Intraseasonal variability near 10°N in the eastern tropical Pacific Ocean

Abstract

[1] New in situ observations from 10°N, 125°W during 1997-1998 show strong intraseasonal variability in meridional velocity and sea surface temperature. The 50-to 100-day oscillations in sea surface height (SSH) have long been recognized as a prominent aspect of oceanic variability in the region of 9-13°N in the eastern Pacific Ocean. We use in situ and satellite data to more fully characterize this variability. The oscillations have zonal wavelengths of 550-1650 km and propagate westward in a manner consistent with the dispersion relation for first baroclinic mode, free Rossby waves in the presence of a mean westward flow. Analysis of 9 years of altimetry data shows that the amplitude of the 50-to 100-day SSH variability at 10°N is largest on 90-115°W, with peak amplitudes occurring around April. Some eddies traveling westward at 10-13°N emanate from near the gulfs of Tehuantepec and Papagayo, but eddies sometimes also appear to intensify well away from the coast while in the North Equatorial Current (NEC). The hypothesis that the intraseasonal variability and its annual cycle are associated with baroclinic instability of the NEC is supported by a spatiotemporal correlation between the amplitude of 50-to 100-day variability and the occurrence of westward zonal flows meeting an approximate necessary condition for baroclinic instability. The notion that baroclinic instability may be involved is further corroborated by the tendency of the NEC to weaken while the eddies intensify, even as the wind works to strengthen the current.

Observations of the Dispersion Characteristics and Meridional Sea Level Structure of Equatorial Waves in the Pacific Ocean

“…The zonal wavenumber-frequency spectrum of a given longitude-time section of SSH (or sum or difference of SSH at opposing latitudes) was estimated by a method similar to that described in Farrar and Weller (2006) using a two-dimensional fast Fourier transform (2DFFT). The longitude-time mean was removed from each longitude-time section before tapering the edges of each section to zero with a half-Hanning window over 220 days and 10°longitude.…”

A multi-dimensional spectral description of ocean variability with applications

“…Furthermore, SSH is closely related to other more dynamically relevant quantities such as the near surface velocity, kinetic energy, and pressure fields. Many altimetric studies focus on particular particular dynamical process such as eddy diffusivity near jets [Ferrari and Nikurashin, 2010], monitor changes in ocean circulation [Farrar and Weller , 2006], or look for evidence of turbulent energy cascades [Scott and Wang, 2005]. Yet a comprehensive description of the three-dimensional spectrum of ocean variability is lacking.…”

Measuring the Global Ocean Surface Circulation with Satellite and In Situ Observations

“…In the tropical Atlantic, studies have addressed equatorial divergence and upwelling dynamics [25], interannual equatorial long waves [26] and the mixed-layer salinity balance [27]. The surface current field was also used to assess the seasonal cycle of diabatic heat storage in subregions of the Pacific [28] and in surface heat flux estimates ( [29] and [30]); the spatial and temporal context of the seasonal-to-interannual timescale currents was examined in order to study intraseasonal variability and dynamics in the tropics ( [31] and [32]). The data are also in use for fishery resource management research, marine animal migrations studies and other maritime applications throughout the tropical Pacific.…”