2002: Seasonal and interannual modulation of mixed layer variability at 08, 1108W. Deep-Sea Res

Long, high resolution time series from the 01, 1101W tropical atmosphere ocean mooring in the eastern equatorial Pacific are used to analyze how warm and cold phases of El Ni *no-Southern Oscillation (ENSO) and the annual cycle modulate the near-surface stratification and sea-surface temperature (SST) diurnal cycle. During the annual warm season (February–April), when solar warming is large and wind mixing weak, the isothermal-mixed layer depth (MLDT) is shallow (typically 10m deep) and the 1m SST diurnal cycle amplitude is large (typically up to 0.41C). Likewise during the remainder of the year when SST is generally cool, typically the diurnal cycle amplitude is less than 0.21C and the isothermal-mixed layer is deeper than 20m. Thus, annual variations in wind and insolation, which lead to an annual cycle in SST, also cause annual modulation of the SST diurnal cycle and near-surface stratification, consistent with one-dimensional mixed layer physics. However, on interannual time scales, mixed layer physics are more complicated. In particular, the diurnal cycle amplitude andMLDT anomalies are out of phase with the SST anomalies. MLDT is anomalously deep and the SST diurnal cycle amplitude is anomalously low during the warm phase of ENSO. On these longer timescales,MLDT tends to be strongly influenced by thermocline-depth variability. In addition, salinity stratified barrier layers large enough to support temperature inversions were often observed at 01, 1101W during the final stage of El Ni *nos. As SST rose above 28.51C during the final stage of the 1997–1998 El Ni *no, a regime shift wa