Tropical Pacific Climate and Its Response to Global Warming in the Kiel Climate Model

M Latif,W Park,A Ströh,N Keenlyside,R Redler,G Madec,E Roeckner

doi:10.1175/2008jcli2261.1

Abstract

Abstract A new, non-flux-corrected, global climate model is introduced, the Kiel Climate Model (KCM), which will be used to study internal climate variability from interannual to millennial time scales and climate predictability of the first and second kind. The version described here is a coarse-resolution version that will be employed in extended-range integrations of several millennia. KCM’s performance in the tropical Pacific with respect to mean state, annual cycle, and El Niño–Southern Oscillation (ENSO) is described. Additionally, the tropical Pacific response to global warming is studied. Overall, climate drift in a multicentury control integration is small. However, KCM exhibits an equatorial cold bias at the surface of the order 1°C, while strong warm biases of several degrees are simulated in the eastern tropical Pacific on both sides off the equator, with maxima near the coasts. The annual and semiannual cycles are realistically simulated in the eastern and western equatorial Pacific, respectively. ENSO performance compares favorably to observations with respect to both amplitude and period. An ensemble of eight greenhouse warming simulations was performed, in which the CO2 concentration was increased by 1% yr−1 until doubling was reached, and stabilized thereafter. Warming of equatorial Pacific sea surface temperature (SST) is, to first order, zonally symmetric and leads to a sharpening of the thermocline. ENSO variability increases because of global warming: during the 30-yr period after CO2 doubling, the ensemble mean standard deviation of Niño-3 SST anomalies is increased by 26% relative to the control, and power in the ENSO band is almost doubled. The increased variability is due to both a strengthened (22%) thermocline feedback and an enhanced (52%) atmospheric sensitivity to SST; both are associated with changes in the basic state. Although variability increases in the mean, there is a large spread among ensemble members and hence a finite probability that in the “model world” no change in ENSO would be observed.

Highlights

Coupled air–sea feedbacks in the tropical Pacific influence its annual mean state, annual cycle, and interannual variability
Kiel Climate Model (KCM) consists of the European Centre for MediumRange Weather Forecasts (ECMWF) Hamburg atmospheric general circulation model version 5 (ECHAM5; Roeckner et al 2003) atmospheric general circulation model (AGCM) coupled to the Nucleus for European Modeling of the Ocean (NEMO; Madec et al 1998; Madec 2008) ocean–sea ice general circulation model, with the Ocean Atmosphere Sea Ice Soil version 3 (OASIS3; Valcke 2006) coupler
In this paper we introduced the newly developed Kiel Climate Model (KCM) and described its performance in the tropical Pacific and response to global warming

Summary

Introduction

Coupled air–sea feedbacks in the tropical Pacific influence its annual mean state, annual cycle, and interannual variability. There is a wide range of model behavior in the tropical Pacific that is reflected in both climatology and variability This applies to the response of the tropical Pacific to global warming (see, e.g., the recent studies by van Oldenborgh et al 2005 and Guilyardi 2006). The model is effectively the generation of the scale interactions experiments (SINTEX) family (Gualdi et al 2003; Guilyardi et al 2003; Luo et al 2005) of models, as each component has been updated to the most recent version This is the first paper describing KCM, and results using a relatively coarse model resolution are presented, with a focus on the tropical Pacific.

Model and experiments

Climatology

ENSO variability

Findings

Summary and discussion