Climate Dynamics https://doi.org/10.1007/s00382-020-05295-2 Pacifc decadal oscillation remotely forced by the equatorial Pacifc and the Atlantic Oceans Zachary F. Johnson1 · Yoshimitsu Chikamoto1 · S.‑Y. Simon Wang1 · Michael J. McPhaden2 · Takashi Mochizuki3,4 Received: 27 September 2019 / Accepted: 13 May 2020 © The Author(s) 2020 Abstract The Pacifc Decadal Oscillation (PDO), the leading mode of Pacifc decadal sea surface temperature variability, arises mainly from combinations of regional air-sea interaction within the North Pacifc Ocean and remote forcing, such as from the tropical Pacifc and the Atlantic. Because of such a combination of mechanisms, a question remains as to how much PDO variability originates from these regions. To better understand PDO variability, the equatorial Pacifc and the Atlantic impacts on the PDO are examined using several 3-dimensional partial ocean data assimilation experiments conducted with two global climate models: the CESM1.0 and MIROC3.2m. In these partial assimilation experiments, the climate models are constrained by observed temperature and salinity anomalies, one solely in the Atlantic basin and the other solely in the equatorial Pacifc basin, but are allowed to evolve freely in other regions. These experiments demonstrate that, in addition to the tropical Pacifc’s role in driving PDO variability, the Atlantic can afect PDO variability by modulating the tropical Pacifc climate through two proposed processes. One is the equatorial pathway, in which tropical Atlantic sea surface tem- perature (SST) variability causes an El Niño-like SST response in the equatorial Pacifc through the reorganization of the global Walker circulation. The other is the north tropical pathway, where low-frequency SST variability associated with the Atlantic Multidecadal Oscillation induces a Matsuno-Gill type atmospheric response in the tropical Atlantic-Pacifc sectors north of the equator. These results provide a quantitative assessment suggesting that 12–29% of PDO variance originates from the Atlantic Ocean and 40–44% from the tropical Pacifc. The remaining 27–48% of the variance is inferred to arise from other processes such as regional ocean-atmosphere interactions in the North Pacifc and possibly teleconnections from the Indian Ocean. Keywords PDO · AMO · Multidecadal variability · Climate model · ENSO 1 Introduction calculated by a statistical decomposition called empirical orthogonal functions (EOFs) (Mantua et al. 1997; Minobe The Pacifc Decadal Oscillation (PDO) is the dominant 1997; Zhang and Levitus 1997). The positive (negative) low frequency climate variability in the North Pacifc and phase of the PDO is characterized by cool (warm) sea sur- is defned as the leading mode of North Pacifc detrended face temperatures (SSTs) in the Kuroshio-Oyashio Exten- ◦ ◦ sea surface temperature anomalies (SSTAs) ( 20 –70 N ) sion (KOE) region and its surrounding warm (cool) SSTs along the west coast of North America (Mantua and Hare * Zachary F. Johnson 2002; Qiu 2002; Newman et al. 2016). Associated with these [email protected] physical ocean changes, the coherent decadal variability is also observed in marine ecosystems and biogeochemical 1 Department of Plants, Soils, and Climate, Utah State cycles in the North Pacifc, such as decadal shifts in fsh University, 4820 Old Main Hill, Logan UT 84322, USA production, plankton biomass, and nutrient concentrations 2 Pacifc Marine Environmental Laboratory/NOAA, Seattle, (Francis and Hare 1994; Mantua and Hare 2002; Chavez WA, USA et al. 2003). In addition to these regional changes, the PDO 3 Department of Earth and Planetary Sciences, Kyushu has shown remote associations through atmospheric tele- University, Fukuoka, Japan connections with multi-decadal drought and pluvial condi- 4 Japan Agency for Marine-Earth Science and Technology, tions over the United States, Canada, Siberia, Australia, and Yokohama, Japan Vol.:(0123456789)1 3 Z. F. Johnson et al. northern South America (Barlow et al. 2001; Tanimoto et al. Interestingly, most CMIP3 and CMIP5 models capture the 2003; McCabe et al. 2004; Hu and Huang 2009; Zanchettin inter-basin impact from the Atlantic to the Pacifc, although et al. 2008; Wang et al. 2009; Taguchi et al. 2012; Wang the strength of the atmospheric response to Atlantic SST et al. 2014; Vance et al. 2015). From the perspective of variability and the subsequent ENSO amplitude show vast these broad impacts, improving our understanding of PDO diversity among the models (Ham and Kug 2015). mechanisms is essential, not only for enhancing PDO pre- The mechanisms of the Atlantic impact on the PDO dictability, but also for extending its application to marine are still under debate, but three major processes have been ecosystem predictions (Mochizuki et al. 2010; Chikamoto proposed: inter-basin interactions, midlatitude jet stream et al. 2013; Kim et al. 2014; Chikamoto et al. 2015a). activity, and an Arctic Ocean process through the Bering Two processes mainly characterize PDO mechanisms: Strait. As described above, Atlantic Ocean variability can regional air-sea interactions in the North Pacifc and remote afect the frequency and amplitude of ENSO events through forcings from other ocean regions (Kwon and Deser 2007; tropical inter-basin interactions (Ham and Kug 2015), which Liu and Alexander 2007; Newman et al. 2016), such as the subsequently infuences the PDO through the atmospheric tropical Pacifc and Atlantic. While it is challenging to sepa- bridge (Alexander 1990). Consistent with this mechanism, rate regional air-sea interactions and remote impacts on the Levine et al. (2018) found that a positive Atlantic Multidec- PDO due to feedback mechanisms (Zhang and Delworth adal Oscillation (AMO) can favor a northward shift in the 2015), there is a scientifc consensus that ENSO variability intertropical convergence zone (ITCZ) through changes in is the primary forcing for the PDO on interannual timescales equatorial trade winds and precipitation anomalies, which through the “atmospheric bridge” and subsequent changes then afect tropical Pacifc SSTs. A similar inter-basin con- in Aleutian low variability (Trenberth et al. 1998; Alexan- nection through a zonal circulation, but in the mid-latitude der et al. 2004; Strong and Magnusdottir 2009). Despite the region instead of the tropics, was also proposed based on previous research on the atmospheric bridge process through the mid-latitude sea level pressure (SLP) see-saw between many observational analyses and modeling experiments the North Atlantic and North Pacifc associated with the (Alexander 1990; Lau and Nath 1994, 1996; Nakamura AMO (Sun et al. 2017; Gong et al. 2020). By contrast to et al. 1997; Barlow et al. 2001; Deser et al. 2004), there is these inter-basin interactions, other studies proposed that large model uncertainty regarding timing and strength of the AMO can afect the PDO through midlatitude jet stream the ENSO-PDO relationship. In simulations from the Cou- activities (Zhang and Delworth 2007; Wu et al. 2011; Oku- pled Model Intercomparison Project (CMIP), maximum mura et al. 2009). In this midlatitude process, three pathways correlation coefcients of the ENSO-PDO relationship in are still under debate: a meridional shift in the midlatitude pre-industrial control simulations range from 0.2–0.9 (Nid- westerlies between the North Atlantic and Pacifc (Zhang heesh et al. 2017), which covers the observational estimate and Delworth 2007), changes in the Arctic Oscillation origi- ∼ ( 0.6), yet there is still signifcant model diversity (Alexan- nating from North Atlantic SSTs (Zhang and Zhao 2015), der et al. 2008; Hu and Huang 2009). In addition to strength, and circumglobal Rossby wave propagation from the tropi- the time lag between the ENSO and PDO indices in CMIP3 cal Atlantic to the North Pacifc via the South Asian jet and CMIP5 models ranges from almost instantaneous to (Okumura et al. 2009; Wu et al. 2011). The third process is 8 months with most CMIP models having a 4–8 month through the Arctic and Bering Strait by introducing fresh- lag. As a result, there is still large uncertainty in the cur- water forcing in the Atlantic Meridional Overturning Cir- rent estimate of PDO variability originating from the tropi- culation (AMOC) (Okumura et al. 2009). Because of these cal Pacifc, which ranges from 25–50% of PDO variance. inter-basin and atmosphere-ocean interactions, appropriate However, some of the uncertainty may simply be internal modeling experiments using a fully coupled atmosphere- variability (Alexander et al. 2002, 2010; Zhang et al. 2018). ocean general circulation model (AOGCM) are required to Uncertainty of the ENSO-PDO relationship may also evaluate the most prominent pathway from the Atlantic to come from the Atlantic remote forcing through inter-basin the North Pacifc. interactions. Recent studies pointed out that Atlantic dec- Whereas just a few studies simulated an Atlantic origin adal variability can afect tropical Pacifc climate variabil- for PDO variability, many models captured the equatorial ity through a modulation of the global Walker circulation Pacifc impact on the PDO through the atmospheric bridge (Rodríguez-Fonseca et al. 2009; Kucharski et al. 2011; Chi- process, including CMIP, Atmospheric Model Intercompari- kamoto et al. 2012; McGregor et al. 2014; Kucharski et al. son Project (AMIP), mixed-layer, partial decoupling, pace- 2016; Li et al. 2015; Cai et al. 2019). On interannual time- maker, and partial assimilation experiments. In the 1990s,