Distinct 17- and 33-Day Tropical Instability Waves in Subsurface Observations*

Abstract

Tropical instability waves (TIWs) within a half-degree of the equator in the Pacific Ocean have been consistently observed in meridional velocity with periods of around 20 days. On the other hand, near 5°N, TIWs have been observed in sea surface height (SSH), thermocline depth, and velocity to have periods near 30 days. Tropical Atmosphere-Ocean (TAO) Project moored equatorial velocity and temperature time series are used to investigate the spatial and temporal structure of TIWs during 3 years of La Niña conditions from 1998 through 2001. Along 140°W, where the TIW temperature and velocity variabilities are at their maxima, these variabilities include two distinct TIWs with periods of 17 and 33 days, rather than one broadbanded process. As predicted by modeling studies, the 17-day TIW variability is shown to occur not only in meridional velocity at the equator, but also in subsurface temperature at 2°N and 2°S, while the 33-day TIW variability is observed primarily in subsurface temperature at 5°N. These two TIWs, respectively, are shown to have characteristics similar to a Yanai wave/surface-trapped instability and an unstable first meridional mode Rossby wave. One implication of such a description is that the velocity variability on the equator is not directly associated with the dominant 33-day variability along 5°N.

Observed Dispersion Relation of Yanai Waves and 17-Day Tropical Instability Waves in the Pacific Ocean

“…While signals of Yanai waves for the wide range of the dispersion curve is not as clear as in the SSH data, broad peaks at around a period of ~17 days and wavelength ~1500 km are evident. These frequencies and wavenumbers are consistent with those associated with TIWs detected from in-situ observations (e.g., Halpern et al 1988;Qiao and Weisberg 1995;Lyman et al 2007). Also, the result is consistent with a previous study suggesting that 17-day TIWs have characteristics similar to an unstable Yanai wave (Lyman et al 2007).…”

“…Since then, many studies identified their generation and propagation, and discussed their characteristics and forcing mechanism in both the ocean and atmosphere (e.g., Wunsch and Gill 1976;Weisberg et al 1979;Itoh and Ghil 1988;Zhu et al 1998). A particular interest to the present study is the in-situ data analysis which shows the structure of tropical instability waves (TIWs) with a ~17-day period that is consistent with a surface trapped Yanai wave (Lyman et al 2007).…”

“…Mixed Rossby-Gravity waves (often referred to as Yanai waves) play an important role in a variety of phenomena in the tropical ocean and atmosphere (e.g., Yanai and Hayashi 1969;Zangvil 1975;Liebmann and Hendon 1990;Takayabu and Nitta 1993;Lyman et al 2007). The dispersion relation and the structure of Yanai waves were derived as one of the eigenmodes of a linearized shallow water equations on an equatorial β-plane (Matsuno 1966).…”

SMOSSea Surface Salinity signals of tropical instability waves

“…In addition, the dominant westward propagation speeds of SSS and SST 17 day signals is 0.6 m/s at 4 N, whereas they are 1.5 m/s at 2 S (not shown). This is consistent with the 17 day TIWs characteristics, which are similar to equatorially trapped Yanai wave with maximum signals between 2 S and 2 N [Lyman et al, 2007;Shinoda, 2012]. At the equator, east of 120 W, the appearance of the fast 1.5 m/s westward 17 day propagations mostly occurs after July (Figures 8b, 8d, 9b, and 9d), at the same time as the appearance of the 33 days poleward propagation (see the latitude-time Hovm€ oller diagrams in Figure 10).…”

The influence of salinity on tropical Atlantic instability waves

“…These tropical instability waves (TIWs) are generated from barotropic and baroclinic instabilities associated with the horizontal and vertical shear of equatorial current systems and related density gradients [e.g., Philander, 1978;Weisberg and Weingartner, 1988;McCreary and Yu, 1992;Masina et al, 1999;Grodsky et al, 2005;Jochum et al, 2004;Lyman et al, 2007;von Schuckmann et al, 2008]. The magnitudes of TIW-related variability as seen from different oceanic parameters exhibits regional and temporal variations, varying on seasonal to interannual time scales in association with changes in the equatorial current systems.…”