Numerical assessment of bathymetric changes caused by the 2004 Indian Ocean tsunami at Kirinda Fishery Harbor,Sri Lanka

Thus far various numerical models have been developed and improved to aid understanding of the sediment transport process due to tsunamis. However, the applicability of these models for the field-scale bathymetric change remains a major issue due to the scarcity of measured bathymetric data immediately before and after tsunamis. This study focuses on assessing the applicability of the sediment transport model by comparing the model results with measured bathymetry data obtained one month before and two months after the 2004 Indian Ocean tsunami at Kirinda Fishery Harbor, Sri Lanka. Obtained model results were compared with measured data along four different transects. In particular, similar to the measured data, the model reproduced the bed level change at the harbor mouth well, although it shows some discrepancy on bathymetric change along the shoreline, which is directly affected by littoral drift. Therefore, it is noted that the divergence of reproducing the local bathymetry change is due to the normal wind wave effect on measured data and the model limitations. Hence we included the wind wave effect in modeled data and the discrepancy between measured and modeled data was reduced. Furthermore, the modeled bed level change indicates a dynamic behavior in terms of the net variation during the tsunami flow, such that deposition dominates in the inflow and erosion dominates in the backflow. Both bed level variation and the suspended load concentration reveal that the large amount of eroded sediment attributable to tsunami waves was in suspended form and was deposited in the nearshore area after the water fluctuation had abated. The model results further indicate that eroded sediment at the initial depth deeper than 11 m might be brought by the incoming tsunami waves and deposited in the nearshore area where the depth is shallower than 7 m.     

Analysis of temporal and spatial characteristics of waves in the Indian Ocean based on ERA-40 wave reanalysis

Based on the 45-year (09/1957-08/2008) European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-40) wave reanalysis dataset, this study analyzes interannual and interdecadal variabilities and intraseasonal oscillations of sea surface wind speed (WS), wind sea wave height (H^w), swell wave height (H^s) and significant wave height (H_s) in the Roaring Forties and tropical waters of the Indian Ocean, to determine swell propagation characteristics. The results show: (1) monthly variabilities of H^s in the Roaring Forties are in good agreement with those in tropical waters of the Indian Ocean; swell plays a dominant role in mixed waves throughout most of the Indian Ocean; and WS, H^w, H^s, and H_s exhibit a significant increasing trend over the 45-year study period. (2) H^s in the Roaring Forties and tropical waters of the Indian Ocean share a common period of 9.8–10.4 years on an interdecadal scale; and WS and H^s in the Roaring Forties and H^s in the tropical waters of the Indian Ocean share a common period of approximately 8 days (weekly oscillation) on an intraseasonal scale. (3) Swell of the Roaring Forties needs approximately 30 h to fully respond to the wind in this region. Approximately 84 h are required for H^s to propagate from the Roaring Forties to the tropical waters of the south Indian Ocean, while it takes approximately 132–138 h for H^s to propagate from the Roaring Forties to the tropical waters of the north Indian Ocean.     

Ectoenzymatic activity in surface waters: A transect from the Mediterranean Sea across the Indian Ocean to Australia

The activities of two hydrolytic enzymes (leucine aminopeptidase and β glucosidase), belonging to the particle-bound enzymatic fraction, were measured in open-sea surface waters. Samples were collected along a transect crossing the Indian Ocean during the early NW monsoon period (November and December 2001). The latitudinal pattern of the ectoenzymatic activities highlighted a generally increasing trend of glycolysis approaching the equator, with significantly higher β glucosidase activity (0.79–3.00 nmol l⁻¹ h⁻¹) within the latitudinal range from 12°N to 16°S. In this area, the surface waters coming from the Indonesian Throughflow and the Bay of Bengal carry a considerable quantity of carbohydrates (38.9–41.9 μg l⁻¹), which stimulated glycolytic activity and its cell-specific rates scaled to bacterial abundance. On the other hand, in the Central Indian Ocean, the proteolytic activity was considerable (0.91–2.03 nmol l⁻¹ h⁻¹), although the particulate proteins did not show significant increases and the dissolved proteinlike signal was one of the lowest of the entire transect (0.7 mg l⁻¹ on average compared to the 1.4–1.6 mg l⁻¹ of the adjacent areas). Therefore, in this area, the two ectoenzymes studied did not respond to the same stimulatory effect (namely the specific substrate concentrations). The time needed for the hydrolysis of macromolecules within the particulate and dissolved organic substrate fractions, although these measures are affected by a number of assumptions starting with the potential nature of the ectoenzymatic determinations, confirms these observations. The Central Indian Ocean displayed the lowest values, from 8 to 26 days for particulate and dissolved organic carbon, respectively. As observed in the equatorial areas of the Atlantic Ocean, the relevant degradation activity of the central area of the Indian Ocean Basin suggests a notable heterotrophy based on a faster turnover of organic substrates.     

Role of the Agulhas Current in Indian Ocean circulation and associated heat and freshwater fluxes

A reduced estimate of Agulhas Current transport provides the motivation to examine the sensitivity of Indian Ocean circulation and meridional heat transport to the strength of the western boundary current. The new transport estimate is 70 Sv, much smaller than the previous value of 85 Sv. Consideration of three case studies for a large, medium and small Agulhas Current transport demonstrate that the divergence of heat transport over the Indian Ocean north of 32°S has a sensitivity of 0.08 PW per 10 Sv of Agulhas transport, and freshwater convergence has a sensitivity of 0.03×10⁹ kg s⁻¹ per 10 Sv of transport. Moreover, a smaller Agulhas Current leads to a better silica balance and a smaller meridional overturning circulation for the Indian Ocean. The mean Agulhas Current transport estimated from time-series current meter measurements is used to constrain the geostrophic transport in the western boundary region in order to re-evaluate the circulation, heat and freshwater transports across 32°S. The Indonesian Throughflow is taken to be 12 Sv at an average temperature of 18°C. The constrained circulation exhibits a vertical–meridional circulation with a net northward flow below 2000 dbar of 10.1 Sv. The heat transport divergence is estimated to be 0.66 PW, the freshwater convergence to be 0.54×10⁹ kg s⁻¹, and the silica convergence to be 335 kmol s⁻¹. Meridional transports are separated into barotropic, baroclinic and horizontal components, with each component conserving mass. The barotropic component is strongly dependent on the estimated size of the Indonesian Throughflow. Surprisingly, the baroclinic component depends principally on the large-scale density distribution and is nearly invariant to the size of the overturning circulation. The horizontal heat and freshwater flux components are strongly influenced by the size of the Agulhas Current because it is warmer and saltier than the mid-ocean. The horizontal fluxes of heat and salt penetrate down to 1500 m depth, suggesting that warm and salty Red Sea Water may be involved in converting the intermediate and upper deep waters which enter the Indian Ocean from the Southern Ocean into warmer and saltier waters before they exit in the Agulhas Current.     

Dianeutral mixing,transformation and transport of the deep water of the Indian Ocean

The realization of North Atlantic Deep Water (NADW) replacement in the deep northern Indian Ocean is crucial to the “conveyor belt” scheme. This was investigated with the updated 1994 Levitus climatological atlas. The study was performed on four selected neutral surfaces, encompassing the Indian deep water from 2000 to 3500 m. The Indian deep water comprises three major water masses: NADW, Circumpolar Deep Water (CDW) and North Indian Deep Water (NIDW). Since NADW flowing into the southwest Indian Ocean is largely blocked by the ridges (the Madagascar Ridge in the east and Davie Ridge in the north in the Mozambique Channel) and NIDW is the only source in the northern Indian Ocean that cannot provide a large amount of volume transport, CDW has to be a major source for the Indian deep circulation and ventilation in the north. Thus the question of NADW replacement becomes that of how the advective flows of CDW from the south are changed to be upwelled flows in the north—a water-mass transformation scenario. This study considered various processes causing motion across neutral surfaces. It is found that dianeutral mixing is vital to achieve CDW transformation. Basin-wide uniform dianeutral upwelling is detected in the entire Indian deep water north of 32°S, somewhat concentrated in the eastern Indian Ocean on the lowest surface. However, the integrated dianeutral transport is quite low, about a net of 0.2 Sv (1 Sv=10⁶ m³ s^-1) across the lowermost neutral surface upward and 0.4 Sv across the uppermost surface upward north of 32°S with an error band of about 10–20% when an uncertainty of half-order change in diffusivities is assumed. Given about 10–15% of rough ridge area where dianeutral diffusivity could be about one order of magnitude higher (10^-4 m² s^-1) due to internal-wave breaking, the additional amount of increased net dianeutral transport across the lowest neutral surface is still within that error band. The averaged net upward transport in the north is matched with a net downward transport of 0.3 Sv integrated in the Southern Ocean south of 45°S across the lowermost surface. With the previous works of You (1996. Deep Sea Research 43, 291–320) in the thermocline and You (Journal of Geophysical Research) in the intermediate water combined, a schematic dianeutral circulation of the Indian Ocean emerges. The integrated net dianeutral upwelling transport shows a steady increase from the deep water to the upper thermocline (from 0.2 to 4.6) north of 32°S. The dianeutral upwelling transport is accumulated upward as the northward advective transport provided from the Southern Ocean increases. As a result, the dianeutral upwelling transport north of 32°S can provide at least 4.6 Sv to south of 32°S from the upper main thermocline, most likely to the Agulhas Current system. This amount of dianeutral upwelling transport does not include the top 150–200 m, which may contribute much more volume transport to the south.     

Community structure across a large-scale ocean productivity gradient: Marine bird assemblages of the Southern Indian Ocean

Our objective was to understand how marine birds respond to oceanographic variability across the Southern Indian Ocean using data collected during an 16-day cruise (4–21 January 2003). We quantified concurrent water mass distributions, ocean productivity patterns, and seabird distributions across a heterogeneous pelagic ecosystem from subtropical to sub-Antarctic waters. We surveyed 5155 km and sighted 15,606 birds from 51 species, and used these data to investigate how seabirds respond to spatial variability in the structure and productivity of the ocean. We addressed two spatial scales: the structure of seabird communities across macro-mega scale (1000 s km) biogeographic domains, and their coarse-scale (10 s km) aggregation at hydrographic and bathymetric gradients. Both seabird density and species composition changed with latitudinal and onshore–offshore gradients in depth, water temperature, and chlorophyll-a concentration. The average seabird density increased across the subtropical convergence (STC) from 2.4 birds km⁻² in subtropical waters to 23.8 birds km⁻² in sub-Antarctic waters. The composition of the avifauna also differed across biogeographic domains. Prions (Pachyptila spp.) accounted for 57% of all sub-Antarctic birds, wedge-tailed shearwaters (Puffinus pacificus) accounted for 46% of all subtropical birds, and Indian Ocean yellow-nosed albatross (Thallasarche carteri) accounted for 32% of all birds in the STC. While surface feeders were the most abundant foraging guild across the study area, divers were disproportionately more numerous in the sub-Antarctic domain, and plungers were disproportionately more abundant in subtropical waters. Seabird densities were also higher within shallow shelf-slope regions, especially in sub-Antarctic waters, where large numbers of breeding seabirds concentrated. However, we did not find elevated seabird densities along the STC, suggesting that this broad frontal region is not a site of enhanced aggregation.     

How does multiscale modelling and inclusion of realistic palaeobathymetry affect numerical simulation of the Storegga Slide tsunami?

The ∼8.15 ka Storegga submarine slide was a large (∼3000 km³), tsunamigenic slide off the coast of Norway. The resulting tsunami had run-up heights of around 10–20 m on the Norwegian coast, over 12 m in Shetland, 3–6 m on the Scottish mainland coast and reached as far as Greenland. Accurate numerical simulations of Storegga require high spatial resolution near the coasts, particularly near tsunami run-up observations, and also in the slide region. However, as the computational domain must span the whole of the Norwegian-Greenland sea, employing uniformly high spatial resolution is computationally prohibitive. To overcome this problem, we present a multiscale numerical model of the Storegga slide-generated tsunami where spatial resolution varies from 500 m to 50 km across the entire Norwegian-Greenland sea domain to optimally resolve the slide region, important coastlines and bathymetric changes. We compare results from our multiscale model to previous results using constant-resolution models and show that accounting for changes in bathymetry since 8.15 ka, neglected in previous numerical studies of the Storegga slide-tsunami, improves the agreement between the model and inferred run-up heights in specific locations, especially in the Shetlands, where maximum run-up height increased from 8 m (modern bathymetry) to 13 m (palaeobathymetry). By tracking the Storegga tsunami as far south as the southern North sea, we also found that wave heights were high enough to inundate Doggerland, an island in the southern North Sea prior to sea level rise over the last 8 ka.     

Wave forces and moments on an intake well

A sea water intake well of size 20 m diameter and 15.5 m height in a water depth of 9.8 m is proposed north of the Visakhapatnam Port for a project to extract magnesia from sea water. A 1:25 scale model of the intake well was tested in the wave basin of the Ocean Engineering Centre, Indian Institute of Technology, Madras to measure the wave forces and moments on the intake well and the variation of water levels inside and outside the well. Accordingly, an intake well model of 0.8 m diameter and 0.62 m height was fabricated and fixed over a false bottom in a wave basin. The well model was subjected to the action of both regular waves for two test conditions, intake well inlet closed during installation and intake well inlet open. The experimental results on wave forces and moments were compared with the results of the Linear Diffraction Theory. The water level inside the well was measured to determine the submergence of suction pipes of pumps and location of the inlet opening of the intake well. The wave crest elevation in front of the well was also measured in order to fix the deck level of the well so as to avoid water overspill onto the deck. The salient results of the present study are presented and discussed in this paper.     

Distribution and burial of organic carbon in sediments from the Indian Ocean upwelling region off Java and Sumatra,Indonesia

Sediments were sampled and oxygen profiles of the water column were determined in the Indian Ocean off west and south Indonesia in order to obtain information on the production, transformation, and accumulation of organic matter (OM). The stable carbon isotope composition (δ¹³C_org) in combination with C/N ratios depicts the almost exclusively marine origin of sedimentary organic matter in the entire study area. Maximum concentrations of organic carbon (C_org) and nitrogen (N) of 3.0% and 0.31%, respectively, were observed in the northern Mentawai Basin and in the Savu and Lombok basins. Minimum δ¹⁵N values of 3.7‰ were measured in the northern Mentawai Basin, whereas they varied around 5.4‰ at stations outside this region. Minimum bottom water oxygen concentrations of 1.1 mL L^?1, corresponding to an oxygen saturation of 16.1%, indicate reduced ventilation of bottom water in the northern Mentawai Basin. This low bottom water oxygen reduces organic matter decomposition, which is demonstrated by the almost unaltered isotopic composition of nitrogen during early diagenesis. Maximum C_org accumulation rates (CARs) were measured in the Lombok (10.4 g C m^?2 yr^?1) and northern Mentawai basins (5.2 g C m^?2 yr^?1). Upwelling-induced high productivity is responsible for the high CAR off East Java, Lombok, and Savu Basins, while a better OM preservation caused by reduced ventilation contributes to the high CAR observed in the northern Mentawai Basin. The interplay between primary production, remineralisation, and organic carbon burial determines the regional heterogeneity. CAR in the Indian Ocean upwelling region off Indonesia is lower than in the Peru and Chile upwellings, but in the same order of magnitude as in the Arabian Sea, the Benguela, and Gulf of California upwellings, and corresponds to 0.1–7.1% of the global ocean carbon burial. This demonstrates the relevance of the Indian Ocean margin off Indonesia for the global OM burial.     

Assessment of tsunami hazard for coastal areas of Shandong Province,China

Shandong province is located on the east coast of China and has a coastline of about 3100 km. There are only a few tsunami events recorded in the history of Shandong Province, but the tsunami hazard assessment is still necessary as the rapid economic development and increasing population of this area. The objective of this study was to evaluate the potential danger posed by tsunamis for Shandong Province. The numerical simulation method was adopted to assess the tsunami hazard for coastal areas of Shandong Province. The Cornell multi-grid coupled tsunami numerical model (COMCOT) was used and its efficacy was verified by comparison with three historical tsunami events. The simulated maximum tsunami wave height agreed well with the observational data. Based on previous studies and statistical analyses, multiple earthquake scenarios in eight seismic zones were designed, the magnitudes of which were set as the potential maximum values. Then, the tsunamis they induced were simulated using the COMCOT model to investigate their impact on the coastal areas of Shandong Province. The numerical results showed that the maximum tsunami wave height, which was caused by the earthquake scenario located in the sea area of the Mariana Islands, could reach up to 1.39 m off the eastern coast of Weihai city. The tsunamis from the seismic zones of the Bohai Sea, Okinawa Trough, and Manila Trench could also reach heights of >1 m in some areas, meaning that earthquakes in these zones should not be ignored. The inundation hazard was distributed primarily in some northern coastal areas near Yantai and southeastern coastal areas of Shandong Peninsula. When considering both the magnitude and arrival time of tsunamis, it is suggested that greater attention be paid to earthquakes that occur in the Bohai Sea. In conclusion, the tsunami hazard facing the coastal area of Shandong Province is not very serious; however, disasters could occur if such events coincided with spring tides or other extreme oceanic conditions. The results of this study will be useful for the design of coastal engineering projects and the establishment of a tsunami warning system for Shandong Province.     

Bacterivory in the northwestern Indian Ocean during the intermonsoon – northeast monsoon period

Bacterial grazing loss rates were studied by radioactive labeling of natural bacteria with L-(4,5-³H) leucine and from the rate of disappearance of bacterial cells in the northwestern Indian Ocean. Bacterivory was measured in a mixed sample that had been combined from various depths across the euphotic zone. Experiments were performed on 26 occasions at 19 stations in the Gulf of Oman and the Arabian Sea during the intermonsoon–northeast monsoon period (November–December 1994). Combined uptake of radiolabeled bacteria (ULB) in 1–8 and 8–100 μm size fractions was somewhat lower than loss of label (LBL) measured in the bacterial fraction (0.2–1.0 μm), suggesting loss of radioactivity from the grazers due to metabolism. The less sensitive rate of disappearance of bacterial cells (LBC) was on average 51% higher than LBL estimates. Results from ULB and LBL measurements revealed that bacterivory was higher in the Gulf of Oman (average loss rate 4.1% h^-1) than in the Arabian Sea where rates were slightly higher inshore (1.7% h^-1) than in the central gyre. Heterotrophic nanoflagellates in the 1–8 μm size fractions were identified as the primary bacterivores. Microzooplankton (8–100 μm) accounted for 33% of total bacterivory in the Gulf of Oman but only 16% in the central Arabian Sea. Time-course experiments conducted at two stations indicated that diel changes in bacterivory may be substantial in the northwestern Indian Ocean.     

Nitrogen assimilation and the f-ratio in the northwestern Indian Ocean during an intermonsoon period

Rates of nitrogen assimilation by phytoplankton were measured at 13 stations along a transect in the northwestern Indian Ocean, from the Gulf of Oman, southwards to approximately 8°N, during November and December 1994. Nitrate (NO₃), ammonium (NH₄) and urea assimilation were measured using simulated in situ ¹⁵N incubation techniques. These measurements were supported by simultaneous rate measurements of primary production using ¹⁴C incubation techniques and detailed vertical distributions of temperature and chlorophyll concentrations. Euphotic zone integrated nitrogen assimilation rates varied between 1.1 and 23.6 mmol N m^-2 day^-1, with generally higher rates occurring at the northern and southern ends of the transect. At the majority of stations ammonium was the preferred nitrogen substrate assimilated; the average integrated assimilation rate of ammonium being 3.7 mmol N m^-2 day^-1 compared to 1.6 and 1.8 mmol N m^-2 day^-1 for urea and nitrate respectively. This general preference is reflected in the low f-ratios, which were ⩽0.52 for all stations and in the relative preference indices (RPI) values which were consistently >1 for ammonium and <1 for nitrate. A further examination of the data has lead to an apparent partitioning of the northwestern Indian Ocean into 2 regions; a region north of 17°30′N and a region south of this, to about 8°N. This division is based on: (i) the relationship between the f-ratio and ambient nitrate levels; (ii) nitrogen assimilation and primary production and (iii) the biomass distribution. It is suggested that this partitioning should be investigated further with the development of biogeochemical provinces in mind and the estimation of f-ratios on much larger, horizontal scales.     

Flux comparison of Eulerian and Lagrangian estimates of Agulhas leakage: A case study using a numerical model

Estimating the magnitude of Agulhas leakage, the volume flux of water from the Indian to the Atlantic Ocean, is difficult because of the presence of other circulation systems in the Agulhas region. Indian Ocean water in the Atlantic Ocean is vigorously mixed and diluted in the Cape Basin. Eulerian integration methods, where the velocity field perpendicular to a section is integrated to yield a flux, have to be calibrated so that only the flux by Agulhas leakage is sampled. Two Eulerian methods for estimating the magnitude of Agulhas leakage are tested within a high-resolution two-way nested model with the goal to devise a mooring-based measurement strategy. At the GoodHope line, a section halfway through the Cape Basin, the integrated velocity perpendicular to that line is compared to the magnitude of Agulhas leakage as determined from the transport carried by numerical Lagrangian floats. In the first method, integration is limited to the flux of water warmer and more saline than specific threshold values. These threshold values are determined by maximizing the correlation with the float-determined time series. By using the threshold values, approximately half of the leakage can directly be measured. The total amount of Agulhas leakage can be estimated using a linear regression, within a 90% confidence band of 12 Sv. In the second method, a subregion of the GoodHope line is sought so that integration over that subregion yields an Eulerian flux as close to the float-determined leakage as possible. It appears that when integration is limited within the model to the upper 300 m of the water column within 900 km of the African coast the time series have the smallest root-mean-square difference. This method yields a root-mean-square error of only 5.2 Sv but the 90% confidence band of the estimate is 20 Sv. It is concluded that the optimum thermohaline threshold method leads to more accurate estimates even though the directly measured transport is a factor of two lower than the actual magnitude of Agulhas leakage in this model.     

Lithogenic and biogenic particle fluxes on the Lomonosov Ridge (central Arctic Ocean) and their relevance for sediment accumulation: Vertical vs. lateral transport

Investigations of lithogenic and biogenic particle fluxes using long-term sediment traps are still very rare in the northern high latitudes and are restricted to the arctic marginal seas and sub-arctic regions. Here data on the variability of fluxes of lithogenic matter, CaCO₃, opal, and organic carbon and biomarker composition from the central Arctic Ocean are presented for a 1-year period. The study was carried out on material obtained from a long-term mooring system equipped with two multi-sampling traps, at 150 and 1550 m depth, and deployed on the southern Lomonosov Ridge close to the Laptev Sea continental margin from September 1995 to August 1996. In addition, data from surface sediments were included in the study. Annual fluxes of lithogenic matter, CaCO₃, opal, and particulate organic carbon were 3.9, 0.8, 2.6, and 1.5 g m⁻² y⁻¹, respectively, in the shallow trap and 11.3, 0.5, 2.9, and 1.05 g m⁻² y⁻¹, respectively, in the deep trap.Both the shallow and the deep trap showed significant variations in vertical flux over the year. Higher values were found from mid-July to the end of October (total mass flux of 75–130 mg m⁻² d⁻¹ in the shallow trap and 40–225 mg m⁻² d⁻¹ in the deep trap). During all other months, fluxes were fairly low in both traps (most total mass flux values <10 mg m⁻² d⁻¹). The interval of increased fluxes can be separated into (1) a mid-July/August maximum caused by increased primary production as documented in high abundances of marine biomarkers and diatoms and (2) a September/October maximum caused by increased influence of Lena River discharge indicated by maximum lithogenic flux and large amounts of terrigenous/fluvial biomarkers in both traps. During September/October, total mass fluxes in the deep trap were significantly higher than in the shallow trap, suggesting a lateral sediment flux at greater depth. The lithogenic flux data also support the importance of sediment input from the Laptev Sea for the sediment accumulation on the Lomonosov Ridge on geological time scales, as indicated in sedimentary records from this region.     

A quantitative analysis of sources for summertime phytoplankton variability over 18 years in the South Shetland Islands (Antarctica) region

Eighteen years of summertime hydrographic and chlorophyll-a (Chl-a) data (～2700 stations) from the South Shetland Islands (Antarctica) region show that a “bell-shaped” (unimodal) distribution of phytoplankton biomass results annually when plotted against the inshore to offshore gradient in surface salinity. The maximum for this unimodal Chl-a distribution corresponds with a shallow upper mixed layer (UML) in iron-rich waters that occurs at salinities ～34. Methods of gradient analysis are used to distinguish sources of variability for bloom development among years. The control of phytoplankton biomass is resolved across the salinity gradient that separates the co-limiting conditions of deep UML depths and low-iron concentrations as opposing end-members. Chlorophyll-fluorescence yield data (a proxy for Fe-stress) showed that at salinities ～34, phytoplankton biomass was unlikely to be limited by Fe. Instead, blooming at salinities ～34 (1.3±1 mg Chl-a m^?3) co-varied with shallow UML depths (41±19 m) that occurred as a function of higher UML temperature (1.5±0.5 °C) among years, and is evidence that atmospheric climate variability impacts summertime phytoplankton biomass and production in this Southern Ocean seascape.     

The Intergovernmental Oceanographic Commission of UNESCO and regional capacity building

This paper concentrates on the past and present interaction between the Intergovernmental Oceanographic Com- mission, donor organisations and developing countries in the development of regional programmes for creating and supporting marine science capabilities. The Global Ocean Observing System and Integrated Coastal Zone Management in relation to fisheries, marine pollution and climate/sea level changes require regional input which depends on South-South and North South cooperation. The Indian Ocean with the Eastern African region, serve as an example for an ongoing process of capacity building and growing partnership in marine science. Through the Intergovernmental Oceanographic Commission, a strong and very beneficial cooperation was established, with several donors, in particular with the Swedish Agency for Research Co- operation with Developing Countries. This tripartite partnership of (a) a regional body with a regionally driven programme, (b) a global coordination mechanism in the form of the Intergovernmental Oceanographic Commission, and (a) a multi-lateral donor in the form of the Swedish Agency for Research Co-operation with Developing Countries is in fact a model for regional development aid. The need for strong leadership of regional bodies is emphasised including a formal intersessional mechanism in the form of a bureau.     

Wave climate variability in the North-East Atlantic Ocean over the last six decades

Ocean surface gravity waves play a major role in many engineering and environmental problems, both in the open ocean and in coastal zones. Therefore, it is essential to improve our knowledge on spatial and temporal variability of wave climate. This study aims at investigating this variability in the North-East Atlantic Ocean (25°W–0°W and 30°N–60° N), using a 57-year hindcast (1953–2009) obtained with a spectral wave model forced with reanalysis wind fields. The hindcast analysis reveals firstly strong seasonal fluctuations of wave climate, with winters characterized by large and long-period waves of mean direction spreading from south-west to north-west, and summers characterized by smaller and shorter-period waves originating from norther directions. From northern (55°N) to southern (35°N) latitudes, the significant wave height (Hs) decreases by roughly 40%, the mean wave direction (Mwd) rotates clockwise by about 25% while the peak period (Tp) only grows by 5%. These three parameters also exhibit a strong inter-annual variability, particularly when winter-means (from 1st of December to 1st of April) are considered. Linear trend analysis over the studied period shows spatially variable long-term trends, with a significant increase of Hs (up to 0.02 m yr^?1) and a counterclockwise shift of Mwd (up to ?0.1° yr^?1) at northern latitude, contrasting with a fairly constant trend for Hs and a clockwise shift of Mwd (up to +0.15° yr^?1) at southern latitudes. Long-term trends of Tp are less significant, with still a slight increase in the north-eastern part of the study area (up to +0.01 s yr^?1). Eventually, a comparison between the inter-annual variability of the winter-means of the three selected wave parameters and the North Atlantic Oscillation (NAO) reveals: (1) a strong positive correlation between Hs and the NAO index at northern latitudes (correlation coefficient up to R = 0.91) and a significant negative correlation at southern latitudes (up to R = ?0.6); (2) no significant correlation for Mwd north of 40°N and a clear positive correlation southward of 40°N (up to R = 0.8) and (3) a northward increasing correlation for Tp (up to R = 0.8). Long-term trends for Hs, Mwd and Tp are finally explained by a significant increase in the NAO index over the studied period.     

Rapid removal of terrigenous dissolved organic carbon over the Eurasian shelves of the Arctic Ocean

The fate of terrigenous dissolved organic carbon (tDOC) delivered to the Arctic Ocean by rivers remains poorly constrained on both spatial and temporal scales. Early reports suggested Arctic tDOC was refractory to degradation, while recent studies have shown tDOC removal to be an active but slow process. Here we present observations of DOC, salinity, δ¹⁸O, and ²²⁸Ra/²²⁶Ra in the Polar Surface Layer (PSL) over the outer East Siberian/Chukchi shelf and the adjacent Makarov and Eurasian basins of the eastern Arctic Ocean. This off-shelf system receives meteoric water, introduced by rivers, after a few years residence on the shelf. Elevated concentrations of DOC (> 120 μM C) were observed in low salinity (~ 27) water over the Makarov Basin, suggesting inputs of tDOC-enriched river water to the source waters of the Transpolar Drift. The regression of DOC against salinity indicated an apparent tDOC concentration of 315 ± 7 μM C in the river water fraction, which is significantly lower than the estimated DOC concentration in the riverine sources to the region (724 ± 55 μM C). To obtain the timescale of removal, estimates of shelf residence were coupled with measurements of dissolved ²²⁸Ra/²²⁶Ra, an isotopic tracer of time since shelf residence. Shelf residence time coupled with DOC distributions indicates a first order tDOC removal rate constant, λ = 0.24 ± 0.07 yr^-1, for the eastern Arctic, 2.5–4 times higher than rates previously observed in the western Arctic. The observed removal of tDOC in the eastern Arctic occurs over the expansive shelf area, highlighting the initial lability of tDOC upon delivery to the Arctic Ocean, and suggests that tDOC is composed of multiple compartments defined by reactivity. The relatively rapid remineralization of tDOC on the shelves may mitigate the strength of the Arctic Ocean atmospheric CO₂ sink if a projected increase in labile tDOC flux occurs.     

The generating mechanisms of the August 17, 1999 İzmit bay (Turkey) tsunami: Regional (tectonic) and local (mass instabilities) causes

The M_W = 7.4 earthquake that affected the northwestern part of Turkey on August 17, 1999, and in particular the gulf of İzmit, had dramatic consequences also as regards tsunami generation. The main cause of the earthquake was a dextral strike-slip rupture that took place along different segments of the western part of the North Anatolian Fault (WNAF). The rupture process involved not only a number of distinct strike-slip fault segments, but also dip-slip ancillary faults, connecting the main transcurrent segments. The general picture was further complicated by the occurrence of subsidence and liquefaction phenomena, especially along the coasts of the İzmit bay and in the Sapanca Lake. Tsunami effects were observed and measured during post-event surveys in several places along both the northern and the southern coasts of the bay. The run-up heights in most places were reported to lie in the interval 1–3 m: but in the small town of Değirmendere, where a local slump occurred carrying underwater buildings and gardens of the waterfront sector, eyewitnesses reported water waves higher than 15 m.The purpose of this work is to investigate on the causes of the tsunami by means of numerical simulations of the water waves. We show that the tsunami was a complex event consisting at least of the combination of a regional event due to tectonic causes and of a local event associated with the mass failure. As to the first, we are able to demonstrate that the observed tsunami cannot be explained only in terms of the sea bottom dislocation produced by the main right-lateral dislocation, but that the prominent contribution comes from the displacement associated with the secondary shallow normal faults. Furthermore, the large waves and effects seen in Değirmendere can be explained as the consequence of the slump. By means of a stability analysis based on an original method making use of the limit equilibrium concept, we show that the slump was highly stable before the earthquake and that it was triggered by seismic waves. Simulation of the tsunami induced by the slump was carried out by a two-step numerical code that computes the landslide motion first, and then the generated water wave propagation. It is shown that the computed local tsunami matches the experimental data.     

Primary productivity,differential size fraction and pigment composition responses in two Southern Ocean in situ iron enrichments

Two in situ iron-enrichment experiments were conducted in the Pacific sector of the Southern Ocean during summer 2002 (SOFeX). The “north patch,” established within the Subantarctic Zone (∼56°S), was characterized by high nitrate (∼21 mmol m⁻³) but low silicic acid (2 mmol m⁻³) concentrations. North patch iron enrichment increased chlorophyll (Chl) by 12-fold to 2.1 mg m⁻³ and primary productivity (PP_EU) by 8-fold to 188 mmol C m⁻² d⁻¹. Surprisingly, despite low silicic acid concentrations, diagnostic pigment and size-fraction composition changes indicated an assemblage shift from prymnesiophytes toward diatoms. The “south patch,” poleward of the Southern Boundary of the Antarctic Circumpolar Current (SBACC) (∼66°S), had high concentrations of nitrate (∼27 mmol m⁻³) and silicic acid (64 mmol m⁻³). South patch iron enrichment increased Chl by 9-fold to 3.8 mg m⁻³ and PP_EU 5-fold to 161 mmol C m⁻² d⁻¹ but, notably, did not alter the phytoplankton assemblage from the initial composition of ∼50% diatoms. South patch iron addition also reduced total particulate organic carbon:Chl from ∼300 to 100; enhanced the presence of novel non-photosynthetic, but fluorescent, compounds; and counteracted a decrease in photosynthetic performance as photoperiod decreased. These experiments show unambiguously that in the contemporary, high nitrate Southern Ocean increasing iron supply increases primary productivity, confirming the initial premise of the Martin Iron Hypothesis. However, despite a 5-fold increase in PP_EU under iron-replete conditions in late summer, the effect of iron on annual productivity in the Southern Ocean poleward of the SBACC is limited by seasonal ice coverage and the dark of polar winter.