The summer Asia–North America teleconnection and its modulation by ENSO in Community Atmosphere Model, version 5 (CAM5)

Abstract

Seasonal forecasts of summer continental United States (CONUS) rainfall have relatively low skill, partly due to a lack of consensus about its sources of predictability. The East Asian monsoon (EAM) can excite a cross-Pacific Rossby wave train, also known as the Asia-North America (ANA) teleconnection. In this study, we analyze the ANA teleconnection in observations and model simulations from the Community Atmospheric Model, version 5 (CAM5), comparing experiments with prescribed climatological SSTs and prescribed observed SSTs. Observations indicate a statistically significant relationship between a strong EAM and increased probability of positive precipitation anomalies over the U.S. west coast and the Plains-Midwest. The ANA teleconnection and CONUS rainfall patterns are improved in the CAM5 experiment with prescribed observed SSTs, suggesting that SST variability is necessary to simulate this teleconnection over CONUS. We find distinct ANA patterns between ENSO phases, with the La Niña-related patterns in CAM5_obsSST disagreeing with observations.Using linear steady-state quasi-geostrophic theory, we conclude that incorrect EAM forcing location greatly contributed to CAM5 biases, and jet stream disparities explained the ENSOrelated biases. Finally, we compared EAM forcing experiments with different mean states using a simple dry nonlinear atmospheric general circulation model. Overall, the ANA pattern over CONUS and its modulation by ENSO forcing are well described by dry dynamics on seasonalto-interannual timescales, including the constructive (destructive) interference between El Niño (La Niña) modulation and the ANA patterns over CONUS.

Subseasonal Great Plains Rainfall via Remote Extratropical Teleconnections: Regional Application of Theory‐Guided Causal Networks

“…Zhou et al, 2020), it would be advantageous to investigate the potential impacts warm or cool phases have on the causal pathways and link magnitudes. It is possible that tropical forcing might influence the predictability of these casual links, such that EAM-forced patterns might be stronger during El Nino years (Malloy & Kirtman, 2022) or ENSO might have a greater causal link to the PNA, Great Plains LLJ, and related Plains rainfall.…”

“…(2020b) to understand subseasonal North American hydroclimate variability, and we focus on the EAM as a regionally significant branch of the ASM based on results from Malloy and Kirtman (2022). The subseasonal patterns related to WNPM and EAM convection in Figure 3 are different from the seasonal EAM‐forced patterns from Malloy and Kirtman (2022), demonstrating the importance of timescale for quantifying impacts (Yang et al., 2020). In addition, the definition/index and spatial scale of the drivers may affect interpretation of results; for instance, the WNPM in Di Capua et al.…”

“…In many of these studies, climate models were used to quantify the monsoon responses, usually by prescribed heating, and were compared to observations (Lopez et al., 2019; Malloy & Kirtman, 2022; Yang et al., 2020). In this case, causality is implied (amongst natural variability or chaos).…”

“…Understanding summertime continental U.S. (CONUS) hydroclimate predictability on the subseasonal‐to‐seasonal (S2S) timescale has been challenging, and relationships between tropical remote forcing and mid‐latitude circulation are difficult to assess due to the overall weak signals of the summer season (Trenberth et al., 1998; S. Zhou et al., 2012). Many studies suggest that Asian summer monsoon (ASM) variability on the seasonal‐to‐interannual timescale, especially over the West North Pacific (WNPM) and/or East Asian Monsoon (EAM) region, can influence CONUS hydroclimate via a quasi‐stationary Rossby wave response (Di Capua et al., 2020b; Kornhuber et al., 2019; Lopez et al., 2019; Malloy & Kirtman, 2022; Yang et al., 2020; Zhu & Li, 2016, 2018). The Great Plains low‐level jet (LLJ) is the prominent transporter of moisture into that region, and large‐scale LLJ anomalies are typically associated with rainfall events (Algarra et al., 2019; Arritt et al., 1997; Cook et al., 2008; Higgins et al., 1997; Malloy & Kirtman, 2020; Nayak & Villarini, 2017; Weaver & Nigam, 2008; Weaver et al., 2009).…”

“…In this case, causality is implied (amongst natural variability or chaos). For example, the EAM heating causes the elongated anomalous ridge over the North Pacific, anomalous trough over western North America, and anomalous ridge over eastern North America from the set of experiments in Malloy and Kirtman (2022). However, there are also ways to quantify causal links via data-driven methods that is, using observations alone.…”