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Ogle, SE, Tamsitt V, Josey SA, Gille ST, Cerovecki I, Talley LD, Weller RA.  2018.  Episodic Southern Ocean heat loss and its mixed layer impacts revealed by the farthest south multiyear surface flux mooring. Geophysical Research Letters. 45:5002-5010.   10.1029/2017gl076909   AbstractWebsite

The Ocean Observatories Initiative air-sea flux mooring deployed at 54.08 degrees S, 89.67 degrees W, in the southeast Pacific sector of the Southern Ocean, is the farthest south long-term open ocean flux mooring ever deployed. Mooring observations (February 2015 to August 2017) provide the first in situ quantification of annual net air-sea heat exchange from one of the prime Subantarctic Mode Water formation regions. Episodic turbulent heat loss events (reaching a daily mean net flux of -294W/m(2)) generally occur when northeastward winds bring relatively cold, dry air to the mooring location, leading to large air-sea temperature and humidity differences. Wintertime heat loss events promote deep mixed layer formation that lead to Subantarctic Mode Water formation. However, these processes have strong interannual variability; a higher frequency of 2 sigma and 3 sigma turbulent heat loss events in winter 2015 led to deep mixed layers (>300m), which were nonexistent in winter 2016.

Holte, J, Talley LD, Gilson J, Roemmich D.  2017.  An Argo mixed layer climatology and database. Geophysical Research Letters. 44:5618-5626.   10.1002/2017gl073426   AbstractWebsite

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 . 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.

Abernathey, RP, Cerovecki I, Holland PR, Newsom E, Mazlo M, Talley LD.  2016.  Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning. Nature Geoscience. 9:596-+.   10.1038/ngeo2749   AbstractWebsite

Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing(1-3), where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch(4-6). While brine rejection from sea ice is thought to contribute to the lower branch(7), the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport(8,9). Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework(10) to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with differential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of similar to 22 x 10(6) m(3) s(-1). These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.

Marshall, J, Andersson A, Bates N, Dewar W, Doney S, Edson J, Ferrari R, Forget G, Fratantoni D, Gregg M, Joyce T, Kelly K, Lozier S, Lumpkin R, Maze G, Palter J, Samelson R, Silverthorne K, Skyllingstad E, Straneo F, Talley L, Thomas L, Toole J, Weller R, Climode G.  2009.  The CLIMODE FIELD CAMPAIGN Observing the Cycle of Convection and Restratification over the Gulf Stream. Bulletin of the American Meteorological Society. 90:1337-1350.   10.1175/2009bams2706.1   AbstractWebsite
Holte, J, Talley L.  2009.  A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation. Journal of Atmospheric and Oceanic Technology. 26:1920-1939.   10.1175/2009jtecho543.1   AbstractWebsite

A new hybrid method for finding the mixed layer depth (MLD) of individual ocean profiles models the general shape of each profile, searches for physical features in the profile, and calculates threshold and gradient MLDs to assemble a suite of possible MLD values. It then analyzes the patterns in the suite to select a final MLD estimate. The new algorithm is provided in online supplemental materials. Developed using profiles from all oceans, the algorithm is compared to threshold methods that use the C. de Boyer Monte gut et al. criteria and to gradient methods using 13 601 Argo profiles from the southeast Pacific and southwest Atlantic Oceans. In general, the threshold methods find deeper MLDs than the new algorithm and the gradient methods produce more anomalous MLDs than the new algorithm. When constrained to using only temperature profiles, the algorithm offers a clear improvement over the temperature threshold and gradient methods; the new temperature algorithm MLDs more closely approximate the density algorithm MLDs than the temperature threshold and gradient MLDs. The algorithm is applied to profiles from a formation region of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The density algorithm finds that the deepest MLDs in this region routinely reach 500 dbar and occur north of the A. H. Orsi et al. mean Subantarctic Front in the southeastern Pacific Ocean. The deepest MLDs typically occur in August and September and are congruent with the subsurface salinity minimum, a signature of AAIW.

Talley, LD.  2008.  Freshwater transport estimates and the global overturning circulation: Shallow, deep and throughflow components. Progress in Oceanography. 78:257-303.   10.1016/j.pocean.2008.05.001   AbstractWebsite

Meridional ocean freshwater transports and convergences are calculated from absolute geostrophic velocities and Ekman transports. The freshwater transports are analyzed in terms of mass-balanced contributions from the shallow, ventilated circulation of the subtropical gyres, intermediate and deep water overturns, and Indonesian Throughflow and Bering Strait components. The following are the major conclusions: 1. Excess freshwater in high latitudes must be transported to the evaporative lower latitudes, as is well known. The calculations here show that the northern hemisphere transports most of its high latitude freshwater equatorward through North Atlantic Deep Water (NADW) formation (as in [Rahmstorf, S., 1996. On the freshwater forcing and transport of the Atlantic thermohaline circulation. Climate Dynamics 12, 799-811]), in which saline subtropical surface waters absorb the freshened Arctic and subpolar North Atlantic surface waters (0.45 +/- 0.15 Sv for a 15 Sv overturn), plus a small contribution from the high latitude North Pacific through Bering Strait (0.06 +/- 0.02 Sv). In the North Pacific, formation of 2.4 Sv of North Pacific Intermediate Water (NPIW) transports 0.07 +/- 0.02 Sv of freshwater equatorward. In complete contrast, almost all of the 0.61 +/- 0.13 Sv of freshwater gained in the Southern Ocean is transported equatorward in the upper ocean, in roughly equal magnitudes of about 0.2 Sv each in the three subtropical gyres, with a smaller contribution of <0. 1 Sv from the Indonesian Throughflow loop through the Southern Ocean. The large Southern Ocean deep water formation (27 Sv) exports almost no freshwater (0.01 +/- 0.03 Sv) or actually imports freshwater if deep overturns in each ocean are considered separately (-0.06 +/- 0.04 Sv). This northern-southern hemisphere asymmetry is likely a consequence of the "Drake Passage" effect, which limits the southward transport of warm, saline surface waters into the Antarctic [Toggweiler, J.R., Samuels, B., 1995a. Effect of Drake Passage on the global thermohaline circulation. Deep-Sea Research 1 42(4), 477-500]. The salinity contrast between the deep Atlantic, Pacific and Indian source waters and the denser new Antarctic waters is limited by their small temperature contrast, resulting in small freshwater transports. No such constraint applies to NADW formation, which draws on warm, saline subtropical surface waters. 2. The Atlantic/Arctic and Indian Oceans are net evaporative basins, hence import freshwater via ocean circulation. For the Atlantic/Arctic north of 32 degrees S, freshwater import (0.28 +/- 0.04 Sv) comes from the Pacific through Bering Strait (0.06 0.02 Sv), from the Southern Ocean via the shallow gyre circulation (0.20 +/- 0.02 Sv), and from three nearly canceling conversions to the NADW layer (0.02 0.02 Sv): from saline Benguela Current surface water (-0.05 +/- 0.01 Sv), fresh AAIW (0.06 0.01 Sv) and fresh AABW/LCDW (0.01 0.01 Sv). Thus, the NADW freshwater balance is nearly closed within the Atlantic/Arctic Ocean and the freshwater transport associated with export of NADW to the Southern Ocean is only a small component of the Atlantic freshwater budget. For the Indian Ocean north of 32 degrees S, import of the required 0.37 +/- 0.10 Sv of freshwater comes from the Pacific through the Indonesian Throughflow (0.23 +/- 0.05 Sv) and the Southern Ocean via the shallow gyre circulation (0.18 +/- 0.02 Sv), with a small export southward due to freshening of bottom waters as they upwell into deep and intermediate waters (-0.04 +/- 0.03 Sv). The Pacific north of 28 degrees S is essentially neutral with respect to freshwater, -0.04 +/- 0.09 Sv. This is the nearly balancing sum of export to the Atlantic through Bering Strait (-0.07 +/- 0.02 Sv), export to the Indian through the Indonesian Throughflow (-0.17 +/- 0.05 Sv), a negligible export due to freshening of upwelled bottom waters (-0.03 +/- 0.03 Sv), and import of 0.23 +/- 0.04 Sv from the Southern Ocean via the shallow gyre circulation. 3. Bering Strait's small freshwater transport of <0.1 Sv helps maintains the Atlantic-Pacific salinity difference. However, proportionally large variations in the small Bering Strait transport would only marginally impact NADW salinity, whose freshening relative to saline surface water is mainly due to air-sea/runoff fluxes in the subpolar North Atlantic and Arctic. In contrast, in the Pacific, because the total overturning rate is much smaller than in the Atlantic, Bering Strait freshwater export has proportionally much greater impact on North Pacific salinity balances, including NPIW salinity. (C) 2008 Elsevier Ltd. All rights reserved.

Shcherbina, AY, Talley LD, Rudnick DL.  2004.  Dense water formation on the northwestern shelf of the Okhotsk Sea: 2. Quantifying the transports. Journal of Geophysical Research-Oceans. 109   10.1029/2003jc002197   AbstractWebsite

A combination of direct bottom mooring measurements, hydrographic and satellite observations, and meteorological reanalysis was used to estimate the rate of formation of Dense Shelf Water (DSW) due to brine rejection on the Okhotsk Sea northwestern shelf and the rate of export of DSW from this region. On the basis of remote sensing data, an estimated 8.6x10(12) m(3) of DSW was formed during the winter of 1999-2000, resulting in a mean annual production rate of 0.3 Sv. According to direct observations, the export rate of DSW during this period varied from negligibly small in autumn to 0.75+/-0.27 Sv in winter (January-February), to 0.34+/-0.12 Sv in spring (March-April). From these observations the mean annual export rate can be estimated to be 0.27 Sv. The same relationships used to obtain the integral estimates were also applied differentially using an advective approach incorporating realistic flow and heat flux fields, which allowed direct comparison with the moored observations. The comparison highlights the importance of along-shelf advection and cross-shelf eddy transport to the accurate parameterization of DSW formation.

Zhang, HM, Talley LD.  1998.  Heat and buoyancy budgets and mixing rates in the upper thermocline of the Indian and global oceans. Journal of Physical Oceanography. 28:1961-1978.   10.1175/1520-0485(1998)028<1961:habbam>;2   AbstractWebsite

Diapycnal and diathermal diffusivity values in the upper thermocline are estimated from buoyancy and heat budgets for water volumes bounded by isopycnals and isotherms, the air-sea interface, and coastline where applicable. Comprehensive analysis is given to the Indian Ocean, with an extended global general description. The Indian Ocean,gains buoyancy in the north (especially in the northeast) and loses buoyancy in the subtropical south. Freshest and least-dense water appears in the Bay of Bengal and isopycnals outcrop southwestward from there and then southward. Computation of diapycnal diffusivity (K-p) starts from the Bay of Bengal, expanding southwestward and southward and with depth. As isopycnals extend equatorward from the northeast and with increasing depth, K-p remains at about 1.3 cm(2) s(-1) for 20.2 sigma(theta) (Bay of Bengal) to 22.0 sigma(theta) (northeast Indian Ocean). Farther south (poleward) and at greater depth, K-p decreases from 0.9 cm(2) s(-1) for 23.0 sigma(theta) (north of 20 degrees S) to 0.5 cm(2) s(-1) for 25.0 sigma(theta) (north of 35 degrees S). Isotherms outcrop poleward from the equator. Diathermal diffusivity values computed from the heat budget are large at the equator and near the surface (4.0 cm(2) s(-1) for 28.5 degrees C isotherm) but decrease rapidly poleward and with depth (1.3 cm(2) s(-1) for 27.0 degrees C). This indicates stronger mixing either near the equator or the surface, or a possible component in the diathermal direction of the larger isopycnal diffusivity, as isotherms do not follow isopycnals in the upper Indian Ocean north of 10 degrees S. For the 21.0 degrees C isotherm? which closely follows isopycnal 25.0 sigma(theta), the heat budget yields a K-theta again of 0.5 cm(2) s(-1), the value of the diapycnal diffusivity. For the Indian-Pacific system, K-rho decreases from 1.3 cm(2) s(-1) for 22.0 sigma(theta) (the warm pool water, depth similar to 60 m) to 0.9 cm(2) s(-1) for 23.0 sigma(theta) (the tropical water between 20 degrees N and 20 degrees S, depth similar to 100 m), and to 0.1 cm(2) s(-1) for 25.0 sigma(theta) (40 degrees N-40 degrees S, depth similar to 170 m). In the eastern tropical Pacific, K-rho = 1.1 cm(2) s(-1) for 21.5 sigma(theta) (depth similar to 25 m) while K-rho = 0.6 cm(2) s(-1) for 22.0 sigma(theta) (depth similar to 35 m). In the Atlantic, K-rho = 0.6 cm(2) s(-1) for 24.0 sigma(theta) between 20 degrees N and 15 degrees S (depth similar to 80 m), and 0.2 cm(2) s(-1) for 25.0 sigma(theta) between 30 degrees N and 35 degrees S (depth similar to 120 m). For the water volume bounded by 25.5 sigma(theta) farther south and north (50 degrees N-40 degrees S), air-sea buoyancy gain in the Tropics is about the size of the buoyancy loss in the subtropics, and the near-zero net flux may not have significance compared to the errors in the data. For 27.5 sigma(theta), which encompasses the large region from about 65 degrees N to the Antarctic (with midocean average depth of 400 m), K-rho is 0.2 cm(2) s(-1). The results indicate that mixing strength generally decreases poleward and with depth in the upper ocean.

Johnson, GC, Talley LD.  1997.  Deep tracer and dynamical plumes in the tropical Pacific Ocean. Journal of Geophysical Research-Oceans. 102:24953-24964.   10.1029/97jc01913   AbstractWebsite

Anomalous middepth plumes in potential temperature-salinity, theta-S, and buoyancy frequency squared, N-2, Originate east of the East Pacific Rise Crest and decay toward the west. Conductivity-temperature-depth (CTD) data from recent hydrographic sections at 15 degrees S and 10 degrees N are used together with meridional sections at 110 degrees, 135 degrees, and 151 degrees W to map these structures. Warm salty plumes west of the rise crest have maxima centered at 2700 m, 10 degrees S and 8 degrees N, and are interrupted by a cold, fresh tongue centered at 2900 m, 2 degrees S. The theta-S anomalies decay to half their peak strength 2800 km to the west of the rise crest, +/-300 km in the meridional, and +/-0.4 km in the vertical. Vertical N-2 minima occur within the plumes, regions of reduced vertical gradients in theta and S. These minima are underlain by maxima near the depth of the rise crest, about 3200 m. The N-2 plumes decay more rapidly to the west of the rise crest than do the theta-S plumes. The N-2 structure is consistent with a pair of stacked gyres in each hemisphere. There are at least three possible mechanisms consistent with some aspects of these features. First, a deep maximum in upwelling somewhere below 2700 m would result in equatorvard and westward interior flow at 2700 m. advecting these plumes along with it. Second, rapid upwelling of warm, salty, unstratified water in the eastern basins could result in westward overflows over the rise crest. Third, upwelling and associated entrainment processes owing to hydrothermal venting could result in stacked counter-rotating gyres west of the rise crest.