A Numerical Study On The Mechanisms Of The Upwelling In The Yellow Sea And East China Sea

The coastal upwelling in the Yellow Sea (YS), East China Sea (ECS), and the western Hainan waters is numerically studied by using the MASNUM (MArine Science and NUmerical Modeling) wave-tide-circulation coupled model, in combination with analysis of the field data obtained from the 973 circulation project, national 126 project, and 908 project, and satellite remotely sensed data. On the basis of successful simulation of the general circulation, numerical experiments have been carried out to explore the formation mechanism of the upwelling, with foci on the frontal upwelling mechanism and the role of tidal movements in inducing upwelling. The study provides new understanding for the traditional mechanism of"tidally induced upwelling (TIU)".A mechanism of TIU in baroclinic mode is proposed in some sea regions with strong tides in China. It is suggested that tidal mixing exerts profound influences on the vertical circulation, as well as coastal upwelling, in some China seas with strong tides, and even serves as the primary inducement for the coastal upwelling. The difference between this mechanism and those responsible for the main eastern boundary upwelling in the world stems from the characteristic of strong tide in China shelf seas. The YS and ECS are among the shelf seas which have the highest tidal energy dissipation rate. The huge tidal energy dissipated in the bottom boundary layer transforms into turbulence, resulting in violent verical mixing which leads to tidal mixing front (TMF) straddling on the sloping bottom in coastal waters. The tidal turbulence is usually strong enough on the shallow side of TMF to homogenize the water from the bottom to the surface, and the whole water column can be fully heated by solar radiation, attaining low density. At the other side of the front, the upper ocean is stratified, and the deep water remains cool and dense. Therefore considerable baroclinic pressure gradient force sets up due to density difference across the TMF, and stimulates a cross-frontal, secondary circulation with apparent upwelling branch occurring usually on the shallow side of the front. Such a mechanism is particularly applicable in the YS, the waters off Yangtze River estuary (YRE) and the western coast of Hainan Island, where strong tidal movement can be found. In the coastal waters off Zhejiang, the upwelling mechanism is somewhat complex because of the relatively weak tidal signals and the influences of the Taiwan Warm Current (TWC). In this dissertation, a summertime upwelling is newly found off the western coast of Hainan by using the baroclinic mechanism of TIU.Similarly, the wintertime salinity front off the ECS coast also induces upwelling. The upwelling, induced by either the TMF in summer or the saliity front in winter, can be classified as"frontal upwelling".This study identifies two upwelling mechanisms associated with tidal movements, the baroclinic and the barotropic mechanism. Besides the baroclinic processes which have been delineated above, upwelling and downwelling can also be induced by a tidal rectification process when barotropic tidal currents flow over complex bottom topography. The residual tidal currents (RTC), even in barotropic aspects, have three-dimensional structure. In the YS and Zhejiang coastal waters, the surface RTC tends to flow offshore while the bottom one moves inversely, thus upwelling is generated near the coast. Without the support from the land, the barotropic upwelling off YRE is produced by the naural uplifting when bottom current flows over bathymetric bumps.1. The summertime upwelling off YRE and Zhejiang coastsTides, TWC, and Yangtze River diluted water (YRDW) are determained as the main inducements of the upwelling off YRE and Zhejiang coasts.In the local region off YRE, tidal mixing plays the the most important role in inducing upwelling, and the baroclinicity resulted from the YRDW also has direct and important effects with a contribution of about 30% to the local upwelling. In Zhejiang coastal waters, both tidal mixing and the encroachment of TWC onto the continent across isobaths are important causations of the upwelling. The TIU resulted from either baroclinic or barotropic process takes in a shape of belt.The existence of proper topography is a premise for the formation of upwelling, either in the case of TIU or the TWC-induced upwelling. A glimpse of the mathematic expression of the SH index, log( H U t3), reminds one the importance of topography in locating the TMF and, subsequently, the occurrence of frontal upwelling.Experimental results make it necessary to reconsider the role of wind in generating the coastal upwelling off Zhejiang coast. Winds may exert a negative influence on the upwelling by the offshore deflection of TWC, impairing its climbing along the sloping bottom. A tentative argument is proposed here that the Zhejiang coastal region is probably not an ideal sea region for wind-driven upwelling like Oregon or Peruvian coastal waters, and the observed low SST belt off Zhejing usually has a distance away from the land, rather than hugging closely to the coast like most cases in typical wind-driven upwelling area.2. The wintertime upwelling off Zhejiang coastsFrontal upwelling off Zhejiang coast is very clearly shown in numerical modeling in winter. Different from the summer case, it is the horizontal gradiet of salinity, rather than temperature, that leads to the formation of the front. In winter, the YRDW is confined within a narrow band off the coasts of Zhejiang and Fujian when the northerly monsoon is dominant in winter. The salinity front between the low salinity coastal water and open sea is quite strong, and a remarkable baroclinic pressure gradient sets up. Accordingly, upwelling will be driven by the similar mechanism to that associated with TMF in summer.Wind and TWC affect upwelling through altering the intensity of salinity front. The notherly wind plays positive role in enhancing upwelling for its driving the YRDW southward, while the net effects of TWC on upwelling are negative since its direction is opposite to that of the low salinity coastal current.3. The upwelling in Yellow Sea and the surface cold patchesThe numerical model produces systematic upwelling belts around the Yellow Sea Cold Water Mass (YSCWM). The highly coincidence between the upwelling belts and TMF in location shows the intrinsic physics of upwelling. As manifested by numerical experiments, the upwelling is mainly induced by tidal motions.Surface cold patches (SCPs) are often observed scattering around YSCWM, especially off the convex coastlines of peninsula tips. The formation mechanism of the SCPs is ascribed to both upwelling and vertical mixing. Almost all the SCPs are accompanied by upwelling strong enough to reach the sea surface, implying the direct contribution of upwelling to the generation of the SCPs. The cross-frontal ascending movement of deep water cools down the bottom of the water column in the frontal zone, and pure vertical mixing may continue to homogenize water column.Another interesting feature of the SCPs is the site-selectivity, meaning their occurrences off convex coastlines. The tidally induced upwelling, both in baroclinic and barotropic aspect, shed some lights on the physics of such site-selectivity. On the one hand, tidal currents tend to be particularly strong off the peninsula tips, resulting in high vertical mixing and, of course, strong TMF and the resultant frontal upwelling. On the other hand, the"centrifugal upwelling mechanism"suggested by Garrett and Loucks (1976) seems to be valid in the Yellow Sea. Numerical modeling indicates that in most cases, the RTC induced by barotropic M2 constituent flows onshore in the bottom layer and offshore in the upper layer, leading to upwelling concentrations off the convex coastlines.4. The detection of the upwelling off Hainan western (southwestern) coast A summertime upwelling region is newly detected off the western and southwestern coast of Hainan Island by applying the baroclinic TIU mechanism in the Gulf of Tonkin, and the existence of this upwelling has been partially verified by both remotely sensed SST climatology and the latest cruise observation.As a matter of fact, the prevailing wind in the Gulf of Tonkin is downwelling favorable in summer. At the same time, study also shows that the coastal waters off west and southwest Hainan are characterized by the maxima of both tidal current and tidal energy dissipation. The TMF-induced upwelling dominates the wind-driven downwelling in a climatological sense, accompanied by a climatologically stable low SST area. Without wind-driven downwelling, numerical modeling brings about approximately 20% increase of the upwelling.