A Mesoscale Ocean–Atmosphere Coupled Pathway for Decadal Variability of the Kuroshio Extension System

Abstract

The Kuroshio Extension (KE) system has been observed to experience a decadal cycle between dynamically stable and unstable states. However, divergent conclusions on its interaction with the atmosphere obfuscate the understanding of its oscillatory nature at the preferred decadal time scale. Here, using satellite observations and the ERA-Interim reanalysis in 2002–16, physical process-oriented diagnoses suggest that the wintertime finer-scale thermodynamic response to mesoscale oceanic surface conditions and slow oceanic Rossby wave adjustment frame a coupled ocean–atmosphere delayed oscillator for the decadal KE variability. During the stable state of the KE system, the downstream KE transition region is rich in mesoscale oceanic warming associated with warm eddies, which induces surface wind convergence and upward motion, probably via the enhanced turbulent mixing. Meanwhile, increased finer-scale diabatic heating in the lower troposphere with abundant moisture supply from warmer water likely facilitates the deep-reaching updraft that adiabatically cools the middle troposphere. The background northwesterly wind helps to spread out the cooling, leading to southward deflection of local atmospheric eddy available potential energy (EAPE) production by baroclinic conversion. Consequently, the synoptic eddy activity displaces southward across the basin with additional energy supply from the increased diabatic production of EAPE downstream. Anomalous synoptic eddy thermal and vorticity forcing eventually fosters the basin-scale equivalent-barotropic cyclonic circulation anomaly, which is further maintained by energy conversion from the background state. The resultant wind-driven negative sea surface height anomalies propagate westward into the upstream KE region with a delay of ∼4 years and can trigger the unstable state of the KE system.

A key role of off-equatorial subsurface temperature anomalies in Tropical Pacific Decadal Variability

“…5). As a result, the state change of the KE partially excites the PMM [59][60][61][62][63] to further strengthen the lowfrequency SST variance (Fig. 3f-h and Supplementary Fig.…”

Observations Reveal Intense Air‐Sea Exchanges Over Submesoscale Ocean Front

“…Additionally, the gradual cross‐front changes in SLP suggest that the pressure adjustment may play a less important role in submesoscale air‐sea interactions, as compared to mesoscale air‐sea interactions (Gan et al., 2023; Minobe et al., 2008). In the next section, we will compare the different processes involved in submesoscale and mesoscale air‐sea interactions using an SST‐forced atmospheric model.…”

“…Decades of observations and climate modeling studies indicated that oceanic fronts shape the intensity of air‐sea fluxes and the movement of overlying atmospheric, especially at mesoscales with cross‐front scales from tens to hundreds of kilometers (Czaja et al., 2019; Kelly et al., 2010; Seo et al., 2023; Small et al., 2008). Through intense heat and moisture release, these oceanic fronts impact the overlying atmospheric weathers by strengthening horizontal gradient of sea level pressure (SLP; Lindzen & Nigam, 1987; Mahrt et al., 2004; Minobe et al., 2008; Kawai et al., 2014) or increasing vertical mixing intensity in the lower atmosphere (Chelton et al., 2004; Gan et al., 2023; Hayes et al., 1989; O’Neill et al., 2010; Schneider, 2020; Schneider & Qiu, 2015; Tanimoto et al., 2011; Wallace et al., 1989; Xie, 2004). In response, cross‐front variability in local wind and associated vertical motions increase within the marine atmospheric boundary layer (MABL; Chelton et al., 2004; Lindzen & Nigam, 1987; Minobe et al., 2008; O’Neill et al., 2010; Spall, 2007; Tanimoto et al., 2011; Xie, 2004; Small et al., 2019; Strobach et al., 2022; Wallace et al., 1989), which favors cloud formations (Takahashi et al., 2021; Tokinaga et al., 2009; Young & Sikora, 2003) and precipitation (Frenger et al., 2013; Minobe et al., 2008).…”