INTRODUCTION
Sea surface temperature variability in the eastern equatorial Pacific occurs primarily within four bands: a diurnal cycle associated with daytime warming and nighttime cooling, a 20-40 day band associated with tropical instability waves, an annual cycle which is dominant despite the fact that the sun crosses the equator twice per year, and a 2-7 year band associated with the El Niño/Southern Oscillation (ENSO) cycle. There has been considerable work showing cross-scale interactions at the lower end of the spectrum, such as the phase locking between the ENSO cycle and the annual cycle and phase locking with tropical instability waves. However, cross-scale interaction with the high end of the spectrum has not been as explored. Understanding how high frequency mixed layer processes are modulated by, and in turn affect lower frequency seasonal and interannual variability is crucial for developing correct parameterizations of these typically subresolution processes.

PROJECT GOALS
In this study we investigate how the warm and cold phases of ENSO and the seasonal cycle modulate and are in turn affected by high frequency mixed layer processes in the eastern equatorial Pacific at 0° 110°W. Specific questions addressed by the study include:

* Are the SST diurnal cycle, short-lived temperature inversions, and near surface stratification modulated by the seasonal and ENSO cycles? If so, what are the processes which control their low frequency modulation?

* During El Niño warm events, how similar is the eastern equatorial Pacific to the western Pacific warm pool? Do mixed layer physics and atmospheric/oceanic boundary layer coupling observed in the warm pool during COARE apply to the eastern equatorial Pacific during El Niños?

* In the eastern equatorial Pacific where the thermocline is relatively shallow, when and when doesn’t mixed layer depth variability track thermocline depth variability? How is the mixed layer depth affected by local processes which create the SST diurnal cycle, short-lived temperature inversions, and near surface stratification?

* What does the low-frequency modulation of the SST diurnal cycle, near surface stratification, and temperature inversions say about the roles of mixing and solar-induced thermal restratification in the seasonal and ENSO cycles?

METHODOLOGY
Because the analysis requires both long and highly resolved surface and near surface time series, our analysis uses data primarily from the 0° 110°W buoy of the Tropical Atmosphere-Ocean (TAO) array. The 0° 110°W buoy in the eastern equatorial Pacific has data extending back to 1982 and has routinely been enhanced with extra instrumentation. In particular, during the peak stage of the 1997-1998 El Niño, the 0° 110°W mooring was instrumented with additional thermistors in the upper 60 m, as well as with a shortwave radiometer, a rain gauge, and a conductivity sensor to measure sea surface salinity. Recently, Wang and McPhaden (1999, 2000a, 2000b) evaluated the seasonal and interannual surface heat budgets using the 0° 110°W data set. Our analysis of the seasonal and interannual modulation of the mixed layer variability is thus complementary to the Wang and McPhaden analyses.

RESULTS AND ACCOMPLISHMENTS
Complex demodulation analyses show that the SST diurnal cycle is modulated both by the seasonal cycle and the ENSO cycle (Fig. 1). During the annual warm season (February-April) the SST diurnal cycle typically has an amplitude larger than 0.4°C, while at other times of the year it is generally less than 0.2°C. The ENSO cycle caused approximately +/- 0.1°C variations in the SST diurnal amplitude, with higher amplitudes during La Niña cool events, and lower amplitudes during El Niño warm events. On both the seasonal and interannual time scales, the diurnal cycle amplitude is approximately 180° out of phase with wind speed variability and solar insolation. Thus, anomalously large diurnal cycle requires both clear skies and light winds, conditions typically present in the far eastern Pacific during the annual warm season and during La Niña cool events.

As might be expected, during periods with a strong diurnal cycle, a large surface stratification developed each afternoon that affected the stratification even on monthly time scales (Fig. 2). Monthly averaged data show a temperature difference of more than 0.5°C between the 1 m and 10 m sensors during the warm season, indicating that on average the mixed layer depth was shallower than 10 m. In contrast, for much of the rest of the year, there was typically a 0.2°C temperature difference and the mixed layer depth (defined in terms of a 0.5°C temperature step from the surface) was on average below 20 m. The seasonal cycle in mixed layer depth is out of phase with the thermocline depth variability and appears to be controlled by local surface heating and turbulent processes, processes which also contribute to the SST diurnal cycle. On interannual time scales the mixed layer depth is highly correlated (0.9) with thermocline depth variability (Fig. 3). While this is probably due to the influence of the thermocline variability on the background buoyancy of the upper water column, it is interesting to note that the local restratification and mixing processes, as inferred by the diurnal cycle modulation, would also produce mixed layer depth variability with similar phasing.

Although there were limited salinity measurements, barrier layers large enough to support temperature inversions were often observed at 0° 110°W during the final stage of the El Niños (Fig. 4). As SST rose above 29°C during the final stage of the 1997-98 El Niño a regime shift was observed (Fig. 5), with large temperature inversions (sometimes up to 80 m thick), a relative increase in SST diurnal cycle amplitude, and large variability in the mixed layer depth. Although the rising thermocline was ultimately responsible for the mixed layer shoaling during the termination of the El Niño, it is likely that the barrier layer formation allowed the warm conditions to remain longer than might otherwise be expected.

FUTURE WORK
Recently, as a part of the PACS Eastern Pacific Investigation of Climate Processes (EPIC), the easternmost TAO line at 95°W has been enhanced with additional moorings (at 3.5°N, 10°N, and 12°N) and with additional sensors (Fig. 6). Thus similar types of surface and mixed layer analyses, will be done at all 10 moorings along 95°W. It is expected that the meridional migration of the ITCZ, the occasional appearance of a double ITCZ, and the formation of stratus over the southern moorings will modulate the mixed layer variability in different ways. Understanding this modulation is critical for understanding the ocean-atmosphere coupling in the cold tongue-ITCZ complex and the formation of meridional gradients in this region.

PUBLICATIONS RESULTING FROM THIS WORK

Cronin, M. F., and W. S. Kessler, Seasonal and interannual modulation of mixed layer variability at 0°,110°W. To be submitted to Deep Sea Res. II.

INSTITUTION
NOAA / Pacific Marine Environmental Laboratory
7600 Sand Point Way NE; Bldg. #3
Seattle WA 98115 USA

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