Why are they important?
It is the vertical circulations associated with submesoscales that makes them so important to the climate system and oceanic productivity. Vertical currents at large scale fronts, such as the subantarctic front, are very small, of the order of millimetres per second. The vertical velocities at submesoscale fronts are more than ten times larger however, reaching up to 100m/day. These vertical currents transport not just heat downwards to the ocean interior but also dissolved gases such as carbon dioxide; the oceans slow the rate of climate change by absorbing 25% of anthropogenic carbon dioxide emissions annually (Sallee et al., 2012).
In the opposite direction, nutrients from beneath the thermocline are rapidly brought up to the ocean surface where the combination of sunlight and nutrient availability leads to the development of plankton blooms. It is for this reason that regions of strong submesoscale activity are often characterised by plankton blooms that have basin-scale implications. A recent study suggests that the North Atlantic spring bloom is initiated not by the warming of the surface layers by sunlight but by the slumping of submesoscale fronts that trap nutrients in the sunlit surface layer and triggers the growth of plankton a month earlier than would have occurred without submesoscale fronts (Mahadevan et al., 2012). Notice in the figure below from Mahadevan et al. (2012) that the chlorophyll is increased in narrow streaks (top right panel) that correspond to the fronts in the surface mixed layer (top left panel); it is precisely these sorts of features that we will aim to measure during the SMILES cruise.
Figure from Mahadevan. 2012
It is only over the last few years that we have been able to identify these smaller features as our technological capability has increased; the resolution of sensors on satellites, for example those used to detect the temperature of the sea surface, is now less than 1km and therefore able to resolve the small scales and weak signatures characterised by submesoscales. Similarly, increases in computational capacity have permitted numerical models to resolve oceanic flows to scales much less than 1km over large regions, thereby allowing us to understand the impact of submesoscale fronts on ocean processes. We therefore have the technical capability to both observe and model these processes over basin scales, thereby understanding their effect over large portions of the oceans.