1st PART








The TAV (Tropical Atlantique Variability) it's the fluctuation of all of the following variations : sea surface temperature (SST), sea-level pressure (SLP), circulation of Hadley of the intertropical convergence zone (ITCZ), and the wind direction changes. The event usually takes place in March, April and may (MAM). All this is dominated by interannual and decadal changes.

The TAV is often called "El Niño of the Atlantic" and is associated with the trade winds as El Niño in the Pacific. Depending on the strength of the trade winds, Southeast, it warms the ocean water alternately southward to Ecuador, then north and back south of the equator.

The Tropical Atlantic Oceanographic index are :

  • The TNA (Tropical North Atlantic) (15-80°W, 6-22°N)

  • The TSA (Tropical South Atlantic) (15°E-35°W 22-2°N)


Click here to see an animation of the
Tropical Atlantic SST anomaly (in kelvin)



Le TNA à gauche et le TSA à droite. Les rectangles en
tirés indiquent où sont calculé les indices du TNA et du
TSA. Les petits cercles jaunes indiquent les positions
d'amarrage de PIRATA [Servain et autres, 1998 ]

The curve of the Tropical Atlantic gradient index, formed by
the difference between the index of the SST of Tropical North
Altantic (TNA) and the Tropical South Atlantic (TSA).



Unlike El Niño or La Niña where their oscillation is from West to East or vice versa, the TAV is from North to South or vice versa.

In response to SST anomaly of the TNA and the TSA, an anomaly of the wind is developed near the Equator due to the gradient of atmospheric pressure at sea level. Due to the Coriolis force, the equatorial South winds evolve in an anomaly of the south-westerly winds in North of the tropical areas of the Atlantic, while in the South of the tropical areas of the Atlantic anomaly is on the southeast winds. This reduces the trade winds in the North East of the TNA and increases in the Southeast of the TSA. Then the heat flow of the ocean surface is increased in the Northern hemisphere due to a reduction caused by a decrease in the speed of the winds. And for the opposite reason, the heat flow is decreased in the Southern Hemisphere. Then the unstable air interaction is opposed by negative reactions. A suitable balance between the positive and negative reactions then causes an oscillation of approximately 12-13 years.

It has been proven that the NAO (North Atlantic Oscillation) has effects on the TNA and so excite the TAV but isn't condition previously required for the variation of the Tropical Atlantic. Moreover TAV also has impact on the NAO according to the variability of NADW (North Atlantic Deep Water) and stability of the MOC ( Meridionale Ocean Circulation of Atlantic) so the thermohaline circulation due to the advection of the SSS (sea surface salinity) anomaly in north of the hot water sector of the MOC .

ENSO has a considerable influence on the Tropical Atlantic variability during the boreal spring. ENSO can increase the variability to both the north and south of Tropical and Equatorial Atlantic. However, this influence is associated with the reaction of the ocean - atmosphere and would be much less effective if this reaction is absent. Approximately 30% of the variation of the TNA index may be explained by the effect of ENSO and 30% by the NAO.

The SST oscillations of the TNA are associated with a natural variation model known as the AMO (Atlantic Multidecadal Oscillation).


The important anomalies of precipitation in the Sahel (Lamb, 1978) in the North East of Brazil (Hastenrath and Heller, 1977) and many others, are related to the variation of the Tropical Atlantic.

The Tropical North Atlantic (TNA) in 15-80°W 6-22°N, is the region where the hurricanes and tropical storms develop. Therefore the variation of the SST of TNA has effects on the number of hurricanes. In 2005 (June to October) it was very high with an anomaly of 33,62°F (+0,90°C) which caused a lot of records on the tropical hurricanes and storms in Atlantic (number, power, size, damage...). Of course it isn't the only phenomenon to influence the number of hurricanes in the North Atlantic.

This graph is made and updated with the NOAA data for the
TNA index and with Unisys Weather for the number of hurricanes.

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