Semiannual Variations in 1, 000‐dbar Equatorial Indian Ocean Velocity and Isotherm Displacements from Argo Data. Issue 12 (24th December 2019)

• Record Type: Journal Article
• Authors: Zanowski, Hannah; Johnson, Gregory C.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 12(2019)
• Page Start: 9507
• Page End: 9516
• Publication Date: 2019-12-24
• Subjects: equatorial Indian Ocean -- semiannual velocity variability -- Argo -- Rossby waves -- isotherm displacements; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015342
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: The semiannual Rossby wave in the equatorial Indian Ocean is analyzed using 1, 000‐dbar horizontal drift and 3‐D isotherm displacement data from Argo floats. The strong semiannual variations in zonal velocity associated with this wave that have been previously studied are confirmed here, and a more complete picture of the wave is provided with the addition of the 1, 000‐dbar meridional velocities and 0‐ to 2, 000‐dbar isotherm displacements. At 1, 000 dbar, semiannual variations in each field are consistent with Rossby wave physics: Zonal velocity is maximal on the equator, isotherm displacement and meridional velocity extrema are colocated with equatorially symmetric zonal velocity phase reversals near ±2.5° in latitude, and all three fields propagate westward over the year. The latitudinal structure of the meridional velocities in the central and eastern Indian Ocean hints at Rossby wave energy in higher meridional modes than the first, but that signature is not apparent in the isotherm displacements or the zonal velocities. With upward and westward phase propagation and downward and westward energy propagation, isotherm displacement vertical sections confirm the presence of a vertically propagating wave that reaches to at least 2, 000 dbar, the deepest extent of the Argo floats. Plain Language Summary: The currents in the equatorial Indian Ocean vary on many different timescales, from months to decades. Changes that occur semiannually, or twice a year, are particularly strong in this region. At the surface, the currents vary in this way because the Asian monsoon causes the winds to reverse direction semiannually. Below the surface, changes in the currents are due in part to long, planetary‐scale waves, known as Rossby and Kelvin waves, that travel both horizontally and vertically in the ocean, in this case trapped along the equator. Here semiannual changes in tropical Indian Ocean currents at 1, 000 m are examined using velocity and isotherm displacement (changes in the depth of constant temperature surfaces) data from Argo floats. In agreement with earlier studies, these data show that Rossby waves are responsible for much of the prominent semiannual variation in the deep east‐west velocities and isotherm displacements. The floats also allow detection of the much subtler semiannual reversals in north‐south velocities on either side of the equator associated with these Rossby waves. By exploring isotherm displacement changes in depth versus both longitude and latitude, it is also shown that the Rossby wave travels vertically, as expected. Key Points: Equatorial Indian Ocean semiannual velocity and isotherm displacement variability are analyzed using Argo data Zonal and meridional velocity at 1, 000 dbar as well as isotherm displacements are consistent with Rossby wave physics The 3‐D isotherm displacement structure suggests a vertically propagating Rossby wave