Critical layer in equatorial-tropical zone and Indian Ocean dipole

A.B. Polonsky1, 2, A.V. Torbinsky1

1Institute of Natural and Technical Systems, RF, Sevastopol, Lenin St., 28


2Branch of Moscow State University in Sevastopol, RF, Sevastopol, Geroyev Sevastopol St., 7

DOI: 10.33075/2220-5861-2019-2-88-92

UDC 551.465


   The Indian Ocean dipole (IOD) is one of the main modes characterizing the interannual variability of the parameters of large-scale interaction of the ocean and the atmosphere in the equatorial zone of the World Ocean.

   The dipole is characterized as antiphase interannual variability of characteristics of the interaction of the ocean and the atmosphere in the western and eastern parts of the equatorial zone of the Indian Ocean. IOD may occur due to the climate change in the Pacific Ocean or develop independently due to the local interaction of the ocean and the atmosphere.

   It was hypothesized that the generation of growing disturbances, without the El Nino – Southern Oscillation (ENSO) events, is possible due to the instability of the systems of zonal currents and the absorption of wave energy in the critical layer. In this layer, the phase velocity of the Rossby waves is equal to the average velocity of the zonal currents.

   In this article the features of the formation of the critical layer in the equatorial-tropical zone of the Indian Ocean as well as months in which it appears are shown. Using this information allows explaining the mechanism of generating IOD events during ENSO events.

   In the work operative reanalysis data of European Centre for Medium-Range Weather Forecasts (ECMF) were used. Monthly profiles of potential temperature, salinity, and the zonal component of the velocity of currents were selected from the ORA-S3 array for the section with coordinates of 3–14°S, 72–74°E. For each month, the average potential density and average Brent – Väisälä frequency were calculated. The phase velocity of Rossby waves was calculated using monthly average Brent – Väisälä values. Next, a comparison was made of monthly average values ​​of the phase velocity of planetary waves with average values ​​of the velocities of the zonal currents.

   The results obtained allow us to conclude that, in the period from May to November, a critical layer is formed in the vicinity of 13° S and 72–74°E, in which the phase velocity of the Rossby waves is equal to the average velocity of the zonal currents. In this layer, in the given period of the year, due to the instability of the zonal currents, generation of growing perturbations, which are the internal Indian Ocean mode not related to ENSO, is possible

Keywords: Indian Ocean dipole, critical layer, interaction between the Indian Ocean dipole events and El-Niño events of different types.

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