Bottom-driven upwelling generated by eastern intensification in closed and semi-closed basins with a sloping bottom

1999 ◽  
Vol 50 (7) ◽  
pp. 613 ◽  
Author(s):  
M. Herzfeld ◽  
M. Tomczak

Cold water is observed in the eastern Great Australian Bight, and the presence of this water is inconsistent with that expected from surface Ekman-induced upwelling resulting from the imposed windfield. A numerical model is employed to investigate this situation under idealized conditions, from which the bottom slope has been identified as the most dominant factor contributing towards the presence of the cold water. A boundary current generated on the eastern side of the idealized bight is attributed to eastern intensification of a predominantly Stommel type, induced by the topographical gradient. It is proposed that the cold water observed at the eastern boundary is the result of upwelling driven by a bottom Ekman-transport convergence in this region, which is in turn the result of a large bottom stress curl maintained by the eastern intensification.

1998 ◽  
Vol 49 (5) ◽  
pp. 439 ◽  
Author(s):  
Qianyu Li ◽  
Brian McGowran

The distribution of planktonic foraminifera along the southern Australian margin is strongly gradational from warmer assemblages in the west to more temperate assemblages in the east. This pattern follows a decreasing temperature gradient generated by the warm Leeuwin Current which flows southward and then eastward on the southern margin. The abundances of tropical and subtropical species including Globorotalia menardii and Globigerinoides trilobus s.l. decrease rapidly after rounding the south-western corner, while the temperate index species Globorotalia inflata increases and subsequently dominates the fauna towards the east, especially on the Lincoln and Lacepede shelves of South Australia. This W–E gradation is also observed in the relict Pleistocene assemblages, indicating that the Leeuwin Current has been influential since at least the last interglacial time. The interaction between the warm and saline water from the Great Australian Bight and cold water from the south produced an assemblage in deeper parts of the Bight with abundant small species such as Globigerina falconensis, Globotur-borotalita rubescens, Neogloboquadrina pachyderma and Turborotalita quinqueloba.


2008 ◽  
Vol 273-276 ◽  
pp. 388-393
Author(s):  
B.T. Min ◽  
S.W. Hong ◽  
J.H. Kim ◽  
I.K. Park ◽  
H.D. Kim

For the study of a steam explosion phenomenon in a nuclear reactor, prototypic corium, a mixture of UO2 and ZrO2 was melted in a cold crucible by applying an induction heating technique. The molten corium was then poured into cold water. It was fragmented into very small particles, so called debris, which enables a very rapid heat transfer to the water. Some cases led to steam explosions by thermal expansion of the water. After the tests, all the debris particles were dried and classified by their size. From the analysis by using EPMA, it was shown that the particles generated by a steam explosion had fine and irregular forms. It is known that real corium (including UO2) hardly leads to a steam explosion, different from pure ZrO2 or metal. A reason for this was previously suggested in that the corium generated hydrogen gas during melt-water interaction, and it enclosed the melt drops to prevent a direct contact of the corium and water. In order to confirm this fact, the debris particles were analyzed with ICP-AES for their typical element contents, EPMA for the homogeneity of the solid solution, XRD for the chemical compounds, and TGA and hydrogen reduction analysis for the percentage of the debris oxidation and reduction. These analyses showed that hydrogen was not directly related to steam explosion. Meanwhile, the material characteristics of the corium compositions are newly suggested to be the most probable reason for the occurrence of a steam explosion so far.


1997 ◽  
Vol 15 (1) ◽  
pp. 65-69
Author(s):  
Wu De-xing ◽  
Lü Hong-min ◽  
Guo Qun ◽  
Lu Zhen

2013 ◽  
Vol 43 (5) ◽  
pp. 1042-1059 ◽  
Author(s):  
Amandine Schaeffer ◽  
Moninya Roughan ◽  
Bradley D. Morris

Abstract The cross-shelf dynamics up- and downstream of the separation of the South Pacific Ocean’s Western Boundary Current (WBC) are studied using two years of high-resolution velocity and temperature measurements from mooring arrays. The shelf circulation is dominated by the East Australian Current (EAC) and its eddy field, with mean poleward depth-integrated magnitudes on the shelf break of 0.35 and 0.15 m s−1 up- and downstream of the separation point, respectively. The high cross-shelf variability is analyzed though a momentum budget, showing a dominant geostrophic balance at both locations. Among the secondary midshelf terms, the bottom stress influence is higher upstream of the separation point while the wind stress is dominant downstream. This study investigates the response of the velocity and temperature cross-shelf structure to both wind and EAC intrusions. Despite the deep water (up to 140 m), the response to a dominant along-shelf wind stress forcing is a classic two-layer Ekman structure. During weak winds, the shelf encroachment of the southward current drives an onshore Ekman flow in the bottom boundary layer. Both the bottom velocity and the resultant bottom cross-shelf temperature gradient are proportional to the magnitude of the encroaching current, with similar linear regressions up- and downstream of the WBC separation. The upwelled water is then subducted below the EAC upstream of the separation point. Such current-driven upwelling is shown to be the dominant driver of cold water uplift in the EAC-dominated region, with significant impacts expected on nutrient enrichment and thus on biological productivity.


Author(s):  
Harry L. Bryden

Continuous observations of ocean circulation at 26°N in the subtropical Atlantic Ocean have been made since April 2004 to quantify the strength and variability in the Atlantic Meridional overturning circulation (AMOC), in which warm, upper waters flow northward and colder deep waters below 1100 m depth return southward. The principal components of the AMOC are northward western boundary current transport in the Gulf Stream and Antilles Current, northward surface Ekman transport and southward thermocline recirculation, all of which are generally considered to be part of the wind-driven circulation. Southward flowing deep waters below 1100 m depth are usually considered to represent the buoyancy-driven circulation. We argue that the Gulf Stream is partially wind-driven but also partially buoyancy-driven as it returns upper waters upwelled in the global ocean back to water mass formation regions in the northern Atlantic. Seasonal to interannual variations in the circulation at 26°N are principally wind-driven. Variability in the buoyancy-driven circulation occurred in a sharp reduction in 2009 in the southward flow of Lower North Atlantic Deep Water when its transport decreased by 30% from pre-2009 values. Over the 14-year observational period from 2004 to 2018, the AMOC declined by 2.4 Sv from 18.3 to 15.9 Sv.


2016 ◽  
Author(s):  
Johannes Karstensen ◽  
Florian Schütte ◽  
Alice Pietri ◽  
Gerd Krahmann ◽  
Björn Fiedler ◽  
...  

Abstract. The physical (temperature, salinity, velocity) and biogeochemical (oxygen, nitrate) structure of an oxygen depleted coherent, baroclinic, anticyclonic mode-water eddy (ACME) is investigated using high-resolution autonomous glider and ship data. A distinct core with a diameter of about 70 km is found in the eddy, extending from about 60 to 200 m depth and. The core is occupied by fresh and cold water with low oxygen and high nitrate concentrations, and bordered by local maxima in buoyancy frequency. Velocity and property gradient sections show vertical layering at the flanks and underneath the eddy characteristic for vertical propagation (to several hundred-meters depth) of near inertial internal waves (NIW) and confirmed by direct current measurements. A narrow region exists at the outer edge of the eddy where NIW can propagate downward. NIW phase speed and mean flow are of similar magnitude and critical layer formation is expected to occur. An asymmetry in the NIW pattern is seen that possible relates to the large-scale Ekman transport interacting with ACME dynamics. NIW/mean flow induced mixing occurs close to the euphotic zone/mixed layer and upward nutrient flux is expected and supported by the observations. Combing high resolution nitrate (NO3−) data with the apparent oxygen utilization (AOU) reveals AOU:NO3− ratios of 16 which are much higher than in the surrounding waters (8.1). A maximum NO3− deficit of 4 to 6 µmol kg−1 is estimated for the low oxygen core. Denitrification would be a possible explanation. This study provides evidence that the recycling of NO3−, extracted from the eddy core and replenished into the core via the particle export, may quantitatively be more important. In this case, the particulate phase is of keys importance in decoupling the nitrogen from the oxygen cycling.


2010 ◽  
Vol 7 (2) ◽  
pp. 919-971
Author(s):  
C. P. Atkinson ◽  
H. L. Bryden ◽  
J. J.-M. Hirschi ◽  
T. Kanzow

Abstract. Since April 2004 the RAPID array has made continuous measurements of the Atlantic Meridional Overturning Circulation (AMOC) at 26° N. Two key components of this system are Ekman transport zonally integrated across 26° N and western boundary current transport in the Florida Straits. Whilst measurements of the AMOC as a whole are somewhat in their infancy, this study investigates what useful information can be extracted on the variability of the Ekman and Florida Straits transports using the decadal timeseries already available. Analysis is also presented for Sverdrup transports zonally integrated across 26° N. The seasonal cycles of Florida Straits, Ekman and Sverdrup transports are quantified at 26° N using harmonic analysis of annual and semi-annual constituents. Whilst Sverdrup transport shows clear semi-annual periodicity, calculations of seasonal Florida Straits and Ekman transports show substantial interannual variability due to variability at non-seasonal frequencies; the mean seasonal cycle for these transports only emerges from decadal length observations. The Florida Straits and Ekman mean seasonal cycles project on the AMOC with a combined peak-to-peak seasonal range of 3.5 Sv. The combined seasonal range for heat transport is 0.40 PW. The Florida Straits seasonal cycle possesses a smooth annual periodicity in contrast with previous studies suggesting a more asymmetric structure. No clear evidence is found to support significant changes in the Florida Straits seasonal cycle at sub-decadal periods. Whilst evidence of wind driven Florida Straits transport variability is seen at sub-seasonal and annual periods, model runs from the 1/4° eddy-permitting ocean model NEMO are used to identify an important contribution from internal oceanic variability at sub-annual and interannual periods. The Ekman transport seasonal cycle possesses less symmetric structure, due in part to different seasonal transport regimes east and west of 50 to 60° W. Around 60% of non-seasonal Ekman transport variability occurs in phase section-wide at 26° N and is related to the NAO, whilst Sverdrup transport variability is more difficult to decompose.


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