New Delhi, Aug. 29 -- We have often heard of the importance of El Niño and La Niña for the Indian monsoons, of the warm Gulf Stream current that warms Europe, and the Labrador Current that brings cold weather to North America. In general, it is well known that ocean currents have determined shipping routes for hundreds of years; it is also known that ocean currents impact climate in various ways, as alluded to above. However, with climate change, the predictability of ocean currents is in question, and their impact can often lead to extreme climate events never seen before. Let us discuss these issues in a three-part article here. In the first part, we will summarise the various ocean currents in the large oceans, and in the second part, we will discuss how climate change is impacting ocean currents and, through this, in turn, is changing rainfall patterns and wind/cyclone speeds and the marine and coastal ecosystem. In the third part, we will discuss the importance of the ocean as a giant heat sink, without which the Earth would have become unlivable a long time back.
Ocean Current Systems
There are many global ocean current systems within which ocean currents operate. In general, oceans regulate the Earth's climate by transporting heat from one region to another. Let us take a look at the various ocean current systems.
'Thermohaline' circulation and the Atlantic Meridional Overturning Circulation (AMOC): Ocean water circulates globally as 'Thermohaline' circulation and is determined by differences in temperature (Thermo) and salinity (Haline). These differences take the warm, salty water from the equator to the poles on the surface, and cold, dense water, which sinks, moves from the poles to the equator. This is also known as the 'Great Ocean Conveyor Belt', which redistributes heat from the tropics to the poles, transports nutrients through the oceans, and takes carbon dioxide to deep oceans, where it is stored. The Atlantic Meridional Overturning Circulation (AMOC), including the Gulf Stream, is a key part of this Thermohaline circulation and keeps Western Europe warmer than it would have been. It also impacts rainfall in Africa and North and South America and moderates the climate in the North Atlantic.
Equatorial Currents and ENSO (El Niño and La Niña Southern Oscillation): This is basically the interaction between the Pacific Ocean and the atmosphere, leading to the El Niño (warm phase) and La Niña (cool phase) currents. El Niño causes warmer-than-usual sea surface temperatures in the eastern Pacific around the tropics. This leads to weakened trade winds, which, in turn, cause more rains in the Pacific but less-than-normal rains in Asia and Australia. Indian monsoons are also affected by the El Niño current. La Niña (cold phase) is the opposite and causes colder-than-usual waters in the eastern Pacific around the tropics, leading to the opposite effect and more rains in Asia/Australia. The Southern Oscillation is the atmospheric part and refers to changes in the atmospheric pressure over the Pacific and its link to changes in ocean temperatures.
The Antarctic Circumpolar Current (ACC): This is the largest and strongest ocean current circulating around Antarctica and isolates it, thus preserving the ice sheets. Apart from connecting the three largest oceans-the Pacific, Atlantic and Indian-it also plays an important part in deep-ocean circulation and nutrient mixing.
Warm and Cold Ocean Currents
Within the ocean current systems, there are warm and cold ocean currents that influence the global climate by carrying heat/cold and nutrients, as well as interacting with the atmosphere. This has implications for marine life, wind systems and rainfall across the globe.
In the Atlantic Ocean, the main warm current is the Gulf Stream and North Atlantic Drift, which carries hot waters to the Western European coast and keeps these countries warmer than they would have been. The important cold currents in the Atlantic are the Labrador Current, which carries cold water from the North Atlantic southwards and meets the Gulf Stream, influencing the climate on the North American coast. Benguela is another cold current that cools the southwest coast of Africa and preserves the major fishing zones there.
In the Pacific Ocean, the Kuroshio Current (which takes its name from the place Kuroshio in Japan) is a warm current and a twin of the Atlantic's Gulf Stream. This ocean current carries warm water to the Japanese coast and warms the East Asian region. The Humboldt (or Peru) Current is the most important cold current in the Pacific and brings cold water from Antarctica towards the Equator, cooling the west coast of the South American continent. It is also extremely important because of its interaction with ENSO, discussed above, and influences the important marine ecosystems in the region. In the years when La Niña is active, trade winds make the Humboldt Current stronger, taking it to the South American coast, causing the cold water to rise along with the nutrients and encouraging the marine ecosystem. The opposite happens in the El Niño years, which reduces phytoplankton growth and affects the marine ecosystem negatively.
In the Indian Ocean, Agulhas is the most important warm current, which carries warm water to Africa's east coast, influencing the climate there. The West Australian Current is the only cold current in the Indian Ocean, which originates from the Antarctic Circumpolar Current and flows northwards along the West Australian coast in the summer, cooling the area.
Published by HT Digital Content Services with permission from Millennium Post.