OS23F-01 INVITED
Overview of global mode waters with emphasis on Subantarctic and North Atlantic Subpolar Mode Waters
Mode waters are thick upper ocean layers that outcrop near the warm side of strong currents and contribute to ventilating much of the global ocean. Volumetrically significant mode waters include the Subtropical Mode Waters of each subtropical gyre, Subantarctic Mode Water (SAMW) north of the Subantarctic Front, and Subpolar Mode Water (SPMW) of the North Atlantic Current and Irminger gyre. While the North Atlantic and North Pacific's Subtropical Mode Waters are associated with extraordinary localized buoyancy loss in addition to a strong current, the other significant mode waters are not associated with extraordinary localized buoyancy loss. However, cumulative buoyancy loss and subduction/entrainment associated with SAMW/SPMW do contribute to significant ventilation of the Indian Ocean and South Pacific thermoclines and the deep North Atlantic. New insights on SAMW formation and downstream ventilation are based on Argo and historical observations in the Indian and South Pacific and in situ wintertime observations in the southeastern Pacific. Consideration is also given to air-sea fluxes and other dynamics as diagnosed from an eddy-permitting Southern Ocean State Estimate. The contributions of eddy diffusion and small scale cross-frontal fluxes can be regionally important.
OS23F-02 INVITED
Upper Ocean Heat Content and Subtropical Mode Water Variations at the Kuroshio Extension Observatory
The Kuroshio Extension Observatory (KEO) surface mooring in the Kuroshio Extension recirculation gyre is funded by NOAA as a contribution to the global network of OceanSITES time series reference sites. KEO was also a integral component of the Kuroshio Extension System Study (KESS) which took place from June 2004 - June 2006 and whose overall goal was to identify and quantify the processes governing the variability of, and interactions between the Kuroshio Extension (KE) and its recirculation gyre. In 2004, when the KEO mooring was first deployed, the jet was quasi-stable with a minimal eddy field and was well to the north of KEO. The recirculation gyre was elongated and mean currents at KEO were westward. During the fall 2004, the seasonal thermocline eroded due to heat loss and intense turbulent mixing associated with several early winter storms. By February 2005, the mixed layer was nearly 400m thick and Subtropical Mode Water (STMW) was exposed to the atmosphere. In mid-2005, the KE eddy activity increased and the KEO buoy has often been in the meander field of the jet. Overall, the top of the main thermocline is shallower than during the first year and is particularly shallow when a cold-core eddy or meander trough is present at the site. Likewise, since the shift in the KE's dynamical state, the KEO salinity field has shown evidence of cross- frontal intrusions. The suite of surface and subsurface measurements at KEO, together with the KESS measurements, thus show how the STMW, upper ocean heat content and sea surface temperature vary on a wide range of time scales due to both local and remote processes.
OS23F-03
Direct Observations of Eighteen Degree Water Formation and Dispersal in the North Atlantic using Acoustically-Tracked Bobbing Floats and Subsurface Profiling Moorings
Eighteen Degree Water (EDW) is the dominant mode water of the North Atlantic subtropical gyre. The
product of convective overturning along the southern periphery of the Gulf Stream, EDW is readily
identifiable by its minimal vertical stratification. Here we report on recent and unique measurements of EDW
formation and dispersal in the western subtropical gyre using two subsurface profiling moorings and an array
of 40 acoustically-tracked isotherm-following bobbing floats. The moorings, located just south of the Gulf
Stream path, show evidence of local EDW formation via convective mixing during the winter months and rapid
restratification through lateral export over the rest of the year. The bobbers, programmed to drift within the
homogenized EDW layer, reveal convoluted dispersal pathways and coherent features unresolved by
previous low-resolution float studies. Bobber- and mooring-derived velocity and stratification measurements
allow direct computation of lateral fluxes of mass, heat, momentum, and potential vorticity.
http://asl.whoi.edu
OS23F-04
Wintertime observations of SubTropical Mode Water formation within the Gulf Stream
We study the structure of SubTropical Mode Water (STMW) within the eastward-flowing Gulf Stream as it forms during strong winter cooling. Shipboard observations using SeaSoar and ADCP reveal that while active mixing by gravitational instabilities is common, large vertical and lateral shears of the Gulf Stream play a central role in determination of the modes of active mixing. Evidence is presented that low static stability and large vertical shear can combine to cause slantwise convection/symmetric instabilities, while the large anticyclonic shears to the south of the Gulf Stream core can cause low absolute vorticity and precondition the Ertel potential vorticity to be small and more susceptible to instabilities. The area of active mixing driven by surface forcing in the presences of shear occupies a swath 50-90km wide immediately south of the Gulf Stream core at the northern edge of the Sargasso Sea: a region previously known to be where deepest winter mixed layers of STMW are found.
OS23F-05 INVITED
Potential Vorticity Dynamics of Mode Water Formation
The defining feature of mode waters is their low potential vorticity (PV). Formation of mode water therefore involves non-conservative processes that reduce the PV. PV is modified by diabatic processes and/or frictional forces. Mode water formation has long been attributed to diabatic processes associated with convection driven by buoyancy loss. Scaling arguments show however that wind-driven frictional forces at ocean fronts can also be effective at forming low PV water when the wind-stress has a component parallel to the frontal jet. For this orientation of the wind, Ekman flow advects water from the dense side of the front over lighter waters, triggering convective mixing and a reduction of stratification and PV. It is hypothesized that this process of PV reduction by winds preconditions large- scale eastward flowing baroclinic currents forced by westerlies such as the Gulf Stream, Kuroshio, and Antarctic Circumpolar Current to form mode waters on their equatorward sides. In this presentation high resolution numerical experiments will be described that illustrate the PV dynamics involved in this wind-driven preconditioning of mode water formation, highlighting the steps involved in the process: the frictional creation of low PV water in the mixed layer; its subsequent subduction by frontal meanders; and its pooling into coherent anticyclonic vortices that take the form of mode water eddies. Unprecedented wintertime observations of the PV of the Gulf Stream and Eighteen Degree Water taken as part of the CLIMODE experiment will be compared with the results of the numerical experiments to test the hypothesis that frontal processes and winds play an important role in mode water formation.
OS23F-06
Sources and Predictability of Anomalies in North Atlantic Subtropical Mode Water Formation and Transformation Using the Walin Framework
While the mean rates of mode water formation and transformation remain elusive owing to the difficulty of estimating each of the terms, the relative contribution of some of the processes can be determined by comparing the forcing in different years. Estimates of any quantity near the highly variable Gulf Stream front require high spatial resolution for accuracy. The calculation of the formation and transformation forcing functions using the Walin framework requires SST and net surface heat flux fields that are well-marched in spatial resolution, because the heat fluxes must be integrated over each SST outcrop. Great improvements have been made in the spatial resolution of these fields in recent years from microwave SST and scatterometer winds, but the high resolution fields span only a short period. Using microwave SST and turbulent fluxes that combine QuikSCAT winds, microwave SST and ECMWF fields in the COARE bulk formulas, a simple method is presented to extend the current high resolution to earlier years and to create matched fields. Monthly estimates of formation and transformation are used to quantify interannual variability and to determine the relative contributions of outcrop area (stratification) and heat flux to mode water transformation anomalies. Correlations between net surface heat flux and outcrop area clearly show the influence of the ocean temperature on the air-sea fluxes. Transformation is shown to depend more critically on outcrop area than on heat flux anomalies. The influence of preconditioning on transformation is examined using early winter values of outcrop area and is shown to have predictive skill for wintertime transformation anomalies.
OS23F-07
Investigation of the subtropical mode water biogeochemical properties in the North Atlantic through a bio-physical coupled model study
The subtropical mode water (STMW) has been analyzed through observed and simulated physical aspects. From a biogeochemical perspective, few studies have been completed while the biogeochemical importance of STMW has been suggested recently from the compilation of various observations: STMW represents a thick subsurface reservoir of nutrients potentially available for the ecosystem. In particular, the careful examination of two summer WOCE sections across the subtropical North Atlantic (Palter et al., 2005) revealed for the first time that STMW is nutrient-poor near the STMW formation region. They suggest that this low-nutrient signature is responsible in part for the low productivity of the subtropical gyre, especially on the western side. We developped an idealized bio-physical configuration of the North Atlantic at 1/3° to investigate the setting and variability of the STMW biogeochemical properties, and its impact on primary production in the North Atlantic. We first show how the exact timing of STMW formation versus the timing of the spring bloom is of primary importance for setting the STMW biogeochemical properties. The model solution highlights the key role played by the export of dissolved organic matter at subsurface by STMW subduction. Then, the presentation identifies how and where the STMW biogeochemical properties control primary production in the North Atlantic through sensitivity analysis. We infer that STMW can be viewed as a nutrient supplier to the surface in obduction regions as well as an efficient carbon export pathway on subduction regions.
OS23F-08
Interannual Variability of the North Pacific Subtropical Mode Water: New Insights from the KESS Profiling Float Program
Forty-eight profiling floats were deployed in the Kuroshio Extension
region since May 2004 as part of the Kuroshio Extension System
Study (KESS) project. By combining the float temperature-salinity
measurements with satellite altimetry data, this study investigates
the role played by mesoscale eddies in controlling the property
changes in North Pacific Subtropical Mode Water (STMW). Following a
3-yr period of low eddy activity in 2002-04, the KE transitioned to
a high eddy kinetic energy state in 2005. This transition is the
result of delayed oceanic response to the 2002 shift in the
basin-scale surface wind forcing in connection with the Pacific
decadal oscillation. By transporting northern-origin, high potential
vorticity KE water into the recirculation gyre, the enhanced eddy
activity affects STMW in two ways: first, it hinders the formation
of deep winter mixed layer (hence the source for STMW) by modifying
the upper ocean stratification and, secondly, it provides a direct
high-PV source to mix with the surrounding low-PV STMW. The eddies's
influence upon STMW is observed to be both significant in magnitude
and efficient in time. Compared to 2004, the PV signal in the core
of STMW was reduced by half in 2005 and this weakening of STMW's
intensity occurred within a period of less than 7 months.
http://www.soest.hawaii.edu/oceanography/bo