Direct and Rectified Effects of Tropical Instability Waves on the Eastern Tropical Pacific Mean State in a Regional Ocean Model

Abstract

Tropical instability waves (TIWs) are oceanic features propagating westward along the northern front of the pacific cold tongue. Observational and modeling studies suggest that TIWs may have a large impact on the eastern tropical Pacific background state from seasonal to interannual time-scales, through heat advection and mixing. However, observations are coarse or limited to surface data, and modeling studies are often based on the comparison of low- vs. high-resolution simulations. In this study, we perform a set of regional high-resolution ocean simulations (CROCO 1/12°) in which we strongly damp (NOTIWs-RUN) or not (TIWs-RUN) TIW propagation, by nudging meridional current velocities in the TIW region toward their monthly climatological values. This approach, while effectively removing TIW mesoscale activity, does not alter the model internal physics in particular related to the equatorial Kelvin wave dynamics. The impact of TIWs on the oceanic mean state is then assessed by comparing the two simulations. While the well-known direct effect of TIW heat advection is to weaken the meridional temperature gradient by warming up the cold tongue (0.34°C/month), the rectified effect of TIWs onto the mean state attenuates this direct effect by cooling down the cold tongue (-0.10°C/month). This rectified effect occurs through the TIW induced deepening and weakening of the Equatorial Undercurrent, that subsequently modulates the mean zonal advection and counterbalances TIWs direct effect. This approach allows quantifying the rectified effect of TIWs without degrading the model horizontal resolution, and may lead to a better characterization of the eastern tropical Pacific mean state and to the development of TIW parameterizations in Earth system models.

Modulation of the Eastern Equatorial Pacific Seasonal Cycle by Tropical Instability Waves

“…In this study, we assess the modulation of the eastern tropical Pacific seasonality by TIWs, using a regional ocean model. Following the methodology of Maillard et al (2022), we compare a 1/12° regional eddy-rich simulation to one where TIWs are removed. We also consider a 1° simulation in which the coarse resolution prevents TIWs to be fully resolved, and a similar configuration where the nonlinear terms in the momentum equation are turned off to reduce the TIW activity even further.…”

“…Indeed, only the amplitude of Kelvin waves is slightly different between TIW‐ and NOTIW‐RUN (see Maillard et al. (2022), their Figures 7c and 7d), which might be due to the TIW‐Kelvin wave interactions (Escobar‐Franco et al., 2022; Holmes & Thomas, 2016) or to the TIW‐induced modification of the mean state. These two eddy‐rich simulations are used to isolate the effect of TIWs on the ocean mean state, by subtracting the NOTIW‐RUN from the TIW‐RUN.…”

“…Following Maillard et al. (2022), in NOTIW‐RUN, to damp TIWs, the meridional currents v in the region of active TIWs are nudged online toward their monthly climatology calculated from TIW‐RUN. This methodology removes most of TIW activity while keeping the equatorial dynamics and in particular the Kelvin wave activity mostly untouched.…”

“…Here, the NDH is defined as the 3-dimensional TIW nonlinear advection of heat − ′ ∇ ′ over the first 50 m, with primes denoting fluctuations from a temporal 30-day running-mean. On average, TIWs warm waters between 0° and 3°N by up to 0.4°C in the first 50 m and cool the northern tropical waters by the same amount, decreasing the northward meridional temperature gradient (see Figures 2a and 2c and Maillard et al, 2022). This process expands the cold tongue in the meridional direction, as depicted by the 27.5°C isotherms of TIW-RUN (solid black line) and NOTIW-RUN (dashed black line), with their meridional extent differing by 100 km at 140°W.…”

“…They also reduce the average strength of equatorial currents and in particular the EUC. This reduces the volume of warm subsurface water brought from the western Pacific into the cold tongue, leading to a rectified cooling effect that partly counterbalances the direct TIW warming through NDH (Maillard et al., 2022). By modulating the heat and momentum budgets, TIWs have a complex impact on the mean flow of active and passive tracers, which together with their strong effect on the vertical oceanic mixing (Holmes & Thomas, 2015; Inoue et al., 2012; Lien et al., 2008; Moum et al., 2009) can substantially influence biogeochemical properties (Eddebbar et al., 2021) in this key region for ecosystems and the global carbon cycle (Takahashi et al., 2009).…”