South China Sea Deep Basin Research Articles

Modulation of Internal Solitary Waves by One Mesoscale Eddy Pair West of the Luzon Strait

Abstract The characteristics of modulated internal solitary waves (ISWs) under the influence of one mesoscale eddy pair in the Luzon Strait, involving one anticyclonic eddy (AE) and one cyclonic eddy (CE) induced by the Kuroshio intrusion, were investigated using a nested high-resolution numerical model in the northeastern South China Sea (SCS). The presence of mesoscale eddies greatly impacts the nonlinear evolution of type-a and type-b ISWs. The eddy pair contributes to distinct wave properties and energy evolutions. Compared to type-b waves, type-a waves display more pronounced modulatory characteristics with a larger spatial scale. CE currents and horizontal inhomogeneous stratification are crucial in modulating the wave behaviors, which induce extremely large-amplitude depression ISWs. The AE thereafter yields retardation effects on the wave energy evolution. The average depth-integrated available potential and kinetic energy showed relative growth rates of −66.12% and −46.07%, respectively, for type-a waves, and −24.26% and −20.15%, respectively, for type-b waves along the propagation path up to the AE core. The deformed and distorted ISW crest lines propagating further northward exhibit a more dramatic shoaling evolution. The maximum total energies of type-a and type-b waves at the north station are approximately 13.5 and 3.5 times, respectively, greater than those at the south station on the continental shelf of the Dongsha Atoll. This work provides essential insights into modulated ISW dynamics under the mesoscale eddy pair within the northeastern SCS deep basin.

The Rossby Normal Mode as a Physical Linkage in a Machine Learning Forecast Model for the SST and SSH of South China Sea Deep Basin

AbstractMachine learning (ML) has been widely applied in Earth sciences, but its black‐box nature still remains. Existing techniques for interpretable ML developed by deep learning researchers are not satisfactory in understanding physical problems. In this study, our objective is to establish the physical linkage between features extracted from ML model and results derived from physical equations that describe the problem. This approach aims to provide a more comprehensible way of understanding an ML model. We select a less complex ML forecasting model, consisting of traditional statistical algorithms, which effectively forecast sea surface temperature anomaly (SSTA) and sea level anomaly (SLA) of the South China Sea (SCS) in 30 days ahead. Here, we focus on the SLA prediction and detect the physical mechanism of model capability to predict SCS SLA. Previous study has identified Rossby normal modes as the physical mechanism for long‐lived eddies in SCS in previous study. We here demonstrate the relationship between the ML model and Rossby normal modes in SCS in terms of temporal variations and spatial distributions of water mass. The model's skill in predicting Rossby normal modes partially explains its skill in predicting SLA, thereby providing a more transparent and interpretable basis for forecasts in SCS.

Reconstruction of Kuroshio intrusion into the South China Sea over the last 40 kyr

The Kuroshio current transports large amounts of heat and water vapor into the South China Sea (SCS) through the Luzon Strait and plays a key role in modulating the surrounding hydroclimate. However, there is no detailed high-resolution history of the Kuroshio intrusion into the SCS preceding the Holocene. Here, clay mineral assemblages and Sr–Nd isotopic values (87Sr/86Sr and εNd) of core CS11 represent the relative abundance of Taiwan and Luzon fluvial sediment contributions in the SCS deep basin, which can be further used as proxies of relative variation of the Kuroshio intrusion into the SCS over the past 40 kyr. Combined with linear sedimentation rate (LSR) and results of previous studies, we demonstrate that variations of Kuroshio currents between the SCS and the open Pacific display opposing trends within each marine isotope stage, similar to the waxing and waning of its modern configuration. The El Niño–Southern Oscillation (ENSO) has acted as the key factor driving variations of the Kuroshio currents over the past 40 kyr. During late marine isotope stage (MIS) 3, the Kuroshio intrusion into the SCS gradually decreased due to decline in intensity of El Niño-like conditions, while an El Niño-like state favored the Kuroshio intrusion into the SCS during MIS 2, in particular during the Last Glacial Maximum (LGM) when Kuroshio transport in the open Pacific weakened. The Kuroshio intrusion into the SCS was largely sluggish in the early Holocene, when a La Niña-like condition prevailed, and then the Kuroshio intrusion was enhanced during the middle to late Holocene under the influence of increased El Niño frequency.

Terrigenous and volcanogenic contribution to the deep basin of the South China Sea: Evidence from trace elements and Sr-Nd isotopes

The deep basin of the South China Sea (SCS) receives terrigenous materials from the surrounding continents and islands, making it an ideal region for studies on source-to-sink transport. The analysis of grain size, major and trace elements, and Sr-Nd isotopes of 130 surface sediment samples throughout the SCS deep basin has been performed for identifying the contribution of terrigenous and volcanic materials. Based on the analysis results, the SCS deep basin can be divided into three geographic areas with different provenances: the northern area of the East Subbasin (NES), the southern region of the East Subbasin (SES), and the Southwest Subbasin (SWS). The predominant provenances of sediments in the NES and SWS are Taiwan rivers and Mekong River, respectively. Sediments from the SES exhibit higher mean grain size, less radiogenic 87Sr/86Sr, and more radiogenic 143Nd/144Nd than those from the NES and SWS. Pinatubo ash input from the Luzon Island may be a predominant source in the SES. The contribution of Pinatubo ash decreased westward from 80% to 20% in the SES. The supply of Pinatubo ash could be considered insignificant to the NES and SWS. Sediments from Taiwan rivers or Mekong River were the subordinate source in the SES. The deep-water current and mesoscale eddies may play a role in transporting terrigenous materials into the SCS deep basin as well as the turbidity current.

Seasonal variation of deep vertical velocity in the South China Sea

On the basis of the Simple Ocean Data Assimilation (SODA) and the Ocean General Circulation Model for the Earth Simulator (OFES), it is surprised that a distinct seasonal variation is found in the deep vertical velocity fields of the South China Sea (SCS). In summer, an upwelling band extending from the northeast to the southwest across the SCS deep basin is mainly located in the interior region of the basin, surrounded by the downwelling areas situated in the marginal region of the basin. In winter, the vertical motion is almost completely reversed, which is also enhanced in its strength and transport. A more extended downwelling band in winter replaces the upwelling band in summer, and the upwelling occurs in the marginal region. The upwelling or downwelling in the interior region of the basin is compensated by the reverse flow near the marginal basin in summer or winter, maintaining the balance of the whole deep basin without water exchange. The seasonal structures of potential density and potential temperature are consistent with the seasonal variation of the SCS deep vertical velocity. In the interior region, their isolines uplift in summer and drop in winter. The seasonal variation of the SCS deep vertical velocity in the interior region also has a good correspondence with that of basin-scale horizontal circulation, the seasonal pattern of which is cyclonic in summer and anticyclonic in winter. Just as the vertical transports always maintain a balanced, the vorticity averaged over the entire basin always remains positive, and the values in winter are one order of magnitude smaller than that in summer. This correspondence can be explained by the Sverdrup relation. The mechanism of the seasonal variation of the SCS deep vertical velocity may be related to the seasonal deepwater overflow through the Luzon Strait.

Variation of Internal Solitary Wave Propagation Induced by the Typical Oceanic Circulation Patterns in the Northern South China Sea Deep Basin

AbstractLarge‐scale circulations are quite typical processes in the northern South China Sea (SCS) deep basin, yet their impacts on the variation of internal solitary waves (ISWs) remains poorly understood. We, here, focus on impacts of the upstream SCS western boundary current (SCSwbc) and three typical circulation patterns associated with different Kuroshio intruding paths on ISW propagation in this region. We show that the ISW modulated by the upstream SCSwbc gets a speedup, which is comparable to that induced by Coriolis effect, while the ISW amplitude shows an obvious reduction. Statistically, there is approximately one‐third time of 23 years (1993–2015) when circulation‐induced changes of wave speed exceed Coriolis‐induced changes. Specifically, the looping circulation pattern has the highest impact on significant speedup and amplitude reduction among the three patterns, while the leaping pattern has the lowest impact. These differences in wave characteristics caused by different circulation patterns result from the wave scattering, focusing and Doppler effects.

Comparison and Calibration of Elemental Measurements in Sediments Using X-Ray Fluorescence Core Scanning with ICP Methods: A Case Study of the South China Sea Deep Basin

Comparison and Calibration of Elemental Measurements in Sediments Using X-Ray Fluorescence Core Scanning with ICP Methods: A Case Study of the South China Sea Deep Basin

Statistical analysis of mesoscale eddy propagation velocity in the South China Sea deep basin

Using mesoscale eddy trajectory product derived from satellite altimetry data from 1993 to 2017, this study analyzes the statistical characteristics of spatiotemporal distribution of mesoscale eddy propagation velocities (C) in the South China Sea (SCS) deep basin with depths >1 000 m. Climatologically, the zonal propagation velocities (cx) are westwards in the whole basin, and the meridional velocities (cy) are southwards in the northwestern basin, and northwards in the southeastern basin. The variation of cy with longitude is consistent with that of the background meridional currents with correlation coefficient R2 of 0.96, while the variation of cx is related both to the background zonal currents and β effect. The propagation velocities characterize significant seasonality with the minimum magnitude occurring in summer and the maximum in winter for cx and C. Interannually, larger values of cx and cy mostly occurred in La Nina years in the negative phase of the Pacific Decadal Oscillation (PDO). Mesoscale eddies move fast at the beginning and end of their life span, i.e., at their growth and dissipation periods, and slowly during their stable “midlife” period. This trend is negatively correlated with the rotating tangential velocity with R2 of −0.93. Eddies with extreme propagation velocities are defined, which are slower (faster) than 1.5 cm/s (15.4 cm/s) and take 1.5% (1.9%) of the total eddies. The extremely slow-moving (fast-moving) eddies tend to appear in the middle (on the edge) of the basin, and mostly occur in summer (winter). The mechanism analysis reveals that the spatiotemporal distributions of the propagation velocities of mesoscale eddies in the SCS are modulated by the basin-scale background circulation.

The Rossby normal modes in the South China Sea deep basin evidenced by satellite altimetry

ABSTRACTThis study investigates the mechanism of mesoscale eddies observed in the South China Sea (SCS) deep basin, which are characterized by high-frequency occurrence, long life, and regular distribution patterns. Geophysical fluid dynamics predicts that for an enclosed ocean basin, there exist the Rossby normal modes if considering the Earth rotation effect. We use a rectangular ocean basin with a uniform depth to simulate the SCS deep basin and obtain 2D (x − y) sea level distribution patterns of the Rossby normal modes, consisting of alternating cyclonic and anticyclonic eddies, as well as their dynamic parameters. The periods of the modes are evidenced by the Fourier period power density spectra of satellite altimeter sea level anomaly (SLA) data from 1993 to 2015. The 2D normal mode patterns are evidenced by SLA images. The phase speeds are evidenced by that derived from the westward movement of the wave/eddy patterns on SLA time-series images.

Impacts of a Mesoscale Eddy Pair on Internal Solitary Waves in the Northern South China Sea revealed by Mooring Array Observations

AbstractBoth internal solitary waves (ISWs) and mesoscale eddies are ubiquitous in the northern South China Sea (SCS). In this study, the authors examine the impacts of mesoscale eddies on the ISWs transiting the northern SCS deep basin that evolve from the steepening internal tide generated in the Luzon Strait, using in situ data collected from a specifically designed mooring array. From November 2013 to January 2014, an energetic mesoscale eddy pair consisting of one anticyclonic eddy (AE) and one cyclonic eddy (CE) propagated across the mooring array. Observations revealed that the amplitude, propagation direction, and speed of the transbasin ISWs were significantly modulated by the eddy pair. When the moorings were covered by the southern portion of the AE, the ISW amplitudes decreased by as much as 67% because of the thermocline deepening along the wave direction and the energy divergence along the wave front. When the moorings were covered by the northern portions of both eddies, the amplitude of ISWs also decreased but to a relatively smaller degree. ISWs propagated the fastest inside the southern portion of the AE, where both the thermocline deepening and eddy currents enhanced the propagation speed of ISWs. Under the influence of the AE (CE) core, ISWs propagated more northward (southward) than usual. The observational results reported here highlight the importance of resolving mesoscale eddies in circulation–internal wave coupled models to accurately predict kinematic characteristics of ISWs.

A fresh look at the deepwater overflow in the Luzon Strait

On the basis of the latest version of a U.S. Navy generalized digital environment model (GDEM-V3.0) and World Ocean Atlas (WOA13), the hydraulic theory is revisited and applied to the Luzon Strait, providing a fresh look at the deepwater overflow there. The result reveals that: (1) the persistent density difference between two sides of the Luzon Strait sustains an all year round deepwater overflow from the western Pacific to the South China Sea (SCS); (2) the seasonal variability of the deepwater overflow is influenced not only by changes in the density difference between two sides of the Luzon Strait, but also by changes in its upstream layer thickness; (3) the deepwater overflow in the Luzon Strait shows a weak semiannual variability; (4) the seasonal mean circulation pattern in the SCS deep basin does not synchronously respond to the seasonality of the deepwater overflow in the Luzon Strait. Moreover, the deepwater overflow reaches its seasonal maximum in December (based on GDEM-V3.0) or in fall (October–December, based on the WOA13), accompanied by the lowest temperature of the year on the Pacific side of the Luzon Strait. The seasonal variability of the deepwater overflow is consistent with the existing longest (3.5 a) continuous observation along the major deepwater passage of the Luzon Strait.

Distortion and broadening of internal solitary wavefront in the northeastern South China Sea deep basin

AbstractInternal solitary waves (ISWs) with peculiar fronts are frequently observed in the world ocean by satellite images, though with quite few explanations. In this study a distorted and broadening ISW front across the northeastern South China Sea deep basin is presented by using synthetic aperture radar (SAR) image. To illustrate this peculiar front, a nonlinear refraction model is developed to simulate and evaluate the effects of realistic bottom topography, current, and stratification on its transformation. Simulated results in realistic oceanic environments show good agreements with this SAR‐observed front. Based on separate and comparative results in different background environments, we demonstrate that the distortion is actually caused by the strong mesoscale currents at periphery of an anticyclonic eddy. Moreover, the broadening is due to the difference in change of wave half width at different rays, which is associated with the different transformation of ISWs across variable bottom topography in the deep basin.

Fluxes of clay minerals in the South China Sea

In order to assess dominant settling processes that change the composition of the detrital clay fraction during transport from neighboring estuaries to a deep sea basin, we studied relative clay mineral abundances and absolute clay mineral fluxes of clay-sized sinking particulate matter collected by eight sediment trap systems deployed from shallow to deep water depth in the South China Sea. This is the first basin-wide study on recent sedimentation processes in the western Pacific marginal seas.Annual averages of relative clay mineral abundances at the shallow traps are temporally more variable and regionally more diverse, resembling those of surrounding drainage basins. In contrast, higher fluxes of material reach the deeper traps. Their characteristics trend temporally and spatially towards uniformity and are enriched with smectite in the entire deep basin.Sinking particulate matter that reaches the shallow traps spends less time in pelagic transport and is affected by monsoonal current reversals. The enrichment in smectite in the deeper traps is a result of longer duration in transport at low velocities, which may increase the effect of differential settling during transport. The trend is caused by lateral advection driven by the cyclonic deep circulation, and this is considered as the main transport process in the northern and central deep basin. The high fluxes in the south-western deep basin could be the result of laterally advected re-suspended sediments from the neighboring shelves.The effects on the composition of the detrital clay fraction caused by oceanographic control, which indirectly include those by differential settling, mask the climatic signal from surrounding drainage basins in the deep basin sediments. This strongly affects the interpretation of the clay mineralogical record in sediments deposited under recent conditions in the South China Sea deep basin.

Depleted deep South China Sea δ13C paleoceanographic events in response to tectonic evolution in Taiwan–Luzon Strait since Middle Miocene

The most distinctive feature of the deep South China Sea (SCS) paleoceanography is the occurrence of long-term depleted deep-sea benthic foraminiferal δ13C values. They are lower than the global and the Pacific composite records in the last 16Ma, especially at 13.2, 10.5, 6.5, 3.0 and 1.2–0.4Ma. This distinct deep SCS paleoceanograhic history coincides with the subduction–collision history in the Taiwan region where waters of the West Pacific (WP) and the SCS exchange. The depleted deep-sea benthic foraminiferal δ13C events indicate that the SCS deep basin became progressively a stagnant environment in the last 16Ma due to either closure of the connection with the WP bottom water or temporary reduction of the WP deep water flowing into the deep SCS. Both the Taiwan accretionary prism and the Luzon arc became the main tectono-morphological barriers for the WP bottom water flowing into the SCS deep basin when eastward subduction of the SCS oceanic lithosphere beneath the Philippine Sea Plate started from the Middle Miocene (18–16Ma). This began a long-term trend of depleted SCS deep-sea benthic δ13C values in the last 16Ma. The oblique arc–continent collision since ~6.5Ma uplifted the Taiwan accretionary prism rapidly above sea level and further isolated the SCS from the open Pacific. The collision simultaneously causes backthrusting deformations in the North Luzon Trough forearc basin and sequentially closes interarc water gates between volcanic islands from north to south. The Loho and the Taitung interarc water gates in the advanced collision zone were closed at ~3.0Ma and ~1.2Ma, coinciding with the very low SCS deep-sea benthic δ13C events at 3.0 and 1.2–0.4Ma, respectively. The Taitung Canyon between the Lutao and Lanyu volcanic islands in the incipient collision zone is semi-closed presently. These closure events also lead to the result that the WP deep water intrudes westward into the SCS principally through the Bashi Channel between the Lanyu and Batan volcanic islands in the subduction zone.

Asymmetry of internal waves and its effects on the ecological environment observed in the northern South China Sea

Ecological parameters, including current velocity, water temperature, and nutrient and chlorophyll a concentrations, accompanied by two internal solitary waves, were observed in the South China Sea deep basin from 2 October to 4 October, 2012. Both solitons belonged to the mode-1 category, which were characterized by a westward velocity exceeding 1ms−1 above 500m and an eastward velocity up to 0.1ms−1 below 500m. The observed structure of the vertical current was asymmetrical. For the first soliton, the downwelling region had a temporal scale of 32min and an average vertical velocity of 3cms−1, whereas the upwelling region had a temporal scale of 24min and an average vertical velocity of 2cms−1. For the second soliton, the temporal scale and average velocity were 36min and 2cms−1 in the downwelling, compared with 26min and 2cms−1 in the upwelling. On the basis of the convection equation, the downward net vertical displacements of water particles ranged from nearly zero to tens of meters with the maximum value in excess of 50m. Such asymmetry caused changes in temperature as high as 2.3°C, nutrient concentrations up to 12.04μmolL−1, and chlorophyll a concentration up to 0.12μgL−1 before and after the passage of the solitons.

Standing wave modes observed in the South China Sea deep basin

AbstractThis study deals with standing wave or seiche events using cruise observations, satellite altimeter data, and theoretical analysis. Cruise missions in summer 2007 and 2009 detected internal oscillation signals in the South China Sea (SCS) deep basin. The signals have average wavelengths of 320 and 390 km and the maximum amplitudes of 50–100 m at layers 500–700 m and 1500–1700 m. Satellite altimeter sea level anomaly (SLA) images and the second intrinsic mode function (IMF2) images derived from the empirical mode decomposition (EMD) analysis show that the observed internal oscillations are a portion of 2‐D seiche modes, which lasted for at least 2 weeks. We recognize that the observed internal oscillation signals represent seiche modes H5,3 and H5,1 derived from a rectangular model ocean basin with a uniform depth, a west‐east length of 1000 km and a north‐south width of 800 km. Statistical analysis of standing wave modes H4,0, H5,1, and H5,3 with the average wavelength of 500, 390, and 320 km indicates that from 1993 to 2012 (1045 weeks), total 94 events with total temporal coverage of 218 weeks are affirmed. The total occurrence frequency is 20.9%. Histograms of annual distributions of seiche events and timespans show an interannual variability of about 9 years, with peak years 1993, 1994, 1998, 2001, and 2011. While monthly distributions show an intraseasonal variability double‐peaked in May and October, transit periods of East Asia monsoon in the SCS.

A summary of special section: regional environmental oceanography in the South China Sea and its adjacent areas (REO-SCS)

The South China Sea (SCS) is an important marginal sea inthe western Pacific Ocean, surrounded by 9 countries:Brunei, Cambodia, China, Indonesia, Malaysia, the Phil-ippines, Singapore, Thailand, and Vietnam. Since ancienttimes, the SCS has served as a convenient navigationwaterway for the Southeast Asian nations to communicatewith each other and with nations of the outside world. Upto now, the SCS has been one of the busiest waterways inthe world because of the weight and high growth rate of theregion in the world economy and trade (Zheng et al. 2006).It is an inexorable trend, therefore, for the oceanographersfrom surrounding countries to pay more and more attentionto the research of regional environmental oceanography inthe SCS and its adjacent areas.The special section ‘‘Regional Environmental Ocean-ography in the South China Sea and Its Adjacent Areas(REO-SCS),’’ which aims to deliver the latest researchresults to the readers, appears in two issues, 4 and 6,Volume 67, Journal of Oceanography. The special sectionincludes 11 original papers and an introduction paper (Huet al. 2011). The key points are summarized as follows.For the mesoscale dynamics studies, Zheng et al. (2011)analyze the interaction of nonlinear Rossby eddies with theKuroshio at the Luzon Strait using the satellite altimetersea level data from 1993 to 2008, indicating that the eddieswith a radius larger than 150 km are strong enough tosignificantly alter the Kuroshio path and are able to modifythe local circulation pattern. Chen et al. (2011a) extend theKorteweg–de Vries equation to the Benjamin–Ono equa-tion and determine the internal solitary wave amplitude inthe SCS deep basin west of the Luzon Strait from satelliteand in situ measurements.There are two papers focused on studying near-inertialoscillation (NIO) in the SCS. Sun et al. (2011a) observe anNIO burst event in the western SCS by an upward-lookingmooring acoustic Doppler current profiler (ADCP) insummer 2004 and reveal that typhoon ‘‘Chanchu’’ is amajor mechanism to trigger the NIO burst event. Sun et al.(2011b) detect strong NIO from observations of threeADCP moorings in the continental shelf of the northernSCS in July 2008 and find that the NIOs are characterizedby red-shifted frequencies.There are four papers related to the current, upwelling,or water mass in the Taiwan Strait and in the Beibu Gulf.Hong et al. (2011) analyze the observational data andnumerical model results and confirm the two-prongednorthward flow in the southern Taiwan Strait in summer.Qiu et al. (2011) analyze the near surface circulation in theTaiwan Strait using 110 satellite-tracked surface driftersfrom 1989 to 2007, showing that almost all winter driftersthat enter the Taiwan Strait eventually move southward.Using a nested circulation model based on the PrincetonOcean Model, Jiang et al. (2011) investigate the charac-teristics and mechanisms of the Southwest upwelling and

Derivation of internal solitary wave amplitude in the South China Sea deep basin from satellite images

In the present study, theories based on the Korteweg–de Vries equation are extended to the Benjamin–Ono equation to allow the determination of internal solitary wave (ISW) amplitude from satellite images. The free surface flow induced by an ISW is derived for deep water. As a coherent structure, the amplitude of the ISW has a unique relation to the convergence/divergence of surface flow, such that the flow convergence/divergence will increase/decrease the backscattering cross section and generate bright/dark bands in satellite images. The distance between bright and dark bands can be related to the amplitude of ISW. To validate the theory, a multi-ship measurement made on 9–11 May 2005 during the spring tide period is used. A systematic approach to determine the thickness and density of the upper and lower layers is also included so that the free surface flow can be determined with a relatively high accuracy.

Diversity and distribution of diazotrophic communities in the South China Sea deep basin with mesoscale cyclonic eddy perturbations

The South China Sea (SCS) is an oligotrophic subtropical marginal ocean with a deep basin and a permanently stratified central gyre. Upwelling and nitrogen fixation provide new nitrogen for primary production in the SCS. This study was aimed at an investigation of phylogenetic diversity and quantification of the diazotroph community in the SCS deep basin, which is characterized by frequent mesoscale eddies. The diazotroph community had a relatively low diversity but a distinct spatial heterogeneity of diversity in the SCS deep basin. The potential for nitrogen fixation consistently occurred during cyclonic eddies, although upwelling of nutrient-replete deep water might have alleviated nitrogen limitation in the SCS. However, diazotrophic proteobacteria were dominant, but neither Trichodesmium nor heterocystous cyanobacterial diatom symbionts. Quantitative PCR analysis using probe-primer sets developed in this study revealed that the nifH gene of the two dominant alpha- and gammaproteobacterial groups was at the highest abundance (up to 10(4) to 10(5) copiesL(-1) ). Trichodesmium thiebautii was detected with an average density of 10(2) trichomesL(-1) in the euphotic waters, while Richelia intracellularis was observed sporadically under the microscope. The unicellular cyanobacterial groups A and B were not detected in our libraries. Our results suggested that diazotrophic proteobacteria were significant components potentially contributing to nitrogen fixation in this oligotrophic marginal ocean ecosystem.

Pathways of mesoscale variability in the South China Sea

The propagation of oceanic mesoscale signals in the South China Sea (SCS) is mapped from satellite altimetric observations and an eddy-resolving global ocean model by using the maximum cross-correlation (MCC) method. Significant mesoscale signals propagate along two major bands of high variability. The northern band is located west of the Luzon Strait, characterized by southwestward eddy propagation. Although eddies are the most active in winter, their southwestward migrations, steered by bathymetry, occur throughout the year. Advection by the mean flow plays a secondary role in modulating the propagating speed. The southern eddy band lies in the southwest part of the SCS deep basin and is oriented in an approximately meridional direction. Mesoscale variability propagates southward along the band in autumn. This southward eddy pathway could not be explained by mean flow advection and is likely related to eddy detachments from the western boundary current due to nonlinear effects. Our mapping of eddy propagation velocities provides important information for further understanding eddy dynamics in the SCS.