A.B. Polonsky, A.V. Torbinskii, A.V. Gubarev
Institute of Natural and Technical Systems, RF, Sevastopol, Lenin St., 28
The aim of this work is to study the impact of the instability of the system of zonal currents on the generation of the Indian Ocean dipole (IOD). For that, the cases of a critical layer occurrence in the southern part of the equatorial-tropical zone of the Indian Ocean within certain months for the period 1979–2018 are identified. In this layer, the phase velocity of neutral waves is equal to the average velocity of zonal currents, and the generation of unstable growing disturbances is most probable.
In the work, the operative reanalysis ORAS5 data of European Centre for Medium-Range Weather Forecasts (ECMWF) on vertical distribution of potential temperature, salinity, and the zonal component of the current velocity for the period 1979 – 2018 were used. Monthly profiles of potential temperature, salinity, and the zonal component of the current velocity were selected from the ORAS5 array for the sections situated between 7.5 – 15.5°S, 50 – 100°E. By these data for each month, using the standard theory of planetary waves, the phase velocity of the lowest baroclinic mode of the Rossby long waves was calculated and the critical layer was determined. For each critical layer, its length was determined. The obtained time series of the critical layer length were compared to the variability of dipole mode index (DMI).
It is shown that most cases of the critical layer occurrence take place in spring, one or two months before the onset of the positive IOD events. This indicates that the presence of instability in the system of the zonal currents can be a reason of the generation of IOD and the asymmetry of the amplitude of the dipole mode index between positive and negative events.
Keywords: Indian Ocean dipole, system of zonal currents, Rossby wave, critical layer.
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