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

Abstract

Long, high resolution time series from the 0°, 110°W tropical atmosphere ocean mooring in the eastern equatorial Pacific are used to analyze how warm and cold phases of El Niño-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 ( MLD T ) is shallow (typically 10 m deep) and the 1 m SST diurnal cycle amplitude is large (typically up to 0.4°C). Likewise during the remainder of the year when SST is generally cool, typically the diurnal cycle amplitude is less than 0.2°C 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 MLD T anomalies are out of phase with the SST anomalies. MLD T is anomalously deep and the SST diurnal cycle amplitude is anomalously low during the warm phase of ENSO. On these longer timescales, MLD T 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°, 110°W during the final stage of El Niños. As SST rose above 28.5°C during the final stage of the 1997–1998 El Niño, 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.