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Holte, JW, Talley LD, Chereskin TK, Sloyan BM.  2012.  The role of air-sea fluxes in Subantarctic Mode Water formation. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007798   AbstractWebsite

Two hydrographic surveys and a one-dimensional mixed layer model are used to assess the role of air-sea fluxes in forming deep Subantarctic Mode Water (SAMW) mixed layers in the southeast Pacific Ocean. Forty-two SAMW mixed layers deeper than 400 m were observed north of the Subantarctic Front during the 2005 winter cruise, with the deepest mixed layers reaching 550 m. The densest, coldest, and freshest mixed layers were found in the cruise's eastern sections near 77 degrees W. The deep. SAMW mixed layers were observed concurrently with surface ocean heat loss of approximately -200 W m(-2). The heat, momentum, and precipitation flux fields of five flux products are used to force a one-dimensional KPP mixed layer model initialized with profiles from the 2006 summer cruise. The simulated winter mixed layers generated by all of the forcing products resemble Argo observations of SAMW; this agreement also validates the flux products. Mixing driven by buoyancy loss and wind forcing is strong enough to deepen the SAMW layers. Wind-driven mixing is central to SAMW formation, as model runs forced with buoyancy forcing alone produce shallow mixed layers. Air-sea fluxes indirectly influence winter SAMW properties by controlling how deeply the profiles mix. The stratification and heat content of the initial profiles determine the properties of the SAMW and the likelihood of deep mixing. Summer profiles from just upstream of Drake Passage have less heat stored between 100 and 600 m than upstream profiles, and so, with sufficiently strong winter forcing, form a cold, dense variety of SAMW.

Sloyan, BM, Talley LD, Chereskin TK, Fine R, Holte J.  2010.  Antarctic Intermediate Water and Subantarctic Mode Water Formation in the Southeast Pacific: The Role of Turbulent Mixing. Journal of Physical Oceanography. 40:1558-1574.   10.1175/2010jpo4114.1   AbstractWebsite

During the 2005 austral winter (late August-early October) and 2006 austral summer (February-mid-March) two intensive hydrographic surveys of the southeast Pacific sector of the Southern Ocean were completed. In this study the turbulent kinetic energy dissipation rate epsilon, diapycnal diffusivity kappa, and buoyancy flux J(b) are estimated from the CTD/O(2) and XCTD profiles for each survey. Enhanced kappa of O(10(-3) to 10(-4) m(2) s(-1)) is found near the Subantarctic Front (SAF) during both surveys. During the winter survey, enhanced kappa was also observed north of the "subduction front,'' the northern boundary of the winter deep mixed layer north of the SAF. In contrast, the summer survey found enhanced kappa across the entire region north of the SAF below the shallow seasonal mixed layer. The enhanced kappa below the mixed layer decays rapidly with depth. A number of ocean processes are considered that may provide the energy flux necessary to support the observed diffusivity. The observed buoyancy flux (4.0 x 10(-8) m(2) s(-3)) surrounding the SAF during the summer survey is comparable to the mean buoyancy flux (0.57 x 10(-8) m(2) s(-3)) associated with the change in the interior stratification between austral summer and autumn, determined from Argo profiles. The authors suggest that reduced ocean stratification during austral summer and autumn, by interior mixing, preconditions the water column for the rapid development of deep mixed layers and efficient Antarctic Intermediate Water and Subantarctic Mode Water formation during austral winter and early spring.

Oka, E, Talley LD, Suga T.  2007.  Temporal variability of winter mixed layer in the mid- to high-latitude North Pacific. Journal of Oceanography. 63:293-307.   10.1007/s10872-007-0029-2   AbstractWebsite

Temperature and salinity data from 2001 through 2005 from Argo profiling floats have been analyzed to examine the time evolution of the mixed layer depth (MLD) and density in the late fall to early spring in mid to high latitudes of the North Pacific. To examine MLD variations on various time scales from several days to seasonal, relatively small criteria (0.03 kg m(-3) in density and 0.2 degrees C in temperature) are used to determine MLD. Our analysis emphasizes that maximum MLD in some regions occurs much earlier than expected. We also observe systematic differences in timing between maximum mixed layer depth and density. Specifically, in the formation regions of the Subtropical and Central Mode Waters and in the Bering Sea, where the winter mixed layer is deep, MLD reaches its maximum in late winter (February and March), as expected. In the eastern subarctic North Pacific, however, the shallow, strong, permanent halocline prevents the mixed layer from deepening after early January, resulting in a range of timings of maximum MLD between January and April. In the southern subtropics; from 20 degrees to 30 degrees N, where the winter mixed layer is relatively shallow, MLD reaches a maximum even earlier in December-January. In each region, MLD fluctuates on short time scales as it increases from late fall through early winter. Corresponding to this short-term variation, maximum MLD almost always occurs 0 to 100 days earlier than maximum mixed layer density in all regions.

Alley, RB, Marotzke J, Nordhaus WD, Overpeck JT, Peteet DM, Pielke RA, Pierrehumbert RT, Rhines PB, Stocker TF, Talley LD, Wallace JM.  2003.  Abrupt climate change. Science. 299:2005-2010.   10.1126/science.1081056   AbstractWebsite

Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.

Flatau, MK, Talley L, Niiler PP.  2003.  The North Atlantic Oscillation, surface current velocities, and SST changes in the subpolar North Atlantic. Journal of Climate. 16:2355-2369.   10.1175/2787.1   AbstractWebsite

Changes in surface circulation in the subpolar North Atlantic are documented for the recent interannual switch in the North Atlantic Oscillation (NAO) index from positive values in the early 1990s to negative values in 1995/96. Data from Lagrangian drifters, which were deployed in the North Atlantic from 1992 to 1998, were used to compute the mean and varying surface currents. NCEP winds were used to calculate the Ekman component, allowing isolation of the geostrophic currents. The mean Ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses. The northeastward flow of the North Atlantic Current is organized into three strong cores associated with topography: along the eastern boundary in Rockall Trough, in the Iceland Basin ( the subpolar front), and on the western flank of the Reykjanes Ridge (Irminger Current). The last is isolated in this Eulerian mean from the rest of the North Atlantic Current by a region of weak velocities on the east side of the Reykjanes Ridge. The drifter results during the two different NAO periods are compared with geostrophic flow changes calculated from the NASA/Pathfinder monthly gridded sea surface height (SSH) variability products and the Advanced Very High Resolution Radiometer (AVHRR) SST data. During the positive NAO years the northeastward flow in the North Atlantic Current appeared stronger and the circulation in the cyclonic gyre in the Irminger Basin became more intense. This was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from AVHRR SST data, which show that during the positive NAO years, with stronger westerlies, the subpolar front was sharper and located farther east. SST gradients intensified in the North Atlantic Current, Irminger Basin, and east of the Shetland Islands during the positive NAO phase, associated with stronger currents. SST differences between positive and negative NAO years were consistent with changes in air-sea heat flux and the eastward shift of the subpolar front. SST advection, as diagnosed from the drifters, likely acted to reduce the SST differences.

Yuan, XJ, Talley LD.  1996.  The subarctic frontal zone in the North Pacific: Characteristics of frontal structure from climatological data and synoptic surveys. Journal of Geophysical Research-Oceans. 101:16491-16508.   10.1029/96jc01249   AbstractWebsite

The subarctic front is a thermohaline structure across the North Pacific, separating colder, fresher water to the north from warmer, saltier water to the south. Levitus's [1982] data and 72 conductivity-temperature-depth/salinity-temperature-depth sections are used to show the spatial and seasonal variations of the climatological frontal zone and the characteristics of the frontal structure in synoptic surveys. The temperature gradient in the mean frontal zone is stronger in the western Pacific and decreases eastward, while the salinity gradient has less variation across the Pacific. The temperature gradient also has larger seasonal variation, with a maximum in spring, than the salinity gradient. The synoptic surveys show that the frontal zone is narrower and individual fronts tend to be stronger in the western Pacific than in the eastern Pacific. Density gradients tend to be more compensated at the strongest salinity fronts than at the strongest temperature fronts. A horizontal minimum of vertical stability is found south of the subarctic halocline outcrop. The northern boundary of the North Pacific Intermediate Water merges with the frontal zone west of 175 degrees W and is north of the northern boundary of the subarctic frontal zone in the eastern Pacific. The shallow salinity minima start within the subarctic frontal zone in the eastern Pacific.

Yuan, XJ, Talley LD.  1992.  Shallow Salinity Minima in the North Pacific. Journal of Physical Oceanography. 22:1302-1316.   10.1175/1520-0485(1992)022<1302:ssmitn>;2   AbstractWebsite

CTD/STD data from 24 cruises in the North Pacific are studied for their vertical salinity structure and compared to bottle observations. A triple-salinity minimum is found in two separated regions in the eastern North Pacific. In the first region, bounded by the northern edge of the subarctic frontal zone and the 34-degrees-N front between 160-degrees and 150-degrees-W, a middle salinity minimum is found below the permanent pycnocline in the density range of 26.0 and 26.5 sigma(theta). This middle minimum underlies Reid's shallow salinity minimum and overlies the North Pacific Intermediate Water (NPIW). In the second region, southeast of the first, a seasonal salinity minimum appears above the shallow salinity minimum at densities lower than 25.1 sigma(theta). The shallow salinity minimum and the NPIW can be found throughout year, while the seasonal minimum only appears in summer and fall. The middle and shallow salinity minima, as well as the seasonal minimum, originate at the sea surface in the northeast Pacific. The properties at the minima depend on the surface conditions in their source areas. The source of the middle minimum is the winter surface water in a narrow band between the gyre boundary and the subarctic front west of 170-degrees-W. The shallow salinity minimum is generated in winter and is present throughout the year. The seasonal salinity minimum has the same source area as the shallow salinity minimum but is formed in summer and fall at lower density and is not present in winter. A tropical shallow salinity minimum found south of 18-degrees-N does not appear to be connected with the shallow salinity minimum in the eastern North Pacific. South of 20-degrees-N, the shallow salinity minimum and the NPIW appear to merge into a thick, low salinity water mass. When an intrusion of high salinity water breaks through this low salinity water mass south of 18-degrees-N, this tropical salinity minimum appears at the same density as the shallow salinity minimum. Though the water mass of the tropical minimum is derived from the water in the shallow salinity minimum, the formation of the vertical minimum is different.