Interannual variability of the upper layer of the tropical Atlantic Ocean from in situ data between 1979 and 1999

Interannual variability of the upper layers of the tropical Atlantic is described based on in situ data. An objective analysis used all available temperature observations of the upper tropical Atlantic between 1979 and 1999 to construct a 4D database. Wind data are used to investigate potential mechanisms which might explain the observed variability. Four remarkable events are described: 1953-1984, 1988-1990, 1994-1995 and 1997-1998. Three of them are characterised as equatorial (1953-1984, 1994-1995, and 1997-1998). The 1988-1990 event is a basin-wide phenomenon which does not involve the same mechanisms as the other three. Results of statistical decomposition in empirical orthogonal functions (EOFs) are discussed. There is no evidence of an inter-hemispheric mode on the depth of the 20 degreesC-isotherm (D20) and heat content comparable to the observed mode for sea surface temperature (SST) fields. Most energetic patterns for D20 and heat content are dominated by the stronger variability in the northern part of the basin. Influences of other climate signals are investigated. Correlations between the winter NAO (North Atlantic Oscillation) index and our standard variables is marginally significant. A positive NAOW (North Atlantic Oscillation of Winter) is associated with SST cooling in a latitude band between 10 degreesN and 20 degreesN. When applied to the El-Nino index, correlations are much more significant. We found two scales of maximum correlation: at the four month lag after the El-Nino mature phase when the thermocline slope and zonal heat content gradient are maximum along the equator, and at the ten month lag after the mature phase of El-Nino when the thermocline slope weakens and the equatorial gradient of heat content vanished. The correlation with a zonal wind index (average between 30 degreesW-35 degreesW and 2 degreesN-2 degreesS) has been computed. Correlation is maximum at the six month lag when the thermocline slope and the zonal heat content gradient are maximum in the equatorial band. This "Atlantic Nino" mode is influenced by the Pacific Ocean's variability and reaches a maximum one year after a warm event in the eastern Pacific.