Examination of the Two Types of ENSO in the NCEP CFS Model and Its Extratropical Associations

Seon Tae Kim,Jin-Yi Yu,Hui Wang,Arun Kumar

doi:10.1175/mwr-d-11-00300.1

Abstract

Abstract Two types of El Niño–Southern Oscillation (ENSO) simulated by the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) model are examined. The model is found to produce both the eastern Pacific (EP) and central Pacific (CP) types of ENSO with spatial patterns and temporal evolutions similar to the observed. The simulated ENSO intensity is comparable to the observed for the EP type, but weaker than the observed for the CP type. Further analyses reveal that the generation of the simulated CP ENSO is linked to extratropical forcing associated with the North Pacific Oscillation (NPO) and that the model is capable of simulating the coupled air–sea processes in the subtropical Pacific that slowly spreads the NPO-induced SST variability into the tropics, as shown in the observations. The simulated NPO, however, does not extend as far into the deep tropics as it does in the observations and the coupling in the model is not sustained as long as it is in the observations. As a result, the extratropical forcing of tropical central Pacific SST variability in the CFS model is weaker than in the observations. An additional analysis with the Bjerknes stability index indicates that the weaker CP ENSO in the CFS model is also partially due to unrealistically weak zonal advective feedback in the equatorial Pacific. These model deficiencies appear to be related to an underestimation in the amount of the marine stratus clouds off the North American coasts inducing an ocean surface warm bias in the eastern Pacific. This study suggests that a realistic simulation of these marine stratus clouds can be important for the CP ENSO simulation.

Highlights

Recent studies have suggested that two different types of interannual sea surface temperature (SST) variability can be observed in the tropical Pacific (Larkin and Harrison 2005a; Yu and Kao 2007; Ashok et al 2007; Kao and Yu 2009; Kug et al 2009)
We examine the properties of the eastern Pacific (EP) and central Pacific (CP) types of El Nino–Southern Oscillation (ENSO) produced by the Climate Forecast System (CFS) model, with particular emphasis on the CP ENSO and its association with extratropical atmospheric variations
It is concluded that the CFS model is able to simulate both the CP and EP types of ENSO with realistic spatial patterns and temporal evolution

Summary

Introduction

The CP type of ENSO has been suggested to produce different climate impacts from the conventional EP type (e.g., Larkin and Harrison 2005a,b; Kao and Yu 2009; Weng et al 2009; Kim et al 2009; Yeh et al 2009; Mo 2010). Kim (2010a), who analyzed observed CP El Nino events since the 1960s to show that CP El Nino events can occur during times when the mean thermocline depth along the equatorial Pacific is above, below, or near normal, and does not show a consistent behavior These studies suggested that the generation of the CP ENSO does not rely on the thermocline variations necessary for the delayed oscillator (Schopf and Suarez 1988; Suarez and Schopf 1988; Battisti and Hirst 1989) or the recharge oscillator (Jin 1997). A major coupled climate model not included in the CMIP3 analysis is the global coupled atmosphere–ocean Climate Forecast System (CFS) model developed at the National Centers for Environmental Prediction (NCEP), which is used for operational seasonal climate prediction The performance of this model in ENSO simulations has been documented and examined by several studies.

Model and data

Two types of ENSO in the CFS

Extratropical associations of the simulated CP ENSO

Findings

Summary and discussion