Variability Off Research Articles

Time series of carbonate system variables off Otaru coast in Hokkaido, Japan

We report several biogeochemical parameters (dissolved inorganic carbon (DIC), total alkalinity (TA), dissolved oxygen (DO), phosphate (PO 4), nitrate + nitrite (NO 3 + NO 2), silicate (Si(OH) 4)) in a region off Otaru coast in Hokkaido, Japan on a “weekly” basis during the period of April 2002–May 2003. To better understand the long-term temporal variations of the main factors affecting CO 2 flux in this coastal region and its role as a sink/source of atmospheric CO 2, we constructed an algorithm of DIC and TA using other hydrographic properties. We estimated the CO 2 flux across the air–sea interface by using the classical bulk method. During 1998–2003 in our study region, the estimated fCO 2sea ranged about 185–335 μatm. The maximum of fCO 2sea in the summer was primarily due to the change of water temperature. The minimum of fCO 2sea in the early spring can be explained not only by the change of water temperature but also the change of nutrients and chlorophyll- a. To clarify the factors affecting fCO 2sea (water temperature, salinity, and biological activity), we carried out a sensitivity analysis of these effects on the variation of fCO 2sea. In spring, the biological effect had the largest effect for the minimum of fCO 2sea (40%). In summer, the water temperature effect had the largest effect for the maximum of fCO 2sea (25%). In fall, the water temperature effect had the largest effect for the minimum of fCO 2sea (53%). In winter, the biological effect had the largest effect for the minimum of fCO 2sea (35%). We found that our study region was a sink region of CO 2 throughout a year (−0.78 mol/m 2/yr). Furthermore, we estimated that the increase of fCO 2sea was about 0.56 μatm/yr under equilibrium with the atmospheric CO 2 content for the period 1998–2003, with the temporal changes in the variables ( T, S, PO 4) on fCO 2sea, thus as the maximum trend of each variable on fCO 2sea was 0.22 μatm/yr, and the trend of residual fCO 2 including gas exchange was 0.34 μatm/yr. This result suggests that interaction among variables would affect gas exchange between air and sea effects on fCO 2sea. We conclude that this study region as a representative coastal region of marginal seas of the North Pacific is special because it was measured, but there is no particular significance in comparison to any other area.

Zooplankton biomass variability off Angola and Namibia investigated by a lowered ADCP and net sampling

Variability in horizontal and vertical zooplankton biomass distribution was investigated down to a maximum depth of 1250 m off Angola/Namibia in August/September 2000 using a combination of net samples and backscatter profiles. The latter were measured by a 300-kHz ADCP mounted on the CTD frame. Thus, we obtained a near constant signal to noise ratio over the entire water column. This approach allowed both, a large spatial coverage and a relative high resolution in horizontal and vertical plane. The volume-backscattering coefficient was calibrated against biomass concentration of size fractionated net samples. In this way differences in shape and scatter properties of organisms were considered in a first approximation. Potential occasional larger signals were smoothed by averaging backscatter intensity over net sampling layers prior to the statistical comparison. The agreement in biomass concentration ranges and in horizontal distribution patterns derived by both methods was used as an additional quality factor. The biomass-rich zone spread deeper in the regions south of the Angola Benguela Frontal Zone (ABFZ) than north of it as a response to coastal upwelling. The diel vertical migration (DVM) caused “centres of the populations” (weighted mean depths) in 122 m (night), 303 m (day), 135 m (dawn), and 154 m (dusk). The average biomass at night within the upper 200 m exceeded those at day by a factor of 3.5. The phenomenon was more pronounced north and seaward of the ABFZ than south and shoreward of it. Minimum oxygen concentrations of about 0.2 ml/l were no barrier for migrating organisms passing the intermediate oxygen minimum zone (OMZ), which is a characteristic feature in the region.

Surface boundary-layer variability off Northern California, USA, during upwelling

Abstract A five-element mooring array is used to study surface boundary-layer transport over the Northern California shelf from May to August 2001. In this region, upwelling favorable winds increase in strength offshore, leading to a strong positive wind stress curl. We examine the cross-shelf variation in surface Ekman transport calculated from the wind stress and the actual surface boundary-layer transport estimated from oceanic observations. The two quantities are highly correlated with a regression slope near one. Both the Ekman transport and surface boundary layer transport imply curl-driven upwelling rates of about 3×10 −4 m s −1 between the 40 and 90 m isobaths (1.5 and 11.0 km from the coast, respectively) and curl-driven upwelling rates about 1.5×10 −4 m s −1 between the 90 and 130 m isobaths (11.0 and 28.4 km from the coast, respectively). Thus curl-driven upwelling extends to at least 25 km from the coast. In contrast, upwelling driven by the adjustment to the coastal boundary condition occurs primarily inshore of the 40-m isobath. The upwelling rates implied by the differentiating the 40-m transport observations with the coastal boundary condition are up to 8×10 −4 m s −1 . The estimated upwelling rates and the temperature–nitrate relationship imply curl-driven vertical nitrate flux divergences are about half of those driven by coastal boundary upwelling.

Application of wavelet transform in the study of coastal trapped waves off the west coast of South America

Wavelet transform and cross wavelet transform were applied for analyzing long time series of sea level and alongshore wind stress to identify intraseasonal variability off western South America and the relations with remote and local forcings. Hydrographic data were used to estimate properties of coastal trapped waves with a theoretical model. For El Niño years, we found the existence of intraseasonal oscillations with periods 20–90 days, between 2S and 27S. At the peak of 91–92 and 97–98 EL Niños, we found perturbations in the northern region, probably associated with remotely forced internal Kelvin waves, with periods 6–11 days and phase velocities 160–260 km/day. Between 12S and 15S, during two El Niño events, our calculations show perturbations which appear to be barotropic shelf waves propagating southward with velocities between 110 and 150 km/day and periods between 30 and 50 days.

Nearshore sea temperature variability off Rottnest Island (Western Australia) derived from satellite data

While satellite‐derived sea‐surface temperatures (SSTs) have been widely used in open ocean monitoring, they have rarely been applied to the nearshore region. In this study, full‐resolution Advanced Very High Resolution Radiometer (AVHRR) satellite data were used to derive a sea temperature climatology for three nearshore sites at Rottnest Island (Western Australia) for the 7‐year period 1995 to 2001. Because of the proximity to the land, image geolocation and careful pixel selection were crucial. The mean annual cycle shows the influence of both air–sea heat flux in the shallow waters and the seasonal strengthening of the tropical Leeuwin Current. Self‐recording temperature loggers were installed on near‐surface buoys for a few months in 2001 to assess how reliably the satellite temperatures could be used in such a dynamic nearshore environment. About 80% of the individual satellite temperatures were within ±0.5°C of the logger measurements (at the same time of day) and over 95% were within ±1°C. Occasionally, however, the satellite temperatures differed from the in situ measurements by more than 2°C and possible reasons for this are discussed. The monthly mean satellite temperatures were generally within 0.3°C of the monthly logger averages, which may be taken as a practical reliability limit for the climatology.

Chlorophyll‐a variability off Patagonia based on SeaWiFS data

Seasonal to interannual variability of satellite derived chlorophyll‐a over the Patagonia shelf and shelf break in the western South Atlantic are studied based on 7 years of ocean‐color data (1998–2004) from the Sea‐Viewing Wide Field‐of‐View Sensor (SeaWiFS). Strong chlorophyll‐a seasonal variability (>4 mg m−3) is observed compared to the open ocean (<1.5 mg m−3). North of 45°S, chlorophyll‐a blooms initiate in early austral spring (September and October), while south of 45°S blooms begin in late spring to early summer (November through January). The spring maximum (>3.5 mg m−3) extends from the midshelf to the shelf break between 37°S and 44°S and southward to 51°S along a narrow shelf break band. In summer the shelf break maximum persists from 37°S to 51°S, and two inner‐shelf blooms develop off Valdés Peninsula and along a near‐coastal band between 46°S and 52°S (>3 mg m−3). Chlorophyll‐a concentrations in the northern midshelf sharply decay in late spring, reaching lowest concentrations in summer (February and March) and a secondary maximum in early winter (June). Though all regions present substantial interannual variations, the bloom locations are stable. The shelf break maximum is located inshore of the front between the low salinity shelf waters and the cold, salty, and nutrient‐rich Malvinas Current waters. The inner shelf maxima are offshore of fronts separating well‐mixed coastal waters from the stratified midshelf. North of 41°S the midshelf bloom is also associated to a bottom trapped thermal front. Thus, all the high chlorophyll‐a regions are associated to well‐defined fronts.

Internal tide spatial variability off western Portugal detected by current meter observations

Long‐term (10–12 months) current meter observations from the western Iberian continental margin are analyzed focusing on oscillations in the internal gravity wave band. Kinetic energy spectra are characterized by background falloff rates with frequency ω ranging from ω−1 to ω−2, superposed by energetic narrow bands at specific frequencies. Significant spectral peaks are found at some of the inertial‐tidal nonlinear interaction frequencies (M2 + f and M4 + f) and at higher semidiurnal harmonics (M4, M6 and M8). Semidiurnal internal tide kinetic energy reveals a spatial distribution with a maximum coherent contribution (44 cm2 s−2) at 1200‐m depth on the mid continental slope and a maximum relative incoherent signal (>60% of the total semidiurnal tidal band energy) at intermediate levels (650 to 1200 m depth and 30 to 130 km from the shelf break).

El Niño variability off Peru during the last 20,000 years

Here we present a high‐resolution marine sediment record from the El Niño region off the coast of Peru spanning the last 20,000 years. Sea surface temperature, photosynthetic pigments, and a lithic proxy for El Niño flood events on the continent are used as paleo–El Niño–Southern Oscillation proxy data. The onset of stronger El Niño activity in Peru started around 17,000 calibrated years before the present, which is later than modeling experiments show but contemporaneous with the Heinrich event 1. Maximum El Niño activity occurred during the early and late Holocene, especially during the second and third millennium B.P. The recurrence period of very strong El Niño events is 60–80 years. El Niño events were weak before and during the beginning of the Younger Dryas, during the middle of the Holocene, and during medieval times. The strength of El Niño flood events during the last millennium has positive and negative relationships to global and Northern Hemisphere temperature reconstructions.

Benthic nitrogen regeneration under oxygen and organic matter spatial variability off Concepción (∼36°S), central Chile

The effects of dissolved oxygen (DO) and organic matter (OM) on several geochemical parameters and nitrogen cycling were evaluated in the upwelling region off central Chile (∼36°S). The sediments of the study area presented: (i) nearly anoxic conditions (2.68 μM) and high chlorophyll-a content (1112 mg g−1) within the Concepción Bay (29 m water depth); (ii) suboxic to oxic conditions (10–129 μM), and from moderate to low chlorophyll-a levels (47–13 mg g−1), in a depth transect outside the bay (88–820 m depth); and (iii) suboxic conditions (5.75–8.9 μM) and low chlorophyll-a content (∼13 mg g−1) in the Itata Canyon area (209 and 362 m depth). Nitrogen processes (in mmol m−2 d−1) varied with depth, in association with changes in fresh OM input and DO content at the transect stations: i.e. decreased ammonification (from 12.9 to 1.0), NH4+ efflux (from 4.8 to 1.2) and denitrification (from 1.3 to 0.3), and increased potential nitrification (from 1.0 to 2.4). Concepción Bay and one of the Itata Canyon stations (209 m) were notably different from the others, indicating heterogeneity of benthic nitrogen cycling in this upwelling region. The bay presented the highest ammonification and lowest denitrification rates. Itata Canyon sediments showed high urea influxes at both sites, although high NH4+ influxes and the highest potential nitrification were only found at the 209 m station. Patterns such as the occurrence of dissolved organic nitrogen effluxes (4.9–75) and higher ammonification rates were found at stations <30 km offshore; influxes (9.7–20.5) and lower ammonification rates were observed at stations >52 km offshore. All the stations presented NO3−+NO2− influxes (0.6–4.3). A nitrogen-cycling imbalance in the NH4+ benthic cycle was found at four stations: NH4+ excesses (30–>100%) at two transect and one of the canyon stations, and a deficit (>100%) at the other canyon station. A small proportion of the NO3−+NO2− influxes (<42%) was explained by the processes studied herein. These imbalances indicated potential incubation biases and the presence of other N processes not accounted for in this study.

Timing and mechanisms of surface and intermediate water circulation changes in the Nordic Seas over the last 10,000 cal years: a view from the North Iceland shelf

The North Iceland shelf bears essential components of the present surface and intermediate circulation of the northern North Atlantic. Instrumental and historical data give evidence of the sensitivity of this domain to broad, regional-scale oceanic and atmospheric anomalies. Our investigation of the paleohydrological variability off Northern Iceland throughout the last 10 000 cal yr suggests that atmospheric forcing alone, through combined changes in strength of the wind stress curl and sea-level atmospheric pressure pattern over the Nordic Seas, is sufficient to explain the recorded changes in origins and dynamics of surface and intermediate water masses. Our biotic proxies, coccoliths and benthic foraminifera, were extracted from a giant piston core (MD99-2269) collected in a shelf trough where sediment accumulated at an excess rate of 2 m/kyr. The mid-Holocene from 6.5 to 3.5 cal kyr BP was a time of peaked carbonate production and subsequent sedimentation, and strong water-column stratification with a thick layer of cold-fresh Arctic surface water overlapping an enhanced flow of Irminger/Atlantic Intermediate water. Applying conditions triggering present-time carbonate plankton blooms in the studied area, we infer that a lowered cyclonic activity associated with decreased winter storms and reduced production of Arctic Intermediate Water in the Iceland Sea were conductive of the recorded mid-Holocene water column structure. The opposite situation (warm Atlantic surface water, low vertically-integrated inflow of Irminger water, abutment of Arctic Intermediate water in deep shelf troughs) characterized the early Holocene as well as a shorter late Holocene period centred at 2 cal kyr BP. The Little Ice Age (ca. 0.2–0.6 cal kyr BP) and a short event at around 3 cal kyr BP stand as times of extreme advection of polar waters and extended sea–ice development. A comparison of the recorded long-term Holocene evolution of water column structure off Northern Iceland with climate and hydrological changes in the north-eastern Atlantic suggests that the strength of Atlantic inflow into the Nordic Seas was subjected to a balance between the Irminger and the Norwegian branches. This balance is thought to be mostly related to changes in the intensity and location of westerly winds and associated atmospheric pressure gradients in the North Atlantic.

Intraseasonal variability of wind-forced coastal upwelling off central Chile (30°S)

Satellite-derived along-shore windstress variability off subtropical Chile at intraseasonal periods has been recently associated with the corresponding wind variability over the tropical Pacific. Along the coastal strip in which satellite winds can not be resolved, wind-driven upwelling might be in principle modulated through large-scale intraseasonal changes in the strength of the subtropical anticyclone and by intraseasonal ocean coastal-trapped waves (CTWs) through associated changes in coastal sea-surface temperatures. Daily anomalies of band-passed outgoing long-wave radiation over the central equatorial Pacific have been selected to represent Madden–Julian Oscillations, as the dominant mode of equatorially sourced, intraseasonal variability in the atmosphere and coastal ocean. Afternoon alongshore pseudo-windstresses at a representative coastal site in central Chile (30°S) describe the upwelling-favourable wind field over a 30–40 km coastal strip. The subtropical anticyclone strength is expressed as a 900-km offshore sea-level pressure (San Felix Island), and the sea-level pressure difference with the coast. CTWs are represented by the smoothed sea-level at 27°S. The intermittent nature of intraseasonal variability led to statistical procedures as Wavelet and Singular-Spectrum (SSA) analyses to search for associations among those series. Cross-correlation analysis of the SSA-reconstructed series with the two principal oscillatory modes (40–100-day, and 28–50-day periods, respectively), shows that intraseasonal variability in the subtropical anticyclone strength, partially associated with the Pacific South America teleconnection pattern, has the best association with coastal windstress variability. CTWs appeared to be important in their association with coastal winds only during strong MJO activity in connection with El Niño in the low-frequency intraseasonal band, while in the high-frequency one a fraction of the ASL variability appears to be locally wind-forced.

Coastal transition zone off Chile

Seven years of subsurface current observations at deep‐sea and continental slope moorings off Chile (30°S), satellite sea surface anomaly, and satellite wind stress are used to examine space, time, and propagation characteristics of the mesoscale eddy activity in the coastal transition zone off Chile. This zone off Chile extends from the coast to ∼600–800 km offshore and can be separated into two latitudinal regions. The first region, between 29° and 39°S off central Chile, is characterized by high eddy kinetic energy and strong but variable equatorward wind stress. The second region, between 19° and 29°S off northern Chile, is characterized by low eddy kinetic energy and weak but persistent equatorward wind stress. Low‐frequency mesoscale variability off central Chile is mainly associated with baroclinic instability of coastal currents and westward propagation of Rossby waves from the eastern boundary and is strongly modulated over El Niño/La Niña cycles. Application of a new rotary wavelet spectrum method to the recording current meter data and the use of current estimates from satellite altimeter data show that mesoscale eddies and meanders in the coastal transition zone fluctuate with periods centered on 120 days and with length scales centered on ∼200 km. Cross‐rotary wavelet analyses of currents and wind stress indicate that fluctuations of coastal currents with periods longer than 220 days propagated westward as free Rossby waves during 1995–1996 and as forced Rossby waves during 1999–2001.

Reproductive strategies of the spiny lobster Panulirus interruptus related to the marine environmental variability off central Baja California, Mexico: management implications

The reproductive strategies of the spiny lobster, Panulirus interruptus, in relation to environmental factors (temperature and upwelling) were determined in Baja California during 1989–1999. Two reproductive indices, R1 and R2, are proposed, based on the rate of occurrence of females with spermatophores and ovigerous females, respectively. P. interruptus spawned one brood annually between April and August. The size at which 50% of females were sexually mature was 72.6 mm carapace length (CL), which is well below the current minimum legal size of 82.5 mm CL. Mating and deposition of spermatophores began under moderate sea surface temperature (SST) and low upwelling conditions, but higher rates of deposition were coincident with low SST and strong upwelling. A high value of R1 generally occurred between 12.8 and 16.8 °C, and in general was inversely correlated with winter–spring temperature. Egg-bearing females were present in waters with a temperature range of 12.5–24 °C, and peaked at 14–19.2 °C. Embryonic development and larval hatching coincided with weakening Ekman transport and seasonal ocean warming. Timing of breeding was strongly temperature dependent. El Niño and La Niña condition impacts were particularly evident at the onset and duration of breeding. Warmer temperatures during El Niño periods accelerated breeding significantly, while the contrary happened under cold temperatures during La Niña. Females responded rapidly to declining winter temperatures and began to mate as early as December or as late as February/March. Similarly, oviposition and hatching were accelerated in response to increasing summer temperatures and finished as early as July and as late as September. The relationship between reproduction and the environment suggests that reproductive pattern and larval recruitment are coupled to dominant physical features of the California Current. Since environmental variability impacts on the onset and duration of breeding, the use of flexible management criteria is recommended, in order to guarantee that the opening and closing dates of the fishing season can be adjusted to take account of this unpredictability. This approach, in conjunction with monitoring of reproductive activity, will benefit the fishery by maximising the species’ annual reproductive success.

Linking nitrogen dynamics to climate variability off central California: a 51 year record based on 15N/14N in CalCOFI zooplankton

Abstract Long-term variability in zooplankton 15N/14N was investigated in two species of calanoid copepods (Calanus pacificus and Eucalanus californicus) and two chaetognaths (Sagitta bierii and Sagitta euneritica) sampled in the spring of selected years from 1951 to 2001 off the central California coast. No statistically significant trend in 15N/14N was detected for any of the four species, with isotopic ratios in 2001 resembling those in copepods and chaetognaths sampled five decades earlier. Zooplankton body lengths also showed no long-term trends. With respect to proposed regime shifts in this region, heterogeneity in 15N/14N was detected only for S. bierii when comparing the periods 1951–1975, 1978–1998, and 1999–2001. In this species the 15N/14N in the most recent, brief period (1999–2001) averaged slightly lower than in the previous period. Three of the four species (C. pacificus, S. bierii, and S. euneritica) showed significant increases in 15N/14N during major El Ninos. El Nino-related enrichment in 15N could arise as a consequence of increased nitrate demand:supply at the base of the food web or advection of 15N-enriched nitrate from more southerly waters. While a range of physical and climate indices were evaluated, anomalies of 15N/14N from the long-term mean were found to be significantly related only to: (i) the Southern Oscillation Index in the case of both chaetognath species, (ii) a regional surface water temperature record (S. bierii only), (iii) an index of wind-driven coastal upwelling for the surface-dwelling C. pacificus, and (iv) variability in the Pacific Decadal Oscillation for the somewhat deeper-dwelling E. californicus. The relationships among each species’ 15N/14N averaged over the total sampling period was: E. californicus≈C. pacificus⪡S. euneritica

Multiscale characterization of the sea temperature in an area of abalone banks (Haliotis spp.) at Bahia Asuncion, Baja California Sur, Mexico

The Baja California abalone is one of the most important Mexican fisheries because of its high economic value. Despite strong management efforts, this fishery has faced dramatic fluctuations attributed mainly to overfishing and variability of the marine climate. In this report we analyze sea temperature variability off Bahía Asunción, Baja California Sur (Mexico), at temporal scales that may affect abalone biological processes, such as mortality, reproduction, recruitment, and individual growth rate. Since the analysis is based on different data sources, we conducted a series of correlation analyses to determine the coherence between them. We report the average daily cycle and its variations throughout the year and between years for the period 1997–2000, the seasonal cycle and interannual deviations for the period 1992–2000, the behavior of the decadal scale variability based on annual values for the period 1959–1997, and the long-term trends after analyzing the entire 20th century. Furthermore, we conducted a spectral density analysis to estimate the proportional contribution of each temporal scale to the entire variability signal. Results from the high-frequency variability (daily to interannual) suggest a relationship between temperature and the abalone reproductive cycles, a potential negative effect of the very strong ENSO events on the postlarvae, and a differential effect of temperature on juveniles of different species. Regarding the low-frequency variability (decadal to century), we show that recent years represent a much warmer period compared to previous decades, suggesting a different biological community structure between periods. The spectral density analysis indicates seasonal variability as the major contributor to the regional variability, followed by the interannual (related to ENSO). There is almost no information of the marine climate in this region, particularly in regard to fine and high-resolution observations. This contribution will provide new quantitative elements for studies dealing with the ecology of these coastal productive systems.

A Large-Scale Seasonal Modeling Study of the California Current System

A high-resolution, multi-level, primitive equation ocean model has been used to investigate the combined role of seasonal wind forcing, seasonal thermohaline gradients, and coastline irregularities on the formation of currents, meanders, eddies, and filaments in the entire California Current System (CCS) region, from Baja to the Washington-Canada border. Additional objectives are to further characterize the meandering jet south of Cape Blanco and the seasonal variability off Baja. Model results show the following: All of the major currents of the CCS (i.e., the California Current, the California Undercurrent, the Davidson Current, the Southern California Countercurrent, and the Southern California Eddy) as well as filaments, meanders and eddies are generated. The results are consistent with the generation of eddies from instabilities of the southward current and northward undercurrent via barotropic and baroclinic instability processes. The meandering southward jet, which divides coastally-influenced water from water of offshore origin, is a continuous feature in the CCS, and covers an alongshore distance of over 2000 km from south of Cape Blanco to Baja. Off Baja, the southward jet strengthens (weakens) during spring and summer (fall and winter). The area off southern Baja is a highly dynamic environment for meanders, filaments, and eddies, while the region off Point Eugenia, which represents the largest coastline perturbation along the Baja peninsula, is shown to be a persistent cyclonic eddy generation region.

Capelin (Mallotus villosus) in the Iceland–East Greenland–Jan Mayen ecosystem

The stock of capelin, Mallotus villosus (Müller), that inhabits the area between Iceland, East Greenland, and the island of Jan Mayen spawns in shallow coastal water south and west of Iceland. Juveniles grow up over the continental shelf off north Iceland and off East Greenland west of the Denmark Strait. The main feeding area of adults is the Iceland Sea, the oceanic area from about 68 to 72°N, between the Jan Mayen Ridge and the East Greenland continental shelf. After the feeding season, the adult stock assembles over the outer shelf off north Iceland and migrates to the spawning grounds along the south and west coasts from December to March. The main oceanographic features of Icelandic waters and the Iceland Sea are described and capelin migrations related to the distribution of water masses and the ocean current systems in the area. In the past two decades there have been large variations in capelin migrations and catchability, especially during the feeding season. However, these variations can only be explained in part by the available environmental data. Year-class abundance appears to be determined by survival during the first winter, in tune with the greater environmental variability off north Iceland than south and west of Iceland, where these capelin spawn and the larvae start drifting. Adult growth is positively related to the flow of Atlantic water into the area north of Iceland, indicating improved feeding conditions in the Iceland Sea when the Irminger Current is strong. There can be large interannual variations in number and weight-at-age in the adult stock. The main predators are whales, seabirds, and fish, especially cod. The combined annual removal by predators is estimated to have been 2.1–3.4 million tonnes in the early 1990s. The mean weight-at-age of cod aged 5–8 years dropped by up to 25–30% when capelin abundance was low in the early 1980s and 1990s. The relatively low mean weight of cod in the past 3 years may well be due to changed distribution and migration of capelin, resulting in reduced access of cod to this most important item in their diet.

Structure and Mechanisms of South Indian Ocean Climate Variability*

A unique open-ocean upwelling exists in the tropical South Indian Ocean (SIO), a result of the negative wind curl between the southeasterly trades and equatorial westerlies, raising the thermocline in the west. Analysis of in situ measurements and a model-assimilated dataset reveals a strong influence of subsurface thermocline variability on sea surface temperature (SST) in this upwelling zone. El Nino-Southern Oscillation (ENSO) is found to be the dominant forcing for the SIO thermocline variability, with SST variability off Sumatra, Indonesia, also making a significant contribution. When either an El Nino or Sumatra cooling event takes place, anomalous easterlies appear in the equatorial Indian Ocean, forcing a westward-propagating downwelling Rossby wave in the SIO. In phase with this dynamic Rossby wave, there is a pronounced copropagation of SST. Moreover, a positive precipitation anomaly is found over, or just to the south of, the Rossby wave-induced positive SST anomaly, resulting in a cyclonic circulation in the surface wind field that appears to feedback onto the SST anomaly. Finally, this downwelling Rossby wave also increases tropical cyclone activity in the SIO through its SST effect. This coupled Rossby wave thus offers potential predictability for SST and tropical cyclones in the western SIO. These results suggest that models that allow for the existence of upwelling and Rossby wave dynamics will have better seasonal forecasts than ones that use a slab ocean mixed layer. The lagged-correlation analysis shows that SST anomalies off Java, Indonesia, tend to precede those off Sumatra by a season, a time lead that may further increase the Indian Ocean predictability.

Mesoscale primary production and bio-optical variability off Antofagasta (23-24º S) during the transition to El Niño 1997-1998

The spatial variability of primary production (PP), chlorophyll-a (Chl-a) and the photosynthetic parameters were studied off Antofagasta, Chile (23-24° S, 70-72° W) during austral summer and winter. Between cruises (January and July 1997), significant changes occurred in the water column, including higher temperatures in the euphotic zone (Z eu ) deepening of thermocline below Z eu , an increase of oxygen concentration and the intrusion of Subtropical Waters with potential limitation of nutrients. These strong physical anomalies due to the transition period of El Nino 1997-1998 appeared in this study area during March 1997. During the July cruise, the El Nino event 1997-1998 was in the middle of its development (IOS–2). The hypothesis that chlorophyll-a concentrations and primary production differ significantly in the coastal areas in the Antofagasta region due to year-round coastal upwelling was tested in this study. Photosynthesis versus irradiance (P-E) experiments were performed daily, using simulated in situ incubations with samples collected within the Z eu . Also in vitro incubations were done at several selected stations. For results analyses, stations were pooled in coastal and oceanic sites according to distance from the narrow shelf and differential influence of local upwelling. Integrated Chl-a values during both cruises were significantly higher at the coastal stations, and since between cruises no differences were found, a mean value of 44 mg Chl-a m can be reported for the coastal area. Daily PP values were significantly different in space and time (P < 0.001), and at the coast also between cruises (P < 0.004) as a result of the high mean coastal value in January, 3,129 mg C m d in comparison to 942 mg C m d in July. The attenuation coefficient k d of photosynthetic active radiation (PAR), determined a significant change in the mean depth of Z eu between coastal and oceanic stations (44 ± 20 and 80 ± 17 m, respectively) during both sampling periods. Notwithstanding the spatial differences in chlorophyll-a concentrations and primary production, the observed weaker upwelling favourable winds during both cruises, the increase in depth of the mixing layer and light limitation in July, and the higher mean values of zooplankton grazing rate during January contributed to the similar abundance of chlorophyll-a in time. Although the El Nino event could negatively affect primary production during July, prevailing space and seasonal variability masked this effect.

Sardine and horse mackerel recruitment and upwelling off Portugal

The results of preliminary investigations on the possible relationships between coastal upwelling variability observed with satellite remote sensing, and sardine and horse mackerel recruitment dynamics in the west coast of Portugal are presented. The analysis of a sardine-recruitment time-series for the period 1976–1998 shows that there has been a decreasing trend since 1983 (with the exceptions of 1991 and 1992), the years from 1993–1995 representing the lowest values in that time series. Horse mackerel exhibited a decrease from 1986 until 1990, and from 1992–1995, the latter representing the lowest value for the period 1986–1998. Sea surface temperature (SST) derived from satellite data were used to compute monthly SST upwelling indices along the Portuguese west coast over a period from 1987–1997. The analysis focused on seasonal and annual upwelling variability off Portugal, and its possible effects on sardine and horse mackerel recruitment to the fishery. An increasing trend in SST upwelling indices during the winter (January–March), especially between 1992 and 1995 was found. The upwelling events observed off Portugal during winter months, which correspond to the spawning season for sardine and horse mackerel, had a negative impact on the recruitment of these fish species. This effect on recruitment could be due to an increase in conditions favourable to the offshore transport of larvae and consequently an increase in their mortality. Prior to 1992, when no upwelling events occurred during winter, recruitment dynamics was closely linked with the spring–summer seasonal upwelling variability off the Portuguese west coast.