Temperature, salinity, and velocity are the key variables that describe the state of the ocean.

We are interested in the density of the ocean too because it determines whether the ocean will encourage vertical motions. The density is a function of both temperature and salinity. As water warms it expands and becomes less dense. Water containing dissolved salts is heavier than fresh (pure) water, so density increases with salinity.

I described the average vertical structure in the ocean using a figure like 5-6 in the text. Because the density  on average increases with depth, the ocean is on average stable. The density increase with depth is relatively small in the upper 100 m or so, where I mentioned before that the ocean tends to be well mixed due to turbulent exchange of heat in this layer. The upper ocean is separated from the deep ocean in a layer where the density decreases sharply with depth. This is called the pycnocline.

The ocean is heated from above, therefore the warmest temperatures are at the surface of the ocean. Because warmer water is lighter, this tends to create a stable environment, with little vertical motion. This is why the deep circulation of the ocean is so sluggish compared to the atmosphere. Like the density, the vertical temperature and salinity profiles have regions where they change very rapidly with depth, known as the thermocline and the halocline, respectively.

A map of the sea surface temperature indicates that the temperature is not constant along latitudes, instead the temperature tends to be relative high in the western boundary currents and low in the eastern sides of the subpolar gyres. The temperature is also quite a bit higher in the northern North Atlantic than in the northern North Pacific. The circulation associated with the deep ocean influences the temperature in the northern North Atlantic in particular.

A temperature cross section through the Atlantic Ocean (Fig 5-8) in the text shows that the temperature near the surface is warmer than it is at depth in the tropics, but this is not apparent in the high latitudes in this figure where the temperature gradient is contoured in  5 K intervals.

A map of the surface salinity indicates that the Atlantic is generally more saline than the the Pacific, and the Arctic Ocean is remarkably low in salinity. The northern North Atlantic is one of two locations where dense water is formed on Earth and it is thought that this is because of its relatively high salinity. (The other dense water formation site is near Antarctica.) The low salinity water to the north in the Arctic threatens to reduce the high salinity and weaken the deep water formation there. Sea ice is another factor in altering the surface density. Sea ice insulates the ocean (prevents it from losing heat) and it produces an intense but short term salinity source when it grows.

Deep water production occurs where the surface water density is high enough. This almost always takes place at high latitudes during the wintertime, when the cold atmosphere extracts huge quantities of heat from the surface ocean and salt is expelled from sea ice growth. If the water becomes dense enough, it will sink to the depths of the ocean in small-scale "chimneys" or downward plumes. Once it sinks, this water spreads throughout the global ocean. This overturning circulation is known as the thermohaline circulation. It typically takes about 1000 years for a parcel of water to sink, flow through the deep ocean and return to the surface. This is an indication of how sluggish the deep ocean circulation is. 

The global ocean thermohaline circulation (also called conveyor belt) is depicted in Fig 5-12 of the text. This circulation contributes to the strength of the Gulf Stream along the east coast of North America. The circulation is responsible for transporting heat into the northern North Atlantic, which maintains sea ice free conditions off the coast of Norway and a more temperature climate in northwestern Europe (Scandinavia mainly) than one would expect without the thermohaline circulation.

There are (at least) three ways the ocean transports heat that we learned about. The thermohaline circulation just mentioned is one. The wind driven circulations move warm water poleward in western boundary currents and cooler water equatorward on their eastern branches. Finally Ekman transport along the equator causes a poleward transport of heat on either side of the equator. In addition to poleward heat transport, the other main climatic influence from the ocean is the moderating effect of the ocean on coastal regions.

In summary:

On timescales of a few years and less:

• The contrast in the heat capacity of the land and ocean has a profound effect on our climate's seasonality and its response to increasing greenhouse gases. 
• The ocean plays a critical role in the El Nino phenomenon, a periodic climate "oscillation" centered in the equatorial Pacific.

• Changes in the global thermohaline circulation, which maintains higher temperature in the North Atlantic when it is strong, can affect temperatures.
• Long-term changes in sea ice coverage can alter the planet's energy budget
• Changes in ocean ecosystems can have a large influence on climate through their role in helping to regulate carbon dioxide in the atmosphere.