A diagnostic model for the large-scale tropical circulation based on moist static energy balance

Abstract

In this study we present a diagnostic model for the large-scale tropical circulation (vertical motion) based on the moist static energy equation for first baroclinic mode anomalies (MSEB model). The aim of this model is to provide a basis for conceptual understanding of the drivers of the large-scale tropical circulation changes or variations as they are observed or simulated in Coupled Model Inter-comparison Project (CMIP) models. The MSEB model is based on previous studies relating vertical motion in the tropics to the driving forces of the tropospheric column heating rate, advection of moisture and heat, and the moist stability of the air columns scaled by the first baroclinic mode. We apply and evaluate the skill of this model on the basis of observations (reanalysis) and CMIP model simulations of the large-scale tropical vertical motion. The model is capable of diagnosing the large-scale pattern of vertical motion of the mean state, annual cycle, interannual variability, model-to-model variations and in warmer climates of climate change scenarios; it has spatial correlations of 0.6–0.8 and nearly unbiased amplitudes for the whole tropics (30° S–30° N). The skills are generally better over oceans at large scales and worse over land regions. For the interannual variation of zonally anomalous and zonal mean circulation in tropical Pacific region, it has temporal correlations ~ 0.8. The model also tends to have an upward motion bias at higher latitudes, but still has good correlations in temporal and spatial variations even at the higher latitudes. It is further illustrated how the MSEB model sensitivities can be used to determine the mechanisms in the models that are responsible for the mean state, seasonal cycle and interannual variability such as El Nino. The model clearly illustrates that the seasonal cycle in the circulation is driven by the incoming solar radiation and that the El Nino shift in the Walker circulation results mainly from the sea-surface temperature changes. Overall, the model provides a powerful diagnostic tool to understand tropical circulation change on larger and longer (> month) time scales.