Temperatures of the Oceans There is a somewhat similar pattern in Ocean temperatures to salinity. There is a discontinuous band of the hottest waters north of the equator in the northern Hemisphere summer, and this migrates southward with the seasons. Total temperature variations are less in the Oceans than on land, because of the high heat capacity of water. It takes a lot of energy to heat up or cool down water. Thus, land surfaces cool and heat daily and seasonally, Oceans much less.
Coastal climates are more mild than central continental climates.
Compare Seattle with Fargo, N.D. as an example. 2 layer system surface layer and deep Ocean, separated by thermocline . The Surface layer is warm, heated by the sun. It is therefore less dense than deeper, colder water. It also generally has lower salinity, which makes it even less dense.