Of the three major oceans—Pacific, Atlantic, and Indian—the Indian Ocean has is the only one that is not open to the northern subtropical regions. This is a consequence of the presence of the Asian landmass restricting the Indian Ocean to south of about 25°N and hence it cannot transport heat gained in the tropics to the higher northern latitudes, as the Pacific and Atlantic oceans do, mainly via their western boundary currents. Furthermore, the Indian Ocean is the only ocean with a low-latitude opening in its eastern boundary and gains additional heat from the tropical Pacific via the Indonesian Throughflow. The unique geography has important implications for the oceanic circulation physics, and consequently for climate and the biogeochemistry of the ocean, giving the Indian Ocean many unique features. Heat is carried southward along the western coast of Australia toward the southern subtropics. The Indian Ocean consequently has a unique system of three-dimensional currents and interactions with the atmosphere that redistribute heat to keep the ocean approximately in a long-term thermal equilibrium (International Clivar Project Office 2006). Further, the strong influence of monsoon systems generates distinct seasonal variations in the upper ocean. Also, previous attempts to measure and simulate the ocean variability reveals rich spectrum of variability spanning from intraseasonal to interan-nual, decadal, and much longer time-scale phenomena. Combination and interaction among these phenomena cause significant climate variability over and around the Indian Ocean. Despite such an important role of the Indian Ocean such as monsoons, climate variability and its impact on global climate change through atmospheric and oceanic teleconnections, a long-term, sustained observing system in the Indian Ocean had not been started. This had left the Indian Ocean as the least observed ocean among the three major basins. Recognizing this observation-gap, an enthusiastic spirit emerged after the OceanObs'99 meeting, resulting in the development of a plan for the Indian Ocean Observing System (IndOOS) under the coordination of the CLIVAR/GOOS Indian Ocean Panel (Meyers and Boscolo 2006). The schematic diagram of IndOOS and the regional Observing system is shown in Fig. 3.7.
The outstanding research issues that need to be addressed with observations to advance the understanding of the role of the Indian Ocean in the climate system and its predictability are (1) Seasonal monsoon variability and the Indian Ocean, (2) In-traseasonal variability, (3) Indian Ocean zonal dipole mode and El Nino-Southern Oscillation, (4) Decadal variation and warming trends in the upper Indian Ocean, and (5) Southern Indian Ocean and climate variability, (6) Circulation and the Indian Ocean heat budget (Indonesian Throughflow, shallow and deep overturning cells), (7) Biogeochemical cycling in the Indian Ocean and (8) Operational oceanography. The status of each element of IndOOS is briefed below.
Indian Ocean Observing System (IndOOS)
Indian Ocean Observing System (IndOOS)
»a i, ! 0 Real-time and near real-time tide qauqe network
' (including the tsunami buoy network)
Process Studies (roo^ Regional Ocean Observing Systems
Fig. 3.7 Schematic chart of IndOOS. Fixed location in-situ observations of IndOOS are indicated in detail, the argo and surface drifters scatter widely within the Indian Ocean, and the satellite measurements cover surface observation of the whole area
The basin-scale mooring array is essential for understanding and identifying their limits of predictability of the role of the ocean in the Monsoon Intraseasonal oscillation (MISO) and Madden-Julian Oscillation (MJO), which are long lasting weather patterns that evolve in a systematic way for periods of four to eight weeks. The intense, long-lasting weather conditions associated with MISO and MJO interact strongly with the temperature and salinity structure of the ocean mixed layer, but the physics is not yet understood nor is it fully built into coupled models. The role of surface currents in the evolution of intraseasonal variation is not known. The air-sea heat and freshwater fluxes are poorly known. The array will provide vital information on these processes. It is also needed to understand the mixed-layer dynamics and the role of currents in interannual variations, such as the IOD. Operational ocean-state estimation, such as the production of daily maps of currents and thermal structure for marine industry and defence, is not possible without the array. While this report is primarily concerned with oceanographic measurements, the meteorological measurements (particularly at moorings) will be extremely valuable to data assimilation issues concerned with weather forecasting and reanalysis efforts.
The sub surface mooring array, called the Research moored Array for African-Asian-Australian Monsoon Analysis and prediction (RAMA) (McPhaden et al. 2009b) consists of a total of 46 moorings, of which 38 are ATLAS/TRITON-type surface moorings. Seven of these surface moorings are selected as surface flux reference sites, with enhanced flux measurements. The surface mooring system can measure temperature and salinity profiles from the surface down to 500 m depth as well as the surface meteorological variables, and the observed data is transmitted in real-time via Argos satellites. In addition to these surface buoys, there are five subsurface ADCP moorings along the equator to observe current profiles in the upper equatorial ocean, and three deep current-meter moorings with ADCPs in the central and eastern equatorial regions. The RAMA array design was evaluated and supported by observing system simulation experiments (Oke and Schiller 2007; Vecchi and Harrison 2007). The array has been implemented rapidly in recent years, largely through bi-national activities involving Japan, India, USA, Indonesia, China, France, Holland and South Africa.
Early observations of this mooring provide invaluable data set for analyses on the Indian Ocean variability (Masumoto et al. 2009 and references therein). For example, a long-term current observation at 90°E on the equator reveals that there is significantly large amplitude intraseasonal variability both in the zonal and meridional components as well as the well-known semiannual and annual variations. Also, RAMA mooring data used to capture subsurface evolution of the three consecutive Indian Ocean Dipole event from 2006 to 2008, with clear negative temperature anomaly at the thermocline depth that appeared a few months before the surface signatures of the IOD events. Mooring data were also used to observe the oceanic response to cyclone "Nargis", which made landfall in Myanmar on 2 May 2008. Intense ocean mixing and significant turbulent heat loss from the ocean surface (~600°W/m2) occurred as Nargis passed near RAMA buoy at 15°N, 90°E in the Bay of Bengal (McPhaden et al. 2009c). Surface moorings from the RAMA array also allowed process studies of the strong upper ocean response to the MJO in the Seychells-Chagos Thermocline Ridge region (Vialard et al. 2008).
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