The bulk of Antarctic Bottom Water flowing northward into the abyssal North Pacific goes through the Samoan Passage, making it a key "choke point" in the overturning circulation of the Pacific. Detailed current meter and hydrographic observations confirm a strong northward flow of 6.0 Sv. In addition, measurements are suggestive of extremely strong mixing (Roemmich et al, 1996) and hydraulic control (Whitehead, 1998; Freeland, 2001). However, many important questions remain unanswered, including the degree of hydraulic control of the flow, the strength of the mixing and the processes leading to it, the role of tidal and near-inertial flows in modulating the flow and the mixing, and the temporal variability of the transport.
A better understanding of each of these is vital to our understanding of the large-scale circulation in the Pacific, as well as to our general understanding of flow through deep passages. We therefore propose a coordinated observational and modeling program to measure and understand flow and mixing through the Samoan Passage, which includes the following elements:
- Two cruises, each including a 25-day focused mooring array with profiling instrumentation and a ship survey with lowered ADCP/CTD and microstructure profilers to resolve the currents and mixing in time and space. The cruises will be separated by 18 months to enable a long monitoring timeseries and a sharper focus in the second cruise based on the analysis of data from the first.
- An 18-month monitoring mooring array deployed between the cruises at the same location as that done 20 years prior (Rudnick, 1997) in order to obtain another direct estimate of the properties and transport through the passage.
- A hierarchy of models of differing complexity and realism, ranging from a 2-D MITgcm configuration used for internal wave and exchange flow problems through regional 3-D runs to evaluate downstream overflow evolution, rotational effects and the influence of remote internal wave sources. Together, the models will bring a larger-scale context to the observations and allow testing of hypotheses and parameter dependencies.
- Detailed multibeam bathymetric mapping of the passage.
The abyssal circulation is expected to influence the ocean's response to climate change and atmospheric carbon dioxide increases on the longest timescales, and the Samoan Passage is clearly a weak point in most large scale ocean models. This work directly addresses the processes that these models will need to simulate in order to improve their predictive capability. In addition, the community recognizes a need for extended monitoring of the Samoan Passage (Gries et al, 2010, Garzoli et al, 2010), in like manner to ongoing efforts in the Atlantic (Line W, RAPID), in order to assess decadal changes to the Pacific overturning circulation. The proposed measurements and modeling are intended to determine the most efficient configuration for such a monitoring array.
A variety of outreach activities are planned in conjunction with the cruises, including documentary filming and exhibits at local science centers. The project will educate a postdoc and graduate student in the US, as well as 4 summer undergraduate fellows. In addition, the inclusion of Canadian and Australian scientists and students in the work will support international collaboration.