OS51A-1285 0800h
Changes in World Ocean Nitrate Availability Through the 20th Century.
Conceptual models linking climate change with fluctuations in fish population abundances are based on how cyclic patterns in air-sea interactions and ocean circulation influence pelagic food web dynamics. The effect of changing mixed layer dynamics on phytoplankton light and nutrient exposure is a prominent focal point in the overall mechanism. The Extended Reconstruction (ER) of Sea Surface Temperature (SST) version one (ERSST.v1) and version two (ERSST.v2) monthly time series from 1854-2003 interpreted with the aid of a historically based Nitrate Depletion Temperature (NDT) matrix provides a tool for examining temporal and spatial patterns in nitrate availability. After a temporal analysis of NDT at four time series stations demonstrated temporal stability, [SST-NDT] was calculated for April and October for the whole ERSST data set to provide indices sensitive either to changing surface nitrate concentration (negative values of [SST-NDT]) or to changing subsurface nitrate stratification (positive values of [SST-NDT]). The records from both months showed that nitrate availability generally decreased throughout the 20th century in association with global warming. In the northern (southern) hemisphere, the greatest losses in nitrate availability occurred in two warming events in the time periods 1909-1937 (1926-1937) and 1977-present (1950-1990). Different areas of the world ocean were affected in each warming event. Prominent exceptions in ERSST.v1 where 20th century nitrate availability actually increased at least in one season were in western parts of the South Indian, the North Pacific, the equatorial Pacific, the South Pacific, the North Atlantic, and the South Atlantic and in eastern parts of the South Pacific and South Atlantic. ERSST.v2 also showed increased 20th century nitrate availability in the eastern subarctic Pacific. The resulting nitrate availability trends in the present analysis agree with reports on regional changes in plant nutrient availability, on increased occurrences of Harmful Algal Blooms, and on changes in marine fisheries and raise issues concerning the resilience of historical pelagic community oscillations in the face of a continuing warming trend and decreasing overall nitrate availability that has spanned the 20th century.
OS51A-1286 0800h
Seasonal Variability of the SeaWiFS Chlorophyll in the Western Tropical Pacific
We use Sea-viewing Wide Field-of-view Sensor (SeaWiFS) observations to document the seasonal cycle of surface chlorophyll concentration in the western tropical Pacific. Essentially, surface waters of this region can be divided into two contrasted ecosystems. The western end of the cold, salty waters of the equatorial divergence with High Nutrient Low Chlorophyll (HNLC) characteristics occupy most of the eastern part of the region while warm, fresh, and oligotrophic waters of the warm pool stand in the western part. Nevertheless, disruption of the oligotrophy may show up at different locations. We reconstruct the seasonal cycle of chlorophyll, SST, winds, and surface currents from satellite data and satellite-derived products by fitting the annual harmonic to the time-series at each grid point. The calculation is done for the 1999-2001 years in order to exclude the 1997-1998 and 2002-2003 ENSO events. An EOF analysis applied to the typical year highlights three major regions with coherent seasonal signal. [1] The oligotrophy/upwelling transition zone undergoes zonal and meridional seasonal displacements. The front is at its westernmost and northernmost (easternmost and southernmost) position during boreal spring (autumn). The equatorial displacements of the front can be explained in terms of advection, whereas the seasonal variability of chlorophyll along the northern (southern) boundary is mainly consistent with the seasonal cycle of the north and south equatorial countercurrents (NECC and SECC). [2] SeaWiFS images show enrichments in the western NECC, especially from February to June. The Chl maximum lags a maximum eastward velocity and a SST minimum, and occurs during the upwelling favorable phase of the wind stress curl. These enrichments can be attributed to advection and/or upwelling associated with current meandering. [3] Near the Solomon Islands, we observe enhancement of Chl concentration either east or southwest of the islands, depending on the direction of surface currents. This may be explained by the island mass effect.
OS51A-1287 0800h
Analysis of Seasonal Chlorophyll-a Using An Adjoint Three-Dimensional Ocean Carbon Cycle Model
The misfit between numerical ocean model and observations can be reduced using data assimilation. This can be achieved by optimizing the model parameter values using adjoint model. The adjoint model minimizes the model-data misfit by estimating the sensitivity or gradient of the cost function with respect to initial condition, boundary condition, or parameters. The adjoint technique was used to assimilate seasonal chlorophyll-a data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite to a marine biogeochemical model HAMOCC5.1. An Identical Twin Experiment (ITE) was conducted to test the robustness of the model and the non-linearity level of the forward model. The ITE experiment successfully recovered most of the perturbed parameter to their initial values, and identified the most sensitive ecosystem parameters, which contribute significantly to model-data bias. The regional assimilations of SeaWiFS chlorophyll-a data into the model were able to reduce the model-data misfit (i.e. the cost function) significantly. The cost function reduction mostly occurred in the high latitudes (e.g. the model-data misfit in the northern region during summer season was reduced by 54%). On the other hand, the equatorial regions appear to be relatively stable with no strong reduction in cost function. The optimized parameter set is used to forecast the carbon fluxes between marine ecosystem compartments (e.g. Phytoplankton, Zooplankton, Nutrients, Particulate Organic Carbon, and Dissolved Organic Carbon). The a posteriori model run using the regional best-fit parameterization yields approximately 36 PgC/yr of global net primary productions in the euphotic zone.
OS51A-1288 0800h
Assimilation of global satellite ocean chlorophyll data: Prospects and Issues
Initial efforts at assimilating satellite ocean chlorophyll data into a global three-dimensional coupled physical/biogeochemical model have indicated promising results. Assimilated chlorophyll fields compare favorably with Sea-viewing Wide Field-of-view Sensor data. Compared to the Vertically Generalized Production Model, global primary production estimates improve from a 19% difference in the free run model to 10% with the assimilation model. Thus the assimilation dramatically improves estimates of biomass, but less so estimates of carbon flux. Additionally, surface nutrient distributions are less realistic is some areas, due to an imbalance between chlorophyll and nutrients caused by the assimilation. Data errors in SeaWiFS are a particularly important problem that adversely affects both the assimilation and the quantification of results.
OS51A-1289 0800h
Real Data and Rapid Results: Ocean Color Data Analysis with Giovanni (GES DISC Interactive Online Visualization and ANalysis Infrastructure)
The NASA Goddard Earth Sciences Data and Information Services Center (GES DISC) has taken a major step addressing the challenge of using archived Earth Observing System (EOS) data for regional or global studies by developing an infrastructure with a World Wide Web interface which allows online, interactive, data analysis: the GES DISC Interactive Online Visualization and ANalysis Infrastructure, or "Giovanni." Giovanni provides a data analysis environment that is largely independent of underlying data file format. The Ocean Color Time-Series Project has created an initial implementation of Giovanni using monthly Standard Mapped Image (SMI) data products from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission. Giovanni users select geophysical parameters, and the geographical region and time period of interest. The system rapidly generates a graphical or ASCII numerical data output. Currently available output options are: Area plot (averaged or accumulated over any available data period for any rectangular area); Time plot (time series averaged over any rectangular area); Hovmöller plots (image view of any longitude-time and latitude-time cross sections); ASCII output for all plot types; and area plot animations. Future plans include correlation plots, output formats compatible with Geographical Information Systems (GIS), and higher temporal resolution data. The Ocean Color Time-Series Project will produce sensor-independent ocean color data beginning with the Coastal Zone Color Scanner (CZCS) mission and extending through SeaWiFS and Moderate Resolution Imaging Spectroradiometer (MODIS) data sets, and will enable incorporation of Visible/Infrared Imaging Radiometer Suite (VIIRS) data, which will be added to Giovanni. The first phase of Giovanni will also include tutorials demonstrating the use of Giovanni and collaborative assistance in the development of research projects using the SeaWiFS and Ocean Color Time-Series Project data in the online Laboratory for Ocean Color Users (LOCUS). The synergy of Giovanni with high-quality ocean color data provides users with the ability to investigate a variety of important oceanic phenomena, such as coastal primary productivity related to pelagic fisheries, seasonal patterns and interannual variability, interdependence of atmospheric dust aerosols and harmful algal blooms, and the potential effects of climate change on oceanic productivity.
http://reason.gsfc.nasa.gov/Giovanni/
OS51A-1290 0800h
Dust Transport to the Surface of the Arabian Sea Followed by Phytoplankton Biomass Increase
Oligotrophic open-ocean regions, including the Arabian Sea during the NE Monsoon, are characterized by low phytoplankton abundance near the surface. Here we report significant increases in micro-algal biomass in the Arabian Sea just a few days after aeolian transport of desert dust to that area. The result is based on weekly averaged remote sensing images of aerosol optical depth and chlorophyll concentrations obtained with the MODIS instrument. Enhanced phytoplankton concentrations were found after four out of five dust events during the two seasons of observations. A similar effect was not observed in images obtained in other regions of the globe in the same period (the Mediterranean; the northern Atlantic). This may be due to the fact that in those areas dust deposition is much more continuous than in the Indian Ocean region we focused on. The stimulation of micro-algal growth may have been direct by enrichment with nutrients associated with dust particles, or indirect by a positive effect of these particles on microbial mineralization of organic matter. This is the first time that the link between desert dust storms and chlorophyll increase has been established repeatedly in a time series of observations.
OS51A-1291 0800h
Size Dependent 4D Structures of Phyto- and Zooplankton Biomasses due to Advective Processes in the Kuroshio Region
Temporal and spatial variations of plankton biomasses were calculated by a three-dimensional ecosystem model with focus on size dependent effects of advective processes in the frontal region of the Kuroshio and the Kuroshio Extension in the western North Pacific. The model consists of a lower trophic-level model with 11 compartments based on NEMURO coupled with an eddy-resolving physical model assimilated to satellite altimetry, and it was driven by a surface forcing from January 1997 through April 2004. Downstream of the Kuroshio, high concentration region of phyto- and zooplankton was distributed along the northern edge of the front, where variation of the biomass was controlled primarily by advective processes due to the stream. Moreover local maxima were formed in convergence zones located downstream (upstream) of the meander ridge (trough), as observed. On the other hand, in the fringe areas of the Kuroshio the variation was affected mainly by in situ biological growth. Consequently in the frontal region, a small difference in initial growth rate between small and large-sized plankton induced a large difference in their biomass, coupled with advective process due to convergence and divergence, cross-frontal current and eddy-stream interaction.
OS51A-1292 0800h
Large-scale Physical Forcing of Chlorophyll Blooms in the Subtropical Pacific
Large chlorophyll blooms observable with ocean color satellite data develop in late summer in the oligotrophic subtropical Pacific, northeast of Hawaii [Wilson, 2003]. These blooms are uncharacterized and the mechanisms that stimulate them are unknown. The blooms are quite large, the largest ones reaching the size of California, but there is significant interannual variation in their size and occurrence. They have been observed in 10 of 16 years of satellite ocean color data, with shorter ones lasting $\sim$6 weeks and longer ones up to 5 months. The two most consistent aspects of the blooms are the timing of their development, occurring in late summer, and their location, centered along $30\deg$N between $140\deg-160\deg$W, approximately $10\deg$ south of the transition zone chlorophyll front (TZCF). Possible biological mechanisms for stimulating these blooms include nitrogen fixation and a biologically-mediated vertical transport of nitrate by {\it Rhizosolenia} mats. The latitudinal locations of the chlorophyll blooms are coincident with the highest observed concentrations of {\it Rhizosolenia} mats. Both processes, nitrogen fixation and mat migration, are sensitive to water column stability, which is linked to physical forcing. Furthermore, the relatively consistent position of the blooms within the Pacific basin suggests large-scale physical forcing from either wind stress or the location of the subtropical front. Here we compare the position of the blooms against basin-scale wind patterns and the location of the subtropical front, both climatologically and interannually, to identify unique characteristics of this region that could lead to the development of the observed chlorophyll blooms. Wilson, C., Late summer chlorophyll blooms in the oligotrophic North Pacific subtropical gyre, {\it Geophysical Research Letters}, 30 (18), 1942, doi:10.1029/2003GL017770, 2003.
http://www.pfeg.noaa.gov/~cwilson/bloom
OS51A-1293 0800h
Sea Surface Temperature and the Brown Shrimp (Farfantepenaeus Aztecus) Population on the Alabama, Mississippi, Louisiana and Texas Continental Shelves
Analysis of the Southeast Area Monitoring and Assessment Program-Gulf of Mexico (SEAMAP-GM) summer and fall trawl survey data covering the Alabama, Mississippi, Louisiana and Texas continental shelves for the period 1987-2000 shows that the annual number n and weight w of the brown shrimp per trawl is positively correlated with annual sea surface temperature (SST) averaged over the continental shelves. Monthly SST correlations with the annual shrimp data are highest in April and May. Past work suggested possible reason for this: juvenile brown shrimp, which mainly develop from postlarvae in coastal estuaries in April and May, grow faster in warmer water and are therefore more likely to escape predators. Since the juvenile shrimp population is a good predictor of the adult shelf shrimp population, and since the estuarine and shelf SST are closely linked, it is reasonable that April and May shelf SST should be positively correlated with the number and weight of brown shrimp per trawl. Hindcasts is made using the simple linear statistical prediction model $n=\alpha + \beta \cdot SSTA $ , where SSTA is the anomalous April-May departure of SST from the annual cycle. The cross-verified correlation between model and observed n is $r=0.77$ . A similar model for w gave $r=0.79$ . In 2004, the model predicts that the number and weight of the brown shrimp should be less than those in 1999 and 2000, but still larger than the average values from 1987 to 2000 (5.92% above normal for the number and 5.64% above for the weight).
OS51A-1294 0800h
A Lagrangian photoresponse model coupled with 2nd-order turbulence closure
Vertical mixing can transport nutrients from deep layer to euphotic zone. Therefore, it plays a very important role for biological productivity. Also, it can transport phytoplankton vertically and varies light exposure history of individual phytoplankton. When phytoplankton_fs response to the ambient light intensity is slow compare to the time scale of vertical mixing, production averaged in space can be varied caused by vertical mixing. In the past, such effects of vertical mixing on the photoresponse of phytoplankton have been considered with constant eddy diffusivity. However, vertical mixing is not independent of time, and variable vertical mixing in time should be examined for the Lagrangian photoresponse. In present study, a 2nd-order turbulence closure approach was coupled with a Lagrangian phytoplankton model, in order to examine the effect of time-dependent vertical eddy diffusion on the photoresponse of phytoplankton in a wind-driven upper mixing layer. In general, stronger wind mixing in a lower-transparency water column contributes to greater phytoplankton production. According to our study, vertical mixing is insignificant for photoinhibition in relatively clear open ocean water, while it can be more important in relatively turbid coastal water. A simple Ekman layer model provided surprisingly similar production to that observed with the 2nd-order closure scheme when the starting distribution of the phytoplankton cells was normalized. Two factors involved in the process are the change in the background stratification and the time dependence of the diffusivity coefficient. The influences of these 2 factors cancel each other to reduce the apparent difference between the total production estimated by the Ekman model compared to that estimated by the 2nd-order closure scheme.
OS51A-1295 0800h
Stochastic Larval Settlement in Nearshore Marine Ecosystems
Key to the predictive understanding of nearshore marine ecosystems is the transport of larvae by ocean circulation processes. Only a very few lucky larvae successfully settle upon suitable habitat and are able to recruit to adult life stages. Methodologies for predicting this source/settlement relationship for larval transport is still very primitive and simple diffusive scaling analyses are used for many important applications. Here, we investigate source/settlement relationships of the larval transport using an idealized model of the coastal Regional Ocean Model System (ROMS) to provide time evolving coastal circulations in which many ($>10^6$) Lagrangian (and quasi-Lagrangian) particles are released and tracked as models of planktonic larvae. The Lagrangian simulation results are used to construct larval dispersal kernels which describe the probability distribution of settlement from a given location. These dispersal kernels are strong functions of several time scales including the planktonic larval duration, the frequency and duration of larval release events, inherent coastal circulation time scales and the planning time over which one wishes to predict changes in nearshore abundances. For typical situations (such as typify nearshore fish stock assessment), larval dispersal will be far from a simple diffusion process. This work provides new insights into the persistence and spatial structure of nearshore fish stock abundances.
OS51A-1296 0800h
Enhanced Primary Productivity at the Subtropical Convergence
The Subtropical Convergence is one of the major frontal systems in the world ocean. It is not just simply a biogeographical boundary, but forms a unique biological habitat of its own. Ocean colour satellite data indicate that it is a region of enhanced chlorophyll a and thus a potential key region of carbon drawdown from the atmosphere. Ship crossings sometimes show peaks in chlorophyll a at the front, but at other times these peaks are absent. If the normal mode of primary productivity at the front consists of intermittent bloom events, as these observations suggest, organisms endemic to this habitat may have to be adapted to a boom-or-bust situation. We have studied the presence of chlorophyll a using daily SeaWiFS images and showed that bloom events with limited spatial and temporal scales are indeed the norm. A coupled physical-biological model simulates this process with a fair degree of verisimilitude. We use this model to investigate the physico-biogeochemical requirements for bloom events and demonstrate that in most cases the limiting factor is intensity of vertical stratification combined with light availability.
OS51A-1297 0800h
Ice core indicates explosive volcanic debris not a potent source of trace metals to environments of deposition
The products of explosive volcanic eruptions (tephra) represent massive additions of earth materials to the ocean and land environments on which they fall. There is a widespread assumption that tephra has high trace element content, and that it can be a potent source of excess trace metals. The source of this assumption may be that the plumes of quiescently degassing volcanoes have in fact been shown to be rich in trace elements. However, we show here by analysis of tephra in old Antarctic ice, reliably preserved and free of any anthropogenic influence, that explosive volcanic ejecta are not a source of enhanced trace elements to the lands or oceans on which they fall. This lack of enrichment of trace elements in products of explosive volcanism is consistent with what is observed in nearly all energetic, disordered, high-entropy processes in nature, in which fractionation is minimal. Volcanic tephra is probably a minor source of iron to the oceans, because iron content is low in the rock types of volcanoes that most commonly explode. Furthermore, tephra is probably only a minor source of soluble silica, because the massive nature of many falls leads to effective burial and sequestration, with the solubility limit of silica in seawater being quickly reached in the region of a fall.