INTRODUCTION: Special Issue on Indian Ocean Climate

Abstract

In the past, great advances in Indian Ocean research have emerged after expeditions by research vessels such as the Discovery in the 1930s or international cooperative observational programs such as the International Indian Ocean Expedition (1960–65) and ship surveys during the World Ocean Circulation Experiment in the mid-1990s. However, Saji et al. (1999) and Webster et al. (1999), in two letters to Nature, with an introduction by Anderson (1999), used existing observational data to bring attention to an air–sea interaction phenomenon across the tropical Indian Ocean. It involved abnormally high sea surface temperatures in the west during the southwest monsoon, and large cold anomalies off Sumatra and associated easterly equatorial wind anomalies that last through the fall. This La Nina–like pattern tends to make the Indian Ocean SST pattern be more like the pattern in Pacific Ocean and Atlantic Ocean tropical regions. The terms Indian Ocean dipole (IOD) mode or Indian Ocean zonal mode (IOZM) have been used for this climate event, and the extent to which this mode is independent from ENSO quickly became controversial. In spite of the controversy, or rather perhaps because of it, the IOD/IOZM discovery has generated an unprecedented interest in Indian Ocean air–sea interaction. The 24 papers in this special issue reflect this revival of interest in the Indian Ocean region. Many of these papers are based on presentations given at the Climate Variability and Predictability Studies (CLIVAR) workshop hosted by the International Pacific Research Center in Honolulu, Hawaii, from 29 November to 3 December 2004. The first group of papers addresses the decadal and interannual variability in the Indian Ocean and its relation to El Nino and La Nina. Part of the controversy as to which extent IOD and ENSO are related depends on the definition of these events. Meyers et al. propose a new method to classify El Nino, La Nina, and positive and negative IOD events and use that method to identify independent and co-occurring events from 1876 to 1999, setting the stage for the subsequent papers in this group. In the paper by Tozuka et al., a 200-yr run of a coupled ocean–atmosphere model is analyzed for decadal variations of sea surface temperature anomaly (SSTA) and heat content over the Indian Ocean. Modulation of an El Nino–like basin-scale SSTA pattern and an independent interannual IOD pattern are found. In the following paper by Song et al., a similar decadal modulation is seen. They explore the feedbacks between oceanic heat content and the atmosphere in a 250-yr-long coupled atmosphere–ocean model over the Indian Ocean with emphasis on composite El Nino and IOD events. Co-occurrence of IOD and El Nino results in particularly strong anomaly patterns. Huang and Shukla, in two papers, use ensemble coupled ocean–atmosphere model runs to determine the link between remote and regional forcing in the Indian Ocean. It is