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Carl Wunsch, Dunxin Hu, and Barbara Grant

Abstract

An inverse method has been applied to the Scorpio sections spanning the subtropical gyre of the South Pacific. In addition to constraints upon geostrophic fluxes of mass and salt, constraints were written for silica, oxygen and “PO”, and the Ekman fluxes and divergence were accounted for explicitly. Nutrient constraints contribute significantly to the system information with oxygen containing information independent of “PO”. Attention is focussed on zonally integrated quantities that are insensitive to differences in modeling assumptions. Overall interior flows are dominated by an equatorward flux of Bottom Water (12 × 109 kg s−1), a poleward return flow at mid-depths, equatorward flow of Circumpolar Intermediate Water, and a convergence of surface waters. The zonal average vertical velocity appears to be everywhere downward with the Bottom Water entraining fluid from above. The net meridional fluxes of silica, phosphate, oxygen and

water are indistinguishable from zero. Production/consumption of the nutrients and oxygen as functions of depth are biologically and chemically reasonable. As a consequence of the “multi-cellular” nature of the flows the meridional beat flux across both sections is very small, typically less than 10% of the North Atlantic values. The best estimate shows weak poleward heat fluxes at 28 and 43°S, but both are formally indistinguishable from zero. Bulk formula results are in conflict with these estimates and their error bars. The models demonstrate, for regions the size of the box defined by the two sections, that three-dimensional advection and biological/chemical processes are adequate to account for the observed property distributions, with interior mixingg at or below the noise level

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Linlin Zhang, Yuchao Hui, Tangdong Qu, and Dunxin Hu

Abstract

Seasonal modulation of subthermocline eddy kinetic energy (EKE) east of the Philippines and its associated dynamics are studied, using mooring measurements and outputs from an eddy-resolving ocean general circulation model for the period from 2000 to 2017. Significantly high EKE appears below the thermocline in the latitude band between 5° and 14°N east of the Philippines. Separated by 10°N, the EKE in the northern and southern parts of the region shows nearly opposite seasonal cycles, with its magnitude reaching a maximum in early spring and minimum in summer in the northern part and reaching a maximum in summer and minimum in winter in the southern part of the region. Further investigation indicates that both baroclinic and barotropic instabilities are essential in generating the subthermocline eddies, but the seasonal variation of subthermocline EKE is mainly caused by the seasonal modulation of barotropic instability. The seasonal modulation of barotropic instability in the northern and southern part of the region is associated with the seasonal evolution of North Equatorial Undercurrent and Halmahera Eddy, respectively.

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Shijian Hu, Dunxin Hu, Cong Guan, Fan Wang, Linlin Zhang, Fujun Wang, and Qingye Wang

Abstract

The interannual variability of the boundary currents east of the Mindanao Island, including the Mindanao Current/Undercurrent (MC/MUC), is investigated using moored acoustic Doppler current profiler (ADCP) measurements combined with a series of numerical experiments. The ADCP mooring system was deployed east of the Mindanao Island at 7°59′N, 127°3′E during December 2010–August 2014. Depth-dependent interannual variability is detected in the two western boundary currents: strong and lower-frequency variability dominates the upper-layer MC, while weaker and higher-frequency fluctuation controls the subsurface MUC. Throughout the duration of mooring measurements, the weakest MC was observed in June 2012, in contrast to the maximum peaks in December 2010 and June 2014, while in the deeper layer the MUC shows speed peaks circa December 2010, January 2011, April 2013, and July 2014 and valleys circa June 2011, August 2012, and November 2013. Diagnostic analysis and numerical sensitivity experiments using a 2.5-layer reduced-gravity model indicate that wind forcing in the western Pacific Ocean is a driving agent in conditioning the interannual variability of MC and MUC. Results suggest that westward-propagating Rossby waves that generate in the western Pacific Ocean (roughly 150°–180°E) are of much significance in the interannual variability of the two boundary currents. Fluctuation of Ekman pumping due to local wind stress curl anomaly in the far western Pacific Ocean (roughly 120°–150°E) also plays a role in the interannual variability of the MC. The relationship between the MC/MUC and El Niño is discussed.

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Dunxin Hu, Shijian Hu, Lixin Wu, Lei Li, Linlin Zhang, Xinyuan Diao, Zhaohui Chen, Yuanlong Li, Fan Wang, and Dongliang Yuan

Abstract

The Luzon Undercurrent (LUC) was discovered about 20 years ago by geostrophic calculation from conductivity–temperature–depth (CTD) data. But it was not directly measured until 2010. From November 2010 to July 2011, the LUC was first directly measured by acoustic Doppler current profiler (ADCP) from a subsurface mooring at 18.0°N, 122.7°E to the east of Luzon Island. A number of new features of the LUC were identified from the measurements of the current. Its depth covers a range from 400 m to deeper than 700 m. The observed maximum velocity of the LUC, centered at about 650 m, could exceed 27.5 cm s−1, four times stronger than the one derived from previous geostrophic calculation with hydrographic data. According to the time series available, the seasonality of the LUC strength is in winter > summer > spring. Significant intraseasonal variability (ISV; 70–80 days) of the LUC is exposed. Evidence exists to suggest that a large portion of the intraseasonal variability in the LUC is related to the westward propagation of mesoscale eddies from the east of the mooring site.

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Shijian Hu, Ying Zhang, Ming Feng, Yan Du, Janet Sprintall, Fan Wang, Dunxin Hu, Qiang Xie, and Fei Chai

Abstract

Variability of oceanic salinity, an indicator of the global hydrological cycle, plays an important role in the basin-scale ocean circulation. In this study, interannual to decadal variability of salinity in the upper layer of the Indian Ocean is investigated using Argo observations since 2004 and data assimilating model outputs (1992–2015). The southeastern Indian Ocean shows the strongest interannual to decadal variability of upper-ocean salinity in the Indian Ocean. Westward propagation of salinity anomalies along isopycnal surfaces is detected in the southern Indian Ocean and attributed to zonal salinity advection anomalies associated with the Indonesian Throughflow and the South Equatorial Current. Composite and salinity budget analyses show that horizontal advection is a major contributor to the interannual to decadal salinity variability of the southern Indian Ocean, and the local air–sea freshwater flux plays a secondary role. The Pacific decadal oscillation (PDO) and El Niño–Southern Oscillation (ENSO) modulate the salinity variability in the southeastern Indian Ocean, with low salinity anomalies occurring during the negative phases of the PDO and ENSO and high salinity anomalies during their positive phases. The Indonesian Throughflow plays an essential role in transmitting the PDO- and ENSO-related salinity signals into the Indian Ocean. A statistical model is proposed based on the PDO index, which successfully predicts the southeastern Indian Ocean salinity variability with a lead time of 10 months.

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