Identification of formation mechanisms of Indian Ocean dipole

by

A.B. Polonsky, A.V. Torbinskii A.V., A.V. Gubarev

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

E-mail: apolonsky5@mail.ru

DOI: 10.33075/2220-5861-2020-2-13-18

UDC 551.465

Abstract:

   The Indian Ocean dipole (IOD) is one of the main regional mechanisms of heat redistribution in the Indian Ocean. In its extreme phases, the dipole manifests itself in intense anomalies in ocean surface temperature and precipitation in the western and eastern parts of the tropical zone of the Indian Ocean. According to recent studies, there are two types of dipole. The first type directly depends on IOD events. The existence of a second non-ENSO type of IOD is usually associated with seasonal climatic variability. Earlier was hypothesize that independent generation of IOD events as an internal mode can arise due to the instability of the system of zonal flows. This instability can occur in the critical layer. In this layer, the phase velocity of Rossby waves coincides with the zonal velocity of average currents. This article refines the characteristics of the critical layer using modern re-analysis data on the potential temperature, salinity and velocity of zonal currents south of the equator in the Indian Ocean. the use of more advanced data will make it possible to obtain more accurate values ​​of the phase velocity of Rossby waves and average zonal currents and, as a result, to clarify the region of formation and the depth of the critical layer in the tropical zone of the Indian Ocean.

   The use of more advanced data will make it possible to obtain more accurate values ​​of the phase velocity of Rossby waves and average zonal currents and, as a result, to clarify the region of formation and the depth of the critical layer in the tropical zone of the Indian Ocean on an average annual and monthly average scale. In work used operative reanalysis ORAS5 data of European Centre for Medium-Range Weather Forecasts (ECMWF) on potential temperature, salinity, and the zonal component of the current velocity for the period 1979 – 2018. Monthly profiles of potential temperature, salinity, and the zonal component of the velocity of currents selected from the ORAS5 array for the section with coordinates of 3.5 – 20.5°S, 45 – 100°E. For each month, the average potential density, average Brent – Väisälä frequency was calculated. The phase velocity of Rossby waves was calculated using monthly average Brent – Väisälä values. Next, a comparison was made of monthly mean values of the phase velocity of planetary waves with average values of the velocities of zonal currents. The coordinates and depths of those points where the phase velocities are equal to the average velocity zonal flows are calculated using interpolation. Thus, was localized a critical layer.

   The results obtained allow us to conclude that critical layer formed on 11 – 12°S in the western part of the pool (49 – 50°E). In this layer, the phase velocity of Rossby waves is equal to the average velocity of zonal currents. It is here that instability in the system of zonal flows is possible. These results will make it possible to distinguish IOD events of different types in the future.

Keywords: Indian Ocean dipole, critical layer, Rossby wave.

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