Equatorial Upwelling Region Research Articles

A deep Eastern Equatorial Pacific thermocline during the early Pliocene warm period

During the early Pliocene warm period (∼4.6–4.2Ma) in the Eastern Equatorial Pacific upwelling region, sea surface temperatures were warm in comparison to modern conditions. Warm upwelling regions have global effects on the heat budget and atmospheric circulation, and are argued to have contributed to Pliocene warmth. Though warm upwelling regions could be explained by weak winds and/or a deep thermocline, the temporal and spatial evolution of the equatorial thermocline is poorly understood. Here we reconstruct temporal and spatial changes in subsurface temperature to monitor thermocline depth and show the thermocline was deeper during the early Pliocene warm period than it is today. We measured subsurface temperature records from Eastern Equatorial Pacific ODP transect Sites 848, 849, and 853 using Mg/Ca records from Globorotalia tumida, which has a depth habitat of ∼50–100m. In the early Pliocene, subsurface temperatures were ∼4–5°C warmer than modern temperatures, indicating the thermocline was relatively deep. Subsurface temperatures steeply cooled ∼2–3°C from 4.8 to 4.0Ma and continued to cool an additional 2–3°C from 4.0Ma to present. Compared to records from other regions, the data suggests the pronounced subsurface cooling between 4.8 and 4.0Ma was a regional signal related to restriction of the Isthmus of Panama, while continued cooling from 4.0Ma to present was likely related to global processes that changed global thermocline structure. Additionally, the spatial evolution of the equatorial thermocline along a N–S transect across ODP Sites 853, 849 and 848 suggests an intensification of the southeast trades from the Pliocene to present. Large-scale atmospheric and oceanographic circulation processes link high and low latitude climate through their influence on equatorial thermocline source water regions and consequently the equatorial thermocline. Through these low latitude/high latitude linkages, changes in the equatorial thermocline and thermocline source water played an important role in the transition from the warm Pliocene to the cold Pleistocene.

Declines in phytoplankton cell size in the subtropical oceans estimated from satellite remotely-sensed temperature and chlorophyll, 1998–2007

Satellite remotely-sensed sea surface temperature (SST) and surface chlorophyll were used to estimate median phytoplankton cell size at monthly and 11km2 resolution over the global ocean, 1998–2007. The temporal dynamics of median phytoplankton cell size were examined for the Pacific equatorial upwelling region and the subtropical oceans. For the equatorial upwelling region, cell size varied coherently with the El Niño Southern Oscillation with smaller (larger) median cell size during El Niños (La Niñas). Specifically, estimated median cell diameter increased by 34% between the 1998 El Nino and the 1999–2001 La Nina. In the subtropical oceans, over the period 1998–2007, median cell diameter exhibited statistically significant linear declines of about 2% in the North and South Pacific, and 4% in the North Atlantic. Pooling the data over all subtropical oceans, over the period 1998–2007, global median cell diameter declined by about 2%.These results suggest that phytoplankton cell size may vary on interannual and decadal scales resulting in changes in food chain length and hence energy transfer to higher trophic levels. Further, a shift to smaller sized phytoplankton has been hypothesized as a response to ocean warming. Thus, this approach, estimating phytoplankton cell size from remotely-sensed temperature and chlorophyll, has the potential to provide global monitoring of an aspect of phytoplankton community structure likely to be responsive to future climate change.

Hadley and Walker Circulations in the Mid-Pliocene Warm Period Simulated by an Atmospheric General Circulation Model

The mid-Pliocene warm period (∼3 million years ago; 3 Ma) is one of the plausible scenarios which provide insight into the climate system in a globally warmer world as projected by climate models for the future. The reconstructed sea surface temperature (SST) by the Pliocene Research, Interpretation and Synoptic Mapping phase 3 (PRISM3) reveals that salient warming occurs in the higher latitudes together with weakening of surface cooling in the equatorial and coastal upwelling regions. The sensitivity of an atmospheric general circulation model (AGCM) is studied by prescribing the surface condition based on the PRISM3 paleoenvironmental reconstructions. The simulated Walker circulation generally slows down, inducing convergent anomaly over the African continent and divergent anomaly over the Asian monsoon region at the lower troposphere; and vice versa at the upper troposphere in response to the reduced east-west gradient of the tropical SST in that period. The ascending branch of the Hadley cell expands poleward, implying a weakening of the meridional circulation in response to the warmer SST in the higher latitudes. To identify the physical reason for the modulation of the wet-dry climatological pattern in low latitudes, additional sensitivity experiments were conducted by changing the configurations of ice-sheet cover, vegetation and zonal patterns of the SST. The results indicate that the reduction of meridional and zonal gradient of tropical SST is the key factor for the expansion of proxy-suggested wetter climate over Africa.

Relationship between time-series diatom fluxes in the central and western equatorial Pacific and ENSO-associated migrations of the Western Pacific Warm Pool

The water masses of the central and western equatorial Pacific can be divided into two parts: the Western Pacific Warm Pool (WPWP) and the Equatorial Upwelling Region (EUR). The behavior of the WPWP plays a significant role in global climate changes such as the El Niño-Southern Oscillation (ENSO), and it drastically modifies the oceanographic conditions in the area every few years. It is important to evaluate changes in time-series diatom fluxes during both the El Niño and the La Niña events. As a part of the Global Carbon Cycle and Related Mapping based on Satellite Imagery (GCMAPS) Program, time-series sediment trap moorings were deployed and recovered along the Equator at seven stations (Sites MT1–MT7) during five R/V Mirai cruises in the central and western Pacific during January 1999–January 2003. The entire length of this study is divided into two phases depending on the oceanographic conditions: the La Niña event (1999 and 2000); and the El Niño event (2002). Site MT3 was located in the WPWP and Sites MT5–MT7 were in the EUR. Annual means of total diatom fluxes increased towards the east in each year. The fluxes observed at Sites MT4–MT6 decreased from the La Niña event to the El Niño event. However, the fluxes observed at Site MT3 in 2001 and 2002 were higher than those in 2000. Total diatom fluxes showed different seasonal patterns at all sites. The diatom assemblages in the WPWP differed from those of the EUR. Pennate diatoms (e.g., Nitzschia bicapitata, Thalassionema nitzschioides) dominated in the WPWP, while the relative abundances of centric diatoms (e.g., Rhizosolenia bergonii, Azpeitia spp., Thalassiosira spp.) were higher than those of pennate diatoms in the EUR. The diatom fluxes during the La Niña event reflected seasonal oscillation of the WPWP in spatial extent. At Site MT3 during El Niño, terrestrial materials appeared to have been transported by subsurface currents, which might be a secondary influence on total diatom fluxes. The spatial extent of the WPWP reached Site MT7 in 2002, when total diatom fluxes decreased in the sediment traps located in the eastern region including Site MT7. Therefore, we conclude that the relationships between the ENSO and diatoms fluxes in the western and central equatorial Pacific can be explained by the geographic (west–east) expansion or contraction of the WPWP.

Pacific Subtropical Cell Response to Reduced Equatorial Dissipation

Abstract Equatorial turbulent diffusivities resulting from breaking gravity waves may be more than a factor of 10 less than those in the midlatitudes. A coupled general circulation model with a layered isopycnal coordinate ocean is used to assess Pacific climate sensitivity to a latitudinally varying background diapycnal diffusivity with extremely low values near the equator. The control experiments have a minimum upper-ocean diffusivity of 10−5 m2 s−1 and are initialized from present-day conditions. The average depth of the σθ = 26.4 interface (z26.4) in the Pacific increases by ∼140 m after 500 yr of coupled model integration. This corresponds to a warming trend in the upper ocean. Low equatorial diffusivities reduce the z26.4 bias by ∼30%. Isopycnal surfaces are elevated from the eastern boundary up to midlatitudes by cooling in the upper several hundred meters, partially compensated by freshening. Entrainment of intermediate water masses from below σθ = 26.4 decreases by ∼1.5 Sv (1 Sv ≡ 106 m3 s−1), mainly in the western tropical Pacific. The Pacific heat uptake (30°S–30°N) from the atmosphere reduces by ∼0.1 PW. This is associated with warmer entrainment temperatures in the eastern equatorial Pacific upwelling region. Equatorward heat transport from the Southern Ocean increases by ∼0.07 PW. Reducing the upper-ocean background diffusivity uniformly to 10−6 m2 s−1 cools the upper ocean from the tropics, but warms and freshens from the midlatitudes. Enhanced convergence into the Pacific of water lighter than σθ = 26.4 compensates the reduction in upwelling of intermediate waters in the tropics. Basin-averaged z26.4 bias increases in the low background case. These results demonstrate basin-scale sensitivity to the observed suppression of equatorial background dissipation. This has clear implications for understanding oceanic heat uptake in the Pacific as well as other important aspects of the climate system. Diapycnal diffusivities due to truncation errors and other numerical artifacts in ocean models may need to be less than 10−6 m2 s−1 in order to accurately represent this effect in climate models.

Seasonal, interannual and event scale variation in North Pacific ecosystems

We synthesize information on changes in ecosystems of the North Pacific at seasonal, interannual, and event time scales. Three approaches are used to cope with inadequate temporal, spatial and trophic resolution in generating this synthesis. First, we use highly spatially and temporally resolved data on physical forcing and chlorophyll (SeaWiFS data from 1998 to 2005) to describe basin-wide spatial patterns and seasonal to interannual time scales. The second approach is to compare time series of zooplankton at selected spatial sites at which sampling resolution is sufficient to describe seasonal biomass/abundance patterns, and where multiple years of data exist to examine interannual variability. The third approach is to infer trophic relationships, and broaden the first two approaches to higher trophic levels, by examining the impacts of several event scale phenomena on many trophic levels, but only over a rather limited geographic region. The 8 years of satellite chlorophyll data clearly show that interannually persistent seasonal patterns exist in most regions in the North Pacific, even in the tropical waters. From frequency analysis (Lomb periodograms), the annual cycle was the strongest in most regions, but in the tropics and eastern boundary current regions, periods greater than 1 year were significant. In mid- to high-latitude regions, periods of less than 1 year were also significant in addition to the annual period indicating double peaks with varying intervals. Seasonal progression of the timing of annual peak chlorophyll concentration in the North Pacific showed a different pattern compared with the Atlantic or Indian Ocean, largely due to the presence of the subarctic high nutrient–low chlorophyll (HNLC) and equatorial upwelling regions, which had later phytoplankton blooms than would have been predicted based on a simple equatorial to pole progression of bloom timing. Seasonal cycles in zooplankton were more or less synchronized (concomitant with or slightly lagged) with those of phytoplankton with a few exceptions. Exceptions occur in the Eastern Subarctic Gyre where annual peaks of chlorophyll occur in autumn, after the peak in zooplankton biomass. Interannual variation of annually averaged chlorophyll in 30 regions show three patterns, one positively related to El Niño, one negatively related to El Niño, and one with longer-scale variation, possibly related to climate regimes. Nine regions did not match any of the three patterns. Interannual variation in zooplankton abundance/biomass from selected regions indicates that El Niño may be the major source of interannual variability with its effects modulated by longer-scale variation, such as by the Pacific Decadal Oscillation. Two well-documented environmental events in the Northern California Current, in 2002 and 2005, exemplify how short-term disruption in atmospheric forcing causes changes in ocean hydrography and circulation that has significant impacts on primary production and ripple effects throughout multiple trophic levels of the ecosystem. We conclude that a closer look at the data often yields interesting results that might not necessarily be gained by considering the broad generalizations. Specifically, we observe that short-term disruptions of the ecosystem at the primary producer level may impact higher trophic levels in nonlinear ways that lead to unpredic impacts when one considers the entire food chain.

Temporal fluxes of radiolarians along the W-E transect in the central and western equatorial Pacific, 1999-2002

Coarse size (>63μm) radiolarian fluxes along the Equator in the central and western Pacific between 145°E and 160°W (Stations MT3, MT4, MT5, MT6 and MT7) were examined during January 1999 to January 2003 in order to evaluate how radiolarian assemblages respond to the El Niño Southern Oscillations (ENSO). Radiolarian fluxes were constrained by the water masses: the Western Pacific Warm Pool (WPWP) located in the west and the Equatorial Upwelling Region (EUR) east of the WPWP in the central equatorial Pacific. In general, radiolarian fluxes gradually increased from the western sites to the eastern sites, responding to the increases in nutrient supply and primary production in the euphotic layer. Notable annual cycles of radiolarian fluxes observed at the two Stations with nearly complete time series (MT3 and MT5) were likely corresponding to the rainy seasons of Papua New Guinea (MT3) and the extent of upwelling intensity in the equatorial Pacific (MT3 and MT5). With transition from La Niña to El Niño during 2001 to 2002, high radiolarian fluxes were coincident with cooler surface water. Among the radiolarian taxa, Lithomelissa sp. group and Pseudocubus obeliscus Haeckel 1887, which were mainly dwelling in the surface water in the EUR, responded distinctively to the WPWP excursion and decreased their fluxes as surface water temperature rose.

Western equatorial Pacific planktic foraminiferal fluxes and assemblages during a La Niña year (1999)

A correlation between foraminiferal community dynamics and environmental conditions may provide a basis for establishing paleoclimatic proxies. We studied planktic foraminiferal shell fluxes and assemblages in samples collected in three time-series sediment trap deployments in the western equatorial Pacific under La Niña conditions from January to November 1999. Eleven species contributed about 90% of the total flux in all traps. Two sites (MT1, MT3) in the Western Pacific Warm Pool region (WPWP) were characterized by common occurrences of the species Globigerinoides ruber, Globigerinoides sacculifer, Globigerinoides tenellus, and Neogloboquadrina dutertrei. Site MT5 farther to the east in the equatorial upwelling region had common occurrences of Globigerina bulloides, Globigerinita glutinata, and Pulleniatina obliquiloculata. Very high abundances of G. bulloides and G. glutinata at MT5 indicate that equatorial upwelling (EU) occurred during the 1999 La Niña. The two western sites have similar assemblage compositions, but MT1 (∼ 135°E) has the highest fluxes (up to ∼ 3800 tests m − 2 day − 1 ), whereas MT3 (∼ 145° E) has fluxes below ∼ 2200 tests m − 2 day − 1 . Relatively high fluxes (up to ∼ 3000 tests m − 2 day − 1 ) occur at site MT5 (∼ 176° E), where upwelling occurred. The differences in faunal composition in the WPWP and EU might be attributable to differences in the way in which nutrients are supplied to the phytoplankton: large amounts of suspended material are supplied to the WPWP by advection of waters passing through the coastal region of an archipelago, whereas upwelling of nutrient-rich waters enhances primary production in the EU. At the westernmost site in the WPWP, a peak in the G. bulloides flux coincided with southward flow of the New Guinea Coastal Current (NGCC) in late February, but the highest G. ruber flux coincided with northward flow of this current in late May. Thus, the differences in species dominance at this location may be caused by monsoon-driven variability in the flow direction of the NGGC.

The value of adding optics to ecosystem models: a case study

Abstract. Many ecosystem models have been developed to study the ocean's biogeochemical properties, but most of these models use simple formulations to describe light penetration and spectral quality. Here, an optical model is coupled with a previously published ecosystem model that explicitly represents two phytoplankton (picoplankton and diatoms) and two zooplankton functional groups, as well as multiple nutrients and detritus. Surface ocean color fields and subsurface light fields are calculated by coupling the ecosystem model with an optical model that relates biogeochemical standing stocks with inherent optical properties (absorption, scattering); this provides input to a commercially available radiative transfer model (Ecolight). We apply this bio-optical model to the equatorial Pacific upwelling region, and find the model to be capable of reproducing many measured optical properties and key biogeochemical processes in this region. Our model results suggest that non-algal particles largely contribute to the total scattering or attenuation (>50% at 660 nm) but have a much smaller contribution to particulate absorption (<20% at 440 nm), while picoplankton dominate the total phytoplankton absorption (>95% at 440 nm). These results are consistent with the field observations. In order to achieve such good agreement between data and model results, however, key model parameters, for which no field data are available, have to be constrained. Sensitivity analysis of the model results to optical parameters reveals a significant role played by colored dissolved organic matter through its influence on the quantity and quality of the ambient light. Coupling explicit optics to an ecosystem model provides advantages in generating: (1) a more accurate subsurface light-field, which is important for light sensitive biogeochemical processes such as photosynthesis and photo-oxidation, (2) additional constraints on model parameters that help to reduce uncertainties in ecosystem model simulations, and (3) model output which is comparable to basic remotely-sensed properties. In addition, the coupling of biogeochemical models and optics paves the road for future assimilation of ocean color and in-situ measured optical properties into the models.

Atlantic Equatorial Undercurrent and associated cold tongue variability

The Atlantic Equatorial Undercurrent (EUC) is studied using a simulation for the period 1990–2002 with a high‐resolution ocean general circulation model. Simulated transports of the EUC that supplies the annual mean upwelling in the central and eastern equatorial Atlantic are in good agreement with new transport estimates derived from ship observations, i.e., 19.9 and 14.0 Sv at 35°W and 23°W, respectively. Although the observations are not conclusive concerning the seasonal cycle of EUC transports, the simulated seasonal cycles fit largely in the observed range. The analysis of the EUC variability associated with interannual boreal summer variability of the equatorial cold tongue showed that cold tongue indices, defined either by near‐surface temperature or steric height anomalies, are anticorrelated with thermocline EUC transport anomalies: A strong EUC corresponds to low near‐surface temperatures and steric heights. The importance of equatorial waves for the cold tongue region is shown: Surface layer transport anomalies at 23°W and 10°W are significantly correlated with both near‐surface temperature and steric height anomalies in the equatorial and coastal upwelling regions, indicating an associated eastward phase propagation along the equator toward the African coast where the signal bifurcates into two poleward branches along the coast and is reflected into a westward propagating wave.

Modeling carbon and silicon cycling in the equatorial Pacific

Abstract The equatorial Pacific is a region of significant particulate inorganic carbon (PIC) and biogenic silica sedimentation, the majority of which is carried out by coccolithophorids and diatoms. We developed an ecosystem model that explicitly includes three phytoplankton functional groups (picoplankton, coccolithophorids, and diatoms), two zooplankton functional groups (microzooplankton and mesozooplankton), nutrients (nitrate NO 3 , ammonium NH 4 , and silicate Si(OH) 4 ), detritus (particulate organic matter, biogenic silica, and PIC), total alkalinity, total CO 2 , and partial pressure of CO 2 at the surface water (pCO 2sea ). The model is capable of reproducing many biogeochemical features for the region, such as high-nutrient low-chlorophyll condition, significant exposure of phytoplankton under grazing controls by zooplankton, and large CO 2 release to the atmosphere. The export ratio of PIC to particulate organic carbon (rain ratio) to the deep water was 0.16, higher than the global-mean values, implying predominant PIC sedimentation in the equatorial Pacific upwelling region. Comparison between calcification and no-calcification model results indicates that when coccolithophorids are present, the community interactions actually induce more diatom biomass, export fluxes of detritus, and CO 2 release to the atmosphere. The model results show remarkable calcification in the subsurface layers, which suggests more field data on calcification processes are needed. Increase of source (120 m depth) Si(OH) 4 concentration associated with the tropical instability waves lead to a linear increase in biogenic silica export. Higher Si(OH) 4 concentration stimulates diatom growth, which causes a decrease in picoplankton because feeding pressure by mesozooplankton switched from picoplankton's grazer, microzooplankton, to the abundant diatoms. Surface coccolithophorid biomass has its maximum at intermediate source Si(OH) 4 concentrations as a result of higher grazing pressure on coccolithophorids and higher NO 3 regulation on coccolithophorid growth, with lower and higher source Si(OH) 4 concentrations, respectively. Surface total alkalinity has its minimum and TCO 2 has its maximum at intermediate source Si(OH) 4 concentrations. The two effects on pCO 2sea result in maximum CO 2 release to the atmosphere and PIC export to the deep water, with nearby standard source Si(OH) 4 concentration of 7.5 (mmol Si m −3 ). The enhanced changes in biogenic silica export flux than in surface diatom biomass, confirmed by the model sensitivity study, suggest sedimented detritus under the equatorial Pacific upwelling region acts as an amplifier of changes in surface properties. The model results suggest that physical forcing, such as tropical instability waves, Kelvin waves, and La Nina, which is capable of changing Si(OH) 4 and iron concentrations in the euphotic zone, significantly affect both carbon and silicon fluxes in the region.

A review of modern coral δ18O and Δ14C proxy records

This paper is a review of published modern coral δ 18O and Δ 14C isotopic records that are at least 30 and 20 years long, respectively. The data are presented to show basin-scale trends in both of these proxy records on decadal-to-centennial timescales. The goal was to qualitatively integrate the general inter-annual-to-centennial timescale variability revealed in these data, as well as the statistical and modeling output results that have been produced using these coral records. While many review papers typically include a representative subset of the data available, this review aims to include as much of the available data as possible. In general, coral δ 18O records show a long-term warming and/or freshening throughout the tropical oceans, and agree with the NOAA Extended Reconstruction Sea Surface Temperature 2 (ERSST) on decadal timescales. In the western Pacific, it is most likely a freshening of the seawater δ 18O that dominates the signal. El Niño Southern Oscillation (ENSO) variability dominates most δ 18O records either by varying local seawater temperature or salinity, depending on the regional oceanography/climatology. Outside of the Pacific, ENSO affects seawater temperature and salinity via atmospheric or oceanic teleconnections. Post-bomb coral Δ 14C records collectively show that the uptake of 14C has been greatest in gyre-water fed sites, followed in descending order by western boundary current areas, equatorial upwelling regions, and eastern tropical Pacific upwelling sites. These surface water Δ 14C values indicate the proportion of surface water and/or the residence time of water at the surface at a given location, and can be used to model water mass mixing rates. Such models have only begun to be run and show that the amount of eastern Pacific water entering the central South Pacific increases during El Niños and that the Indonesian throughflow is supplied year-round by the North Pacific. Comparing ocean circulation models with coral Δ 14C-modelled circulation enables researchers to explore the mechanisms that drive seawater Δ 14C variability and fine-tune their models. In addition, our comparison between the rate of coral Δ 14C increase between 1960 and 1970 and total anthropogenic CO 2 uptake rates show general agreement, demonstrating the value of coral records in understanding past carbon fluxes. Overall, coral δ 18O and Δ 14C proxy records represent natural archives of seawater conditions and are critical for studying the natural variability in local and regional patterns within, and teleconnection patterns between, the tropics, extra-tropics, temperate, and Polar Regions on intra-annual-to-centennial timescales.

Testing the silica leakage hypothesis with sedimentary opal records from the eastern equatorial Pacific over the last 150 kyrs

We have measured 230Th‐normalized opal fluxes in several cores from the eastern equatorial Pacific (EEP) to test the validity of the “silica leakage” hypothesis, which purports that redistribution of silicic acid from the Southern Ocean to the low latitudes was responsible for a significant portion of the reduction in atmospheric carbon dioxide (CO2) during the last glacial period. The silica leakage hypothesis predicts higher opal fluxes in the EEP and lower opal fluxes in the Southern Ocean during periods of low atmospheric CO2. These predictions are not borne by the sedimentary record during glacial oxygen isotope stage 2 (OIS 2, 13–27 kyrs B.P.). However, we find a prominent opal flux maximum in the EEP in the middle of OIS 3 (ca. 40–60 kyrs BP) coinciding with low opal fluxes in several cores from the subantarctic zone. This observation is consistent with silica leakage from the Southern Ocean to the equatorial upwelling region during OIS 3, when both low dust flux and extended sea ice could have contributed to limiting diatom productivity in the Southern Ocean. Since this event is not associated with a clear minimum in the Vostok ice record of CO2, its impact on atmospheric CO2 appears to be small.

A generalized model of pelagic biogeochemistry for the global ocean ecosystem. Part II: Numerical simulations

This paper presents a global ocean implementation of a multi-component model of marine pelagic biogeochemistry coupled on-line with an ocean general circulation model forced with climatological surface fields (PELAgic biogeochemistry for Global Ocean Simulations, PELAGOS). The final objective is the inclusion of this model as a component in an Earth System model for climate studies. The pelagic model is based on a functional stoichiometric representation of marine biogeochemical cycles and allows simulating the dynamics of C, N, P, Si, O and Fe taking into account the variation of their elemental ratios in the functional groups. The model also includes a parameterization of variable chlorophyll/carbon ratio in phytoplankton, carrying chl as a prognostic variable. The first part of the paper analyzes the contribution of non-local advective–diffusive terms and local vertical processes to the simulated chl distributions. The comparison of the three experiments shows that the mean chl distribution at higher latitudes is largely determined by mixing processes, while vertical advection controls the distribution in the equatorial upwelling regions. Horizontal advective and diffusive processes are necessary mechanisms for the shape of chl distribution in the sub-tropical Pacific. In the second part, the results have been compared with existing datasets of satellite-derived chlorophyll, surface nutrients, estimates of phytoplankton community composition and primary production data. The agreement is reasonable both in terms of the spatial distribution of annual means and of the seasonal variability in different dynamical oceanographic regions. Results indicate that some of the model biases in chl and surface nutrients distributions can be related to deficiencies in the simulation of physical processes such as advection and mixing. Other discrepancies are attributed to inadequate parameterizations of phytoplankton functional groups. The model has skill in reproducing the overall distribution of large and small phytoplankton but tends to underestimate diatoms in the northern higher latitudes and overestimate nanophytoplankton with respect to picoautotrophs in oligotrophic regions. The performance of the model is discussed in the context of its use in climate studies and an approach for improving the parameterization of functional groups in deterministic models is outlined.

Latitudinal changes in the standing stocks of nano- and picoeukaryotic phytoplankton in the Atlantic Ocean

The latitudinal distributions of picoeukaryote phytoplankton (PEUK), coccolithophores (COCCO), cryptophytes (CRYPTO) and other nanoeukaryote phytoplankton (NEUK) were studied in the Atlantic Ocean between 49°N and 46°S in September–October 2003 and April–June 2004 by flow cytometry. Phytoplankton abundance and carbon (C) biomass varied considerably with latitude and down through the water column. Abundance and C biomass of all eukaryotic groups studied were highest in North and South Atlantic temperate waters and in the Mauritanian Upwelling off the west coast of Africa, where the total C biomass of eukaryotic phytoplankton smaller than 10 μm reached almost 150 mg C m−3. Phytoplankton in the Equatorial Upwelling region was concentrated well below the surface at 50–80 m, with total C biomass in this layer being approximately 4 times that in the mixed layer. The North and South Atlantic Gyres supported much lower eukaryotic phytoplankton C biomass, with total eukaryote C biomass only reaching 2–3 mg C m−3, peaking below 100 m. Of the four eukaryote groups studied, the PEUK were the most abundant, reaching densities of up to 40,000 cells cm−3. They often contributed between 25% and 60% of total C biomass, particularly in the deep chlorophyll maxima of the different oceanic regions and also in the South Atlantic temperate waters, both in austral spring and autumn. NEUK also contributed significantly to C biomass. They generally dominated in the mixed layer, where they contributed 65–85% of total C biomass in the subtropical gyres and in North Atlantic temperate waters. CRYPTO and COCCO were generally less abundant. CRYPTO attained highest abundance in the Southern Temperate waters of over 500 cells cm−3 on both cruises. COCCO were often undetectable but on the European continental shelf abundance reached up to 2600 cells cm−3 during AMT 14. The C biomass standing stock of eukaryotic phytoplankton (<10 μm) for the Atlantic Ocean as a whole was estimated to be 80 million tonnes C during AMT 13, approximately one-third of total phytoplankton C biomass in the Atlantic Ocean.

The Atlantic Meridional Transect (AMT) Programme: A contextual view 1995–2005

The aims of the Atlantic Meridional Transect (AMT) programme [www.amt-uk.org] are to quantify the nature and causes of ecological and biogeochemical variability in the planktonic ecosystems of the Atlantic Ocean, and to assess the effects of this variability on biological carbon cycling and air–sea exchange of radiatively active gases and aerosols. Marine and atmospheric data have been collected twice a year along a 13,500 km transect in the Atlantic Ocean between 50°N and 52°S since 1995. The cruise track enables biogeochemical measurements to be made within the poorly studied North and South Atlantic oligotrophic gyres as well as within equatorial and coastal upwelling regions. The range of ecosystems sampled during AMT has facilitated the calibration and validation of newly developed optical, microbiological, molecular and analytical techniques and provided a testbed for comparative ecology and the development of atmospheric and oceanographic models. This paper describes the rationale and methodology of the programme. Upper-ocean measurements of density, nitrate and chlorophyll a (Chl a) are presented to illustrate seasonal, inter-annual and decadal variability in hydrography. Seasonal distributions of dissolved nitrous oxide (N2O) and methane (CH4) are used to derive estimates of the sea–air flux of these gases in the South Atlantic Gyre. Observations made during AMT and published since 2000 are reviewed, and the key findings are highlighted. The extent to which the programme aims have been achieved is discussed and improvements for the future suggested.

Missing organic carbon in Eocene marine sediments: Is metabolism the biological feedback that maintains end‐member climates?

Ocean chemistry is affected by pCO2 in the atmosphere by increasing the dissolution of solid calcium carbonate and elevating the dissolved inorganic carbon concentrations in seawater. Positive feedbacks between the ocean and atmosphere can maintain high atmospheric pCO2 and affect global climate. We report evidence for changes in the oceanic carbon cycle from the first high‐quality organic carbon (Corg) data set of Eocene sediments beneath the equatorial Pacific upwelling region (Leg 199 of the Ocean Drilling Program). Eocene Corg mass accumulation rates (MARs) are 10 times lower than Holocene rates, even though expected Corg MARs estimated from biogenic‐barium MARs (an indicator of biological production) equal or exceed modern fluxes. What happened to the missing Corg? Recent advances in ecology and biochemical kinetics show that the metabolism of nearly all animals, marine and terrestrial, is positively correlated by first principles to environmental temperatures. The approximately 10°C abyssal temperature difference from Eocene to Holocene should have radically reduced pelagic Corg burial, as we observe. We propose that higher basal metabolism and nutrient utilization/recycling rates in the Eocene water column and surface sediments precluded Corg sediment burial in the pelagic ocean. Increased rates of metabolism, nutrient utilization, and lowered Corg sedimentation caused by increased temperature may have acted as a biological feedback to maintain high atmospheric pCO2 and hothouse climates. Conversely, these same parameters would reverse sign to maintain low pCO2 when temperatures decrease, thereby maintaining “icehouse” conditions during cold climate regimes.

Latitudinal distribution of microbial plankton abundance, production, and respiration in the Equatorial Atlantic in autumn 2000

Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll- a (Chl- a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated 14C primary production ranged from ∼200 mg C m −2 d −1 in the TTS region to ∼1300 mg C m −2 d −1 in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl- a m −2, respectively). Picophytoplankton were the main contributors (>60%) to both Chl- a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m −2 d −1) three-fold larger than net production measured in the TTS region (370 mg C m −2 d −1). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.

Response of tropical African and East Atlantic climates to orbital forcing over the last 1.7 Ma

Abstract Records of organic matter accumulation, organic carbon isotopic composition and iron content covering the last 1.7 Ma are presented for the Congo Fan Ocean Drilling Program (ODP) Site 1075, and are compared with their counterparts from ODP Site 663 in the equatorial upwelling region. They are discussed with regard to variations in African precipitation and Congo River discharge and in the context of changes in trade-wind-driven marine productivity for the tropical Atlantic at periodicities typical of Milankovitch forcing. On the Congo Fan, elevated total organic carbon mass accumulation rates (TOC MAR) and Fe intensities occur predominantly during interglacial periods when the African monsoon was most intense. Band-pass filtering applied to TOC MAR shows distinct precessional variations, indicating that the African climate was largely controlled by low-latitude insolation changes. Only for the last 0.6 Ma, an interval of enhanced glacial-interglacial climate changes, is the precessional TOC MAR signal superimposed by a strong 100 ka oscillation. In contrast, variations in terrestrial iron input to the Congo Fan indicate pronounced 100 ka variance already well before global glacial-interglacial cycles increased in amplitude between 0.9 and 0.6 Ma. Obliquity cycles in the Fe signal are strongly expressed for the last 0.9 Ma. The highest amplitudes in the precessional variance of fluvial Fe input occur when amplitudes in the 100 ka oscillation were at intermediate levels and reveal a 800 ka cycle in phase shift with respect to precessional forcing. Together with a pronounced 800 ka signal in the 100 ka amplitude variations during the last 1.7 Ma, the Congo Fan iron record therefore suggests that eccentricity modulation of the low-latitude insolation directly influenced the equatorial African monsoon system and probably the weathering conditions on land. It further suggests that low-latitude precessional forcing and monsoonal response in the tropics might have played an important role for 100 ka cycles in global climate well before huge continental ice sheets existed.

Relative roles of the equatorial upper ocean zonal current and thermocline in determining the timescale of the tropical climate system

In this study, it is demonstrated that the amplitude of the equatorial upper-ocean zonal current anomaly induced by the fast-varying wind forcing (shorter than a year) is much greater than that induced by the slowly varying wind forcing (longer than 2 year), and the center of maximum zonal current anomaly shifts from the central Pacific to the western Pacific with an increase in the timescale of wind forcing. As a result, the zonal advective feedback (the zonal advection of mean sea surface temperature by anomalous current) in a slowly varying climate system becomes weaker and barely induces a low-frequency mode such as El Nino-Southern Oscillation. On the other hand, both amplitude and zonal location of the maximum thermocline anomaly are little changed by the change in the timescale of wind forcing – confined at the strong equatorial upwelling region of the eastern Pacific. Accordingly, the thermocline feedback (the vertical advection of anomalous subsurface temperature by the mean upwelling) is more favorable to generate a low-frequency mode.