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Liping Yin, Fangli Qiao, and Quanan Zheng

Abstract

Using five mooring array observations in the coastal water of the East China Sea (ECS) in winter 2006, the authors identify three kinds of low-frequency waves using the ensemble empirical mode decomposition (EEMD) method. The analysis indicates that the periods of the waves varied from 2 to 10 days, which are consistent with coastal-trapped wave (CTW) modes: the Kelvin wave (KW) mode, the first shelf wave (SW1) mode, and the second shelf wave (SW2) mode. An analytical model is established and the dispersion relation of the waves from the analytical method agrees well with the observations. The wind-forced, coastal-trapped wave theory is then applied. The calculation shows that over a wide shelf, the forcing term of wind stress curl plays an important role in shaping the CTW. Numerical solutions reproduce the sea level variation and the alongshore current. The results show that the sea level variation mainly resulted from the KW mode, but the alongshore current resulted from both the KW and SW1 modes.

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Chung-Ru Ho, Xiao-Hai Yan, and Quanan Zheng

The variabilities of the upper layer of the western Pacific warm pool (WPWP) were observed using satellite infrared data from 1982 to 1991 and altimeter data from November 1986 to September 1989. The warm pool was defined as the area where the sea surface temperatures are above 28°C. The eastern boundary oscillation, the centroid movement, and the upper-layer volume variation of the WPWP were intensively studied. Spectral analysis revealed that the eastern boundary oscillation of the WPWP was related to the El Niño event and the annual cycle. The centroid of the WPWP traced an ellipselike trajectory during a year and moved counterclockwise in most years. However, in 1982 and 1986, the years of the onset of El Niño events, the movements were clockwise. The upper-layer volume of the WPWP was divided latitudinally into three sections. The annual cycles in the northern (from 3° to 30°N) and southern (from 3° to 30°S) sections were dominant. No annual cycle was found in the equatorial section (from 3°s to 3°N), but the volume of warm water in the equatorial Pacific increased during the 1986/87 El Niño event. The equatorial section was further divided into eastern and western sectors along 165°W. During the 1986/87 El Niño event, the volume of warm water increased in the eastern sector, but the variation was smaller in the western sector than that in the eastern sector. During the 1988 La Niña event, the warm water volumes decreased in both sectors.

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R. Dwi Susanto, Quanan Zheng, and Xiao-Hai Yan

Abstract

The mean of the sea level deviation data derived from the TOPEX/Poseidon altimeter in the equatorial Pacific, between 10°S and 10°N, and between 120°E and 78°W, from cycles 2 to 136 (3 October 1992–2 June 1996), are extracted using a maximum–minimum average method. Then, two-dimensional (2D) sea level deviation time series are developed to visualize the dynamics of equatorial waves. The complex singular value decomposition (CSVD) method is applied to decompose these 2D time series into empirical orthogonal modes. Using this method, zonal and meridional structures, propagation directions, periods, and propagation speeds of these empirical modes are obtained.

The first empirical mode is propagating westward, and its structure is asymmetric to the equator. It has an average phase speed c = −0.6 m s−1 within 4°–6°N and c = −0.4 m s−1 within 6°–8°S, respectively, and a period of 15 months, which is associated with an interannual Rossby wave. The second empirical mode is propagating eastward along the equator and has a phase speed of 2.5 m s−1 and a period of 7 months, which is associated with an equatorial Kelvin wave.

The asymmetric feature of the empirical Rossby wave, which is also observed in the equatorial Pacific, may suggest that the background currents and wind fields in the equatorial Pacific Ocean affect its propagation. The amplitude of the empirical Kelvin mode increases as it propagates eastward. This is associated with an eastward shoaling of the thermocline depth along the equatorial Pacific Ocean. The results of both empirical modes are consistent with those predicted by the theory of Kelvin and Rossby waves and closely represent the actual features of both waves observed in the equatorial Pacific Ocean. Therefore, the CSVD is a suitable method for revealing the dynamics of equatorial waves.

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Lingling Xie, Quanan Zheng, Jiwei Tian, Shuwen Zhang, Ying Feng, and Xiaofei Yi

Abstract

This study deals with the physical properties and 3D structures of the wave motions with finite wavelengths of O(100–550) km in the tropical western North Pacific and their variation as propagating from the Pacific to the South China Sea (SCS) using conductivity–temperature–depth observations taken in October and November 2005 and concurrent satellite altimeter data. Three wave components with wavelength bands of O(100), O(200), and O(550) km are derived from the isopycnal undulation signals along 21°, 18°, and 15°N using the ensemble empirical mode decomposition analysis. Their maximum amplitudes are over 100 m in the layer of 1000–2000 m. Phase speeds are derived from cruise-observed vertical profiles of zonal-mean geostrophic flow velocity and the Brunt–Väisälä frequency based on linear quasigeostrophic wave theory with background flow and topography. The speeds are also derived from concurrent sea level anomaly data with the objective Radon transform method. They are close to that of the first baroclinic mode of theoretical solutions, implying that the observed wave motions possess the physical properties of Rossby waves (RWs). The vertical structures of the first generalized modes are derived from cruise observations at three sections. It is shown that the RWs continuously propagate from the Pacific to the SCS, and the available potential energy of RWs 1 and 2 intensify 3–4 times in the Luzon Strait and the SCS compared to that in the Pacific.

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Xiao-Hai Yan, Yun He, W. Timothy Liu, Quanan Zheng, and Chung-Ru Ho

Abstract

The centroid motion of the Western Pacific warm pool for 1982–83, 1986–87 and 1991–93 El Niño–Southern Oscillation events was investigated by analyzing satellite multichannel sea surface temperature and satellite pathfinder sea surface temperature data in conjunction with wind data from The Florida State University. The authors found that the direction of motion of the warm pool’s center changes shortly before the onset of a new El Niño event and that this change was quite different during the 1991–93 El Niño than during the 1982–83 and 1986–87 El Niño events. The possible cause of these changes and differences may be the El Niño–related wind pattern shifts in the western equatorial Pacific Ocean before and during the events, and the seasonal phase-lock of the El Niño events.

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Han-Tsung Chen, Xiao-Hai Yan, Ping-Tung Shaw, and Quanan Zheng

Abstract

A semispectral primitive equation model is used to study the effects of wind stress and bottom topography on the meandering of a jet over the continental margin. The jet is specified initially as a geostrophic current (similar to the Kuroshio Current) supported by the temperature field. The intrusion of water from the meandering jet over the continental margin is not only dominated by topographic effects but also enhanced by northerly winds. The latter induces an onshore Ekman transport, which causes surface water in the meandering jet to intrude onto the continental margin. Lighter water is then carried downward to the deeper ocean through the hydrostatic and topographic boundary layers. Some subsurface water in the meandering jet also intrudes onto the continental slope and shelf and then rejoins the main stream. Under the influence of southerly wind, surface water in the meandering jet migrates offshore toward the deeper ocean in the Ekman layer, while subsurface water in the meandering jet intrudes onto the continental slope and shelf in the topographic boundary layers. In this case, heavier water is lifted to the sea surface through the topographic boundary and hydrostatic layers. The result suggests that the path of the Kuroshio north of Taiwan is strongly influenced by both the topographic effects and Ekman drift. The intrusions of the meandering Kuroshio surface and subsurface waters are steered and determined by wind directions.

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Quanan Zheng, Xiao-Hai Yan, Chung-Ru Ho, and Chang-Kou Tai

Abstract

The effects of strong meridional shear of mean flow on propagation of long waves having a wavelength of about 1000 km and period of 25 days observed in the equatorial Pacific and Atlantic Oceans are analyzed in this paper. Information from the weekly multichannel sea surface temperature images derived from the Advanced Very High Resolution Radiometer on board the NOAA series satellites from January 1986 to March 1992 is used for the statistics of the visual parameters of the waves. The characteristic scales of the waves are estimated based on the results by previous investigators and the statistical results of the present study. Solving a potential vorticity equation, the authors obtain a dispersion relation with an additional term depending on the flow shear strength compared with that of the free Rossby wave. The theoretical model provides a good explanation for the variable behavior of long-wave propagation.

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Jiayi Pan, Xiao-Hai Yan, Young-Heon Jo, Quanan Zheng, and W. Timothy Liu

Abstract

It has been difficult to estimate the sensible heat flux at the air–sea interface using satellite data because of the difficulty in remotely observing the sea level air temperature. In this study, a new method is developed for estimating the sensible heat flux using satellite observations under unstable conditions. The basic idea of the method is that the air–sea temperature difference is related to the atmospheric convergence. Employed data include the wind convergence, sea level humidity, and sea surface temperature. These parameters can be derived from the satellite wind vectors, Special Sensor Microwave Imager (SSM/I) precipitable water, and Advanced Very High Resolution Radiometer (AVHRR) observations, respectively. The authors selected a region east of Japan as the test area where the atmospheric convergence appears all year. Comparison between the heat fluxes derived from the satellite data and from the National Centers for Environmental Prediction (NCEP) data suggests that the rms difference between the two kinds of sensible heat fluxes has low values in the sea area east of Japan with a minimum of 10.0 W m−2. The time series of the two kinds of sensible heat fluxes at 10 locations in the area are in agreement, with rms difference ranging between 10.0 and 14.1 W m−2 and correlation coefficient being higher than 0.7. In addition, the National Aeronautics and Space Administration (NASA) Goddard Satellite- Based Surface Turbulent Flux (GSSTF) was used for a further comparison. The low-rms region with high correlation coefficient (>0.7) was also found in the region east of Japan with a minimum of 12.2 W m−2. Considering the nonlinearity in calculation of the sensible monthly means, the authors believe that the comparison with GSSTF is consistent with that with NCEP data.

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Lingling Xie, Enric Pallàs-Sanz, Quanan Zheng, Shuwen Zhang, Xiaolong Zong, Xiaofei Yi, and Mingming Li

Abstract

Using the generalized omega equation and cruise observations in July 2012, this study analyzes the 3D vertical circulation in the upwelling region and frontal zone east of Hainan Island, China. The results show that there is a strong frontal zone in subsurface layer along the 100-m isobath, which is characterized by density gradient of O(10−4) kg m−4 and vertical eddy diffusivity of O(10−5–10−4) m2 s−1. The kinematic deformation term S DEF, ageostrophic advection term S ADV, and vertical mixing forcing term S MIX are calculated from the observations. Their distribution patterns are featured by banded structure, that is, alternating positive–negative alongshore bands distributed in the cross-shelf direction. Correspondingly, alternating upwelling and downwelling bands appear from the coast to the deep waters. The maximum downward velocity reaches −5 × 10−5 m s−1 within the frontal zone, accompanied by the maximum upward velocity of 7 × 10−5 m s−1 on two sides. The dynamic diagnosis indicates that S ADV contributes most to the coastal upwelling, while term S DEF, which is dominated by the ageostrophic component S DEFa, plays a dominant role in the frontal zone. The vertical mixing forcing term S MIX, which includes the momentum and buoyancy flux terms S MOM and S BUO, is comparable to S DEF and S ADV in the upper ocean, but negligible below the thermocline. The effect of the vertical mixing on the vertical velocity is mainly concentrated at depths with relatively large eddy diffusivity and eddy diffusivity gradient in the frontal zone.

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Guang-Bing Yang, Changshui Xia, Xia Ju, Quanan Zheng, Yeli Yuan, Xue-Jun Xiong, and Fangli Qiao

Abstract

Previous in-situ observations have suggested that bottom water temperature variations in shelf seas can drive significant ocean bottom heat flux (BHF) by heat conduction. The BHF-driven bottom water temperature variations, however, have been overlooked in ocean general circulation models. In this study, we established a sea-sediment fully coupled model through incorporating the BHF. The coupled model included a sediment temperature module/model, and the BHF was calculated based on the sediment heat content variations. Meanwhile, we applied temporally varying BHF in the calculation of the bottom water temperature, which further determined the sediment temperature. The two-way coupled BHF process presents a more complete and physically reasonable heat budget in the ocean model and a synchronously varying sediment temperature profile. The coupled model was validated using a one-dimensional test case, and then it was applied in a domain covering the Bohai and Yellow Seas. The results suggest that when a strong thermocline exists, the BHF can change the bottom water temperature by more than 1°C because the effects of the BHF are limited to within a shallow bottom layer. However, when the water column is well mixed, the BHF changes the temperature of the entire water column, and the heat transported across the bottom boundary is ventilated to the atmosphere. Thus, the BHF has less effect on water temperature and may directly affect air-sea heat flux. The sea-sediment interactions dampen the amplitude of the bottom water temperature variations, which we propose calling the seabed dampening ocean heat content variation mechanism (SDH).

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