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Zonal heat advection can be estimated from the moored velocity data at 110°W (Figure 5) and a measure of the zonal temperature gradient, T.
The vertically averaged zonal velocity of the mixed layer is u. This velocity was estimated using the gridded, low-pass-filtered time series shown in Figure 5b, using the mixed layer depth shown in Figure 5a. A time series of T was estimated from the moored temperature measurements at 110°W and 140°W (the 140°W time series are discussed in MH). Since the local temperature gradient at 110°W may be poorly estimated by this large-scale temperature difference, we also computed the zonal heat advection assuming a constant T = -6.6 × 10°C km which is the time averaged gradient between 110°W and 140°W observed by the moored array. The time series of the two estimates of zonal advection are shown in Figure 6b. There is little difference between the constant and time-varying T cases, indicating that most of the change in zonal advection is associated with zonal velocity fluctuations. It should be noted that uncertainty in the zonal temperature gradient can affect the amplitude but is unlikely to affect the direction of the zonal heat advection. On the time scales considered here, SST increases to the west.
In boreal spring (March-May) the normally westward surface current in the eastern Pacific reverses (Figure 5) and warm water is advected into the region. This eastward current nearly coincides with the maximum SST; hence the change in mixed layer heating (Figure 6a) and the zonal heat advection tend to be out of phase. In January-February of each year, the mixed layer was warming while the zonal advection was tending to export heat from the region. This effect was particularly pronounced in December 1986 to April 1987, when the heat content of the mixed layer was increasing by up to 100 W m (in January) and the zonal advection was persistently trying to cool the region. In April 1987, SST reached a maximum and the mixed layer heat content began to decrease; at this time the zonal surface current switched direction and the advective heat flux was warming the eastern Pacific.
The only extended period when mixed layer heating and zonal advection were in phase was in August to December 1986, during the onset of warm conditions in the eastern Pacific. MH pointed out that the zonal surface current during this period was anomalously weak (although still westward) and could contribute towards the development of an SST anomaly by reduced cooling. The mixed layer at this time was at its deepest level (Figure 5 a) and the vertically averaged mixed layer velocity was actually eastward. This contributed to the observed warming; however, the total rate of mixed layer heating was far greater than could be accounted for by zonal advection alone. On average, the mean zonal advective heat flux was -3.5 W m and the standard deviation was 15 W m.
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