Variability In Physical Forcing Research Articles

Role of Aerosols in Spring Blooms in the Central Yellow Sea During the COVID-19 Lockdown by China

Reduced amounts of aerosols blowing into the Yellow Sea (YS), owing to the temporary lockdown of factories in China during COVID-19, resulted in a 15% decrease in spring chlorophyll-aconcentration (CHL) in March 2020 compared to its mean March values from 2003 to 2021. Particularly, the effect of land-based AOD is insignificant compared with that of atmospheric aerosols flowing into the YS, as indicated by the currents and wind directions. Hence, the main objective of this study was to understand the relationship between atmospheric aerosols and CHL by quantitatively considering relevant environmental changes using a Random Forest (RF) algorithm. Various input physical forcing variables to RF were employed, including aerosol optical depth (AOD), sea surface temperature (SST), mixed layer depth (MLD), wind divergence (WD), and total precipitation (TP). From the RF-based analysis, we estimated the relative contribution of each physical forcing variable to the difference in CHL during and after the COVID-19 lockdown period. The sensitivity of the RF model to changes in aerosol levels indicated positive effects of increased amounts of aerosols during spring blooms. Additionally, we calculated the quantitative contribution of aerosols to CHL changes. When SST was warmer and TP was lower than their climatology in March 2020, CHL increased by 0.22 mg m-3and 0.02 mg m-3, respectively. Conversely, when MLD became shallower and AOD was lower than their climatology, CHL decreased as much as 0.01 mg m-3and 0.20 mg m-3. Variations in WD caused no significant change in CHL. Overall, the specific estimations for reduced spring blooms were caused by a reduction in aerosols during the COVID-19 lockdown period. Furthermore, the RF developed in this study can be used to examine CHL changes and the relative role of significant environmental changes in biological blooms in the ocean for any normal year.

Isotope constraints on seasonal dynamics of dissolved and particulate N in the Pearl River Estuary, south China

Isotope measurements were performed on dissolved NO3−, NH4+ and suspended particulate total N along a salinity gradient in the Pearl River Estuary (PRE) to investigate seasonal changes in main N sources and its biogeochemical processing under the influence of monsoon climate. Our data revealed that municipal sewage and re-mineralized soil organic N were the major sources of DIN (NO3− and/or NH4+) in freshwater during winter and summer, respectively, whereas phytoplankton biomass was a major component of PN in both seasons. In low salinity waters (<2–3), nitrification was proved to be a significant NO3− source via NH4+ consumption, with N isotope effects of −15.3‰ in summer and −23.7‰ in winter for NH4+ oxidation. The contribution of nitrification to the total NO3− pool was smaller in summer than in winter, most likely due to freshwater dilution. At mid-salinities (3–20), δ15N values of PN were similar to those of NO3− and NH4+ in summer, reflecting a strong coupling between assimilation and remineralization. In winter, however, higher δ15NNH4 but lower δ15NNO3 than δ15NPN were observed, even though δ15NPN was similar between summer and winter. Intense sediment-water interaction and resuspension of sediments during winter appeared largely responsible for the decoupling. At high salinities, the greater enrichment in δ18ONO3 than in δ15NNO3 (up to 15.6‰) in winter suggests that atmospheric deposition may contribute to NO3− delivery during the dry season. Overall, these results show the importance of seasonal variability in physical forcing on biological N sources and its turnover processes in the highly dynamic river-dominated estuary. This article is protected by copyright. All rights reserved.

Longitudinal variability of organic nutrients in the North Atlantic subtropical gyre

We combine modelled timescales of ocean circulation with satellite-retrieved and in situ biogeochemical observations collected in spring along 24.5°N in the subtropical North Atlantic. Longitudinal gradients in the distribution of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) and in other biogeochemical parameters are associated with the longitudinal variability in physical forcing and in the eastward increase of the timescale of advective transport. The western (West of 70°W) and eastern (East of 30°W) margins of the subtropical gyre appear influenced by the productive regions of the Gulf Stream and upwelling zones off Africa, respectively. Within the oligotrophic zone between 70 and 31°W, at approximately 46°W there is a change in the nutrient-controlling factors from the western ultraoligotrophic with barely any seasonal cycle to an eastern oligotrophic environment with a more intense mixed layer dynamics. The allochthonous supply of semilabile-DOP may be important in the western sector of the oligotrophic gyre (approx. 70–46°W) where, together with the combination of shallow mixed layers, almost permanent stratification and high water temperatures create a niche for the growth of diazotrophs, which we detect from space. Turnover estimates exceeding 3yr suggest that even reactive fractions of DON are unlikely to be a significant N source. In the eastern sector of the oligotrophic gyre (46–31°W), transit timescales longer than 3 years suggest that the allochthonous supply of the semilabile DOP is negligible due to its exhaustion. Here, an intense mixed layer dynamics favours nutrient supply from below the mixed layer. We speculate that longitudinal variability in physical forcing and gradients in the timescale of advection, combined with distinct turnover timescales of reactive fractions of DON and DOP, drive diverse phytoplankton assemblages and surface nitrogen fixation gradients across our region of investigation.

Diel patterns of total suspended solids, turbidity, and water transparency in a highly turbid, shallow lake (Laguna Chascomús, Argentina)

The effects of external physical forcing variables (solar radiation and winds) on short-term dynamics of total suspended solids (TSS), Chlorophyll-a (Chl-a), turbidity levels, and water transparency were studied during 15 days in a highly turbid, shallow lake (Laguna Chascomus, Argentina). Water samples were taken three times per day (8, 14 and 20 h.). Solar radiation and wind velocity showed a repeatedly bell-shaped diurnal pattern, with significant higher values during morning and afternoon, respectively. TSS and turbidity displayed a general decreasing trend during the sampling period, while water transparency showed the opposite trend. Also Chl-a displayed a decreasing trend and was closely correlated to TSS levels. We assayed a first-order kinetics model to detrend the series, obtaining the rate of change during the night, morning, and afternoon. We observed higher values on afternoon compared to morning for TSS, Chl-a, and turbidity levels and the opposite pattern for water transparency. We conclude that this pattern may result from a combination of biological activity, as it took place after a period of intense photosynthetic activity, together with resuspension by winds during the afternoon, (windiest time of the day).

Predicting climate effects on Pacific sardine

For many marine species and habitats, climate change and overfishing present a double threat. To manage marine resources effectively, it is necessary to adapt management to changes in the physical environment. Simple relationships between environmental conditions and fish abundance have long been used in both fisheries and fishery management. In many cases, however, physical, biological, and human variables feed back on each other. For these systems, associations between variables can change as the system evolves in time. This can obscure relationships between population dynamics and environmental variability, undermining our ability to forecast changes in populations tied to physical processes. Here we present a methodology for identifying physical forcing variables based on nonlinear forecasting and show how the method provides a predictive understanding of the influence of physical forcing on Pacific sardine.

A comparative analysis of coastal and shelf‐slope copepod communities in the northern California Current system: Synchronized response to large‐scale forcing?

The synchrony between coastal and shelf‐slope copepod communities was investigated in the northern California Current (NCC) system, a strong upwelling zone, using time series of zooplankton sampled from a nearshore station (9 km offshore, water depth 62 m) and a shelf‐slope station (46 km offshore, water depth 297 m). Long‐term trends and seasonal changes were constructed for the dissimilarity index (Euclidean distance) between the two stations and for the biomass of three different copepod assemblages at the two stations: cold neritic, southern, and warm neritic copepods. The dissimilarity between the community structures of the two stations showed little variation in the long‐term trend, but exhibited a clear seasonal pattern. All three copepod assemblages showed similar long‐term trends in relation to the large‐scale forcing as indexed by the Pacific Decadal Oscillation at both stations, but variations in the long‐term trend at the nearshore station were much higher than the offshore station. Most copepod groups exhibited regular seasonal patterns at both stations except southern copepods at the nearshore station. All three copepod assemblages exhibited more pronounced seasonal fluctuations at the nearshore station compared with the slope station, and this difference is likely driven by higher productivity nearshore fueled by nutrient‐enriched upwelled water. Copepods in the inshore and offshore waters in the NCC ecosystem showed synchronized response to the large‐scale variability in physical forcing and copepods in the coastal waters were more responsive to local perturbations than were those in the slope waters.

Multiple Factors driving Variability of CO2 Exchange Between the Ocean and Atmosphere in a Tropical Coral Reef Environment

In this paper, we present the results of the first automated continuous multi-year high temporal frequency study of CO2 dynamics in a coastal coral reef ecosystem. The data cover 2.5 years of nearly continuous operation of the CRIMP-CO2 buoy spanning particularly wet and dry seasons in southern Kaneohe Bay, a semi-enclosed tropical coral reef ecosystem in Hawaii. We interpret our observational results in the context of how rapidly changing physical and biogeochemical conditions affect the pCO2 of surface waters and the magnitude and direction of air–sea exchange of CO2. Local climatic forcing strongly affects the biogeochemistry, water column properties, and gas exchange between the ocean and atmosphere in Kaneohe Bay. Rainfall driven by trade winds and other localized storms generates pulses of nutrient-rich water, which exert a strong control on primary productivity and impact carbon cycling in the water column of the bay. The “La Nina” winter of 2005–2006 was one of the wettest winters in Hawaii in 30 years and contrasted sharply with preceding and subsequent drier winter seasons. In addition, short-term variability in physical forcing adds complexity and helps drive the response of the CO2–carbonic acid system of the bay. Freshwater pulses to Kaneohe Bay provide nutrient subsidies to bay waters, relieving the normal nitrogen limitation of this system and driving phytoplankton productivity. Seawater pCO2 responds to the blooms as well as to physical forcing mechanisms, leading to a relatively wide range of pCO2 in seawater from about 250 to 650 μatm, depending on conditions. Large drawdowns in pCO2 following storms occasionally cause bay waters to switch from being a source of CO2 to the atmosphere to being a sink. Yet, during our study period, the southern sector of Kaneohe Bay remained a net source of CO2 to the atmosphere on an annualized basis. The integrated net annual flux of CO2 from the bay to the atmosphere varied between years by a factor of more than two and was lower during the wet “La Nina” year, than during the following year. Over the study period, the net annualized flux was 1.80 mol C m−2 year−1. Our CO2 flux estimates are consistent with prior synoptic work in Kaneohe Bay and with estimates in other tropical coral reef ecosystems studied to date. The high degree of climatological, physical, and biogeochemical variability observed in this study suggests that automated high-frequency observations are needed to capture the short-, intermediate-, and long-term variability of CO2 and other properties of these highly dynamic coastal coral reef ecosystems.

Surface layer variability in the Ross Sea, Antarctica as assessed by in situ fluorescence measurements

Phytoplankton fluorescence, temperature and salinity were measured from December through February using in situ instruments deployed at two locations in the southern Ross Sea, Antarctica during the austral summers of three consecutive years (2003–2004, 2004–2005, and 2005–2006) to assess the short-term, seasonal and interannual variations in phytoplankton biomass and oceanographic conditions. The seasonal climatologies of physical forcing variables were also determined from satellite measurements, and the data from the two sites compared to the 2000–2009 mean. In situ fluorometers were deployed at three depths at 77°S, 172.7°E and 77.5°S, 180°. Significant differences between the two sites were consistently observed, confirming the anticipated high level of spatial and temporal heterogeneity. Chlorophyll fluorescence was maximal in late December, and generally decreased rapidly to modest levels in January and February. However, during 1 year (2003–2004) a secondary bloom was found, with summer maxima being similar to those observed during spring. Fluorescence displayed a strong diel cycle, with strong quenching during periods of maximum irradiance. The magnitude of this reduction was large (the minimum average fluorescence was 25% of the daily mean) and decreased with depth. Fluorescence varied interannually, with the absolute levels and temporal patterns being different among years. The two sites had different temperature/salinity properties as measured at 24 m, and both variables changed with time. During 2004–2005 we were able to continuously measure the photosynthetic quantum efficiency of PSII ( F v / F m ) at 11 m, which revealed a minimum in December, and an increase in January, whereas the absolute fluorescence ( F o ) decreased simultaneously. We suggest that this reflected a mixing event, whereby available irradiance increased, allowing a short period of growth in a more favorable optical environment. While substantial variations from the mean physical forcing were observed, the linkage of these physical variations with fluorescence was not always clear. Short-term (over 24-h) changes in fluorescence occurred, and were likely related to advective events. Wind events altered fluorescence in the surface layer, and these redistributed phytoplankton in the surface. The variability in chlorophyll fluorescence and physical forcing over a variety of scales in the Ross Sea provides insights into temporal–spatial coupling of phytoplankton.

A predictive model for satellite‐derived phytoplankton absorption over the Louisiana shelf hypoxic zone: Effects of nutrients and physical forcing

We investigated environmental forcing mechanisms of phytoplankton absorption near the Mississippi River delta using multiyear satellite data. An algorithm for the phytoplankton absorption coefficient (aph) was developed from in situ measurements and applied to ocean color imagery. We employed a suite of chemical and physical forcing variables, including surface currents. For satellite‐derived aph time series (2002–2004), correlation and stepwise regression analyses revealed the most important forcing variables of aph on the Louisiana shelf. Areally, Mississippi River discharge and nitrate concentration ([NO3]) were the two most important predictors of aph over the hypoxic zone (defined by its maximum extent). River discharge was important in a band stretching from the Mississippi River delta to the Louisiana‐Texas border. Riverine [NO3] and wind magnitude best predicted aph in nearshore waters, and solar radiation and SST were most important farther offshore over the hypoxic zone, indicating upwelled nutrient sources to phytoplankton. A multiple linear regression model performed well in resolving seasonal and interannual aph variability in model development years (2002–2004) (mean error of 18%, over all pixels and months) and in predicting shelf‐wide aph patterns in 2005 (mean error of 32%). Our results strongly suggest that in recent years, stratification and vertical mixing, in addition to riverine [NO3], play a primary role in regulating phytoplankton biomass over the hypoxic zone. As well, a springtime model experiment showed that aph over the hypoxic zone can differ by an average absolute 37% from its average scenario owing to changes solely in environmental variables other than NO3 flux.

Statistical models for sediment/detritus and dissolved absorption coefficients in coastal waters of the northern Gulf of Mexico

We developed statistically-based, optical models to estimate tripton (sediment/detrital) and colored dissolved organic matter (CDOM) absorption coefficients ( a sd , a g ) from physical hydrographic and atmospheric properties. The models were developed for northern Gulf of Mexico shelf waters using multi-year satellite and physical data. First, empirical algorithms for satellite-derived a sd and a g were developed, based on comparison with a large data set of cruise measurements from northern Gulf shelf waters; these algorithms were then applied to a time series of ocean color (SeaWiFS) satellite imagery for 2002–2005. Unique seasonal timing was observed in satellite-derived optical properties, with a sd peaking most often in fall/winter on the shelf, in contrast to summertime peaks observed in a g . Next, the satellite-derived values were coupled with the physical data to form multiple regression models. A suite of physical forcing variables were tested for inclusion in the models: discharge from the Mississippi River and Mobile Bay, Alabama; gridded fields for winds, precipitation, solar radiation, sea surface temperature and height (SST, SSH); and modeled surface salinity and currents (Navy Coastal Ocean Model, NCOM). For satellite-derived a sd and a g time series (2002–2004), correlation and stepwise regression analyses revealed the most important physical forcing variables. Over our region of interest, the best predictors of tripton absorption were wind speed, river discharge, and SST, whereas dissolved absorption was best predicted by east–west wind speed, river discharge, and river discharge lagged by 1 month. These results suggest the importance of vertical mixing (as a function of winds and thermal stratification) in controlling a sd distribution patterns over large regions of the shelf, in comparison to advection as the most important control on a g . The multiple linear regression models for estimating a sd and a g were applied on a pixel-by-pixel basis and results were compared to monthly SeaWiFS composite imagery. The models performed well in resolving seasonal and interannual optical variability in model development years (2002–2004) (mean error of 32% for a sd and 29% for a g ) and in predicting shelfwide optical patterns in a year independent of model development (2005; mean error of 41% for a sd and 46% for a g ). The models provide insight into the dominant processes controlling optical distributions in this region, and they can be used to predict the optical fields from the physical properties at monthly timescales.

Short‐term variability in physical forcing in temperate reservoirs: effects on phytoplankton dynamics and sedimentary fluxes

Summary1. The effects of wind events on phytoplankton dynamics were investigated in two temperate reservoirs.2. Meteorological forcing, change in physical and chemical structure of the water column and biological responses of phytoplankton communities were followed for 3 weeks in three seasons.3. Depending on the season, the phytoplankton response differed in response to nutrient and light conditions, and to the intensity of stratification and mixing.4. We demonstrated that, on a time scale of a few days, wind events can modify phytoplankton dynamics, in terms of size structure and exported biomass. An increase of mixing favoured the largest size class and disadvantaged the smallest size class, while an increase in stratification had the opposite effects. The short‐term change in size structure was reflected in the sedimentary fluxes but with a time lag.

Introduction to special section on North Pacific Carbon Cycle Variability and Climate Change

This article compares the major findings of eight synthesis papers resulting from a workshop to examine carbon cycle variability and climate change in the tropical and extratropical North Pacific Ocean. The workshop's intent was to encourage scientific exchange, synthesis activities, and the development of new ideas among scientists from different countries and backgrounds who do not regularly collaborate, but share a common interest in the Pacific. Six of the papers focus on air‐sea CO2 flux variability on timescales from years to decades. While the average North Pacific surface waters show increases in CO2 that are comparable to the increase in atmospheric CO2 over time, considerable temporal and spatial variations are observed. A recurring theme of these studies is that throughout most of the Pacific, interannual to decadal variations in CO2 fluxes are primarily controlled by variations in physical forcing with changes in the biological cycle being of secondary importance. The dominant mode of variability is confirmed to be associated with the El Niño–Southern Oscillation phenomenon, particularly in the tropical Pacific. Extratropical air‐sea CO2 flux variability is substantially smaller, primarily because dissolved inorganic carbon and temperature driven variations in the seawater pCO2 tend to compensate each other. Temporal variations in the wind fields are also important for assessing the variability in air‐sea CO2 fluxes. The two papers addressing carbon and oxygen cycle changes within the water column also found that variability in physical forcing can explain the magnitude and patterns of observed variability in apparent oxygen utilization and other tracers.

Variability in retention of Calanus finmarchicus in the Nordic Seas

Abstract Using a regional ocean circulation model and particle tracking, we have studied the probability of the copepod Calanus finmarchicus being retained within the Nordic Seas' population as a function of its initial location, its vertical migration pattern, and the interannual variability in physical forcing. Defining a retention index in terms of the number of particles remaining within the Nordic Seas divided by the initial number of particles released, we found that spatial location had the greatest effect on the retention index during the study period, 1988–1991. Variability as a result of differences in physical forcing among years and among different seasonal vertical migration patterns had smaller but similar effects. The seasonal vertical migration behaviours with the highest advective loss rates and the most sensitive to interannual physical forcing were those that ascended early and descended late from a shallow summer depth. Average retention within the Nordic Seas was 0.40 after one year in simulations with diffusion and advection, and 0.42 in simulations with advection only. The average retention at the end of the four-year sequence was 0.10 and 0.12 with and without diffusion, respectively. Particles located in the western areas of the Nordic Seas had the highest retention, while those along the Norwegian coast showed little or no retention after four years. Initial location has a larger influence on retention than interannual variability in advective fields. C. finmarchicus offspring tend to reside in areas different from their parents, with different probabilities of retention. This spatial variability in retention rate is also experienced as inter-generational variability by members of the population. Model results suggest that almost all of the C. finmarchicus that are advected into the Barents Sea originate from off the Norwegian coast. Thus, predicting C. finmarchicus inflow into the Barents Sea requires knowledge of their abundance on the Norwegian Shelf.

The role of physical forcing in initiation of spring blooms in the northeast Atlantic

In this paper, the relationship between changes in the intensity of the spring bloom and changes in the physical forcing, mediated by the upper ocean mixing, is examined and illustrated. An idealized mixed-layer model coupled with an NPZD model with two different detritus compartments provides a conceptual framework, predicting the biological response to vertical mixing anomalies in the ocean–surface boundary layer. The model of coupled biological–hydrodynamic processes is used to search for general physical principles of phytoplankton bloom regulation in open ocean waters. The study addresses three questions: How does an individual winter differ from climatologically averaged ones? What are the dynamics of stratification in open ocean systems as influenced by variable atmospheric forcing? How does phytoplankton growth respond to these dynamics? The calculations suggest that changes in the balance of the atmospheric forcing on a daily scale could lead to less pronounced fluctuations of the mixed layer in winter as during most of the day the heat input by the sun exceeds or compensates the heat loss by nonsolar heat flux components (i.e. transient or moderate conditions). Furthermore, the model results show that the springtime shallowing of the mixed layer is not typically a smooth transition, but is interrupted by mixing events stimulated by passage of weather systems. The frequency and intensity of these synoptic events varies from year to year due to shift in storm tracks and modes of atmospheric circulation. The simulations suggest that such synoptic events (storms) associated with time scales of several days, and the period of the year when the heat flux changes sign, are important for the development of the phytoplankton population. Enhanced or reduced storm frequency associated with changes in vertical mixing intensity during the late winter/spring period leads to an increase/decrease in amplitude and duration of the bloom caused by enhanced/reduced nutrient supply or retard it due to light limitation. Finally, supported by the model results and observations, the role of interannual variability in physical forcing on timing, amplitude and structure of the phytoplankton bloom for the period 1989–1997 is examined.

Controls on new production: the role of iron and physical processes

A coupled nitrogen–iron model is developed from simple mass balance equations to address fundamental controls on new production (NP) and how they relate to nitrate concentrations in high-nitrate-low-chlorophyll environments such as the central and eastern equatorial Pacific. The model demonstrates that a wide range ofrelationships between NP and mixed-layer nitrate is possible due to partial decoupling ofnitrogen and iron cycles in the upper ocean. The shapes and slopes ofthese relationships are determined by the mode ofphysical forcing that introduces the most variability to the iron supply for a set of observations. When variability of upwelling fluxes is dominantly determined by changes in nutricline depth over changes in upwelling velocity, NP is a linear function of mixed-layer nitrate; whereas when variability in upwelling velocity dominates, NP takes on a hyperbolic function of nitrate. Fluctuations in atmospheric iron supply also introduce distinct patterns between NP and mixed-layer nitrate concentrations. Variability in nitrogento-iron ratios in exported organic matter, however, are not likely to drive large changes in NP or nitrate because of expected covariation with N:Fe ratios in upwelled source waters. Within surface waters of the equatorial Pacific, variability in physical forcing exists over a wide range of temporal and spatial scales. Tropical instability waves, equatorially trapped internal gravity waves and daily fluctuations in zonal winds all predominantly drive short-term variability in upwelling velocities with minor changes to nutricline depth. El Ni * no cycles and Kelvin waves produce longer-term variability in nutricline depth with minimal changes in upwelling velocities. Comparisons ofobserved NP-to-nitrate relationships for individual cruises to the observed modes ofphysical forcing match model predictions quite well, with Zonal Flux and Flupac serving as end-member examples of these processes. These results support those from statistical studies (Aufdenkampe et al., Deep-Sea Research (2002) 2619– 2648, Aufdenkampe et al., Global Biogeochemical Cycles 15 (2002) 101–113), by offering a mechanistic explanation for observations that the relationship between NP and nitrate is more variable between cruises than within cruises. r 2002 Elsevier Science Ltd. All rights reserved.

The annual cycle and biological effects of the Costa Rica Dome

The Costa Rica Dome is similar to other tropical thermocline domes in several respects: it is part of an east–west thermocline ridge associated with the equatorial circulation, surface currents flow cyclonically around it, and its seasonal evolution is affected by large-scale wind patterns. The Costa Rica Dome is unique because it is also forced by a coastal wind jet. Monthly climatological fields of thermocline depth and physical forcing variables (wind stress curl and surface current divergence) were analyzed to examine the structure and seasonal evolution of the dome. The annual cycle of the dome can be explained by wind forcing in four stages: (1) coastal shoaling of the thermocline off the Gulf of Papagayo during February–April, forced by Ekman pumping on the equatorward side of the Papagayo wind jet; (2) separation from the coast during May–June when the intertropical convergence zone (ITCZ) moves north to the countercurrent thermocline ridge, the wind jet stops, and the North Equatorial Countercurrent extends toward the coast on the equatorward flank of the ridge; (3) countercurrent thermocline ridging during July–November, when the dome expands to the west as the countercurrent thermocline ridge shoals beneath a band of cyclonic wind stress curl on the poleward side of the ITCZ; and (4) deepening during December–January when the ITCZ moves south and strong trade winds blow over the dome. Coastal eddies may be involved in the coastal shoaling observed during February–March. A seasonally predictable, strong, and shallow thermocline makes the Costa Rica Dome a distinct biological habitat where phytoplankton and zooplankton biomass are higher than in surrounding tropical waters. The physical structure and biological productivity of the dome affect the distribution and feeding of whales and dolphins, probably through forage availability.

Physical control of primary productivity on a seasonal scale in central and eastern Arabian Sea

Usingin situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing during winter resulted in one-and-a-half times higher production in 1997 than in 1995.

A pelagic ecosystem model calibrated with BATS data

Mechanistic models of ocean ecosystem dynamics are of fundamental importance to understanding and predicting the role of marine ecosystems in the oceanic uptake of carbon. In this paper, a new pelagic ecosystem model that is descended from the model of Fasham et al. (Journal of Marine Research, 99 (1990) 591–639) (FDM model) is presented. During model development, the FDM model was first simplified to reduce the number of variables unconstrained by data and to reduce the number of parameters to be estimated. Many alternative simplified model formulations were tested in an attempt to fit 1988–1991 Bermuda Atlantic Time-series Study (BATS) data. The model presented here incorporates the changes found to be important. (i) A feature of the FDM physics that gives rise to a troublesome fall bloom was replaced. (ii) A biodiversity effect was added: the addition of larger algal and detrital size classes as phytoplankton and detrital biomasses increase. (iii) A phytoplankton physiological effect was also added: the adjustment of the chlorophyll-to-nitrogen ratio by phytoplankton in response to light and nutrient availabilities. The new model has only four state variables and a total of 11 biological parameters; yet it fits the average annual cycle in BATS data better than the FDM model. The new model also responds reasonably well to interannual variability in physical forcing. Based on the justification for changes (i)--(iii) from empirical studies and the success of this simple model at fitting BATS data, it is argued that these changes may be generally important. It is also shown that two alternative assumptions about ammonium concentrations lead to very different model calibrations, emphasizing the need for time series data on ammonium.