Seasonal variation of the surface North Equatorial Countercurrent (NECC) in the western Pacific Ocean

The North Equatorial Countercurrent(NECC) is an important zonal fl ow in the upper circulation of the tropical Pacifi c Ocean, which plays a vital role in the heat budget of the western Pacifi c warm pool. Using satellite-derived data of ocean surface currents and sea surface heights(SSHs) from 1992 to 2011, the seasonal variation of the surface NECC in the western tropical Pacifi c Ocean was investigated. It was found that the intensity(INT) and axis position(Y_(CM)) of the surface NECC exhibit strikingly different seasonal fl uctuations in the upstream(128°–136°E) and downstream(145°–160°E) regions. Of the two regions, the seasonal cycle of the upstream NECC shows the greater interannual variability. Its INT and Y CM are greatly infl uenced by variations of the Mindanao Eddy, Mindanao Dome(MD), and equatorial Rossby waves to its south. Both INT and YC M also show semiannual signals induced by the combined effects of equatorial Rossby waves from the Central Pacifi c and local wind forcing in the western Pacifi c Ocean. In the downstream region, the variability of the NECC is affected by SSH anomalies in the MD and the central equatorial Pacifi c Ocean. Those in the MD region are especially important in modulating the Y CM of the downstream NECC. In addition to the SSH-related geostrophic fl ow, zonal Ekman fl ow driven by meridional wind stress also plays a role, having considerable impact on INT variability of the surface NECC. The contrasting features of the variability of the NECC in the upstream and downstream regions refl ect the high complexity of regional ocean dynamics.     

太平洋赤道潜流起源的模式研究

基于一个高分辨率准全球海洋模式HYCOM(HYbrid Coordinate Ocean Model),研究了热带西太平洋海域赤道潜流的起源。结果表明:赤道潜流在大约130°E处开始,流核位于225 m、2°N附近,最大流速超过15 cm/s,体积输运约1.6×106 m3/s,其水源来自棉兰老海流;在东部140°E断面,赤道潜流的北部主要是由棉兰老海流提供(9.7×106 m3/s),其南部主要是来自新几内亚沿岸潜流(9.1×106 m3/s)。     

Research on the response of the upper layer heat structure in the Western Pacific Warm Pool to the mean Madden-Julian Oscillation

1　Introduction　TheMadden JulianOscillation (MJO)isastrongatmosphericconvection phenomenonoccurringovertheEasternIndianOceanandtheTropicalWesternPacific,usuallyinregionswithseasurfacetempera tures (SSTs)over 2 9℃ .Theeastwardmovingofalarge scalecirculat…     

Seasonal heat budget in the tropical Western Pacific Ocean in a global GCM II. In five subregions

The general features of the seasonal surface heat budget in the tropical western Pacific Ocean, 20° S–20°N, western boundary −160°E, were documented by Qu (1995) using a high-resolution general circulation model (GCM, Semtner & Chervin, 1992) and existing observations. Close inspection of the smaller areas, with the whole region further partitioned into six parts, showed different mechanisms balance the seasonal surface heat budget in different parts of the region. The results of study on five subregions are detailed in this article. In the equatorial (3°S–3°N) and North Equatorial Countercurrent (3°N–9°N) region, the surface heat flux does not change significantly throughout the year, so the surface heat content is determined largely by vertical motion near the equator and roughly half due to horizontal and half due to vertical circulation in the region of the North Equatorial Countercurrent (NECC). In the other subrigions (9°N–20°N, 20°S–11°S and 11°S–3°S), however, in addition to ocean dynamics, surface heat flux can also play a major role in the seasonal variation of sea surface temperature (SST). The remotely forced baroclinic waves and their effect on the surface heat storage in the model are also investigated. Comparison with observations indicates that the model wave activities are reasonably realistic. Contribution No. 2396 from the Institute of Oceanology, Chinese Academy of Sciences. This study was supported by the Australian CSIRO Division of Oceanography and the National Natural Science Foundation of China (No. 49176255)     

Response of the equatorial western Pacific thermohaline structure to wind variation

During the Global Weather Experiment oceanographic measurements were recorded during winter and summer in the western Pacific region 5°S−5°N, 160°E−175°E. The variations of the upper ocean temperature and salinity fields were produced by the large seasonal and spatial wind fluctuations. The vertical temperature structure of the thermocline at the equator, the meridional slope of the thermocline south of the equator, and the northward penetration of high salinity water were related to the direction and intensity of the zonal wind-stress. (NOAA Pacific Marine Environmental Laboratory) Contribution No. 1307 from the Institute of Ocean., Academia Sinica. Received Sept. 3, 1985     

Seasonal variability of salinity budget and water exchange in the northern Indian Ocean from HYCOM assimilation

Based on HYbrid Coordinate Ocean Model (HYCOM) assimilation and observations, we analyzed seasonal variability of the salinity budget in the southeastern Arabian Sea (AS) and the southern part of the Bay of Bengal (BOB), as well as water exchange between the two basins. Results show that fresh water flux cannot explain salinity changes in salinity budget of both regions. Oceanic advection decreases salinity in the southeastern AS during the winter monsoon season and increases salinity in the southern BOB during the summer monsoon season. In winter, the Northeast Monsoon Current (NMC) carries fresher water from the BOB westward into the southern AS; this westward advection is confined to 4°-6°N and the upper 180 m south of the Indian peninsula. Part of the less saline water then turns northward, decreasing salinity in the southeastern AS. In summer, the Southwest Monsoon Current (SMC) advects high-salinity water from the AS eastward into the BOB, increasing salinity along its path. This eastward advection of high-salinity water south of the India Peninsula extends southward to 2°N, and the layer becomes shallower than in winter. In addition to the monsoon current, the salinity difference between the two basins is important for salinity advection.     

Statistical study on relationship of El Niño events and the hydrography along 137°E in summer

Principal component analysis (PCA) used by meteorologists and oceanographers is a powerful tool for the analysis of the spatial and temporal variability of physical fields. This study is aimed at applying “quasi-local PCA for singular factor” to make the cumulative percentage for the first principal component as great as possible, so that the many-dimension problem can be reduced to a single-dimension one, and at combining PCA with stepwise regression analysis to parameterize the relationship between El Niño events and the anomalies in hydrographic factors along 137°E in summer. The results show that the hydrography on 30–50 m levels at 7–9°N along 137°E in summer is very closely correlated with El Niño events because of the thermocline movement caused by enhanced upwelling in this area during El Niño years.     

Variabilities of surface current in the tropical Pacific Ocean

The Simple Ocean Data Assimilation (SODA) package is used to better understand the variabilities of surface current transport in the Tropical Pacific Ocean from 1950 to 1999. Seasonal variation, internnual and decadal variability analyses are conducted on the three major surface currents of the Tropical Pacific Ocean: the North Equatorial Current (NEC), the North Equatorial Countecurrent (NECC), and the South Equatorial Current (SEC). The transport of SEC is quite larger than those of NEC and NECC. The SEC has two maximums in February and August. The NEC has a small annual variation. The NECC has a maximum in October and is very weak in March and April. All currents have remarkable interannual and decadal variabilities. The variabilities of the NEC and the SEC related to the winds over them well, but the relationship between the NECC and the wind over it is not close. Analysis related to El Niño-Southern Oscillation (ENSO) suggests that before El Niño (La Niña) the SEC is weaker (stronger) and the NECC is stronger (weaker), after El Niño (La Niña) the SEC is stronger (weaker) and the SEC is weaker (stronger). There is no notable relationship between the NEC and ENSO.     

Distribution of the tropical Pacific surface zonal wind anomaly and its relation with two types of El Niño

El Nio events with an eastern Pacific pattern(EP) and central Pacific pattern(CP) were first separated using rotated empirical orthogonal functions(REOF).Lead/lag regression and rotated singular value decomposition(RSVD) analyses were then carried out to study the relation between the surface zonal wind(SZW) anomalies and sea surface temperature(SST) anomalies in the tropical Pacific.A possible physical process for the CP El Ni o was proposed.For the EP El Ni o,strong westerly anomalies that spread eastward continuously produce an anomalous ocean zonal convergence zone(ZCZ) centered on about 165°W.This SZW anomaly pattern favors poleward and eastward Sverdrup transport at the equator.For the CP El Nio,westerly anomalies and the ZCZ are mainly confined to the western Pacific,and easterly anomalies blow in the eastern Pacific.This SZW anomaly pattern restrains poleward and eastward Sverdrup transport at the equator;however,there is an eastward Sverdrup transport at about 5°N,which favors the warming of the north-eastern tropical Pacific.It is found that the slowness of eastward propagation of subsurface warm water(partly from the downwelling caused by Ekman convergence and the ZCZ) is due to the slowdown of the undercurrent in the central basin,and vertical advection in the central Pacific may be important in the formation and disappearance of the CP El Nio.     

The Western Boundary Current of the pacific and its role in the climate

On the basis of the CTD data gathered by the R/VScience I in each Oct. of 1986–1988 and the winter averaged temperature anomaly in southeast China, the interannual variability of the Western Boundary Current (WBC) is examined in terms of volume transport by inverse calculation and its role in the climate is studied by statistical method. The estimated transport is 50, 20, and 33×10⁶ m³/s for the Kuroshio and 24, 34, and 36×10⁶ m³/s for the Mindanao Current (MC) in October of 1986, 1987, and 1988, respectively. The WBC is the biggest channel in the ocean for transporting heat poleward and plays an extremely important role in establishing and maintaining the global heat balance. Results showed that meridional heat transport by the Kuroshio northeast of Luzon apparently dominates coldness or warmness in winter in southeast China. Two phenomena observed in the western Pacific but not in the western Atlantic are the warm pool and the equatorward flowing MC which, together with the North Equatorial Counter-current (NECC) may play an important role in preventing the warm water from extending to the north. So in order to understand the dynamics of the warm pool formation and evolution, the MC and NECC must be studied as well as the Equatorial Current. Contribution No. 1791 from the Institute of Oceanology, Academia Sinica.     

OUTFLOW OF SUSPENDED MATERIALS FROM THE CHANGJIANG RIVER MOUTH

ImODUrmNJ0GFS(JointGlobaldrinFLuxStudy:l99()-2OO()isaworldwidenawhprogramfocusingontheoasns'buharolewhentheC0,prmtageintheairincreasesandtheatmOspheretemPeraturebo.ThefluxofsuspendedrnateriaIs(SM)intheEastChinaScaisboortanttotheworld'srnatterchaltfon,astheChangiiangRiver,thelargestriverinAsia,dischargesbillionsoftOnsoftheidWhterintoit.ManystudAshavebocondtalonSMmovementncarChina'scoastalseas.Yang(l983)studiedthefine%rainedsededtSfromtheChangiiangandHuangheRiversWhileQin(l983)fo…     

Current observations in the southern Yellow Sea in summer

Current data from three moored Acoustic Doppler Profilers (ADPs) deployed in the southern Yellow Sea at sites A (1-24.17°E, 34.82°N), B (122.82°E, 35.65°N) in summer 2001 and site C (120.85°E, 34.99°N) in summer 2003 were analyzed in this paper. Features of the tidal and residual currents were studied with rotary spectral and cross-spectral methods. Main achievements were as follows: 1) Tides dominated the currents. At sites A and B, the semidiurnal tidal current was basically homogeneous in the whole depth, taking a clockwise rotation at site A, and near-rectilinear counterclockwise rotation at site B; while the diurnal tidal current was strong and clockwise near the surface, but decreased and turned counterclockwise with depth; at site C, semidiurnal tidal current dominated and diurnal current took the second, both of which were counterclockwise and vertically homogeneous. Inertial motion contributed to the clockwise component of diurnal fluctuations; 2) The 3-5d fluctuation of residual current w     

Spike in phytoplankton biomass in Greenland Sea during 2009 and the correlations among chlorophyll-a,aerosol optical depth and ice cover

The distributions and correlations of chlorophyll-a(Chl-a),aerosol optical depth(AOD)and ice cover in the southeast Arctic Ocean-Greenland Sea(10°W–10°E,70°–80°N)between 2003 and 2009 were studied using satellite data and statistical analyses.Regression analysis showed correlations between Chl-a and AOD,Chl-a and ice cover,and AOD and ice cover with different time lags.The time lag of Chl-a and AOD indicated their long-term equilibrium relationship.Peaks in AOD and Chl-a and generally occurred in May and July,respectively.Despite the time lag,the correlation between Chl-a and AOD in the study region was as high as 0.7.The peak gap between Chl-a and AOD shifted for about 6 weeks during 2003–2009.In the summer and autumn of 2009,Chl-a and AOD levels were much higher than during the other years,especially in the northern band of the study region(75°–80°N).The driving forces for this localized increase in phytoplankton biomass could be mainly attributed to the very high rate of ice melting in spring and early summer and the high wind speed in autumn,together with the increased deposition of aerosol throughout the year.The unusually high AOD in the spring of 2003 was mainly due to a massive fi re in Russia,which occurred in the fi rst half of the year.Over the 7 years of the study,the sea surface temperature generally decreased.This may have been due to the release of dimethylsulfi de into the air,excreted in large amounts from abundant phytoplankton biomass,and its subsequent reaction,form large amounts of aerosol,and resulting in regional cooling.     

Statistic characteristics of thermal structure in the southern Yellow Sea in summer

Based on the temperature data along 34°N, 35°N and 36°N sections in August from 1977 to 2003, the structure and formation of the Southern Yellow Sea Cold Water Mass (SYSCWM) and its responses to El Nino events are analyzed. Results show that: (1) There exist double cold cores under the main thermocline along the 35°N and 36°N sections. Also, double warm cores exist above the main thermocline along the 36°N section. (2) Thermocline dome by upwelling separates the upper warm water into two parts, the eastern and western warm waters. Additionally, the circulation structure caused by upwelling along the cold front and northeastward current along the coast in summer is the main reasons of double warm cores along the 36°N section. The intermediate cold water is formed in early spring and moves eastward slowly, which results in the formation of the western one of double cold cores. (3) Position of the thermocline dome and its intensity vary interannually, which is related to El Nino events. However, the     

An approach to prediction of the South China Sea summer monsoon onset

In the present paper, correlation between the South China Sea summer monsoon （SCSSM） onset and heat content in the upper layer of the warm pool in the western Pacific Ocean is examined using the Scripps Institution of Oceanography dataset for the period of 1955-1998 and an approach to prediction the SCSSM onset is proposed. Correlation showes that there exists interdecadal variability of the SCSSM onset demarcated by 1970 with the largest correlation coefficient in the area west of the center of the warm pool rather than near its centers, implying certain effect from other factors involved besides ENSO. As the correlation is poor for the period before 1970, the heat content anomaly of the warm pool after 1970 is used to indicate early or late onset of the SCSSM beforehand. An ideal representative area （1°×1°） for the warm pool heat content was determined with its center at 3°N/138°E. The nearest TAO （TAO-Tropical Atmosphere Ocean-array） mooring to the center is at 2°N/137°E, and chosen to calculate the heat content for prediction. It is suggested that the TAO mooring at 2°N/137°E could be used to predict the SCSSM onset with the heat content in the upper layer, if the correlation between the SCSSM onset and the heat content of the warm pool runs like that of after 1970. On the other hand, if the situation does like the one before 1970, the representative station is determined at 13°S/74°E with relatively poor correlation, meaning that the warm pool in the western Pacific Ocean plays more important role in the SCSSM onset than the Indian Ocean.     

Simulation of double cold cores of the 35°N section in the Yellow Sea with a wave-tide-circulation coupled model

Based on the MASNUM wave-tide-circulation coupled numerical model, the temperature structure along 35°N in the Yellow Sea was simulated and compared with the observations. One of the notable features of the temperature structure along 35°N section is the double cold cores phenomena during spring and summer. The double cold cores refer to the two cold water centers located near 122°E and 125°E from the depth of 30m to bottom. The formation, maintenance and disappearance of the double cold cores are discussed. At least two reasons make the temperature in the center (near 123°E) of the section higher than that near the west and east shores in winter. One reason is that the water there is deeper than the west and east sides so its heat content is higher. The other is invasion of the warm water brought by the Yellow Sea Warm Current (YSWC) during winter. This temperature pattern of the lower layer (from 30m to bottom) is maintained through spring and summer when the upper layer (0 to 30m) is heated and strong thermocline is formed. Large zonal span of the 35°N section (about 600 km) makes the cold cores have more opportunity to survive. The double cold cores phenomena disappears in early autumn when the west cold core vanishes first with the dropping of the thermocline position. Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No. 49736190).     

Interpretation of sea surface wind interannual vector EOFs over the China seas

Using interpolation and averaging methods, we analyzed the sea surface wind data obtained from December 1992 to November 2008 by the scatterometers ERS-1, ERS-2, and QuikSCAT in the area of 2°N–39 °N, 105°E–130°E, and we reported the monthly mean distributions of the sea surface wind field. A vector empirical orthogonal function (VEOF) method was employed to study the data and three temporal and spatial patterns were obtained. The first interannual VEOF accounts for 26% of the interannual variance and displays the interannual variability of the East Asian monsoon. The second interannual VEOF accounts for 21% of the variance and reflects the response of China sea winds to El Niño events. The temporal mode of VEOF-2 is in good agreement with the curve of the Niño 3.4 index with a four-month lag. The spatial mode of VEOF-2 indicates that four months after an El Niño event, the southwesterly anomalous winds over the northern South China Sea, the East China Sea, the Yellow Sea, and the Bohai Sea can weaken the prevailing winds in winter, and can strengthen the prevailing winds in summer. The third interannual VEOF accounts for 10% of the variance and also reflects the influence of the ENSO events to China Sea winds. The temporal mode of VEOF-3 is similar to the curve of the Southern Oscillation Index. The spatial mode of VEOF-3 shows that the northeasterly anomalous winds over the South China Sea and the southern part of the East China Sea can weaken the prevailing winds, and southwesterly anomalous winds over the northern part of the East China Sea, the Yellow Sea, and the Bohai Sea can strengthen the prevailing winds when El Niño occurs in winter. If El Niño happens in summer, the reverse is true.     

Future projection of East China Sea temperature by dynamic downscaling of the IPCC_AR4 CCSM3 model result

Future temperature distributions of the marginal Chinese seas are studied by dynamic downscaling of global CCSM3 IPCC_AR4 scenario runs.Different forcing fields from 2080-2099 Special Report on Emissions Scenarios(SRES) B1,A1,and A2 to 1980-1999 20C3M are averaged and superimposed on CORE2 and SODA2.2.4 data to force high-resolution regional future simulations using the Regional Ocean Modeling System(ROMS).Volume transport increments in downscaling simulation support the CCSM3 result that with a weakening subtropical gyre circulation,the Kuroshio Current in the East China Sea(ECS) is possibly strengthened under the global warming scheme.This mostly relates to local wind change,whereby the summer monsoon is strengthened and winter monsoon weakened.Future temperature fluxes and their seasonal variations are larger than in the CCSM3 result.Downscaling 100 years’ temperature increments are comparable to the CCSM3,with a minimum in B1 scenario of 1.2-2.0°C and a maximum in A2 scenario of 2.5-4.5°C.More detailed temperature distributions are shown in the downscaling simulation.Larger increments are in the Bohai Sea and middle Yellow Sea,and smaller increments near the southeast coast of China,west coast of Korea,and southern ECS.There is a reduction of advective heat north of Taiwan Island and west of Tsushima in summer,and along the southern part of the Yellow Sea warm current in winter.There is enhancement of advective heat in the northern Yellow Sea in winter,related to the delicate temperature increment distribution.At 50 meter depth,the Yellow Sea cold water mass is destroyed.Our simulations suggest that in the formation season of the cold water mass,regional temperature is higher in the future and the water remains at the bottom until next summer.In summer,the mixed layer is deeper,making it much easier for the strengthened surface heat flux to penetrate to the bottom of this water.     

A modified method to estimate eddy diffusivity in the North Pacific using altimeter eddy statistics

The method proposed by Stammer (1998) is modified using eddy statistics from altimeter observation to obtain more realistic eddy diffusivity (K) for the North Pacific. Compared with original estimates, the modified K has remarkably reduced values in the Kuroshio Extension (KE) and North Equatorial Counter Current (NECC) regions, but slightly enhanced values in the Subtropical Counter Current (STCC) region. In strong eastward flow areas like the KE and NECC, owing to a large difference between mean flow velocity and propagation velocity of mesoscale eddies, tracers inside the mesoscale eddies are transported outside rapidly by advection, and mixing length L is hence strongly suppressed. The low eddy probability (P) is also responsible for the reduced K in the NECC area. In the STCC region, however, L is mildly suppressed and P is very high, so K there is enhanced. The zonally-averaged K has two peaks with comparable magnitudes, in the latitude bands of the STCC and KE. In the core of KE, because of the reduced values of P and L, the zonally-averaged K is a minimum. Zonally-integrated eddy heat transport in the KE band, calculated based on the modified K, is much closer to the results of previous independent research, indicating the robustness of our modified K. The map of modified K provides useful informationfor modeling studies in the North Pacific.     

Observations of Kuroshio intrusion into the South China Sea

To discuss the intrusion of the Kuroshio into the SCS, we examined the mixing between the North Pacific and South China Sea (SCS) waters based on in-situ CTD data collected in August and September 2008 and the moored ADCP data taken from mid September 2008 to early July 2009. The CTD survey included four meridional sections from 119°E to 122°E around the Luzon Strait, during which pressure, temperature, and salinity were measured. The CTD data show that the isopycnal surface tilted from the SCS to the North Pacific; and it was steeper in the lower layers than in the upper ones. Meanwhile, we found strong vertical mixing taken place in the areas near 121°E. The Kuroshio in high temperature and salinity intruded westward through Luzon Strait. The frequency of buoyancy was one order of magnitude greater than that of the common ones in the ocean, suggesting stronger stratification in the northeastern SCS. On the other hand, the long-term ADCP data show that before late October 2008, the direction of water flow in the SCS was eastward, and from November 2008 to late February 2009, it turned northwestward in the layers shallower than 150 m, while remained unchanged in deep layers from 200 to 450 m. From March to June 2009, the direction shifted with increasing depth from northward to southward, akin to the Ekman spiral. EOF analysis of the current time series revealed dominant empirical modes: the first mode corresponded to the mean current and showed that the Kuroshio intrusion occurred in the upper layers only from late December to early March. The temporal coefficient of the first and the second mode indicated clearly a dominant signal in a quasi-seasonal cycle.