Frank Bahr

Massive phytoplankton blooms under Arctic Sea ice

In midsummer, diatoms have taken advantage of thinning ice cover to feed in nutrient-rich waters. Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

Interaction of a Slope Eddy with the Shelfbreak Front in the Middle Atlantic Bight

Abstract Spring conditions at the shelf break in the Middle Atlantic Bight mark the transition period between the generally well-mixed shelf water in winter and the highly stratified shelf conditions during summer. A high-resolution hydrographic survey made during early May 1996 is used to describe the thermohaline and velocity structure of the shelfbreak front. The front was strongly affected by the presence of a slope eddy immediately offshore of the front. The eddy, which had a diameter of 25 km, was anticyclonic with onshore/offshore flows of 0.2 m s−1 on opposing sides. On the western side of the eddy, where the flow seaward of the front was predominantly onshore, the front was very steep and the frontal jet was particularly strong, with maximum near-surface velocities of 0.5 m s−1. On the eastern side of the eddy, the front was drawn offshore and was much less steep, with near-surface velocities of only 0.2 m s−1. A surprising feature was the presence of a second jet over the foot of the front, shor...

Hydrographic structure and transport of the Oyashio south of Hokkaido and the formation of North Pacific Intermediate Water

Hydrographic structure and transport of the Oyashio south of Hokkaido were described with conductivity-temperature-depth and lowered acoustic Doppler current profiler (LADCP) survey performed in June 1998. The southwestward Oyashio transport just off the Hokkaido coast was 10.1 Sv in the density of 26.6–27.5σθ, in which 2.5 Sv was from the Okhotsk Sea and 7.6 Sv was from the Western Subarctic Gyre (WSAG). The Oyashio northeastward countercurrent was 4.9 Sv. The cross-gyre Oyashio transport in the area from the east coast of Hokkaido to the Subarctic Front was estimated to be 5.2 Sv; 2.4 Sv in 26.6–27.0σθ was mainly composed of low potential vorticity Okhotsk Sea water and 2.8 Sv in 27.0–27.5σθ mostly from WSAG, suggesting that the Okhotsk Sea water (WSAG water) would contribute to the formation of the upper (lower) part of the North Pacific Intermediate Water (NPIW). The Oyashio water was lower in oxygen than in the subtropical areas south of the Subarctic Front in the density of 26.9–27.6σθ, possibly because of the absence of the Antarctic Intermediate Water (AAIW) influence with the relatively high oxygen water that might be transported along the western boundary of the North Pacific Ocean and along the Kuroshio Extension, increasing the oxygen in the areas south of the Subarctic Front. Just south of the Subarctic Front, a cold- and fresh-core anticyclonic eddy was observed with a salinity minimum in the core, suggesting one possible formation process of NPIW. The density of the potential vorticity minimum in the southwestward Oyashio near Hokkaido was at around 26.65σθ, which was lower than 26.8–26.9σθ observed in the early 1990s. This is possibly because the potential vorticity vertical profiles in the Okhotsk Sea and WSAG significantly changed corresponding to the water mass regime shift occurred in the Subarctic Pacific in the mid-1990s [Kawasaki, 1999].

A large-amplitude meander of the shelfbreak front during summer south of New England : observations from the Shelfbreak PRIMER experiment

[1] In order to examine spatial and temporal variability of the shelfbreak front during peak stratification, repeated surveys using a towed undulating vehicle (SeaSoar) are used to describe the evolution of shelfbreak frontal structure during 26 July to 1 August 1996 south of New England. Spatial correlation (e-folding) scales for the upper 60 m of the water column were generally between 8 and 15 km for temperature, salinity, and velocity. Temporal correlation scales were about 1 day. The frontal variability was dominated by the passage of a westward propagating meander that had a wavelength of 40 km, a propagation speed of 0.11 m s -1 , and an amplitude of 15 km (30 km from crest to trough). Along-front geostrophic velocities (referenced to a shipboard acoustic Doppler current profilers) were as large as 0.45 m s -1 , although subject to significant along-front variations. The relative vorticity within the jet was large, with a maximum 0.6 of the local value of the Coriolis parameter. Seaward of the front, a small detached eddy consisting of shelf water was present with a diameter of approximately 15 km. Ageostrophic contributions to the velocity field are estimated to be as large as 0.3 m s -1 in regions of sharp curvature within the meander. These observations strongly suggest that during at least some time periods, shelfbreak exchange is nonlinear (large Rossby number) and dominated by features on a horizontal scale of order 10 km.

Meso- to Large-Scale Structure of Subducting Water in the Subtropical Gyre of the Eastern North Atlantic Ocean

Abstract As part of an observational study of ocean subduction, SeaSoar surveys were carried out following subducting water tagged by bobber floats during their deployment in the spring of 1991 and again in the fall of 1992 and spring of 1993. In the latter two cases, real-time tracking limitations made it difficult to accurately locate bobbers, so the focus was on SeaSoaring over a few of the floats that were deployed in the shallow, near-surface layer initially. A total of 13 surveys were made over the course of the experiment. They were designed to map the mesoscale (in the meteorological sense) or submesoscale, as the term is generally used in oceanography. In addition, opportunity was taken to acquire other hydrographic data collocated with the floats in order to fill in the gaps in the above temporal sampling. The density mode locally found in the northeastern part of the North Atlantic subtropical gyre is Madeira mode water, with a mode near a potential density of 26.5. Results indicate that follow...

Mixing in the meandering Kuroshio Extension and the formation of North Pacific Intermediate Water

Synoptic hydrographic and velocity data were obtained in the first meander crest and trough of the Kuroshio Extension (KE) from measurements aboard the R/V Soyo-maru in late May and early June 1998. Measurements indicated that the eastward flowing KE had significant (up to 20 cm s−1) flow at 1400 m depth and that a subsurface velocity maximum could be seen on the equatorward side of the current. Large salinity differences were found on potential densities from 26.2 to 27.2 kg m−3 that reflected cold, fresh waters of northern origin and warm, salty subtropical waters. Approaching the first meander crest, northward moving water was saltier than southward moving water at the same density on the downstream side of the crest, approaching the first trough. The velocity-salinity covariance was high and consistent with an eddy diffusivity of order 104 m2 s−2. With an assumption of quasi-steady meanders, supported by altimeter results, density advection can be used to infer vertical velocities of 50–100 m d−1 in the meanders. Ascending water was found to be saltier than the descending water, especially in a potential density range about 26.8 kg m−3 within the core of North Pacific Intermediate Water (NPIW)). However, the two patterns of horizontal velocity-salinity covariance and vertical velocity-salinity covariance were not consistent with one another. The mixing of Oyashio and Kuroshio source waters, thought to be critical to the formation of NPIW, is clearly tied to cross-frontal flows within the meandering KE, with Kuroshio water ejected northward as water parcels approach the first meander crest and Oyashio water injected southward into the flow as parcels approach the first trough.

Shelfbreak circulation and thermohaline structure in the northern south China Sea-contrasting spring conditions in 2000 and 2001

Two intensive, high-resolution hydrographic surveys during April 2000 and May 2001 are used to characterize the thermohaline and current structure at the shelfbreak in the South China Sea. In 2000, a strong anticyclonic circulation was present in the northern portion of the South China Sea with strong onshore currents east of Dongsha Island. The flow became polarized along isobaths as it encountered shallow water, with northeastward flows of over 0.9 m/s along steep topography. The flow was driven by strong density contrasts between waters of the outer shelf and upper slope. Shelf water was both cooler and more fresh than the water offshore, which had salinities close to that of Kuroshio water. In contrast, the mean flow in the northern South China Sea was predominantly cyclonic in 2001. Flow over the slope was to the southwest at up to 0.2 m/s. The water mass properties of the outer shelf and upper slope were similar, so that there were not the strong cross-shelf density gradients present as in 2000. A potential difference between the water mass structure of the two years was the difference in cooling during the preceding winters. In December, 1999, unusually strong cooling may have resulted in cooler shelf waters relative to the following year. The ASIAEX study area may be a particularly sensitive region to both seasonal and interannual variability, as it is near a bifurcation point associated with the Kuroshio Intrusion into the South China Sea.

Alongshore currents and mesoscale variability near the shelf edge off northwestern Australia

[1] Shelf break conditions and alongshore flow off northwestern Australia are studied during the strongly evaporative conditions of austral winter 2003. Present results, along with those of previous authors, confirm that a poleward, fresh Leeuwin current core is normally found near the shelf break. Salinity increases alongshore toward the southwest. Although there is no obvious shelf break front, there is a persistent offshore upward tilting of isopycnals in the depth range of 100–150 m. Repeated mesoscale surveys were made at the shelf edge during 8 days when the shelf break flow was equatorward. Waters offshore of the shelf break contain a rich baroclinic ageostrophic eddy field, with typically 10 km length scales, and the eddy patterns in the upper 60 m are uncorrelated with those below 90 m. The two depth horizons yield energy transfers from mean to eddy potential energy and appear to represent distinct finite amplitude instabilities on upper ocean and 180 m mean velocity cores, respectively. A linear stability model supports the existence of the two vertically separated instability types.

Seasonal variation of the Beaufort shelfbreak jet and its relationship to Arctic cetacean occurrence

Using mooring time series from September 2008 to August 2012, together with ancillary atmospheric and satellite data sets, we quantify the seasonal variations of the shelfbreak jet in the Alaskan Beaufort Sea and explore connections to the occurrences of bowhead and beluga whales. Wind patterns during the 4 year study period are different from the long-term climatological conditions that the springtime peak in easterly winds shifted from May to June and the autumn peak was limited to October instead of extending farther into the fall. These changes were primarily due to the behavior of the two regional atmospheric centers of action, the Aleutian Low and Beaufort High. The volume transport of the shelfbreak jet, which peaks in the summer, was decomposed into a background (weak wind) component and a wind-driven component. The wind-driven component is correlated to the Pt. Barrow, AK alongcoast wind speed record although a more accurate prediction is obtained when considering the ice thickness at the mooring site. An upwelling index reveals that wind-driven upwelling is enhanced in June and October when storms are stronger and longer-lasting. The seasonal variation of Arctic cetacean occurrence is dominated by the eastward migration in spring, dictated by pack-ice patterns, and westward migration in fall, coincident with the autumn peak in shelfbreak upwelling intensity.

Acoustics and oceanographic observations collected during the QPE Experiment by Research Vessels OR1, OR2 and OR3 in the East China Sea in the Summer of 2009

Abstract : This document describes data, sensors, and other useful information pertaining to the ONR sponsored QPE field program to quantify, predict and exploit uncertainty in observations and prediction of sound propagation. This experiment was a joint operation between Taiwanese and U.S. researchers to measure and assess uncertainty of predictions of acoustic transmission loss and ambient noise, and to observe the physical oceanography and geology that are necessary to improve their predictability. This work was performed over the continental shelf and slope northeast of Taiwan at two sites: one that was a relatively flat, homogeneous shelf region and a more complex geological site just shoreward of the shelfbreak that was influenced by the proximity of the Kuroshio Current. Environmental moorings and ADCP moorings were deployed and a shipboard SeaSoar vehicle was used to measure environmental spatial structure. In addition, multiple bottom moored receivers and a horizontal hy drophone array were deployed to sample transmission loss from a mobile source and ambient noise. The acoustic sensors, environmental sensors, shipboard resources, and experiment design, and their data, are presented and described in this technical report.