Abstract
An intermediate coupled model (ICM) yields a successful real-time prediction of the sea surface temperature (SST) evolution in the tropical Pacific during the 2010–12 La Niña event, whereas many other coupled models fail. It was previously identified that the thermocline effect on the SST (including vertical advection and mixing), as represented by water temperature entrained into the mixed layer (Te) and its relationship with the thermocline fluctuation, is an important factor that affects the second-year cooling in mid-late 2011. Because atmospheric wind forcing is also important to ENSO processes, its role is investigated in this study within the context of real-time prediction of the 2010–12 La Niña event using the ICM in which wind stress anomalies are calculated using an empirical model as a response to SST anomalies. An easterly wind anomaly is observed to persist over the western-central Pacific during 2010–11, which acts to sustain a horse shoe-like Te pattern connecting large negative subsurface thermal anomalies in the central-eastern regions off and on the equator. Sensitivity experiments are conducted using the ICM to demonstrate how its SST predictions are directly affected by the intensity of wind forcing. The second-year cooling in 2011 is not predicted to occur in the ICM if the easterly wind anomaly intensity is weakly represented below certain levels; instead, a surface warming can emerge in 2011, with weak SST variability. The results of the current study indicate that the intensity of interannual wind forcing is equally important to SST evolution during 2010–11 compared with that of the thermocline effect. To correctly predict the observed La Niña conditions in the fall of 2011, the ICM needs to adequately represent the intensity of both the wind forcing and the thermocline effects.
Highlights
The El Niño-Southern Oscillation (ENSO) is a natural climate phenomenon centered in the tropical Pacific, which influences climate variability and predictability worldwide
The loop of the positive thermocline feedback includes two elements, one related to the anomalous temperature of water entrained into the mixed layer (Te), which affects the sea surface temperature (SST) and is affected by the thermocline; another element is associated with wind forcing, which affects the ocean and is affected by the SST
We find that the explicit Te parameterization based on the relation with sea level (SL) in the intermediate coupled model (ICM) enhances the ability to forecast the 2010–11 La Niña events
Summary
The El Niño-Southern Oscillation (ENSO) is a natural climate phenomenon centered in the tropical Pacific, which influences climate variability and predictability worldwide. Very few coupled models make a good prediction of the 2010–11 cold SST conditions in the tropical Pacific; see the IRI web site and the summary online at http://www.nws.noaa.gov/ost/climate/STIP/r+d/ board_op4.html This difficulty represents a great challenge to the ENSO research and prediction communities, indicating a clear need to understand why so many coupled models fail to forecast the second-year cooling at a lead time of 6 months or so and to find a way to effectively improve real-time ENSO predictions. Interannual anomalies of Te and τ are determined by their statistical models, written respectively as Te = αTe·F1(SL) and τinter = ατ·F2(SST), in which F1 and F2 are the relationships between Te and SL and between τ and SST derived using statistical methods from historical data; ατ and αTe are two parameters introduced to represent their intensities (they are tunable and can be prescribed as a constant in coupled simulations) Note that this ICM has been routinely used for real-time ENSO prediction since 2003
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