Consequences of the Tropical Pacific Zonal Sea Surface Temperature Gradient Discrepancy: What Can the Cane-Zebiak Model Tell Us?

Most general circulation models (GCMs) project a weakening of the mean zonal sea surface temperature (SST) gradient across the tropical Pacific under future warming. However, this gradient has instead strengthened over time in the real world.

How does this discrepancy affect model projections of future changes to El Niño-Southern Oscillation (ENSO) events? Answering this requires an understanding of the relationship between the background state and ENSO properties (spatial pattern, amplitude, frequency, and asymmetry). The Cane-Zebiak model is particularly well-suited to this problem: since its dynamics are linearized about a specified climatological mean state, it offers a clear separation between the background state and ENSO anomalies, while also avoiding the persistent biases in the climatological mean states simulated by GCMs. In this study, we impose a series of changes to the zonal SST gradient of the mean state in the Cane-Zebiak model to develop a theory for how ENSO responds to changes in the background state of the tropical Pacific.

We find that weakening the zonal SST gradient enhances the variance of the NINO3 index, while a strengthened gradient reduces variance but increases skewness. The spatial pattern of ENSO anomalies also responds, with a stronger gradient enhancing Central Pacific activity. The substantial response to the imposed changes in our experiments indicates that the magnitude of the zonal SST gradient is a key influence on ENSO properties, implying that projections of ENSO under future climate change from models that do not reproduce the observed mean state changes should be interpreted with caution.