Despite its potential importance in the global climate system, mixing properties of the North Pacific low‐latitude western boundary current system (LLWBC) remained unsampled until very recently. We report here on the first measurements of turbulence microstructure associated with these currents, made in the western boundary region of the tropical North Pacific east of the Philippines. The results suggest that thermocline mixing in the North Pacific LLWBC is generally weak with the diapycnal diffusivity κρ∼O(10−6) m2 s−1. This is consistent with predictions from internal wave‐wave interaction theory that mixing due to internal wave breaking is significantly reduced at low latitudes. Enhanced mixing is found to be associated with a permanent cyclonic eddy, the Mindanao Eddy, but mainly at its south and north flanks. There, κρ is elevated by an order of magnitude due to eddy‐induced geostrophic shear. Mixing in the eddy core is at the background level with no indication of enhancement.

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1The surface circulation of the tropical Pacific Ocean is characterized by alternating zonal 2 currents, such as the North Equatorial Current (NEC), North Equatorial Countercurrent 3 (NECC), South Equatorial Current (SEC), and South Equatorial Countercurrent (SECC). 4In-situ measurements of subsurface moorings and satellite observations reveal pronounced 5 intraseasonal variability (ISV; 20-90 day) of these zonal currents in the western tropical 6Pacific Ocean (WTPO). The amplitude of ISV is the largest within the equatorial band 7 exceeding 20 cm s -1 , and decreases to ~10 cm s -1 in the NECC band and further to 4-8 cm s -1 8 in the NEC and SECC. The ISV power generally increases from high frequencies to low 9 frequencies and exhibits a peak at 50-60 day in the NECC, SEC, and SECC. These variations 10 are faithfully reproduced by an ocean general circulation model (OGCM) forced by satellite 11 winds, and parallel model experiments are performed to gain insights into the underlying 12 mechanisms. It is found that large-scale ISV (> 500 km) is primarily caused by atmospheric 13 intraseasonal oscillations (ISOs), such as the Madden-Julian oscillation (MJO), through wind 14 stress forcing. These signals are confined within 10°S-8°N, mainly as baroclinic ocean wave 15 responses to ISO winds. For scales shorter than 200 km, ISV is dominated by ocean internal 16 variabilities with mesoscale structures. They arise from the baroclinic and barotropic 17 instabilities associated with the vertical and horizontal shears of the upper-ocean circulation. 18 The ISV exhibits evident seasonal variation, with larger (smaller) amplitude in boreal winter 19 (summer) in the SEC and SECC. 20 Pacific basin. The South Equatorial Current (SEC) refers broadly to the westward flows 45 existing between 18°S-3°N over much of the western and central Pacific basin, as driven by 46 the equatorial trade winds (Reid 1986). It transports a huge amount of surface warm water to 47 Manuscript (non-LaTeX) Click here to download Manuscript (non-LaTeX) WTP.Uisv.Wangetal.figures.pdf

Intermediate water exchange in the northwest tropical Pacific is explored with the temperature, salinity, and current measurements of a mooring system deployed at 8°N, 127.05°E during 2010–2014. For the first time, prominent semiannual variability (SAV; with the maximum power at ~ 187 days) of subthermocline meridional flow along the Mindanao coast is revealed. A significant correlation between meridional flow and salinity is found at intermediate depths. This provides direct evidence for the alternating transports of South Pacific and North Pacific Intermediate Waters by northward and southward undercurrents, respectively. Further analysis with an eddy‐resolving ocean general circulation model demonstrates that the SAV is generated locally near the western boundary, manifesting as large‐scale subthermocline recirculation and leading to alternating northward and southward flows near the Mindanao coast, which plays an efficient role in the intermediate water exchange of the northwest tropical Pacific. Mechanisms underlying the observed SAV are discussed.

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The ocean currents of the tropical Pacific Ocean vary with El Niño‐Southern Oscillation (ENSO) cycles. A mooring time series obtained during 2014–2018 in the western Pacific reveal that interannual variability extends to Lower Equatorial Intermediate Current (LEIC). The LEIC velocity anomalies are significantly correlated with the Niño‐3.4 index at an 11‐month lag. Monthly velocity data from the global ocean physical reanalysis product and from a linear continuously stratified ocean model during 1993–2018 capture the 2015–2016 signal and are used to identify the underlying mechanism. During El Niño (La Niña) events, the direct wind forcing in the central tropical Pacific and the reflection of Kelvin waves in the eastern Pacific during the first autumn‐winter period contribute comparably to an eastward (westward) current anomaly in the western Pacific during the second autumn‐winter period. This process is achieved mainly through the generation of the second baroclinic mode and off‐equatorial westward‐propagating downwelling (upwelling) Rossby waves.

This study was performed at three eutrophic rivers in Southeast China aiming to determine the magnitude and patterns of dissolved N 2 O concentrations and fluxes over a seasonal (in 2009) and diurnal (24 h) temporal scale. The results showed that N 2 O concentrations varied from 0.28 to 0.38 (mean 0.32±0.04), 0.29 to 0.46 (mean 0.37±0.07), and 2.07 to 3.47 (mean 2.84±0.63) μg N-N 2 O L −1 in the Fengle, Hangbu and Nanfei rivers, respectively, in the diurnal study performed during the summer of 2008. The study found that mean N 2 O concentration and estimated N 2 O flux (67.89 ± 6.71 µg N-N 2 O m −2 h −1 ) measured from the Nanfei River with serious urban wastewater pollution was significantly higher than those from the Fengle and the Hangbu Rivers with agricultural runoff. In addition, the seasonal study during June and December of 2009 also showed that the mean N 2 O concentration (10.59±14.67 μg N-N 2 O L −1 ) and flux (236.87±449.74 µg N-N 2 O m −2 h −1 ) observed from the Nanfei River were significantly higher than those from the other two rivers. Our study demonstrated both N 2 O concentrations and fluxes exhibited seasonal and diurnal fluctuations. Over three consecutive days during the summer of 2008, N 2 O accumulation rates varied within the range of 3.91-7.21, 2.76-15.71, and 3.23-30.03 µg N-N 2 O m −2 h −1 for the Fengle, Hangbu and Nanfei Rivers, respectively, and exponentially decreased with time.

Meridional ocean current in the northwestern Pacific was documented by seven subsurface moorings deployed at 142°E during August 2014-October 2015. A sandwich structure of the tropical meridional overturning circulation (TMOC) was revealed between 0–6°N that consists of a surface northward flow (0–80 m), a thermocline southward flow (80–260 m; 22.6–26.5 σθ), and a subthermocline northward flow (260–500 m; 26.5–26.9 σθ). Based on mooring data, along with satellite and reanalysis data, prominent seasonal-to-interannual variations were observed in all three layers, and the equatorial zonal winds were found to be a dominant cause of the variations. The TMOC is generally stronger in boreal winter and weaker in summer. During 2014–2015, the TMOC was greatly weakened by westerly wind anomalies associated with the El Niño condition. Further analysis suggests that the TMOC can affect equatorial surface temperature in the western Pacific through anomalous upwelling/downwelling and likely plays a vital role in the El Niño-Southern Oscillation (ENSO).