Edvin Aldrian Æ Dmitry Sein Æ Daniela Jacob Lydia Du¨menil Gates Æ Ralf Podzun Modelling Indonesian rainfall with a coupled regional model Received: 11 May 2004 / Accepted: 16 September 2004 / Published online: 14 May 2005 Ó Springer-Verlag 2005 Abstract Long-term high-resolution coupled climate model simulations using the Max Planck Institute Re- gional Climate Model and the Max Planck Institute Ocean Model have been performed with boundary forcings from two reanalyses: firstly from the European Centre for Medium-Range Weather Forecasts, and secondly from the joint reanalysis of the National Cen- ters for Environmental Prediction and the National Center for Atmospheric Research. This study employs a special coupling setup using a regional atmospheric model and a global ocean model. The latter model ap- plies a special conformal grid from a bipolar orthogonal spherical coordinate system, which allows irregular positions of the poles and focuses on the detail over the Maritime Continent. The coupled model was able to simulate stable and realistic rainfall variabilities without flux correction and at two different ocean resolutions. The coupled system is integrated for a period between 1979 and 1993 and the results are then compared to those from uncoupled runs and from observation. The results show improved performance after coupling: a remarkable reduction of overestimated rainfall over the sea for the atmospheric model and of warm SST biases for the ocean model. There is no significant change in rainfall variability at higher ocean model resolution, but the ocean circulation shows less transport variability within the Makassar Strait in comparison to observa- tions. 1 Introduction The maritime continent, Indonesia, is the largest archi- pelago and mostly covered by ocean. Study concerning the temporal and spatial variation of rainfall of Indo- nesia using a regional climate model (RCM) has been reported by Aldrian et al. (2004). One problem in sim- ulating rainfall of the region is the appropriate land–sea representation (Aldrian et al. 2003). The area is highly complex with large ocean coverage and chains of islands. Intense ocean atmosphere interactions take place at the ocean surface in this most convective region of the world. Due to large ocean areas, such processes will be important in modeling the climate of the region, because the local sea surface temperature (SST) is among the major factors that drive rainfall variability across the Maritime Continent (e.g., Nicholls 1979; Hackert and Hastenrath 1986; Hendon 2003; Aldrian and Susanto 2003). With a stand alone (uncoupled) atmospheric or ocean model, such processes cannot be simulated ade- quately. The uncoupled atmospheric model uses the spatially and temporally prescribed and interpolated SST, while the uncoupled ocean model uses the ocean surface fluxes calculated using empirical formulae. Such configurations disregard dynamical interactions that occur at the ocean surface. An integrated or coupled ocean/atmosphere model gives more realistic dynamics close to the ocean surface, where ocean atmospheric exchanges take place at higher frequency determined by the coupling setup. Regional climate studies using a coupled ocean/atmosphere model for the maritime continent are relatively new. Our approach is to use a high-resolution, regional atmospheric model coupled to an ocean model with an This paper has not been published or considered by any other journal in any language. E. Aldrian Æ D. Sein Æ D. Jacob Æ L. D. Gates Æ R. Podzun Max Planck Institut fu¨r Meteorologie, Bundesstraße 55, 20146 Hamburg, Germany E. Aldrian (&) Agency for the Assessment and Application of Technology, BPPT, Jakarta, Indonesia E-mail: edvin@bppt.go.id Tel.: +49-40-41173313 Fax: +49-40-41173391 L. D. Gates National Science Foundation, 4201 Wilson Blvd, Arlington, VA 22230, USA Climate Dynamics (2005) 25: 1–17 DOI 10.1007/s00382-004-0483-0