The Nepheloid Layer Effect on the Bottom Boundary Layer and Sediment Transport in Coastal Seas

Significant Achievements, Anticipated Outcomes and Future Work

The effect of freshwater inflow on the development of spring blooms in the shallow estuarine embayment. Low vertical mixing rates are a key physical condition associated with the development of phytoplankton spring blooms in coastal and ocean waters. Vertical mixing rates in shallow coastal regimes are decreased not only by thermal stratification but also by haline stratification due to river runoff. In this paper, the main physical processes involved in the onset of phytoplankton spring blooms are examined in a tide-dominated estuarine embayment, characterized by weak current velocities, using a 1-d ecosystem model (the European Regional Seas Ecosystem Model) coupled with a 3-d physical model (the Princeton Ocean Model coupled with a sediment transport model) via the off-line method. Simulation results show that a reduction in vertical mixing, caused by the episodic input of buoyant, freshwater inflows from a reservoir during the period of neap tides, is the main physical controlling process on the occurrence of spring algal blooms. Furthermore, sensitivity tests using (i) layered and (ii) depth-averaged monthly vertical eddy diffusivity values reveal that the timing of phytoplankton spring blooms in the model is strongly affected by the parameterization of vertical diffusivity.

Inter-annual variability of the Gargano plume in the southern Adriatic Sea, Italy. The Adriatic Sea general circulation model (AIM) was implemented to study the Gargano bottom density plume in the Southern Adriatic Sea (SAS), Italy and its inter-annual variability. The model has been run with realistic wind stress, surface heat flux and river runoff forcings continuously for a period from 1 January 1999 to 31 December 2002. The study found that, the Gargano bottom density plume, typically as found in spring 2000, is a bottom density current driven by a perturbation in density as a result of successive Bora cooling events in the Northern Adriatic Sea (NAS) from previous autumn and winter. The density current advects cold and less saline NADdW (Northern Adriatic Deep Water) from the NAS to the SAS, and the propagation speed of the plume can reach 0.1 m s-1 with a downslope component in order of 0.05 m s-1. The southward mass and heat transport associated with the Gargano plume can be calculated to be 0.08 Sv and -30 W m-2 at a cross section near Gargano Peninsula, and was about 15% of the NADdW and heat content change produced in the previous winter. One of most important findings of this work is the inter-annual variability in the production of NADdW in the NAS, thus the formation of the Gargano bottom density plume in the SAS. The numerical study demonstrated that a sustained minimum heat loss of 175 W m-2 from November to January is required to produce adequate volume of NADdW thus density perturbation to form a density current that is strong enough to reach the Gargano Peninsula in the following spring. This minimum requirement of heat loss equates to 8 consecutive strong Bora events in this period. In addition to the autumn and winter surface cooling, summer preconditioning of the NAS density by the lower Po River runoff is another important factor in the Gargano plume generation and may cause its inter- annual variability. A wet summer results in large Po River runoff and decreases the NAS density. Thus additional heat loss is required for adequate NAdDW production in the following winter. Finally, the study also demonstrated that heat budget balance of the NAS during the winter season is between the surface heat loss, horizontal net heat inflow and resulting heat content decrease, and the horizontal advection played a minor role in the heat budget closure.

We are doing comparison between the Adriatic Sea tidal model results on a single processor and parallelized code for four processors. We found that there are some minor differences. Once we idetify and understand the significance of this difference, we will shift the code to APAC for further testing.

The future will see a continuation of our current projects, with a continued emphasis on the properties of both well- mixed and stratified oceanic bottom boundary layers and on the modelling of sediment distributions and fluxes in estuaries and coastal seas.

One aim for the future is to model the nepheloid layer dynamics in order to develop conceptual and numerical models that describe and predict the cross-shelf transport of fluidized sediments on continental shelves, and their roles in turbid marine ecosystems. This project, DESTINY-I, is at the forefront of bottom boundary layer and sediment transport research.

DESTINY-I (Design of an integrated coasts and river basin management system for the East China and Yellow Sea-I) has been approved by the Italian Ministry for Environment and Territory for funding 2006-2008 (Euro 3.5 million). The project will be formally announced during the first 2006 Sino-Italian Facility (SIF) Steering Committee meeting in Beijing, China, for Chinese in-kind co-financing (Euro 3.5 million). The aims of the project are to design and construct a marine observational-modelling-assimilation module, a river basin management module and a flood- forecasting module for China seas and their coasts.

Computational Techniques Used

The coastal sea sediment transport model is based on the three-dimensional Princeton Ocean Model, which solves the primitive equations for surface elevation, momentum, temperature and salinity variables.

ERSEM is a generic, biomass-based, ecosystem model, describing the biogeochemical processes occurring in the water column and sediments, as well as their interactions, in terms of the cycling of carbon, nitrogen, phosphorus and silicon within the marine ecosystem. The biological state variables are aggregated into functional groups according to their trophic level. In some cases the functional groups such as phytoplankton are subdivided.

Publications, Awards and External Funding

External Funding and Awards

The effect of the fluid-mud layer on ocean dynamics in coastal seas (with Prof J. Morrison) UNSW@ADFA RTS Scholarship Funding Scheme 2006-2008 $60,111

Nepheloid layers and their role in the coastal oceanography in the Changjiang (Yangsze) River estuary (with L.A. Wong) Endeavour Australia Cheung Kong Awards, Australian Department of Education, Science & Training 2006 $26,900

Publications

Wang, X.H., 2005, Circulation of the northern Adriatic Sea (Italy) due to a Bora event in January 2001: a numerical model study. Ocean Modelling, 10, 253-271.

Byun, D.S. and Wang, X.H. 2005, Numerical studies on the dynamics of tide and sediment transport in the western tip of the southwest coast, Korea. Journal of Geophysical Research, 110 (C03011), 10.1029/2004JC002459.

Wang, X.H., Byun, D.S., Wang, X.L. and Cho, Y.K., 2005, Modelling tidal currents in the sediment stratified coastal seas. Continental Shelf Research, 25, 655-665.

Byun, D.S., Wang, X.H., Hart, D. and Cho,Y.K. 2005, The effect of freshwater inflow on the development of spring blooms in the shallow estuarine embayment. Estuarine, Coastal and Shelf Science 65, 351-360.

Wang, X.H., Byun, D.S., Wang, X. and Cho, Y.K., 2005, On the changes of tidal characteristics due to sediment-induced stratification in a macro-tidal coastal sea. In Proceedings of 16th Biennial Australian Institute of Physics Congress, Canberra, Australia 30 January-4 February 2005, Extended Abstract, 17-20.

Malačič, V., Mozetič, P., 2005. Modeling of the spread of an effluent and the overturning length scale near an underwater source in the Northern Adriatic. Journal of Chemical Information and Modeling, 45, 6, 1491-1501.

Malacic, V. and B. Petelin, 2006. Winter circulation of the Gulf of Trieste (northern Adriatic). Acta Adriatica, in press.

Celio, M., Malacic, V., Cermelj B. and Comici C, 2006. The coastal scale observing system component of ADRICOSM: Gulf of Trieste, in press.

Malacic V., Celio M., Cermelj B., Bussani A., and C. Comici, 2006. Inter-annual evolution of seasonal thermohaline properties in the Gulf of Trieste (Northern Adriatic) 1991-2003. J. Geophys. Res. (C), in press.