the Slope Current at the northwest European shelf break ) are principally driven by surface buoyancy forcing, owing to the combined effects of heat and freshwater exchange between ocean and atmosphere. Moving into mid-latitudes, some boundary currents (e.g. the Florida Current) to broad weak flows spanning several hundred kilometres (e.g. Current width ranges considerably, from narrow swift flows spanning a few kilometres (e.g. Current speeds are typically in the range of 10–100 cm s −1. Such currents are quasi-steady, subject to some seasonality, particularly in wind forcing, e.g. These return flows comprise the ocean currents of leading importance for the long-distance migration of marine organisms. Northward return flow in the gyre is confined to a western boundary region, governed by frictional processes, and is consequently swift. At this scale in the subtropics, the ‘subtropical gyre’ is set up by the ‘Sverdrup transport’, which is a broad equatorward flow across the subtropics. These gyres are major currents that occur at the ocean basin scale. For example, in the North Atlantic, the ‘subpolar gyre’ is used by Atlantic salmon and the ‘subtropical gyre’ is used by sea turtles. For juvenile stages of marine organisms, small size may limit their capability to swim actively against currents, so ‘going with the flow’ would be an efficient means of migrating to distant foraging grounds while maximizing growth and development. Many species have been shown to use ocean currents as migratory pathways. įor aerial organisms, global wind patterns are a strong determinant of long-distance migratory routes, but in the sea, it is the prevailing oceanographic features, such as circulation patterns, that are believed to be important in determining the distribution and connectivity of populations. It is therefore suggested that global migrators, such as transoceanic migratory birds, may be useful as biological indicators of climate and oceanic health. climate) that may have consequences for migratory behaviour and species survival. These studies go beyond curiosity, as anthropogenic changes to the environment are affecting large-scale processes (e.g. The iconic questions of where eels go to spawn, and how sea turtles and salmonids navigate and the factors that shape their migratory routes continue to drive scientific investigation. Migratory animals exploit different locations at different stages in their life: a strategy so effective at optimizing resource use that the cost of travel is worthwhile. Long-distance migration remains one of nature's wonders. Increased stormy weather predicted under climate change scenarios suggests an increasing role of storms in dispersal of sea turtles and other marine groups with life-stages near the ocean surface. As such, storms may be a route by which unexpected areas are encountered by juveniles which may in turn shape adult migrations. However, in some cases, unusual occurrences of juveniles are more readily explained by storm events and we show that juvenile turtles may be displaced thousands of kilometres from their expected dispersal based on prevailing ocean currents. Here, we use multi-disciplinary oceanographic, atmospheric and genetic mixed stock analyses to show that juvenile turtles are encountered ‘downstream’ at sites predicted by currents. Sea turtles are one of these paradigmatic long-distance travellers, with hatchlings thought to be dispersed by ocean currents and adults often shuttling between distant breeding and foraging grounds. For many species, there is broad-scale dispersal of juvenile stages and/or long-distance migration of individuals and hence the processes that drive these various wide-ranging movements have important life-history consequences.
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