OS13B-1177
Transport and Variability on CLIVAR Section I6S from South Africa to Antarctica
In February-March 2008 the CLIVAR Repeat Hydrography cruise occupied the I6S line south of Africa along 30E to the Antarctic margin. This section is a nominal repeat of the WOCE section occupied in February- March 1996. A number of water mass and flow characteristics and statistics are examined to investigate the link between the mixed layer and the interior, particularly near fronts, and to characterize the state of the major water masses on the section. We examine property differences between the two sections on neutral density surfaces and consider mechanisms for the observed changes, including previously inferred trends, interannual variability, or mesoscale eddy activity. Total transport estimates are made using shipboard and lowered ADCP data along with CTD data. Special attention is given to boundary currents at the northern and southern end of the section, and the identification of their water mass components and transports. An additional section was occupied at the southern end across the slope to help constrain the Antarctic Slope Current transport.
OS13B-1178
SUBDUCTION OF SAMW/AAIW IN THE UPPER CELL OF THE SOUTHERN OCEAN
The intensity and location of the upper overturning cell in the Southern Ocean is uncertain, for instance if the cell is associated with subduction centered on the Subantarctic Front or the Polar Front. Argo data provide a new view of the Southern Ocean interior and allow for the first time to resolve the seasonal cycle of the upper ocean. This new dataset is used to revisit the role of the mean flow in subducting water masses in the vicinity of the Antarctic Circumpolar Current. New eddy diffusivity estimates are also used to revise the main balances of the 2-d overturning cell, and it is found that horizontal buoyancy mixing plays an important role near the main fronts of the ACC. The new estimate of kappa allows us to also revisit the estimate of the eddy induced advection through the parameterization of Gent and McWilliam (1990). Eddy diffusivity in the Southern Ocean has been estimated from several approaches in order to quantify the role of eddies on mixed layer heat and mass budgets, and to revisit the role of eddies on the upper cell of the meridional overturning circulation. We find 2 main convergences of the 2-d upper cell in the ACC, associated with AAIW and SAMW subduction. The eddy induced advection tends to balance the subduction induced by Ekman and lateral geostrophic induction. We also investigate the regional patterns of the subduction along the circumpolar belt.
OS13B-1179
Air-sea Fluxes in Subantarctic Mode Water and Antarctic Intermediate Water Formation
Recent observed freshening of Antarctic Intermediate Water (AAIW) may signal the continued onset of climate change. However, the implications of this freshening are difficult to deduce without knowledge of AAIW's formation mechanisms. Two hydrographic surveys from the Southeast Pacific Ocean provide high quality, synoptic views of the Subantarctic Mode Water (SAMW) and AAIW formation region during winter and summer. The winter cruise, from August 23 to October 5, 2005, occupied 135 full depth CTD/Rosette stations and deployed 371 XCTDs. This data set is used to assess the role of atmospheric forcing in forming the deep SAMW and AAIW mixed layers. Forty-two SAMW mixed layers deeper than 400 m were observed on the winter cruise. The mixed layer potential density varies by 0.05 kg m(-3) along the front, such that two clusters of deep SAMW mixed layers are visible in T-S diagrams. The deepest mixed layers occur immediately north of the Subantarctic Front (SAF) and are associated with oceanic heat loss to the atmosphere (maximum of 250 W m(-2)). To assess the importance of air-sea fluxes in SAMW and AAIW formation, the observed fluxes are used to verify model heat fluxes (NCEP and ECMWF). The model fluxes are compared to backwards heat flux calculations and used to force a one-dimensional mixed layer model, KPP, to model the mixed layer's seasonal cycle and examine the down-front changes in SAMW. This analysis is contrasted with a preliminary examination of cross-frontal transport and its effect on the deep SAMW and AAIW mixed layers.
OS13B-1180
Subantartcic mode water formation estimated from data assimilating model simulations
Subantarctic Mode Waters (SAMW) are thick winter mixed layers that form on the equatorward side of the Subantarctic Front (SAF). Formation rates are first estimated using a Walin-type analysis, requiring surface buoyancy fluxes. These fields are generally poor for the Southern Ocean, mainly because of the poor data coverage and near-cancellation of freshwater and heat components. Two data-assimilating ocean models, which provide dynamically consistent fields including surface fluxes and salinity, yield SAMW formation estimates from the Walin analysis which are more similar to hydrographic estimates than the corresponding results obtained from two widely-used buoyancy flux estimates (NCEP-NCAR REanalysis 1, ECMWF operational model) and also to the recent Large and Yeager (2008) buoyancy flux estimate. The two state estimates are the high resolution (1/6 degree) Southern Ocean State Estimate (SOSE; Mazloff, 2008) and the 1 degree resolution Ocean Comprehensible Atlas (OCCA; Forget et al., 2008). The years 2005 and 2006 are available from SOSE. Both estimates correctly reproduce the density classes of SAMW as determined by hydrography (sigma theta of 27.1 for Pacific SAMW and 26.8 for the Indian SAMW). Water mass transformation and formation rate maps show that formation occurs in June-July-August in the region of high transformation across the warmer flank of the layer. This region coincides with that of deep winter mixed layers as estimated from Argo floats (Dong et al., 2008). SAMW subduction rates are determined independently using the ocean interior properties and flow, and are comparable to the Walin-analysis rates. Formation is predominantly localized to the region of high surface buoyancy loss. The high resolution SOSE output shows that locally lateral eddy buoyancy fluxes can play an important role.
OS13B-1181
North Atlantic Water Mass Formation as Depicted by Coupled Climate Models
We evaluate the realism of North Atlantic (NA) water mass properties in two fully coupled one-degree class Community Climate System Model 3.0 (CCSM3) simulations using Eulerian spectral (CCSM-T85) and finite- volume (CCSM-FV) atmospheric dynamical cores. We calculate the water mass transformation rates (WMTR) for the entire North Atlantic (NA) and separately for the Nordic Seas using 20-year monthly time series of sea surface temperature, sea surface salinity, surface heat and freshwater fluxes due to the atmosphere-ocean and ice-ocean interactions. Compared to the CCSM3-T85 the CCSM3-FV has higher WMTR in narrower density ranges. It's subpolar mode water (σθ= 27.5 kg.m-3) formation rate is 25Sv while in CCSM-T85 it is 20Sv. In the Nordic Seas the CCSM-FV formation rate is about 7.5 Sv versus 6 Sv in CCSM-T85 in the σθ range 27.-28.35 kg.m-3. These results will be validated with observational estimates based on surface fluxes and sea surface temperature from European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data and Polar science center Hydrographic Climatology (PHC) sea surface salinity monthly climatology. As expected the dominant formation process is the buoyancy flux caused by the atmosphere-ocean heat exchange. The effect of ice-ocean buoyancy fluxes is to reduce the WMTR mainly due to the freshwater component (freshening during melting). We investigate the interannual variability of the NA WMTR related to the North Atlantic Oscillation (NAO) for both models. The ocean response to the NAO mode is better simulated in the CCSM-FV with intensified water mass production during the negative phase when the winter convection in the Nordic Seas is stronger.
OS13B-1182
Studies of North Atlantic Eighteen Degree Water formation and dissipation during the CLIMODE experiment using a data-model synthesis
A data-model synthesis is used to quantify the cycle of Eighteen Degree Water (EDW) formation and dissipation and the underlying driving mechanisms. The MITgcm and its adjoint is used to dynamically interpolate Argo profiles, SST and altimetric observations during the CLIMODE experiment. Air sea fluxes and the initial T/S fields are used as controls to minimize the misfit between the model and the observations in a least squares procedure facilitated by the adjoint model. The model can be thought of as an interpolation tool which yields an evolving solution which is very close to Argo observations. This allows us to monitor V(t), the time evolution of the volume of fluid between 17 and 19 degrees. We find that EDW is formed by air-sea heat fluxes in the western part of the subtropical gyre, just south of the Gulf Stream. The formation rate peaks in February when the EDW layer is thickened by convection due to buoyancy loss. EDW is destroyed by air-sea heat fluxes and mixing from spring to summer over the entire subtropical gyre. Our results suggest that the net annual-mean formation rates of EDW associated with air-sea fluxes are in the range 3 to 5 Sv. As more and more of the CLIMODE in-situ data are drawn in to our analysis, uncertainties in the V(t) curve will be reduced and we can use the system to refine our estimates of the relative importance of air-sea fluxes and interior mixing in setting the evolution of V(t).
OS13B-1183
Transformation and pathway of Subpolar Mode Water in the northeastern Atlantic
The transformation and the pathway of Supolar Mode Water (SPMW) in the northeastern Atlantic is investigated from observational data (air-sea flux data, National Oceanography Center Southampton, NOCS; hydrographic data, World Ocean Circulation Experiment, WOCE; Lagrangian data, National Oceanographic and Atmospheric Administration,NOAA and Atlantic Climate Change Experiment, ACCE). The surface diapycnal flux is computed in terms of water mass transformation driven by air-sea interaction. It is shown that the diapycnal flux is the dominant source of SPMW. Furthermore, since the surface diapycnal volume flux is not constant for each density class, we suggest that processes of subduction and obduction occur through the permanent pycnocline. Thus, the water mass involved in the SPMW transformation continuously exchanges with the ocean interior. SPMW transformation processes occur along the main path of each of the several branches of North Atlantic Current (NAC). The Rockall Trough branch of the NAC carries 27.3σθ, 27.4σθ, and 27.5σθ SPMW toward the Iceland-Faroe Front. In the Iceland Basin, the Subarctic Front along the western flank of the Rockall Plateau carries a similar sequence of SPMW. The western side of the Central Iceland Basin branch of the NAC veers westward and joins the East Reykjanes Ridge Current, feeding the 27.5σθ SPMW on the Reykjanes Ridge.
OS13B-1184
Evolution of Eighteen Degree Water and deep mixed layer oxygen and properties from profiling floats during CLIMODE
As part of the CLIMODE observations of Eighteen Degree Water (EDW) formation and destruction, 9 profiling
floats equipped with optical oxygen sensors as well as CTDs profiled temperature, salinity and oxygen every
5 days to 500 m and every 10 days to 1800 m during 2006 and 2007 in the Gulf Stream region. Thick (200-
450 m) late winter mixed layers with EDW properties were observed close to the Gulf Stream at about 52W,
and in a separate cluster up to several hundred kilometers south of the Gulf Stream at about 62W, within the
westward recirculation. Relatively thick winter mixed layers at 14C were observed north of the Gulf Stream,
and also adjacent to the North Atlantic Current much farther to the east. Mixed layer oxygen saturation is a
robust indicator of deep mixing: it is depressed in the thick winter mixed layers and increases abruptly upon
cessation of deep mixing.
EDW layers for these floats, defined by low potential density gradient, were thickest during and just after late-
winter outcropping. EDW thickness declined by about 100 m when mixing ceased, with the EDW layer center
also shifting physically downward. Oxygen saturation decreased in the EDW by about 10 percent from late
winter to late summer, indicating annual aging of the water mass followed by renewal again in the winter.
The oxygen-equipped floats provide an excellent opportunity to study the seasonal evolution of oxygen in the
upper ocean in the Sargasso Sea. A well-developed oxygen minimum, centered at about 27.2 sigma theta
and originating in the tropical Atlantic, underlies the EDW at about 800 m (south of the Gulf Stream). Above
the EDW were the seasonally-evolving mixed layer with a thin underlying oxygen maximum layer in
spring/summer and a thin oxygen minimum layer in fall/early winter, associated with
photosynthesis/respiration. Although the EDW's absolute oxygen content decreased throughout the year
following late winter formation, the EDW was marked by a slight vertical oxygen maximum once the overlying
seasonal oxygen minimum developed.
http://www-
pord.ucsd.edu/climode_apexfloats/
OS13B-1185
Observations of Near-inertial Motions During CLIMODE
Addressing the CLIMODE goals of building understanding of the formation and dissipation mechanisms of EDW, we investigate energetic near-inertial motions forced by the passage of wintertime storms. Two EM- APEX profiling floats deployed during February of 2007 in the equatorward flank of the Gulf Stream returned ~ hourly vertical profiles of horizontal velocity, temperature and salinity. Near-inertial amplitude and frequency are estimated for the float velocities using spectral methods, as well the extended complex demodulation technique. SeaSoar and ship ADCP sections about the float tracks provide estimates of the local relative vorticity. Examination of the observations shows energetic (~ 40 cm/s) near-inertial oscillations in the surface mixed layer and stratified interior in the vicinity of the Gulf Stream. The estimated frequency of these oscillations is less than local f at times, and the depression of frequency can be accounted for by the local relative vorticity derived from the ship data. Vertical wavenumber spectra indicate that the energy is predominantly downward propagating, which is consistent with surface wind forcing. Velocity profiling data from two bottom anchored CLIMODE moorings provide longer-term information about near-inertial motions in the Gulf Stream region, including estimates of the wind energy input to inertial motions as well as mixing rates inferred from Richardson number estimates.
OS13B-1186
Mixing rates across and in the Gulf Stream
Microstructure measurements across the Gulf Stream (GS) were taken as a part of the CLIMODE February 2007 cruise to observe temporal and spatial variability of mixing and its impact on formation of Eighteen Degree Water (EDW). Owing to strong winds and low temperatures, enhanced mixing was found throughout: in the surface mixed layer, in the entrainment zone at the base of the mixed layer, and in the thermocline. North of the GS, mixed layers were relatively shallow, about 50 m, and surface forcing produced such strong turbulence in the entrainment zone that the accompanying heat flux was comparable to that at the surface. Magnitudes ranged from -1000 to 1000 W/m2. Some of this mixing may contribute to forming EDW, but the surface foot print of the formation region along the north edge of the GS is too small for it to be a major source. Farther south, mixed layers were 300 to 500 m deep and entrainment fluxes ranged from - 100 to -10 W/m2. Although entrainment was much less than north of the GS, the much larger area where the fluxes were found suggests that they may produce significant amount of EDW. Diapycnal diffusivities in the GS thermocline averaged about 10-4 m2/s, approximately ten times levels previously observed during other seasons. Some of the most intense mixing was in regions of strong shear where Richardson numbers computed from ADCP profiles with 16 m bins dropped below 1. The banded structure of the horizontally-coherent shear and the dominance of clockwise rotation with increasing depth indicate downward propagating near-inertial waves were responsible for much of the enhanced mixing. Internal wave scaling of observed dissipation rates worked well throughout the thermocline, with only a few discrepancies greater than a factor of two. The K-profile parameterization (KPP) for the thermocline under- predicted dissipation rates at high Richardson numbers and over-predicted them at low values.
OS13B-1187
Mode Water Potential Vorticity Forcing Characteristics
We are examining the different contributions to the air-sea potential vorticity (PV) forcing in the outcrops connected to the Subtropical Mode Water in the North Atlantic Ocean, for the years 2004 to 2006. The diabatic, frictional and fresh water flux contributions are considered. Their relative importance is discussed. Different model outputs have also been used in the same attempt. The global 1/4° DRAKKAR simulation, the assimilating 1° ECCO atlas, and a 1/8° idealized configuration. We verify the potential vorticity balance in an isopycnal range corresponding of the mode water, applying the impermeability theorem. The relative contribution of advection, diffusion and eddies, in the redistribution of the PV, is addressed.
OS13B-1188
Atmospheric Boundary Layer Observations Made in Support of CLIMODE
The National Center for Atmospheric Research (NCAR) operated a balloon borne radiosonde sounding
system and a wind profiler radar on the R/V Knorr in support of the CLIMODE project. The cruises took
place in February and March 2007, and made multiple traverses of northwestern boundary of the Gulf
Stream off the coast of New England. The radisondes and wind profiler were deployed to examine the
response of the atmospheric boundary layer in this region of very strong air-sea exchange. The radiosondes
were typically launched at three to six hourly intervals, whereas the wind profiler operated continuously. In
addition to measuring the wind and atmospheric reflectivity, the profiler included a Radio Acoustic Sounding
System (RASS) to measure virtual temperature aloft. Strong signals were seen in all systems. For example,
crossing into the warmer waters of the Gulf Stream produced a strong response in the atmosphere, with both
a warming and deepening of the atmospheric boundary layer. Soundings and atmospheric reflectivity
sometimes showed complex structure with multiple inversions and reflectivity gradients, representing the
history of the atmosphere as it passed over waters of varying temperature and air-sea exchange.
http://www.eol.ucar.edu/rtf/projects/climode/
OS13B-1189
Air-Sea Surface Conditions in the Gulf Stream From In-Situ Observations
As part of the CLIMODE observational program, a fixed surface mooring was deployed in the Gulf Stream (38N, 65W) for 15 months. Air-sea measurements (1 minute sampling), including radiation, and subsurface oceanographic data (5 minutes sampling) were collected. These high quality and long term measurements are the first of their kind in this region where high winds and strong currents are common. Air-sea fluxes were computed using the COARE bulk parameterization. Wintertime oceanic heat loss events in excess of 1500 W/m2 occurred during cold air outbreaks when SST was high. We describe changes of the marine boundary layer surface conditions as the Gulf Stream front moved back and forth past the mooring. Very rapid and large changes in SST were observed that concurred with atmospheric conditions. Air-sea coupling such as wind intensification and cloud coverage is discussed. Comparison with NWP products is also shown. Subsurface temperature data is presented and show the mixed layer and mode water evolution below the mooring.
OS13B-1190
Improvements to Bulk Formulae for Momentum, Heat and Mass Exchange at High Winds using CLIMODE Data
The NSF sponsored CLIvar MOde Water Dynamic Experiment (CLIMODE) is designed to investigate the
formation, evolution, storage, and dispersal of Eighteen Degree Water (EDW), the subtropical mode water of
the North Atlantic. The Gulf Stream region of the North Atlantic is a very attractive region for air-sea
interaction research as the region experiences the largest net wintertime heat loss over the global ocean with
climatological estimates approaching 400 W/m2. Near surface winds of 15 m/s (30 knots) were
commonly encountered during the 2007 field program with a maximum winds speed of approximately 25 m/s
(50 knots). These high wind events drove surface stresses that exceeded 1.0 N/m2.
An overall objective of the CLIMODE program is to improve our estimates EDW formation through air-sea
exchange. Therefore, accurate measurement of the heat, mass and momentum fluxes is of crucial
importance to CLIMODE investigations. This was accomplished using three highly instrumented platforms
during the field program to provide surface fluxes using the direct covariance technique: a moored 3-m
discus buoy, a research vessel for surveys, and a drifting Air-Sea Interaction Spar (ASIS). The ship and
ASIS packages included Direct Covariance Flux Systems (DCFS) used in the development of the COARE
bulk algorithm. A low-power version of the DCFS was deployed on the discus mooring and operated
successfully for over a year.
Fluxes from these platforms have been combined with data taken from stabilized platforms and fixed towers in
previous field studies to validate and improve the COARE 3.0 bulk algorithm. The combined data set
provides a wide range of wind speed, sea state and atmospheric stability conditions to improve the bulk flux
formula. Direct estimates of the drag coefficient are in good agreement with COARE 3.0 over moderate wind
conditions. However, there are differences at the lowest and highest wind speeds where COARE 3.0
overestimates and underestimates the drag, respectively. Fairly simple modifications to the COARE 3.0
parameterization provide excellent agreement with the merged data set.
Similarly, estimates of the transfer coefficients for sensible and latent heat show differences with COARE 3.0
at high winds. Of particular interest are the extreme conditions encountered during cold air outbreaks that
drive combined latent and sensible heat fluxes exceed 1200 W/m2. These enormous heat fluxes are
driven by a combination of high winds are significant air-sea temperature and humidity differences. The data
is now being used to reduce the uncertainty in the Stanton and Dalton number at wind speeds greater than
15 m/s.
http://www.marinesciences.uconn.edu/faculty/edson.html
OS13B-1191
The Role of Convective Clouds in Setting Air-sea Fluxes Over mode Water Formation Regions
The evolution of the marine boundary layer during cold-air outbreaks over the Gulf stream is examined using a cloud resolving, large-eddy simulation model. The model is initialized using an idealized sounding based on observations from the 2007 CLIMODE cruise. Simulations produce a cloud-topped boundary layer characterized by strong convective plumes and spreading stratocumulus clouds along the boundary layer top. Boundary layer growth is controlled by cloud top entrainment, radiative cooling of the cloud tops, and evaporation of cloud and precipitation into the overlying atmosphere. We find that relatively low humidity values near the surface, which are necessary for strong latent heat fluxes, are maintained by the continual expansion of the boundary layer in the entrainment layer. Clouds play a critical role in this process by generating both strong radiative and evaporative cooling of the relatively warmer and drier air above the boundary layer, thereby providing a sink for increased boundary layer moisture. Total cloud coverage is inversely related to the sea surface temperatures (SST), with higher SST's generating more isolated cumulus convection versus more uniform stratocumulus clouds over cooler SST's.
OS13B-1192
Episodic Mode Water Formation: Atmospheric Controls and Trends
We explore the intraseasonal variability of North Atlantic mode water formation in the CLIMODE region. We observe that wintertime sensible and latent heat fluxes from CLIMODE region surface waters are characterized by episodic high flux events due to cold air outbreaks from North America. Up to 60% of the November-March (NDJFM) total sensible heat flux and 45% of latent heat flux occurs on these high flux days. On average of 40% (33%) of the total NDJFM sensible (latent) heat flux takes place during just 17.5% (20%) of the days. Over the last 60 years, the total NDJFM sensible and latent heat fluxes over the CLIMODE region have increased, due to an increased number and intensity of high flux event days. Recent cold air outbreaks are characterized by increasingly colder surface air conditions over the eastern seaboard of North America and slightly warmer ocean SSTs, leading to increased flux rates. An increase of meridional wind variance in the CLIMODE region, indicative of more intense and rapid north-south advection of arctic air masses, is also observed over the last 60 years and may explain the surface air cooling trend over eastern North America.
OS13B-1193
Chromophoric DOM as a Tracer of North Atlantic Subtropical Mode Water Formation
Global surveys of chromophoric dissolved organic matter (CDOM) profiles have revealed that subtropical
mode water layers represent a local minimum in the CDOM profile, reflecting their source in surface waters
that have been bleached by solar radiation. Biogeochemical changes in CDOM below the surface layer have
also been shown to be relatively slow, suggesting the use of CDOM as a semiconservative tracer of mode
water formation. Estimates of CDOM formation rate in the mode water from concurrent measurements of
CDOM and CFCs and salinity could be used to separate the biogeochemical signal from that of mixing. In this
study, we examine mode water formation and transport using a 7-year record of CDOM concentrations in the
subtropical mode water and sections from the CO2/CLIVAR Repeat Hydrography program. We used
this in conjunction with hydrographic and biogeochemical data collected by the BATS and Repeat
Hydrography programs, as well as nutrient data from the same sites. We found that profiles of CDOM
concentrations at Bats tended to be inversely proportional to those of excess nitrogen (DINxs), reflecting
remineralization throughout the water column. Furthermore, typical CDOM profiles were altered when the
mode water was compressed due to factors such as eddies, indicating a need to consider such factors in
characterizing CDOM distribution in areas where mode water exists.
http://www.icess.ucsb.edu/bbop/bbop.html
OS13B-1194
Biogeochemical Evidence of Large Vertical Eddy Diffusivity Associated With Subtropical Mode Water of the North Pacific
Based on the extensive profiling float observation carried out as part of the Kuroshio Extension System Study (KESS), Qiu et al. (2006) reported large vertical eddy diffusivity (2-5 x10-4 m2s-1) near the upper boundary of Subtropical Mode Water (STMW). This large diffusivity possibly have an impact on subsurface redistribution of heat, nutrients and dissolved gas components, etc., in the subtropical ocean. On the other hand, recent measurement of turbulent kinetic energy dissipation rate by Mori et al. (2008) indicates much smaller vertical eddy diffusivity (10-7 – 10-5 m2s-1) over the whole depth range of STMW. However, the direct comparison between the estimation by Qiu et al. and that by Mori et al. is possibly inappropriate because the former is based on the PV change over a couple of months and the latter on the instantaneous turbulent measurements. We carried out physical and biogeochemical observation to examine the vertical diffusivity near the top of STMW using a profiling float. The profiling float, which was equipped with a fluorometer and a dissolved oxygen sensor along with temperature and salinity sensors, was deployed in the STMW formation region and acquired quasi-Lagrangian, 5-day-interval time-series records from March to July in 2006. The time-series distribution of chl.a showed a sustained and sizable deep chlorophyll maximum just above the upper boundary of the STMW throughout early summer. Vertically integrated chlorophyll in this period was consistently ranging from 15-30 mgm-2, indicating sustained primary production and a continuous supply of nutrients ranging from 10-20 mgNm-2day-1. The time-series data indicate no sporadic events to supply nutrients and instead support, along with vertical profiles of nitrate obtained by ship-board measurements near the float, the large vertical diffusivity reported by Qiu et al. Since our estimation of vertical diffusivity is based on temporal evolution of primary production over several weeks, it is fairly consistent with the estimation by Qiu et al. The results of ongoing float observations will be also presented.
OS13B-1195
Mixed layer depth variations in the Kuroshio Extension in relation to Japanese sardine
Mixed layer depths in and south of the Kuroshio Extension changed from deep to shallow states in the late- 1980s and early-1990s. This change corresponded to the collapse of Japanese sardine (Sardinops melanostictus). This mixed layer shoaling was accompanied by lower temperature in 200-400m depths and higher temperature near the surface. Wintertime field survey in 2006 (Hakuho-maru KH06-1 cruise) demonstrated that late winter maximum mixed layer depth reach deeper with the greater isothermal depth of 14-15degC and higher temperature in 300-400m depth. High-resolution ocean model hindcast data suggests that the accelerated near-surface Kuroshio/Kuroshio Extension associated with the elevated sea-surface height anomaly enhanced the heat transport near the surface. This greater heat advection near the surface overrides the cooler subtropical mode water that was created in the previous years possibly causes the shallower winter mixed layer and collapse of the Japanese sardine.
OS13B-1196
Vertical Structures of the North Pacific Mode Waters
Vertical structures of mode waters in the North Pacific are investigated using the Argo data from 2001 until 2007. As parameters of vertical structures, vertical gradient of water properties and Turner angle are used. Four mode waters in the North Pacific, Subtropical Mode Water (STMW), Eastern STMW (ESTMW), Central Mode Water (CMW) and Transition Region Mode Water (TRMW) are defined as thick (> 100 dbar) low potential vorticity (PV < 2.0 x 10-10 m-1 s-1) waters with specific water properties within specific areas. Temperature gradient and density gradient of SMTW is in linear relationship and salinity gradient of STMW is very small. Density gradient of STMW is mostly determined by temperature gradient. On the other hand, gradient of salinity and that of temperature of CMW, ESTMW and TRMW are relatively large and take various values in wide ranges. It is suggested that these three mode waters have density compensating stratification of temperature and salinity. In the low-PV portion of STMW, Turner angle is less than 60 degree and vertically homogeneous. Tu in the low-PV portion of CMW is also mostly homogeneous with 60-70 degree. Tu of ESTMW and TRMW, on the other hand, have different structure in upper and lower side of low-PV portion. In the lower side of ESTMW and in the upper side of TRMW, Tu is larger than 70 degree on average, indicating active salt finger type convection. In the upper side of ESTMW and in the lower of TRMW, Tu is less than 70 degree, suggesting relatively stable stratification. Salt finger type convection possibly modifies temperature and salinity of ESTMW and TRMW from the lower and upper side of those, respectively. T/S modification by double diffusive convection appears relatively small in case of STMW and CMW.
OS13B-1197
Seasonal and Interannual Variation of North Pacific Subtropical Mode Water in 2003- 2006
Temperature and salinity data from 2003 through 2006 from Argo profiling floats have been analyzed to
examine the formation and circulation of the North Pacific Subtropical Mode Water (STMW) and the
interannual variation of its properties over the entire distribution region. STMW is formed in late winter in the
zonally-elongated recirculation gyre south of the Kuroshio and its extension, which extends north of
28°N, from 135°E to near the date line. The recirculation gyre consists of several anticyclonic
circulations, in each of which thick STMW with a characteristic temperature is formed. After spring, the thick
STMW tends to be continually trapped in the respective circulations, remaining in the formation region. From
this stagnant pool of thick STMW, some portion seeps little by little into the southern region, where
southwestward subsurface currents advect relatively thin STMW as far as 20°N to the south and just
east of Taiwan to the west. The STMW formed in the recirculation gyre becomes colder, less saline, and
denser to the east, with an abrupt change of properties across 140°E and a gradual change east of
140°E. The STMW formed east of 140°E exhibits coherent interannual variations, increasing its
temperature by ~1°C from 2003 through 2006 and also increasing its salinity by ~0.05 from
2003 through 2005. These property changes are clearly detected in the southern region as far downstream
as just east of Taiwan, with reasonable time lags.
http://ocg.ori.u-
tokyo.ac.jp/member/eoka/
OS13B-1198
Ice-Ocean Interactions, Tidal Mixing, and Dense Shelf Water Formation Within the Sea of Okhotsk
The dense water that forms along the shallow shelves within the Sea of Okhotsk is a major source of North Pacific Intermediate Water (NPIW), which plays an important role in the circulation of the North Pacific Ocean. The production of this dense shelf water (DSW) depends upon the sea ice cover within the sea, as well as upon the details of coastal polynyas and the strong tidal mixing that occurs on the shelves. In order to investigate these processes and their relation to the formation of DSW, we have coupled ROMS (Regional Ocean Modeling System), a terrain following regional ocean model, and CSIM (Community Sea-Ice Model), a dynamic-thermodynamic sea ice model. We perform an analysis of sea-ice properties and study the effects of tidal mixing within the model. A tidal mixing front forms similar to observations when tides are included. The front impacts both the formation and the export off the shelf of the DSW. We also quantify the dense water formation in our model and compare our results to observations of water mass and sea-ice properties within the Sea of Okhotsk.