Seasonality of the Pacific decadal oscillation - Climate Prediction ...

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1. Introduction The Pacific decadal oscillation (PDO) is the leading empirical orthogonal function (EOF) of monthly mean sea surface temperature (SST) anomalies in the North Pacific (Mantua et al. 1997; Zhang et al. 1997). The causes of the PDO are the subject of many previous studies. Proposed mechanisms responsible for the spatial and temporal characteristics of the PDO include stochastic forcing of the ocean by the atmosphere (Alexander 2010), coupling of the midlatitude ocean-atmosphere system (Barnett et al. 1999), the reemergence of deep oceanic mixed layer temperature anomaly leading to a multi-year persistence of the PDO (Alexander et al. 1999), tropical forcing by the El Niño–Southern Oscillation (ENSO; Newman et al. 2003; An et al. 2007; Shakun and Shaman 2009), and the variability of the Aleutian low and Kuroshio Extension (Qiu 2003; Schneider and Cornuelle 2005), among others. Similar to ENSO, the PDO is associated with broad impacts on global climate (Mantua and Hare 2002). However, unlike the influence of ENSO, the PDO-related precipitation and temperature anomalies in North America are not strongest in winter (Minobe 2000; Cayan et al. 2001; Hu and Huang 2009). It is also well known that tropical SST variability related to ENSO influences the PDO through the atmospheric bridge mechanism (Alexander et al. 2002). Specifically, ENSO can significantly alter the variability of the Aleutian low. The latter in turn affects the PDO. Furthermore, this “tropical-extratropical” linkage between ENSO and the PDO occurs on interannual timescales, and is likely to follow the seasonality in the evolution of both ENSO SST anomalies in the tropical Pacific and the characteristics of tropical-extratropical teleconnection (Newman and Sardeshmukh, 1998). 2
The PDO can also modulate the impact of ENSO on extratropical climate (e.g., Gershunov and Barnett 1998; Wang et al. 2008; Birk et al. 2010) by altering the background mean flow (on which tropical-extratropical teleconnection depend) on decadal time-scales. Further, based on the seasonality of the PDO, such a modulation may also have a seasonality. What factors determine the characteristic seasonal evolution of the PDO With much focus on the low-frequency evolution of the PDO, seasonal variations in the PDO characteristics have received less attention. Diagnosis and budget analysis with oceanic models indicate that atmospheric heat flux (Dawe and Thompson 2007), the variability of the Aleutian low and anomalous advection of mean SST (Chhak et al. 2009) are important for the seasonal evolution of the PDO. However, the understanding is far from comprehensive. A better understanding of the seasonality of the PDO is important for potential improvements in our understanding of its predictability and prediction skill, its global associations, and its decadal modulation of the impact of ENSO on extratropical climate. This study aims to characterize the seasonality of the PDO and potential mechanisms. Our hypothesis is that the PDO seasonality is dominated by the combined influence of the seasonal variations in the atmospheric forcing and oceanic mixed layer depth (MLD). Our emphasis is on the PDO evolution from spring to summer because as will be shown, there are significant changes in 1) the seasonal characteristics of the PDO, and 2) the strength of the atmospheric forcing and MLD between the two seasons, both of which play an important role in the variability of midlatitude SST (Alexander and Penland 1996). 3
Page 1 and 2: AMERICAN METEOROLOGICAL SOCIETY Jou
Page 3: ABSTRACT The seasonality of the Pac
Page 7 and 8: ealizations provides an estimate of
Page 9 and 10: more connection with SST variabilit
Page 11 and 12: and April the cold temperature anom
Page 13 and 14: wind over the North Pacific region
Page 15 and 16: first EOF of the 1000-hPa zonal win
Page 17 and 18: heat flux. This indicates that shor
Page 19 and 20: SST response to the surface wind le
Page 21 and 22: Barnett, T. P., D. W. Pierce, R. Sa
Page 23 and 24: Newman, M. E., and P. D. Sardeshmuk
Page 25 and 26: Figure Captions Fig. 1. Spatial pat
Page 27 and 28: Fig. 1. Spatial patterns of SST ass
Page 29 and 30: Fig. 3. Same as in Fig. 2, but base
Page 35 and 36: Fig. 9. Seasonal variations of area

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