We present the first results from a densely instrumented mooring array upstream of the Denmark Strait sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September 2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kg m −3 approaching the sill. The mean transport of overflow water over the length of the deployment was 3.54 ± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique based on the hydrographic properties of the water, the transports of these two sources are found to be 2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport * Corresponding author. is more consistent than the transport in either branch; compensation takes place among the pathways that maintains a stable total overflow transport. This is especially true for the two East Greenland Current branches whose transports vary out of phase with each other on weekly and longer time scales. We argue that wind forcing plays a role in this partitioning.
The global thermohaline circulation is an important part of Earth's climate system. Cold, dense water formed in the Nordic Seas enters the Atlantic Ocean as overflows across the sills of the Greenland‐Scotland Ridge. The Denmark Strait Overflow (DSO) is one of the main sources of North Atlantic Deep Water. Until now the DSO has been believed to be stable on interannual timescales. Here, for the first time, evidence is presented from a 4‐year program of observations showing that overflow transports in 1999/2000 were approximately 30% higher than previous estimates. Later, transports decreased remarkably during the observation period, coincident with a temporary temperature increase of about 0.5°C.
Tests of an unmanned airborne system (UAS) for surveys of marine mammals were conducted near Port Townsend, Washington. Sixteen surveys were conducted over a 10-d period to find 128 simulated whale targets (4 to 9 per survey). Various weather conditions were encountered, and searchwidths and altitudes were varied to establish optimal search parameters for future surveys. Logistic regression models were applied to estimate how detection rates were influenced by target color, degree of target inflation, shutter speed, searchwidth, and Beaufort wind force. Beaufort wind force was the strongest predictor of detection rates with color and degree of target inflation also included in the model that best fit these data. Overall detection rates of simulated large whale profiles using UASs were similar to published estimates of detection rates during manned aerial surveys for marine mammals, except the search area was much smaller (narrow strip width) when using the UAS. The best detection rates were obtained when Beaufort wind force was lowest (~ 2). The UAS tested showed promise for replacing manned aerial surveys for monitoring distribution and abundance of large marine mammals; however, improvements are required before the UAS would be an efficient tool for detection of all species. Side-by-side comparisons are needed between the UAS and manned aircraft to evaluate any differences in detection rates from the two platforms.
In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability, no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill.
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