Effects of the PDO phase on the tropical belt width

Abstract

Recent studies have shown that the Tropical Belt (TB) has progressively expanded since at least the late 1970´s. This trend has been largely attributed to the radiative forcing due to GHG increase and stratospheric ozone depletion, even if an influence of Sea Surface Temperature (SST) anomalies has been also suggested. In this work we investigate the impact of the Pacific Decadal Oscillation (PDO) on the TB width. The study is performed by using both AMIP (Atmospheric Model Intercomparison Project) and idealized simulations, produced by NCAR/CAM3 GCM, and reanalysis data (ERA-Interim, ERA-40 and MERRA). Reanalysis show that a switch of the PDO from a positive to a negative phase can lead to a significant TB expansion during the equinoxes. This effect, indicating a possible PDO contribution to the widening that characterized the TB width during the last decades, is not correctly reproduced by model simulations. Deficiencies in the sensitivity of model simulated convective processes to SST anomalies are suggested as a possible cause of the TB widening underestimation.