Kathleen A. Donohue

Observations of the Subtropical Mode Water Evolution from the Kuroshio Extension System Study

Abstract Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σθ ≃ 25.5 kg m−3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of ...

Mapping Circulation in the Kuroshio Extension with an Array of Current and Pressure Recording Inverted Echo Sounders

Abstract The Kuroshio Extension System Study (KESS) aimed to quantify processes governing the variability of and the interaction between the Kuroshio Extension and the recirculation gyre. To meet this goal, a suite of instrumentation, including 43 inverted echo sounders equipped with bottom pressure gauges and current meters [current and pressure recording inverted echo sounders (CPIES)], was deployed. The array was centered on the first quasi-stationary meander crest and trough east of Japan, which is also the region of highest eddy kinetic energy. KESS was the first experiment to deploy a large quantity of these new CPIES instruments, and it was unique in that the instruments were deployed in water depths (5300–6400 m) close to their limit of operation. A comprehensive narrative of the methodology to produce mesoscale-resolving four-dimensional circulation fields of temperature, specific volume anomaly, and velocity from the KESS CPIES array is provided. In addition, an improved technique for removing p...

Wavenumber Spectrum in the Gulf Stream from Shipboard ADCP Observations and Comparison with Altimetry Measurements

The wavenumber spectra for velocity and temperature in the Gulf Stream region are calculated from a decade (1994‐2004) of shipboard acoustic Doppler current profiler (ADCP) measurements taken as part of the Oleander Project. The velocity and temperature spectra have comparable magnitude, in terms of the kinetic and potential energy, and both indicate a k 23 slope in the mesoscales. In contrast, the corresponding velocity spectrum determined from satellite altimetry sea surface heights yields a significantly higher energy level and a k 22 slope. The discrepancy between altimeter-derived and directly measured velocity spectra suggests that altimetric velocity probably is contaminated by noise in sea surface height measurement. Also, the k 23 slope, which appears to be in agreement with two-dimensional quasigeostrophic turbulence theory, does not support the contemporary surface quasigeostrophic theory. These results highlight large gaps in the current understanding of the nature of surface geostrophic turbulence.

Program Studies the Kuroshio Extension

The Kuroshio Extension system links to North Pacific climate through its role in subtropical-subpolar exchange, the formation and distribution of mode waters, and the intensification of the extratropical storm track across the North Pacific. The Kuroshio Extension System Study (KESS) offers a window into these processes through integrated measurements of the ocean and atmosphere and through modeling efforts (Figure 1). The northward flowing waters of the Kuroshio western boundary current leave the Japanese coast to flow eastward as a free jet—the Kuroshio Extension. The Extension forms a vigorously meandering boundary between the warm subtropical and cold northern waters.

On the variability of Gulf Stream transport from seasonal to decadal timescales

Given the Gulf Stream’s central role in the North Atlantic’s wind-driven and meridional overturning circulation (MOC), there is considerable interest in measuring mass and heat flux to sufficient accuracy that their variability can be quantified with some degree of confidence. Here we combine high-resolution direct measurements of upper ocean transport from the last 17 years of Oleander ADCP data with previously published estimates of baroclinic transport to examine Gulf Stream transport variability over the last 80 years just downstream of where the current separates from the U.S. east coast. By far the greatest source of variability occurs on short time scales related to the meandering of the current and energetic eddy field to either side such that the inherent uncertainty of a single transport estimate is 15% with respect to an annual mean. The annual cycle of layer transport at 55-m depth has a maximum increase of 4.3% of the mean in September while the annual cycle at 205 m reaches a maximum of only 1.5% in July. A running low-pass filter indicates transport variations of only a few percent of the mean on inter-annual and longer time scales although swings as large as 10–12% over a few years can occur. The length of the time series now reveals a significant correlation between the NAO index and near-surface transport in the Gulf Stream. No significant trend in transport can be detected from either the last 17 years of directly measured surface currents, or from hydrographic sections starting in the 1930’s. It follows therefore that the upper branch of the MOC, the other major component of Gulf Stream transport at the Oleander line, must have been quite stable over the last 80 years.

Comparison of three velocity sections of the Agulhas Current and Agulhas Undercurrent

Lowered acoustic Doppler current profiles (LADCP) from an early March 1995 cruise across the Agulhas Current show a swift, narrow undercurrent flowing northeast along the continental slope. Neither this Agulhas Undercurrent nor the adjacent deep extension of the Agulhas Current are evident from measurements of water properties alone, and their absence from the conventional referencing of geostrophic current estimates biases net southward transport estimates high by several sverdrups. Here we refine the original transport calculation by removing barotropic tides and by estimating instrumental and sampling errors. Two additional LADCP sections, from cruises in late March and June 1995, also show the undercurrent and the deep extension of the Agulhas. Differences in the current structure are evident. The Agulhas Current extends throughout the water column in March, but extends only to 2300 m depth in June. Additionally, the current extends further offshore in March. Of the three available LADCP sections, only those from early March and June have sufficient sampling to calculate the net southward transport of the Agulhas Current and Undercurrent. The two estimates, 78±3 and 76±2 Sv, are nearly identical. Consideration of water properties on density surfaces shows that although the undercurrent carries intermediate water with Red Sea Water influence northward, the bulk of this water mass is flowing southward, away from its source, in the Agulhas Current.

On the long‐term stability of Gulf Stream transport based on 20 years of direct measurements

In contrast to recent claims of a Gulf Stream slowdown, two decades of directly measured velocity across the current show no evidence of a decrease. Using a well-constrained definition of Gulf Stream width, the linear least square fit yields a mean surface layer transport of 1.35 × 105 m2 s−1 with a 0.13% negative trend per year. Assuming geostrophy, this corresponds to a mean cross-stream sea level difference of 1.17 m, with sea level decreasing 0.03 m over the 20 year period. This is not significant at the 95% confidence level, and it is a factor of 2–4 less than that alleged from accelerated sea level rise along the U.S. Coast north of Cape Hatteras. Part of the disparity can be traced to the spatial complexity of altimetric sea level trends over the same period.

Interannual variations in upper-ocean transport by the Gulf Stream and adjacent waters between New Jersey and Bermuda

Since the fall of 1992, an acoustic Doppler current profiler mounted on a freighter, the CMV Oleander, has been measuring upper-ocean currents between New Jersey and Bermuda on a weekly basis. The extensive database that results from the frequent, systematic, and sustained sampling enables the exploration of a number of questions regarding currents in the northwest Atlantic. This paper reports on interannual variations in transport in the Gulf Stream and adjacent waters. The repeat sampling greatly increases the ability to discern even rather subtle variations in near-surface transport and explore their possible causes. The transect is divided into three subregions: the Gulf Stream is defined by a high velocity core with an instantaneous width set by where the downstream component of velocity changes sign; the Slope Sea exists between the Gulf Stream and the continental shelfbreak; the Sargasso Sea lies between the Gulf Stream and Bermuda. These three regions exhibit quite different signatures of variability. Over the eleven years of operation to date annual averages of Gulf Stream transport have a standard deviation of 6% but a 23% peak-to-peak range. No discernable trend in transport is evident in the eleven-year record. The westward transport in the Slope and Sargasso seas can both vary by a factor two in magnitude but they have quite different temporal characteristics: the Slope Sea transport changes take place gradually whereas the Sargasso Sea exhibits much larger variations on shorter time scales. It is conjectured that the Slope Sea time scales are set by high-latitude buoyancy-related forcing, whereas the Sargasso Sea and Gulf Stream variability reflects tropical and subtropical mechanical forcing. The lateral position of the Gulf Stream exhibits a correlated behavior with westward transport in the Slope Sea. When Slope Sea transport increases, the Gulf Stream shifts to the south with a concomitant hint of increased Gulf Stream transport. The southward shift of the Gulf Stream may be part of a dynamical response to this increased circulation in the Slope Sea since the Slope Sea flow is blocked in the west by the Gulf Stream at Cape Hatteras suggesting that the path of the Gulf Stream is governed more by thermohalinethan wind-driven forcing. The fast time scales of transport in the stream, on the other hand, point to wind-driven forcing from the tropics and subtropics. Thus Gulf Stream position and transport would appear to be driven by quite different physical processes. 1. Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island, 02881, U.S.A. 2. Corresponding author. email: trossby@gso.uri.edu 3. Marine Sciences Research Center, State University of New York, Stony Brook, New York, 11794, U.S.A. Journal of Marine Research, 63, 203–226, 2005

Baroclinic Transport Time Series of the Antarctic Circumpolar Current Measured in Drake Passage

AbstractThe first multiyear continuous time series of Antarctic Circumpolar Current (ACC) baroclinic transport through Drake Passage measured by moored observations is presented. From 2007 to 2011, 19 current- and pressure-recording inverted echo sounders and 3 current-meter moorings were deployed in Drake Passage to monitor the transport during the cDrake experiment. Full-depth ACC baroclinic transport relative to the bottom has a mean strength of 127.7 ± 1.0 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) with a standard deviation of 8.1 Sv. Mean annual baroclinic transport is remarkably steady. About 65% of the baroclinic transport variance is associated with time periods shorter than 60 days with peaks at 20 and 55 days. Nearly 28% of apparent energy in the spectrum computed from transport subsampled at the 10-day repeat cycle of the Jason altimeter results from aliasing of high-frequency signals. Approximately 80% of the total baroclinic transport is carried by the Subantarctic Front and the Polar Front. Partition...

Mean Antarctic Circumpolar Current transport measured in Drake Passage

The Antarctic Circumpolar Current is an important component of the global climate system connecting the major ocean basins as it flows eastward around Antarctica, yet due to the paucity of data it remains unclear how much water is transported by the current. Between 2007 and 2011 flow through Drake Passage was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution. Annual mean near-bottom currents are remarkably stable from year to year. The mean depth-independent, or barotropic transport, determined from the near-bottom current meter records was 45.6 Sv with an uncertainty of 8.9 Sv. Summing the mean barotropic transport with the mean baroclinic transport relative to zero at the seafloor of 127.7 Sv gives a total transport through Drake Passage of 173.3 Sv. This new measurement is 30% larger than the canonical value often used as the benchmark for global circulation and climate models.