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[1] A case of extreme meteorologically forced fluctuation of net flow through the Strait of Gibraltar is analysed. The Atlantic water inflow was interrupted during some days and the net flow reached a peak of À1.5 Sv towards the Atlantic Ocean. In spite of the rapid increase of atmospheric pressure that triggered this episode, the expected inflow contribution to the net flow induced by pressure variation should not exceed 75% of the mean inflow, insufficient to reverse the inflow. Wind stress acting on the upper layer can induce important inflow and therefore net flow fluctuations. A simple model is proposed in which wind stress intensity as a function of frequency determines the inflow response. For low frequency fluctuations and moderate wind speeds the model predicts a gain that, if added to the atmospheric pressure effect, could bring the inflow fluctuation peak beyond the mean, thus explaining the inflow interruption.

We construct d-dimensional pure simplicial complexes and pseudo-manifolds (without boundary) with n vertices whose combinatorial diameter grows as c d n d−1 for a constant c d depending only on d, which is the maximum possible growth. Moreover, the constant c d is optimal modulo a singly exponential factor in d. The pure simplicial complexes improve on a construction of the second author that achieved c d n 2d/3 . For pseudo-manifolds without boundary, as far as we know, no construction with diameter greater than n 2 was previously known.

We introduce topological prismatoids, a combinatorial abstraction of the (geometric) prismatoids recently introduced by the second author to construct counter-examples to the Hirsch conjecture. We show that the "strong d-step Theorem" that allows to construct such large-diameter polytopes from "non-d-step" prismatoids still works at this combinatorial level. Then, using metaheuristic methods on the flip graph, we construct four combinatorially different non-d-step 4-dimensional topological prismatoids with 14 vertices. This implies the existence of 8-dimensional spheres with 18 vertices whose combinatorial diameter exceeds the Hirsch bound. These examples are smaller that the previously known examples by Mani and Walkup in 1980 (24 vertices, dimension 11).Our non-Hirsch spheres are shellable but we do not know whether they are realizable as polytopes.2000 Mathematics Subject Classification. 52B05, 52B12, 90C60, 90C05.

Three oceanographic surveys carried out in the Sicilian Channel during the spawning season (June to July) of anchovy (Engraulis encrasicolus) showed a close relationship between anchovy reproductive strategy and important hydrographic structures. A time series of satellite-derived sea surface temperature images of the Sicilian Channel were analysed by means of empirical orthogonal functions and the dominant empirical modes were studied in detail. The first empirical mode captured much of the original variance and reproduced the trajectory of the Atlantic Ionian Stream (AIS), the principal hydrodynamic feature of the area. The time coefficients of modes 1 and 2 had seasonal signals which, when combined, accounted for the enhancement of the thermal front, clearly visible off Cape Passero (southernmost coast of Sicily) during summer. As the area constituted the principal nursery ground of the Sicilian Channel anchovy, the combination of the time coefficients of these modes was considered a potential indicator of the food particle concentration usually associated with oceanic fronts, which provided the energy requirements for larval growth. Mode 3 described the north/south displacements of the mean AIS trajectory, which modified the surface temperature regime of the anchovy spawning habitat. Therefore, the time coefficients of this mode were used as a potential indicator of anchovy spawning habitat variability. The capability of time coefficients of modes 2 and 3 to modify the main pattern depicted by mode 1 were tested successfully against in situ oceanographic observations.