Stephen C. Riser

Decadal Spinup of the South Pacific Subtropical Gyre

Abstract An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment (WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170°W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40°S, 170°W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and...

Nitrate supply from deep to near-surface waters of the North Pacific subtropical gyre

Concentrations of dissolved inorganic carbon (DIC) decrease in the surface mixed layers during spring and summer in most of the oligotrophic ocean. Mass balance calculations require that the missing DIC is converted into particulate carbon by photosynthesis. This DIC uptake represents one of the largest components of net community production in the world ocean. However, mixed-layer waters in these regions of the ocean typically contain negligible concentrations of plant nutrients such as nitrate and phosphate. Combined nutrient supply mechanisms including nitrogen fixation, diffusive transport and vertical entrainment are believed to be insufficient to supply the required nutrients for photosynthesis. The basin-scale potential for episodic nutrient transport by eddy events is unresolved. As a result, it is not understood how biologically mediated DIC uptake can be supported in the absence of nutrients. Here we report on high-resolution measurements of nitrate (NO3−) and oxygen (O2) concentration made over 21 months using a profiling float deployed near the Hawaii Ocean Time-series station in the North Pacific subtropical gyre. Our measurements demonstrate that as O2 was produced and DIC was consumed over two annual cycles, a corresponding seasonal deficit in dissolved NO3− appeared in water at depths from 100 to 250 m. The deep-water deficit in NO3− was in near-stoichiometric balance with the fixed nitrogen exported to depth. Thus, when the water column from the surface to 250 m is considered as a whole, there is near equivalence between nutrient supply and demand. Short-lived transport events (<10 days) that connect deep stocks of nitrate to nutrient-poor surface waters were clearly present in 12 of the 127 vertical profiles.

Net production of oxygen in the subtropical ocean.

The question of whether the plankton communities in low-nutrient regions of the ocean, comprising 80% of the global ocean surface area, are net producers or consumers of oxygen and fixed carbon is a key uncertainty in the global carbon cycle. Direct measurements in bottle experiments indicate net oxygen consumption in the sunlit zone, whereas geochemical evidence suggests that the upper ocean is a net source of oxygen. One possible resolution to this conflict is that primary production in the gyres is episodic and thus difficult to observe: in this model, oligotrophic regions would be net consumers of oxygen during most of the year, but strong, brief events with high primary production rates might produce enough fixed carbon and dissolved oxygen to yield net production as an average over the annual cycle. Here we examine the balance of oxygen production over three years at sites in the North and South Pacific subtropical gyres using the new technique of oxygen sensors deployed on profiling floats. We find that mixing events during early winter homogenize the upper water column and cause low oxygen concentrations. Oxygen then increases below the mixed layer at a nearly constant rate that is similar to independent measures of net community production. This continuous oxygen increase is consistent with an ecosystem that is a net producer of fixed carbon (net autotrophic) throughout the year, with episodic events not required to sustain positive oxygen production.

Deep convection and brine rejection in the Japan Sea

Received 15 October 2002; accepted 13 January 2003; published 19 February 2003. [1] Direct water mass renewal through convection deeper than 1000 m and the independent process of dense water production through brine rejection during sea ice formation occur at only a limited number of sites globally. Our late winter observations in 2000 and 2001 show that the Japan (East) Sea is a part of both exclusive groups. Japan Sea deep convection apparently occurs every winter, but massive renewal of bottom waters through brine rejection had not occurred for many decades prior to the extremely cold winter of 2001. The sites for both renewal mechanisms are south of Vladivostok, in the path of cold continental air outbreaks. INDEX TERMS: 4283 Oceanography: General: Water masses; 4243 Marginal and semienclosed seas; 4223; 4215 Climate and interannual variability (3309). Citation: Talley, L. D., V. Lobanov, V. Ponomarev, A. Salyuk, P. Tishchenko, I. Zhabin, and S. Riser, Deep convection and brine rejection in the Japan Sea, Geophys. Res. Lett., 30(4), 1159, doi:10.1029/ 2002GL016451, 2003.

AUTONOMOUS PROFILING FLOATS: WORKHORSE FOR BROAD-SCALE OCEAN OBSERVATIONS

The autonomous profiling float has been a revolutionary development in oceanography, enabling global broad-scale ocean observations of temperature, salinity, velocity, and additional variables. The Argo float array applies this new technology to provide unprecedented measurements of the global upper ocean in near real time, with no period of exclusive use. It builds on its predecessors, the upper ocean thermal networks of the 1970’s to 1990’s— extending the spatial domain and depth range, improving the accuracy, and adding salinity and velocity. Precision satellite measurements of sea surface height, as made by the Jason1 altimeter, combine with Argo data in a dynamically complementary description of sea level variability and its subsurface causes. The broad-scale Argo float array is a central element in the international infrastructure for ocean research. A comprehensive ocean observing system can be constructed from floats, together with satellite measurements, improved measurements of air-sea fluxes, moored time-series in the tropics and other special locations, shipboard hydrography, and high resolution measurements in fronts, eddies and boundary currents from autonomous gliders. One of the primary objectives of the observing system is to close the oceanic budgets of momentum, heat, and freshwater on seasonal and longer time-scales in order to understand the role of the ocean in the climate system.

An Advanced Method to Estimate Deep Currents from Profiling Floats

Abstract Subsurface ocean currents can be estimated from the positions of drifting profiling floats that are being widely deployed for the international Argo program. The calculation of subsurface velocity depends on how the trajectory of the float while on the surface is treated. The following three aspects of the calculation of drift velocities are addressed: the accurate determination of surfacing and dive times, a new method for extrapolating surface and dive positions from the set of discrete Argos position fixes, and a discussion of the errors in the method. In the new method described herein, the mean drift velocity and the phase and amplitude of inertial motions are derived explicitly from a least squares fit to the set of Argos position fixes for each surface cycle separately. The new method differs from previous methods that include prior assumptions about the statistics of inertial motions. It is concluded that the endpoints of the subsurface trajectory can be estimated with accuracy better tha...

Long-Term Nitrate Measurements in the Ocean Using the in situ Ultraviolet Spectrophotometer: Sensor Integration into the APEX Profiling Float

AbstractReagent-free optical nitrate sensors [in situ ultraviolet spectrophotometer (ISUS)] can be used to detect nitrate throughout most of the ocean. Although the sensor is a relatively high-power device when operated continuously (7.5 W typical), the instrument can be operated in a low-power mode, where individual nitrate measurements require only a few seconds of instrument time and the system consumes only 45 J of energy per nitrate measurement. Operation in this mode has enabled the integration of ISUS sensors with Teledyne Webb Researchs Autonomous Profiling Explorer (APEX) profiling floats with a capability to operate to 2000 m. The energy consumed with each nitrate measurement is low enough to allow 60 nitrate observations on each vertical profile to 1000 m. Vertical resolution varies from 5 m near the surface to 50 m near 1000 m, and every 100 m below that. Primary lithium batteries allow more than 300 vertical profiles from a depth of 1000 m to be made, which corresponds to an endurance near f...

A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo

Using observations from the Argo array of profiling floats, the large-scale circulation of the upper 2000 decibars (db) of the global ocean is computed for the period from December 2004 to November 2010. The geostrophic velocity relative to a reference level of 900db is estimated from temperature and salinity profiles, and the absolute geostrophic velocity at the reference level is estimated from the trajectory data provided by the floats. Combining the two gives the absolute geostrophic velocity on 29 pressure surfaces spanning the upper 2000db of the global ocean. These velocities, together with satellite observations of wind stress, are thenusedtoevaluateSverdrupbalance,thesimplecanonicaltheoryrelatingmeridionalgeostrophictransport to wind forcing. Observed transports agree well with predictions based on the wind field over large areas, primarily in the tropics and subtropics. Elsewhere, especially at higher latitudes and in boundary regions, Sverdrup balance does not accurately describe meridional geostrophic transports, possibly due to the increased importance of the barotropic flow, nonlinear dynamics, and topographic influence. Thus, while it provides an effective framework for understanding the zero-order wind-driven circulation in much of the global ocean, Sverdrup balance should not be regarded as axiomatic.

Sverdrup Balance and the Cyclonic Gyre in the Sea of Okhotsk

Abstract It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean–Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s−1 near the western boundary and slow northward flow with an average speed of approximately 2 cm s−1 in the east. Climatological data show that along zonal sections at 50°–53°N isopycnal surfaces g...

The Bering Slope Current System Revisited

Mean circulation and water properties within the Aleutian Basin of the Bering Sea are investigated using hydrographic and subsurface park pressure displacement data from a regional array of 14 profiling CTD floats. After 10 days drifting at 1000 dbar, each float measures temperature and salinity profiles as it rises to the surface and then transmits these data via satellites, which also make several fixes of the float surface positions before it sinks again. Every fourth cycle, the floats sink from 1000 dbar to a 2000-dbar target just prior to ascent to measure deeper profiles. The 1000-dbar displacements estimated from the float surface position fixes reveal a coherent few-centimeters-per-second northwestward flow along the northeastern boundary, the deep signature of the Bering Slope Current. Middepth water property distributions are consistent with cyclonic advection of warm water from the south around the basin, eastward in the Aleutian North Slope Current, and then northwestward in the Bering Slope Current. Geostrophic transport estimates relative to 1000 dbar also show cyclonic motion, although with significant noise, likely owing to the influence of mesoscale eddies. The mean alongslope geostrophic transport of the Bering Slope Current is determined between 0 and 1900 dbar relative to 1000 dbar and then combined with mean along-slope velocities at 1000 dbar. The result is an absolute geostrophic transport estimate with 95% confidence intervals for the along-slope current offshore of the 1000-m isobath and between 0 and 1900 dbar of 5.8 (61.7) 3 106 m3 s 21.