Since the pioneering work of Hasselmann (1976) there has been considerable scientific debate as to how much coupled ocean-atmospheric climate variability can be simply explained as a red noise ocean mixed-layer response to white noise atmospheric forcing. For example, modeling studies into causes of mid-latitude ocean variability often focus on to what extent the variability involves coupled ocean-atmosphere feedbacks (Latif and Barnett 1994; Latif and Barnett 1996; Latif et al. 1996; Blade 1997; Latif 1998; Barsugli and Battisti 1998; Barnett et al. 1999; Bretherton and Battisti, 2000; Ferreira et al. 2001; Liu et al. 2002; Wu et al. 2003) versus the uncoupled response to atmospheric stochastic white noise forcing (Hasselmann 1976; Frankignoul and Hasselmann 1977; Frankignoul et al. 1997; Jin 1997; Saravanan and McWilliams 1998; Weng and Neelin 1998; Xie et al. 2000; Miller and Schneider 2000; Seager et al. 2001; Schneider et al. 2002; Qiu 2003; Wu and Liu 2003). The coupled feedbacks are either viewed as a generalization of the Hasselman (1976) theory to include local air-sea interactions which amplify the low frequency response without any preferred time scale (Barsugli and Battisti 1998; Bretherton and Battisti 2000) or as involving a “delayed oscillator” due to ocean memory whereby the variability has some preferred time scale (i.e., Latif and Barnett 1994; 1996; Schneider et al. 2002).
Generally, the coupled air-sea feedbacks are stable requiring atmospheric stochastic forcing, and the inclusion of ocean dynamics is thought to enhance the variability (Barnett et al. 1999). The uncoupled stochastic forcing of the ocean includes a number of proposed physical mechanisms for the preferred low frequency. These mechanisms include oceanic advection processes associated with the mid-latitude gyre (Saravanan and McWilliams 1998), an atmospheric pattern of forcing with a preferred length scale or position (Weng and Neelin 1998; Qiu 2003; Wu and Liu 2003), the dynamical adjustment of the extra tropical ocean circulation via long baroclinic Rossby waves (Jin 1997), and Ekman pumping (Frankignoul et al. 1997; Xie et al. 2000; Miller and Schneider 2000; Seager et al. 2001; Schneider et al. 2002). Another possibility is that tropical forcing via some atmospheric “bridge” acts as a source of North Pacific decadal variations, which may or may not be amplified by coupled feedbacks (Nitta and Yamada 1989; Trenberth 1990; Graham et al. 1994; Zhang et al. 1997; Alexander 1992; Miller et al. 1994; Jacobs et al. 1994).
Recently, Kirtman and Shukla (2002) introduced the interactive ensemble coupling strategy, which is ideally suited for testing Hasselmann’s (1976) hypothesis within the context of a coupled GCM. The basic idea of the interactive ensemble strategy is to use ensembles of atmospheric realizations to reduce the stochastic forcing felt at the air-sea interface. Yeh and Kirtman (2004a) used the interactive ensemble approach to diagnose SST variability in the North Pacific. Applying the interactive ensemble approach leads to a reduction in the SSTA variance that is proportional to the number of ensemble members indicating that unstable coupled feedbacks are highly unlikely. While this has been the conventional wisdom for some time, this was the first time it has been demonstrated by removing the noise at the air-sea interface within the context of a CGCM. In addition, the impact of the internal atmospheric dynamics at the air-sea interface masks out much of the tropical-midlatitude SST teleconnections on interannual timescales, particularly at the northern flank of the sub-tropical gyre. Once this interference is reduced (i.e., by applying the interactive ensemble technique), tropical- midlatitude SST teleconnections are more easily detected (Yeh and Kirtman, 2004a). The variability associated with the “North Pacific Mode” was found to be unconnected to tropical variability, consistent with Deser and Blackmon (1995), Barnett et al. (1999) and Pierce et al. (2000).
