Climate fluctuations of tropical coupled systems — The role of ocean dynamics

Abstract

The tropical oceans have long been recognized as the most important region for large-scale ocean-atmosphere interactions, giving rise to coupled climate variations on several time scales. During the TOGA decade, the focus of much tropical ocean research was on understanding El Niño-related processes and on development of tropical ocean models capable of simulating and predicting El Niño. These studies led to an appreciation of the vital role the ocean plays in providing the memory for predicting El Niño and thus making seasonal climate prediction feasible. With the end of TOGA and the beginning of CLIVAR, the scope of climate variability and predictability studies has expanded from the tropical Pacific and ENSO-centric basis to the global domain. In this paper we discuss the progress that has been made in tropical ocean climate studies during the early years of CLIVAR. The discussion is divided geographically into three tropical ocean basins with an emphasis on the dynamical processes that are most relevant to the coupling between the atmosphere and oceans. For the tropical Pacific, we assess the continuing effort to improve our understanding of large- and small-scale dynamics for the purpose of extending the skill of ENSO prediction. We then go beyond the time and space scales of El Niño and discuss recent research activities on the fundamental issue of the processes maintaining the tropical thermocline. This includes the study of Subtropical Cells (STCs) and ventilated thermocline processes, which are potentially important to the understanding of the low-frequency modulation of El Niño. For the tropical Atlantic, we examine the dominant oceanic processes that interact with regional atmospheric feedbacks as well as the remote influence from both the Pacific El Niño and extratropical climate fluctuations, giving rise to multiple patterns of variability distinguished by season and location. We also discuss the potential impact of Atlantic thermohaline circulation on Tropical Atlantic Variability (TAV). For the tropical Indian Ocean, we examine local and remote mechanisms governing low-frequency sea-surface temperature variations. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, we focus on the active role of ocean dynamics in a seasonally locked east-west internal mode of variability, known as the Indian Ocean Dipole (IOD). We discuss influences of the IOD on climatic conditions in Asia, Australia, East Africa and Europe. While the attempt throughout is to give a comprehensive overview of what is known about the role of the tropical oceans in climate, the fact of the matter is that much remains to be understood and explained. The complex nature of the tropical coupled phenomena and the interaction among them argue strongly for coordinated and sustained observations, as well as additional careful modeling investigations in order to further advance our current understanding of the role of tropical oceans in climate.