The Impact of Convection on ENSO: From a Delayed Oscillator to a Series of Events

Abstract

The NCAR Community Climate System Model, version 3 (CCSM3) exhibits persistent errors in its simulation of the El Niño-Southern Oscillation (ENSO) mode of coupled variability. The amplitude of the oscillation is too strong, the dominant 2-yr period too regular, and the width of the sea surface temperature response in the Pacific too narrow, with positive anomalies extending too far into the western Pacific. Two changes in the parameterization of deep convection result in a significant improvement to many aspects of the ENSO simulation. The inclusion of convective momentum transport (CMT) and a dilution approximation for the calculation of convective available potential energy (CAPE) are used in development integrations, and a striking improvement in ENSO characteristics is seen. An increase in the periodicity of ENSO is achieved by a reduction in the strength of the existing "short-circuited" delayed-oscillator mode. The off-equatorial response is weaker and less tropically confined, largely as a result of the CMT and an associated redistribution of zonal momentum. The Pacific east-west structure is improved in response to the presence of convective dilution and cooling provided by increased surface fluxes. The initiation of El Niño events is fundamentally different. Enhanced intraseasonal surface stress variability leads to absolute surface westerlies and a cooling-warming dipole between the Philippine Sea and western Pacific. Lag-regression analysis shows that intraseasonal variability may play a significant role in event initiation and maintenance as opposed to being a benign response to increased SSTs. Recent observational evidence appears to support such a leading relationship.

Uncertainty quantification and parameter tuning in the CAM5 Zhang‐McFarlane convection scheme and impact of improved convection on the global circulation and climate

“…modifications [Gregory et al, 1997;Neale et al, 2008;Richter and Rasch, 2008;Neale et al, 2010]. Here we introduce several equations to provide a proper context for the important parameters being evaluated and optimized in this study.…”

“…A dilute CAPE with an entraining parcel has been introduced in the ZM convection scheme to bring more sensitivity of the CAPE calculation to tropospheric humidity [Neale et al, 2008].…”

“…[13] In CAM5, the deep convective process is parameterized by the ZM deep convection scheme, which was originally developed by Zhang and McFarlane [1995], with additional modifications [Gregory et al, 1997;Neale et al, 2008;Richter and Rasch, 2008;Neale et al, 2010] (see more details in Appendix A). Some convection parameterizations [e.g., Emanuel and Zivkovic-Rothman, 1999;Donner et al, 2001] explicitly include contributions to precipitation from cloud elements usually labeled as stratiform clouds, including components that are not explicitly associated with precipitation in vigorous up and downdrafts.…”

Nonlinear Controls on the Persistence of La Niña*

“…For example, ENSO exhibited a biennial selfsustained oscillation in CCSM3, a previous version of the model. However, since version 3.5, CCSM represents ENSO as a series of events, owing to improved simulation of the meridional extent of the wind anomalies and their associated ocean response (Neale et al 2008). In the real world, warm events also appear to arise from a stable basic state excited by stochastic forcing (Penland and Sardeshmukh 1995;Thompson and Battisti 2001;Kessler 2002 Jin et al 2007); however, our choice of white Gaussian noise with a 1-month decorrelation time scale produces an adequate simulation of the CCSM4 ENSO, given the simplicity of the model.…”

How well do coupled models replicate ocean energetics relevant to ENSO?

“…For instance, several studies use simplified ENSO models driven by coupled GCM outputs to understand the simulated ENSO properties or changes thereof van Oldenborgh 2006, 2009;Dewitte et al 2007;Jin et al 2006;Kim and Jin 2010). Others use coupled GCM experiments to understand the effect of specific parameterizations on the simulated ENSO (Meehl et al 2001;Neale et al 2008;Guilyardi et al 2009b;). …”