OS41C-01 08:30h
Tidal Surface Currents Observed by High-Frequency Radars in the Kauai Channel, and Implications for Internal Tide Generation.
Two high-frequency radars were deployed on the west shore of Oahu, Hawaii, from September 2002 to May 2003, as a component of the Hawaii Ocean Mixing Experiment (HOME). Surface currents were obtained every 20 to 30 minutes during the 9-month period, up to 100 km offshore with a spatial resolution of 2 km. The observation area lies just south of the Kauai channel ridge, an internal tide generation hot spot. The main tidal constituents were extracted at each grid point by least-square fit. The resulting current ellipses parameters for the M2 constituent are in good agreement with numerical model results over most of the domain, although the modeled amplitudes are generally higher than the observed. Fourier analysis of the detided observed currents reveal that a significant portion of energy remains in narrow bands around the main tidal frequencies. Mesoscale currents and stratification variability, which are not taken into account in the model, are thought to be responsible for this non-coherent tidal energy, which must be taken into account in the global estimation of energy conversion from barotropic to baroclinic tides.
OS41C-02 08:45h
How Water Crosses the Equatorial Vorticity Barrier in the Eastern Pacific
Cross equatorial exchange of water parcels involves a change in sign of Coriolis parameter and therefore also of potential vorticity. This implies a potential vorticity barrier at the equator. However, Sverdrup theory allows for breakage of this barrier, through the curl of the wind stress; an OGCM also shows breakage in the East Pacific, but very different in magnitude and structure from the pure Sverdrup relation. Clearly then, some element of friction or forcing must be added to the water parcels and Kessler et. al. (2003) have shown that the curl of the nonlinear and friction terms do become significant in this region. Despite considerable efforts in collecting observations over this region, little can be ascertained as to meridional flow due to the large variability in the region (Johnson, McPhaden and Firing 2001). In particular, variability in Tropical Instability Waves is an order of magnitude greater than the mean flow. Models suggest however that this cross hemispheric flow occurs and some (eg. Fukumori 2004) suggest that water parcels are able to cross the equator due to some mechanism associated with intra-annual variations. We discuss the role of windstress, friction and nonlinearity in this region, on various timescales, and from phenomena such as Tropical Instability Waves. These appear to cause sufficient changes to potential vorticity, allowing cross equatorial flow.
OS41C-03 09:00h
Transmission of Equatorial Waves Through a Strait
Numerical and analytical solutions are presented for the transmission of equatorial Rossby and Kelvin waves through a narrow strait penetrating a meridional barrier. Regardless of the number of free Rossby modes available in the basin, a controlling factor is shown to be simply the interference of Kelvin waves within the strait.
OS41C-04 09:15h
Eddies Around Fiji and French Polynesia Islands: Implication for Phytoplankton Growth
Recent studies have shown the importance of eddies in supplying nutrients to the surface water where photosynthesis can be possible. The role of eddies is even more crucial for the areas like the South Pacific Subtropical Gyre where surface water are poor in nutrients. The existence of these eddies can be due to different processes. Among them, the Rossby waves generated in the east of the South Pacific Ocean propagate westward and eddies also result from the interaction of the mean flow with the chains of seamounts and islands. Using approximately 10 years of T/P-ERS altimetry data, we analyse the eddy field around Fiji and French Polynesia islands. It is found that the eddy shedding is dependent on the strength of the mean flow and is strongly perturbed by the 1997/98 El Nino-La Nina event. Implication of these eddies for the phytoplankton growth is discussed.
OS41C-05 09:30h
Ocean Circulation Around Fiji Islands
Ocean circulation around Fiji Islands in the spatial domain of 10$^\circ$S to 25$^\circ$S latitude and 170$^\circ$E to 190$^\circ$E longitude is being investigated using Topex / Poseidon and ERS 1/2 derived sea surface height anomaly (SSHA) data fields from October, 1992 to August, 2002 and available ARGO floats data. The variability of the circulation is diagnosed with the Empirical Orthogonal Functions. The first 4 modes encompass 57% of the variability. The first and the second modes are dominated by the ENSO signal. The seasonal signal is only visible on modes 3 and 4. Geostrophic velocities were calculated combining SSHA and the Levitus climatological data. The geostrophic flow pattern shows that during ENSO cycle, the characteristic flow pattern is the major westward (eastward) flow in the northern (southern) part of the study region. During the La Nina phase, the flow pattern in the northern part of the study region, however, reverses and that in the southern part broadly remains eastward. On the seasonal time-scale, the dominant geostrophic flow in the northern part of the study region during the months of early summer is westward, unlike the eastward flow during early months of winter. In the southern part of the region, the major flow is eastward regardless of winter and summer conditions. The variability of this flow pattern on seasonal and inter-annual time scale was related to the steric changes caused by the movement of western equatorial Pacific `warm pool' and the associated movement of the South Pacific Convergence Zone (SPCZ). The ocean circulation around Fiji is also investigated using ARGO float data and compare with the altimetry derived circulation.
OS41C-06 09:45h
Circulation of the Suva Lagoon in the Fiji Islands
A 3-D hydrodynamic model has been used to determine the water circulation in the Suva lagoon of Fiji Islands. The hydrodynamic equations in this model use the Boussinesq and hydrostatic approximations and are based on the alternating direction implicit method (ADI) and the finite difference scheme for time and space discretisation. The -coordinate system is used to represent the position on the vertical grid. The study area covers 500 km2 and has average depth of 25 m. The effects of the M2, S2, K1, O1 tidal constituents, wind and major river-runoff are included in the model. The model is verified using field data collected in Laucala Bay. The verification procedure involves comparing salinity profile measurements and model output. Field data shows highly turbulent salinity in shallow waters, the verification results indicate that the model predictions are more accurate for depths greater than 15 meters. The results show the river-runoff plays a dominant role in determining the physical water properties and current circulation. The salinity for the entire lagoon water column decreases during the rainy season while during dry season only areas closer to the river mouth are influenced. The wind induced vertical mixing produce homogeneous water columns within 1km from shore along the coastline while in deeper areas of the lagoon, only the upper 6m is affected significantly. The model prediction for high-tide + 1 hr shows formation of two circulation layers. The surface layer of the river flows seaward, entering Rewa Delta and Vunidawa River. The Vunidawa river runoff enters Laucala Bay, flowing along the coast towards Nukubuco passage. The bulk of the water in Laucala Bay exits through the Nukubuco passage. The water flowing along the coast flows through the Nasese channel to enter into Suva harbour subsequently going out through the Suva passage. The current speed for most of Laucala Bay is $\sim$ 15 cm s-1 while the current speed in the harbour is mostly below 10 cm s-1. A key occurrence noted during the field measurements was calmer water in the harbour than wind exposed Laucala Bay. This is clearly simulated by model showing stronger currents in surface currents in Laucala Bay. The circulation in the lower layer moves in opposition to the surface layer circulation. The water enters through Nukubuco passage flows Northeast then turns counterclockwise and flows along the coast and through Nasese channel. Similarly for Suva passage, the water enters into the harbour in the bottom layer, flowing into Nasese channel. The bottom current speeds range from 5-11 cm s-1 for Laucala Bay and less than 5 cm s-1 for the harbor. Results for the ebbing tide show that the bulk of the surface water flows seaward through the Nukubuco passage and the Nasese channel, whereas the bottom water layer flows landward, primarily entering through the Nukubuco passage and the Suva passage.