Australia: The Land Where Time Began

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Atlantic Ocean - Tropical Warm Events

In the eastern equatorial Atlantic sea surface temperatures (SSTs) vary from year to year. It is suggested by reanalysis data and simulations by models that some of the warm events can be driven by warm water advection from north of the Equator.

Droughts and floods are among the wide range of climatic impacts that be caused by variations of sea surface temperatures in tropical oceans. The ENSO, anomalous warmth in the eastern basin when El Niņo holds sway and cooling when La Niņa dominates, of the tropical Pacific Ocean is the main driver for interannual fluctuations of temperatures. In the eastern equatorial Atlantic Ocean there are also periods during which there are unusually warm or cold temperatures occur, their climatic impacts being most pronounced in western Africa (Xie & Carton, 2004; Reason & Rounalt, 2006; Binet, 2001). Trade wind changes and the onset of planetary-scale oceanic waves are similar to the mechanisms of the El Niņo and La Niņa  events. Atlantic Niņos is the term used for anomalous warming events in the Atlantic cold tongue. It has been shown that in addition to the El Niņo-like forcing that had been previously identified as being responsible for the temperature anomalies in the equatorial Atlantic, the advection of warm water from the north drives some Atlantic warm events (Richter et al., 2013).

The dominant mode of variability in the tropical Pacific Ocean is the El Niño/Southern Oscillation. Variation of similar magnitude occur in the tropical Atlantic Ocean, covering a range of timescales, that interact with each other. In the eastern tropical Atlantic the seasonal variation in the SSTs reach about 4oC between April to August, which is the most pronounced variation, though this seasonal cycle doesn't repeat perfectly every year. Modulations occurring from year to year result in interannual temperature anomalies associated with development and strength the cold tongue, which is an area of cooler water developing on and slightly south of the Equator in the northern summer, between the prime meridian and 20oW.

In times of the Atlantic El Niño, periods during the cold tongue is unusually warm, it is often caused by mechanisms that cause the warming related to the El Niño in the eastern Pacific (Keenlyside & Latif, 2007). A series of eastward-propagating waves, with wavelengths of the order of thousands of kilometres, are triggered by a trade wind weakening over the western equatorial basin. They push the ocean layer that separates the warm upper ocean from the cooler deep ocean down when the waves reach the cold tongue, allowing the warm water to collect in the region that is usually occupied by the cold tongue. The atmospheric gradient over the equatorial Atlantic drops when the difference in temperature between the cold tongue and the western basin declines, the trade winds then weakening still further. Warm anomalies near Angola and Namibia results from the unusual warmth propagating southwards along the coast of west Africa (Florenchie et al., 2003).  

Another mechanism has been proposed for causing warm anomalies in the region of the cold tongue (Richter et al., 2013). The use of observational and reanalysis data by Richter et al. has allowed them to identify warm events that were preceded by trade winds in the western basin that were unusually strong, though the Atlantic El Niņos require the development of trade winds that are unusually weak. According to the authors1 canonical forcing by the Atlantic Niņo cannot explain these events. Another difference between the non-canonical events and the Atlantic Niņos is that surface warming in the eastern edge of the tropical Atlantic, centred at about 15oN, precedes the non-canonical events. It has been suggested by Richter et al. that these non-canonical events can be explained by the advection of warm water from the north. According to this proposal, in the northern tropical Atlantic the wind pattern is changed by anomalous warmth and this leads to the generation of a subsurface warm anomaly just to the north of the Equator. This warm water is then advected by meridional currents into the cold tongue with the result that the equatorial sea surface temperature anomaly has the appearance of an Atlantic Niņo. The heat budget analysis of Richter et al. clearly shows the importance of this advection. Because of the weakness of the meridional currents in this region, and the convoluted pathways of water particles in the equatorial current system, the direct evidence of an adjective pathway to the Equator from the northern tropical Atlantic is weak.

It is not unprecedented for equatorial temperatures to have an influence from the northern tropical Atlantic. In the northern tropical Atlantic a warm anomaly can be an expression of the Atlantic meridional mode with a decadal-timescale climate mode that manifests itself in the sea surface temperature as a cross-equatorial gradient in sea surface temperature, as well as being a typical response of the El Niņo events. A chain of planetary-scale waves is set off by the associated equatorial wind anomaly with the result that there is cooling, followed by warming, of the Atlantic equatorial cold tongue (Foltz & McPhaden, 2010; Lübbeck & McPhaden, 2012).

It has been demonstrated that as a consequence of warm water advection from north of the Equator Atlantic Niņos can develop regardless of strong easterly winds (Richter el al., 2013). It is demonstrated by this that there a further processes, some of which are possibly still unknown, that are capable of generating anomalous warm events in the basin of the eastern tropical Atlantic and the associated climatic effects in western Africa.

Sources & Further reading

  1. Lubbecke, Joke F. "Climate Science: Tropical Atlantic Warm Events." Nature Geosci 6, no. 1 (01//print 2013): 22-23.


Author: M. H. Monroe
Last updated 18/02/2013

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