The effects of low-frequency spiciness anomalies on the El-Niño-Southern Oscillation

Abstract

Low-frequency subsurface spiciness anomalies generated in the northern and southern subtropics of the Pacific Ocean advect geostrophically westward and equatorward where they often enter the equatorial undercurrent (EUC). Assuming the spiciness anomalies in each hemisphere are generated independently, the temperature gradient across the pycnocline, ∂T/∂ᵖ, may vary significantly on decadal time scales in the EUC as a result of convergent spiciness anomalies. Observations of ∂T/∂ᵖ from an eight-year time series of Argo float profile data show variability around the mean value of approximately 10% in the warm pool region where the EUC forms. Decadal variability may be significantly larger. As spiciness anomalies propagate east in the EUC, they may affect the sea surface temperature in the cold tongue region, which is sensitive to the subsurface temperature and vertical displacements of the pycnocline. Using a modified Zebiak-Cane ENSO model, the effect of ∂T/∂ᵖ variability on ENSO is studied. Results show that ∂T/∂ᵖ variability in the NINO3 region similar to that shown by Argo observations has a significant affect on ENSO variance; as ∂T/∂ᵖ increases (decreases), ENSO variance increases (decreases). This result is primarily due to an increase in the growth rate of ENSO resulting from an increase in the sensitivity of the thermocline feedback and the non-linearity of the system. These results indicate that for accurate forecasting of ENSO, low-frequency spiciness variability in the subtropical and western equatorial Pacific should be considered.