Nonlinear Response of Summertime Synoptic-Scale Disturbance Intensity over the Tropical Western North Pacific to ENSO Amplitude

Abstract

El Ni & ntilde;o - Southern Oscillation (ENSO) exhibits nonlinearity in its amplitude and impacts. This study investigates the dependence of summertime synoptic -scale disturbance (SSD) intensity over the tropical western North Paci fi c (TWNP) on the ENSO amplitude. A tendency of nonlinearity exists in the observed response of the TWNP SSD intensity to the amplitude of tropical central -eastern Paci fi c (CEP) sea surface temperature (SST) anomalies in boreal summer. Numerical experiments are conducted with an atmospheric general circulation model with linearly varying tropical CEP SST anomalies imposed to illustrate the nonlinearity exclusively induced by changes in the ENSO amplitude. A linear increase in the amplitude of El Ni & ntilde;o - like SST anomalies results in a nonlinear enhancement of SSD intensity over the TWNP, manifested as the increase in SSD intensity at a rate larger than expected by linear response with an eastward shift. This is attributed to the nonlinear intensi fi cation of anomalous ascent over the TWNP induced by tropical convection response to positive tropical CEP SST anomalies and the nonlinear effect of anomalous convection on the synoptic -scale activity. In contrast, as La Ni & ntilde;a - like SST anomalies increase linearly, the SSD intensity over the TWNP decreases at a rate slower than expected from a linear response and even reaches saturation with little longitudinal shift. Due to the thermodynamic control on the occurrence of deep convection in the tropics, enhanced negative SST anomalies do not induce additional changes in anomalous descent over the tropical CEP. Thus, the TWNP SSD intensity no longer decreases with further increase in tropical CEP cold SST anomalies.