10
2 Frontal types
2.6 Fronts Associated with the Convergence or Divergence
of Water Masses at High Seas
In the Pacific, a westward current driven by the Trade winds is located roughly
between 5°S and 5°N. The Earth rotation leads to divergence of the Ekman layer
away from the equator, which is compensated by upwelling from subsurface layers
(Fig. 2.3b). The upwelling in turn is compensated by equatorward flows below the
mixed layers in both hemispheres (Wyrtki and Kilonsky 1984; Johnson et al. 2001).
Upwelling of cool subsurface water forms a cold tongue along the equator: the
equatorial upwelling and creates an extended thermal front of moderate intensity.
This thermal front shows some degree of seasonality in response to the seasonal
pattern of the trade winds (Mann and Lazier 2006). A relatively strong upwelling
system referred to as the Antarctic Divergence is observed in the Southern Ocean.
This system is caused by opposing southern hemisphere mid-latitude westerlies and
high-latitude easterlies. Here the winds and the Earth rotation drive a flow divergence in the upper layers. Along the line of strongest wind stress curl separating
Fig. 2.3 Frontal types. a Plume front (southern hemisphere); b divergence front; c frontal eddies
(top panel) show eddies formation from a meandering current, w warm core eddy; c cold core
eddy. Lower panels show fronts (f) at convergences created in cyclonic or anticyclonic (southern hemisphere) eddies, modified from Mann and Lazier (2006) and Bakun (2006a); d topographically controlled front (greenish areas indicate convergence frontal zones), modified from
Wolanski and Hamner (1988). f front; ss sea surface; t thermocline; w wind; arrows are currents;
dashed lines in a and b are lines of equal density
2 Frontal types
2.6 Fronts Associated with the Convergence or Divergence
of Water Masses at High Seas
In the Pacific, a westward current driven by the Trade winds is located roughly
between 5°S and 5°N. The Earth rotation leads to divergence of the Ekman layer
away from the equator, which is compensated by upwelling from subsurface layers
(Fig. 2.3b). The upwelling in turn is compensated by equatorward flows below the
mixed layers in both hemispheres (Wyrtki and Kilonsky 1984; Johnson et al. 2001).
Upwelling of cool subsurface water forms a cold tongue along the equator: the
equatorial upwelling and creates an extended thermal front of moderate intensity.
This thermal front shows some degree of seasonality in response to the seasonal
pattern of the trade winds (Mann and Lazier 2006). A relatively strong upwelling
system referred to as the Antarctic Divergence is observed in the Southern Ocean.
This system is caused by opposing southern hemisphere mid-latitude westerlies and
high-latitude easterlies. Here the winds and the Earth rotation drive a flow divergence in the upper layers. Along the line of strongest wind stress curl separating
Fig. 2.3 Frontal types. a Plume front (southern hemisphere); b divergence front; c frontal eddies
(top panel) show eddies formation from a meandering current, w warm core eddy; c cold core
eddy. Lower panels show fronts (f) at convergences created in cyclonic or anticyclonic (southern hemisphere) eddies, modified from Mann and Lazier (2006) and Bakun (2006a); d topographically controlled front (greenish areas indicate convergence frontal zones), modified from
Wolanski and Hamner (1988). f front; ss sea surface; t thermocline; w wind; arrows are currents;
dashed lines in a and b are lines of equal density