The world’s ocean currents play a critical role in climate. Those currents are influenced by atmospheric conditions. These interactions between the oceans and atmosphere determine how vital rain and snow are distributed around the globe.
Precipitation patterns are critical to agriculture and the natural food webs upon which all life depends. We must understand how human activities cause changes that affect the strength, direction and volume of water moving in oceanic currents
Currents are generally distinguished by whether they are deep or near the surface. Each arises from different causes.
Shallow currents, generally to depths of about 100 meters (300 feet), are driven by friction between prevailing winds and the ocean’s surface. These currents generally are faster moving than deep currents, but move smaller volumes of water.
Patterns in prevailing winds around the globe are the result of the rotation of the Earth and variations in the intensity of sunlight striking the Earth’s surface at different latitudes. The intensity of sunlight in any region changes throughout the year as a consequence of the Earth being tilted relative to the plane defined by its orbit around the sun. The resulting atmospheric wind patterns impose themselves on ocean surfaces.
In a reciprocal fashion ocean surface temperatures dictate the development of ocean storms and the paths they take. Every hurricane season we witness how the strength and destructive power of hurricanes are subject to this effect.
Patterns of rain and snowfall across the planet, from Southeast Asia’s monsoons to North African droughts, have consequences for those living both near and far from coasts.
Stable ocean currents have made life as we know it in northern Europe possible. One of the better known surface currents is the Gulf Stream. It is a narrow, relatively deep and fast moving current traveling 25 to 75 miles per day.
It carries warm waters from the tropics north to the shores off western England and Europe. Though London is nearly as far north as Moscow, its winter climate is temperate rather than frigid because of the effects of the Gulf Stream.
In contrast to wind-dominated shallow currents, deep-ocean currents are driven by variations in water density at ever greater depths. This arises from changes, or gradients, in both temperature (thermo) and salinity (haline) with depth. Hence, the alternative name for these currents, thermohaline circulation.
Here again, we see interactions between the atmosphere and ocean. In the Polar Regions very cold atmospheric temperatures give rise to formation of sea ice. When ice forms, salt is left behind. Saltier seawater is denser, so it sinks.
Sinking of dense masses of water causes the water below it to be displaced or pushed aside. Warmer surface water is drawn in to replace the sinking water until it cools and sinks itself. The resulting deep-ocean currents move large volumes of water at much slower speeds than wind-driven surface currents.
Two other effects contort ocean currents into the patterns we see. As currents move from equatorial to more polar latitudes they are deflected by a phenomenon called the Coriolis force (a phenomena arising from conservation of angular momentum). In the northern hemisphere currents are routed in an eastward direction, and westward in southern climes.
Additionally, the topology (shape) of the ocean bottoms and the continents themselves create barriers that redirect flow as local conditions dictate.
The combined effect of the Coriolis force and continental boundaries set the oceans into circular flow patterns between the continents. In the northern hemisphere the flow is clockwise, while in the southern hemisphere it is counterclockwise.
In each of the three large oceanic basins — Pacific, Atlantic and Indian — these circular patterns are called gyre. Examining flows across the entire globe however shows there to be significant flow between the gyre in each ocean basin. The result can be thought of as a system of interlocking global conveyor belts.
The importance of this system to life on Earth is evident when one realizes it is responsible for redistributing not just heat but nutrients and carbon dioxide from one climatic zone to another.
The average depth of the oceans is 3.7 kilometers (2.3 miles). Yet the top few meters of the oceans store as much heat energy as the entire atmosphere. With such reserves of heat energy, disruptions of oceanic currents can dramatically affect climatic patterns.
Even with the relatively small size of the Gulf Stream, the temperate climate Europeans enjoy depends on the warmth it brings. The lives of many billions of people are intimately dependent upon historical, seasonal rainfalls or river flows from melting snow in nearby mountains. Across the planet the distribution of rain and snow are strongly dependent on climatic changes driven, in part, by ocean currents.
Of great importance to all living things is the recharging of ocean waters with nutrients as they pass through ocean depths. Oceanic algae and seaweed are at the base of the global food chain. Their vitality is dependent upon these cool, nutrient rich waters.
Finally, warm surface waters arriving at the poles are depleted of nutrients and carbon dioxide. Carbon dioxide dissolves in cold water more readily than warm. Upon being chilled, the frigid ocean water takes on carbon dioxide before it sinks and begins recirculating.
The oceans have about 18 times the amount of carbon dioxide as the atmosphere. Scientists are attempting to understand how disruption of ocean currents might affect the amount of CO2 absorbed in the oceans.
This is likely to impact the carbon cycle, which is an important element in maintaining stable global temperatures. Such a shift could push global warming toward ever more catastrophic consequences for humanity.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.