Poleward of the (Sub)tropics, the annual excess precipitation
over evaporation increases, and thus salinity
decreases, with latitude. As a result, the values of AT
generally decrease with increasing latitude (Figure 1). Seasonal
changes in the intensity of the convective mixing here
are additional key factors that act to change surface AT in a
measurable way. During seasonal cooling, the intensive
vertical mixing brings deep waters rich in CaCO3-driven
AT to the surface, and thus increases surface AT. During
seasonal warming, however, the shoaling of the mixed layer
makes the contribution of AT-rich deep waters to the change
in surface AT minimal.