Qingye Wang

Pacific western boundary currents and their roles in climate

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents—in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow—has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.

Intraseasonal variability of the subthermocline current east of Mindanao

The intraseasonal variability (ISV) of the subthermocline current east of Mindanao was characterized and shown to be caused by the activity of subthermocline eddies using mooring observations at 8 degrees N, 127.03 degrees E and a high-resolution numerical model. The ISV of the observed current east of Mindanao is vertically coherent in the upper 940 m but is significantly intensified below the thermocline. The ISV amplitude (8 cm s(-1)) of zonal subthermocline current is comparable with that (11 cm s(-1)) of the meridional current, revealing the nature of active eddies. The ISV of the subthermocline current was caused by the subthermocline eddies from three different pathways. The subthermocline eddies propagating along approximately 10 degrees N-11 degrees N contributed more to the ISV of the subthermocline current east of Mindanao than did those eddies propagating westward along 8 degrees N or northwestward from the New Guinea coast. Subthermocline eddies mainly exist south of the bifurcation latitude of the North Equatorial Current in the western tropical Pacific, and their generation and propagation mechanisms are briefly discussed.

Variations of Luzon Undercurrent from observations and numerical model simulations

Significant intraseasonal variability (ISV) of about 45-80 days and seasonal variation of the Luzon Undercurrent (LUC) at 18 degrees N are studied using direct current measurements and a high-resolution global Hybrid Coordinate Ocean Model. The variations of the LUC are vertically coherent with those of Kuroshio Current both on intraseasonal and seasonal time scales. The ISV of the LUC is dominated by eddies with diameters of about 200-300 km and extending from sea surface to intermediate layer east of Luzon Island. The LUC becomes strong (weak) when cyclonic (anticyclonic) eddies occur. The eddies east of Luzon Island mainly originate from the bifurcation point (similar to 13 degrees N) of the North Equatorial Current. These eddies propagate northwestward at a typical propagation speed of about 0.16 m s(-1) along the east coast of Philippines, gradually strengthen and pass the Luzon coast, and continue northward to Luzon strait. On seasonal time scale, the LUC is strong (weak) in boreal winter (summer), and this variation is related to the seasonal evolution of large-scale ocean circulation east of Philippines mainly controlled by local wind forcing.

Decadal variations of Pacific North Equatorial Current bifurcation from multiple ocean products

In this study, we examine the decadal variations of the Pacific North Equatorial Current (NEC) bifurcation latitude (NBL) averaged over upper 100 m and underlying dynamics over the past six decades using 11 ocean products, including seven kinds of ocean reanalyzes based on ocean data assimilation systems, two kinds of numerical simulations without assimilating observations and two kinds of objective analyzes based on in situ observations only. During the period of 1954-2007, the multiproduct mean of decadal NBL anomalies shows maxima around 1965/1966, 1980/1981, 1995/1996, and 2003/2004, and minima around 1958, 1971/1972, 1986/1987, and 2000/2001, respectively. The NBL decadal variations are related to the first Empirical Orthogonal Function mode of decadal anomalies of sea surface height (SSH) in the northwestern tropical Pacific Ocean, which shows spatially coherent variation over the whole region and explains most of the total variance. Further regression and composite analyzes indicate that northerly/southerly NBL corresponds to negative/positive SSH anomalies and cyclonic/anticyclonic gyre anomalies in the northwestern tropical Pacific Ocean. These decadal circulation variations and thus the decadal NBL variations are governed mostly by the first two vertical modes and attribute the most to the first baroclinic mode. The NBL decadal variation is highly positively correlated with the tropical Pacific decadal variability (TPDV) around the zero time lag. With a lead of about half the decadal cycle the NBL displays closer but negative relationship to TPDV in four ocean products, possibly manifesting the dynamical role of the circulation in the northwestern tropical Pacific in the phase-shifting of TPDV.

Long‐term trend of Pacific South Equatorial Current bifurcation over 1950–2010

This study investigates the long-term change of the Pacific South Equatorial Current (SEC) bifurcation latitude (SBL) over 1950-2010 with Simple Ocean Data Assimilation version 2.2.4. Results indicate that the SBL averaged within upper 200 m has migrated southward at 0.020 degrees S yr(-1), comparable in magnitude with -0.024 degrees N yr(-1) for the North Equatorial Current bifurcation latitude (NBL). The SEC transport into the Coral Sea has increased. Due to the southward SBL migration, most of the increased SEC water was transported equatorward, contributing to the Equatorial Undercurrent intensification. Experiments with a nonlinear 1.5 layer reduced gravity model indicate that the southward migration of SBL is mainly caused by positive Ekman flux divergence trend in the eastern tropical South Pacific, while that of NBL is caused by negative Ekman flux divergence trend in the western tropical North Pacific.

Observation of the abyssal western boundary current in the Philippine Sea

Mooring observations were conducted from July 16, 2011 to March 30, 2012 east of Mindanao, Philippines (127°2.8′E, 8°0.3′N) to observe the abyssal current at about 5 600 m deep and 500 m above the ocean bottom. Several features were revealed: 1) the observed abyssal current was highly variable with standard deviations of 57.3 mm/s and 34.0 mm/s, larger than the mean values of -31.9 and 16.6 mm/s for the zonal and meridional components, respectively; 2) low-frequency current longer than 6 days exhibited strong seasonal variation, flowing southeastward (mean flow direction of 119.0° clockwise from north) before about October 1, 2011 and northwestward (mean flow direction of 60.5° counter-clockwise from north) thereafter; 3) the high-frequency flow bands were dominated by tidal currents O1, K1, M2, and S2, and near-inertial currents, whose frequencies were higher than the local inertial frequency. The two diurnal tidal constituents were much stronger than the two semidiurnal ones. This study provides for the first time an observational insight into the abyssal western boundary current east of Mindanao based on long-term observations at one site. It is meaningful for further research into the deep and abyssal circulation over the whole Philippine Sea and the 3D structure of the western boundary current system in this region. More observational and high-resolution model studies are needed to examine the spatial structure and temporal variation of the abyssal current over a much larger space and longer period, their relation to the upper-layer circulation, and the underlying dynamics.

Structure and Variability of the North Equatorial Current/Undercurrent from Mooring Measurements at 130°E in the Western Pacific

The mean structure and variability of the North Equatorial Current/Undercurrent (NEC/NEUC) are investigated with one-year Acoustic Doppler Current Profilers measurements from 4 subsurface moorings deployed at 10.5°N, 13°N, 15.5°N, and 18°N along 130°E in the western Pacific. The strong westward flowing NEC ranges from the sea surface down to 400 m, and the mean zonal velocity of the NEC at 10.5°N is around −30 cm/s at the depth of 70 m. Eastward flowing NEUC jets are detected below the NEC at 10.5°N and 13°N, and the depth of the NEUC could reach at least 900 m. The mean velocity of the NEUC is around 4.2 cm/s at 800 m. No eastward undercurrents are observed at 15°N and 18°N. The mooring measurements also reveal a strong intraseasonal variability of the currents at all 4 mooring sites, and the period is around 70–120 days. The vertical structure of this intraseasonal variability varies at different latitudes. The variability of the NEUC jets at 10.5°N and 13°N appears to be dominated by subthermocline signals, while the variability of the currents at 15.5°N and 18°N is dominated by surface-intensified signals.

Observed water current and transport through Qiongzhou Strait during August 2010

The velocity structure of the residual current across an entire section of the Qiongzhou Strait (QS) in summer is presented for the first time. Shipboard Acoustic Doppler Current Profile measurements, from the mid-region of the QS (110.18°E), were collected on 1–4 August 2010. The diurnal tidal currents had their maximum amplitudes between 4.24 and 20.24 m. Their amplitude along the major axis ranged from approximately 0.55 m/s in the middle part of the strait (20.15°N) to 0.84 m/s in the north part of the strait (20.20°N). Both anticlockwise and clockwise tidal current rotations exist in the QS. During the observation period (neap tide), a significant westward residual current occupied almost the entire study section. Two velocity cores of westward current were observed at the northern part and near the deepest trough, although an eastward current appeared in the middle part of the transect. The deepest core was located near 62 m at 20.13°N, with a maximum velocity of −0.34 m/s. The shallower core was located at approximately 16 m at 20.20°N, with a maximum velocity of −0.33 m/s. The estimated total volume of water transported through the QS was −0.16 Sv. This value is an important boundary condition, applicable to numerical models studying coastal ocean circulation in the northwestern South China Sea.

Three-dimensional structure of mesoscale eddies in the western tropical Pacific as revealed by a high-resolution ocean simulation

The three-dimensional structure of mesoscale eddies in the western tropical Pacific (6°S–20°N, 120°E–150°E) is investigated using a high-resolution ocean model simulation. Eddy detection and eddy tracking algorithms are applied to simulated horizontal velocity vectors, and the anticyclonic and cyclonic eddies identified are composited to obtain their three-dimensional structures. The mean lifetime of all long-lived eddies is about 52 days, and their mean diameter is 147 km. Two typical characteristics of mesoscale eddies are revealed and possible dynamic explanations are analyzed. One typical characteristic is that surface eddies are generally separated from subthermocline eddies along the bifurcation latitude (~13°N) of the North Equatorial Current in the western tropical Pacific, which may be associated with different eddy energy sources and vertical eddy energy fluxes in subtropical and tropical gyres. Surface eddies have maximum swirl velocities of 8–9 cm s−1 and can extend to about 1500 m depth. Subthermocline eddies occur below 200 m, with their cores at about 400–600 m depth, and their maximum swirl velocities can reach 10 cm s−1. The other typical characteristic is that the meridional velocity component of the eddy is much larger than the zonal component. This characteristic might be due to more zonal eddy pairs (two eddies at the same latitude), which is also supported by the zonal wavelength (about 200 km) in the high-frequency meridional velocity component of the horizontal velocity.