Latitudinal Transition Research Articles

Making sense of chromosome polymorphisms in two leptysmine grasshoppers.

The touchstone of the 'New Synthesis' was population cytogenetics -rather than genetics - due to the abundant polymorphic inversions in the genus Drosophila. Grasshoppers were not a material of choice because of their conservative karyotypes. However, nowadays seven species of Acrididae were described for polymorphic centric fusions, five of them in South-America. Leptysma argentina and the likely biocontrol of water-hyacinth Cornops aquaticum are semiaquatic Leptysminae (Acrididae: Orthoptera), polymorphic for centric fusions, supernumerary segments and a B-chromosome. We sought to demonstrate the operation of natural selection on them, by detecting: (I) latitudinal clines; (II) regression on environmental variables; (III) deviation from null models, such as linkage equilibrium; (IV) seasonal variation; (V) comparison between age classes and (VI) selection component analyses. All of them were confirmed in L. argentina, just (I) and (II) in C. aquaticum. Furthermore, the relationship between karyotype, phenotype and recombination was confirmed in both species. Karyotype-phenotype relationship may be due to the body enlargement the fusions are associated with, along with a latitudinal transition in voltinism. Karyotype-related recombination reduction in both species may help explain all fusion clines, although there is probably more than one factor at work. No effects were noticed for a supernumerary segment in L. argentina, but it is ubiquitous and certainly non-neutral. C. aquaticum is poised for introduction in South-Africa as a biocontrol of water-hyacinths; the recent discovery of four more segment polymorphisms may imply more chromosomal markers to make sense of its genetic system.

Opposing Patterns of Altitude-Driven Pollinator Turnover in the Tropical and Temperate Americas.

AbstractAbiotic factors (e.g., temperature, precipitation) vary markedly along elevational gradients and differentially affect major groups of pollinators. Ectothermic bees, for example, are impeded in visiting flowers by cold and rainy conditions common at high elevations, while endothermic hummingbirds may continue foraging under such conditions. Despite the possibly far-reaching effects of the abiotic environment on plant-pollinator interactions, we know little about how these factors play out at broad ecogeographic scales. We address this knowledge gap by investigating how pollination systems vary across elevations in 26 plant clades from the Americas. Specifically, we explore Cruden's 1972 hypothesis that the harsh montane environment drives a turnover from insect to vertebrate pollination at higher elevations. We compared the elevational distribution and bioclimatic attributes for a total of 2,232 flowering plants and found that Cruden's hypothesis holds only in the tropics. Above 30°N and below 30°S, plants pollinated by vertebrates (mostly hummingbirds) tend to occur at lower elevations than those pollinated by insects. We hypothesize that this latitudinal transition is due to the distribution of moist, forested habitats favored by vertebrate pollinators, which are common at high elevations in the tropics but not in the temperate Americas.

Latitudinal transition of epipelagic mesozooplankton in the northwestern Pacific in winter

The northwestern Pacific (NWP) is a hotspot of marine biodiversity study, and zooplankton is a crucial secondary producer in the marine ecosystem. It is of utmost importance to do extensive study on the distribution of zooplankton community in the NWP. The distribution of epipelagic zooplankton community in the 143–146°E section between the equator and 36°N in winter was examined in this study. The findings indicated that the Kuroshio extension, the North Pacific Subtropical Gyre, the North Equatorial Current and the North Equatorial Countercurrent were the four main ocean currents in the NWP that regulated the latitudinal transition of epipelagic mesozooplankton and split the sample section into four station groups. The key factors influencing zooplankton's geographic distribution were temperature, primary productivity, and current movement. In general, as latitude increased, zooplankton abundance and biomass first decreased and subsequently flourished in the NWP. Diversity indexes and indicator species also revealed the difference across the communities in four station groups. The body length of zooplankton declined gradually from north to south under the combined influence of temperature and primary production, with the minimum in the oligotrophic subtropical zone and a sub-peak in the high primary production area near the equator. Additionally, the abundance of zooplankton was higher in the winter than in the summer due to seasonal fluctuations in the properties of the ocean currents. This study elucidated the control effects of ocean currents on the latitudinal distribution of zooplankton, supplemented records of the geographic distribution and body length characteristics of zooplankton communities in the NWP, and provided the basis for further research on the ecological role of zooplankton communities and the global changes of marine ecosystems.

The Effect of an Equatorial Continent on the Tropical Rain Belt. Part II: Summer Monsoons

Abstract The TRACMIP (Tropical Rain Belts with an Annual Cycle and Continent Model Intercomparison Project) ensemble includes slab-ocean aquaplanet control simulations and experiments with a highly idealized narrow tropical continent (0°–45°W, 30°S–30°N). We compare the two setups to contrast the characteristics of oceanic and continental rainbands and investigate monsoon development in GCMs with CMIP5-class dynamics and physics. Over land, the rainy season occurs close to the time of maximum insolation. Other than in its timing, the continental rainband remains in an ITCZ-like regime akin to deep-tropical monsoons, with a smooth latitudinal transition, a poleward reach only slightly farther than that of the oceanic ITCZ (about 10°), and a constant width throughout the year. This confinement of the monsoon to the deep tropics is the result of a tight coupling between regional rainfall and circulation anomalies: ventilation of the lower troposphere by the anomalous meridional circulation is the main limiting mechanism, while ventilation by the mean westerly jet aloft is secondary. Comparison of two subsets of TRACMIP simulations indicates that a low heat capacity determines, to a first degree, both the timing and the strength of the regional solsticial circulation; this lends support to the choice of idealizing land as a thin slab ocean in much theoretical literature on monsoon dynamics. Yet, the timing and strength of the monsoon are modulated by the treatment of evaporation over land, especially when moisture and radiation can interact. This points to the need for a fuller exploration of land characteristics in the hierarchical modeling of the tropical rainbands.

Microevolutionary dynamics show tropical valleys are deeper for montane birds of the Atlantic Forest

Tropical mountains hold more biodiversity than their temperate counterparts, and this disparity is often associated with the latitudinal climatic gradient. However, distinguishing the impact of latitude versus the background effects of species history and traits is challenging due to the evolutionary distance between tropical and temperate assemblages. Here, we test whether microevolutionary processes are linked to environmental variation across a sharp latitudinal transition in 21 montane birds of the southern Atlantic Forest in Brazil. We find that effective dispersal within populations in the tropical mountains is lower and genomic differentiation is better predicted by the current environmental complexity of the region than within the subtropical populations. The concordant response of multiple co-occurring populations is consistent with spatial climatic variability as a major process driving population differentiation. Our results provide evidence for how a narrow latitudinal gradient can shape microevolutionary processes and contribute to broader scale biodiversity patterns.

Latitudinal transition of mesopelagic larval fish assemblages in the eastern central Atlantic

This study presents a broad-scale view of larval fish species distributions in the eastern Central and North Atlantic and places it in a hydrographic context. Pelagic fish larvae, including metamorphic stages, were sampled to 1000 m depth across a 46° latitudinal transect from the equator to the Bay of Biscay. By analysing species composition and relating it to hydrographic parameters, we were able to identify 5 assemblages: Equatorial, Tropical, Subtropical, Temperate-bloom and Temperate-nobloom. We also categorized 4 groups of species by association with specific hydrographic parameters. Tropical species, including Vinciguerria nimbaria, Ceratoscopelus warmingii, and Hygophum macrochir, are associated with high sea surface temperature. Tropical-subtropical species, such as Diogenichthys atlanticus, Benthosema suborbitale and Electrona risso, are associated with high temperature in the upper 200 m, i.e. the epipelagic zone. The Temperate species group, dominated by Benthosema glaciale and Maurolicus muelleri, is associated with high chlorophyll-a in the epipelagic zone, and these species were abundant where the spring bloom occurred. Species with the broadest latitudinal distributions, such as Cyclothone braueri and Argyropelecus hemigymnus, here termed Cosmopolitan species, are associated with high salinity in the epipelagic zone. The Cape Verde Frontal Zone seems to act as a one-way distributional barrier, confining many Tropical larval species to the South Atlantic Central Water mass, while allowing more northerly-distributed species to cross southward. This may be related to a dependence of Tropical species on high near-surface water temperatures, which decrease sharply at the Cape Verde Frontal Zone. An oxygen minimum zone had no significant effect on larval abundance or species richness, although there were some taxonomic differences. For example, phosichthyids, including Vinciguerria nimbaria, were not found within the oxygen minimum zone, while the family Melamphaidae thrived. Our results suggest that the dominant hydrographic factors that affect the distribution of fish larvae in the eastern Central and North Atlantic vary by latitude and by species. The Cape Verde Frontal Zone and oxygen minimum zone play complex roles, affecting different taxonomic groups in various ways.

Detection of the Oyashio and Kuroshio fronts under the projected climate change in the 21st century

We evaluate the Oyashio and Kuroshio fronts latitudinal transition under the projected climate change scenarios using eddy resolving regional ocean climate projection products from 1981 to 2100. The regional ocean climate projections are produced based on dynamical downscaling of four CMIP5 models and two RCP experiments RCP2.6 and RCP8.5. Two approaches of the fronts detection methods are compared. One is conventional approach based on subsurface specific isotherm and another is newly proposed approach based on the Oyashio and Kuroshio water temperature and salinity (TS) profiles that may change as the global warming progresses. It is found that the Oyashio TS profile rapidly shift to be high both in temperature and salinity in RCP8.5 cases while the Kuroshio TS profile shows small change toward the end of the twenty-first century in all cases. Northward shift of the Oyashio and Kuroshio fronts reaches 2° northward by the late twenty-first century in extreme case of RCP8.5 cases from the temperature-based definition. On the other hand, the northward shifts of fronts are less than 1° when we use the TS-based definition. The comparison of both fronts positions with the surface velocity distributions suggest that the TS-based definition captures transitions of the fronts better than the temperature-based definition.

Space-Time Variability of the Rainfall over Sahel: Observation of a Latitudinal Sharp Transition of the Statistical Properties

The rain statistics of 0–45° N area including equatorial, Sahelian, and mid-latitude regions, are studied using the probability distributions of the duration of rainy and dry events. Long time daily data set from ground measurements and satellite observations of rain fields are used. This technique highlights a sharp latitudinal transition of the statistics between equatorial and all other regions (Sahel, mid-latitude). The probability distribution of the 8° S to 8° N latitude band shows a large-scale organization with a slow decreasing (power law decrease) distributions for the time and space size of rain events. This observation is in agreement with a scaling, or macro turbulent, behavior of the equatorial regions rain fields. For the Sahelian and mid-latitude regions, our observations are clearly not in agreement with this behavior. They show that the largest rain systems have a limited time and space size (well described with a decreasing exponential distribution). For these non-equatorial regions it is possible to define a local characteristic duration and a characteristic horizontal size of the large rain events. These characteristics time and space scales of observed mesoscale convective systems could be a sensible indicator for the detection of the possible trend of rain distribution properties due to anthropogenic influence.

The geographic pattern of nest height in a neotropical arboreal termite

Nasutitermes corniger tends to construct its distinctive, conspicuous nests at the bases of trees in the northern West Indies versus well above ground on trunks or branches in Trinidad & Tobago and further south. By means of data from 32 localities from the Equator to 17° north latitude, we describe the geographic pattern of nest-height variation. The localities are from both continental areas (mainland and continental islands) and oceanic islands. The latter are all to the north of the former. If nest-height variation is mainly or solely due to abiotic (climatic) factors, a more or less monotonic south–north decline is predicted. If it is mainly due to biotic factors, a change in the trend is expected at a distinct latitudinal transition. The data show an overall tendency for nest height to decrease further from the Equator. However, there is a distinct change in the slope, consistent with the biotic hypothesis. Against expectation, this does not occur between Tobago (continental island) and Grenada (oceanic island) but between Venezuela (continental mainland) and Trinidad (continental island). The biotic factors affecting nest-site in this species are not known. However, the data are consistent with the hypothesis that the species involved are affected by biotic relaxation during the several millennia since Trinidad and Tobago isolated from the mainland.

WES feedback and the Atlantic Meridional Mode: observations and CMIP5 comparisons

The Atlantic Meridional Mode (AMM) is the dominant mode of tropical SST/wind coupled variability. Modeling studies have implicated wind-evaporation-SST (WES) feedback as the primary driver of the AMM’s evolution across the Atlantic basin; however, a robust coupling of the SST and winds has not been shown in observations. This study examines observed AMM growth, propagation, and decay as a result of WES interactions. Investigation of an extended maximum covariance analysis shows that boreal wintertime atmospheric forcing generates positive SST anomalies (SSTA) through a reduction of surface evaporative cooling. When the AMM peaks in magnitude during spring and summer, upward latent heat flux anomalies occur over the warmest SSTs and act to dampen the initial forcing. In contrast, on the southwestern edge of the SSTA, SST-forced cross-equatorial flow reduces the strength of the climatological trade winds and provides an anomalous latent heat flux into the ocean, which causes southwestward propagation of the initial atmosphere-forced SSTA through WES dynamics. Additionally, the lead-lag relationship of the ocean and atmosphere indicates a transition from an atmosphere-forcing-ocean regime in the northern subtropics to a highly coupled regime in the northern tropics that is not observed in the southern hemisphere. CMIP5 models poorly simulate the latitudinal transition from a one-way interaction to a two-way feedback, which may explain why they also struggle to reproduce spatially coherent interactions between tropical Atlantic SST and winds. This analysis provides valuable insight on how meridional modes act as links between extratropical and tropical variability and focuses future research aimed at improving climate model simulations.

One lump or two? Explaining a major latitudinal transition in reproductive allocation in a viviparous lizard

Summary In viviparous ectotherms, the interval between reproductive bouts is often extended by long gestation times, preventing multiple reproductive events per annum. We assessed the potential roles of physiological adaptation and environmental constraints in driving an unusual case of geographic variation in life history, in the viviparous lizard (Eulamprus quoyii), which has either one or two reproductive bouts per annum, depending on the geographic location of the population. Using dynamic energy budget theory, we developed an integrated model of the energetics of growth and reproduction in this lizard, and applied it in conjunction with biophysical calculations of body temperature and activity time across its geographic range to predict reproductive frequency. Our model indicated that geographic variation in body temperature alone (i.e. environmental constraints) explained the observed pattern of litter frequency, suggesting that differences in energy allocation among populations were unlikely to be a major cause of differences in litter frequency in E. quoyii. It also suggested that natural selection should favour fixation of litter size in the transition zone.

APPLICATION OF A SOLAR WIND MODEL DRIVEN BY TURBULENCE DISSIPATION TO A 2D MAGNETIC FIELD CONFIGURATION

Although it is widely accepted that photospheric motions provide the energy source and that the magnetic field must play a key role in the process, the detailed mechanisms responsible for heating the Sun's corona and accelerating the solar wind are still not fully understood. Cranmer et al. (2007) developed a sophisticated, 1D, time-steady model of the solar wind with turbulence dissipation. By varying the coronal magnetic field, they obtain, for a single choice of wave properties, a realistic range of slow and fast wind conditions with a sharp latitudinal transition between the two streams. Using a 1D, time-dependent model of the solar wind of Lionello et al. (2014), which incorporates turbulent dissipation of Alfv\'en waves to provide heating and acceleration of the plasma, we have explored a similar configuration, obtaining qualitatively equivalent results. However, our calculations suggest that the rapid transition between slow and fast wind suggested by this 1D model may be disrupted in multidimensional MHD simulations by the requirement of transverse force balance.

Latitudinal discontinuity in thermal conditions along the nearshore of central-northern Chile.

Over the past decade, evidence of abrupt latitudinal changes in the dynamics, structure and genetic variability of intertidal and subtidal benthic communities along central-northern Chile has been found consistently at 30–32°S. Changes in the advective and thermal environment in nearshore waters have been inferred from ecological patterns, since analyses of in situ physical data have thus far been missing. Here we analyze a unique set of shoreline temperature data, gathered over 4–10 years at 15 sites between 28–35°S, and combine it with satellite-derived winds and sea surface temperatures to investigate the latitudinal transition in nearshore oceanographic conditions suggested by recent ecological studies. Our results show a marked transition in thermal conditions at 30–31°S, superimposed on a broad latitudinal trend, and small-scale structures associated with cape-and-bay topography. The seasonal cycle dominated temperature variability throughout the region, but its relative importance decreased abruptly south of 30–31°S, as variability at synoptic and intra-seasonal scales became more important. The response of shoreline temperatures to meridional wind stress also changed abruptly at the transition, leading to a sharp drop in the occurrence of low-temperature waters at northern sites, and a concurrent decrease in corticated algal biomass. Together, these results suggest a limitation of nitrate availability in nearshore waters north of the transition. The localized alongshore change results from the interaction of latitudinal trends (e.g., wind stress, surface warming, inertial period) with a major headland-bay system (Punta Lengua de Vaca at 30.25°S), which juxtaposes a southern stretch of coast characterized by upwelling with a northern stretch of coast characterized by warm surface waters and stratification. This transition likely generates a number of latitude-dependent controls on ecological processes in the nearshore that can explain species-specific effects, and add strength to the suggestion of an oceanography-driven, major spatial transition in coastal communities at 30–31°S.

Nitrogen fixation strategies can explain the latitudinal shift in nitrogen-fixing tree abundance.

The rarity of symbiotic nitrogen-fixing trees in higher-latitude compared to lower-latitude forests is paradoxical because higher-latitude soils are relatively N poor. Using national-scale forest inventories from the United States and Mexico, we show that the latitudinal abundance distribution of N-fixing trees (more than 10 times less abundant poleward of 35 degrees N) coincides with a latitudinal transition in symbiotic N-fixation type: rhizobial N-fixing trees (which are typically facultative, regulating fixation to meet nutritional demand) dominate equatorward of 35 degrees N, whereas actinorhizal N-fixing trees (typically obligate, maintaining fixation regardless of soil nutrition) dominate to the north. We then use theoretical and statistical models to show that a latitudinal shift in N-fixation strategy (facultative vs. obligate) near 35 degrees N can explain the observed change in N-fixing tree abundance, even if N availability is lower at higher latitudes, because facultative fixation leads to much higher landscape-scale N-fixing tree abundance than obligate fixation.

Key controls on the seasonal and interannual variations of the carbonate system and air‐sea CO2 flux in the Northeast Atlantic (Bay of Biscay)

AbstractBiogeochemical variations of surface water in the Northeast Atlantic (Bay of Biscay) were examined using high‐frequency underway measurements combined with monthly sampling of carbon‐related variables. The mechanisms controlling seasonal CO2 variability were investigated by distinguishing the contributions of biological and physical processes to the monthly changes in dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2). The seasonality of DIC (47–81 µmol kg−1) had a single peak with a winter maximum primarily driven by vertical mixing and a summer minimum driven by spring biological removal. Non‐Redfield C:N uptake was observed in the nutrient‐depleted summer but not during the spring bloom. In the North Atlantic, pCO2 seasonality shows a latitudinal transition: from the temperature‐dominated oligotrophic subtropical gyre to the subpolar region where pCO2 is dominated by changing concentrations of DIC. In the midlatitude Bay of Biscay, the annual cycle of pCO2 (61–75 µatm) showed a double‐peak distribution. The summer pCO2 peak was mainly driven by temperature increase, while the winter peak resulted from the dominant effect of entrainment of subsurface water. Interannual variations of DIC were more pronounced in winter and were driven by the changes in the strength of winter mixing. Higher wintertime concentrations and seasonal amplitudes of DIC were observed in cold years when the mixed‐layer depths were deeper, which appears to be associated with negative phases of the North Atlantic Oscillation. The Bay of Biscay shows a decrease of CO2 uptake in 2008–2010 (−0.97 and −0.75 mol m−2 yr−1) compared to 2002–2004 (−1.47 and −1.68 mol m−2 yr−1).

Interdecadal changes of vegetation transition zones and their responses to climate in Northeast China

According to the concept of ecoclimatic factor’s guarantee rate, this study purposes to quantitatively analyze the interdecadal changes of the four major vegetation transition zones in Northeast China based on the warmth index first developed by Kira and the humidity index put forward by Xu during the period 1961–2007. On this basis, the responses of the interdecadal changes of vegetation transition zones to climate were analyzed. The results indicated that latitudinal vegetation transition zones had been moving laggingly to higher latitude (shifting upwards in altitude) and that longitudinal vegetation transition zones had been shifting slowly toward humid area in the whole sequence, whereas the shifts were fluctuant in stages. The changes of vegetation transition zones of Northeast China expressed in distribution and bandwidth responded to climate change, not only in the whole sequence but also in stages.

Detecting the critical periods that underpin interannual fluctuations in the carbon balance of European forests

The interannual variability of CO2 exchange by forest ecosystems in Europe was analyzed at site and regional scales by identifying critical periods that contributed to interannual flux anomalies. Critical periods were defined as periods in which monthly and annual flux anomalies were correlated. The analysis was first conducted at seven European forest flux tower sites with contrasting species and climatic conditions. Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE), a generic process‐based model, represented fairly well most features of the critical period patterns and their climate drivers at the site scale. Simulations at the scale of European forests were performed with ORCHIDEE integrated at a 0.25° spatial resolution. The spatial and temporal distributions of critical periods for canopy photosynthesis, ecosystem respiration, and net ecosystem exchange (NEE) as well as their underlying climate drivers were analyzed. The interannual variability in gross primary productivity (GPP) was explained by critical periods during spring and summer months. In contrast, the interannual variability in total ecosystem respiration (TER) was explained by critical periods occurring throughout the year. A latitudinal contrast between southern and northern Europe was observed in the distributions of critical periods for GPP and TER. The critical periods were positively controlled by temperature in northern Europe and by soil water availability in southern Europe. More importantly, the latitudinal transition between temperature‐driven and water‐driven critical periods for GPP varied from early spring to late summer. Such a distinct seasonal regime of critical periods was less clearly defined for TER and NEE. Overall, the critical periods associated with NEE variations and their meteorological drivers followed those associated with GPP.

Spatial variation of wetland woods in the latitudinal transition to arid regions: a multiscale approach

AbstractAim In order to investigate the occurrence of wetland woods in the latitudinal transition to arid regions in south‐western Europe, we studied species patterns (richness and abundance), examined floristic differences between woods along the latitudinal gradient, and determined the relative influence of the underlying environmental drivers of plant variation at various scales.Location The Atlantic coastal belt of the Iberian Peninsula along the entire latitudinal gradient (44–38° N).Methods Large‐scale surveys were carried out in search of woods located in flats or depressions with prolonged waterlogging. Stands were selected for study when they displayed a continuous tree structure and little sign of human disturbance. Sampling included plant inventories in 114 plots, in which presence and abundance cover were recorded for all vascular and bryophyte species. Both diversity and composition were used to investigate plant species patterns. Gamma and alpha diversity (species richness) values were used to compare Ibero‐Atlantic wetland woods with other European woods. Species richness was modelled as a function of environmental variation at regional and finer scales (landscape and local scale), using linear mixed‐effects models and model selection based on the Akaike information criterion. Hierarchical clustering and ordination using perennial species were used to detect floristic differences between sites. Partial canonical correspondence analyses were performed to determine the relative importance of each set of environmental drivers in structuring the vegetation trends at regional and finer scales.Results A significant proportion of wetland woods occurred in the transition to the Mediterranean region. Ibero‐Atlantic wetland woods displayed low gamma and alpha diversity compared with other woods. Species richness was strongly influenced by finer‐scale variables, in particular distance to rivers, whereas regional variables were less influential. Based on tree dominance, the classification revealed five vegetation types, but the majority of stands (86%) were included in woods dominated by Salix atrocinerea Brot. and Alnus glutinosa (L.) Gaertner. Species abundances were correlated with both regional and finer‐scale hydrological variables, which explained 37.5% of the variation, 11.9% of which corresponded to regional and 18.5% to finer‐scale environmental descriptors.Main conclusions True wetland woods persist in the transition to arid regions of south‐western Europe. The latitudinal gradient influences the spatial variation of species, but local hydrological variables were found to play a significant role in both diversity and compositional patterns.

Population dynamics, distribution and secondary production of the brown shrimp <i>Crangon crangon</i> (L.) in a southern European estuary. Latitudinal variations

The population structure, growth, distribution and secondary production of the brown shrimp Crangon crangon (L. 1758) were examined between June 2003 and August 2005 at five stations in the Mondego Estuary, Portugal, using a 2-m beam trawl. Occurrence of immature shrimps suggested almost continuous reproduction with two main recruitment periods in November and March/April; this recruitment pattern is unique and represents a latitudinal transition in the species’ life cycle. Severe drought in winter 2004 caused spatial shifts along the north arm of the estuary regarding post-larval settlement in spring 2005. Mean growth of all cohorts was 0.06 mm TL day -1 , much lower than rates calculated elsewhere. Secondary production (P) of C. crangon was estimated at 4.6 mg AFDW m -2 y -1 (P/B– ratio of 8.7) in 2003/2004 and at 2.4 mg AFDW m -2 y -1 (P/B– ratio of 10.9) in 2004/2005. High abundances of individuals, predominantly juveniles, were observed in spring/summer with peaks in June 2003, April 2004 and July 2005. This study shows that this species dynamics must be analysed in a latitudinal context, meaning that the contribution of each component of the systems can vary according to environmental and climate variables.

Magnetic cloud distortion resulting from propagation through a structured solar wind: Models and observations

Numerical simulations of magnetic clouds (MCs) propagating through a structured solar wind suggest that MC‐associated magnetic flux ropes are highly distorted by inhomogeneities in the ambient medium. In particular, a solar wind configuration of fast wind from high latitudes and slow wind at low latitudes, common at periods close to solar minimum, should distort the cross section of magnetic clouds into concave‐outward structures. This phenomenon has been reported in observations of shock front orientations, but not in the body of magnetic clouds. In this study an analytical magnetic cloud model based upon a kinematically distorted flux rope is modified to simulate propagation through a structured medium. This new model is then used to identify specific time series signatures of the resulting concave‐outward flux ropes. In situ observations of three well studied magnetic clouds are examined with comparison to the model, but the expected concave‐outward signatures are not present. Indeed, the observations are better described by the convex‐outward flux rope model. This may be due to a sharp latitudinal transition from fast to slow wind, resulting in a globally concave‐outward flux rope, but with convex‐outward signatures on a local scale.