Event Off Research Articles

Thirty-two-year ocean–atmosphere coupled downscaling of global reanalysis over the Intra-American Seas

This study examines the oceanic and atmospheric variability over the Intra-American Seas (IAS) from a 32-year integration of a 15-km coupled regional climate model consisting of the Regional Spectral Model (RSM) for the atmosphere and the Regional Ocean Modeling System (ROMS) for the ocean. It is forced at the lateral boundaries by National Centers for Environmental Prediction-Department of Energy (NCEP-DOE R-2) atmospheric global reanalysis and Simplified Ocean Data Assimilation global oceanic reanalysis. This coupled downscaling integration is a free run without any heat flux correction and is referred as the Regional Ocean–Atmosphere coupled downscaling of global Reanalysis over the Intra-American Seas (ROARS). The paper examines the fidelity of ROARS with respect to independent observations that are both satellite based and in situ. In order to provide a perspective on the fidelity of the ROARS simulation, we also compare it with the Climate Forecast System Reanalysis (CFSR), a modern global ocean–atmosphere reanalysis product. Our analysis reveals that ROARS exhibits reasonable climatology and interannual variability over the IAS region, with climatological SST errors less than 1 °C except along the coastlines. The anomaly correlation of the monthly SST and precipitation anomalies in ROARS are well over 0.5 over the Gulf of Mexico, Caribbean Sea, Western Atlantic and Eastern Pacific Oceans. A highlight of the ROARS simulation is its resolution of the loop current and the episodic eddy events off of it. This is rather poorly simulated in the CFSR. This is also reflected in the simulated, albeit, higher variance of the sea surface height in ROARS and the lack of any variability in the sea surface height of the CFSR over the IAS. However the anomaly correlations of the monthly heat content anomalies of ROARS are comparatively lower, especially over the Gulf of Mexico and the Caribbean Sea. This is a result of ROARS exhibiting a bias of underestimation (overestimation) of high (low) clouds. ROARS like CFSR is also able to capture the Caribbean Low Level Jet and its seasonal variability reasonably well.

Seasonality of sporadic physical processes driving temperature and nutrient high‐frequency variability in the coastal ocean off southeast Australia

Physical processes forced by alongshore winds and currents are known to strongly influence the biogeochemistry of coastal waters. Combining in situ observations (moored platforms, hydrographic surveys) and satellite data (sea surface wind and sea surface height), we investigate the transient occurrence of wind‐driven upwelling/downwelling and current‐driven upwelling events off southeast Australia. Remote‐sensed indices are developed and calibrated with multiannual time series of in situ temperature and current measurements at two shelf locations. Based on archives up to 10 years long, climatological analyses of these indices reveal various latitudinal regimes with respect to seasonality, magnitude, duration of events, and their driving mechanisms (wind or current). Generally, downwelling‐favorable winds prevail in this region; however, we demonstrate that up to 10 wind‐driven upwelling days per month occur during spring/summer at 28–33.5°S and up to 5 days in summer further south. Current‐driven upwelling upstream of the East Australian Current separation zone (∼32°S) occurs twice as often as downstream. Using independent in situ data sets, we show that the response of the coastal ocean is consistent with our climatology of shelf processes: upwelling leads to a large range of temperatures and elevated nutrient concentrations on the shelf, maximized in the wind‐driven case, while downwelling results in destratified nutrient‐poor waters. The combination of these sporadic wind‐ and current‐driven processes may drive an important part of the high‐frequency variability of coastal temperature and nutrient content. Our results suggest that localized nutrient enrichment events of variable magnitude are favored at specific latitudes and seasons, potentially impacting coastal ecosystems.

Predictability of the Ningaloo Niño/Niña

The seasonal prediction of the coastal oceanic warm event off West Australia, recently named the Ningaloo Niño, is explored by use of a state-of-the-art ocean-atmosphere coupled general circulation model. The Ningaloo Niño/Niña, which generally matures in austral summer, is found to be predictable two seasons ahead. In particular, the unprecedented extreme warm event in February 2011 was successfully predicted 9 months in advance. The successful prediction of the Ningaloo Niño is mainly due to the high prediction skill of La Niña in the Pacific. However, the model deficiency to underestimate its early evolution and peak amplitude needs to be improved. Since the Ningaloo Niño/Niña has potential impacts on regional societies and industries through extreme events, the present success of its prediction may encourage development of its early warning system.

Progressive changes in rifting directions in the Campania margin (Italy): New constrains for the Tyrrhenian Sea opening

Current models for the opening of the Tyrrhenian Sea invoke a unique extension vector throughout the rift to drift process, accommodated by margin parallel faults, but the role of the Campania margin faults in the opening of the Tyrrhenian back-arc basin is still poorly constrained. Gaeta Bay, located on the Campania margin, was investigated through the interpretation of seismic reflection and borehole data together with a stratigraphic correlation of dated outcropping units. The interpretation of a seismic grid using seismic and sequence stratigraphy and structural geology approaches in a dedicated GIS environment led to the generation of 2-D models of relevant geological surfaces, isochron maps, a 3-D digital model of the subsurface and to the reconstruction of the geological evolution of the margin with a resolution of 100ka. Gaeta Bay is formed by three basins (northern, central and southern) and features a complex stratigraphic architecture: in the northern and central basins a syn-rift deposit (unit PP) is buried by the oldest aggradational deposit (unit A) that fills the accommodation space; a post 0.7Ma succession (units B and C) filled the northern basin with a lateral aggradational geometry; a syn-tectonic wedge (unit B) was deposited in the central basin between 0.7 and 0.4Ma; post 0.4Ma thick deposits (unit C) are testimony to the collapse of the southern basin. A correlation between the geology of the bathyal basin and that of the Campania margin was established using a CROP seismic section that extended from the Vavilov basin to Gaeta Bay.Based on original and literature data we propose a kinematic evolution of the Tyrrhenian Sea upper plate over the last 10Ma. This evolution consists of older extensional events (stages 1–2) off Sardinia and in the Vavilov basin (leaving the Campania margin unaffected). Younger events (stages 3–5) developed in the eastern (with a Pliocene–Quaternary change of the extension direction along the Campania Margin) and central Tyrrhenian region.This paper suggests that the Tyrrhenian Sea opening was a polyphase rifting with a migration of activity and a change of style over time (including a period of lithospheric detachment faulting).

Sea Surface Warming and its Implications for Harmful Algal Blooms off Oman

The analysis of the sea surface temperature (SST) data for the period from 1961 to 2010, showed the SST regime changes off Oman on different temporal scales. A rapid warming phase is conspicuous in the SST off Muscat and off Masirah along Oman after 1984. The shifts in the SST occurred several times on shorter time scales prior to 1984 but a notable shift in the SST occurred after 1984 when the mean annual SST increased by 0.53℃ off Masirah and by 0.32℃ off Muscat. The amplitude and period of heating/cooling are smaller and shorter off Muscat compared to Masirah with increasing summer warming and decreasing winter cooling. The multivariate ENSO index (MEI) and the annual mean SST at the study locations showed better correlation (R 2 =0.3) with April-May MEI compared to the other months. The highs and lows of SST at both the locations responded with stronger El Nino and La Nina events respectively. The frequency of harmful algal bloom (HAB) events off Oman closely followed the warming and cooling patterns of the sea surface. The frequency of HAB events increased with increasing SST during present decade (2001~2010). The increase in toxic or potentially harmful HAB events may be the consequence of upper ocean stratification forced by increasing SST.

Propagation of Atlantic Ocean swells in the north Indian Ocean: a case study

Abstract. An analysis of altimeter significant wave height data of May 2007 revealed the occurrence of an extreme weather event off southern tip of South Africa in the Atlantic Ocean, and generation of a series of very high swells at 40° S. These swells propagated towards northeast and broke over La Réunion island in the Indian Ocean on 12 May 2007. The wave model WAVEWATCH III was used to study the propagation of these swells in the Indian Ocean. The model was validated for the Indian Ocean using moored buoy data at 12 locations and merged altimeter wave data. The wave model accurately reproduced the event of May 2007. Swell heights, of the order of 15.0 m, at the generation area reduced to 6.0 m near La Réunion island. This study shows that the swells generated in the Roaring Forties of the Atlantic Ocean (between 15° to 80° E longitude) propagate in the NE/NNE direction towards the north Indian Ocean, and wave characteristics of the Arabian Sea are least influenced compared to that of Bay of Bengal, when swells from the Atlantic Ocean enter the Indian Ocean. The double peak spectrum extracted for the Bay of Bengal indicates that one of the peaks is due to swells generated off southern tip of South Africa.

Upwelling response to atmospheric coastal jets off central Chile: A modeling study of the October 2000 event

The spatial and temporal variability of nearshore winds in eastern boundary current systems affect the oceanic heat balance that drives sea surface temperature changes. In this study, regional atmospheric and oceanic simulations are used to document such processes during an atmospheric coastal jet event off central Chile. The event is well reproduced by the atmospheric model and is associated with the migration of an anomalous anticyclone in the southeastern Pacific region during October 2000. A robust feature of the simulation is a sharp coastal wind dropoff, which is insensitive to model resolution. As expected, the simulated oceanic response is a significant sea surface cooling. A surface heat budget analysis shows that vertical mixing is a major contributor to the cooling tendency both in the jet core area and in the nearshore zone where the magnitude of this term is comparable to the magnitude of vertical advection. Sensitivity experiments show that the oceanic response in the coastal area is sensitive to wind dropoff representation. This is because total upwelling, i.e., the sum of coastal upwelling and Ekman pumping, depends on the scale of wind dropoff. Because the latter is much larger than the upwelling scale, coastal wind dropoff has only a weak positive effect on vertical velocities driven by Ekman pumping but has a strong negative effect on coastal upwelling. Interestingly though, the weakening of coastal winds in the dropoff zone has a larger effect on vertical mixing than on vertical advection, with both effects contributing to a reduction of cooling.

An Improved High-Resolution SST Climatology to Assess Cold Water Events off Florida

Cloud filters developed for high-resolution (1-km) Advanced Very High Resolution Radiometer (AVHRR) satellite-derived sea surface temperature (SST) observations are generally inadequate to capture extreme cold events. Such events impacted shallow waters in Florida Bay and other coastal regions in January 2010 with fatal consequences for large numbers of corals and associated organisms. Raw AVHRR images were reprocessed to understand whether historical knowledge of daily and interannual SST variations could be used to derive a practical cloud-filtering technique. This approach, however, misidentified valid water temperature pixels in nearly 20% of 2703 images collected during the month of January for each year between 1995 and 2010. To create an improved SST climatology, this cloud-filtering method was combined with manually delineated overrides of falsely masked regions. During the January 2010 cold event, this climatology indicated negative SST anomalies of up to 11.6°C in the Big Bend region and 14°C in Florida Bay, with high spatial heterogeneity throughout. Our findings highlight the need for improved autonomous cloud-masking techniques to detect cold events in near real time.

The potential of laser scanning cytometry for early warning of algal blooms in desalination plant feedwater

When algal blooms (such as red tide) occur, early detection of increased algal growth in intake water offers desalination plant operators advanced warning of severe fouling and provides the necessary time for preventative measures. Monitoring basic water quality measurements such as turbidity, silt density index (SDI), dissolved oxygen, and bulk fluorescence can approximate algal activity; however, these measurements lack the resolution needed to accurately track growth throughout all phases of a bloom event. This study investigates the use of a laser scanning cytometer (LSC) that detects algal cells over a wide range of concentrations (50–150,000 cells/ml), and characterizes cell shape, size, and distribution parameters through fluorescence signal imaging. Water samples are filtered to collect algae on a 0.2-μm membrane surface. Sample volume is variable so cells can be concentrated. The membrane surface is scanned with a 635-nm diode laser and the signal is processed to generate a cell count and distribution image. LSC performance was comparable to fluorescence microscopy and flow cytometry over a range of concentrations that may be encountered during a bloom event. The applicability of LSC was demonstrated using laboratory-grown cultures and seawater samples taken during a bloom event off of the coast of Long Beach, CA.

An easterly tip jet off Cape Farewell, Greenland. I: Aircraft observations

AbstractAn easterly tip jet event off Cape Farewell, Greenland, is described and analysed in considerable detail. In Part I of this study (this paper) comprehensive aircraft‐based observations are described, while in Part II of this study numerical simulations and a dynamical analysis are presented. The easterly tip jet of 21 February 2007 took place during the Greenland Flow Distortion experiment. It resulted through the interaction of a barotropic synoptic‐scale low pressure system in the central North Atlantic and the high topography of southern Greenland. In situ observations reveal a jet core at the coast with peak winds of almost 50 m s−1, about 600–800 m above the sea surface, and of 30 m s−1 at 10 m. The depth of the jet increased with wind speed from ∼1500 m to ∼2500 m as the peak winds increased from 30 to 50 m s−1. The jet accelerated and curved anticyclonically as it reached Cape Farewell and the end of the barrier. The easterly tip jet was associated with a tongue of cold and dry air along the coast of southeast Greenland, general cloud cover to the east, and cloud streets to the south of Cape Farewell. Precipitation was observed during the low‐level components of the flight. The very high wind speeds generated a highly turbulent atmospheric boundary layer and resulted in some of the highest surface wind stresses ever observed over the ocean. Copyright © 2009 Royal Meteorological Society

Co-seismic and post-seismic pore-fluid pressure changes in the Philippine Sea plate and Nankai decollement in response to a seismogenic strain event off Kii Peninsula, Japan

AbstractNew pressure data from a pair of Ocean Drilling Program (ODP) hydrologic borehole observatories at ODP Sites 1173 and 808, located off Japan in the subducting Philippine Sea plate and in the nearby Nankai accretionary prism, respectively, show clear signals associated with an earthquake swarm off the Kii Peninsula that began on September 5, 2004, roughly 220 km away from the observatory sites. At Site 1173, formation pressures rose by 1.0–1.5 kPa at the time of the largest earthquake (Mw= 7.5), then continued to rise to a total anomaly of 4 kPa during the following 200–300 days. These transients are inferred to reflect co-seismic and slow continuing volumetric contraction of the plate by amounts of roughly 0.2 × 10−6and 0.5 × 10−6, respectively. The sign of the estimated strain is consistent with that predicted with a seismic-moment-constrained elastic half-space dislocation model, but the amplitude is much larger, by roughly a factor of 6 at the time of the main earthquake, and by nearly a factor of 20 when the total pressure-estimated strain at the end of the post-seismic period is compared to that estimated from the total cumulative seismic moment including aftershocks. The simplest inference that can be drawn is that a large component of aseismic slip occurred in the epicentral area. At Site 808, pressure at the deepest monitoring zone just above the subduction decollement fell at the time of the largest earthquakes. This may reflect shear-induced dilatation which would be consistent with strain-or velocity-hardening behaviour along this seaward-most part of the subduction thrust interface.

Toward an integrated coastal sea-ice observatory: System components and a case study at Barrow, Alaska

The morphology, stability and duration of seasonal landfast sea ice in Alaska's coastal zone is changing alongside large-scale ice thinning and retreat. The extent and complexity of change at the local level requires an integrated observing approach to assess implications of such change for coastal ecosystems and communities that rely on or make use of the sea-ice cover. Barrow, Alaska is an example of a community that experiences and utilizes a broad range of sea-ice types and conditions. The local population is increasingly forced to adapt to less stable sea ice, loss of multiyear ice and a shorter ice season. We are working toward an integrated coastal ice observatory to monitor landfast and adjacent pack ice and to maximize the usefulness of information to the community. The observatory includes: (1) satellite remote-sensing datasets distributed in near real-time; (2) a coastal sea-ice radar and webcam that monitor ice movement and evolution; (3) a mass-balance site that provides temperature profiles and thickness information for ice and snow; (4) sea-level measurements; (5) periodic ice thickness surveys using direct drilling and electromagnetic induction sounding; and (6) a program of regular, undirected observations by Iñupiat sea-ice experts. We examine two significant landfast ice breakout events off Barrow in spring of 2007. During these events, Barrow's subsistence whaling community partook in a successful hunting season observing and responding to these breakout events and their impacts on ice stability. Using local expert knowledge to parse geophysical datasets obtained from the observatory has provided deeper insight into different approaches for assessing ice stability, and integrating information on ice growth, origin, morphology, and dynamics, as well as winds, weather, and currents.

Surface temperature, chlorophyll and advection patterns during a summer upwelling event off central Portugal

Satellite-derived sea surface temperature (SST) and chlorophyll ([Chl]) concentration maps are used together with numerical models to study the advection patterns observed during a summer upwelling event off central Portugal, a region characterized by a complex coastline and bathymetry. It is shown that the model solutions realistically reproduce the main patterns of spatial and temporal SST variability, namely the 2 – 2.5 ∘ C decrease in the active upwelling areas during the wind intensification phase, the 1-day lag between the peak of the northerlies and minimum SST, and the small SST variability in the sheltered embayments. The expected circulation features are reproduced in the model solutions: the strong along-slope flows, the development of filaments and separated jets, shadow areas downstream of main capes, frontal-scale instabilities and the rapid onset of coastal counter currents along the inshore zone during relaxation. The evolution of the oceanographic conditions, specifically the mixed layer depth and horizontal velocity, and the advection patterns obtained from a Lagrangian model are compared with the observed SST and [Chl] variability. The results imply that the potential phytoplankton accumulation/growth (PPAG) areas are characterized by surface temperatures lower than 16.5 ∘ C , mixed layer depth and horizontal velocity less than 30 m and 0.3 m/s, respectively. It is shown that remotely-sensed [Chl] concentration patterns, namely its asymmetric distribution relative to SST during the intense wind phase, is primarily related to the mixed layer depth and secondly to the horizontal velocity. The trajectories obtained with the Lagrangian model confirmed earlier suggestions of the presence of recirculation cells, downstream of the capes where the coastal jet separates from the coast, and revealed that significant vertical displacements occur in these cells.

Bio-optical characteristics of a phytoplankton bloom event off Baja California Peninsula (30–31°N)

A phytoplankton bloom was detected in the Southern California Current System, off the Baja California Peninsula (Mexico) on June 2003 with chlorophyll- a concentration (TChl a) of 10.13 mg m −3. Two stations (D1 and D2) were sampled on June 24, and D2 was resampled 6 days later; chlorophyll- a concentration had decreased by about one half. LAC MODIS-Chl a images were obtained and showed the spread of the bloom on the day after sampling. The phytoplankton community consisted primarily of dinoflagellate temporary cysts, mainly at the surface and at 5 m in station D1. Two Pseudo-nitzschia species ( P. australis, P. seriata) were also very abundant. Samples from the bloom had a specific phytoplankton absorption coefficient ( a ph * ( λ ) ) lower than the rest of the samples. Values varied from 0.0186 to 0.0455 m 2 mg −1 for a ph * (440) and from 0.0092 to 0.0294 m 2 mg −1 for a ph * (675), with ratios a ph * (440): a ph * (675) ranging from 0.99 to 2.20. These low ratios were associated with the combined effect of packaging, and with the relatively high ratios of fucoxanthin, peridinin, diadinoxanthin and chlorophyll- c2 to TChl a. Samples from the surface and 5 m depth at station D1 had higher ratios of Perid:TChl a (0.12–0.32) than the rest of the samples, suggesting that cysts have similar Perid:TChl a as free-living dinoflagellates. An unusual absorption spectrum with a broad maximum around 480–500 nm was associated with the high proportion of cysts and diatoms. The slope of the spectra between 443 and 488 nm was a good index to differentiate bloom samples containing high proportions of dinoflagellate temporary cysts. Further investigation of the absorption properties of dinoflagellate cysts is needed in order to detect these waters by remote sensing. Although much work is still necessary to understand and explain the bio-optical properties of a bloom, the present study is the first assessment off the Baja California coast to simultaneously consider aspects such as absorption properties, pigment composition and to include a spatial evaluation of the extension of a bloom with satellite images.

Phytoplankton community in the upwelling event off central colombian Caribbean sea

To evaluate the structure of the phytoplankton commu-nity under upwelling and non-upwelling conditions in the Colombian central Caribbean coast and to establish its conditions, compared with similar oceanographic events on a regional scale, four samples, taken between July and September 2006, two in upwelling and two in non-upwelling condition, were carried out along the water column (0-50m) following the Eurelian method. Water samples with the Van Dorn bottle and horizontal tows were taken to determine phytoplankton composition, density, dominance and diversity. It was found that dia-toms were the dominant group, specifically species of the genera Skeletonema, Chaetoceros, Bacteriastrum and Thalassionema, showing densities higher than 50cel/L, o evaluate the structure of the phytoplankton commu-nity under upwelling and non-upwelling conditions in the Colombian central Caribbean coast and to establish its conditions, compared with similar oceanographic events on a regional scale, four samples, taken between July and September 2006, two in upwelling and two in non-upwelling condition, were carried out along the water column (0-50m) following the Eurelian method. Water samples with the Van Dorn bottle and horizontal tows were taken to determine phytoplankton composition, density, dominance and diversity. It was found that dia-toms were the dominant group, specifically species of the genera Skeletonema, Chaetoceros, Bacteriastrum and Thalassionema, showing densities higher than 50cel/L.

The Low-Level Jet off the West Coast of Subtropical South America: Structure and Variability

Abstract The subtropical anticyclone over the southeast Pacific drives low-level southerly flow along the west coast of South America. In turn, the alongshore flow induces coastal upwelling that supports a wealth of fishery resources. Within this region, satellite data, marine reports, and coastal observations indicate the existence of a southerly coastal jet (i.e., a maximum of wind speed) off central Chile (26°–36°S). The mean features and variability of this southerly jet is documented in this work using 4 yr of satellite-derived sea surface winds, complemented by satellite-derived cloud amount fields and atmospheric reanalysis. Furthermore, analysis of in situ data and model results of a well-defined jet event during October 2000 allows a preliminary description of the jet’s three-dimensional structure and a comparison with the northerly jet off the coast of California. Southerly jet events off central Chile occur year-round, but they are more frequent during spring–summer (over 60% of the time). The jet is characterized by an elongated maximum of surface wind speed (∼10 m s−1) with its axis at about 150 km off the coast and a cross-shore scale of about 500 km. The two Quick Scatterometer (QuikSCAT) fields per day (a.m. and p.m. passes) allow a rough estimate of the amplitude of the diurnal cycle of the surface winds, which appears to be remarkably small in the region of the jet. The jet events are associated with the passage of a midlatitude ridge over the southeast Pacific strengthening the subtropical anticyclone. Upstream and over the jet region the coastal deck of stratocumulus clouds tends to dissipate in contrast to an increase in cloudiness downstream of the jet. In the case study the jet core resides at the top of the marine boundary layer (MBL)/inversion layer. Weak offshore flow prevails above the jet axis, and even weaker onshore flow prevails in the MBL. Consistent with its subtropical location the jet is embedded in a region of large-scale subsidence; nevertheless a mesoscale area of mean upward motion is simulated just downstream of the jet core.

A study of the response of chlorophyll- a biomass to a winter upwelling event off Western Iberia using SeaWiFS and in situ data

Observations of a winter upwelling event off Western Iberia shelf/slope in the area of influence of the Western Iberia Buoyant Plume (WIBP) were conducted in February 2000. Spatial patterns and time evolution of the chlorophyll- a (chl- a) biomass are analysed, based on in situ and satellite data. SeaWiFS-derived chl- a concentration L2 products were used to track the chlorophyll front and estimate its westward migration velocity (maximum up to 29 km day −1), as well as to characterize the frontal system and its evolution. A method associating the type of spectral signature of a pixel to the fraction of chlorophyll probed by SeaWiFS enabled the estimation of the chl- a biomass within error intervals. High chlorophyll concentrations (for wintertime) were observed over the shelf and slope, up to large distances to the coast. Due to the WIBP, a shallow Ekman layer developed, being nearly coincident with the stratified upper meters. The transport comprised westward advection and stretching of the plume, with little entrainment with the offshore deep mixed layer waters. The relative enlargement of the total area of the Inside-Front Zone (IFZ) during the upwelling event was roughly accompanied by the maintenance of the average biomass per unit of area, considering the water column up to depths of interest. This suggests that there was a net increase of chl- a biomass inside the water column associated with the IFZ, roughly proportional to the increase in the IFZ area. Retention of phytoplankton in the shallow stratified nutrient-rich waters of the WIBP was a key factor for this increase in chl- a biomass.

The magnetism of a glacial aeolianite sequence from Lanzarote (Canary Islands): coupling between luvic calcisol formation and Saharan dust trapping processes during wet deposition events off northwestern Sahara

SUMMARY In order to better document the climatic origin of pedogenized loess deposits west of Sahara, rock-magnetic measurements were performed on a Last Glacial coastal sand sequence from La Mala (LM) quarry (Lanzarote, Canary Islands) containing six interbedded loess–palaeosol units. Hysteresis and susceptibility data point to a coarse-grained magnetic enhancement in the coastal sand, which contrasts with the superparamagnetic (SP) to pseudo-single domain (PSD) behaviour of the Saharan loess and palaeosol. High- and low-temperature experiments show that oxidized titanomagnetite contributions dominate in the sand, while fine-grained (SP to PSD) iron oxidation products such as maghemite and goethite are evidenced in the Saharan loess/palaeosol units. At room temperature, the detrital PSD–multidomain titanomagnetite contribution of local origin is tentatively estimated from the AC-field dependence of magnetic susceptibility. Surface oxidation of detrital spinel grains and authigenesis of fine-grained iron oxides (including SP goethite) are proposed to explain the magnetic properties of the primarily fine, Saharan-dust-bearing material. The dry, local climate of the present-day and Late Holocene means that loess deposits are not preserved. The six pedogenized loess units, however, point to dust trapping under semi-arid, wetter conditions, probably illustrating periods of reduced latitudinal temperature gradients and climate variability of the North Atlantic climate, respectively. These findings suggest that both the Canary palaeosol and its content of (ultra)fine iron oxide might be constrained by (wet) deposition and trapping of fine Saharan dust.

The coherent variability of African river flows : composite climate structure and the Atlantic circulation

The composite structure of the ocean and atmosphere around Africa is studied in the context of river flow variability. Annual streamflows are analysed for the Blue and White Nile, Congo, Niger, Senegal, Zambezi, and Orange Rivers, and inflow to Lake Malawi. Spectral energy is concentrated in 6.6- and 2.4-year bands. The interannual variability of many river flows is significantly cross-correlated (p < .05) over the period 1950-1995, following removal of the mean trend. A combined river flow index indicates that flows were highest in 1961, 1962, 1968, 1977, 1978 and lowest in 1971, 1972, 1982, 1983, 1991. Forming a composite of differences between high- and low-flow years, SST anomalies and other atmospheric fields are investigated to better understand climatic factors driving hydrological extremes. During high flows, NCEP reanalysis data reveal a composite La Nina event off Peru-Ecuador. SST patterns reveal an inter-hemispheric dipole in the Atlantic (eg. warm - north) and below normal SST in the west Indian Ocean during years with high flow. The equatorial east Atlantic undergoes warming through the ‘composite year’ in a manner consistent with its opposing response to the Indo-Pacific La Nina. Tropical upper winds are easterly and symmetrical about the equator, and may explain why inter-annual variability of river flows south of the equator are correlated with those of the north at 6-month lag. Low level westerly winds are greater during high flow years, particularly along the Guinea coast. Differences of OLR and upper velocity potential demonstrate two distinct centers of action either side of the tropical Atlantic. It is concluded that hydrological events over Africa and South America are sensitive to tropical Atlantic coupling with the global El Nino – Southern Oscillation signal.

ENSO effects on primary production off Baja California

Long-term time series of total primary production (PT) from 1969 through 2002, associated with ENSO (El Niño/Southern Oscillation) events are presented for the region off northern Baja California. PT was calculated using an empiric model in six inshore and offshore grids of the CalCOFI-IMECOCAL network in the southern California Current. In general, PT anomalies were positive from 1970 to 1975, with a period average of 0.024 GtC yr–1 for the study lines. PT changed to negative anomalies after ENSO 1976–77, with a decrease in productivity for at least 20 years, reinforced by the ENSO events of 1982–84, 1987–88, 1992–93 and 1997–98. Annual time series of PT anomalies show that during the ENSO events, values diminished by ~20% in the study area. Long-term PT changes (1976–97) originated a reduction of 0.007 GtC yr–1 (~70%), in relation to the higher production of the early 1970s (1970–75). Mean PT during 1997–98 was 0.014 GtC yr–1, increasing during 1999–2002 (0.023 GtC yr–1) toward similar values calculated for the early 1970s. Superposed Epoch Analysis proved the statistical association between the ENSO events and PT, with the probability of high production one year before (–1), diminishing during the key year (0), and recovering one year after (+1) the event off Baja California. Complete (1950–2002) PT time series showed greatest variance in the 1.43-year period. Small pelagic fish biomass estimated for inshore stations of lines 90 and 107 dropped below 200 × 103 tons after the 1976–77 period, with a recovering trend after 2000.