Mesoscale variability in the northeastern tropical Pacific: Forcing mechanisms and eddy properties

Abstract

[1] The generation mechanisms and properties of the mesoscale eddies in the northeastern tropical Pacific (NETP) are studied using a series of numerical solutions and satellite observations. The spatial and temporal resolution of mountain wind jets over the Gulfs of Tehuantepec and Papagayo are essential for an accurate modeling of the regional mesoscale variability. Three previously proposed eddy generation mechanisms are the local transient wind forcing, the combined low-frequency wind and boundary forcing, and the remote equatorial Kelvin wave forcing. In our model, the transient wind-forcing was represented by wind fields with synoptic variability, and the remote equatorial Kelvin wave forcing by 5-day open boundary conditions with interannual variability. Solutions with and without high-frequency forcings are contrasted to evaluate the contributions of the three mechanisms. The combined low-frequency wind and boundary forcing is the primary eddy forcing mechanism in the NETP. In the Tehuantepec region, the high-frequency wind-forcing contributes more to the mesoscale variability than the remote equatorial Kelvin wave forcing, while in the Papagayo region, their contributions are comparable. Eddies in the Tehuantepec and Papagayo regions are larger in both amplitude and size, and are more likely to deflect equatorward than eddies in the rest of the NETP. The maximum temperature anomaly of eddies is at around 50 m depth. Eddies with lifetimes >6 weeks are more likely anticyclonic, while eddies with lifetimes <6 weeks are more likely cyclonic. The anticyclone-dominance for large-amplitude long-lived eddies is the combined consequence of wind-driven asymmetric dipole spin-up and the evolutionary fragility of large-amplitude cyclones.

Statistical Characteristics of Mesoscale Eddies in the North Pacific Derived from Satellite Altimetry

“…Kurian et al [41] observed eddies in the California Current system and indicated that many of eddies formed near the coast with high eddy kinetic energy (0.018 m 2 /s 2 ). In the northeastern tropical Pacific, the ocean is strongly modulated by three powerful wind jets over the Tehuantepec, Papagayo, and Panama Gulfs [45]. Therefore, there are three bands of strong mesoscale eddy variability off the coast of Central America from the Tehuantepec, Papagayo, and Panama Gulfs [42].…”

Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

“…These seasonal differences indicate different eddy generation mechanisms at play in the TANWA during the different seasons. Different mechanisms for the generation of eddies in eastern boundary upwelling regions have been proposed (e.g., Liang et al, 2012). Barotropic and baroclinic instabilities of the near-coastal currents (Pantoja et al, 2012) triggered by, e.g., the passage of poleward propagating coastal trapped waves (Zamudio et al, 2001(Zamudio et al, , 2007, wind perturbations (Pares-Sierra et al, 1993), or interactions of the large-scale circulation with the bottom topography (Kurian et al, 2011) are the main processes identified for the eddy generation in eastern boundary upwelling regions.…”

“…The eddy generation in the northeastern Pacific Ocean, off California and Mexico including the California Current System, was studied with high-resolution models applied to reproduce observed characteristics of the eddy field (Liang et al, 2012;Chang et al, 2012). These studies not only highlight hotspots of eddy generation associated with local wind fluctuations (e.g., over the Gulf of Tehuantepec and Papagayo), but also suggest an important role of lowfrequency wind and boundary forcing.…”

Time-Dependent Eddy-Mean Energy Diagrams and Their Application to the Ocean

“…This diagram, though simple, is to our knowledge, the first published time-dependent energy diagram for eddy-mean flow interactions. Previous studies mostly evaluate the time-mean, eddy-mean flow interactions energetics, with a focus on the EKE budget (e.g., Biastoch and Krauss 1999;Marchesiello et al 2003;Mata et al 2006;Liang et al 2012). This diagram reveals that during eddy-mean flow interactions, the energy exchange actually occurs between K M and K R , K R and K E , P R and P R , and P R and P E .…”

“…Therefore, the source, sink, and transformation of energy in the global ocean has received much attention (e.g., Wunsch and Ferrari 2004;Ferrari and Wunsch 2009;Zemskova et al 2015). Many studies exist about energetics during eddy-mean flow interactions, and most of these focus on characterizing the spatial structures and magnitude of energy conversion rates between the mean and eddies from a long-term, time-mean perspective (e.g., von Storch et al 2012;Liang et al 2012;Chen et al 2014a;Kang and Curchitser 2015). Efforts in diagnosing the energy cascade diagram in the frequencywavenumber domain, as summarized by Ferrari and Wunsch (2009), also mainly focus on a long-term, timemean perspective (e.g., Scott and Wang 2005;Scott and Arbic 2007;Arbic et al 2012).…”