Abstract
Atmospheric transient eddy dynamical forcing (TEDF)-driven midlatitude unstable air–sea interaction has recently been recognized as a crucial positive feedback for the maintenance of the extratropical decadal variabilities. Our recent theoretical work (Chen et al., Clim Dyn https://doi.org/10.1007/s00382-020-05405-0, 2020) has characterized such an interaction through building an analytical midlatitude barotropic atmospheric model coupled to a simplified upper oceanic model. This study extends the analytical model to including a two-layer quasi-geostrophic baroclinic atmospheric model and then identifies the roles of vertical distributions of atmospheric TEDF and diabatic heating in midlatitude unstable air–sea interaction. It is found that midlatitude air–sea coupling with more realistic vertical profiles of atmospheric TEDF and diabatic heating destabilizes oceanic Rossby wave modes over the entire range of zonal wavelengths, in which the most unstable coupled mode features an equivalent barotropic atmospheric low (high) pressure over a cold (warm) oceanic surface. Spatial structure and period of the most unstable mode are more consistent with the observation than those from in previous model. Although either TEDF or diabatic heating alone can lead to a destabilized coupled mode, the former makes a dominant contribution to the instability. The increase of low-layer TEDF stimulates the instability more effectively if the TEDF in upper layer is larger than in lower layer, while the TEDF in either high or low layers can individually cause the instability. The surface heating always destabilizes the air–sea interaction, while the mid-level heating always decays the coupled mode. The results of this study further confirm the TEDF-driven positive feedback mechanism in midlatitude air–sea interaction proposed by recent observational and numerical experiment studies.
Highlights
Observational studies have revealed that there is a significant decadal variability in the midlatitude North Pacific ocean–atmosphere system (Trenberth 1990; Graham et al 1994; Minobe 1997; Mantua et al 1997; Enfield and MestasNunez 1999; Zhu and Yang 2003; Fang et al 2006)
Since atmospheric transient eddy dynamical forcingdriven midlatitude unstable air–sea interaction is recognized as a crucial positive feedback for the maintenance of the extratropical decadal variabilities, our recent theoretical work (Chen et al 2020, hereinafter CFY2020) has characterized such an interaction through building an analytical midlatitude barotropic atmospheric model coupled to a simplified upper oceanic model
The results provide a theoretical support for the eddy-driven midlatitude unstable air–sea interaction hypothesis
Summary
Observational studies have revealed that there is a significant decadal variability in the midlatitude North Pacific ocean–atmosphere system (Trenberth 1990; Graham et al 1994; Minobe 1997; Mantua et al 1997; Enfield and MestasNunez 1999; Zhu and Yang 2003; Fang et al 2006). Fang and Yang (2016) identified the features of those PV sources associated with PDO and quantitatively diagnosed their effects on the atmospheric anomalies Based on their analyses, a positive feedback mechanism for midlatitude unstable air–sea interaction in the North Pacific was hypothesized as follows. The midlatitude air–sea interaction, in which the oceanic front and the atmospheric transient eddy are the indispensable ingredients, can provide a positive feedback mechanism for the development and maintenance of the observed decadal anomalies in the midlatitude North Pacific ocean–atmosphere system This hypothesis has been confirmed by the later observational and modeling studies (Wang et al 2017, 2019; Tao et al 2020; Zhang et al 2020).
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