Monthly period oscillations in the Pacific North Equatorial Countercurrent

Abstract

Monthly period oscillations in temperature and velocity in the vicinity of the North Equatorial Countercurrent along 140°W are examined using moored time series data collected during 1988–1989. These oscillations, which presumably derive their energy from instability of the large‐scale shear of the zonal equatorial current system, are found to be energetic across a broad band of periods of about 15–50 days. Along 7°N, wavelike motions at these periods propagate westward with a phase speed of about 30–40 cm s−1 and a zonal wavelength of 750–1150 km. The oscillations are approximately in geostrophic balance with the thermal field, with the velocity vector rotating anticyclonically around westward propagating high sea level centers. Zonal volume transport variations associated with these waves between 5° and 9°N at 20–180 m depth have a standard deviation of about 4.1 Sv, with peak‐to‐peak changes in some instances approaching 20 Sv. Maximum temperature variance along 140°W occurs between 5° and 7°N at 100–150 m depth in a region of strong mean vertical and meridional temperature gradient associated with a thermocline sloping upward to the north. At the surface, maximum temperature variability occurs from 2° to 5°N in the vicinity of the strong sea surface temperature front north of the equator. Analysis of the upper ocean temperature balance at 7°N, 140°W reveals that meridional advection is important both in the surface layer and in the thermocline. Vertical advection, estimated using vertical velocity calculated from a simplified linear vorticity balance, is also important in the thermocline, where it contributes a variance comparable to that of meridional advection. Zonal advection is likely to be significant in the upper 80 m, below which it is probably of secondary importance compared to meridional and vertical advection. One consequence of advection in the temperature balance is that variance is shifted toward lower frequencies in temperature vis‐a‐vis velocity spectra. This “red shift” may account for the apparent difference between periods for instability waves inferred from near‐equatorial meridional velocity spectra, and slightly longer periods reported for oscillations of the sea surface temperature front north of the equator based on satellite imagery.

The influence of salinity on tropical Atlantic instability waves

“…Thousand kilometer zonal scale propagating waves with typical periods of tens of days have been observed in the tropical Pacific and Atlantic Oceans, affecting the physical and biogeochemical properties of the ocean and its interaction with the atmosphere [e.g., Legeckis, 1977;Legeckis and Reverdin, 1983;McPhaden et al, 1984;Hayes et al, 1989;Luther and Johnson, 1990;Yoder et al, 1994;Qiao and Weisberg, 1995;McPhaden, 1996;Xie et al, 1998;Chelton et al, 2000;Liu et al, 2000;Polito et al, 2001;Lee et al, 2012]. 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].…”

Pacific subtropical‐tropical thermocline water exchange in the National Centers for Environmental Prediction ocean model

“…For the meridional current, because its magnitude is about 1 order smaller than the zonal current, the comparison between simulations and observations becomes even more challenging. For example, the TOGA-TAO mooring at 7øN, 140øW shows that the meridional current is poleward above 120 m and equatorward from 120 m to the data available depth of 300 m [McPhaden, 1996]. However, the NCEP meridional current is poleward above 40 m and equatorward between 40 m and 250 m. Thus the results presented here based on the NCEP simulations may still need to be verified by basin-wide, long-term climatological observations, especially for the LLWBC transport, since the geostrophic balance is no longer valid within the western boundary and the Indonesia Throughway is eliminated in the NCEP model.…”

“…It has recently been recognized that subtropical-tropical water exchange plays an important role in climate variability at decadal and longer timescales [Gu and In the meantime, the existence of an equatorward interior pathway is implied in the anomalous oceanic geopotential thickness [Levitus and Oort, 1977], equatorward geostrophic flows [Pazan and White, 1987;McPhaden and Fine, 1988;Toole et al, 1988], and mooring currents [McPhaden, 1996]. Simulations of oceanic general circulation models (OGCMs) show that subtropical water can be transported to the equatorial ocean in both the LLWBC and the interior ocean [Liu, 1994;Liu et al, 1994;McCreary and Lu, 1994;Rothstein et al, 1998].…”

A note on the vertical scales of temperature and salinity and their signature in dynamic height in the western Pacific Ocean: Implications for data assimilation

“…The analysis is performed in terms of principal components, or empirical orthogonal functions (EOFs). EOFs have been used in the past to analyze dynamical processes in the vertical in studies using a wide range of data sources from time series recorded at individual moorings [Hayes and Halpern, 1984;McPhaden, 1996] The following section describes the data set and the analysis. Section 3 presents the vertical decomposition of the temperature and salinity fields and their related signatures in dynamic height.…”