Midlatitude unstable air-sea interaction with atmospheric transient eddy dynamical forcing in an analytical coupled model

Abstract

While recent observational studies have shown the critical role of atmospheric transient eddy (TE) activities in midlatitude unstable air-sea interaction, there is still a lack of a theoretical framework characterizing such an interaction. In this study, an analytical coupled air-sea model with inclusion of the TE dynamical forcing is developed to investigate the role of such a forcing in midlatitude unstable air-sea interaction. In this model, the atmosphere is governed by a barotropic quasi-geostrophic potential vorticity equation forced by surface diabatic heating and TE vorticity forcing. The ocean is governed by a baroclinic Rossby wave equation driven by wind stress. Sea surface temperature (SST) is determined by mixing layer physics. Based on detailed observational analyses, a parameterized linear relationship between TE vorticity forcing and meridional second-order derivative of SST is proposed to close the equations. Analytical solutions of the coupled model show that the midlatitude air-sea interaction with atmospheric TE dynamical forcing can destabilize the oceanic Rossby wave within a wide range of wavelengths. For the most unstable growing mode, characteristic atmospheric streamfunction anomalies are nearly in phase with their oceanic counterparts and both have a northeastward phase shift relative to SST anomalies, as the observed. Although both surface diabatic heating and TE vorticity forcing can lead to unstable air-sea interaction, the latter has a dominant contribution to the unstable growth. Sensitivity analyses further show that the growth rate of the unstable coupled mode is also influenced by the background zonal wind and the air-sea coupling strength. Such an unstable air-sea interaction provides a key positive feedback mechanism for midlatitude coupled climate variabilities.

Roles of vertical distributions of atmospheric transient eddy dynamical forcing and diabatic heating in midlatitude unstable air–sea interaction

“…The eddy-to-mean flow feedback has been found to be a key approach for midlatitude SST to affect the atmosphere by a number of recent studies (Wang et al 2017(Wang et al , 2019Tao et al 2020). 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 (Fang and Yang 2016;Chen et al 2020). Our recent theoretical work by Chen et al (2020) has characterized such an interaction through building an analytical midlatitude barotropic atmospheric model coupled to a simplified upper oceanic model.…”

“…As a theoretical development, this study extends the analytical coupled air-sea model of Chen et al (2020) 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. In the extended coupled model, both the atmospheric and oceanic perturbations satisfy quasi-geostrophic dynamics and are governed by their respective linearized QGPV equations on a midlatitude beta plane.…”

“…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. In the analytical coupled model, the atmospheric component is governed by the barotropic QGPV equation with transient eddy vorticity forcing which is parameterized to be linearly proportional to the second-order meridional derivative of SST anomaly in terms of observational analyses.…”

Observational Characteristics of Wintertime Extreme Surface Turbulent Heat Flux Events in the North Pacific Subtropical Region

“…In addition to the SAFZ, recent high‐resolution observational data also show a subtropical oceanic frontal zone (STFZ) existing to the south of SAFZ, which is probably associated with Subtropical Countercurrent (Yamanaka et al., 2008). Though with weaker intensity compared with SAFZ, the STFZ sits in the subtropical region with higher SST and latent heat, suggesting another key region of wintertime air‐sea interactions by recent studies (e.g., Q. Chen et al., 2019; L. Chen et al., 2020; Fang & Yang, 2016; Huang et al., 2020; Zhang et al., 2020).…”

Roles of vertical distributions of atmospheric transient eddy dynamical forcing and diabatic heating in midlatitude unstable air-sea interaction

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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 previous theoretical work by Chen et al (2020) characterizes such an interaction with building an analytical midlatitude barotropic atmospheric model coupled to a simplified upper oceanic model. This study firstly extends the analytical model to a two-layer quasi-geostrophic baroclinic atmospheric model coupled to a simplified upper oceanic model and then identifies the roles of vertical distributions of atmospheric TEDF and diabatic heating in midlatitude unstable air-sea interaction.

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