Ocean Currents: how do they affect the climate?
The ocean currents are vital for the environment as they bring to the surface and around the globe nutrients for microorganisms creating the sea food chain. Moreover, the movement of the warm water towards the poles plays a vital role in the regulation of the climate. There is several sources for the movements of the water in the seas: winds on the ocean surface arising from changes in air pressure, tides caused by gravity impacting the beaches, rotation of the earth which affects the global directions of the currents and changes in water density which cause the vertical loop called thermohaline effect.
However, two major groups can be identified: the superficial one with the winds blowing impacting the changes of surface currents and tides, and deep ocean currents, mostly affected by the thermohaline (thermo = temperature, haline = salt) effect which moves warmer water towards the surface and colder water to the bottom. Currents are also impacted by the air pressure of the planet, which is higher in the poles and lower close to the equator. This discrepancy constructs the Coriolis effect, winds of a low pressure system (equator) swirl counterclockwise north bound and clockwise south bound. These big lopes in ocean currents are also called gyres. Whereas the intrinsic system of conjoining surface with deep water currents created by the thermohaline effect is called Global Conveyor Belt which creates a thousand year long circulation around the whole globe.
A very interesting example of natural phenomenon occurring in the sea and impacting the climate globally is El Nino y La Nina. In a neutral situation, the cold water in South America is blown by the trade wind upwards and westbound on the equator towards South East Asia, making the climate of Indonesia and Australia warmer and therefore also more humid than South America. During La Nina, this event is accentuated and rainfall in Australia is more severe. Oppositely, El Nino has the opposed effect: the weakening of the wind brings back warm water from Australia to Eastern Pacific resulting in droughts in the Southern Pacific and more rainfall in the Americas. Moreover, El Nino causes a decrease in nutrients in the water of Western Pacific and therefore seafood decline as the warm water smoothers the abundance of the supplements once carried to the surface by the cold currents.
The two events are always related to each other, experiencing first El Nino, which can last even longer than one year, and consequently La Nina, which may even dure up to two or more years. The two events together form the so called ENSO (El Niño-Southern Oscillation).
What is the cause of the changes in trade wind strength?
The strengthening or weakening of the trade winds, which consequently are the cause of El Nino or La Nina, derives from the variations of pressure gradient in the atmosphere. Consequently, warmer sea surface also acts to decrease atmospheric pressure by transferring heat to the air and making it less dense (function of pressure is = temperature * density).
Is there predictability of this natural phenomenon?
To a certain extent yes, however there is limit to its predictability. There is still an open debate in the scientific community as to whether the initial studies of the event were constructed according to a correct model, whereas others argue that predictability is "limited by the uncertainties of atmospheric noise. Our analysis suggests that the evolution of El Niño is controlled to a larger degree by self-sustaining internal dynamics than by stochastic forcing. Model-based prediction of El Niño therefore depends more on the initial conditions than on unpredictable atmospheric noise." (Dake, 2004).
What's the impact by climate change on this phenomenon?
According to the study of Wang 2019 published on peer reviewed journal PNAS (Proceedings of the National Academy of Science), the frequency and intensity of El Nino "will increase significantly if the projected central Pacific zonal SST (Sea Surface Temperature) gradients become enhanced." The model developed based on the study of the changing properties of ENSO during the 20th century (Coupled Model Intercomparison Project phase 5 CMIP5) still assumes some doubts on the projected changes of the SST gradient, however it confirms that anthropogenic forces influencing the climate will directly affect the natural phenomenon of El Nino as well, resulting in possibly an increased amplitude extreme events.
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