Acknowledgements: TAO Project Office

The work I will be presenting today on the seasonal and interannual modulation of mixed layer variability uses data from the 0 110W TAO mooring which is a very interesting data set, not only because of it was the first TAO mooring and therefore has one of the longest record, but also because it has historically been enhanced with additional sensors, such as current meters, thermistors with increased vertical resolution, radiometers, rain gauges, etc... Recently, Mike McPhaden's graduate student, Weimin Wang, used these data to compute the seasonal and interannual heat balance. This analysis I will be presenting today, is complementary to their analysis. Billy and I have recently submitted the manuscript to Deep Sea Research.

This figure shows the time-longitude plot of equatorial band SST and wind speed. Time extends from 1986 at the top to the present at the bottom. Dots at the top and bottom represent the mooring sites. The 0 110W mooring extends years prior to this. During the Aug 1997 deployment at 0 110W, there were thermistors at 5 m intervals from the surface to 60 m. So this is our starting point in the analysis. Notice that this is during the final stages of the 1997-98 El Nino, when warm water extended across the entire Pacific basin along the equator and the stronger trade winds were shifted eastward.

This slide shows hourly averages of some of the variables measured at 0 110W during the August 1997-February 1998 deployment: SST, 1 m to 5 m temperature stratification, wind speed, rain rate, sea surface salinity, mixed layer depth (estimated as the depth where the temperature is 0.5C cooler than at the surface), and the depth of the 20C isotherm depth, which represents the thermocline.

Notice that as SST rose above 28C, a regime shift occurred: with increased rainfall, weaker winds, and an increased diurnal cycle in SST and near-surface stratification. Also, if you look closely at the 2nd panel, you'll see that at times, the 5 m temperature was WARMER than the 1 m SST. These grey stripes represent times when large (0.2C) temperature inversions, lasting longer than 24 hours occurred. These temperature inversions must be supported by a salinity stratification, and therefore imply the presence of a barrier layer. Indeed, during these events, SSS were anomalously low. Periods with temperature inversions represent only a subset of the time that barrier layers were present here since isothermal barrier layers cannot be detected without subsurface salinity.

In summary, as the SST rose above 28C, the east Pacific began to resemble the western Pacific warm pool, both in its mean structure and high-frequency variability.

How does this regime shift relate to seasonal and interannual variability? how prevalent are these conditions in the long record?

This figure shows the long time series (1980-1998) temperature inversions estimated from the daily averaged data. The dotted line is the Southern Oscillation Index -- negative values indicate El Nino warm event, positive values indicate La Nina cold events. The stripe across the top indicates periods when we can do this calculation.

Temperature inversions, and thus barrier layers, are not exclusive of El Ninos, although they were most prevalent during the final stages of the 2 large El Ninos of the century (1982-83 & 1997-98). In particular, as in the 1997-98 El Nino discussed earlier, in almost every case, the inversions occurred in months December - March.

Since barrier layers tend to inhibit surface cooling due to mixing, we might speculate that these inversions and barrier layers might have helped extend the surface warm conditions, even as the thermocline was returning to its normal shallow state.

The regime shift also showed an increase in the SST diurnal cycle amplitude. However, was this because of the El Nino or because of the approaching vernal equinox, when the noontime sun is directly overhead at the equator?

To address this, Billy did a diurnal cycle complex demodulation on the hourly SST. Essentially, this fits a sinusoid to the diurnal cycle and allows the amplitude and phase to vary in time. The amplitude is about half the peak-to-peak diurnal variation. This diurnal cycle amplitude time series is shown as the dark line in the top panel. As we all know, you can get a large diurnal amplitude on calm (low wind speed) sunny days. The dotted line is windspeed and there's a strong anticorrelation as expected.

This relation holds on both seasonal (shown in the bottom panel) and interannual time scales (shown in the middle panel). In March, when the sun is overhead and wind speeds are typically weak at the equator, the diurnal amplitude is about 0.4C. The rest of the year it is about half that value. Likewise, on interannual timescales there is a +/- 0.1 variation, with higher diurnal cycle amplitude during La Nina cold events when skies are clear and the trades are shifted westward so that winds at 0 110W are weak.

In this slide we have compared the diurnal amplitude to the wind speed. However, there are many implications in regard to the surface heat fluxes, SST, and near surface stratification.

[MEGHAN: NOW MOVE THIS SLIDE TO THE 2nd PROJECTOR]

This figure is based on Weimin Wang's dissertation. In this he shows various commonly used seasonal climatologies of the net surface heat flux at 0 110W, including the net surface heat flux measured using 0 110W TAO data and the COARE bulk algorithm. Notice that there is a large spread and different phasings. Of particular note is that despite the austral equinox in September, there is not a 2nd peak in the TAO net surface heat flux, due to the reduction in the solar radiation by the stratus. One might expect that if there was a strong surface warming in later months (as some of these other products show), then there might be an increase in the SST diurnal cycle at those times. But there isn't. The phasing of the TAO flux (maximum in March) is consistent with the observed seasonal modulation of the SST diurnal cycle.

On another note... because latent and sensible heat flux depend upon SST, the diurnal cycle in these fluxes might also have similar seasonal and interannual modulation.

Other implications....

This figure shows the long timeseries of OLR, ws, SST, SST - 10 m temperature, MLD, and Z20. The seasonal cycles and +/- 95% confidence levels are shown in the right panels.

The OLR and Z20 don't have significant seasonal cycles. However, the other variables have a seasonal cycle that is simliar in phase to the diurnal cycle: During March, the WARM season, SST is high, winds are weak, there is a large SST diurnal cycle (as we have already shown), and the top 10 m is highly stratified causing the mixed layer to be shallow. These phasings suggest that on seasonal time scales, the mixed layer is decoupled from the thermocline and the surface heat balance is dominated by top-down processes.

In this figure, the seasonal cycle has been removed from the long time series to highlight the interannual variability. The SOI is shown over the SST. During ENSO warm events, winds are stronger at 0 110W, and during the final stages, there are deep convective clouds. Consequently the SST diurnal cycle is weak. The reverse occurs during La Nina cold events. Recall on seasonal time scales the near surface stratification and mixed layer depth were in phase with the SST diurnal cycle, consistent with top-down processes. This is also true on interannual time scales. However, on interannual time scales, the mixed layer is also affected by the large-scale variations in the thermocline depth, that work in the same direction as the top-down processes to make the mixed layer shallow during La Nina cold events and deep during El Nino warm events.

Conclusion

* Regime shift during 1997-98 ENSO. As SST rose above 28.5C, eastern Pac resembled WP

* Temperature inversions (barrier layers) during final stages of El Nino (implications: extend warming?)

* SST diurnal cycle has seasonal and interannual modulation...
(implications for MLD, Q0, Qlat, Qsen, C02 flux)

Meghan F. Cronin Pacific Marine Environmental Laboratory 7600 Sand Point Way NE Seattle, WA 98115 USA

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