Reply to: A critical examination of a newly proposed interhemispheric teleconnection to Southwestern US winter precipitation

Graph-Guided Regularized Regression of Pacific Ocean Climate Variables to Increase Predictive Skill of Southwestern U.S. Winter Precipitation

“…1b). To distinguish between the two different hydroclimatologies of the northern part and the central/ southern part of the SWUS, previous studies have used different approaches, such as focusing on the area below a certain latitude (Liu et al 2018) or considering only the climate divisions for which a specific predictor (e.g., the Niño-3.4 index, an ENSO index) exhibits a significant relation with precipitation (Mamalakis et al 2018). Here, we distinguish between the two precipitation regimes using an area-weighted principal component (PC) analysis.…”

“…Physically, El Niño (or La Niña) events typically associate with persistent low (or high) atmospheric pressure patterns over the northeastern Pacific (a teleconnection that materializes via quasi-stationary Rossby waves; Trenberth et al 1998;Castello and Shelton 2004), and thus disturb the location and strength of the wintertime jet stream, which can then bring more (or fewer) winter storms to the SWUS, leading to wet (or dry) conditions over the SWUS and dry (or wet) conditions over the northwestern United States. However, recent research shows that the ENSO effect on the atmospheric pressure and (consequently) on precipitation over the eastern Pacific and North America has been decreasing in strength during the last 3-4 decades, while many studies have highlighted to a greater or lesser extent that the western Pacific climatic state (e.g., SSTs) has been a stronger driver of precipitation variability over North America (Wang et al 2014;Baxter and Nigam 2015;Teng and Branstator 2017;Seager et al 2017;Swain et al 2017;Myoung et al 2018;Mamalakis et al 2018). On a similar note, new research (Johnson et al 2019) shows that during the last 3-4 decades, western Pacific SSTs have been important players in affecting the connection between the tropical atmospheric circulation and the eastern tropical Pacific SSTs, during weak ENSO events, which highlights changes in the tropical Pacific dynamics (see also Mamalakis et al 2019).…”

“…However, recent research shows that the ENSO effect on the atmospheric pressure and (consequently) on precipitation over the eastern Pacific and North America has been decreasing in strength during the last 3-4 decades, while many studies have highlighted to a greater or lesser extent that the western Pacific climatic state (e.g., SSTs) has been a stronger driver of precipitation variability over North America (Wang et al 2014;Baxter and Nigam 2015;Teng and Branstator 2017;Seager et al 2017;Swain et al 2017;Myoung et al 2018;Mamalakis et al 2018). On a similar note, new research (Johnson et al 2019) shows that during the last 3-4 decades, western Pacific SSTs have been important players in affecting the connection between the tropical atmospheric circulation and the eastern tropical Pacific SSTs, during weak ENSO events, which highlights changes in the tropical Pacific dynamics (see also Mamalakis et al 2019). Whether these changes in precipitation teleconnections and Pacific dynamics are a result of internal multidecadal climate variability or anthropogenic forcing is still not clear.…”

“…We explore the prediction skill of our methodology for the case study of predicting precipitation over the southwestern United States (SWUS), focusing on the winter season (specifically, November-March), when the majority of precipitation occurs. Despite the increasing attention that it has received over the years (Schonher and Nicholson 1989;McCabe and Dettinger 1999;Gershunov and Cayan 2003;Schubert et al 2016;Madadgar et al 2016;Liu et al 2018;Hao et al 2018;Zhang et al 2018;Mamalakis et al 2018;Pan et al 2019), early and accurate prediction of winter precipitation in SWUS remains a challenge, with significant implications for the region's population and economy (Howitt et al 2014(Howitt et al , 2015AghaKouchak et al 2015;Medellín-Azuara et al 2016). Traditional climatic drivers of SWUS precipitation (e.g., ENSO) explain just a small fraction of the interannual variability of precipitation totals, which in some cases are determined by a small number of winter storms (Dettinger et al 2011;Dettinger and Cayan 2014).…”

“…Traditional climatic drivers of SWUS precipitation (e.g., ENSO) explain just a small fraction of the interannual variability of precipitation totals, which in some cases are determined by a small number of winter storms (Dettinger et al 2011;Dettinger and Cayan 2014). Moreover, it is known that the ENSO relationship with SWUS climate undergoes multidecadal fluctuations (McCabe and Dettinger 1999;Yu et al 2012), with many recent studies pointing out that it has been losing strength in the recent decades, while the western Pacific climatic state is gaining in importance (Wang et al 2014;Baxter and Nigam 2015;Teng and Branstator 2017;Seager et al 2017;Swain et al 2017;Myoung et al 2018;Mamalakis et al 2018;Lee et al 2018). The special difficulty of this problem also arises because the SWUS lies within a transition zone between the subtropics and the midlatitudes (i.e., 308-408N).…”