John M. Toole

Beaufort Gyre freshwater reservoir: State and variability from observations

[1] We investigate basin-scale mechanisms regulating anomalies in freshwater content (FWC) in the Beaufort Gyre (BG) of the Arctic Ocean using historical observations and data collected in 2003–2007. Specifically, the mean annual cycle and interannual and decadal FWC variability are explored. The major cause of the large FWC in the BG is the process of Ekman pumping (EP) due to the Arctic High anticyclonic circulation centered in the BG. The mean seasonal cycle of liquid FWC is a result of interplay between the mechanical (EP) and thermal (ice transformations) factors and has two peaks. One peak occurs around June–July when the sea ice thickness reaches its minimum (maximum ice melt). The second maximum is observed in November–January when wind curl is strongest (maximum EP) and the salt input from the growing ice has not yet reached its maximum. Interannual changes in FWC during 2003–2007 are characterized by a strong positive trend in the region varying by location with a maximum of approximately 170 cm a � 1 in the center of EP influenced region. Decadal FWC variability in the period 1950–2000 is dominated by a significant change in the 1990s forced by an atmospheric circulation regime change. The center of maximum FWC shifted to the southeast and appeared to contract in area relative to the pre-1990s climatology. In spite of the areal reduction, the spatially integrated FWC increased by over 1000 km 3 relative to climatology.

Finescale parameterizations of turbulent dissipation

Fine- and microstructure data from a free fall profiler are analyzed to test models that relate the turbulent dissipation rate (e) to characteristics of the internal wave field. The data were obtained from several distinct been previously available. Observations from the ocean interior with negligible large-scale flow were examined to address the buoyancy scaling of e. These data exhibited a factor of 140 range in squared buoyancy frequency (N 2 ) with depth and uniform internal wave characteristics, consistent with the Garrett-Munk spectrum. The magnitude of e and its variation with N(∼N 2 ) was best described by the dynamical model of Henyey et al. A second dynamical model, by McComas and Muller, predicted an appropriate buoyancy scaling but overestimated the observed dissipation rates. Two kinematical dissipation parametrizations predicted buoyancy scalings of N 3/2 ; these are shown to be inconscient with the observations. Data from wave fields that depart from the canonical GM description are also examined an interpreted with reference to the dynamical models. The measurements came from a warm core ring dominated by strong near-inertial shears, a region of steep topography exhibiting high-frequency internal wave characteristics, and a midocean regime dominated at large wavelengths by an internal tide. Of the dissipation predictions examined, those of the Henyey et al. model in which eN − 2 scales as E 2 , where E is the nondimensional spectral shear level, were most consistent with observations. Nevertheless, the predictions for these cases exhibited departures from the observations by more than an order of magnitude. For the present data, these discrepancies appeared most sensitive to the distribution of internal wave frequency, inferred here from the ratio of shear spectral level to that for strain. Application of a frequency-based correction to the Henyey et al. model returned dissipation values consistent with observed estimates to within a factor of 2. These results indicate that the kinetic energy dissipation rate (and attendant turbulent mixing) is small for the background Garrett and Munk internal wave conditions (0.25eN −2 ∼ 0.7 × 10 − 5 m 2 s − 1). Dissipation and mixing become large when wave shear spectral levels are elevated, particularly by high-frequency waves. Thus, internal wave reflection/generation at steep topographic features appear promising candidates for achieving enhanced dissipation and strong diapycnal mixing in the deep ocean that appears required by box models and advection-diffusion balances

Estimates of Diapycnal Mixing in the Abyssal Ocean

Profiles of diapycnal eddy diffusivity to a maximum depth of 4000 meters were derived from ocean velocity and temperature microstructure data obtained in conjunction with separate experiments in the Northeast Pacific and Northeast Atlantic oceans. These profiles indicate that in the ocean interior where the internal wave field is at background intensity, the diapycnal eddy diffusivity is small (on the order of 0.1 x 10–4 meters squared per second) and independent of depth, in apparent contradiction with large-scale budget studies. Enhanced dissipation is observed in regions of elevated internal wave energy, particularly near steeply sloping boundaries (where the eddy diffusivity estimates exceed 1 x 10–4 meters squared per second). These results suggest that basin-averaged mixing rates may be dominated by processes occurring near the ocean boundaries.

A hydrographic section across the subtropical South Indian Ocean

Features of the water-property and circulation fields at the southern limit of the continentally bounded Indian Ocean are described on the basis of a transoceanic hydrographic section occupied along roughly Lat. 32°S by the R.R.S. Charles Darwin in November-December 1987. Primary observations consisted of 106 full-depth CTD/O2 stations with discrete measurements of the concentrations of dissolved silica, phosphate and nitrate. The section lies in the southern part of the South Indian subtropical gyre; water-property features in the upper kilometer indicate that the northward interior flow is predominantly in the eastern half of the ocean there, consistent with the forcing pattern of wind-stress curl. The southward return flow is the Agulhas Current, whose transport at Lats 31–32°S is estimated as 85 × 106 m3 s−1. Circumpolar Deep Water flows northward to fill the greater deep Indian Ocean by means of western-boundary currents in the Crozet Basin, Central Indian Basin and Perth Basin. North Atlantic Deep Water entering directly from the mid-latitude South Atlantic is almost entirely confined to the south-western Indian Ocean (Mozambique Basin, Natal Valley) by the topography of the Madagascar Ridge and Mozambique Channel. Geostrophic transport figures are presented based on a zero-velocity surface constructed along the section from the tracer-property evidence of where deep water was moving northward and where southward. Ekman transport, deduced from shipboard acoustic-Doppler profiler measurements, as well as synoptic and historical wind stress data, is found to be small (about 1 × 106 m3 s−1 northward). Net transport (geostrophic and Ekman) across the section is estimated to be 7 × 106 m3 s−1 southward, which implies a similarly sized Indonesia throughflow. Ambiquity in the geostrophic referencing scheme, and the magnitude of baroclinic eddy noise on the section, suggest this figure in uncertain by at least ±10 × 106mm3 s−1. The calculations obtain a figure for net transport of water below 2000 dbars of 27 × 106 m3 s−1 northward, which specifies an average upwelling speed at the 2-km level north of 30°S of 6.9 × 10−5 cm s−1. This estimate, perhaps uncertain by 20–30%, nonetheless contributes to growing evidence for an anomalously vigorous meridional circulation in the Indian Ocean. The associated calculations of heat and fresh water flux divergences demonstrate that the Indian Ocean thermohaline circulation essentially expresses a conversion of bottom and deep water to mid-depth thermocline, and near-surface water.

Arctic Ocean Warming Contributes to Reduced Polar Ice Cap

Analysis of modern and historical observations demonstrates that the temperature of the intermediate-depth (150–900 m) Atlantic water (AW) of the Arctic Ocean has increased in recent decades. The AW warming has been uneven in time; a local 1°C maximum was observed in the mid-1990s, followed by an intervening minimum and an additional warming that culminated in 2007 with temperatures higher than in the 1990s by 0.24°C. Relative to climatology from all data prior to 1999, the most extreme 2007 temperature anomalies of up to 1°C and higher were observed in the Eurasian and Makarov Basins. The AW warming was associated with a substantial (up to 75–90 m) shoaling of the upper AW boundary in the central Arctic Ocean and weakening of the Eurasian Basin upper-ocean stratification. Taken together, these observations suggest that the changes in the Eurasian Basin facilitated greater upward transfer of AW heat to the ocean surface layer. Available limited observations and results from a 1D ocean column model support this surmised upward spread of AW heat through the Eurasian Basin halocline. Experiments with a 3D coupled ice–ocean model in turn suggest a loss of 28–35 cm of ice thickness after 50 yr in response to the 0.5 W m−2 increase in AW ocean heat flux suggested by the 1D model. This amount of thinning is comparable to the 29 cm of ice thickness loss due to local atmospheric thermodynamic forcing estimated from observations of fast-ice thickness decline. The implication is that AW warming helped precondition the polar ice cap for the extreme ice loss observed in recent years.

Internal Tide Reflection and Turbulent Mixing on the Continental Slope

Abstract Observations of turbulence, internal waves, and subinertial flow were made over a steep, corrugated continental slope off Virginia during May–June 1998. At semidiurnal frequencies, a convergence of low-mode, onshore energy flux is approximately balanced by a divergence of high-wavenumber offshore energy flux. This conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide. It is suggested that elevated near-bottom mixing (Kρ ∼ 10−3 m2 s−1) observed offshore of the supercritical continental slope arises from the reflection of a remotely generated, low-mode, M2 internal tide. Based on the observed turbulent kinetic energy dissipation rate ϵ, the high-wavenumber internal tide decays on time scales O(1 day). No evidence for internal lee wave generation by flow over the slopes corrugations or internal tide generation at the shelf break was found at this site.

Buoyancy Forcing by Turbulence above Rough Topography in the Abyssal Brazil Basin

Abstract Observations of turbulent dissipation above rough bathymetry in the abyssal Brazil Basin are presented. Relative to regions with smooth bathymetry, dissipation is markedly enhanced above rough topography of the Mid-Atlantic Ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors. Furthermore, mixing levels in rough areas are modulated by the spring–neap tidal cycle. Internal waves generated by barotropic tidal flow over topography are the likely mechanism for supplying the energy needed to support the observed turbulent dissipation. A model of the spatial and temporal patterns in the turbulent dissipation rate is used to constrain the diapycnal advection in an inverse calculation for the circulation in an area of rough bathymetry. This inverse model uses both beta-spiral and integrated forms of the advective budgets for heat, mass, and vorticity, and contains sufficient information to resolve the full three-dimensional flow. The inverse model solution re...

Observations of the Pacific North Equatorial Current Bifurcation at the Philippine Coast

Abstract Hydrographic surveys were conducted off the Philippine coast in September 1987 and April 1988 as part of the United States/Peoples Republic of China cooperative research program. These cruises sampled the western Pacific Ocean where the North Equatorial Current (NEC) meets the western boundary and divides into the Kuroshio and Mindanao Currents. The requirement for mass conservation within a region enclosed by stations is utilized here to obtain absolute circulation fields for the two surveys. In both realizations, the surface flow of the NEC was observed to bifurcate near latitude 13°N; NEC flow poleward of this latitude turned north as the Kuroshio while flow to the south fed the Mindanao Current. Most striking was a twofold increase in the strength of the current system in spring 1988 as compared with fall 1987. We note that the observations in fall 1987 were obtained during the height of the 1986/87 El Nino, while those in spring 1988 were during a cold phase of the El Nino/Southern Oscillat...

Tidally Driven Vorticity, Diurnal Shear, and Turbulence atop Fieberling Seamount

Abstract Fine- and microstructure profiles collected over Fieberling Seamount at 32°26′N in the eastern North Pacific reveal a variety of intensified baroclinic motions driven by astronomical diurnal tides. The forced response consists of three phenomena coexisting in a layer 200 m thick above the summit plain: (i) an anticyclonic vortex cap of core relative vorticity − 0.5f, (ii) diurnal fluctuations of ±15 cm s−1 amplitude and 200-m vertical wavelength, and (iii) turbulence levels corresponding to an eddy diffusivity κe ≅ 10 × 10−4 m2 s−1. The vortex cannot be explained by Taylor–Proudman dynamics because of its − 0.3fN2 negative potential vorticity anomaly. The ±0.3f fortnightly cycle in the vortex’s strength suggests that it is at least partially maintained against dissipative erosion by tidal rectification. The diurnal motions are slightly subinertial, turning clockwise in time and counterclockwise with depth over the summit plain. They also exhibit a fortnightly cycle in their amplitude, pointing to...

Mean Circulation of the Upper Layers of the Western Equatorial Pacific Ocean

E and on O.soN at l42°E. No variation in its transport (15.0 x 10 6 m 3 s·l) is found between those longitudes. We fmd indication of the Equatorial Undercurrent at 137°E-0.75°N in the geostrophic field. The northern and southern Subsurface Countercurrents are clearly identified by extrema of eastward velocity at l65°E around 3°N and 3°S (250 dbar). No evidence of a southern Subsurface Countercurrent is found at l42°E. At 137°E the northern Subsurface Countercurrent is not characterized by a local extrema of eastward velocity: the North Equatorial Countercurrent seems to extend from the surface to 400 dbar with a south ward shi ft of its core. Analysis of the seasonal variability at l6s o E indicates that the E~atorial Undercurrent transport increases by a factor 2 between January (10.7 X 10 6 m 3 s-l) and JuIy (21.5 x 10 m 3 s·l) and the Equatorial Interrnediate Current transport is halved (6.3 x 10 6 m 3 s·l in January, 3.5 x 10 6 m 3 s·l in JuIy). In contrast, the transport of the Subsurface Countercurrents does not vary substantially between those two months. The meridional distributions of salinity and potential vorticity show that the axes of the main eastward currents are associated with strong meridional property gradients, not with property extrema. The eastward currents thus represent a barrier to the riorthward extension of the high salinity Tropical Water. Relatively weak meridional gradients of salinity and potential vorticity are observed in the westward directed South Equatorial Current and Equatorial Interrnediate Current.