The 40-50-day variability of sea level and thermocline in the equatorial Atlantic during 2002-2003

Satellite and in situ observations show strong 40-50-day spectral peaks in sea level and thermocline across the equatorial Atlantic Ocean during 2002-2003, a period when continuous PIRATA data are available in several locations along the equator. The 40-50-day variability of sea level and thermocline dominates the 20-40-day variations of the tropical instability waves (TIWs) in 3oS-3oN equatorial region. Data analysis combined with a hierarchy of OGCM and linear model (LM) experiments are used to understand the dynamics of the 40-50-day oscillations. The models are forced by daily QuikSCAT winds.

Results from the OGCM and LM suggest that the 40-50-day sea surface height anomaly (SSHA) and thermocline variability within 3oS-3oN are equatorial Kelvin waves forced by the 40-50-day winds, which also exhibit 40-50-day spectral peaks along the Atlantic equator for the period of interest in both QuikSCAT and PIRATA data. The SSHA and thermocline variations are determined from the first and second baroclinic mode Kelvin waves excited by the zonal wind stress, with the second baroclinic mode playing a more important role. Away from the equator (3o-5oN), oceanic variability at 40-50-day periods result mainly from the TIWs. Further analysis is needed to understand the origin of the 40-50-day winds, themocline variability on SST and its feedback to winds. This study may have an important implication to Atlantic Nino of 2002 (R. Fu, person. commu., 2006), when strong intraseasonal thermocline variability precedes its onset.