Equatorial Upwelling Research Articles

Carbon isotopic composition of the C37:2 alkenone in core top sediments of the South Atlantic Ocean: Effects of CO2and nutrient concentrations

We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone‐based carbon isotopic fractionation (εp) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found εp positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43−] and [NO3−]. However, the relationship between εp and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. [1997]that the isotopic fractionation in haptophytes is related to nutrient‐limited growth rates. The relatively high variability of the εp−[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth‐limiting resources such as light intensity or micronutrient concentrations.

Slowdown of the meridional overturning circulation in the upper Pacific Ocean.

Decadal temperature fluctuations in the Pacific Ocean have a significant effect on marine ecosystems and the climate of North America. The physical mechanisms responsible for these fluctuations are poorly understood. Some theories ascribe a central role to the wind-driven meridional overturning circulation between the tropical and subtropical oceans. Here we show, from observations over the past 50 years, that this overturning circulation has been slowing down since the 1970s, causing a decrease in upwelling of about 25% in an equatorial strip between 9 degrees N and 9 degrees S. This reduction in equatorial upwelling of relatively cool water, from 47 x 10(6) to 35 x 10(6) m3 s(-1), is associated with a rise in equatorial sea surface temperatures of about 0.8 degrees C. Another effect of the slowing circulation is a reduction in the outgassing of CO2 from the equatorial Pacific Ocean-at present the largest oceanic source of carbon dioxide to the atmosphere.

A mechanism for decadal changes of ENSO behavior: roles of background wind changes

This study explains why a number of El Nino properties (period, amplitude, structure, and propagation) have changed in a coherent manner since the late 1970s and why these changes had almost concurred with the Pacific decadal climate shift. Evidence is presented to show that from the pre-shift (1961–1975) to the post-shift (1981–1995) epoch, significant changes in the tropical Pacific are found in the surface winds and temperature, whereas changes in the thermocline are uncertain. Numerical experiments with the Cane and Zebiak model demonstrate that the decadal changes in the surface winds qualitatively reproduce the observed coherent changes in El Nino properties. The fundamental factor that altered the model's El Nino is the decadal changes of the background equatorial winds and associated upwelling. The annual cycle is also necessary for the mean state to modulate El Nino. From the pre- to post-shift epoch, the changes in the background winds and upwelling modify the structure of the coupled mode (eastward displacement of the equatorial westerly anomalies) by reallocating anomalous atmospheric heating and SST gradient along the equator. This structural change amplifies the ENSO cycle and prolongs the oscillation period by enhancing the coupled instability and delaying transitions from a warm to a cold state or vice versa. The changes in the mean currents and upwelling reduce the effect of the zonal temperature advection while enhance that of the vertical advection; thus, the prevailing westward propagation is replaced by eastward propagation or standing oscillation. Our results suggest a critical role of the atmospheric bridge that rapidly conveys the influences of extratropical decadal variations to the tropics, and the possibility that the Pacific climate shift might have affected El Nino properties in the late 1970s by changing the background tropical winds and the associated equatorial upwelling.

Sea-surface temperature estimates in the Southeast Pacific based on planktonic foraminiferal species; modern calibration and Last Glacial Maximum

Estimates of sea-surface temperatures based on foraminiferal faunal species suggest that the Eastern Equatorial Pacific Ocean was 3–5°C cooler during the Last Glacial Maximum (LGM) than at present. Analysis of new cores from the Southeast Pacific reveals a likely source of ice-age cooling in variations of the Peru Current. Off southern Peru, LGM ocean temperatures were 6–8°C cooler than at present, consistent with substantial cooling on land inferred from regional glacier advances and ice-core data. In the Southeast Pacific, ice-age foraminiferal assemblages have good modern analogs, and transfer functions that define assemblages based on ancient samples yield results similar to those based on coretop samples. During the LGM, subpolar species dominate the Eastern Boundary Current off Peru and extend to the equator. In contrast, the range of the equatorial upwelling species remains roughly constant. We infer from these data and a heat budget model that equatorward advection of cool water, more than equatorial upwelling, drove LGM cooling of the Eastern Tropical Pacific Ocean.

One-dimensional ecosystem model of the equatorial Pacific upwelling system. Part I: model development and silicon and nitrogen cycle

Abstract A one-dimensional ecosystem model was developed for the equatorial Pacific upwelling system, and the model was used to study nitrogen and silicon cycle in the equatorial Pacific. The ecosystem model consisted of 10 components (nitrate, silicate, ammonium, small phytoplankton, diatom, micro- and meso-zooplankton, detrital nitrogen and silicon, and total CO2). The ecosystem model was forced by the area-averaged (5°S–5°N, 90°W–180°, the Wyrtki Box) annual mean upwelling velocity and vertical diffusivity obtained from a three-dimensional circulation model. The model was capable of reproducing the low-silicate, high-nitrate, and low-chlorophyll (LSHNLC) conditions in the equatorial Pacific. The linkage to carbon cycle was through the consumption of assimilated nitrate and silicate (i.e. new productions). Model simulations demonstrated that low-silicate concentration in the equatorial Pacific limits production of diatoms, and it resulted in low percentage of diatoms, 16%, in the total phytoplankton biomass. In the area of 5°S–5°N and 90°W–180°, the model produced an estimated sea-to-air CO2 flux of 4.3 mol m−2 yr−1, which is consistent with the observed results ranging of 1.0–4.5 mol m−2 yr−1. The ammonium inhibition played an important role in determining the nitrogen cycle in the model. The modeled surface nitrate concentration could increase by a factor of 10 (from 0.8 to 8.0 mmol m−3) when the strength of the ammonium inhibition increased from ψ=1.0 to 10.0 (mmol m−3)–1. The effects of both micro- and meso-zooplankton grazing were tested by varying the micro- and meso-zooplankton maximum grazing rates, G1max and G2max. The modeled results were quite sensitive to the zooplankton grazing parameters. The current model considered the role of iron implicitly through the parameters that determine the growth rate of diatoms. Several iron-enrichment experiments were conducted by changing the parameter α (the initial slope of the photosynthetic rate over irradiance at low irradiance), KSi(OH)4 (half-saturation concentration of silicate uptake by diatom), and μ2max (the potential maximum specific diatom growth rate) in the regulation terms of silicate uptake by diatom. Within the first 5 days in the modeled iron-enrichment experiment, the diatom biomass increased from 0.08 to 2.5 mmol m−3, more than a factor of 30 increase. But the diatom populations crashed 2 weeks after the experiment started, due to exhaustion of available silicate and increased mesozooplankton population. The modeled iron-enrichment experiments produced several ecological behaviors similar to these observed during the IronEx-2.

Latitudinal comparisons of equatorial Pacific zooplankton

Abstract Zooplankton biomass and rates of ingestion, egestion and production in the equatorial Pacific Ocean along 140°W and 180° exhibit maximum values in the High-Nutrient Low-Chlorophyll (HNLC) zone associated with equatorial upwelling (5°S–5°N) as compared to the more oligotrophic regions to the north and south. Zooplankton biomass and rates are not usually highest on the equator, but increase “downstream” of the upwelling center as the zooplankton populations exhibit a delayed response to enhanced phytoplankton production. The vertical distribution of zooplankton biomass in the equatorial HNLC area tends to be concentrated in surface waters and is more uniform with depth in oligotrophic regions to the north and south of the equatorial upwelling zone. In general, the amount of mesozooplankton (>200 μm) carbon biomass is approximately 25% of estimated phytoplankton biomass and 30% of bacterial biomass in the HNLC area of the equatorial Pacific Ocean. Zooplankton grazing on phytoplankton is low in the equatorial Pacific Ocean, generally −1 for 64–200 μm zooplankton to 0.08 d −1 for 1000–2000 μm zooplankton. Thus, the numerical and functional response of equatorial zooplankton to increases in phytoplankton production are more rapid than normally occurs in sub-tropical and temperate waters. Potential zooplankton fecal pellet production, estimated from metabolic demand, is approximately 1.6 times the estimated gravitational carbon flux at 150 m in the zone of equatorial upwelling (5°S–5°N) and 1.1 times the export flux in the more oligotrophic regions to the north and south. The active flux of carbon by diel migrant zooplankton in the HNLC zone is a minor fraction of the gravitational flux (2% at 140°W, 4% at 180°) but increases in the more oligotrophic regions to the north and south where there is a deeper mixed layer and a greater relative proportion of diel migrant zooplankton.

One-dimensional ecosystem model of the equatorial Pacific upwelling system. Part II: sensitivity analysis and comparison with JGOFS EqPac data

Abstract A one-dimensional model of the equatorial Pacific upwelling ecosystem that incorporates two phytoplankton components, two grazers, and three nutrients, Si(OH)4, NO3, and NH4 (Chai et al., Deep-Sea Research II (2002) 2713–2745), was designed to consider the effects of Si(OH)4 limitation on the diatom growth and ecosystem functioning. Model output was obtained for a range of source concentrations of Si(OH)4, 3–15 mmol m−3, coinciding with the range measured at 120 m depth during JGOFS EqPac. NO3 was held at 12 mmol m−3, reflecting the relatively greater concentrations of NO3 compared to Si(OH)4 in the JGOFS data. The model was shown to function as a chemostat-like system with the loss rates, provided largely from zooplankton grazing, controlling growth rates of the phytoplankton. When different source concentrations of Si(OH)4 were applied, surface concentrations of Si(OH)4 varied within a narrow range compared to NO3 as would occur in a chemostat with limiting Si(OH)4 and non-limiting NO3 in the feed water. Vertical profiles of nutrients compared well with field data. Model results are compared with field data for new and total nitrogen production and export of N, Si, and C, and with other models, although none consider Si(OH)4 specifically. The model suggests that the stability of the equatorial system with its narrow range of biological and chemical variables is conferred by the action of diatoms providing food for mesozooplankton whose grazing also depletes the picoplankton. Diatoms increase with source Si(OH)4 concentrations, and picoplankton population and NO3 consumption decrease, resulting in a maximum surface TCO2 and increased CO2 flux to the atmosphere at intermediate source Si(OH)4 concentrations. Diatoms function in the equatorial system as a silica pump to export silica. This means that sedimented biogenic silica under the equatorial upwelling area should be viewed as an amplifier of changes in surface properties, with important consequences to paleoequatorial productivity.

Polychaete species diversity in the central Pacific abyss: local and regional patterns, and relationships with productivity

We investigated the relationship between productivity and local species diversity, and the degree of species turnover, at 8 sites on the central equatorial Pacific abyssal plain. The 8 sites span a 4-fold difference in seafloor particulate organic carbon (POC) flux and, hence, community productivity. The sites are similar in water depth (4300 to 5100 m), degree of isolation from terrigenous influences, and hydrodynamic regime. Three sites lie under the influence of equatorial upwelling, and are subject to enhanced deep POC flux derived from high overlying primary production. The remaining sites lie beneath the oligotrophic north Pacific gyre. The number of polychaete species collected at a single site ranged from 14 to 113, with at least 90% apparently being new to science. We found no evidence for the purported unimodal relationship between productivity and diversity seen in other ecosystems, including deep-sea slopes, and found only weak evidence of a monotonic increase in diversity with productivity. Rates of species turnover were low over scales of ~200 to 3000 km for the dominant polychaete species in the communities, and all sites were dominated by a core group of biogeographically widespread, locally abundant species. In contrast, there was little between-site similarity in the long list of rare species found at each site, implying either a high turnover of rare species at 200 to 3000 km scales, or incomplete sampling of the rare species list at each site. More intensive sampling studies using both morphological and molecular methods are needed to resolve the distribution patterns of rare species in the Pacific abyss. Local polychaete species diversity beneath equatorial Pacific upwelling (measured by rarefaction) appears to be unusually high for the deep sea, exceeding by at least 10 to 20% that measured in abyssal sites in the Atlantic and Pacific, and on the continental slopes of the North Atlantic, North Pacific, and Indian Oceans.

Meridional asymmetry of source nutrients to the equatorial Pacific upwelling ecosystem and its potential impact on ocean–atmosphere CO2 flux; a data and modeling approach

Si(OH)4, NO3, and TCO2 are shown to be distributed asymmetrically in a north/south direction about the equatorial Pacific using data from WEPOCS III and JGOFS EqPac cruises. Equatorial SiOH4 concentrations are shown to be the product of both geochemical and physical interactions with chemical processes, occurring in at least three regions remote from the equatorial Pacific, and physical delivery processes from the equatorial undercurrent (EUC) to the surface layer varying over a range of time scales. The EUC was partitioned into upper and lower portions, the upper providing source water to the central upwelling area and the lower crossing the Pacific without upwelling and thought to reenter the surface along the coast of Peru and to the eastern equatorial upwelling area. The source waters from the North Pacific, the north equatorial countercurrent (NECC) and from the South Pacific, the New Guinea coastal undercurrent (NGCUC) also were partitioned according to source for the upper and lower EUC. Mean concentrations and ranges of nutrients for each source partition were obtained from field data. Current flow and advective data output from a 3-D physical model were used with the field nutrient data to calculate nutrient fluxes into the EUC. Although the inflow of water from the north and south were approximately equal, the stronger asymmetric distribution of Si(OH)4 compared to NO3 resulted in identifying the South Pacific source as only 30% of the total supply of Si(OH)4 to the EUC and the cause of a low Si(OH)4:NO3 condition. These results suggest a coupling between Southern Ocean productivity, equatorial productivity, and the efflux of CO2 to the atmosphere from the equatorial upwelling system.

On the response of calcareous dinoflagellates to oligotrophy and stratification of the upper water column in the equatorial Atlantic Ocean

Large numbers of calcareous dinoflagellate cysts and the vegetative calcareous coccoid species Thoracosphaera heimii are generally found in sediments underlying oligotrophic and/or stratified (sub)surface water environments. It is difficult to distinguish between the relative importance of these two environmental parameters on calcareous cyst and T. heimii distribution as they usually covary, but this information is essential if we want to apply cysts properly in the reconstruction of palaeoenvironments and past surface water hydrography. In the multi-proxy core GeoB 1523-1 from the Ceará Rise region in the western equatorial Atlantic Ocean (covering the past 155 ka), periods of greatest oligotrophy are not synchronous with periods of greatest stratification (Rühlemann et al., Mar. Geol. 135 (1996) 127–152; Mulitza et al., Geology 25 (1997) 335–338; Mulitza et al., Earth Planet. Sci. Lett. 155 (1998) 237–249), giving us the unique opportunity to differentiate between the effects of both parameters on cyst accumulation. The calcareous cyst record of the core reflects prominent increases in accumulation rate of nearly all observed species only during the nutrient-enriched but more stratified isotopic (sub)stages 5.5, 5.3, 5.1 and 1. In this respect, the distribution trends in the core are more similar to those of the eastern equatorial upwelling region (GeoB 1105-4) than they are to those of the oligotrophic north-eastern Brazilian continental slope (GeoB 2204-2), even though temporal changes in bioproductivity are principally in antiphase between the eastern and western equatorial regions. We conclude that stratification of the upper water column and the presence of a well-developed thermocline are probably the more important factors controlling cyst distribution in the equatorial Atlantic, whereas the state of oligotrophy secondarily influences cyst production within a well-stratified environment.

Distribution of diatoms along the equatorial transect in the western and central Pacific during the 1999 La Niña conditions

Abstract Diatoms in the surface waters were sampled at 22 stations (7°N, 134°E–0°, 170°W) in the western and central equatorial Pacific during December 1998–January 1999 in order to determine the distribution, abundance and composition of taxa. In addition, two types of oceanographic condition and nutrient regimes were investigated. The observed diatom distribution is significantly constrained by the water masses: the Western Pacific Warm Pool (WPWP) located in the west and the EUR (Equatorial Upwelling Region) located in the east. Along a transect from west to east, total standing stocks slightly increased. While the standing stock of pennate diatoms was almost uniform throughout the sampled stations, that of centric diatoms increased in the EUR. In addition, a more diverse species assemblage was found in the EUR than in the WPWP. Nitzschia bicapitata, a pennate diatom, was the dominant species and occurred ubiquitously both in the WPWP and the EUR. Thalassionema nitzschioides, another pennate species, was also present, with high relative abundance throughout the sampled stations. The majority of centric taxa, such as the Thalassiosira complex, Chaetoceros atlanticus var. neapolitana and Rhizosolenia bergonii, occurred only in the EUR. Such a clear-cut distribution of the key taxa is attributed to changes in environmental conditions from west to east along the Equator: increase in salinity, nutrients, and decrease in temperature. Thus, the diatom assemblages in the western and central equatorial Pacific reflect the hydrographic and nutritional conditions of the surface waters.

Microbial community structure and variability in the tropical Pacific

Abstract The spatially extensive tropical Pacific includes regions that are limited by macronutrients or iron, and is thus broadly representative of open-ocean systems in which microbial communities predominate. Despite strong physical forcing due to the El Nino-Southern Oscillation cycle and the local effects of tropical instability waves, microbial abundances from a variety of JGOFS and related studies show similar, modest levels of variability in the high-nutrient, low-chlorophyll (HNLC) equatorial upwelling region, the oligotrophic, western Pacific Warm Pool, and the North Pacific central gyre. Mean 0–50 m abundances of some of the groups distinguished by flow cytometry are significantly enhanced in the HNLC region, including heterotrophic bacteria (HBACT; 720,000 versus 440,000 cells ml −1 ), Synechococcus spp. (SYN; 9800 versus 2000 cells ml −1 ) and pico-eukaryotic algae (PEUK; 6300 versus 800 cells ml −1 ). However, Prochlorococcus spp. (PRO) are slightly more abundant in the low-nitrate regions (180,000 versus 150,000 cells ml −1 ). The higher HNLC concentrations of SYN and PEUK are part of a broader expansion of the phytoplankton community over the relatively constant PRO base when the limiting nutrient (iron) pool is increased. Elevated biomass and production of phytoplankton and the greater availability of DOC presumably explain the higher HNLC abundances of HBACT. The mean biomass (±standard deviation) of bacterial populations for cross-equatorial transects (14.1±2.8 μg C l −1 ) is similar to that in the subtropics (11.6±2.7 μg C l −1 ), with cruise variations falling generally within a 2-fold range. Heterotrophs comprise a significantly higher mean percentage of total prokaryote biomass (59±9%) in the HNLC region than in the low-nutrient subtropics (42±6%). The biomass production of photosynthetic bacteria (PRO and SYN) in the central equatorial Pacific is conservatively twice that of HBACT, but total carbon flux through bacteria (44–75% of phytoplankton 14 C-production) is dominated by the high respiration, hence carbon demand, of heterotrophs. Given, the very different growth limiting factors (Fe, N, P, and organic carbon) among the various subregions and microbial groups in the tropical Pacific, it seems unlikely that direct controls on growth rate are sufficiently precise to account for the relatively low microbial variability observed. Among factors affecting loss rates, the regulatory role of viral lysis remains largely unexplored, as in most open-ocean systems. However, there is relatively good evidence, including the grazing response to the IronEx II perturbation and multi-level cascade influences, that protistan grazer are generally able to suppress large excursions in microbial abundance and biomass. The key elements of such a control mechanism are size or surface-chemistry characteristics that link the dynamics of different microbial populations to common (nanoflagellate) predators and the fact that such predators are held well below their maximum growth rate potential at ambient food concentrations. This latter point, in particular, ensures a rapid and approximately linear increase in protistan growth and grazing pressure up to prey concentrations many times ambient levels.

Basin-scale latitudinal patterns of copepod grazing in the Atlantic Ocean

Size-fractionated copepod abundance and ingestion rates were investigated along a 50°S-50°N latitudinal transect, during the Atlantic Meridional Transect (AMT) 4, 5 and 6 cruises (boreal spring-autumn 1997, boreal spring-summer 1998). Copepod abundance was higher at high latitudes in spring, near northwest Africa, in the equatorial and Benguela upwelling systems, and in the Subtropical Convergence, and lower in oligotrophic gyres. Gut contents were not related to phytoplankton biomass or production. Gut evacuation rate averaged 0.03 min -1 , and was not related to latitude or body size. Conservative estimates of copepod community total ingestion rates ranged between 3.4 and 173 mg C m -2 day -1 for AMT4, 1.6-252 mg C m -2 day -1 in AMT5 and 10-160 mg C m -2 day -1 in AMT6. Maximum values were always in the upwelling regions, the subtropical convergence and high latitudes in the Northern Hemisphere during boreal spring. Calculated ingestion rates translate into average daily minimal consumption values of 2. 07%, 1.89% and 2.6% of total chlorophyll stock, or 8. 02%, 14.5% and 12.9% of total primary production ingested daily on AMT4, 5 and 6 respectively. Grazing impact increases considerably if we consider ingestion of phytoplankton larger than 2 μm, especially under the influence of the Equatorial and North African upwellings, where copepod ingestion represents up to 30% of the biomass and >100% of production by large cells.

Palaeoenvironmental and palaeoceanographic controls on black, laminated mudrock deposition: example from Devonian–Carboniferous strata, Alberta, Canada

Abstract Sedimentological, palaeontological and ichnological analyses indicate that heightened episodes of equatorial upwelling, in tandem with an expanded and intensified oxygen minimum zone, promoted deposition of upper Famennian to lower Tournaisian laminated, organic-rich mud rocks of the Exshaw Formation on the Alberta cratonic platform. Exshaw mud rocks represent the culmination of an upper Devonian transgression that commenced with deposition of open marine carbonates of the Big Valley Formation. The transgression was abruptly halted at the Devonian–Carboniferous boundary by a rapid eustatic sea-level fall and the consequent deposition of near shore marine to shelfal bioturbated siltstones and sandstones of the Exshaw Formation. The siltstones grade into laminated, organic-lean mud rocks to the north in more distal offshore shelf settings. These mudrocks reflect a period of low primary production but continued anoxia of bottom waters. Continuation of anoxic conditions resulted from development of restricted circulation in the epicontinental sea due to the establishment of a westward-located physical barrier. Following this event was a relative sea-level rise that led to deposition of black, laminated mudrocks of the Banff Formation.

Tropical teleconnection and local response to SST anomalies during the 1997–1998 El Niño

The quasi‐equilibrium tropical circulation model (QTCM) is used to examine the response to various sea surface temperature (SST) anomalies in the tropical oceans during the 1997–1998 El Niño. Both local and remote responses are noted. The negative precipitation anomalies to the north and south of the El Niño‐Southern Oscillation (ENSO) enhanced precipitation region are largely a response to the warm SST anomalies in the central and eastern Pacific. However, in the western Pacific and maritime continent, reduction of rainfall is mainly caused by local cold SST anomalies. In the winter of the 1997–1998 El Niño, strong warm SST anomalies in the Indian Ocean contributed to the local enhanced rainfall. They affect precipitation anomalies in central, eastern, and southern Africa. The drought in northern South America is clearly a remote response to ENSO warm SST anomalies in the Pacific, while the SST anomalies in the Atlantic also impact the drought. The tropical Pacific cold SST anomalies surrounding the ENSO warm anomalies appear not to be caused by surface flux changes associated with atmospheric teleconnection (in simulations with specified SST in the ENSO warm region and a mixed‐layer ocean model elsewhere). Atmospheric circulation tends to spread the warm anomalies, but ocean dynamics appears also to be important for both cold and warm SST anomalies outside the equatorial upwelling region. In both the regional specified SST experiments and mixed‐layer experiments, relative subsidence tends to occur within convective zones, and it is generally localized. On the other hand, tropospheric temperature and wind anomalies spread much farther. The typical spatial scale of the remote response in temperature and wind fields tends to be larger than the dominant scale of the precipitation response. The remote response in precipitation anomalies does not appear to be related to temperature and wind anomalies in a simple manner.

Atlantic paleobathymetry, paleoproductivity and paleocirculation in the late Albian: the benthic foraminiferal record

Spatial distribution patterns of benthic foraminifers in upper Albian sediments from 25 DSDP/ODP sites and 31 onshore sections of the North and South Atlantic Ocean are used to generate paleobathymetric reconstructions and to identify areas of high primary production such as coastal and equatorial upwelling zones. New paleobathymetric estimates are provided for DSDP/ODP sites and onshore locations that are not situated on oceanic crust. Paleobathymetric reconstructions indicate shallow water exchange between the North and South Atlantic but show the existence of a deep-water connection between the western and eastern Tethys (>2500 m) through the Gibraltar Gateway. Strikingly, there is no evidence for a strong latitudinal gradient in deep-water benthic foraminiferal distribution during the late Albian: South Atlantic assemblages show close affinity to North Atlantic and Tethyan assemblages, exhibiting only a minor degree of provincialism. Biogeographic patterns reveal a distinct asymmetry in late Albian paleoproductivity for the North Atlantic. As for the present day, the eastern margins of the Atlantic were generally more productive than the western margins, and a belt of enhanced carbon flux export to the seafloor can be traced around the north African coast, which probably corresponded to a zone of vigorous coastal upwelling. By contrast, assemblage composition in the South Atlantic generally reflects mesotrophic to oligotrophic conditions. Benthic foraminiferal distribution patterns, thus, provide robust proxy data to test predictions from paleocirculation and paleobathymetric models for the mid-Cretaceous Atlantic Ocean and adjacent margins.

Equatorial upwelling in the western Pacific warm pool

Vertical velocity on the equator in the western Pacific warm pool is investigated using data from the Coupled Ocean‐Atmosphere Response Experiment enhanced monitoring array (EMA) centered at 0°, 156°E. The data consist of hourly subsurface horizontal velocity time series from August 1991 until April 1994. Vertical velocity is calculated using horizontal velocity components and the application of the continuity equation. During the first year, from March 1992 until February 1993, data are available from five moorings of the EMA and thus provide nine different combinations of moorings from which to calculate vertical velocity. Four moorings were available during the remaining time period. Random errors are found to be <10−5 m s−1, while systematic errors (finite difference error, systematic instrument error, and error due to surface extrapolation) may be larger. It is suggested that errors, including finite difference errors, are not larger than the vertical velocity estimate. The estimates of vertical velocity are valid on spatial scales the size of the array (∼400 km) and timescales longer than a few days. They reveal a seasonal cycle manifested during a moderate El Niño. Results indicate upwelling, on average from 70 m down to 250 m over the 2 year time period, being slightly stronger in 1992 coincident with the stronger El Niño year. The divergence of horizontal velocity components, resulting in positive vertical velocity, is due to geostrophic divergence on the equator produced from a westward directed zonal pressure gradient force. Meridional divergence and zonal wind stress are uncorrelated, suggesting that Ekman convergence due to local westerly winds is only of partial influence. Consequently, downwelling is not found near the surface, where contributions from local winds and geostrophic divergence are in opposition. This estimate of vertical velocity indicates that water is upwelled in the warm pool from much deeper than but with comparable magnitude to the central and eastern Pacific.

Response of the equatorial Pacific to chlorophyll pigment in a mixed layer isopycnal ocean general circulation model

The influence of phytoplankton on the upper ocean dynamics and thermodynamics in the equatorial Pacific is investigated using an isopycnal ocean general circulation model (OPYC) coupled with a mixed layer model and remotely sensed chlorophyll pigment concentration from the Coastal Zone Color Scanner (CZCS). In the equatorial Pacific heat accumulation due to a higher abundance of chlorophyll pigments in the equatorial Pacific leads to a decrease of the mixed layer thickness. This generates anomalous westward geostrophic currents north and south of the equator. In the western equatorial Pacific, these anomalous geostrophic currents merge into and strengthen the equatorial undercurrent (EUC), supplying water mass from the 200 m depth to the eastern equatorial Pacific. This chlorophyll‐induced response of the undercurrent enhances upwelling around 110W, resulting in a lower sea surface temperature (SST) than without chlorophyll. Thus, thermal gradients due to absorption of solar radiation by phytoplankton may contribute remotely to equatorial upwelling in the eastern Pacific.

Simulated Response of the Atmosphere‐Ocean System to deforestation in the Indonesian Archipelago

The climatic effects of large‐scale deforestation in the Indonesian Archipelago are investigated using a fully coupled atmosphere‐ocean model— the Fast Ocean Atmosphere Model (FOAM). Extremely rapid rates of deforestation across the Archipelago motivate this study. We compare two simulations (one with fixed ocean temperatures, another where the ocean responds to changing atmospheric conditions), to investigate the potential for oceanic feedbacks on deforestation‐induced climate change. With fixed sea surface temperatures, evaporation decreases over land but increases over the surrounding oceans where the wind speeds increase. Regional deep convection is enhanced and precipitation increases over the islands. With an interactive ocean the initial decrease in evaporation over the deforested land reduces convergence and increases the easterlies in the Bay of Bengal. More intense equatorial upwelling cools the surface temperatures over that region and reduces ocean evaporation. Regional convection is less intense and precipitation drops by 9% over deforested land.

Variability of surface water fCO2 during seasonal upwelling in the equatorial Atlantic Ocean as observed by a drifting buoy

The fugacity of carbon dioxide (fCO2) in tropical Atlantic surface waters was hourly monitored by a drifting carbon interface ocean atmosphere (CARIOCA) buoy from June to September 1997 during strong seasonal equatorial upwelling. The buoy drifted along the northern side of the equatorial cold tongue from 0.2°S, 7.5°W to 0.2°N, 12.5°W (June 20 to July 3). An inverse trend between temperature and fCO2 reflected mixing between cold upwelled water with high fCO2 and warm tropical surface water with lower fCO2. The fCO2 maxima reflected the strength of the upwelling. Subsequently, the buoy crossed the cold tongue toward the southwest from 0.2°N, 12.5°W to 4.5°S, 20.1°W (July 3 to August 7). During this crossing, warming increased surface water fCO2. While fCO2 was always above 400 μatm, the air‐sea CO2 flux was highest in the southern part of the cold tongue as a result of the spatial distribution of the CO2 exchange coefficient. A variable diel cycle of surface‐water fCO2 with an amplitude up to 3.4 μatm was attributed to the combined effects of diel changes in temperature and stratification, biological activity, and oceanic CO2 release from a shallow daytime mixing layer. At 4.5°S, 20.1°W a sharp rise of temperature, a decrease of fCO2, and a maximum fluorescence marked the exit of the region with a strong upwelling signature. Finally, the buoy drifted westward from 4.5°S, 20.1°W to 2.8°S, 25.0°W (August 7 to September 15). This study has demonstrated the potential of autonomous CARIOCA buoys to monitor the evolution and high‐frequency variability of surface water fCO2 within large‐scale oceanic processes.