Large-scale estimation of transport from the Pacific to the Indian Ocean

The objective of this model-data intercomparison is to determine the feasibility of deriving an useful index for fluctuations in the Pacific to Indian Ocean throughflow volume transport. Due to insufficient direct observations and the present limitations in numerical models, accurate estimation of variations in the throughflow transport on seasonal to interannual timescales is not yet possible; however, an index based on weighted, monthly mean sea level anomalies in different regions of the western Pacific and eastern Indian Oceans is presented. Numerical model results and sea level from the TOPEX/POSEIDON altimeter show that the large-scale pressure gradient forcing of the throughflow is controlled by the Pacific Ocean side on interannual timescales, and by a combination of Indian Ocean and Pacific Ocean processes on seasonal to annual timescales. The model throughflow is maximum in boreal summer (11 Sv) and minimum in boreal winter (4 Sv) with a 9-year mean of 7.4 Sv. These values are within the range of various estimations of throughflow transport, and they agree in phase. Of the 7.4 Sv model transport, almost 1.8 Sv is due to direct, local wind forcing (based on Ekman calculations). Interannual fluctuations from El Ni\~no-Southern Oscillation activity are associated with increases in throughflow transport during cold events and decreases in transport during warm events. Using empirical orthogonal function analysis and results from previous studies, an index of the throughflow variability is developed using model sea level and model transport. Sea level in four regions is found to be sufficient to index the model throughflow variations: south of Java, northwest of Australia, in the Pacific warm pool and off the coast of the Philippines. A regression technique applied to the model sea level at these locations yields an index which correlates with the model throughflow at 0.83. The same weights applied to sea level in similar regions from the TOPEX/POSEIDON altimeter correlate with the model throughflow transport at a level of 0.78.