Seasonal variation occurs at the sea surface.
In summer this latitudinal belt is characterised by relatively uniform sea surface temperatures mainly through an increase in sea surface temperatures in the northern tropics and subtropics. The seasonality in sea surface temperatures is largest in mid latitudes and lowest in the tropical ocean near the equator. Sea surface salinity shows distinct maxima in mid latitudes and low values near the equator and in high latitudes. The pattern remains about the same in summer and winter. Seasonality in sea surface salinity is mainly characterised by spatial variability in low latitudes while conditions are more uniform and stable in mid latitudes.