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Underestimated Responses of Walker Circulation to ENSO-Related SST

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Underestimated Responses of Walker Circulation to ENSO-Related SST Wang et al. Geosci. Lett. (2021) 8:17 https://doi.org/10.1186/s40562-021-00186-8 RESEARCH LETTER Open Access Underestimated responses of Walker circulation to ENSO-related SST anomaly in atmospheric and coupled models Xin‑Yue Wang1,2, Jiang Zhu1,2, Chueh‑Hsin Chang3, Nathaniel C. Johnson4,5, Hailong Liu1,2, Yadi Li1,2, Chentao Song1,2, Meijiao Xin1,2, Yi Zhou1,2 and Xichen Li1,6* Abstract The Pacifc Walker circulation (WC) is a major component of the global climate system. It connects the Pacifc sea surface temperature (SST) variability to the climate variabilities from the other ocean basins to the mid‑ and high latitudes. Previous studies indicated that the ENSO‑related atmospheric feedback, in particular, the surface wind response is largely underestimated in AMIP and CMIP models. In this study, we further investigate the responses in the WC stream function and the sea level pressure (SLP) to the ENSO‑related SST variability by comparing the responses in 45 AMIP and 63 CMIP models and six reanalysis datasets. We reveal a diversity in the performances of simulated SLP and WC between diferent models. While the SLP responses to the El Niño‑related SST variability are well simulated in most of the atmospheric and coupled models, the WC stream function responses are largely underestimated in most of these models. The WC responses in the AMIP5/6 models capture ~ 75% of those in the reanalysis, whereas the CMIP5/6 models capture ~ 58% of the responses. Further analysis indicates that these underestimated circula‑ tion responses could be partially attributed to the biases in the precipitation scheme in both the atmospheric and coupled models, as well as the biases in the simulated ENSO‑related SST patterns in the coupled models. One should pay special attention to these biases when studying the WC or the tropical atmosphere–ocean interactions using numerical models. Keywords: Pacifc Walker circulation, ENSO, AMIP5/6 and CMIP5/6 models, Atmosphere–ocean interaction, Model bias Introduction Carpenter 1982; Tanaka et al. 2004) and impacts global Te Pacifc Walker circulation (WC) is a large-scale zonal climate through atmospheric teleconnections (Lau and overturning circulation in the atmosphere, which is char- Yang 2003; Power et al. 1999). During an El Niño or La acterized by rising motion and deep convection over the Niña event, anomalous SST in equatorial Pacifc leads equatorial western Pacifc and Maritime Continent, and to changes of the WC, whereas the WC strengthens or strong easterly surface winds over the equatorial Pacifc weakens the SST gradient in turn (Wang et al. 2016). (Bjerknes 1969; Lau and Yang 2003). On interannual Te so-called Bjerknes feedback is the most funda- time scales, variations in the WC are tightly connected mental mechanism to maintain tropical Pacifc air–sea to El Niño–Southern Oscillation (ENSO, Rasmusson and interaction. Te WC is the essential medium between the atmos- phere and the ocean (Lau and Yang 2003). Interaction *Correspondence: [email protected] 1 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, between WC and SST determines regional-to-global China climate variability (Li et al. 2016; Wang et al. 2012). Te Full list of author information is available at the end of the article capability of present climate models to simulate these © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Wang et al. Geosci. Lett. (2021) 8:17 Page 2 of 13 interactions and feedbacks is a long-lasting research to ENSO-related SST variability in the boreal winter topic that directly afects the reliability of climate predic- (December, January and February; DJF) in both AMIP tions and projections. In particular, the Coupled Model and CMIP models by comparing with six independent Intercomparison Project and the Atmospheric Model reanalysis datasets. In previous studies, surface wind, Intercomparison Project phase 5 and 6 (CMIP5/6 and sea level pressure and mass stream function are usu- AMIP5/6; Gates et al. 1999; Taylor et al. 2012) multi- ally used to examine the change of Pacifc Walker cir- model simulation results may help us to examine the culation in observations and models (e.g., Kociuba and skill of these models in simulating the feedbacks between Power 2015; Vecchi and Soden 2007; Ma et al. 2016; Yu tropical ocean and atmosphere. and Zwiers 2010). And surface wind responses to ENSO Recently, a series of studies suggested that the simu- SST anomalies in climate models are underestimated lated zonal SST gradient over the equatorial Pacifc, the (Bayr et al. 2020; Hayashi et al. 2020). However, the biases zonal surface wind feedback, the variability of east–west in reproduced responses of sea level pressure and mass sea level pressure gradient, and the negative shortwave stream function to ENSO-related SST variability in the cloud feedback, etc., are all underestimated by most of state-of-the-art climate models remain unclear and are the CMIP models compared with observations (Bayr worth exploring. By statistical analysis, we found that et al. 2020; Bellenger et al. 2014; Davey et al. 2002; DiN- models show a diversity in reproducing the responses ezio et al. 2013; Kociuba and Power 2015; Mechoso to ENSO-related SST. Te responses of sea level pres- et al. 1995; Stocker et al. 2013). In particular, the wind sure simulated by AMIP5/6 and CMIP5/6 models are feedback is underestimated in both AMIP5 and CMIP5 reasonable. However, the responses of mass stream func- models (Bayr et al. 2020). Te weaker zonal surface wind tion are signifcantly underestimated in AMIP and CMIP feedback and surface-heat fux feedback are associated models compared to that in reanalyses. Further, we show with deep convection over the equatorial central Pacifc evidences that, the biases in the precipitation responses and further involve SST bias over the equatorial Pacifc in both the atmospheric and coupled models, as well as (Bayr et al. 2020). Meanwhile, the underestimated zonal the biases in the simulated ENSO-related SST patterns surface wind feedback in CMIP models reduces the in the coupled models, are important contributors to the biases in nonlinear dynamical heating along the equato- underestimation. rial Pacifc thermocline, declining modeled ENSO asym- metry (Hayashi et al. 2020). In addition, the coupled models also sufer from the double Intertropical Conver- Data and methods gence Zone (ITCZ) problem (Zhang et al. 2015). A recent In this study, we represent the ENSO-related Pacifc SST study indicated that the metrics of ENSO processes are forcing with the Niño3.4 index (spatial-mean SST anom- not improved from CMIP5 to CMIP6 (Planton et al. alies over the area 5° S–5° N, 170° W–120° W), which is 2020), consistent with Hayashi et al. (2020). Te above a common metric for measuring the intensity of ENSO. biases may potentially infuence the interactions between Many studies considered the zonal gradient of Sea Level the WC and the tropical Pacifc SST variability. Many Pressure (SLP) in the equatorial Pacifc as an impor- studies have quantifed the ENSO-related atmospheric tant parameter characterizing the intensity of the WC, feedback biases in AMIP and CMIP models (Bayr et al. defned by the Southern Oscillation Index (SOI, the dif- 2020; Hayashi et al. 2020; Planton et al. 2020). However, ference of the spatial-mean SLP between the west Pacifc the responses and the feedbacks of SLP and zonal mass (5° S–5° N, 160°–80° W) and the east Pacifc (5° S–5° stream function to the ENSO-related SST forcing in the N, 80° E–160° E); Kociuba and Power 2015; Vecchi and state-of-the-art atmosphere and coupled models have not Soden 2007). In addition, zonal mass stream function is been well investigated. SLP and zonal mass stream func- used to depict the structure of the WC, as defned in Yu tion are important measures of the WC, and the change and Zwiers (2010) and Yu et al. (2012): of these two variables are also key components of ENSO p = a�ϕ feedback. � uDdp, g 0 Considering that ENSO events are typically phase- locked, the strongest SST and circulation anomalies over where Ψ is the zonal mass stream function, a is the Earth the central-eastern equatorial Pacifc typically appears radius, Δ ϕ is the width of the 5° S–5° N band along the in boreal winter (Rasmusson and Carpenter 1982; Wang equator in radians, g is the gravitational acceleration,uD is et al. 2016). In this study, we attempt to revisit the ENSO the divergent component of the zonal wind, and p is the feedback biases in AMIP and CMIP simulations in pressure. Te results of zonal mass stream function above terms of the WC that was defned by the SLP and mass 100 hPa are not shown in this study, because the stream stream function, and quantify the responses of the WC function is nearly zero above 100 hPa.
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