Future challenges in shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 359

large over most of the Arctic shelf ( 0.5 m s 1 ), though they contain a high

proportion of the kinetic energy of the flow, contributing to mixing in the boundary
layers. Tides can also be locally important in increasing ice formation by opening
leads and polynyas, so providing more area for ice formation and influencing the
amount of dense water created (Hannah et al., 2009 ; Postlethwaite et al., 2011). As
well as exerting a major influence on water column stability, the large freshwater
input to the Arctic shelf seas will also bring with it substantial inputs of nutrients,
organic carbon and other components from terrestrial sources. A further radical
difference from the temperate case is the process of brine rejection which occurs
during the freezing of salt water. As ice is formed mainly from freshwater, the salt
left behind remains in the ambient seawater and increases its salinity and hence its
density. The importance of this process and the potential of the heavy water formed
to cascade into deeper water was first recognised by the explorer Fridtjof Nansen
(Nansen, 1906 ).
Understanding the impact of these additional processes on the structure and
circulation of the Arctic shelves and the response of the biological system presents
a demanding but interesting challenge for the future. The practical difficulties of
working in these high latitude regions are substantial but there is considerable
motivation. The large areas of the Arctic shelf which are free from ice during
the summer are postulated to make a large contribution to carbon fixation
(Bates, 2006 ), with the potential to change markedly in a warming climate (Arrigo
et al., 2008 ).
Moreover, the physical processes occurring on the shelf play a big role in the
transformation of water masses in the Arctic. This is shown in the map in Fig. 11.4 .
Atlantic water, which enters the Arctic Ocean via the Fram Strait, circulates in an
anticlockwise flow around the rim of the deep basin. As it does so, the relatively
warm Atlantic water is progressively cooled through heat loss to the overlying
surface water and through interaction with the waters of the adjacent shelf. Away
from the immediate vicinity of the slope, measurements indicate that the upward heat
flux from the warm rim current through the stable halocline is due to the process of
double diffusive convection and is too small to account for the observed cooling
(Lenn et al., 2009 ). The main contribution to the cooling is postulated to come
from convective stirring by heavy saline water produced by brine rejection on the
shelf (e.g. Turner, 2010 ). In this way, the little understood shelf processes in the
Arctic combine to influence the properties of the return flow of cold Arctic water
through the Fram Strait. This flow is an important part of global overturning
circulation since it constitutes the primary source for the formation of North Atlantic
Deep Water.

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11.2.2 Tropical ROFIs

The shelf seas located in or near the tropics also present an interesting scientific
challenge. Many shelf seas in tropical regions are of limited area, extending only a
small distance (

50 km or less) from the coastline. This is especially true of the

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