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Radon-222 (Rn-222) is used as a transport tracer of forest canopy–atmosphere CO2 exchange in an old-growth, tropical rain forest site near km 67 of the Tapajos National Forest, Para, Brazil. Initial results, from month-long periods at the end of the wet season (June–July) and the end of the dry season (November–December) in 2001, demonstrate the potential of new Rn measurement instruments and methods to quantify mass transport processes between forest canopies and the atmosphere. Gas exchange rates yield mean canopy air residence times ranging from minutes during turbulent daytime hours to greater than 12 h during calm nights. Rn is an effective tracer for net ecosystem exchange of CO2 (CO2 NEE) during calm, night-time hours when eddy covariance-based NEE measurements are less certain because of low atmospheric turbulence. Rn-derived night-time CO2 NEE (9.00±0.99 μmol m−2 s−1 in the wet season, 6.39±0.59 in the dry season) was significantly higher than raw uncorrected, eddy covariance-derived CO2 NEE (5.96±0.51 wet season, 5.57±0.53 dry season), but agrees with corrected eddy covariance results (8.65±1.07 wet season, 6.56±0.73 dry season) derived by filtering out lower NEE values obtained during calm periods using independent meteorological criteria. The Rn CO2 results suggest that uncorrected eddy covariance values underestimate night-time CO2 loss at this site. If generalizable to other sites, these observations indicate that previous reports of strong net CO2 uptake in Amazonian terra firme forest may be overestimated.

Synopsis Across the geographic area that a species uses for reproduction, the density of breeding individuals is typically highest in locations where ecological factors promote reproductive success. For migratory animals, fitness depends, in part, on producing offspring that migrate successfully to habitats suitable for the next life-history stage. Thus, natural selection might favor reproduction in locations with conditions that facilitate the migration of offspring. To investigate this concept, we studied the Kemp’s ridley sea turtle (Lepidochelys kempii) to determine whether coastal areas with the highest levels of nesting have particularly favorable conditions for hatchling migration. We modeled the passive drift of young Kemp’s ridley turtles from seven nesting regions within the Gulf of Mexico to foraging grounds using the particle-tracking program ICHTHYOP and surface-current output from HYCOM (HYbrid Coordinate Ocean Model). Results revealed that geographic regions with conditions that facilitate successful migration to foraging grounds typically have higher abundance of nests than do regions where oceanographic conditions are less favorable and successful migration is difficult for hatchlings. Thus, our findings are consistent with the hypothesis that, for the Kemp’s ridley turtle and perhaps for other migrants, patterns of abundance across the breeding range are shaped in part by conditions that promote or impede the successful migration of offspring.

The SYNoptic Ocean Prediction (SYNOP) experiment was designed to provide an accurate understanding of the energetic mesoscale processes in the Gulf Stream. The Central Array measured velocity and temperature throughout the water column, with horizontal extent large enough nearly to span the meander envelope and Eulerian mean structure of the jet at 68°W. The 55- to 70-km mooring spacing resolved mesoscale eddy interactions with the Gulf Stream, and the 26-month duration allowed stable estimation of long-term mean fields. Six steep meander troughs propagated into or developed within the array, each lasting around 30-60 days, thus impressing a small mean trough near 68°W in the predominantly eastward currents at jet level (1000 m and above). At the deep level (3500m) the mean flow was southwest at the shoreward sites shallower than 4300 m, but it flowed cyclonically around a mean low-pressure anomaly affecting all the deeper offshore sites. The eddy kinetic energy per unit mass (E K ) decreased by a factor of about 2.5 with each depth increment from 400 to 700 to 1000 m but was only a factor of 2 smaller at 3500 m than at 1000 m. Values of E K in the upper central jet (400 m) were 100 to 230 mJ kg -1 and were 4-13 mJ kg -1 at 3500 m. Overall, E K in the upper 1000m at 68°W was higher than previously published values at 55°W. Two extended case studies of meander propagation through the array demonstrate the development and intensification of deep cyclonic and anticyclonic flows beneath the Gulf Stream. The cyclonic flow at 3500m, associated with amplifying meander troughs, often exceeded 0.35 ms -1 , which was much larger than the typical 0.05 ms -1 deep mean velocities.

The SYNoptic Ocean Prediction (SYNOP) experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the Gulf Stream. In the SYNOP Inlet Array off Cape Hatteras and in the Central Array near 68°W modred observations were collected from October 1987 through August 1990. The Inlet Array measured the surface path and bottom currents where the Gulf Stream leaves the continental margin to enter the deep water regime; small amplitude propagating and growing meanders characterized the variability there. The Central Array measured velocity and temperature (as a proxy for density) at four levels in the water column, as well as the upper and deep level streamfunctions, all with mesoscale resolution. Near 70°W the path envelope exhibited a relative node, confined within a 40-km band 55% of the time. Near 68°W the path envelope was over 3 times as wide, due to several elongated (“steep”) meander troughs and relatively steep meander crests. The crests typically propagated downstream without much growth. The troughs often stalled near 68°W, steepened, and persisted for one to several months. Two cases evolved into “S-shaped” paths and subsequently formed rings. Even the time-averaged fields showed a small trough in the mean path and thermocline structure. Whereas meanders of 20- to 60-day periods had similar spectral levels throughout 70°–67°W, meanders with long periods (>85 day) accounted for the local minimum in variance at 70°W. Bottom pressure and velocity observations revealed repeated periods of intense (swirl speeds > 0.30 m s−1) abyssal eddies; the time-averaged deep currents exhibited a mean cyclone centered 30 km offshore and downstream of the upper layer mean trough. The cross-stream slope of the thermocline steepened linearly with path curvature, consistent with gradient wind balance. Structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form.

An analysis of the structure and transport of the Gulf Stream is undertaken using direct current meter observations from a 13-mooring array deployed near 68°W from June 1988 to August 1990. The analysis is based on a “stream-coordinate” approach, in which velocities are rotated into a local, downstream coordinate frame and averaged according to their relative cross-stream location within the current. The picture so obtained represents the average synoptic structure of the Gulf Stream, rather than the Eulerian-averaged structure in which the current is weakened and broadened by lateral meandering of the current and adjacent recirculations. Many familiar features of the Gulf Stream are reproduced in the analysis, including an asymmetric velocity profile with larger shear on the cyclonic (shoreward) side of the current, an offshore displacement of the velocity core with depth, and a subsurface velocity maximum on the offshore side of the current. Westward recirculations are also seen on both sides of the Gulf Stream. Maximum downstream speeds at the axis of the Gulf Stream reach approximately 2.0 m/s at the surface and 0.7 m/s at 1000 m, roughly twice the corresponding Eulerian-averaged values. The analysis also reveals a deep extension of the Gulf Stream at 3500 m depth with a width of 130 km and average speeds of 3–4 cm/s. The transport of the Gulf Stream in the stream-coordinate frame is 113±8 Sv, approximately 30% larger than the Eulerian-averaged transport of 88 Sv. On the basis of these results and other recent studies the downstream transport increase of the Gulf Stream and the inflow structure to the Gulf Stream are reconsidered. It is concluded that approximately 30 Sv, or over half of the transport increase between Cape Hatteras and 68°W, is fed by inflow from the northern side of the Gulf Stream and that this inflow is concentrated near Cape Hatteras and 68°W, where the Gulf Stream flows steeply across isobaths converging from the north.

The first direct measurements of turbulence in an oceanic convective mixed layer were obtained during a cold-air outbreak. The uniformity of the turbulent dissipation rate and its relation to the surface buoyancy flux are similar to observations of convection in the atmosphere.