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Climate variability over the Tropical and North Pacific Ocean (TPO and NPO, respectively) modulates marginal sea variability. The South China Sea (SCS), the largest marginal sea in the western NPO, is an outstanding example of a region that responds quickly to climate change. However, there is considerable uncertainty regarding the response of the deep SCS to large-scale climate variability. Multivariate empirical orthogonal function analysis revealed three prominent modes of interconnected temperature anomaly fluctuations within the TPO and NPO. These coherent modes highlight the interactive dynamics among climate variations and reveal their modulation mechanisms for previously less explored potential temperature variabilities in the deep SCS. On the atmospheric bridge, external forces modify the upper-layer Luzon Strait Transport (LST) by adjusting the Ekman transport and Kuroshio intrusion. For the oceanic pathway, climate variations disturb the deep-layer LST by adjusting the barotropic flows in the upper layer. © 2024. The Authors.

We comprehensively investigate the interannual variability of multi-scale motions in the northern South China Sea (NSCS) and associated influences from the terrestrial, atmospheric, and oceanic processes over the period 1994-2018 based on observations and a numerical simulation. We find that the interannual variabilities of the NSCS circulation and hydrographic properties, in the form of a cross-scale interactive dynamic system, are sensitive to the tropical climate variability represented by the El Nino-Southern Oscillation (ENSO). In coastal areas with less than 30 m depth, the anomalous warm (cold) and less (more) saline waters are regulated by the rate of Pearl River discharge due to the predominant precipitation (evaporation) and positive (negative) heat-flux anomaly in the Pearl River catchment in El Nino (La Nina) years. Controlled by the interannual variabilities of slope current and atmospheric forcings, the shelf current at 30-200 m depth responds asymmetrically to the ENSO, which establishes a stronger northeastward flow anomaly in El Nino years than the southwestward flow anomaly in La Nina years. We further show that the interannual variability of the slope current is jointly governed by the atmospheric and oceanic processes that tend to offset each other in interannual timescales. In particular, the weakened (strengthened) cyclonic wind stress curl over the ocean basin tends to moderate (intensify) the slope current in the NSCS, while this process is mitigated by the stronger (weaker) Kuroshio intrusion in El Nino (La Nina) years.

The mean (sub-tidal) circulations in the Pearl River Estuary (PRE) and over the adjacent shelf are interactive. They are driven by multi-forcing of winds, tides, and the buoyancy of river discharge. Utilizing a validated three-dimensional, high resolution numerical model, we find that the circulation in the PRE during summer is dominated by an advective gravitational two-layer circulation in the upper estuary and the landward part of the lower PRE, where the surface flow pattern also varies with the upwelling winds. The circulation in the seaward part of the lower PRE is governed by both gravitational circulation and geostrophic intrusive current from the shelf. The pattern and intensity of these circulations are largely modulated by variable wind forcing. The cross-shore upwelling shelf circulation off the PRE enhances the water exchange rate between the shelf and the PRE, but the net intrusive transport into the PRE is negatively correlated with the intensity of upwelling-favorable wind stress. Relatively strong water exchange rate between the shelf and estuary occurs during upwelling, which reduces the flushing time of the estuary. Although the ebbing/flooding tide strengthens/weakens the eastward alongshore upwelling current, tidal effect on the upwelling circulation and on the net transport between the shelf and estuary is not significant over the sub-tidal period. The shelf influences the estuary mainly through the intrusions of the shelf waters at the western bank and along the two navigation channels of the PRE; and they are governed by geostrophic cross-shelf upwelling circulation and by gravitational intrusive currents due to pressure gradients, yielded by the alongshore variation of the upwelling shelf circulation and by the buoyancy forcing of the river plume, respectively. (C) 2013 Elsevier Ltd. All rights reserved.

A numerical simulation of the buoyant river plume over the Pearl River Estuary (PRE) and adjacent shelf during a typical upwelling favorable wind period of the summer monsoon is utilized to explore the responses of the plume to wind and tide forcing. The model is forced with time-dependent river discharge, wind and tide, and it shows reasonable ability to capture the basic structure and responses of the plume. Additional numerical experiments that are forced without either wind or tide are used to evaluate the relative importance of wind and tide in generating plume variability. Results show that the vertical structure of the plume and the strength of the stratification in the estuary are determined by the combination of the buoyancy forcing associated with river discharge and tidal forcing, and vary with the advection process, while the horizontal shape and spreading of the plume over the shelf are highly influenced by the wind-driven coastal current, and are more susceptible to the change of vertical mixing. Mechanical energy analysis in each dynamical region (upper, middle, lower estuary, and shelf) reveals that this is because the system mainly gains energy from tide (wind) in the estuary (shelf), and loses energy to the bottom friction (internal-shear mixing) in the estuary (shelf). The largest forcing and dissipation terms in the middle PRE, and at the entrances of smaller estuaries such as Huang Mao Hai, are due to tidal forcing, which enables the middle PRE to serve dynamically as the entrance of an estuary, where the transition of the river plume into coastal buoyancy current usually takes place. In addition, the mixing efficiency increases from upper PRE to the shelf and from strong to weak mixing period, thus the plume in the well-mixed upper estuary is not as sensitive to the changes of wind and tide as that over the highly stratified shelf. (C) 2014 Elsevier B.V. All rights reserved.

Numerical studies of the tides in the South China Sea (SCS) and circulations in the Pearl River Estuary (PRE) and adjacent shelf are carried out by developing a numerical modeling system that includes a tidal circulation model in the SCS basin, a conceptual and a realistic three-dimensional coupled estuary-shelf circulation models forced by wind stress, tides, and river discharge around the PRE. The high resolution tidal model utilizes a physically sensible domain and assimilates Topex/Poseidon (T/P) data through an efficient generalized inverse scheme. The coupled estuary-shelf modeling is a physically novel study that investigates the interactive roles of the wind, tide and buoyancy forcing and dynamic processes of buoyant plume, estuarine circulation and shelf circulations in the PRE region. Tidal simulation in the SCS well simulates the observed feature and provides tidal forcing for the subsequent coupled estuarine-shelf circulation models around the PRE, and investigates the dynamics and energy of tides in a semi-enclosed sea. The study, for the first time, reveals the amplified K1 tide in the SCS basin as a result of the Helmholtz resonance. Analysis of tidal energy shows that the energy dissipates mostly in the LS and strong dissipation of M2 tide also occurs in the Taiwan Strait (TS). The work rate of the tidal generating force in the SCS basin is negative for M2 and positive for K1. The different responses of the M2 and K1 tides in the SCS are largely controlled by the intruding directions of the tides from the Pacific, the tidal frequency, the wavelengths, the local geometry and the bottom topography. A conceptual coupled estuary-shelf model that utilizes representative, but idealized forcing and topography is developed to better identify the fundamental but extremely complex physical processes in a simple but physically well-defined system. Results show that a buoyancy-driven anti-cyclonic eddy is developed inside the idealized PRE under both the gravita

A three-dimensional, primitive equation numerical model is utilized to study the responses of the river plume and estuarine circulation to the wind and tidal forcing in an idealized Pearl River Estuary (PRE) with simplified geometry and physical forcing. This process-oriented study shows that without any other physical forcing, the buoyant-driven circulation, under the Earth rotation effect, forms an anticyclonic eddy in a funneled estuary with a concave bathymetry. However, this eddy disappears with additional wind or tidal forcing. The movement of the buoyant plume is retarded, and the thickness of the plume is largely increased by the tidal forcing as a result of the great enhancement of the vertical mixing inside the estuary and around the mouth of the estuary on the shelf. On the other hand, with wind forcing, the plume moves faster along the seaward branch of the wind-induced estuarine circulation with a narrower width confined by the current. And the wind-induced circulation increases the intrusion of the oceanic saltier water into the estuary, especially in upwelling case, and the consequently enhanced stratification along the axis of the estuary reduces the vertical mixing and the thickness of the plume. The study reveals the interactive roles of the buoyant plume, tidal, and wind forcing on the circulation in the estuary and adjacent shelf waters.