To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

The volume transport of the Agulhas Current was measured over a 3-yr period by an array of seven current meter moorings and four current-and pressure-recording inverted echo sounders (CPIES) deployed at 348S. CPIES extended the array farther offshore in order to capture, for the first time, the full Agulhas Current during meander events. Transports derived from CPIES are well correlated with overlapping current meter transports (0.89). The Eulerian mean current is 219 km wide and 3000 m deep, with peak surface speeds of 1.8 m s 21 and a weak northward undercurrent on the continental slope below 1200 m. A new algorithm to capture the western boundary jet transport at each time step T is defined as the poleward transport out to the first maximum of the vertically integrated velocity beyond the half-width of the mean jet. The mean transport of the Agulhas Current jet, so defined, is 284 Sverdrups (Sv; 1 Sv [ 10 6 m 3 s 21 ) with a standard error of 2 Sv. Sampling and instrumental errors are explicitly estimated and amount to an additional 9 Sv. A more traditional estimate, based on net transport integrated to a fixed distance offshore T box , gives a mean transport of 277 6 5 Sv. This transport is 10 Sv greater than an equivalent transport at 328S, corresponding to a latitudinal increase equal to that predicted by Sverdrup dynamics. The time series of T and T box show important differences during solitary meander events and at longer time scales. In terms of an annual cycle, the Agulhas Current appears strongest during austral summer, a similar phase to the Gulf Stream and Kuroshio.

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[1] Observations of dissolved oxygen (DO) concentration, salinity, and temperature, during summer of 2004, at three levels on two moorings in the area of western Long Island Sound that is prone to seasonal hypoxia are described. Ship surveys in the area reveal that the DO concentration below the pycnocline decreases at approximately 2.4 mM m À3 d À1 throughout the summer. We show that this is the net result of oscillations in the rate of change of the DO concentration with periods of 3 to 7 days. During intervals of declining DO concentration, the rate of change is consistent with previous estimates of the rate of community respiration. Since there is insufficient light for photosynthesis below the pycnocline, increasing DO concentration (ventilation) must be a consequence of either vertical mixing or horizontal advection from regions of higher concentration. Analysis of the covariation of DO, salinity, and temperature and knowledge of the mean property distributions allow us to associate most ($80%) of the ventilation intervals with increased vertical mixing. Comparison of DO and wind stress measurements suggest that it is the component in the along-sound direction that controls the occurrence of ventilation, perhaps through modification of the rate of stratification by the density-driven circulation. We conclude that the spatial and temporal variability of vertical mixing is crucial to understanding the duration and extent of hypoxia in the Long Island Sound estuary.

For the first time, the temperature transport of the Agulhas Current is quantified in a time series. Over a 25-month mooring deployment at 34°S, seven tall moorings were instrumented to measure current velocity, temperature, and salinity. Current and pressure-recording inverted echosounders were used to extend geostrophic velocity, temperature, and salinity records to 300 km offshore. In the mean, the current transports 3.8 PW of heat southwards relative to 0°C: -76 Sv at a transport weighted temperature of 12.3°C. A 0.9 PW standard deviation in temperature transport is due to variability in both volume transport and the temperature field. Meandering of the current core dominates variability in the temperature field by warming temperatures offshore and cooling temperatures near the coast. However, meandering has a limited impact on the temperature transport, which varies more closely with a deepening and broadening of the current associated with an inshore isotherm shoaling and an offshore isotherm deepening. Stronger southward temperature transports correspond to a deeper current transporting more volume, yet at a cooler transport weighted temperature. Seasonality is not observed in the temperature transport time series, possibly due to the offsetting effects of cooler temperatures during times of seasonally stronger volume transports. Although volume transport and temperature transport are highly correlated, the large variability in transport weighted temperature means that using volume transport alone to infer temperature transport results in an error which could be as large as 24% of the South Indian Ocean heat transport.