Water-property distributions along an eastern Pacific hydrographic section at 135W

As part of the World Ocean Circulation Experiment, full-depth CTD/hydrographic measurements with high horizontal and vertical resolutions were made in June-August 1991 along a line extending from 34N to 33S at a nominal longitude of 135W with an additional short leg that connects it to the California coast roughly along 34N. The line spans the major part of the subtropical and intertropical circulation regime of the eastern North and South Pacific. The primary purpose of this paper is to present vertical sections of various properties from CTD and discrete water-sample measurements along this line and to give an overview of some important features as a basis for more comprehensive basin-scale studies. These features include: the frontal structures found in the surface-layer salinity field in the North Pacific; relatively high-salinity water that dominates the subpycnocline layer between the equator and 17N; troughs of the subpycnocline isopycnals for 26.8–27.5 σ found at 12N and 12.5S; a permanent thermostad at 9–10°C observed between 4.5N and 15N; the pycnostad of the Subantarctic Mode Water centered at 27.0–27.05 σ and developed south of 22S; two types of the Antarctic Intermediate Water representing the subtropical and equatorial circulation regimes; a thick tongue of high silica centered at 3000 m (45.8 σ4) and extending southward across the entire section; deep (2000–3000 m) westward flows at 5–8N and 10–15S separated by an eastward flow at 1–2S; and dense, cold, oxygen-rich, nutrient-poor bottom waters, which are associated with fracture zones and believed to represent the pathways of eastward flows into the Northeast Pacific Basin of the bottom waters separated from the northward-flowing western boundary undercurrent. This work once again demonstrates the usefulness of long lines of high-quality, high-resolution hydrographic stations such as the one described herein in advancing the understanding of the large-scale ocean circulation

Instabilities and radiation of thin, baroclinic jets

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982.Microfiche copy available in Archives and ScienceBibliography: leaves 228-233.Lynne D. Talley.Ph.D

Water-mass distributions in the western South Atlantic; A section from South Georgia Island (54S) northward across the equator

A long CTD/hydrographic section with closely spaced stations was made in February–April 1989 in the western Atlantic Ocean between 0°40′N and South Georgia (54S) along a nominal longitude of 25W. Vertical sections of various properties from CTD and discrete water-sample measurements are presented and discussed in terms of the large-scale circulation of the South Atlantic Ocean. One of the most important results is the identification of various deep-reaching fronts in relation to the large-scale circulation and the distribution of mode waters. Five major fronts are clearly defined in the thermal and salinity fields. These are the Polar (49.5S), Subantarctic (45S), Subtropical (41–42S), Brazil Current (35S) Fronts, and an additional front at 20–22S. The first three are associated with strong baroclinic shear. The Brazil Current Front is a boundary between the denser and lighter types of the Subantarctic Mode Water (SAMW), and the 20–22S front marks the boundary between the anticyclonic subtropical and cyclonic subequatorial gyres. The latter front coincides with the northern terminus of the high-oxygen tongue of the Antarctic Intermediate Water (AAIW) and also with the abrupt shift in density of the high-silica tongue originating in the Upper Circumpolar Water and extending northward. Two pycnostads with temperatures 20–24°C are observed between 10S and 25S with the denser one in the subtropical and the other lighter one in the subequatorial gyre. A weak thermostad centered at 4°C occurs in the AAIW between the Subtropical Front and the Subantarctic Front and shows characteristics similar to the densest variety of the SAMW. Another significant result is a detailed description of the complex structure of the deep and bottom waters. The North Atlantic Deep Water (NADW) north of 25S contains two vertical maxima of oxygen (at 2000 m and 3700 m near the equator) separated by intervening low-oxygen water with more influence from the Circumpolar Water. Each maximum is associated with a maximum of salinity and minima of nutrients. The deeper salinity maximum is only weakly defined and is limited to north of 18S, appearing more as vertically uniform salinity. South of 25S the NADW shows only a single maximum of salinity, a single maximum of oxygen, and a single minimum of each nutrient, all lying close together. The salinity maximum south of 25S and the deeper oxygen/salinity maximum north of 11S are derived from the same source waters. The less dense NADW containing the shallower extrema of characteristics turns to the east at lower latitudes and does not reach the region south of 25S. The southward spreading of the NADW is interrupted by domains of intensified circumpolar characteristics. This structure is closely related to the basin-scale gyre circulation pattern. The Weddell Sea Deep Water is the densest water we observed and forms a relatively homogeneous layer at the bottom of the Georgia and Argentine Basins. The bottom layer of the Brazil Basin is occupied by the vertically and laterally homogeneous Lower Circumpolar Water

An Argo mixed layer climatology and database

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 44 (2017): 5618–5626, doi:10.1002/2017GL073426.A global climatology and database of mixed layer properties are computed from nearly 1,250,000 Argo profiles. The climatology is calculated with both a hybrid algorithm for detecting the mixed layer depth (MLD) and a standard threshold method. The climatology provides accurate information about the depth, properties, extent, and seasonal patterns of global mixed layers. The individual profile results in the database can be used to construct time series of mixed layer properties in specific regions of interest. The climatology and database are available online at http://mixedlayer.ucsd.edu. The MLDs calculated by the hybrid algorithm are shallower and generally more accurate than those of the threshold method, particularly in regions of deep winter mixed layers; the new climatology differs the most from existing mixed layer climatologies in these regions. Examples are presented from the Labrador and Irminger Seas, the Southern Ocean, and the North Atlantic Ocean near the Gulf Stream. In these regions the threshold method tends to overestimate winter MLDs by approximately 10% compared to the algorithm.National Science Foundation (NSF) Grant Numbers: OCE-0327544, OCE-0960928, OCE-1459474; NOAA Grant Number: NA10OAR43101392017-12-1

Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea

Abstract: The Scotia Sea is the site of one of the largest spring phytoplankton blooms in the Southern Ocean. Past studies suggest that shelf‐iron inputs are responsible for the high productivity in this region, but the physical mechanisms that initiate and sustain the bloom are not well understood. Analysis of profiling float data from 2002 to 2017 shows that the Scotia Sea has an unusually shallow mixed‐layer depth during the transition from winter to spring, allowing the region to support a bloom earlier in the season than elsewhere in the Antarctic Circumpolar Current. We compare these results to the mixed‐layer depth in the 1/6° data‐assimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixed‐layer variability in the region. Results indicate the importance of lateral advection of Weddell Sea surface waters in setting the stratification. A Lagrangian particle release experiment run backward in time suggests that Weddell outflow constitutes 10% of the waters in the upper 200 m of the water column in the bloom region. This dense Weddell water subducts below the surface waters in the Scotia Sea, establishing a sharp subsurface density contrast that cannot be overcome by wintertime convection. Profiling float trajectories are consistent with the formation of Taylor columns over the region's complex bathymetry, which may also contribute to the unique stratification. Furthermore, biogeochemical measurements from 2016 and 2017 bloom events suggest that vertical exchange associated with this Taylor column enhances productivity by delivering nutrients to the euphotic zone

Subantarctic mode water in the southeast Pacific : effect of exchange across the Subantarctic Front

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 118 (2013): 2052–2066, doi:10.1002/jgrc.20144.This study considered cross-frontal exchange as a possible mechanism for the observed along-front freshening and cooling between the 27.0 and 27.3 kg m − 3 isopycnals north of the Subantarctic Front (SAF) in the southeast Pacific Ocean. This isopycnal range, which includes the densest Subantarctic Mode Water (SAMW) formed in this region, is mostly below the mixed layer, and so experiences little direct air-sea forcing. Data from two cruises in the southeast Pacific were examined for evidence of cross-frontal exchange; numerous eddies and intrusions containing Polar Frontal Zone (PFZ) water were observed north of the SAF, as well as a fresh surface layer during the summer cruise that was likely due to Ekman transport. These features penetrated north of the SAF, even though the potential vorticity structure of the SAF should have acted as a barrier to exchange. An optimum multiparameter (OMP) analysis incorporating a range of observed properties was used to estimate the cumulative cross-frontal exchange. The OMP analysis revealed an along-front increase in PFZ water fractional content in the region north of the SAF between the 27.1 and 27.3 kg m − 3 isopycnals; the increase was approximately 0.13 for every 15° of longitude. Between the 27.0 and 27.1 kg m − 3 isopycnals, the increase was approximately 0.15 for every 15° of longitude. A simple bulk calculation revealed that this magnitude of cross-frontal exchange could have caused the downstream evolution of SAMW temperature and salinity properties observed by Argo profiling floats.NSF Ocean Sciences grant OCE-0327544 supported L.D.T., T.K.C., and J.H. and funded the two research cruises; NSF Ocean Sciences grant OCE-0850869 funded part of the analysis. BMS’s contribution to this work was undertaken as part of the Australian Climate Change Science Program, funded jointly by the Department of Climate Change and CSIRO.2013-10-2

The role of air-sea fluxes in Subantarctic Mode Water formation

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 117 (2012): C03040, doi:10.1029/2011JC007798.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°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.NSF Ocean Sciences grant OCE-0327544 supported LDT, TKC, and JH and funded the two research cruises. BMS’s contribution to this work was undertaken as part of the Australian Climate Change Science Program, funded jointly by the Department of Climate Change and Energy Efficiency and CSIRO. The QuikSCAT wind mapping method [Kelly et al., 1999], used to create the Kelly flux product, was sponsored by NASA’s Ocean Vector Winds Science. NCEP Reanalysis data were provided by the NOAA/OAR/ESRL PSD. WHOI’s OAFlux project is funded by the NOAA Climate Observations and Monitoring (COM) program.2012-09-2

Freshwater displacement effect on the Weddell Gyre carbon budget

This work was funded by NSF's Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) Project under NSF awards PLR-1425989 and OPP-1936222. G.A.M was additionally supported under UKRI Grant MR/W013835/1. M.R.M. also acknowledges support from NASA grant 80NSSC20K1076 and NSF grants OCE-1924388 and OPP-2149501.The Weddell Gyre mediates carbon exchange between the abyssal ocean and atmosphere, which is critical to global climate. This region also features large and highly variable freshwater fluxes due to seasonal sea ice, net precipitation, and glacial melt; however, the impact of these freshwater fluxes on the regional carbon cycle has not been fully appreciated. Using a novel budget analysis of dissolved inorganic carbon (DIC) mass in the Biogeochemical Southern Ocean State Estimate, we highlight two freshwater-driven transports. Where freshwater with minimal DIC enters the ocean, it displaces DIC-rich seawater outwards, driving a lateral transport of 75 ± 5 Tg DIC/year. Additionally, sea ice export requires a compensating import of seawater, which carries 48 ± 11 Tg DIC/year into the gyre. Though often overlooked, these freshwater displacement effects are of leading order in the Weddell Gyre carbon budget in the state estimate and in regrouped box-inversion estimates, with implications for evaluating basin-scale carbon transport.Publisher PDFPeer reviewe