OS43A-01
Spin-up of South Pacific Subtropical Gyre Freshens and Cools the Upper Layer of the Eastern South Pacific Ocean
The general circulation in the South Pacific Ocean is dominated by the subtropical gyre, which manifests itself through elevated mean dynamic topography at its center. Gyre circulation consists of the westward South Equatorial Current, a narrow poleward western boundary current, the East Australian current, the eastward South Pacific Current streaming along the South Tropical Front (centered at around 40°S in the western ocean basin and at 30-35°S in the eastern basin), and the Humboldt Current System, a broad equatorward eastern boundary current, (in the literature, also referred to as the Peru/Chile Current) (Tomczak and Godfrey 1994; Levitus 1982; Reid 1986). The volume transport of upper water (700 m) between the Pacific coast of South America and the East Pacific Rise amounted to 18 Sv across 32.5°S (WOCE section P06) and 14 Sv across 17°S (WOCE section P21) (Tsimplis et al. 1998), emphasizing the importance of equatorward transport by this eastern boundary current system. This boundary current also plays a vital role in the fresh water budget by advecting fresher Subantarctic Surface Water northward thus forming Eastern South Pacific Transition Water (Emery and Meincke 1986). Here, temperature and salinity from the upper 200 m of the water column in the South Pacific Ocean were compared basin wide along 32°30'S between 2003 and 1992, based on two vertically and horizontally high resolution hydrographic repeat-sections involving 227 station pairs (WOCE, BEAGLE). Additionally, the seasonal cycles of the upper water column temperature and salinity between 90- 140°W and 30-35°S were established utilizing more than 1500 ARGO profiles from 2003 to 2006. The surface waters (0-200 m) of the eastern South Pacific Ocean, on average and seasonally adjusted, were clearly fresher in 2003 by 0.14 PSU. The seasonally adjusted, depth integrated temperature was 0.25°C colder in the same region. We further concluded a spin-up of the South Pacific subtropical gyre circulation since the fading of the 1997/98 El Niño, as referred from observations of satellite-born mean sea level anomalies, caused by intensified wind stress over the South Pacific. We relate the observed temperature and salinity changes in the eastern South Pacific to a spin-up of the subtropical gyre circulation caused by intensified winds.
OS43A-02 INVITED
Continental Shelf Currents off Perth, Western Australia
An Acoustic Doppler Current Profiler, ADCP, was moored a few meters above the bottom at the 70 m isobath out from the Perth, Western Australia for a little over a year in 2000/2001. It recorded currents in 4 m depth bins every hour as well as the temperature at the instrument. The ADCP was put at the 70 m isobath so that it would come under the influence of both the wind-driven Capes Current in summer and perhaps incursions by the Leeuwin Current in all seasons. The record shows both these currents and is rich with the effects of other phenomena including upwelling, downwelling, sea/land breezes, summer and winter storms, and the passing of submesoscale eddies on the continental shelf. We discuss the ADCP data and complement them with satellite images, wind and sea level data.
OS43A-03
Onshore Boulder Deposits Near Krakatau Volcano, Indonesia
Onshore coral blocks were found at two sites in 60 km radial distance from Krakatau volcano which erupted and triggered tsunamis in Sunda Strait in the years 416 and 1883. The greatest of the 1883 eruptions generated a tsunami that ran up as high as 36m and reportedly moved coral boulders larger than 300 m3. At one of these sites, Gubug Garam on the east shore of Lampung Bay, Sumatra (S 5° 36' E 105° 24'), more than 40 boulders rest on a 70m wide horizontal coral platform in the inter tidal region. Their volumes range from 1 m3 to > 4m3. Their long axes increasingly align with the trend of the coastline the farther the blocks were transported. Inland, on ground underlain by beach sand and volcano-clastic deposits, more than 20 coral blocks up to 3 m3 as much as 500 m from the shore were found. Most of these boulders are embedded in the sediments. Digging beneath six of the boulders, it was found that they rest on top of pumice and tuff, which in turn overlies former topsoil. Above the pumice and tuff, the material surrounding the blocks is the present topsoil which consists of clayey silt or silty clay with some shell and branching coral fragment. The other site, near Anyer, Java (S 6° 4' E 105° 53'), has a similar topographic setting. The reef platform, 75 m wide, is littered with more than 20 blocks as large as 3 m3. Inland, as much as 150 m from the coastline, 5 very large coral blocks, the largest measuring 300 m3 were found. As at Gubug Garam, these blocks rest on pumice and tuff that are underlain by former topsoil. We thus infer that the blocks at both locations where displaced during the same event.
OS43A-04
Sandy Deposits of the 2006 Java Tsunami
A field reconnaissance was performed out a week after Java tsunami on July, last year. It focused on documentation of sedimentary deposit as the first step to study delineate tsunami history in extend to tsunami hazard assessments. In a region with tremendous tsunami thread, the 2006 Java tsunami still gave a shock with at least 650 death tolls at 300 km affected coast line. The slight felt shaking onshore and unobvious leading recession of sea water were probably the reason as these were the reverse indications of tsunami to the lessons learned from the great Sumatra 2004 event. In addition, although tsunami of similar size have struck the southern coast of Java at least twice in the last century, in 1921 and 1994, tsunami history and the potential threat is still unknown to local population. Three shore normal transects across coastal plain were made in the two affected sites to exhibit the recent tsunami deposit thickness, internal layering, and landward extent. During the documentation, we also discovered two additional sand beds at the end of 2006 deposit landward extent in greater depth. All this information may provides preliminary information to better identify the geologic traces of earlier Javanese tsunami which in turn may help constructing Java's tsunami history.
OS43A-05
Coastal Sedimentation And Risks Of Tsunami Associated With 26Th December 2004 In The Kanyakumari Coast Tamil Nadu, India
The Tsunami signatures were formulated from the field evidence from 26th December 2004 tsunami surge in the Kanyakumari coast. The impact of Tsunami can be identified form the preservation of geomorphic signatures and sedimentary deposits. The more common signature of Tsunami in the deposition of sand with thickness of 20cm towards landward and are sandwiched between finer material and debris on flat coastal plains. Black sands were transported by the strong Tsunami wave flow across the coastal vegetation and could be seen as deposits discontinuous pencil thin lenses in the landward. Thicker units were characterized by a series of lighter minerals fining upwards stacked one upon each other. Each unit indicates the single wave in the Tsunami wave train. Further the relative size of sediment in each site gives and indication of the magnitude of each Tsunami wave. Coarse marine sand mixed with pebbles in landward. tapering sheets was noticed. The Tsunami characteristics were heavily dependent upon the configuration of the coastline. Run up heights In the many locations were three times greater than the initial height of the wave at shore. The damage of concrete roof and Manakudi Bridge indicates the flow velocities and force of tsunami. The Tsunami deposited 15 to 20 can thick sand splays behind sand dunes of chothavilai, pallam and Azhikal coast were seen. The dump deposits were observed around obstacles and road erosion also occurred with formation of turbulent vortices around obstacles and in channelised backwash. Flow velocities were interpreted form these sediment features with those of structural and building damages. A careful observation in the fields indicate that the coast of Kanyakamari is more susceptible to tsunami run up, flooding and inundation. The type of offshore bathymetry and coastal setting are prone to tsunami. The tsunami flood across river inlet and offshore bathymetry is steep. The river mouth surfaces lying only a few metters above sea level have allowed tsunami to penetrate long distances inland. Tsunami wave approached the shore rapidly and with most of their energy intact in manakudi and colachel. The wave at shallow depth of river inlet / creek has traveled less whereas the deep area experienced the more flood. The residents living along the river banks were affected by the tsunami. The risk is very high. Tsunami has an affinity for head lands. They are not blocked by cliffs. The wave energy is concentrated here by wave refraction. Amplitude of wave on headlands is two to three folds relative to an adjacent embayed beach.
OS43A-06
Origin and Transport Mechanism of Iodine-129 to the Japan Sea
Iodine-129 is a long-lived radioisotope with a half life of 1.57 „e 107 years and produced naturally (129Inatural) by cosmic ray-induced spallation of xenon and spontaneous fission of uranium. Anthropogenic 129I has two main sources of releases into the environment during the last 60 years: one is nuclear weapons testing (129INWT) and the other is nuclear fuel reprocessing plants (129INFRP). Because of its long half life, anthropogenic 129I has a potential using as a tracer of the migration behavior of iodine for the last several decades. In this presentation, we discuss not only the origin but also the transport mechanism of 129I to the Japan Sea. Seawater samples were collected at the Toyama Bay and a region off Sekine in or near the Japan Sea. The concentrations of 129I in these samples were determined by accelerator mass spectrometry. The observed concentrations exceed the amounts expected from 129Inatural and 129INWT. The total fraction of 129Inatural and 129INWT is only a few percent. The majority of the concentration must come primarily from nuclear fuel reprocessing plants in Europe. This result indicates a rapid distribution of 129I through atmospheric transport on a global scale. A depth profile of 129I in a seawater column at the Toyama Bay shows that the 129I maximum is in a mixed layer and decrease with depth. The inventory of 129I in the Toyama Bay is four times higher than the Gulf of Mexico which has almost the same depth as the Toyama Bay. This higher inventory probably reflects: 1) the seawater rapid sinking in the Japan Sea, 2) the difference of sampling locations associated with a distance from 129I released points and latitudinal distribution and 3) the differences of sampling dates before which integrated emissions from nuclear fuel reprocessing plants differed.
OS43A-07
N-uptake and f-ratio Characteristics of the Indian Ocean
The Indian Ocean, an ideal laboratory for oceanographic studies, consists of three different biogeochemical provinces: the most productive Arabian Sea, Oligotrophic equatorial Indian Ocean and nutrient rich "HNLC" region in south. The present study investigates the N-uptake and f-ratio characteristics of these three provinces of the Indian Ocean and asses its potential role in the Global Carbon Cycle. Here we present 15N based results from the eastern Arabian Sea, equatorial Indian Ocean and the Southern Indian Ocean. The Arabian Sea, one of the most biologically productive regions of the world ocean, is driven by seasonally reversing Southwest and Northeast monsoons. Development of Noctiluca bloom during winter is well documented in literature but data available on the nitrogen uptake and f-ratios is limited. Our results of 15N based productivity from the Arabian Sea, in conjunction with earlier reported results, reveal consistent high column N-uptake and f-ratios over consecutive years during bloom conditions; mean N-uptake and f-ratio are $20.1 (± 4) mmol N m-2 d-1 and 0.86 (±0.06) respectively. Though N-uptake values are comparable with the values reported (23.2 mmol N m-2 d-1), f-ratios are significantly higher suggesting efficient utilization of available nitrate in the water column. Persistent high productivity and f-ratios over years during a bloom suggest more efficient and strong biological pump in the Arabian Sea compared to other regions of the world ocean. The increased production in the surface layer causes formation of the most severe oxygen minimum zone in this part of the world which may cause vigorous denitrification resulting in the formation of radiatively inert N2 and a potent green house gas N2O and thus can contribute significantly to the global warming. The Indian Sector of the Southern Ocean is traditionally regarded as a low productive area. Our measurements show that even though the productivity of this region is low, the f-ratio is moderately high (mean=0.49). The plot of total N-uptake (on x-axis) and nitrate uptake (on y-axis) shows very significant correlation between the two: y = (0.63 ± 0.06) x - (0.66 ± 0.42) (coefficient of determination, r2 = 0.95). The slope of line of regression (0.63) suggests the maximum possible value of f-ratio for this zone. This signifies that a large part of productivity could get transported to deeper ocean and thus this area has the potential to play a significant role in atmospheric carbon sequestration.
OS43A-08
Variability of the Atlantic meridional overturning circulation during 2004-2005 as observed from the 26°N RAPID-MOC Array
The Atlantic Meridional Overturning Circulation (MOC) strength and variability are estimated from the first year of the joint U.K./U.S. RAPID-MOC Array across 26°N in the Atlantic. The overall measurement strategy relies on deep water endpoint "dynamic height" moorings on either side of the basin to monitor the basin-wide geostrophic shear, combined with observations from clusters of moorings up the western (Bahamas) and eastern (African) continental margins, and direct measurements of the flow through the Straits of Florida by electromagnetic cable. Ekman transports derived from satellite winds are then combined with the geostrophic and direct current observations and an overall mass conservation constraint to continuously estimate the basin- wide MOC strength and vertical structure. Precision bottom pressure gauges were also employed to monitor absolute transports including barotropic circulation. The annual mean strength and standard deviation of the MOC for the period from March 2004 to March 2005 was 18.1±5.1 Sv, with instantaneous (daily) values varying over a range from nearly 10-30 Sv. The Florida Current, Ekman, and mid-ocean geostrophic transport are found to contribute about equally to the variability in the upper ocean limb of the MOC. Independent estimates of the five main contributions to net mass transport across the section (Florida Current, Bahamas western boundary, interior ocean geostrophic internal and external components, and Ekman transport) show a near balance on time scales longer than 20 days, indicating the system is working and is producing reliable estimates of the MOC. The large range of instantaneous values for the MOC strength in this one year of observations suggests that MOC estimates derived from one-time hydrographic sections are likely to be seriously aliased by short-term variability. Although the short-term variability of the MOC is large, the standard error in this one-year estimate derived from the autocorrelation statistics of the time series is approximately 1.5 Sv. Thus the array should be capable of resolving interannual variability or trends of the order of a few Sverdrups.