Warning: This writeup was done to help organize
my thoughts prior to the presentation.
The actual presentation may have been quite different!
Acknowledgements: TAO Project Office
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?
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.
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]
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.
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.
* 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)
Note that CO2 flux estimates are sensitive to SST and what one calls SST. Can 5 m or 10 m temperature be used as the SST? During the warm season and during La Nina cold events there is strong diurnal warm layer.
Meghan F. Cronin
Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle, WA 98115 USA
Meghan Cronin's Home Page
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