Reconstruction of the western Pacific warm pool SST since 1644 AD and its relation to precipitation over East China

Based on coral proxies we reconstructed the western Pacific warm pool sea surface temperature (SST) since 1644 AD. High-frequency reconstructions are based on eight high-pass filtered coral series and raw reconstructions are derived from eight unfiltered coral series, respectively. Validation and comparison with other SST/temperature series show that the reconstructed warm pool SST is highly reliable. The leading periods of warm pool SST are ～2.1, ～2.3, ～2.9, ～3.6, ～3.8, and 80.7-year during the last ～360 years. The warm pool SST exhibits some obvious long-term trends: an upward trend of 0.04°C per century for the period of 1644–1825, while a decreasing trend of 0.24°C per century for the period of 1826–1885, and then a remarkable warming trend of 0.28°C per century taking place between 1886 and 2006. Especially, the SST shows the strongest trend of 0.67°C increase per century during the last 50 years, a warming unprecedented since 1644 AD. On interannual timescale, the connections between ENSO and the warm pool SST are robust during the reconstruction period. There are significant correlations between the warm pool SST and summer precipitation of the Yellow River basin and Huaihe River basin; the correlation coefficients are ?0.44 in reconstruction period (1880–1949 AD) and ?0.46 in instrumental period (1950–2005 AD) respectively. This relationship is also found between flood-drought index and the warm pool SST during the past 360 years, and their correlation coefficients are ?0.20 in reconstruction period and ?0.46 in instrumental period respectively, significant at the 0.01 level. On interdecadal timescale, this connection is more robust, and the correlation coefficient of the low-pass filtered components is ?0.42 during the whole period (1644–2000 AD). When the warm pool is warmer than normal, the precipitation is usually below the normal in the Yellow River and Huaihe River basin. On the contrary, when the warm pool is colder than the normal, there may be more precipitation. The reconstructed warm pool SSTs provide useful information and reference for further research on climate change mechanism in East China.

REGIONAL FEATURES OF LONG-TERM SST VARIATION IN THE WESTERN PACIFIC WARM POOL AREA

The main features of SST interannual and long term variations in the Western Pacific Warm Pool area were studied by using regression analysis, running t test and spectral analysis methods based on monthly mean SST data in the Pacific during 1950-1998. The results showed that the SST inter annual and long term variations in the Western Pacific Warm Pool area had evident regional features. There were significant differences in variation range, phase, period, occurrence time of abrupt variation between SST in the eastern area (east of 160°E) and SST in the western area (west of 160°E).

NUMERICAL STUDY OF INFLUENCE OF THE SSTA IN WESTERN PACIFIC WARM POOL ON RAINFALL IN THE FIRST FLOOD PERIOD IN SOUTH CHINA

A brief introduction of a global atmospheric circulation model CCM3, which is used to simulate the precipitation in China, the height and the flow fields of the atmosphere, is made and the reliability of simulation is analyzed. According to the negative correlation between rainfall in the first flood period in South China (FFSC) and sea surface temperature anomalies (SSTA) in a key region in western Pacific warm pool (West Region), two sensitive experiments are designed to investigate the effects of the latter on the former and the possible physical mechanism is discussed. It is found that in cold water (warm water) years, the rainfall in South China (SC) is far more (less) than normal, while the rainfall in the middle and low reaches of the Yangtze River is relatively less (more). The best correlative area of precipitation is located in Guangdong Province. It matches the diagnostic result well. The effect of SSTA on precipitation of FFSC is realized through the abnormality of atmospheric circulation and tested by a P-σnine-layer regional climate model. Moreover, the simulated result of the P-σmodel is basically coincident with that of the CCM3.

30-60-day Oscillations of Convection and Circulation Associated with the Thermal State of the Western Pacific Warm Pool during Boreal Summer

This study focuses on the characteristics of the 30 60-day oscillation (MJO) associated with the interannual variability of the thermal state in the western Pacific warm pool. The composite results show that, the amplitude of MJO convection over the tropical western Pacific tends to intensify (reduce) in the WARM (COLD) case. The negative correlations between MJO convection in the WARM and in the COLD cases are examined to be significant over most of the Asian-Pacific region. The evolutions of MJO convection and lower circulation, on the one hand, exhibit larger differences between the WARM and COLD cases, but on the other hand, display a unique feature in that a well-developed MJO cyclone (anticyclone) is anchored over the Asian-Western Pacific domain at the peak enhanced (suppressed) MJO convection phase over the western Pacific warm pool, either in the WARM or in the COLD case. This unique feature of MJO shows a Gill-type response of lower circulation to the convection and is inferred to be an inherent appearance of MJO. The context in the paper suggests there may exist interactions between MJO and the interannual variability of the thermal state in the western Pacific warm pool.

UPPER OCEAN RESPONSE TO SURFACE MOMENTUM AND FRESHWATER FLUXES IN THE WESTERN PACIFIC WARM POOL

A series of numerical experiments are carried out to study the tropical upper ocean response to combined momentum and buoyancy forcing, with emphasis on the three-dimensional thermohaline structure in the western Pacific warm pool. In response to climatological winds, heat fluxes and freshwater input, the model is able to simulate the salient dynamic and hydrographic features of the tropical Pacific Ocean and their seasonal variability. In response to idealized episodic westerly wind bursts and rainfall, the simulated upper ocean conditions compare favorably with available observations, thus enabling us to identify important physical processes involved. Local forcing, vertical mixing and meridional advection dominate the salt and heat budgets in the warm pool on short time scales, but it is necessary to include the saline water coming from the east with the South Equatorial Current to close the salt budget on seasonal and longer time scales. Strong westerly wind bursts generate a swift eastward equatorial jet and a pair of meridional circulation cells with convergence at the equator. This results in an equatorward advection of relatively fresh water from the north and a depression of the thermocline at the equator. Heavy rainfall reduces the surface mixed layer depth by creating a shallow halocline, thus trapping the momentum and heat inputs near the surface. The remote influences of the episodic momentum and buoyancy fluxes are very different. Westerly wind bursts can generate large downstream disturbances in both dynamic and thermal fields through the propagation of equatorial waves, while the effect of rainfall is mostly confined to the forcing area.

Seasonal variability in the thermohaline structure of the Western Pacific Warm Pool

Using the 28℃ isotherm to define the Western Pacific Warm Pool （WPWP）, this study analyzes the seasonal variability of the WPWP thermohaline structure on the basis of the monthly-averaged sea temperature and salinity data from 1950 to 2011, and the dynamic and thermodynamic mechanisms based on the monthly-averaged wind, precipitation, net heat fluxes and current velocity data. A△T=-0.4℃ is more suitable than other temperature criterion for determining the mixed layer （ML） and barrier layer （BL） over the WPWP using monthly-averaged temperature and salinity data. The WPWP has a particular thermohaline structure and can be vertically divided into three layers, i.e., the ML, BL, and deep layer （DL）. The BL thickness （BLT） is the thickest, while the ML thickness （MLT） is the thinnest. The MLT has a similar seasonal variation to the DL thickness （DLT） and BLT. They are all thicker in spring and fall but thinner in summer. The temperatures of the ML and BL are both higher in spring and autumn but lower in winter and summer with an annual amplitude of 0.15℃, while the temperature of the DL is higher in May and lower in August. The averaged salinities at these three layers are all higher in March but lower in September, with annual ranges of 0.41-0.45. Zonal currents, i.e., the South Equatorial Current （SEC） and North Equatorial Counter Current （NECC）, and winds may be the main dynamic factors driving the seasonal variability in the WPWP thermohaline structure, while precipitation and net heat fluxes are both important thermodynamic factors. Higher （lower） winds cause both the MLT and BLT to thicken （thin）, a stronger （weaker） NECC induces MLT, BLT, and DLT to thin （thicken）, and a stronger （weaker） SEC causes both the MLT and BLT to thicken （thin） and the DLT to thin （thicken）. An increase （decrease） in the net heat fluxes causes the MLT and BLT to thicken （thin） but the DLT to thin （thicken）, while a stronger （weaker） precipitation favors thinner （thicker） MLT but thicker （thinner） BLT and DLT. In addition, a stronger （weaker） NECC and SEC cause the temperature of the three layers to decrease （increase）, while the seasonal variability in salinity at the ML, BL, and DL might be controlled by the subtropical cell （STC）.

Atmospheric Circulations and Sea Surface Temperatures Related to the convection over the western Pacific Warm Pool on the Interannual Scale

The difference is examined in atmospheric circulation and Sea Surface Temperatures (SSTs) in the trop-ics and subtropics between weak and strong convection over the tropical western Pacific warm pool (signified as WPWP). The WPWP is chosen as the region (110–160°E, 10–20°N), where the Outgoing Longwave Radiation (OLR) shows a great year-to-year variance. A composite study was carried out to examine the differences in atmospheric circulation and SSTs between weak and strong convection over WPWP. First, NCEP/NCAR re-analysis data and satellite-observed OLR data are used to examine the differences. ERA data, in which the OLR data are calculated, are then used for re-examination. The composite results show that the differences are remarkably similar in these two sets of data. The difference in circulations between weak and strong convection over WPWP is significantly associated with westward extension of the North Pacific subtropical anticyclone and stronger westerlies at the northwestern edge of the subtropical anticyclone. It also corresponds with the significant easterly anomaly and the descent anomaly in situ, i.e., over the WPWP. The most prominent characteristics of the difference of SSTs between weak and strong convection over the WPWP are the significant positive SST anomalies in the Indian Ocean, the Bay of Bengal and the South China Sea. In WPWP, however, there are only weak negative SST anomalies. Thus, the anomaly of OLR over WPWP is weakly associated with the SST anomalies in situ, while closely associated with the SST anomalies west of WPWP. Key words Convection over the western Pacific warm pool - Northwest Pacific subtropical high - Sea surface temperatures This study was supported by the “ National Key Programme for Developing Basic Sciences” G1998040900 Part 1.

Possible Origins of the Western Pacific Warm Pool Decadal Variability

In this study, the impacts of the Pacific Decadal Oscillation （PDO） and the Atlantic Multidecadal Oscillation （AMO） on the western Pacific warm pool （WPWP） were investigated. Our results show that the WPWP is linked with the PDO and the AMO at multiple time scales. On the seasonal time scales, the WPWP and the PDO/AMO reinforce each other, while at decadal time scales the forcing roles of the PDO and the AMO dominate. Notably, a positive PDO tends to enlarge the WPWP at both seasonal and decadal time scales, while a positive AMO tends to reduce the WPWP at decadal time scales. Furthermore, the decadal variability of the WPWP can be well predicted based on the PDO and AMO.

Sea surface temperature and salinity reconstruction based on stable isotopes and Mg/Ca of planktonic foraminifera in the western Pacific Warm Pool during the last 155 ka

Changes in sea surface temperature （SST）, seawater oxygen isotope （δ18Osw）, and local salinity proxy （δ18Osw-ss ） in the past 155 ka were studied using a sediment core （MD06-3052） from the northern edge of the western Pacific Warm Pool （WPWP）, within the flow path of the bifurcation of the North Equatorial Current. Our records reveal a lead-lag relationship between paired Mg/Ca-SST and δ18O during Termination II and the last interglacial period. Similarity in SST between our site and the Antarctic temperature proxy and in CO2 profile showed a close connection between the WPWP and the Antarctic. Values of 818Osw exhibited very similar variations to those of mean ocean δ18Osw, owing to the past sea-level changes on glacial-interglacial timescale. Calculated values of δ18O reflect a more saline condition during high local summer insolation （SI） periods. Such correspondence between δ18O and local SI in the WPWP may reflect complex interaction between ENSO and monsoon, which was stimulated by changes in solar irradiance and their influence on the local hydrologic cycle. This then caused a striking reorganization of atmospheric circulation over the WPWP.

Dynamic mechanism of interannual zonal displacements of the eastern edge of the western Pacific warm pool

The eastern edge of the western Pacific warm pool （WPWP） in the upper layer （shallower than 50m） exhibits significant zonal displacements on interannual scale. Employing an intermediate ocean model, the dynamic mechanism for the interannual zonal displacement of the WPWP eastern edge in the upper layer is investigated by diagnosing the dynamic impacts of zonal current anomalies induced by wind, waves （Kelvin and Rossby waves）, and their boundary reflections. The interannual zonal displacements of the WPWP eastern edge in the upper layer and the zonal current anomaly in the equatorial Pacific west of ll0~W for more than 30 years can be well simulated. The modeling results show that zonal current anomalies in the central and eastern equatorial Pacific are the dominant dynamic mechanism for the zonal displacements of the eastern edge of the upper WPWP warm water. Composite analyses suggest that the zonal current anomalies induced by waves dominate the zonal displacement of the WPWP eastern edge, whereas the role played by zonal wind-driven current anomalies is very small. A sensitivity test proves that the zonal current anomalies associated with reflected waves on the western and eastern Pacific boundaries can act as a restoring force that results in the interannual reciprocating zonal motion of the WPWP eastern edge.

Heat center of the western Pacific warm pool

A heat center （HC） of the western Pacific warm pool （WPWP） is defined, its variability is examined, and a possible mechanism is discussed. Analysis and calculation of a temperature dataset from 1945-2006 show that the mean position of the HC during this period was near 0.4°S/169.0°E, at 38.0 m depth. From a time series of the HC, remarkable seasonal variability was found, mainly in the meridional and vertical directions. Interannual variabilities were dominant in the zonal and vertical directions. In addition, semiannual variation in the HC depth was discovered. The longitude of the HC varies with ENSO events, and its latitude is weakly related to ENSO on time scales shorter than a decade. The variation of the HC longitude leads the Nifio-3 index by about 3-4 months, and its depth lags the index for approximately 3 months. It is concluded that the HC depth results from a combination of its longitudinal and latitudinal variations. Low-pass-filtered time series reveal that the HC has moved eastward since the mid 1980s.

Zonal displacement of western Pacific warm pool and zonal wind anomaly over the Pacific Ocean

The thermal condition anomaly of the western Pacific warm pool and its zonal displacement have very important influences on climate change in East Asia and even the whole world. However, the impact of the zonal wind anomaly over the Pacific Ocean on zonal displacement of the warm pool has not yet been analyzed based on long-term record. Therefore, it is important to study the zonal displacement of the warm pool and its response to the zonal wind anomaly over the equatorial Pacific Ocean. Based on the NCDC monthly averaged SST (sea surface temperature) data in 2°×2° grid in the Pacific Ocean from 1950 to 2000, and the NCEP/NCAR global monthly averaged 850 hPa zonal wind data from 1949 to 2000, the relationships between zonal displacements of the western Pacific warm pool and zonal wind anomalies over the tropical Pacific Ocean are analyzed in this paper. The results show that the zonal displacements are closely related to the zonal wind anomalies over the western, central and eastern equatorial Pacific Ocean. Composite analysis indicates that during ENSO events, the warm pool displacement was trigged by the zonal wind anomalies over the western equatorial Pacific Ocean in early stage and the process proceeded under the zonal wind anomalies over the central and eastern equatorial Pacific Ocean unless the wind direction changes. Therefore, in addition to the zonal wind anomaly over the western Pacific, the zonal wind anomalies over the central and eastern Pacific Ocean should be considered also in investigation the dynamical mechanisms of the zonal displacement of the warm pool.

Phylogenetic analysis of bacterial community in deep-sea sediment from the western Pacific “warm pool”

A depth profile of bacterial community structure in one deep-sea sediment core of the western Pacific ＂warm pool＂ （WP） was investigated and compared with that in a sediment sample from the eastern Pacific （EP） by phylogenetic analysis of 16S rDNA fragments. Five bacterial 16S rDNA clone libraries were constructed, and 133 clones with different restriction fragment length polymorphism （RFLP） patterns were sequenced. A phylogenetic analysis of these sequences revealed that the bacterial diversity in a sample from the WP was more abundant than that in the EP sample. The bacterial population in the sediment core of WP was composed of eight major lineages of the domain bacteria. Among them the γ-Proteobacteria was the predominant and most diverse group in each section of WP sediment core, followed by the α-Proteobacteria. The genus Colwellia belonging to γ-Proteobacteria was predominant in this sample. The shift of bacterial communities among different sections of the WP sediment core was δ-, ε-Proteobacteria, and Cytopahga-Flexibacteria-Bacteroides （CFB） group. The ratios between them in the bacterial communities all showed inversely proportional to the depth of sediment. The sequences related to sulphate reducing bacteria （SRB） were detected in every section. The bacterial community structure in this sediment core might be related to the environmental characteristics of the surface seawater of the western Pacific WP.

Movements of the Western Pacific Warm Pool Centroid and Their Relationship to Sea Surface Temperature Changes in Nino Regions

By using monthly historical sea surface temperature (SST) data for the yearsfrom 1950 to 2000, the Western Pacific Warm Pool (WPWP) climatology and anomalies are studied inthis paper. The analysis of WPWP centroid (WPWPC) movement anomalies and the Nino-3 region SSTanomalies(SSTA) seems to reveal a close, linear relation between the zonal WPWPC and Nino-3 regionSSTA, which suggests that a 9° anomaly of the zonal displacement from the climatological positionof the WPWPC corresponds to about a 1℃ anomaly in the Nino-3 region area-mean SST. This studyconnects the WPWPC zonal displacement with the Nino-3 SSTA, and it may be helpful in betterunderstanding the fact that the WPWP eastward extension is conducive to the Nino-3 region SSTincrease during an El Nino-Southern Oscillation (ENSO) event.

INTERANNUAL VARIATION IN HEAT CONTENT OF THE WESTERN PACIFIC WARM POOL AND ITS EFFECT ON EASTERN ASIAN CLIMATE ANOMALIES

Using the 1980-2010 winter GODAS oceanic assimilations, study is conducted of the winter heat content(HC) established in the subsurface layer(5 to 366 m in depth) over the western Pacific warm pool(WP), followed by investigating the HC spatiotemporal characteristics, persistence and the impacts on the climate anomalies of neighboring regions. Results are as follows: 1) the pattern of integral consistency is uncovered by the leading EOF1(PC1) mode of HC interannual variability, the year-to-year fluctuation of the time coefficients being well indicative of the interannual anomaly of the WP winter subsurface-layer thermal regime. The HC variation is bound up with El Ni觡o-Southern Oscillation, keeping pronounced autocorrelation during the following two seasons and more, with the persistence being more stable in comparison to sea surface temperature anomaly in the equatorial middle eastern Pacific; 2) the winter HC anomalies produce lasting effect on the WP thermal state in the following spring and summer and corresponding changes in the warm water volume lead to the meridional transport and vertical exchange of warm water, which exerts greater impacts upon the sea surface temperature/heat flux over the warm pool per se and neighboring regions, especially in the Philippine Sea during the posterior spring and summer; 3) the increase in the winter HC corresponds to the spring outgoing longwave radiation(OLR) decrease and richer precipitation over the waters east to the Philippine Sea and the resultant convective heating anomalies are responsible for the rise of geopotential isobaric surfaces over tropical and subtropical western North Pacific, thereby producing effect on the western Pacific subtropical high(anomaly). Subsequently, the sea-surface heat flux exchange is intensified in the warm pool, a robust anomalous cyclone shows up at lower levels, air-sea interactions are enhanced and abnormal convective heating occurs, together making the winter HC anomalies even more closely associated with the variation in the summer subtropical high. As a result, the WP winter HC can be used as an effective predictor of the variation in spring/summer western Pacific subtropical high and the strength of summer monsoon over the northwestern Pacific.

Northward expansion of the western Pacific Warm Pool in late 1990s and early 2000s

Based on 48-year (1958-2006) ocean reanalysis data of Simple Ocean Data Assimilation and 23-year (1984-2006) global ocean-surface heat flux products developed by the Objectively Analyzed Air-Sea Heat Flux Project, meridional variation of the western Pacific Warm Pool (WPWP) is addressed. The results show that there is a significant expansion of the northern edge of the WPWP in the late 1990s and early 2000s. This variation is mainly within 120°E-160°E by 8°N-20°N, we define this region (120°E-160°E by 8°N-20°N) as the core region. Furthermore, analyses on upper ocean heat budget show that the short wave radiation plays a key role in the northward expansion of the northern edge of the WPWP in the core region. It is proved that the northward expansion may be caused by the change of the mixed layer which became shallower in 1994-2006 compared with 1984-1993 in the study region. The short wave radiation flux distribution within the shallower mixed layer leads to a positive anomaly in seawater temperature, promoting the northward expansion of the WPWP.

Variation of Air-Sea Heat Fluxes over the Western Pacific Warm Pool Area and Its Relationship with the South China Sea Summer Monsoon Onset

Based on oceanic and atmospheric parameters retrieved by satellite remote sensing using a neural network method, air-sea heat fluxes over the western Pacific warm pool area were calculated with the advanced the advanced Coupled Ocean-Atmosphere Response Experiment 3.0 (COARE3.0) bulk algorithm method. Then, the average annual and interannual characteristics of these fluxes were analyzed. The rela- tionship between the fluxes and the South China Sea (SCS) summer monsoon onset is highlighted. The results indicate that these fluxes have clear temporal and spatial characteristics. The sensible heat flux is at its maximum in the Kuroshio area, while the latent heat flux is at its maximum in the North Equatorial Current and Kuroshio area. The distribution of average annual air-sea heat fluxes shows that both sensible and latent heat fluxes are maximized in winter and minimized in summer. The air-sea heat fluxes have obvious interannual variations. Correlation analysis indicates a close lag-correlation between air-sea heat fluxes in the western Pacific warm pool area and at the SCS summer monsoon onset. The lagcorrelation can therefore predict the SCS summer monsoon onset, providing a reference for the study of precipitation related to the monsoon.

A preliminary study on an upper ocean heat and salt content of the western Pacific warm pool region

On the basis of Argo profile data of the temperature and salinity from January 2001 to July 2014, the spatial distributions of an upper ocean heat content(OHC) and ocean salt content(OSC) of the western Pacific warm pool(WPWP) region and their seasonal and interannual variations are studied by a cyclostationary empirical orthogonal function(CSEOF) decomposition, a maximum entropy spectral analysis, and a correlation analysis.Probable reasons for variations are discussed. The results show the following.(1) The OHC variations in the subsurface layer of the WPWP are much greater than those in the surface layer. On the contrary, the OSC variations are mainly in the surface layer, while the subsurface layer varies little.(2) Compared with the OSC, the OHC of the WPWP region is more affected by El Ni?o-Southern Oscillation(ENSO) events. The CSEOF analysis shows that the OHC pattern in mode 1 has strong interannual oscillation, with eastern and western parts opposite in phase. The distribution of the OSC has a positive-negative-positive tripole pattern. Time series analysis shows that the OHC has three phase adjustments with the occurrence of ENSO events after 2007, while the OSC only had one such adjustment during the same period. Further analysis indicates that the OHC variations are mainly caused by ENSO events, local winds, and zonal currents, whereas the OSC variations are caused by much more complex reasons. Two of these, the zonal current and a freshwater flux, have a positive feedback on the OSC change in the WPWP region.

10-25-Day Intraseasonal Variations of Convection and Circulation Associated with Thermal State of the Western Pacific Warm Pool during Boreal Summer

This study focuses on the characteristics of 10-25-day oscillation associated with the interannual variability of the thermal state in the western Pacific warm pool. The time series of 10-25-day oscillation shows a distinct feature between warm (WARM case) and cold (COLD case) summers over the western Pacific warm pool. The significant negative relationship between the time series of 10-25-day convection anomalies in Warm and Cold cases appears over most of Asian-Pacific region manifesting the interactions between the convection on interannual and 10-25-day intraseasonal time scales. At the peak and trough stages of 10-25-day convection oscillation, a Gill-type low-level atmospheric circulation anomaly, cyclonic or anticyclonic. appears northwest of the convection anomaly. This relationship between the convection and circulation exists both in Warm case and in Cold case. However, at other stages rather than the peak and trough stages, there is no Gill-type circulation response, and the circulation anomaly shows a distinct feature between the Warm and Cold cases, although the convection oscillation exhibits a roughly similar feature.

The Heat Balance in the Western Equatorial Pacific Warm Pool during the Westerly Wind Bursts: A Case Study

The responses of sea surface temperature (SST) in the western equatorial Pacific warm pool to the westerly wind bursts (WWBs) play an important role in the relationship between WWB and ENSO. By using data collected from eight buoys of TOGA (Tropical Ocean-Global Atmosphere)- COARE (Coupled Ocean-Atmosphere Response Experiment), the heat balances of the upper ocean in the western equatorial Pacific around 0 degrees, 156 degreesE during two WWB events were calculated according to Stevenson and Niiler's (1983) method. In both events, SST increased before and after the WWBs, while decreased within the WWBs. The SST amplitudes approximated to 1 degreesC. Although sometimes the horizontal heat advections may become the biggest term in the heat balance, the variation of SST was dominated by the surface heat flux. On the other aspect, some different features of the two events are also revealed. The two cases have different variation of mixed layer depth. The depth of mixed layer is almost double in the first case (35 in to 70 m), which is caused by Ekman convergence, while only 10m increments due to entrainment in the second one, There are also differences in the currents structure. The different variations of thermal and currents structure in the mixing layers accounted for the different variation of the heat balance during the two events, especially the advection and residue terms. The seasonal variation of SST in this area is also investigated simply. The first WWB event happened just during the seasonal transition. So we considered that it is a normal season transition rather than a so-called anomaly. That also suggested that the seasonal distinction of the WWB is worthy of more attention in the researches of its relationship to ENSO.