Antarctic Intermediate Water and Subantarctic Mode Water Formation in the Southeast Pacific: The Role of Turbulent Mixing

2010 ◽  
Vol 40 (7) ◽  
pp. 1558-1574 ◽  
Author(s):  
Bernadette M. Sloyan ◽  
Lynne D. Talley ◽  
Teresa K. Chereskin ◽  
Rana Fine ◽  
James Holte
Keyword(s):  
Mixed Layer ◽  
Austral Summer ◽  
Buoyancy Flux ◽  
Intermediate Water ◽  
Austral Winter ◽  
Mode Water ◽  
Subantarctic Mode Water ◽  
Southeast Pacific ◽  
Water Formation ◽  
Summer Survey

Abstract 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 ε, diapycnal diffusivity κ, and buoyancy flux Jb are estimated from the CTD/O2 and XCTD profiles for each survey. Enhanced κ of O(10−3 to 10−4 m2 s−1) is found near the Subantarctic Front (SAF) during both surveys. During the winter survey, enhanced κ 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 κ across the entire region north of the SAF below the shallow seasonal mixed layer. The enhanced κ 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 × 10−8 m2 s−3) surrounding the SAF during the summer survey is comparable to the mean buoyancy flux (0.57 × 10−8 m2 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.

A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation*

2009 ◽  
Vol 26 (9) ◽  
pp. 1920-1939 ◽  
Author(s):  
James Holte ◽  
Lynne Talley
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Gradient Methods ◽  
Temperature Threshold ◽  
Intermediate Water ◽  
Mode Water ◽  
Southwest Atlantic ◽  
Argo Data ◽  
Southeastern Pacific ◽  
Subantarctic Mode Water

Abstract 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 Montégut 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.

scholarly journals Subantarctic Mode Water Formation, Destruction, and Export in the Eddy-Permitting Southern Ocean State Estimate

2013 ◽  
Vol 43 (7) ◽  
pp. 1485-1511 ◽  
Author(s):  
Ivana Cerovečki ◽  
Lynne D. Talley ◽  
Matthew R. Mazloff ◽  
Guillaume Maze
Keyword(s):  
Southern Ocean ◽  
Buoyancy Flux ◽  
Differential Heat ◽  
Diapycnal Mixing ◽  
Mode Water ◽  
State Estimate ◽  
Surface Formation ◽  
Subantarctic Mode Water ◽  
The Pacific ◽  
The Difference

Abstract Subantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air–sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air–sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air–sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air–sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air–sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air–sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss.

Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

2021 ◽  
Vol 34 (10) ◽  
pp. 3927-3953
Author(s):  
Motoki Nagura
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Surface Mixed Layer ◽  
The South ◽  
South Indian Ocean ◽  
Mode Water ◽  
Horizontal Advection ◽  
South Indian ◽  
Subantarctic Mode Water

AbstractThis study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

scholarly journals An observational study of the evolution of the atmospheric boundary-layer over Cabo Frio, Brazil

2007 ◽  
Vol 25 (8) ◽  
pp. 1735-1744 ◽  
Author(s):  
S. H. Franchito ◽  
V. Brahmananda Rao ◽  
T. O. Oda ◽  
J. C. Conforte
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Mixed Layer ◽  
Coastal Upwelling ◽  
Vertical Mixing ◽  
Austral Summer ◽  
Sea Breeze ◽  
Austral Winter ◽  
Atmospheric Mixed Layer ◽  
Cabo Frio

Abstract. The effect of coastal upwelling on the evolution of the atmospheric boundary layer (ABL) in Cabo Frio (Brazil) is investigated. For this purpose, radiosounding data collected in two experiments made during the austral summer (upwelling case) and austral winter (no upwelling case) are analysed. The results show that during the austral summer, cold waters that crop up near the Cabo Frio coast favour the formation of an atmospheric stable layer, which persists during the upwelling episode. Due to the low SSTs, the descending branch of the sea-breeze circulation is located close to the coast, inhibiting the development of a mixed layer mainly during the day. At night, with the reduction of the land-sea thermal contrast the descending motion is weaker, allowing a vertical mixing. The stable ABL favours the formation of a low level jet, which may also contribute to the development of a nocturnal atmospheric mixed layer. During the austral winter, due to the higher SSTs observed near the coast, the ABL is less stable compared with that in the austral summer. Due to warming, a mixed layer is observed during the day. The observed vertical profiles of the zonal winds show that the easterlies at low levels are stronger in the austral summer, indicating that the upwelling modulates the sea-breeze signal, thus confirming model simulations.

scholarly journals Variability of Subantarctic Mode Water and Antarctic Intermediate Water in the Drake Passage during the Late-Twentieth and Early-Twenty-First Centuries

2009 ◽  
Vol 22 (13) ◽  
pp. 3661-3688 ◽  
Author(s):  
Alberto C. Naveira Garabato ◽  
Loïc Jullion ◽  
David P. Stevens ◽  
Karen J. Heywood ◽  
Brian A. King
Keyword(s):  
Sea Ice ◽  
Drake Passage ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Intermediate Water ◽  
The West ◽  
Mode Water ◽  
Subantarctic Mode Water ◽  
Turbulent Heat Fluxes ◽  
The Antarctic

Abstract A time series of the physical and biogeochemical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantial variability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variations in wintertime air–sea turbulent heat fluxes and net evaporation modulated by the El Niño–Southern Oscillation (ENSO). Despite their spatial proximity, the AAIW in the Drake Passage has a very different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the Bellingshausen Sea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air–sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula. A prominent exception to the prevalent modes of SAMW and AAIW formation occurred in 1998, when strong wind forcing associated with constructive interference between ENSO and the southern annular mode (SAM) triggered a transitory shift to an Ekman-dominated mode of SAMW ventilation and a 1–2-yr shutdown of AAIW production. The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by ∼0.3°C) and salinified (by ∼0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed. In contrast, AAIW underwent a net freshening (by ∼0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a ∼30-yr oscillation in regional air–sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation, with a SAM-driven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, a freshening of the Winter Water ventilating AAIW was brought about by increased precipitation and a retreat of the winter sea ice edge, which were seemingly forced by an interdecadal trend in the SAM and regional positive feedbacks in the air–sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemisphere climate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider South Atlantic and suggest that these modes may have contributed significantly to the hemispheric-scale changes undergone by those waters in recent decades.

scholarly journals Variability and Linkages of New Guinea Coastal Undercurrent and Lower Equatorial Intermediate Current

2008 ◽  
Vol 38 (8) ◽  
pp. 1780-1793 ◽  
Author(s):  
Masaki Kawabe ◽  
Yuji Kashino ◽  
Yoshifumi Kuroda
Keyword(s):  
Austral Summer ◽  
New Guinea ◽  
Intermediate Water ◽  
Austral Winter ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Power Spectral ◽  
Spectral Peaks ◽  
Second Period ◽  
Equatorial Intermediate Current

Abstract Velocity at depths of 700–800 m was measured between September 1998 and October 2002 at 2.5°S, 142°E off the New Guinea coast and at 0°, 138°E to examine the New Guinea Coastal Undercurrent (NGCUC) and the current on the equator carrying Antarctic Intermediate Water (AAIW). Velocity characteristics before November 1999 were markedly different from those after November 1999. The typical state occurred during the second period: the intermediate NGCUC and the Lower Equatorial Intermediate Current (LEIC) varied markedly with an annual cycle in opposite phases. In austral winter, the NGCUC flowed west-northwestward strongly (14 cm s−1, 285°T), especially in May–July during which the LEIC disappeared and eddylike equatorial variations with periods of 20–60 days were significant. In austral summer, the LEIC flowed westward strongly (12 cm s−1, 270°T), especially in October–December, whereas the NGCUC reversed its direction repeatedly to flow east-southeastward in November–February. Thus, the intermediate NGCUC and LEIC are present stably in austral winter and summer, respectively. These variations of the currents must change the pathway of AAIW seasonally. The state during the first period was atypical: the current on the equator flowed eastward strongly (13.0 cm s−1, 81°T), that is, no LEIC was present, and the NGCUC flowed west-northwestward strongly (14.8 cm s−1, 280°T) without changing direction. The atypical state may be related to the 1998–99 La Niña. In addition, power spectral peaks at periods of 14–35 days of meridional velocity at the equator suggest that intermediate tropical instability waves are generated in October–December in the typical state.

scholarly journals Rapid acidification of mode and intermediate waters in the southwest Atlantic Ocean

2014 ◽  
Vol 11 (5) ◽  
pp. 6755-6792
Author(s):  
L. A. Salt ◽  
S. M. A. C. van Heuven ◽  
M. E. Claus ◽  
E. M. Jones ◽  
H. J. W. de Baar
Keyword(s):  
Dissolved Inorganic Carbon ◽  
Buffering Capacity ◽  
Intermediate Water ◽  
Mode Water ◽  
Maximum Increase ◽  
Southwest Atlantic ◽  
Subantarctic Mode Water ◽  
Anthropogenic Component ◽  
Rate Of Increase ◽  
Total Pool

Abstract. Observations along the southwest Atlantic WOCE A17 line made during the Dutch GEOTRACES-NL program (2010–2011) were compared with historical data from 1994 to quantify the changes in the anthropogenic component of the total pool of dissolved inorganic carbon (ΔCant). Application of the extended Multi Linear Regression (eMLR) method shows that the ΔCant from 1994 to 2011 has largely remained confined to the upper 1000 dbar. The greatest changes occur in the upper 200 dbar in the SubAntarctic Zone (SAZ), where a maximum increase of 37 μmol kg−1 is found. South Atlantic Central Water (SACW) experienced the highest rate of increase in Cant, at 0.99 ± 0.14 μmol kg−1 yr−1, resulting in a rate of decrease in pH of −0.0016 yr−1. The highest rates of acidification relative to ΔCant, however, were found in SubAntarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The low buffering capacity of SAMW and AAIW combined with their relatively high rates of Cant increase of 0.53 ± 0.11 μmol kg−1 yr−1 and 0.36 ± 0.06 μmol kg−1 yr−1, respectively, will lead to rapid acidification in the SAZ and simultaneously reduce the chemical buffering capacity of this significant CO2 sink.

scholarly journals Circulation and Water Mass Modification in the Brazil–Malvinas Confluence

2010 ◽  
Vol 40 (5) ◽  
pp. 845-864 ◽  
Author(s):  
Loic Jullion ◽  
Karen J. Heywood ◽  
Alberto C. Naveira Garabato ◽  
David P. Stevens
Keyword(s):  
Deep Water ◽  
Subtropical Gyre ◽  
Strong Interactions ◽  
Intermediate Water ◽  
Mode Water ◽  
Brazil Current ◽  
Subantarctic Mode Water ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Argentine Basin

Abstract The confluence between the Brazil Current and the Malvinas Current [the Brazil–Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.4° (0.08), 0.3° (0.02), and 0.6°C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.4° (0.07) and 0.7°C (0.11), respectively. To quantify those ACC–subtropical gyre interactions, a box inverse model surrounding the confluence is built. The model diagnoses a subduction of 16 ± 4 Sv (1 Sv ≡ 106 m3 s−1) of newly formed SAMW and AAIW under the subtropical gyre corresponding to about half of the total subduction rate of the South Atlantic found in previous studies. Cross-frontal heat (0.06 PW) and salt (2.4 × 1012 kg s−1) gains by the ACC in the BMC contribute to the meridional poleward heat and salt fluxes across the ACC. These estimates correspond to perhaps half of the total cross-ACC poleward heat flux. The authors’ results highlight the BMC as a key region in the subtropical–ACC exchanges.

Sign in / Sign up

Export Citation Format

Share Document