Equatorial Atlantic Research Articles

Evaluating the segmented post-rift stratigraphic architecture of the Guyanas continental margin

Segmentation of the Guyanas continental margin of South America is inherited from the dual-phase Mesozoic rifting history controlling the first-order post-rift sedimentary architecture. The margin is divided into two segments by a transform marginal plateau (TMP), the Demerara Rise, into the Central and Equatorial Atlantic domains. This paper investigates the heterogeneities in the post-rift sedimentary systems at a mega-regional scale (>1000 km). Re-sampling seven key exploration wells and scientific boreholes provides new data (189 analysed samples) that have been used to build a high-resolution stratigraphic framework using multiple biostratigraphic techniques integrated with organic geochemistry to refine the timing of 10 key stratigraphic surfaces and three megasequences. The results have been used to calibrate the interpretation of a margin-scale two-dimensional seismic reflection dataset, and to build megasequence isochore maps, structural restorations and gross depositional environment maps at key time intervals of the margin evolution. Our findings revise the dating of the basal succession drilled by the A2-1 well, indicating that the oldest post-rift sequence penetrated along the margin is late Tithonian age (previously Callovian). Early Central Atlantic carbonate platform sediments passively infilled subcircular-shaped basement topography controlled by the underlying basement structure of thinned continental crust. Barremian–Aptian rifting in the Equatorial Atlantic, caused folding and thrusting of the Demerara Rise, resulting in major uplift, gravitational margin collapse, transpressional structures and peneplanation of up to 1 km of sediment capped by the regional angular Base Albian Unconformity. Equatorial Atlantic rifting led to margin segmentation and the formation of the TMP, where two major unconformities developed during the intra Late Albian and base Cenomanian. These two unconformities are time synchronous with oceanic crust accretion offshore French Guiana and in the Demerara–Guinea transform, respectively. A marine connection between the Central and Equatorial Atlantic is demonstrated by middle Late Albian times, coinciding with deposition of the organic-rich source rock of the Canje Formation (average total organic carbon 4.21%). The succession is variably truncated by the Middle Campanian Unconformity. Refining the stratigraphic framework within the context of the structural evolution and segmentation of the Guyanas margin impacts the understanding of key petroleum system elements. Supplementary material : Photographs of sandstone petrography thin sections (Fig. S1); calcareous nannofossil plates (Fig. S2); palynology reports for A2-1 and FG2-1 (Fig. S3); taxonomy description of new species; sample table and organic geochemistry results (Table S1); and nannofossil distribution charts (Table S2) are available at https://doi.org/10.6084/m9.figshare.c.5280490

Deep structure of the Pará-Maranhão/Barreirinhas passive margin in the equatorial Atlantic (NE Brazil)

The Pará-Maranhão/Barreirinhas margin, North Brazil, is a pull-apart passive margin, with two strike-slip borders, formed during the opening of the Equatorial Atlantic Ocean during Cretaceous time. Its geometry and evolution are speculative due to the lack of information on the crustal structure and the crustal nature. We present here the E-W profiles of the MAGIC (Margins of brAzil, Ghana and Ivory Coast) deep seismic experiment, a joint project between French and Brazilian universities, research institutes and the industry. Fifty-six Ocean Bottom Seismometers (OBS) and a 4.5 seismic streamer were deployed at sea along 2 of the 5 MAGIC profiles. One profile was extended onshore by installing 8 land stations. We perform forward modelling through combined interpretation of the multichannel seismic and of the main reflected and refracted of these phases recorded by the OBSs. The final P-wave velocity models reveal distinct structural domains from onshore Brazil towards the Atlantic Ocean characterized by variations of the crustal thicknesses and velocities: (1) an unthinned continental crust below the São Luís Craton, where the crust is 33 km thick, (2) a 60 km wide necking domain below the Ilha de Santana Platform; (3) offshore, east of the continental slope, a 10 km-thick deep sedimentary basin underlain by a 5 km thick crust with velocity of 6.2–6.9 km/s that we interpret as an exhumed lower continental crust, on the top of an Anomalous Velocity Layer (AVL) probably made of intrusions of mantle-derived melts into the lower continental crust, or a mixture of them; (4) eastwards, the limit of the previous domain is marked by NW-SE aligned volcanoes and the disappearance of the AVL. The sedimentary succession becomes thinner (6 km) overlaying a proto-oceanic crust characterized by seismic velocities higher than “normal” oceanic crust in its upper part, but in continuity with the velocity described in the previous domain; (5) followed by a more characteristic but thin oceanic crust.The middle/lower continental crust seems not only to have a crucial role in the genesis of the passive margin but also to be involved in the genesis of the first oceanic crust. The passage to a typical oceanic crust seems to have occurred progressively by steps: first in the deeper layer by the setup of more and more intrusions of mantle-derived melts at the base of the crust or mixture of exhumed lower crust and mantle, producing a domain of proto-oceanic crust, then by the emplacement of an upper 1-2 km-thick layer with typical oceanic characteristics.

Impacts of Tropical North Atlantic and Equatorial Atlantic SST Anomalies on ENSO

AbstractThe Sea Surface Temperature Anomaly (SSTA) in tropical Atlantic during boreal spring and summer shows two dominant modes: a basin-warming and a meridional dipole mode, respectively. Observational and coupled model simulations indicate that the former induces a Pacific La Niña in the succeeding winter whereas the latter cannot. The basin-warming forcing induces a La Niña through a Kelvin wave response and the associated wind-evaporation-SST-convection (WESC) feedback over the northern Indian Ocean (NIO) and Maritime Continent (MC). Anomalous Kelvin wave easterly interacts with the monsoonal westerly, leading to a warm SSTA and a northwest-southeast oriented heating anomaly in NIO/MC, which further induces easterly and cold SSTA over the equatorial Pacific. In contrast, the dipole forcing has little impact on the Indian and Pacific Oceans due to the offsetting of the Kelvin wave to the asymmetric Atlantic heating.Further observational and modeling studies towards the Tropical North Atlantic (TNA) and Equatorial Atlantic (EA) SSTA modes indicate that the TNA (EA) forcing induces a CP- (EP-) type ENSO. In both cases, the Kelvin wave response and the WESC feedback over the NIO/MC are important in conveying the Atlantic’s impact. The difference lies in distinctive Rossby wave responses – A marked westerly anomaly appears in the equatorial eastern Pacific (EEP) for the TNA forcing (due to its westward location) while no significant wind response is observed in EEP for the EA forcing. The westerly anomaly prevents a cooling tendency in EEP through anomalous zonal and vertical advection according to a mixed-layer heat budget analysis.

Atlantic Equatorial Undercurrent intensification counteracts warming-induced deoxygenation

The tropical Atlantic upper-ocean circulation experiences multiannual to decadal changes associated with different climate modes and is simultaneously adjusting to climate warming. The most energetic current in the tropical Atlantic is the Equatorial Undercurrent (EUC), which flows eastwards along the Equator. On the basis of long-term moored observations, we show that the EUC strengthened by more than 20% from 2008 to 2018. The intensification of the EUC is associated with increasing subsurface oxygen concentrations and a thickening of the upper-ocean oxygenated layer in the equatorial Atlantic. These changes counteract climate-warming-induced deoxygenation in the region. The EUC strengthening is found to be mainly forced by trade wind changes in the western tropical North Atlantic. A 60-yr dataset reveals that the recent oxygen increase in the upper equatorial Atlantic is associated with multidecadal variability. This variability is characterized by low oxygen concentrations in the 1990s and early 2000s, and high oxygen concentrations in the 1960s and 1970s. The observed oxygen variability seems to be linked to a compression and expansion of the habitat of tropical pelagic fish, and must be accounted for when evaluating the possible consequences of deoxygenation for marine ecosystems and fisheries.

An observational equatorial Atlantic Ocean constraint on Indian monsoon precipitation projections

This study based on the historical simulations and Representative Concentration Pathway 8.5 experiments in the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble finds that models with lesser (larger) summer mean precipitation wet bias over the equatorial eastern Atlantic (EEA) Ocean tend to project larger (lesser) increase in summer monsoon precipitation over central India (CIND) under increased greenhouse gas forcing. This excessive summer precipitation error over the equatorial Atlantic is physically consistent with the present understanding of the cold tongue error over the EEA and its teleconnection with Indian summer monsoon in the CMIP5 models. The present analysis applied an observational constraint of the EEA precipitation on this present–future emergent relationship to reduce the inter-model spread in the projected summer monsoon precipitation over CIND. This proposed emergent constraint resulted in more robust magnitude of projected increases in future summer monsoon precipitation over CIND.

Impacts of aerosols produced by biomass burning on the stratocumulus‐to‐cumulus transition in the equatorial Atlantic

AbstractThe impact of aerosols produced by biomass burning on the stratocumulus‐to‐cumulus transition (SCT) in the equatorial Atlantic is studied using satellite‐based and reanalysis data for the month of June. The month of June is highlighted because it represents monsoon onset as well as the largest sea surface temperature gradient in the summer, which is the peak season of tropical African biomass burning. Boundary layer deepening and increasing temperatures put the location of the SCT within the Gulf of Guinea. Satellite retrievals indicate that the bulk of aerosols occur near 1,500 m in altitude, either above or below the boundary layer depending on latitudinal position. Changes in smoke loading over the Gulf of Guinea due to greater transport from southern Africa leads to increases in low‐level cloud cover above cloud decks and decreases when mixed within the boundary layer. Further south, we find significant changes to temperature, cloud top height, tropospheric stability and moisture availability near maximum aerosol loading. In addition, changes in vertical velocity during dirty conditions further reinforce changes in tropospheric stability. These effects combine to shorten the SCT in space during increased aerosol loading episodes.

The Spatial Distribution of Aeolian Dust and Terrigenous Fluxes in the Tropical Atlantic Ocean Since the Last Glacial Maximum

AbstractThe flux of terrestrial material from the continents to the oceans links the lithosphere, hydrosphere, and biosphere through physical and biogeochemical processes, with important implications for Earth's climate. Quantitative estimates of terrigenous fluxes from sources such as rivers, aeolian dust, and resuspended shelf sediments are required to understand how the processes delivering terrigenous material respond to and are influenced by climate. We compile thorium‐230 normalized232Th flux records in the tropical Atlantic to provide an improved understanding of aeolian fluxes since the Last Glacial Maximum (LGM). By identifying and isolating sites dominated by aeolian terrigenous inputs, we show that there was a persistent meridional gradient in dust fluxes in the eastern equatorial Atlantic at the LGM, arguing against a large southward shift of the intertropical convergence zone during LGM boreal winter. The ratio of LGM to late‐Holocene232Th fluxes highlights a meridional difference in the magnitude of variations in dust deposition, with sites <10°N showing larger changes over time. This supports an interpretation of increased trade wind strength at the LGM, potentially combined with differential changes in soil moisture and reductions in higher altitude summer winds. Our results also highlight the persistent importance of continental margins as sources of high terrigenous flux to the open ocean. This is especially evident in the western tropical Atlantic, where study locations reveal the primary influence of the South American continent up to >700 km away, characterized by232Th fluxes approximately twice as large as aeolian‐dominated sites in the east.

Diabatic heating governs the seasonality of the Atlantic Ni\xf1o

The Atlantic Niño is the leading mode of interannual sea-surface temperature (SST) variability in the equatorial Atlantic and assumed to be largely governed by coupled ocean-atmosphere dynamics described by the Bjerknes-feedback loop. However, the role of the atmospheric diabatic heating, which can be either an indicator of the atmosphere’s response to, or its influence on the SST, is poorly understood. Here, using satellite-era observations from 1982–2015, we show that diabatic heating variability associated with the seasonal migration of the Inter-Tropical Convergence Zone controls the seasonality of the Atlantic Niño. The variability in precipitation, a measure of vertically integrated diabatic heating, leads that in SST, whereas the atmospheric response to SST variability is relatively weak. Our findings imply that the oceanic impact on the atmosphere is smaller than previously thought, questioning the relevance of the classical Bjerknes-feedback loop for the Atlantic Niño and limiting climate predictability over the equatorial Atlantic sector.

Relating model bias and prediction skill in the equatorial Atlantic

We investigate the impact of large climatological biases in the tropical Atlantic on reanalysis and seasonal prediction performance using the Norwegian Climate Prediction Model (NorCPM) in a standard and an anomaly coupled configuration. Anomaly coupling corrects the climatological surface wind and sea surface temperature (SST) fields exchanged between oceanic and atmospheric models, and thereby significantly reduces the climatological model biases of precipitation and SST. NorCPM combines the Norwegian Earth system model with the ensemble Kalman filter and assimilates SST and hydrographic profiles. We perform a reanalysis for the period 1980–2010 and a set of seasonal predictions for the period 1985–2010 with both model configurations. Anomaly coupling improves the accuracy and the reliability of the reanalysis in the tropical Atlantic, because the corrected model enables a dynamical reconstruction that satisfies better the observations and their uncertainty. Anomaly coupling also enhances seasonal prediction skill in the equatorial Atlantic to the level of the best models of the North American multi-model ensemble, while the standard model is among the worst. However, anomaly coupling slightly damps the amplitude of Atlantic Niño and Niña events. The skill enhancements achieved by anomaly coupling are largest for forecast started from August and February. There is strong spring predictability barrier, with little skill in predicting conditions in June. The anomaly coupled system show some skill in predicting the secondary Atlantic Niño-II SST variability that peaks in November–December from August 1st.

Role of the eastern Pacific-Caribbean Sea SST gradient in the Choco low-level jet variations from 1900-2015

In this article, we propose a novel approach for assessing the effects of sea surface temperature (SST) variations in the eastern Pacific and the Caribbean Sea on the Choco low-level jet (CJ) intensity over the 1900-2015 period that involved defining the interbasin gradient index (IGR) between these 2 oceanic basins. We also studied the effects on rainfall in northwestern South America and Central America in the high CJ season during September-November (SON). Wavelet coherence analysis showed high consistency between CJ and IGR on an interannual scale of 2-8 yr. Precipitation increased over central, western, and northern Colombia and most of Central America during strong CJ (SCJ) and decreased during weak CJ (WCJ) events, which occurred, respectively, in the negative IGR (NIGR) and positive IGR (PIGR) phases. NIGR is associated with anomalous cooling in the tropical Pacific and warming in the equatorial Atlantic; opposite patterns are observed during PIGR. Also, the CJ and the Caribbean low-level jet (CLLJ) showed reversed intensities such that as one strengthened, the other weakened and vice versa. Our results indicate that the low-frequency SST anomalies in the North Atlantic affect the IGR and low-level jet intensities associated with changes in large-scale circulation modulated by the Atlantic multidecadal oscillation (AMO). Indeed, positive precipitation anomalies during the SCJ under NIGR were more accentuated and extensive in the warm AMO (WAMO) than in the cold AMO (CAMO) phase. Conversely, negative precipitation anomalies during WCJ under PIGR were more accentuated and extensive in the CAMO than in the WAMO.

Global Ocean Wind‐Wave Model Hindcasts Forced by Different Reanalyzes: A Comparative Assessment

AbstractFour global wind wave hindcasts based on the third‐generation spectral wave model WAVEWATCH III forced by ERA5, ERA‐Interim, National Centers of Environmental Prediction and Climate Forecast System Reanalysis (CFSR) and MERRA2 reanalyzes for 1980–2019 are presented. Absolute magnitudes of both 10‐m wind speeds and significant wave heights are the largest in the CFSR and CFSR‐based hindcast respectively, particularly in the extratropical regions. The differences in annual extremes between CFSR and ERA5 (and corresponding hindcasts) amount to 2.4 ms−1 for wind speeds and 1.2 m for wave heights. CFSR and CFSR‐based hindcast also stand out with respect to remaining datasets, showing mostly negative trends in both mean and extreme characteristics. ERA5‐, ERA‐Interim‐, and MERRA2‐based hindcasts show annual positive trends in wave heights in 1980–2019, in particular in the Southern Ocean (up to 0.15 m decade‐1 for extreme wave heights in MERRA2‐based hindcast) as well as in the central Pacific, equatorial and tropical Atlantic, and Indian Oceans. All four hindcasts agree on the positive linear trends in the North Atlantic midlatitudes and subtropics, western South Atlantic midlatitudes and the tropical South Pacific, and the negative trends in the eastern North Pacific midlatitudes. Areas with significant linear trends in wave heights generally coincide with those for wind speeds, however, they have larger spatial coverage. The interannual variability in significant wave heights is the most consistent across different hindcasts in the eastern midlatitudes of both hemispheres. The performed hindcasts provide a useful background for studies on both present and future wave climates.

A Ternary Mixing Model Approach Using Benthic Foraminifer δ13C‐δ18O Data to Reconstruct Late Pliocene Deep Atlantic Water Mass Mixing

AbstractLate Pliocene deep Atlantic δ13C data have been interpreted as evidence for enhanced Atlantic Meridional Overturning Circulation (AMOC) compared to the present, but this hypothesis is not supported by the Pliocene Model Intercomparison Project (PlioMIP). Here, we adopt a new approach to assess variability in deep ocean circulation based on paired stable carbon (δ13C) and oxygen isotopes (δ18O) of benthic foraminifera, both (semi)conservative water mass tracers. Assuming that deep Atlantic benthic δ13C‐δ18O variability is predominantly driven by mixing, we extrapolate the δ13C‐δ18O data outside the sampled range to identify the end‐members. At least three end‐members are needed to explain the spatial δ13C‐δ18O variability in the deep North Atlantic Ocean: two Northern Component Water (NCW) and one Southern Component Water (SCW) water masses. We use a ternary mixing model to quantify the mixing proportions between SCW and NCW in the deep Atlantic Ocean. Our analysis includes new data from Ocean Drilling Program Sites 959 and 662 in the eastern equatorial Atlantic and suggests that the AMOC cell was deeper during the M2 glacial than during late Pliocene interglacials. Moreover, we identify a new cold and well‐ventilated water mass that was geographically restricted to the southeast Atlantic Ocean between 3.6 and 2.7 Ma and did not contribute significantly to the δ13C‐δ18O variability of the rest of the basin. This high‐δ13C high‐δ18O water mass has led to the misconception of a reduced latitudinal δ13C gradient. Our analyses show that the late Pliocene δ13C gradient between NCW and SCW was similar to the present‐day value of 1.1‰.

Global phylogeography suggests extensive eucosmopolitanism in Mesopelagic Fishes (Maurolicus: Sternoptychidae)

Fishes in the mesopelagic zone (200–1000 m) have recently been highlighted for potential exploitation. Here we assess global phylogeography in Maurolicus, the Pearlsides, an ecologically important group. We obtained new sequences from mitochondrial COI and nuclear ITS-2 from multiple locations worldwide, representing 10 described species plus an unknown central South Pacific taxon. Phylogenetic analyses identified five geographically distinct groupings, three of which comprise multiple described species. Species delimitation analyses suggest these may represent four species. Maurolicus muelleri and M. australis are potentially a single species, although as no shared haplotypes are found between the two disjunct groups, we suggest maintenance of these as two species. Maurolicus australis is a predominantly southern hemisphere species found in the Pacific, Indian and southern South Atlantic Oceans, comprising five previously allopatric species. M. muelleri (previously two species) is distributed in the North Atlantic and Mediterranean Sea. Maurolicus weitzmani (previously two species) inhabits the eastern equatorial Atlantic, Gulf of Mexico and western North and South Atlantic. Maurolicus mucronatus is restricted to the Red Sea. No Maurolicus have previously been reported in the central South Pacific but we have identified a distinct lineage from this region, which forms a sister group to Maurolicus from the Red Sea.

Plankton net mesh size influences the resultant diversity and abundance estimates of copepods in tropical oligotrophic ecosystems

Quantitative assessment of planktonic organisms is a key issue in understanding biodiversity, biomass, and carbon fluxes in marine ecosystems during the ongoing Anthropocene. However, the implications of the choice of plankton sampling equipment in tropical marine ecosystems have not been fully addressed. The goal of this study was to investigate the abundance and diversity of copepods derived from two different mesh sizes, 120 and 300 μm, to determine differences that may change our perception of the actual role of the key planktonic organisms in tropical marine ecosystems, due to the fact that missing information may be a real problem in trophic estimations (e.g. benthic-pelagic coupling processes in coastal areas). Samples were collected along 650 km of coastline in the Equatorial Atlantic. The average abundance of copepods calculated using the 120 μm net was five times higher than that of the 300 μm net. However, species richness was higher when using the 300 μm net compared to that of the 120 μm. Using the 300 μm net, the number of exclusive taxa (not found in the 120 μm mesh sampling) was higher. The Venn diagram showed that 10% of the copepod taxa were recorded exclusively in 120 μm net, whereas only 30% occurred in the 300 μm net. To improve our understanding of the structure and functioning of tropical marine ecosystems, plankton nets with smaller mesh openings should always be used to estimate abundance because of the dominance of small organisms in the nutrient-poor food webs, giving a new perspective on the available energy in water column and benthic processes of suspension feeding organisms. The absence of these smaller nets will produce an inaccurate picture of the plankton communities and their contributions to other trophic levels, including the blue carbon budget estimates worldwide.

Deep structure of the Demerara Plateau: From a volcanic margin to a Transform Marginal Plateau

The Demerara Plateau (offshore Suriname and French Guiana) is located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic Oceans. The study of its crustal structure is fundamental to understanding its tectonic history, its relationship with the adjacent oceanic domains and to enlightening the formation of Transform Marginal Plateaus (TMPs). This study presents two wide-angle seismic velocity models from the MARGATS cruise seismic experiment, and adjacent composite seismic reflection lines. The plateau itself is characterized by a 30 km thick crust, subdivided into three layers, including a high velocity lower crust (HVLC). The velocities and velocity gradients do not fit values of typical continental crust but could fit with volcanic margin or Large Igneous Province (LIP) type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is likely composed of extrusive volcanic rocks of the same magmatic origin, forming thick seaward dipping reflector sequences tilted to the west. This SDR complex was emplaced during hotspot related volcanic rifting preceding the Jurassic opening of the Central North Atlantic and forming the present-day western margin of the plateau. The internal limit of the SDR complex corresponds to the future limit of the eastern margin. The Demerara Plateau would therefore be an inherited Jurassic volcanic margin boarding the Central Atlantic. This margin was reworked during the Cretaceous at the eastern limit of the Jurassic SDR complex, creating the present-day northern transform margin and the eastern divergent margin along the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies such as hotspots and superposed tectonic phases in the history of TMPs, which share a great number of characteristics with Demerara.

Impact of Systematic GCM Errors on Prediction Skill as Estimated by Linear Inverse Modeling

Statistical prediction of tropical sea surface temperatures (SSTs) is performed using linear inverse models (LIMs) that are constructed from both observations and general circulation model (GCM) output of SST. The goals are to establish a baseline for tropical SST predictions, to examine the extent to which the skill of a GCM-derived LIM is indicative of that GCM’s skill in forecast mode, and to examine the linkages between mean state bias and prediction skill. The observation-derived LIM is more skillful than a simple persistence forecasts in most regions. Its skill also compares well with some GCM forecasts except in the equatorial Pacific, where the GCMs are superior. The observation-derived LIM is matched or even outperformed by the GCM-derived LIMs, which may be related to the longer data record available for GCMs. The GCM-derived LIMs provide a fairly good measure for the skill achieved by their parent GCMs in forecast mode. In some cases, the skill of the LIM is actually superior to that of its parent GCM, indicating that the GCM predictions may suffer from initialization problems. A weak-to-moderate relation exists between model mean state error and prediction skill in some regions. An example is the eastern equatorial Atlantic, where an erroneously deep thermocline reduces SST variability, which in turn affects prediction skill. Another example is the equatorial Pacific, where skill appears to be linked to cold SST biases in the western tropical Pacific, which may reduce the strength of air–sea coupling.

Resolution dependence of CO2-induced Tropical Atlantic sector climate changes

A long-standing problem in state-of-the-art climate models is the Tropical Atlantic (TA) warm sea surface temperature (SST) bias, which goes along with major biases in large-scale atmospheric circulation. Here we show that TA-sector climate changes forced by increasing atmospheric carbon dioxide (CO2) levels are sensitive to model resolution. Two versions of a climate model employing greatly varying atmospheric resolution and exhibiting very different warm bias strength are compared. The version with high atmospheric resolution features a small SST bias and simulates an eastward amplified SST warming over the equatorial Atlantic, in line with the observed SST trends since the mid-20th century. On the contrary, the version with coarse atmospheric resolution exhibits a large SST bias and projects relatively uniform SST changes across the equatorial Atlantic. In both model versions, the warming pattern resembles the pattern of interannual SST variability simulated under present-day conditions. Atmospheric changes also vastly differ among the two climate model versions. In the version with small SST bias, a deep atmospheric response is simulated with a major change in the Walker circulation and strongly enhanced rainfall over the equatorial region, whereas the atmospheric response is much weaker and of rather different character in the model with large SST bias. This study suggests that higher atmospheric resolution in climate models may enhance global warming projections over the TA sector.

Late Aptian to early Cenomanian dinoflagellate cysts from Agadir Basin, southwestern Morocco: Biostratigraphy and palaeoenvironment

The Agadir Basin (Moroccan Atlantic margin) is characterized by its fossil richness, including organic-walled microfossils. The palynological analysis of the middle Cretaceous succession from the Tamzargout section and EGA.1 well reveals the presence of well-preserved and diverse assemblage, rich in dinoflagellate cysts. The assemblages contain stratigraphic marker taxa and show significant chronostratigraphic events. Based on these bioevents and on the comparison with different palaeogeographic domains, a determination of age was proposed. The biogeographic characteristics of most cosmopolitan taxa in this study confirm the establishment of new oceanic communication related to the opening of the equatorial Atlantic. The Albian-Cenomanian transition has been recognized only in EGA.1 well, whereas the Aptian-Albian transition, the early-mid Albian and the mid-late Albian have been defined in both the section and the well. The recorded associations are comparable to those of neighboring deposits and those of the Tethyian and Atlantic domains. The vertical distribution of the organic matter in the sediments of the Agadir Basin and the compilation of all palaeoenvironmental proxies allowed identifying an Albian transgression with specific palaeoenvironments in each interval. Neritic marine environments alternated with short phases of continental and oceanic influence is deduced for the late Aptian-early Cenomanian in this basin. The late Aptian-early to mid Albian recorded an outer neritic palaeoenvironment under oceanic influence; the mid to late Albian showed littoral conditions during a regressive phase fluctuating to middle to outer neritic environment; finally the late Albian-early Cenomanian transition was characterized by a middle neritic palaeoenvironment under coastal influence.

Why Do Benguela Niños Lead Atlantic Niños?

AbstractWe investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern‐equatorial Atlantic, the Atlantic Niños, and the occurrence of Benguela Niños along the southwestern Angolan coast. While it is commonly agreed that both events are associated with equatorial and subsequent coastal‐trapped wave propagations driven remotely by a relaxation of the trade‐winds, it is surprising that SST anomalies off Angola tend to precede the ones in the eastern‐equatorial sector by ~1 month. To explain this counterintuitive behavior, our methodology is based on the experimentation with a Tropical Atlantic Ocean model. Using idealized wind‐stress perturbations from a composite analysis, we trigger warm equatorial and coastal events over a stationary and then, seasonally varying ocean mean‐state. In agreement with the linear dynamics, our results show that when the interannual wind‐stress forcing is restricted to the western‐central equatorial Atlantic, the model yields equatorial events leading the coastal ones. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor the seasonal phasing of the events explains the observed temporal sequence. Only if wind‐stress anomalies are also prescribed in the coastal fringe, the coastal warming precedes the eastern‐equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Niños. A weaker South‐Atlantic Anticyclone initiates the coastal warming before the development of eastern‐equatorial SST anomalies. Then, equatorward coastal wind anomalies, driven by a convergent anomalous circulation located on the warm Atlantic Niño, stop the remotely forced coastal warming prematurely.

Regional atmospheric response to the Benguela Niñas

AbstractWe investigate how the atmosphere is affected by the cold sea surface temperature (SST) anomalies of the Benguela Niñas using reanalysis data and a high‐resolution atmospheric model. A composite analysis of reanalysis data based on five Benguela Niña events (5 years out of 39 years for 1979–2017) reveals that the rainfall along the Angolan coast is reduced significantly and other anomalies of precipitation are detected over the African continent and equatorial Atlantic. Those anomalies can be explained by the anomalies of vertically‐integrated moisture flux (VIMF) and its divergence and convergence. Additionally, the Namibian low‐level cloud and sea level pressure (SLP) are enhanced around the ABFZ by the Benguela Niñas. A simulation of the model forced by the cold SST anomalies of the Benguela Niñas reproduces the reduction (enhancement) of rainfall and VIMF divergent (convergent) anomaly over the Angolan coastal region (Gulf of Guinea). However, the other anomalies over the African continent are not significant. It is suggested that the effects of the Benguela Niñas are limited along southwestern coast of Africa. Composite analysis of reanalyses of rainfall anomalies associated with Benguela Niña events shows pattern over the other regions which might be induced by larger‐scale atmospheric anomaly. This large‐scale atmospheric anomaly can be linked with the South Atlantic Anticyclone and/or forced by the teleconnection from the tropical Pacific, for instance. The increment of the Namibian low‐level cloud and SLP is simulated by the numerical experiments consistently. The low‐level cloud becomes more frequent between 950 and 900 hPa and the radiative cooling by longwave radiation is reinforced during the Benguela Niñas events. In contrast, the cloud formation around 850 hPa is reduced and radiative cooling is weakened. It is indicated that this change in the cooling rate possibly induces the strengthening in the inversion layer over the cold SST anomalies.