Rectified Effects of Regional Current Feedback on Large-Scale Air-Sea Interactions and ENSO

Recent observational and modeling studies have shown that coupled atmospheric and oceanic mesoscale processes exert a significant influence on ENSO dynamics. However, the spatial resolution of the latest generation of climate models is still insufficient to fully resolve mesoscale air-sea interactions. Furthermore, climate models exhibit significant biases in the simulation of the tropical Pacific mean state, which can affect the ability of the model to accurately reproduce ENSO variability. In particular, the Current Feedback to the Atmosphere (CFB) slows the large-scale mean circulation by reducing the mean energy input from the atmosphere to the ocean, while at the mesoscale it causes the "eddy killing" mechanism: a damping of eddies by ~30%, caused mainly by the transfer of energy from ocean currents to the atmosphere.

In this study, we perform a set of regional high-resolution oceanic (1/12°) and atmospheric (1/4°) coupled simulations, in which the CFB is considered (CTRL) or not (NOCFB), to quantify the impact of mesoscale air-sea interactions on the Pacific mean state and ENSO. The coupled simulations cover the entire Pacific basin (90°E-70°W) and the tropics (30°S-30°N) for the period 1980-2020. The impact of CFB on the oceanic mean state and ENSO is then assessed by comparing the two simulations. The CTRL simulation effectively reproduces the mean state and seasonal cycle of the tropical climate, specially the sea surface temperature (SST) pattern with an accurate representation of the warm pool and cold tongue extension. Additionally, the model properly simulates the equatorial current system, the equatorial thermocline, and key atmospheric characteristics of the tropics (e.g., ITCZ-SPCZ). These features enable the model to successfully represent the ENSO seasonal phase, including the onset, development, and termination of the most intense El Niño events (e.g., 1982/83 and 1997/98).

We show that, in addition to the slowdown of the equatorial current system from the surface to ~100 m depth and the reduction of equatorial eddy kinetic energy through decreased barotropic and baroclinic energy conversion, CFB enhances the equatorial zonal SST gradient by warming the western Pacific by up to 0.3°C and cooling the eastern Pacific by about 0.4°C. These effects directly impact the mean state of precipitation and net heat flux across the warm pool and cold tongue. Consequently, the ENSO asymmetry and nonlinearity are reduced by approximately 10% in the NOCFB simulation, thereby underscoring the rectifying effects of the CFB on large-scale SST patterns in the tropical Pacific basin.