Fangguo Zhai

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.

Interannual variability of transport and bifurcation of the North Equatorial Current in the tropical North Pacific Ocean

The relationship of the interannual variability of the transport and bifurcation latitude of the North Equatorial Current (NEC) to the El Niño-Southern Oscillation (ENSO) is investigated. This is done through composite analysis of sea surface height (SSH) observed by satellite altimeter during October 1992–July 2009, and correspondingly derived sea surface geostrophic currents. During El Niño/La Niña years, the SSH in the tropical North Pacific Ocean falls/rises, with maximum changes in the region 0–15°N, 130°E–160°E. The decrease/increase in SSH induces a cyclonic/anticyclonic anomaly in the western tropical gyre. The cyclonic/anticyclonic anomaly in the gyre results in an increase/decrease of NEC transport, and a northward/southward shift of the NEC bifurcation latitude near the Philippine coast. The variations are mainly in response to anomalous wind forcing in the west-central tropical North Pacific Ocean, related to ENSO events.

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.

Statistical characteristics and formation mechanism of the Lanyu cold eddy

The statistical characteristics and formation mechanism of the Lanyu cold eddy were examined using satellite data from 1993 to 2013. The statistical characteristics of the Lanyu cold eddy in this paper are given for the first time to the best of our knowledge. It is found that Lanyu cold eddies occurred seven times in total during the period examined, and that this eddy generally moves from southeast to northwest and gradually decays when it approaches the island of Taiwan. Next, we estimated the eddy lifetime, diameter, strength and straight line travel distance. Composite analyses of sea surface height anomaly and geostrophic current demonstrate that the formation of the Lanyu cold eddy mainly results from the combined action of the Kuroshio loop and an anticyclonic eddy east of Lanyu Island. Thus, our study provides a new insight into our understanding of the formation mechanism of the Lanyu cold eddy.

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.

Seasonal variability of the North Equatorial Current transport in the western Pacific Ocean

Seasonal variability of the North Equatorial Current (NEC) transport in the western Pacific Ocean is investigated with ECMWF Ocean Analysis/Reanalysis System 3 (ORA-S3). The result shows that NEC transport (NT) across different longitudes in the research area shows a similar double-peak structure, with two maxima (in summer and winter), and two minima (in spring and autumn). This kind of structure can also be found in NEC geostrophic transport (NGT), but in a different magnitude and phase. These differences are attributable to Ekman transport induced by the local meridional wind and transport caused by nonzero velocity at the reference level, which is assumed to be zero in the NGT calculation. In the present work, a linear vorticity equation governing a 1.5-layer reduced gravity model is adopted to examine the dynamics of the seasonal variability of NGT. It is found that the annual cycle of NGT is mainly controlled by Ekman pumping induced by local wind, and westward-propagating Rossby waves induced by remote wind. Further research demonstrates that the maximum in winter and minimum in spring are mostly attributed to wind east of the dateline, whilst the maximum in summer and minimum in autumn are largely attributed to that west of the dateline.

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.

Sea surface height variations in the Mindanao Dome region in response to the northern tropical Pacific winds

Sea surface height (SSH) variability in the Mindanao Dome (MD) region is found to be one of the strong variations in the northern Pacific. It is only weaker than that in the Kuroshio Extension area, and is comparable to that in the North Pacific Subtropical Countercurrent region. Based on a 1.5-layer reduced gravity model, we analyzed SSH variations in this region and their responses to northern tropical Pacific winds. The average SSH anomaly in the region varies mainly on a seasonal scale, with significant periods of 0.5 and 1 year, ENSO time scale 2–7 years, and time scale in excess of 8 years. Annual and long-term variabilities are comparably stronger. These variations are essentially a response to the northern tropical Pacific winds. On seasonal and ENSO time scales, they are mainly caused by wind anomalies east of the region, which generate westward-propagating, long Rossby waves. On time scales longer than 8 years, they are mostly induced by local Ekman pumping. Long-term SSH variations in the MD region and their responses to local winds are examined and discussed for the first time.

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.

Interannual wave height variability in the Yellow Sea

This paper investigates the interannual wave climate variability in the Yellow Sea (YS) using the Simulating Waves Nearshore (SWAN) wave model. Interannual variations of both the monthly mean and extreme wave conditions are weaker than their seasonal counterparts. Meanwhile, both exhibit significant seasonal variations. The interannual variability of the seasonal mean wave condition is strongest in winter and weakest in summer. It basically decreases both northward and shoreward in all four seasons. However, the longitudinal position of the maximum variability shifts zonally with seasons. On the other hand, the interannual variability of the seasonal extreme wave condition is strongest roughly in fall, but weakest in winter in most regions of the YS except in the southeastern YS, where the largest variability appears in summer. As for the spatial distribution, the largest variability occurs in the middle YS in spring, while in the southeastern YS in other three seasons. Both of them are dominantly controlled by the local sea surface wind forcing with possibly minor contributions from the western Pacific Ocean. The climate variability modes possibly at work are also discussed.