The biennial variability of Indian Ocean heat flux

The interannual variability of Indian Ocean heat transport is studied using an intermediate, two-and-a-half-layer thermodynamically active ocean model, with the mixed layer imbedded within the upper model layer. The model is run for 41 years (1958-1998) with 5 days output, and is forced by NCEP reanalysis fields, producing an extended time series of the components of the heat budget of the northern Indian Ocean. The anomaly time series possesses variability in interannual time-scales (from 1.5 to 4 years), including a strong biennial signal. The signal is stronger in the southern ocean in the first model layer, and in the northern ocean in the second layer. The codependence between the heat transport and Ekman transport interannual variability is investigated. A strong coherency in a broadband between 1 and 2.5 years in the southern ocean, and a very weak coherency in the northern ocean was found. The biennial signal is found to be the strongest from 1973 to 1988. In 1970 there is a shift toward lower frequency variability (about 4 years), followed by a gradual return to higher frequencies around 1975. A similar shift is observed in the Ekman transport time series south of the Equator. In addition, the relationship between the variability of the heat transport (north-south variability) and the Indian Ocean dipole index (east-west variability) was studied. The dipole variability appears to be most closely related to the second layer heat transport in the south, as well as Ekman transport south of the equator.