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Samoan Passage: results

Mapping cruise 2011

The results of a multi-beam echosounder survey carried out in November 2011 with R/V Kilo Moana provide us an updated and extended high resolution version of the Samoan Passage bottom topography. Parts of the Samoan Passage bathymetry were measured with a multi-beam by Rudnick (1997), but due to instrument failure some areas were only covered with single-beam measurements in this survey.

The 2011 echosoundings agree with the Rudnick multi-beam data in the overlapping regions. A comparison between multi-beam, single-beam and the newly available Smith & Sandwell bathymetry with a grid resolution of half a minute shows that the latter is the better choice over the single beam data for the areas not covered by the two multi-beam measurement.

Figure 1: Bottom depths within the red encircled area were gathered during the 2011 mapping cruise. The straight white line gives the region of the Rudnick (1997) multi-beam survey, the dashed white line shows the area measured by Rudnick with a single beam due to instrument failure. Remaining areas show the bottom topography as given by the newly available Smith & Sandwell bathymetry dataset with a grid resolution of 30 seconds.

Comparison between multi-beam echosounder data from 2011, Rudnick (1997) bathymetry and Smith & Sandwell bathymetry.

Figure 2: Bottom topography along 9.0°S and 9.5°S from Smith & Sandwell 30'' (blue), Rudnick (1997) multi- and single-beam echosoundings (black), and recent multi-beam echosoundings from November 2011.

Pathways cruise 2012

The "Pathways" cruise in summer 2012 was a great success! We measured the flow at many locations and in different ways: With moorings anchored to the seafloor for the duration of our cruise, from the ship while being on a fixed position and even from the moving ship by towing our instruments behind us - at 5 km depth! Glenn Carter's vertical microstructure profiler went autonomously close to the seafloor, measuring turbulence on it's way, and we had to catch it when it came back to the surface.

The information about the bottom topography gained during the mapping cruise provided extremely helpful guidance in selecting the spots that we measured. And we kept on using the multi-beam during this cruise, too, so now we mapped out almost the entire region in great detail.

The following figures show some of the data that we collected during the "Pathways" cruise along with first results of our analysis.

Figure 3: Upper Panel: Location of stations along the eastern channel. Lower Panel: Potential temperature along the main channel in the east. Cold water enters the channel from the south. As it flows northwards it decends to greater depths, especially after the second sill. A lot of mixing happens and no water colder than 0.7°C exits the channel towards the north. Since the Samoan Passage carries the majority of the waters feeding the deep parts of the North Pacific, this mixing affects the bottom water temperatures of a whole big ocean basin.

Figure 4: Northward Velocity (colors) and dissipation of turbulent kinetic energy (black & blue shaded profiles) along the eastern channel. The flow accelerates at both of the two sills - at the northern constriction it reaches velocities of about 0.5 m/s as it descends to greater depths. The black profiles show the strength of the mixing measured by Glenn's vertical microstructure profiler. While in the temperature data we con only conclude that there must be mixing going, here we measure it directly.

Figure 5: Transport per unit width in temperature classes at each station where we measured the temperature and velocity of the water from the ship. The left figure shows the flow of the coldest water. Almost all of it takes it's way through the main channel in the east. None of this water exits the channel - this is because it is mixed with warmer water along it's way and thereby changes it's temperature to more than 0.7°C. It then shows up in the middle figure in the warmer temperature class as it exits the channel. Water of this temperature is also flowing through the smaller channels in the western region - more than we expected to find! The warmest waters shown here in the right panel are relatively quiescent as there is not much volume transport in this temperature class.