Seasonal Variability of Thermohaline Front in the Central South China Sea

An upper layer thermohaline front across the South China Sea (SCS) basin from the South Vietnamese coast (around 15°N) to Luzon Island (around 19°N) has been identified using the Navy's open domain Generalized Digital Environmental Model (GDEM) monthly mean temperature and salinity data on a 0.5° × 0.5° grid. This front does not occur at the surface in summer. The strength of this front is around 1°C/100 km at the surface and 1.4°C/100 km at the subsurface (50 m deep). A cross-basin current, inverted using the P-vector method, is associating with the front. Meandering and eddies have been identified along this current. Seasonal and vertical variabilities of the thermohaline structure across this front are reported in this paper. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and Variability of Mesoscale Perturbations of Ocean Currents in the Drake Passage and Scotia Sea

Oceanology - Satellite altimeter observation data collected over 22 years are used to analyze the structure and variability of mesoscale fluctuations of ocean currents in the Drake Passage and...     

Vertical Thermohaline Structure,Water Masses,and Large-Scale Fronts in the Southwest Atlantic and Neighboring Antarctic Water Areas

The vertical profiles of temperature and salinity are classified by using the contemporary array of actual and monthly average archive oceanographic data. A scheme of decomposition of the investigated water area into zones according to the typical features of vertical stratification of the thermohaline field is proposed. The criterion of maximum of the horizontal temperature and salinity gradients is used to select and map the principal large-scale frontal boundaries. The parameters of fronts and some regularities of their seasonal variability are described and the data on water masses interacting in the analyzed system of fronts are presented.     

On the detectability of internal tides in Drake Passage

The Southern Ocean hosts significant topographic mixing that might be associated with internal tides. Tidal signals are evident in bottom temperature at 1000 m in Drake Passage, suggesting that internal tides with an amplitude of between ∼20 and 200 m may be present. Various necessary conditions for internal tide generation show that the steep topography in and around Drake Passage can initiate internal tides, and recent global tide models have suggested this region to generate very large interface displacements. Here, we present an attempt to detect internal tides in Drake Passage. During the last 10 years, combinations of bottom pressure recorders and inverted echo sounders have been deployed in the region. The bottom pressure recorders measure predominantly the barotropic tide; the inverted echo sounders measure travel time from sea bed to sea surface and therefore are influenced both by sea level (barotropic tide) and internal sound speed (internal tide). By subtracting one from the other, the internal tide should be detectable. Although the technique works successfully around Hawaii, it does not prove the existence of large internal tides in Drake Passage. The detectability of the internal tidal signal in Drake Passage is investigated using a six-layer one-dimensional model to simulate the bottom pressure and travel time signals of a semi-diurnal tide. The temperature and salinity stratification in Drake Passage is sufficiently weak that large vertical excursions are necessary to produce a signal in travel time detectable above the noise in Drake Passage. An internal tide of at least 70 or 20 m in northern and southern Drake Passage, respectively, would be detected. The fact that these are, perhaps surprisingly, not detected by the combination of bottom pressure and travel time, constrains the internal tides in Drake Passage to be ∼20 m in southern Drake Passage, and between 20 and 70 m in northern Drake Passage. The model also predicts that satellite altimetry would not be able to detect internal tides in Drake Passage, but would in the Brazil Basin and Hawaii regions.     

Dynamics of the current system in the southern Drake Passage

It has long been seen from satellite ocean color data that strong zonal gradients of phytoplankton biomass persistently occur in the southern Drake Passage during austral summer and fall, where the low productivity Antarctic Surface Water (ASW) within the Antarctic Circumpolar Current (ACC) region transforms to the high productivity water. An interdisciplinary cruise was conducted in February and March 2004 to investigate potential physical and biogeochemical processes, which are responsible for transporting nutrients and metals and for enhancing primary production. To explore physical processes at both the meso- and large-scales, surface drifters, a shipboard Acoustic Doppler Current Profiler and conductivity–temperature–depth sensors were used. Analyzing meso- and large-scale hydrography, circulation and eddy activities, it is shown that the topographic rise of the Shackleton Transverse Ridge plays the key role in steering an ACC branch southward west of the ridge, forming an eastward ACC jet through the gap between the ridge and Elephant Island and causing the offshelf transport of shelf waters approximately 1.2 Sv from the shelf near Elephant Island. High mesoscale eddy activities associated with this ACC southern branch and shelf waters transported off the shelf were found. The mixing between the iron-poor warmer ASW of the ACC and iron-rich waters on the shelf through horizontal transport and vertical upwelling processes provides a physical process which could be responsible for the enhanced primary productivity in this region and the southern Scotia Sea.     

Antarctic bottom water in the Scotia Sea and the Drake Passage

The quantitative features and circulation of the Antarctic bottom water (AABW) in the Scotia Sea are investigated using an original procedure for the determination of the boundaries between the water masses. It is shown that the AABW is effectively transferred across the Antarctic Circumpolar Current (ACC) from the regions on the south flank of this current where the AABW penetrates into the Scotia Sea. This transfer results in the abyssal water cooling and freshening in the Yaghan Basin of the north Scotia Sea. Some rises and depressions in the bottom relief of the western and northern Scotia Sea are important features that impact the AABW transfer. It is shown that there is an additional path of the AABW transit transport to the North Atlantic passing through the western Scotia Sea. The existence of the semienclosed cyclonic abyssal water circulation in the South Shetland Trench and the westward transport of the Atlantic AABW along the Antarctic slope foot into the Pacific are proved.     

Particulate carbohydrate fluxes in the Bransfield Strait and the Drake Passage

Carbohydrate fluxes were determined for five drifting sediment traps (max. 30 h) in the Bransfield Strait and one moored trap array (52 days) in the Drake Passage in December/January 1980. In a Phaeocystis-dominated zone in the Bransfield Strait the carbohydrate composition was comparable to that of water column suspended material with absolute fluxes of 9.0 and 56.7 mg carbohydrate carbon m⁻²d⁻¹. In Bransfield Strait waters dominated by bacillariophyceae, fluxes were 75.4, 92.6 and 276.8 mg carbohydrate carbon m⁻²d⁻¹ with a pronounced dominance of glucose. This is attributed to the presence of the reserve polysaccharide chrysolaminarin in diatom resting spores. Drake Passage fluxes were 1–2 orders of magnitude lower with a distinct degradation of glucose with depth.     

Circumpolar bottom water in the Scotia Sea and the Drake Passage

The quantitative properties and circulation of the lower layer of circumpolar water in the Scotia Sea with density 28.16 < γ ⁿ < 28.26 (potential temperature 0.9° > θ > 0.2°C) are investigated using the original procedure for determination of boundaries between water masses. The primary objective of this work is data analyses of four Russian sections, which were occupied in the vicinity of the Shackleton Fracture Zone in 2003, 2005, and 2007. It is shown that the ridges in the Hero and Shackleton fracture zones essentially constrain overflow of the lower layer of circumpolar water, and thereby, they produce the conditions to the east of the Shackleton Ridge for transformation (freshening and warming) of this layer reaching the northern side of the Antarctic Circumpolar Current. These ridges also promote formation of several quasi-permanent and semi-enclosed abyssal and deep-water eddies adjacent to these ridges. The estimation of overflow of the lower part of the investigated layer with density 28.23 < γ ⁿ < 28.26 (0.9° > θ > 0.2°C) through the Shackleton Ridge based on LADCP measurements in 2007 is 0.5 Sv (0.1 Sv) to the east (west). The upper part of the overflow is estimated as 8.0 (7.9) Sv. Thus, the total transport of the lower layer of circumpolar water through the ridge is practically zero. It is confirmed by LADCP measurements carried out on the section across the Drake Passage in 2003.     

Long-term trends and interannual variability of temperature in Drake Passage

Temperature data collected over the last 36 years (1969–2004) in Drake Passage are used to examine interannual temperature variation and long-term trends in the upper ocean. To reduce the effect of variation from different sampling locations and temporal variability introduced by meridional shifts in the Polar Front (PF), the data were divided into two sub-regions north (3800 temperature profiles) and south (3400) of the PF. Temperature anomalies were formed by removing a temporal mean field for each profile in each sub-region at 100 m depth intervals from the surface to 700 m. North of the PF, statistically significant warming trends of 0.02 °C yr⁻¹ were observed that were largely depth-independent between 100 and 700 m. A statistically significant cooling trend of −0.07 °C yr⁻¹ was observed at the surface south of the PF, which was smaller (−0.04 °C yr⁻¹) but still statistically significant when possible seasonal sampling biases were accounted for. The observed cooling at the surface and warming at depth is largely consistent with a poleward shift of the PF due to enhancement of westerly winds in the Southern Ocean, as recently suggested by models and observations. The observed annual temperature anomalies in the upper 400 m north of the PF and in the upper 100 m south of the PF are highly correlated to variability in sea ice, and also to climate indices of the Antarctic Oscillation and the El Niño Southern Oscillation. Variability in sea ice and temperature anomalies lag El Niño variability in the Pacific, with a phasing consistent with the observed cyclical patterns of sea ice and sea surface temperature associated with the Antarctic Circumpolar Wave or Antarctic Dipole Mode in the Southern Ocean. In contrast, the sea ice variability and temperature anomalies at all depths north of the PF and at 0–100 m depth south of the PF were primarily coincident with, or led the Antarctic Oscillation Index. No significant correlations were found with the large-scale climate variability indices in southern Drake Passage below 100 m depth, which is occupied by upper Circumpolar Deep Water (uCDW). This water mass is not formed locally, is largely isolated from the surface, and exhibits vertical and lateral homogeneity. Hence changes may be difficult to detect in the available measurements, and climate variation in the source water regions of uCDW may take a long time to reach Drake Passage.     

德雷克海峡上层海洋温度的年代际变化

采用经验模分解方法分析了德雷克海峡上层海洋温度的年代际变化特征.结果表明德雷克海峡上层海洋平均温度在1976-1986年以平均每年0.124 ℃的速度下降,而在1987-1998年以每年0.144 ℃的速度上升.与南方涛动指数的年代际时间尺度分量的相关性分析结果表明,热带太平洋海面气压场的变化对德雷克海峡上层海洋温度的年代际变化有一定影响.     

Seasonal cycle of circulation in the Antarctic Peninsula and the off-shelf transport of shelf waters into southern Drake Passage and Scotia Sea

The seasonal cycle of circulation and transport in the Antarctic Peninsula shelf region is investigated using a high-resolution (∼2 km) regional model based on the Regional Oceanic Modeling System (ROMS). The model also includes a naturally occurring tracer with a strong source over the shelf (radium isotope ²²⁸Ra, t_1/2=5.8 years) to investigate the sediment Fe input and its transport. The model is spun-up for three years using climatological boundary and surface forcing and then run for the 2004–2006 period using realistic forcing. Model results suggest a persistent and coherent circulation system throughout the year consisting of several major components that converge water masses from various sources toward Elephant Island. These currents are largely in geostrophic balance, driven by surface winds, topographic steering, and large-scale forcing. Strong off-shelf transport of the Fe-rich shelf waters takes place over the northeastern shelf/slope of Elephant Island, driven by a combination of topographic steering, extension of shelf currents, and strong horizontal mixing between the ACC and shelf waters. These results are generally consistent with recent and historical observational studies. Both the shelf circulation and off-shelf transport show a significant seasonality, mainly due to the seasonal changes of surface winds and large-scale circulation. Modeled and observed distributions of ²²⁸Ra suggest that a majority of Fe-rich upper layer waters exported off-shelf around Elephant Island are carried by the shelfbreak current and the Bransfield Strait Current from the shallow sills between Gerlache Strait and Livingston Island, and northern shelf of the South Shetland Islands, where strong winter mixing supplies much of the sediment derived nutrients (including Fe) input to the surface layer.     

Currents in the Drake Passage based on observations in 2007

Currents in the Drake Passage are studied using the data of a hydrographic section along the Shackleton Transverse Ridge observed in November 2007. The distribution of the velocity component normal to the section was computed by two methods: (a) directly on the basis of the lowered acoustic Doppler current profiler (LADCP) data; (b) by the calculation of geostrophic currents based on the CTD data with estimating the depth averaged velocity on the basis of the LADCP data. The main results of the research are the relatively low estimated value of the Antarctic Circumpolar Current (ACC) transport, which accounts for 65–70% of the ACC transports in December 2003 and November 2005, and the confirmation of the existence of several abyssal currents confined to the deep passages of the bottom topography.     

Distribution of chaetognaths in the central part of the Drake Passage

The mesoscale distribution of two chaetognath species, Eukrohnia hamata and Sagitta gazellae, found in the Drake Passage in October and November of 2008 was studied using a stationary grid in the area of the Antarctic Polar Front. The distance between the stations was around ten miles. The most abundant chaetognath species was Eukrohnia hamata, which accounted for 86.7% of all the individuals collected. In the upper 200 m, the average abundance of chaetognaths decreased from 2.1 to 0.8 ind./m³, and their average biomass decreased from 1.25 to 0.25 mg/m³ over the transition through the front from the north to the south. The high heterogeneity of the spatial chaetognath distribution in the area was demonstrated: the abundance of the chaetognaths could increase or decrease over 20 miles by a factor of 45 (6.8 and 0.15 ind./m³). In addition, two peaks were revealed in the size structure of the E. hamata population, probably indicating two periods of reproduction in this species in the Drake Passage.     

Seasonal Variations of Water Masses and Sea Level in the Southwestern Part of the Okhotsk Sea

A new grid data set for the southwestern part of the Okhotsk Sea was compiled by using all the available hydrographic data from the Japan Oceanographic Data Center, World Ocean Atlas 1994 and the other additional data sources with the resolution of about 10 km. We examine the seasonal variations of areas and volumes of Soya Warm Current Water (SWCW) and East Sakhalin Current Water (ESCW) and show that the exchanges of these water masses drastically occur in April and November. The peculiar variation of sea level in this region is also related with the water mass exchange. Sea level at the Hokkaido coast of the Okhotsk Sea reaches its minimum in April about two months later than in the case of ordinary mid-latitude ocean, and its maximum in December besides the summer peak. The winter peak of sea level in December is caused by the advent of fresh and cold ESCW which is accumulated at the subsurface layers (20–150 m) through the Ekman convergence by the prevailing northerly wind. Sea level minimum in April is caused by the release of the convergence and the recovery of dense SWCW that is saline and much colder than that in summer.     

Absolute geostrophic currents in the Drake Passage based on observations in 2003 and 2005

Geostrophic currents in the Drake Passage are studied using the data of two hydrographic sections across the passage occupied in December 2003 and November 2005 along with satellite altimetry data. A conclusion is reached that the altimetry correction of the geostrophic currents has advantages compared to the correction made on the basis of the lowered acoustic Doppler current profiler data. A number of new results about the structure and intensity of the ocean currents in the Drake Passage are obtained; the main one is the distinguishing of several abyssal currents of westward direction confined to deep passages in the bottom topography.     

Currents in the Western Drake Passage according to the observations in January of 2010

The Western Drake Passage current system is investigated using the CTD, LADCP, and SADCP data of the cross Drake section carried out in January 2010. A complicated current structure consisting of the six Antarctic Circumpolar Current (ACC) jets as well as the system of slope and abyssal currents was revealed. The most interesting result is the identification of the abyssal quasi-geostrophic spurts in the northern part that probably are generated by abyssal eddy fragments, which are an imperative part of the meandering ACC.     

Seasonal Variability of Water and Sea-Ice Circulation in the Arctic Ocean in a High-Resolution Model

Izvestiya, Atmospheric and Oceanic Physics - In this work, results of modeling the intra-annual variability of water and ice circulation in the Arctic Ocean by means of the INMIO4.1...