Description: Understanding and predicting the interannual variations of the whole monsoon climate system has been, and will continue to be, one of the major reasons for studying the oceanography of the Indian Ocean; but there are other reasons. Knowledge about Indian Ocean current systems may have diverse practical applications, from fisheries through search and rescue to management of Exclusive Economic Zones. Our discussion mainly concerns the open ocean and the climate applications, but the results are important for most continental shelves of the Indian Ocean region on all but the shortest timescales. We start by discussing what we know now of the Indian Ocean’s mean annual cycle, painfully gleaned from sparse observations over the last four decades. This data base for understanding the interannual variability of the Indian Ocean climate has not been adequate until very recently; however, this data base is in the process of expanding radically, due to the availability of four new tools. These are: satellite data (altimeter, wind stress); surface flux products, from weather forecast reanalyses; output of fine-scale numerical models, driven with those fluxes; and data from profiling floats. As we will see in various talks, this is revolutionising our understanding of variability in the Indian Ocean. CLIVAR’s Asian-Australian Monsoon Panel is starting to plan a programme of further observations, to coincide with a useful conjunction of observation satellites in 2003. This will be aimed at filling the larger remaining gaps in our understanding of Indian Ocean dynamics, (with emphasis on understanding its role in the monsoon cycle).

Description: Published

Description: We explore the path between Indian Ocean observations and monsoon dynamics, the societal impacts of interannual climate variations and applications of resource predictions in southeastern Africa, the Mascarene Islands, India, southeast Asia and Australia. Recent progress in understanding ocean dynamics associated with SST variation is reviewed. The global El Niño-Southern Oscillation (ENSO) affects monsoon winds and ocean temperatures in a manner consistent with, but lagging, the Pacific. The ENSO influence often propagates across the tropical Indian Ocean from Africa to Indonesia, modulating the tropospheric moisture flux over the Indian Ocean and rainfall in surrounding continents. An east-west dipole in SST anomalies and monsoon rainfall is identified and related to the atmospheric Walker Cell. It appears partially in response to global ENSO conditions during build-up phase (July-Nov.). The eastern ‘node’ is confined near Sumatra, whilst the western centre of action extends from the Maldives to the Seychelles Islands. Correlations indicate that the strength of ENSO in the Indian Ocean region has decreased in recent decades, while large scale, spatial and temporal patterns suggest independent variations of the Indian Ocean. Apart from annual variations of the monsoon and year-to-year fluctuations of climate, short-term weather events have a dramatic impact on countries around the Indian Ocean. Recent floods in southern (2000) and eastern (1997- 98) Africa and southeast Asia (1998) are related to SST patterns and localised atmospheric responses. Predictions of the future availability of food and water resources, and short-term forecasts of storm events are some of the decision tools that can be offered through information from ocean data. The close relationship between regional SST anomalies and impacts on the food and water resources of surrounding countries provides a strong motivation for sustained observations in the Indian Ocean. A specific design plan for the observational network is proposed, linking eastern and western efforts in an efficient manner.

Description: The Leeuwin Current, a warm, poleward flowing eastern boundary current, dominates the surface circulation off the west coast of Australia and has profound influence on regional marine ecosystem and fisheries recruitment. In this study, the seasonal and interannual variations of upper ocean heat balance in the Leeuwin Current region are analyzed by using an eddy-resolving numerical model simulation, as a first step to quantify the climate impacts on regional ocean thermodynamics and marine ecosystem. The volume transport and heat advection of the Leeuwin Current are stronger during the austral winter on the seasonal cycle and are stronger during a La Nina event on the interannual scale. On both seasonal and interannual timescales, the mixed layer heat budget off the west coast of Australia is predominantly balanced between the variations of the Leeuwin Current heat advection and heat flux across the air-sea interface. On the interannual timescale, the variation of the Leeuwin Current heat advection tends to lead that of the air-sea (latent) heat flux by two months, which is likely a reflection of advection timescales of the Leeuwin Current and its eddy field. The interannual variation of the average February–April sea surface temperature off the west coast of Australia, which is crucial for the larval settlement of western rock lobster, is mostly influenced by the Leeuwin Current heat advection as well as the ocean memory from the previous austral winter, with the air-sea heat exchange playing a buffering role.

Description: 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 Tropical Ocean Global Atmosphere (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 Climate Variability and Prediction (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 the progress that has been made in tropical ocean climate studies during the early years of CLIVAR is discussed. 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, the continuing effort to improve understanding of large- and small-scale dynamics for the purpose of extending the skill of ENSO prediction is assessed. This paper then goes beyond the time and space scales of El Niño and discusses 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, the dominant oceanic processes that interact with regional atmospheric feedbacks are examined 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. The potential impact of Atlantic thermohaline circulation on tropical Atlantic variability (TAV) is also discussed. For the tropical Indian Ocean, local and remote mechanisms governing low-frequency sea surface temperature variations are examined. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, this study focuses on the active role of ocean dynamics in a seasonally locked east–west internal mode of variability, known as the Indian Ocean dipole (IOD). Influences of the IOD on climatic conditions in Asia, Australia, East Africa, and Europe are discussed. 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 the current understanding of the role of tropical oceans in climate.