Impact of diurnal atmosphere–ocean coupling on tropical climate simulations using a coupled GCM Yoo-Geun Ham Æ Jong-Seong Kug Æ In-Sik Kang Æ Fei-Fei Jin Æ Axel Timmermann Received: 7 October 2008 / Accepted: 21 April 2009 / Published online: 12 May 2009 Ó Springer-Verlag 2009 Abstract The impacts of diurnal atmosphere–ocean (air– sea) coupling on tropical climate simulations are investi- gated using the SNU coupled GCM. To investigate the effect of the atmospheric and oceanic diurnal cycles on a climate simulation, a 1-day air–sea coupling interval experiment is compared to a 2-h coupling experiment. As previous studies have suggested, cold temperature biases over equatorial western Pacific regions are significantly reduced when diurnal air–sea coupling strategy is imple- mented. This warming is initiated by diurnal rectification and amplified further by the air–sea coupled feedbacks. In addition to its effect on the mean climatology, the diurnal coupling has also a distinctive impact on the amplitude of the El Nino-Southern Oscillation (ENSO). It is demon- strated that a weakening of the ENSO magnitude is caused by reduced (increased) surface net heat fluxes into the ocean during El Nino (La Nina) events. Primarily, decreased (increased) incoming shortwave radiation during El Nino (La Nina) due to cloud shading is responsible for the net heat fluxes associated with ENSO. Keywords Diurnal coupling Á Coupled GCM Á ENSO Á Tropical climate simulation 1 Introduction Since coupled GCMs become indispensable tools for studying and predicting a variety of atmospheric and oce- anic phenomena, a number of studies have been devoted to improve the performance of coupled GCMs (Arakawa and Schubert 1974; Betts and Miller 1986; Randall et al. 1992; Suarez et al. 1983; Williamson and Olson 1994; Wu and Yanai 1994; Yu et al. 1994; Neelin and Yu 1994; Mcwil- liams and Gent 1980; Mellor and Yamada 1974; Moorthi and Suarez 1992; Griffies et al. 2000; Pacanowski and Philander 1981; Delworth et al. 2006). However, accurate modeling of the climate mean state and its variability is an extremely difficult challenge until now. Some studies suggest that the deficiency of GCMs in terms of repro- ducing climate processes originates from deficiencies in the parameterization of atmospheric convection and related processes (Neale and Slingo 2003; Lee et al. 2007). From an oceanic perspective, modeling sub-grid scale turbu- lence, and mixing, via eddy parameterizations are likely to be primary sources for model errors (Noh and Kim 1999; Mellor and Blumberg 2004). In addition, inaccurate air–sea coupling strategies could be another source of model deficiencies. For example, coupled model suffers serious climate drifts, if heat, momentum, and water fluxes are not conserved globally. Hence, air–sea coupling has to be represented as accurately as possible. There have been many studies to improve air–sea cou- pling strategies. For example, allowing for a more accurate representation of air–sea momentum exchanges, Luo et al. (2005) significantly reduces the equatorial SST cold bias of their coupled GCM. In addition, methods for calculating momentum drag coefficients have evolved from using simple bulk formula to complex methods that capture the influence of surface waves (Moon et al. 2003; Powell et al. Y.-G. Ham Á I.-S. Kang School of Earth and Environment Sciences, Seoul National University, Seoul, Korea J.-S. Kug Á F.-F. Jin Á A. Timmermann School of Ocean and Earth Sciences and Technology, University of Hawaii, Hawaii, USA J.-S. Kug (&) Korea Ocean Research and Development Institute, Ansan, Korea e-mail: jskug@kordi.re.kr 123 Clim Dyn (2010) 34:905–917 DOI 10.1007/s00382-009-0586-8