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Southern Oscillation Events (southern + oscillation_event)
Selected AbstractsVariable strength of top-down effects in Nothofagus forests: bird predation and insect herbivory during an ENSO eventAUSTRAL ECOLOGY, Issue 4 2009C. NOEMI MAZIA Abstract Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. However, evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variation in top-down forces created by environmental heterogeneity. We examined the magnitude of predation effects on foliar damage by chewing insects and mean leaf size, by excluding birds from saplings in ,dry' and ,wet'Nothofagus pumilio forests in the northern Patagonian Andes, Argentina. The experiment lasted 2 years encompassing a severe drought during the La Niña phase of a strong El Niño/Southern Oscillation event, which was followed by unusually high background folivory levels. Insect damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased folivory rates in both forests but did not affect tree leaf size. In the ensuing season (2000), leaf damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size. The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss by dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing folivory than at facilitating leaf area growth. Our results indicate that bird-initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above-normal herbivory, and also support the view that large-scale climatic events influence the strength of trophic cascades. [source] From small-scale habitat loopholes to decadal cycles: a habitat-based hypothesis explaining fluctuation in pelagic fish populations off PeruFISH AND FISHERIES, Issue 4 2004Arnaud Bertrand Abstract The Peru-Humboldt Current system (HCS) supports the world's largest pelagic fisheries. Among the world's eastern boundary current systems, it is the most exposed to high climatic stress and is directly affected by El Niño and La Niña events. In this volatile ecosystem, fish have been led to develop adaptive strategies in space and time. In this paper, we attempt to understand the mechanisms underlying such strategies, focusing on the El Niño 1997,98 in Peru from which an extensive set of hydrographic, capture and acoustic survey data are available. An integrated analysis of the data is crucial, as each has substantial shortcomings individually; for example, both catch data and acoustic surveys may easily lead to wrong conclusions. Existing hypotheses on anchovy and sardine alternations lead us to a ,habitat-based' synthetic hypothesis. Using our data, an integrated approach evaluated how fish responded to habitat variation, and determined the consequences in terms of fish-population variability. Various factors occurring at a range of different spatio-temporal scales were considered: interdecadal regime (warm ,El Viejo'/cool ,La Vieja' decadal scale); strength and the duration of the El Niño Southern Oscillation event (interannual scale); population condition before the event (interannual scale); fishing pressure and other predation (annual scale); changes in reproductive behaviour (intra-annual scale); presence of local upwelling (local scale). During El Niño 1997,98, anchovy was able to exploit a small-scale temporal and spatial ,loophole' inside the general unfavourable conditions. Moreover, sardine did not do better than anchovy during this El Niño and was not able to take advantage of the ,loophole' opened by this short-term event. Our results question the traditional view that El Niño is bad for anchovy and good for sardine. [source] Seasonal and interannual variation of bacterial production in lowland rivers of the Orinoco basinFRESHWATER BIOLOGY, Issue 11 2004María M. Castillo Summary 1. We examined the influence of hydrologic seasonality on temporal variation of planktonic bacterial production (BP) in relatively undisturbed lowland rivers of the middle Orinoco basin, Venezuela. We sampled two clearwater and two blackwater rivers over 2 years for dissolved organic carbon (DOC), chlorophyll, phosphorus and bacterial abundance to determine their relationship to temporal variation in BP. 2. Dissolved organic carbon concentration was greater in blackwater (543,664 ,m) than in clearwater rivers (184,240 ,m), and was generally higher during periods of rising and high water compared with low water. Chlorophyll concentration peaked (3 ,g L,1) during the first year of study when discharge was lowest, particularly in blackwater rivers. Soluble reactive phosphorus (SRP) was very low in the study rivers (<3.8 ,g L,1) and concentration increased during low water. 3. Average BP was higher in clearwater (0.20,0.26 ,g C L,1 h,1) than in blackwater rivers (0.14,0.17 ,g C L,1 h,1), although mean bacterial abundance was similar among rivers (0.6,0.8 × 106 cells mL,1). 4. Periods of higher chlorophyll a concentration (low water) or flushing of terrestrial organic material (rising water) were accompanied by higher BP, while low BP was observed during the period of high water. 5. Interannual variation in BP was influenced by variations in discharge related to El Niño Southern Oscillation events. 6. Seasonal variation in BP in the study rivers and other tropical systems was relatively small compared with seasonal variation in temperate rivers and lakes. In addition to the low seasonal variation of temperature in the tropics, low overall human disturbance could result in less variation in the inputs of nutrients and carbon to the study rivers compared with more disturbed temperate systems. [source] Characterizing interannual variations in global fire calendar using data from Earth observing satellitesGLOBAL CHANGE BIOLOGY, Issue 9 2005César Carmona-Moreno Abstract Daily global observations from the Advanced Very High-Resolution Radiometers on the series of meteorological satellites operated by the National Oceanic and Atmospheric Administration between 1982 and 1999 were used to generate a new weekly global burnt surface product at a resolution of 8 km. Comparison with independently available information on fire locations and timing suggest that while the time-series cannot yet be used to make accurate and quantitative estimates of global burnt area it does provide a reliable estimate of changes in location and season of burning on the global scale. This time-series was used to characterize fire activity in both northern and southern hemispheres on the basis of average seasonal cycle and interannual variability. Fire seasonality and fire distribution data sets have been combined to provide gridded maps at 0.5° resolution documenting the probability of fire occurring in any given season for any location. A multiannual variogram constructed from 17 years of observations shows good agreement between the spatial,temporal behavior in fire activity and the ,El Niño' Southern Oscillation events, showing highly likely connections between both phenomena. [source] Patterns of convection in the tropical pacific and their influence on New Zealand weatherINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2002John W. Kidson Abstract Characteristic patterns of convection in the tropical Pacific Ocean have previously been inferred from analysis of outgoing longwave radiation (OLR), and associated with year-to-year variations in El Niño (EN),Southern Oscillation events. This study examines both the effects of these convection patterns on the New Zealand climate, and the more general influence of tropical convection on the New Zealand sector of the Southern Hemisphere. The Southern Hemisphere circulation, as a whole, is found to be most strongly influenced by equatorial convection near the Philippines, and in a broad band over the central Pacific. Where increased convection occurs west of 160°E, La Niña-like (LN) conditions prevail. When the anomalous convective activity is located near the dateline, in ,moderate' EN conditions, SW flow prevails over New Zealand. This gives way to stronger WSW anomalies as the centre of convection is displaced further eastwards and a second centre of reduced convection becomes prominent west of the dateline in strong EN (EN+) events. The changes in wind regimes over the New Zealand region implied by the hemispheric 1000 hPa height fields are supported by mean sea-level pressure differences between a number of New Zealand and adjacent island stations. Indices of the zonal flow show a weak reduction in strength of the westerlies for LN OLR composites, and no apparent effects for EN composites, whereas EN+ conditions strongly favour above-normal westerlies. The meridional flow over New Zealand is skewed towards more frequent southerlies in both the EN and EN+ composites, whereas LN conditions favour northerly flow anomalies. A change is also observed in the frequency of New Zealand-area ,weather regimes'. Enhanced convection centred on 5°S and east of the dateline, as found in the EN+ composites, leads to an increase in zonal regimes and a corresponding decrease in blocking regimes. The direct influence of tropical OLR variations on New Zealand temperature and precipitation has also been assessed. These indicate that the response is not simply one of degree. Different spatial anomaly patterns in the climatic elements result from the varying regional circulation patterns, and these need to be considered if present climate-forecasting schemes are to be improved. Copyright © 2002 Royal Meteorological Society. [source] El Niño,southern oscillation events and associated European winter precipitation anomaliesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2005D. Pozo-Vázquez Abstract The winter precipitation anomalies in the European area have been analysed over the period 1900,98 based on the El Niño,Southern oscillation (ENSO) state. A set of winter and autumn ENSO events is first selected using the Sea-Surface temperature (SST) data of the Niño 3 region, with the constraint that the ENSO event is well developed during the winter and autumn of study, and that it is an extreme event. Cold and warm ENSO events and periods that can be regarded as normal are selected. For the selected winter ENSO events and for the winter following the selected autumn ENSO events, composites of European winter precipitation anomalies have been obtained and compared with each other. A study of the consistency among events of the relationship between ENSO and precipitation anomalies was also carried out. The analysis of the winter precipitation anomalies based on the selected winter ENSO events shows the existence, for the European area and during La Niña events, of a statistically significant precipitation anomaly pattern with positive precipitation anomalies north of the British Isles and in the Scandinavian area and negative anomalies in southern Europe, resembling the precipitation pattern associated with the positive phase of the North Atlantic oscillation (NAO). Particularly, for the southwestern area of the Iberian Peninsula, the negative anomaly reaches 20% of the winter average precipitation. The consistency analysis shows that this precipitation pattern is not the result of a few major events, but rather that it is stable and qualitatively similar to that found during the positive phase of the NAO. A non-linear response to ENSO is found in the eastern Mediterranean area: negative precipitation anomalies are found, having similar amplitude anomalies, both during El Niño and La Niña events. The analysis of the precipitation anomalies for the winter following the selected autumn ENSO events shows very similar results to those found for the previous analysis, thus suggesting the existence of a potential source of seasonal forecasting of European precipitation. An analysis of the sensitivity of the precipitation anomalies to the strength of the ENSO events shows that, when the strength of the ENSO increases, the magnitude of the rainfall anomalies does not change, but the area influenced and the coherence between events do increase slightly. Copyright © 2005 Royal Meteorological Society [source] | |||||||||||||