods 1.1 Data sources 1.1.1 Argo data This paper adopts the gridded Argo temperature and salinity profile data provided by the Scripps Institution of Oceanography, UC San Diego (Dean et al., 2009; Wu et al., 2011). The coverage scope of the data is 60 S 60 N, 0 360 E, and the horizontal resolution is 1 1 (longitude latitude). There are vertically 58 layers in total (2.5 m for the surface layer and 1975 m for the bottom layer). The vertical spacing of the data grows with increasing depth (the spacing is 10 m above 200 m, 20 m for 200 500 m, 50 m for 500 1350 m, and 75 100 m for 1400 1975 m), and the time sequence is Jan 2004 to Dec 2010. The temperature, salinity and pressure profile information of the tropical Pacific ocean (30S 30 N, 120 E 120 W, Figure 1) is used as the basic data to analyze the essential features and volume variations of the WPWP. In addition, absolute geostrophic currents in the study region are also calculated by Argo profile data. 1.1.2 Surface geostrophic currents inversed by AVISO data We adopted the surface geostrophic current information inversed by satellite altimeter data to verify the results of the absolute geostrophic current calculated using the P- vector method. The inversion information was derived from the 1/3 1/3 gridded data provided by Archiving Validation and Interpretation of Satellite Oceanographic (AVISO). These data integrated different observational information from satellites such as TOPEX/POSEIDON, JASON, ERS1/2 and so on. The first advantage of this dataset is that it combined information from upward, downward, and connecting tracks. Another advantage is that the data precision is high, even at crossover points, which benefits from the consideration of the gradients between tracks 1). 1.2 P-vector method Direct measurements of ocean currents are relatively difficult and expensive and thus, existing in-situ current data are rare. Although numerical simulation techniques have been evolving constantly, the current numerical models have great uncertainty in the simulation of ocean currents. Therefore, using hydrologic observational data to derive geostrophic current has become a common method. Many inverse methods such as spiral method (Huang et al., 1994), P-vector method (Chu, 1994, 1995, 2000), and improved inverse mode method (Wang et al., 2001) have successively been developed in ocean circulation researches. In recent decade, Argo program has gradually established and maintained a global ocean observation network, enabling the acquisition of numerous high-resolution in-situ temperature and salinity profile data, which are very useful in the study of the WPWP and ocean circulation study. The P-vector method used to compute absolute geostrophic current was proposed by Chu (1995). Its basic principle is based on the -Spiral method and its governing equations are g z u u0 ˆd z, f (1) z 0 0 y g z ˆ v v0 dz, f (2) z0 x 0 1) MDT_CNES-CLS09 was produced by CLS Space Oceanography Division and distributed by Aviso, with support from Cnes (http://www.aviso. oceanobs.com/).

2272 Figure 1 Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 Study region and locations of all Argo floats deployed during 2004 2010. where (u, v) and (u0, v0) are the geostrophic speeds at any depth Z and the reference depth Z0, respectively, ˆ refers to seawater density, 0 refers to the average seawater density, and f=2 sin represents the Coriolis effect. In the context where the two necessary conditions (allelic vortex surface does not overlap the isopycnic surface, and the horizontal component of speed exhibits deflection with increasing depth, i.e., spiral structure) are fully satisfied, the unit vector P shall be defined as q. q small when they are calculated by the P-vector method, the currents of the 500 m are adopted as the reference plane to calculate the absolute geostrophic currents of 0 500 m once again. Furthermore, because the Coriolis force at the equator is almost zero, it cannot satisfy the geostrophic relation and therefore, the P-vector method is not applicable. To overcome this shortcoming, the absolute geostrophic currents at 5 S and 5 N sections are taken to represent the currents within the scope of 0 5 S and 0 5 N (Wyrtki, 1981). (3) 1.3 Calculation and verification of absolute geostrophic currents Because the unit vector meets q 0, any V=(u, v, w) is parallel to the unit vector P, i.e. Figure 2 shows the average absolute geostrophic currents at 2.5 m of the Pacific Ocean (45S 45 N, 120 E 80 W) from Jan 2004 to Dec 2010 which are calculated by the P-vector method (for the convenience of verification, we have expanded the longitudinal and latitudinal range compared with Figure 1). As can be seen, the principal surface current systems of the Pacific Ocean, such as the NEC, NECC, subtropical circulation, Kuroshio Current, and Kuroshio Extension, are all embodied in the figure, especially the features of the Kuroshio Current and Kuroshio Extension. Moreover, it can be seen that the bifurcation of the NEC is around 14 N. To verify the reliability of the absolute geostrophic currents, we correlate the currents at 2.5 m with the surface geostrophic currents inversed from the AVISO satellite altimeter data, as shown in Figure 3. Here, the result of the comparison between latitudinal currents is given. As can be seen, the correlation coefficients of the low latitude areas P= V=r(x, y, z)p. (4) Thus, the absolute geostrophic currents could be obtained. Moreover, when using the P-vector method, the second order of differential quotient in density field should be computed. However, its precision would be affected by extensive internal waves and mesoscale eddies existing within the ocean, and noises from instrumental measurements, and consequently could influence the calculation of geostrophic currents. To address this problem, Chu (1994, 1995, 2000, 2001) proposed the optimization scheme of the least squares method to reduce the errors arising from the calculation process to a minimum. The reader is referred to the references for more detailed information of the P-vector method. It is important to note that, because the absolute geostrophic currents in the upper mixed layer are relatively

Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2273 Figure 2 Average absolute geostrophic currents at 2.5 m. Figure 3 Correlation coefficients between absolute geostrophic currents derived from Argo (2.5 m, latitudinal) and AVISO (surface, latitudinal) data. along both sides of the equator and in the northwestern Pacific Ocean area are all above 0.4, and some are even as high as 0.8 (having passed the significance test of =0.05), whereas the correlation coefficients in the northeastern and southern Pacific areas are relatively low, which is probably due to insufficient Argo temperature and salinity profiles in these regions, and the difference of the compared levels between the two. By comparing (figure omitted) the long-term sequences of surface (2.5 m) latitudinal geostrophic currents with the inversion results of the AVISO satellite altimeter data at the station (8 N, 137 E), it could be found that the latitudinal absolute geostrophic currents calculated from Argo data shows more or less the same trend as the variations shown

2274 Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 by AVISO data. 2 Three-dimensional structure of the WPWP and its volume variation 2.1 Features of climatic WPWP Here we define the WPWP s temperature higher than or equal to 28 C. Figures 4 and 5 show the 3D structure of the climatic state (the climatic state here refers to both the annual and monthly temperatures being no less than 28 C) of the warm pool (2004 2010) and its boundary distribution for different depths, respectively. As indicated, the area of the WPWP gradually shrinks and inclines southeastwards. Below 100 m, nearly the entire WPWP is located south of the equator. At 120 m, it is located mainly within the region of 3 7 S, 160 180 E. The eastern boundary of the WPWP can extend as far as 140 W, while the southern and northern boundaries can almost both reach 20. The climatic WPWP can be as deep as 120 m, thus, its volume, calculated according to Figures 4 and 5, is 1.86 10 15 m 3. This is quite close to the assessment by Wyrtki (1989), who assumed a WPWP depth, area, and volume of 80 m, 25 10 12 m 2, and 20 10 14 m 3, respectively. 2.2 Volume variation of the WPWP Figure 6 illustrates the volume (integral from the bottom irregular boundary to the surface irregular boundary as shown in Figure 5) variation of the WPWP. Figure 6(a) shows the annual variation of the WPWP, whereas Figure 6(b) shows the long-term monthly variation of the WPWP volume from 2004 to 2010 (solid line) and the results after 12-month moving average (asterisk line). As demonstrated by Figure 6(a), the annual variation of the WPWP volume shows a typical double-peak structure. The volume of the WPWP increases gradually in spring (Mar May) by about 0.4 10 15 m 3. The first peak value appears in June (~2.2 10 15 m 3 ), following which the WPWP volume initially declines slightly and then rises slowly during summer (Jun Aug). During the autumn (Sep Nov), in October, the second peak value (~2.3 10 15 m 3 ) in the annual variation of the WPWP volume appears, following which the WPWP volume declines significantly by about 0.5 10 15 m 3 into winter (Dec Feb). These features of the annual variation of WPWP volume are embodied in trace of each year (see the blue solid line in Figure 7(b)). The double-peak structure of the semiannual variation of the WPWP is related to the seasonal variation of solar radiation. On the seasonal scale, solar radiation affects the vertical structure and area of the WPWP and consequently, also its volume (Schneider et al., 1996; Meng et al., 2002) As can be seen from Figure 6(b) (solid line), the annual volume variation that featured with a double-peak structure is obvious in each year. The lower values of WPWP volume for 2004 2007 and for 2009 2010 are similar; all are around 1.8 10 15 m 3 with small fluctuations. The lower value for 2008 is the lowest (~1.6 10 15 m 3 ) among all the years. With regard to the maximum values, those for 2005 2007 are similar (~2.3 10 15 m 3 ), the high values for 2004 and Figure 4 Three dimensional structure of climatic (2004 2010) WPWP.

Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2275 Figure 5 Boundary distribution of climatic (2004 2010) WPWP for different depths. Figure 6 Volume variations of the WPWP. (a) is the annual variation. The solid line in (b) shows the monthly variation from 2004 to 2010; the asterisk line in (b) shows the result after 12-month smoothing average.

2276 Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2009 are somewhat larger (~2.4 10 15 m 3 ), whereas the high values for 2008 and 2010 (<2.2 10 15 m 3 ) are quite different from those of the other years. Regarding the annual variation of WPWP volume, 12-month moving averaged curve (asterisk line in Figure 6(b)) and the maximum entropyspectrum analysis (figure omitted) of the time sequence both show that it has a period of quasi-biennial inter-annual variation. The inter-annual signal further shows that the volume of the WPWP declined significantly during the La Niña event of 2007 2008 (Climate Center of NOAA, (http:// www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso stuff/ensoyears.shtml, henceforth the same), and rose significantly during the El Niño events of 2004 and 2009 2010. Kim (2012) highlighted that ENSO events have a strong correlation with both the volume and the latitudinal variation of the WPWP, and that it exhibits lagged effects of about 5 6 months on the intensity of the WPWP. However, they did not mention the response of the WPWP volume to the ENSO events. We shall discuss that later. 3 Discussion on warm water sources and volume variation mechanisms of the WPWP According to the irregular eastern and western boundaries of the climatic WPWP of different depths shown in Figures 4 and 5, and the average absolute geostrophic currents for several years given above, we have calculated the inflow and outflow of the warm water in the latitudinal and the longitudinal direction, respectively. It is important to note that, in order to discuss warm water sources of the WPWP, the calculations regarding the flows are all performed under the controlling condition that the water temperature is above 28 C. Thus, it means that the entering currents can provide warm water for the WPWP, and the exiting currents would bring warm water out of the WPWP. 3.1 warm water entering and exiting the WPWP at meridional direction (climatic state) Table 1 shows the situation regarding warm water entering and exiting the WPWP via the eastern and western boundaries. Firstly, as shown, within the 2.5 50 m range, the warm water entering the WPWP is greater than the exiting one which means that the WPWP mainly receives warm water within this depth range. Meanwhile, the warm water entering the WPWP is mainly from the east, which is clearly revealed by Table 1. And, in the same depth range, the warm water exiting from the eastern boundary is also greater than that exiting in the west. Secondly, within the depth range of 100 120 m, the WPWP still receives warm water from its eastern boundary, whereas the warm water inflow across the western boundary declines remarkably. While regarding the outflow of the warm water, its net transportation across the western boundary represents a loss of warm water from the WPWP, whereas the warm water exiting from the eastern boundary within this depth range is very little. We also calculated the amount of warm water entering and exiting the WPWP via the eastern and western boundaries for the entire thickness. As shown in Figure 7, the warm water entering from the east is about 39 Sv and that exiting is about 31 Sv. And the warm water entering via the western boundary is about 13 Sv and that exiting is about 18 Sv. 3.2 warm water entering and exiting the WPWP at zonal direction (climatic state) Table 2 shows the inflows and outflows of warm water entering and exiting the WPWP via the southern and northern boundaries. Similar to the latitudinal calculation, the results here are also performed under the controlling that the water temperature is above 28 C. As can be seen, within the depth range of 2.5 50 m, the warm water entering the WPWP from the northern boundary is a little more than that entering from the southern boundary, and the exact same thing also happened in the exiting circumstance. Within the depth range of 100 120 m, irrespective of entering or exiting, the volumes (the flows are both lower than 1 Sv) are much less than those within the 2.5 50 m. In particular, the warm water entering from the northern boundary shows a remarkable decline. Within this depth range, warm water is obtained mainly from the south, whereas the outflows have more or less the same values at both the northern and southern boundaries. We also calculated the total inflows and outflows of each layer via the southern and northern boundaries of the WPWP. The inflow of warm water from the northern boundary is Table 1 Warm water entering and exiting the WPWP at different depths Depth (m) Entering from the east Entering from the west Exiting from the east Exiting from the west 2.5 2.65 0.32 1.08 0.18 50 3.18 1.29 2.80 0.58 100 1.22 0.16 0.06 1.37 110 1.45 0.02 0 1.56 120 1.36 0 0 1.39

Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2277 Figure 7 Amount of warm water (unit: Sv) entering and exiting the WPWP for the entire thickness. Table 2 Warm water entering and exiting the WPWP at different depths Depth (m) Entering from the north Entering from the south Exiting from the north Exiting from the south 2.5 1.19 0.78 0.82 1.11 50 1.45 1.11 1.25 1.28 100 0.02 0.92 0.29 0.40 110 0 0.48 0.22 0.31 120 0 0.46 0.02 0.09 about 12 Sv and the outflow is about 11 Sv, whereas the inflow from the southern layer is about 14 Sv and the outflow is about 12 Sv, as shown in Figure 8. 3.3 Annual and inter-annual variations of net transportation of warm water Same as shown in Figure 6, we depicted the annual and interannual variations of the latitudinal and longitudinal net transportation (inflows minus outflows for the total layers) of warm water entering and exiting the WPWP in Figure 9. Figure 9(a) shows the annual variation of the net transportation in the zonal direction, while Figure 9(b) shows the long-term monthly variations of the net transportation from Jan 2004 to Dec 2010 (solid line) and the outcome after 12-month moving average (dashed line). Figure 9(c) and (d) presents the situation on longitudinal variations. As can be seen, during January and December, the net transportations of warm water meridionally entering and exiting the WPWP are negative, indicating that the WPWP mainly loses warm water in this direction on annual time scale. Furthermore, the annual variations also show a relatively clear double-peak structure. The first maximal value (about 38 Sv) of outflows appears in February, and then

2278 Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 Figure 8 Amount of warm water (unit: Sv) entering and exiting the WPWP for the entire thickness. Figure 9 Variations of latitudinal and longitudinal net transportations (unit: Sv) entering and exiting the WPWP. (a) and (b) showing the latitudinal variations; (c) and (d) showing the longitudinal variations. The dashed line represents the outcome after 12-month moving average.

Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 2279 gradually declines until July. During that time, the variation between February and March is quite rapid, whereas from March to July it is quite slow. The amount of decline is about 31 Sv, and the minimal value (about 6 Sv) is achieved in July. From early summer to early autumn, the net transportation of warm water starts to rise again, and the second maximal value (about 25 Sv) appears in October. Subsequently, it goes down again. Figure 9(b) shows that during January 2004 and December 2010, the latitudinal net transportations fluctuate frequently (solid line) and the variations in the La Niña event of 2007/2008 and El Niño event of 2009 are so different from normal years (2005 2006). In the La Niña event, the net transportation of warm water increases, but it drops significantly during the El Niño event and the WPWP even obtains warm water. This is related to the eastward expansion of the entire layer of the warm pool during the El Niño event and the westward contraction of the entire layer of the WPWP during the La Niña event (Zhang et al., 2004; Qi et al., 2008). The longitudinal net transportations are both positive values, indicating that the WPWP mainly gains warm water from this direction, and the annual variation presents an approximate double-peak structure. The first extreme value appears in April June and the second in August October. The amounts of warm water gained latitudinally by the WPWP in January March, July, and November are the smallest (about 50 Sv). Moreover, during March July and July November, the variations of the net transportation are similar. The maximum value (about 80 Sv) appears in the first month and that is maintained at a similar level for about 2 months before it drops to a low value in the fourth month. The red solid line in Figure 9(d) indicates that the latitudinal net transportation of warm water has obvious inter-annual variation, especially during the ENSO event. The La Niña events beginning in 2007 and late 2010 both witness the latitudinal increases in the amount of warm water. Furthermore, we also analyzed the correlation between WPWP volume and the net transportation of warm water (latitudinal added longitudinal) on annual time scale, and found that the correlation coefficient between them at the same phase is 73.78% (having passed the test of significance). This indicates that the annual variation of warm pool volume is not only influenced by solar radiation, but also by the water advection passing through the WPWP. On inter-annual time scale, the correlation coefficient between the two is much lower, and the correlation between the longitudinal net transportation and the Niño3.4 index is also low. However, the variation of the latitudinal net transportation has a correlation coefficient as high as 66.78% with the Niño3.4 index, indicating that the latitudinal water advection is more strongly influenced by the ENSO event. 3.4 Inflow and outflow via each boundary Here we discuss the transportation of warm water entering and exiting the WPWP from each boundary. We found that the inflow and outflow via the western boundary are both less than that via the eastern boundary (figures omitted), indicating that the longitudinal gain and loss of warm water is more obviously influenced by the eastern area to the WPWP than that to its west. Meanwhile, through the western boundary, the warm water exiting the WPWP is far greater than entering, for the ITF, SEC, and NEC can bring warm water of the WPWP to the Indian Ocean and higher latitudes of the Pacific Ocean. However, detailed analysis shows that during the two La Niña events in 2007 and 2010, the warm water exiting the WPWP from the western boundary declined remarkably, whereas that exiting from the eastern boundary rose significantly, which could be related to the eastward rise of thermocline during ENSO events (Long et al., 1998; Li et al., 1999; Chao et al., 2003). Moreover, regarding the latitudinal transportation, irrespective of the boundary, the inflow of warm water is greater than the outflow, indicating that the WPWP mainly gains warm water in the south-north direction. And, warm water gained via the southern boundary is much greater than that received across the northern boundary, suggesting that the WPWP mainly gains warm water from the tropical and subtropical circulations to the south of the equator. 4 Summary This paper adopted Argo profile data to analyze the threedimensional structure and volume variations of the WPWP, and established that the annual variations of the WPWP volume present a typical double-peak structure. Moreover, the warm water entering latitudinally the WPWP is sourced mainly via the upper layer of the eastern boundary, and the latitudinal flow exiting the WPWP is mainly via the middle and lower layers of the WPWP across the western boundary. In the longitudinal direction, the WPWP mainly gains warm water from the tropical and subtropical circulations to the south of the equator, and the quantities of warm water exiting the WPWP across the southern and northern boundaries are quite similar. Say generally, the WPWP mainly loses warm water in the latitudinal direction, while it receives plenty of warm water in the south-north direction. The WPWP volume and the net transportation of warm water entering and exiting the WPWP show a relatively strong correlation on annual time scale, however, the correlation between the two on inter-annual time scale is relatively weak. But, in respect of the inter-annual time scale, warm water advection in the longitudinal direction is more strongly influenced by ENSO events than that in the latitudinal one. During the two strong La Niña events in 2007 and 2010, the longitudinal warm water transport was so different compared with normal years. It affected the supply of warm water and led to a variation of the WPWP volume during the ENSO events. Actually, as Wyrtki (1989) pointed out, it

2280 Wu X F, et al. Sci China Earth Sci September (2014) Vol.57 No.9 is difficult to measure the volume variations of the WPWP, and harder still to measure the inflows and outflows. So, even though we use the P-vector method and Argo data to calculate the flows, there will still be some inherent errors. For example, the geostrophic balance is not suitable at the equator, while the upper layers of the WPWP step cross the equator and thus, the simplified calculation of the geostrophic currents at the equator will result in imbalance of net transportation went through the WPWP. In spite of these, our analysis could still help understand the volume variations and sources of warm water of the WPWP. This work was supported by the Special Program for the National Basic Research (Grant No. 2012FY112300). We are grateful to SOED HPCC of the Second Institute of Oceanography, State Oceanic Administration for support and assistance. 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