How do ocean currents affect weather and climate?
Ocean currents affect weather and climate by means of ocean heat and salinity; by means of ocean water evaporation; by means of ocean current systems. It is conversely true that wind from the atmosphere affects ocean currents and waves.
There is a relationship between ocean currents, weather and climate because of the interaction of the ocean and the atmosphere. Water in the ocean absorbs solar energy (heat) slowly and retains that heat for a long time, which helps moderate global climate by spreading heat more evenly across the globe. For example, in coastal communities heat from the ocean, absorbed over the summer months, helps keep areas on nearby land warmer in autumn and winter as that heat energy is transferred to the atmosphere. Thus, heat from the ocean affects the circulation of the atmosphere, which affects weather and climate.
Weather is the conditions of the atmosphere on a specific day in a specific area and it is affected by the amount of precipitation, humidity, air pressure, the temperature of the day, etc. Since moisture from the ocean evaporates, it adds to the amount of moisture in the atmosphere; ocean currents affect weather and climate by affecting levels and kinds of precipitation active along the pattern of ocean currents. Weather and climate are affected as seen when cold dry air, which has higher pressure than warm moist air, encounters a front of cold dry air: the conditions are set for precipitation in the form of rain or snow depending on temperature of the climate zone.
Climate is measured in a specific place over a long period of time and reflects a typical set of conditions for that area. For instance, the climate of a tropical rain forest has similar temperatures year round with great amounts of precipitation usually on a daily basis resulting in 80-180 inches of precipitation per year.
The ocean currents are compared to an ocean conveyor belt that has two functions. The ocean current conveyor belt (thermohaline circulation) moves global ocean water from top to bottom based on density, which is the combination of temperature (warm, cold) and salinity (saltiness), and from the poles to the equator based on the movement created by up-down thermohaline circulation and by wind current. Weather patterns "are driven largely by ocean currents." For instance, the relatively warm climate of Europe is affected by the fact that the Gulf Stream water pushes into the North Atlantic and distributes heat to Europe because this water originated in the tropics and its heat is transferred to the atmosphere moderating the climate.
The pattern of this conveyor belt fueled by density (temperature plus salinity) is that cold, deep water is transported from lower ocean depths to warmer, increasingly salty, surface ocean water. The surface water becomes more dense as it gains salinity (saltiness) and eventually sinks. The sinking water is replace by fresh rain and river water. It eventually gets cold and salty and sinks. When this dense water sinks, deep ocean waters initiate global ocean current cycles: polar surface water "eventually becomes cold and salty enough to sink. This initiates the deep-ocean currents...." This flow of ocean currents affects the atmosphere, which thus affects the weather and climate of different regions.