Abstract
The structure and dynamics of decadal anomalies in the wintertime midlatitude North Pacific ocean–atmosphere system are examined in this study, using the NCEP/NCAR atmospheric reanalysis, HadISST SST and Simple Ocean Data Assimilation data for 1960–2010. The midlatitude decadal anomalies associated with the Pacific Decadal Oscillation are identified, being characterized by an equivalent barotropic atmospheric low (high) pressure over a cold (warm) oceanic surface. Such a unique configuration of decadal anomalies can be maintained by an unstable ocean–atmosphere interaction mechanism in the midlatitudes, which is hypothesized as follows. Associated with a warm PDO phase, an initial midlatitude surface westerly anomaly accompanied with intensified Aleutian low tends to force a negative SST anomaly by increasing upward surface heat fluxes and driving southward Ekman current anomaly. The SST cooling tends to increase the meridional SST gradient, thus enhancing the subtropical oceanic front. As an adjustment of the atmospheric boundary layer to the enhanced oceanic front, the low-level atmospheric meridional temperature gradient and thus the low-level atmospheric baroclinicity tend to be strengthened, inducing more active transient eddy activities that increase transient eddy vorticity forcing. The vorticity forcing that dominates the total atmospheric forcing tends to produce an equivalent barotropic atmospheric low pressure north of the initial westerly anomaly, intensifying the initial anomalies of the midlatitude surface westerly and Aleutian low. Therefore, it is suggested that the midlatitude ocean–atmosphere interaction can provide a positive feedback mechanism for the development of initial anomaly, in which the oceanic front and the atmospheric transient eddy are the indispensable ingredients. Such a positive ocean–atmosphere feedback mechanism is fundamentally responsible for the observed decadal anomalies in the midlatitude North Pacific ocean–atmosphere system.
Highlights
Since the mid-1990s, a variety of observational studies have discovered the existence of the midlatitude climate variabilities on decadal to inter-decadal time scales in both the atmosphere and the ocean, such as the Pacific Decadal Oscillation (PDO) over North Pacific and the North Atlantic Oscillation (NAO) over North Atlantic (Trenberth 1990; Graham et al 1994; Kushnir 1994; Minobe 1997; Mantua et al 1997; Enfield and Mestas-Nunez 1999; Zhu and Yang 2003a; Deser et al 2004; Fang et al 2006)
To understand the mechanism responsible for the decadal climate anomalies, here we further identify the decadal anomalies of the forcing sources including the diabatic heating, transient eddy heating and transient eddy vorticity forcing over the North Pacific
A detailed analysis of the 3-D distribution and configuration relationship of decadal climate anomalies in the North Pacific has been conducted in this study, showing that corresponding to a basin scale sea surface temperature (SST) cooling for a positive PDO phase, the atmospheric geopotential height aloft dispalys an equivalent barotropic low anomaly throughout the troposphere
Summary
Since the mid-1990s, a variety of observational studies have discovered the existence of the midlatitude climate variabilities on decadal to inter-decadal time scales in both the atmosphere and the ocean, such as the Pacific Decadal Oscillation (PDO) over North Pacific and the North Atlantic Oscillation (NAO) over North Atlantic (Trenberth 1990; Graham et al 1994; Kushnir 1994; Minobe 1997; Mantua et al 1997; Enfield and Mestas-Nunez 1999; Zhu and Yang 2003a; Deser et al 2004; Fang et al 2006). The atmospheric circulation anomalies associated with large-scale SST anomalies on decadal timescale exhibit an equivalent barotropic structure in the vertical direction, with lows (highs) above cold (warm) water (Kushnir et al 2002; Namias and Cayan 1981; Cayan 1992; Deser and Blackman 1993) The focus will be on the dynamical processes of atmospheric response to the midlatitude SST anomalies associated with the oceanic front variations through thermal and dynamical forcing sources that include the direct diabatic heating and indirect transient eddy forcing. A hypothesis for unstable ocean–atmosphere interaction in the midlatitudes is proposed In this hypothesis, the role of the atmospheric transient eddy forcing induced by the oceanic front in how the midlatitude SST anomalies affect the atmospheric anomalies is emphasized.
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