Seasonal and interannual modulation of mixed layer variability at 0°, 110°W

Long, high resolution time series from the 0 degrees, 110 degreesW tropical atmosphere ocean mooring in the eastern equatorial Pacific are used to analyze how warm and cold phases of El Nino-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 10 in deep) and the 1 m SST diurnal cycle amplitude is large (typically up to 0.4 degreesC). Likewise during the remainder of the year when SST is generally cool, typically the diurnal cycle amplitude is less than 0.2 degreesC and the isothermal-mixed layer is deeper than 20 m. 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 and MLDT 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 0 degrees, 110 degreesW during the final stage of El Ninos. As SST rose above 28.5 degreesC during the final stage of the 1997-1998 El Nino, a regime shift was observed, with large temperature inversions, a relative increase in SST diurnal cycle amplitude, and large variability in the mixed-layer depth. It is likely that barrier layers (inferred from temperature inversions) allowed warm conditions to remain, even as the thermocline and mixed-layer depths shoaled. (C) 2001 Elsevier Science Ltd. All rights reserved.