The demise of semidiurnal internal tides in the Equatorial Pacific: incoherence or dissipation?

The demise of semidiurnal internal tides in the Equatorial Pacific: Incoherence or dissipation? Maarten Buijsman 1 , Brian K. Arbic 2 , James G. Richman 3 , and Luis Zamudio 3 1 University of Southern Mississippi, Stennis Space Center, MS maarten.buijsman@usm.edu 2 University of Michigan, Ann Arbor, MI james.richman. ctr@nrlssc.navy. mil 3 Florida State University, Tallahassee, FL luis.zamudio. ctr@nrlssc.navy. mil Abstract The jets in the equatorial Pacific Ocean of a realistically-forced global circulation model yield a strong loss of phase coherence in semidiurnal internal tides that propagate equatorward from the French Polynesian islands and Hawaii. The loss of phase coherence is attributed to the time variability in the sheared background flow and stratification associated with the zonal jets. The incoherence due to background flow variability occurs on monthly time scales, whereas the incoherence due to stratification occurs on seasonal time scales. For time scales longer than a year the coherent energy flux approaches zero values at the equator, while the total flux, comprising the sum of the coherent, incoherent, and cross-term fluxes, remains non-zero. The 1/12.5 ◦ model simulation suggests that low-frequency jets do not noticeable enhance the dissipation of the internal tide, but merely decohere and scatter it. Thus, the model results presented here suggest that the apparent demise of coherent internal tides seen in satellite altimetry maps may be due to incoherence rather than dissipation. Introduction Internal gravity waves generated by barotropic tidal motions over underwater topography, also referred to as internal tides, fill the world’s oceans (Ray and Mitchum, 1997; Alford, 2003; Ar- bic et al., 2004). The internal tides that propagate long distances are mostly vertical low-mode waves (Alford, 2003), while the higher mode waves generally dissipate near their generation sites. Satellite altimetry of the ocean surface provides near-global coverage of low-mode internal tides (Ray and Mitchum, 1997; Ray and Zaron, 2011; Shriver et al., 2012). Satellite altimeter maps show internal tides propagating across basins for thousands of kilometers. However, the equato- rial Pacific ocean appears to be a barrier of sorts for internal tides. According to altimeter maps (see Figure 7a of Shriver et al. (2012)), neither beams that propagate southward from Hawaii or beams that propagate northward from the French Polynesian islands, cross the equatorial Pacific. This may imply that the equatorial zonal jets, and associated meridional gradients in stratification, cause a strong dissipation of the equatorward propagating internal tides. Another potential explanation for the apparent lack of internal tides at the equator is incoherence. The internal tide sea surface height signals are generally extracted from altimetry with a harmonic least-squares analysis (Ray and Mitchum, 1997; Zhao et al., 2012; Shriver et al., 2012), and only the part of the total signal that is coherent with the barotropic narrow-band tidal forcing is retained. The residual signal is defined as the incoherent or non-stationary part. A question that is addressed in this paper is how much of the apparent “dissipation” at the equator can be attributed to the internal tides becoming incoherent after passing through the equatorial Pacific jets, and thus being missed by the least squares analysis. Because of their O(10 day) sampling times, satellite altimeters do not easily allow for estimation of the incoherent tide amplitude at a particular location in the ocean. In contrast, eddy-resolving numerical models with realistic tidal and atmospheric forcing allow for such local estimates (Zaron and Egbert, 2014; Shriver VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29 - Sept. 1, 2016