Description: Recent SST and atmospheric circulation anomaly data suggest that the 2015/16 El Niño event is quickly decaying. Some researchers have predicted a forthcoming La Niña event in late summer or early fall 2016. From the perspective of the modulation of tropical SST by solar activity, the authors studied the evolution of the 2015/16 El Niño event, which occurred right after the 2014 solar peak year. Based on statistical and composite analysis, a significant positive correlation was found between sunspot number index and El Niño Modoki index, with a lag of two years. A clear evolution of El Niño Modoki events was found within 1–3 years following each solar peak year during the past 126 years, suggesting that anomalously strong solar activity during solar peak periods favors the triggering of an El Niño Modoki event. The patterns of seasonal mean SST and wind anomalies since 2014 are more like a mixture of two types of El Niño (i.e. eastern Pacific El Niño and El Niño Modoki), which is similar to the pattern modulated by solar activity during the years following a solar peak. Therefore, the El Niño Modoki component in the 2015/16 El Niño event may be a consequence of solar activity, which probably will not decay as quickly as the eastern Pacific El Niño component. The positive SST anomaly will probably sustain in the central equatorial Pacific (around the dateline) and the northeastern Pacific along the coast of North America, with a low-intensity level, during the second half of 2016.

Description: As an important external forcing, the effect of the 11-yr solar cycle on the tropical Pacific decadal variability is an interesting question. Here, we systematically investigate the phase-locking of the atmosphere and ocean covariations to the solar cycle in the tropical Pacific and propose a new mechanism to explain these decadal covariations. In both observation/reanalysis datasets and a solar cycle forced sensitivity experiment (named the SOL experiment), the ocean heat content anomalies (OHCa; 300 m) resemble a La Niña–like pattern in the solar cycle ascending phase, and the Walker circulation shifts westward. In the declining phase, the opposite is true. The accumulative solar irradiation directly contributes to this coherent decadal variability via changing the warm water volume and the solar-related heat is redistributed by the ocean dynamic processes. During the 11-yr solar cycle, the Pacific Walker circulation anomalies maintain the OHCa in the western equatorial Pacific and work as negative feedback for the eastern Pacific to help the OHCa phase transition. In addition, oceanic meridional heat transport via the subtropical cells and the propagation of off-equatorial Rossby waves also provide a lagged negative feedback to the OHCa phase transition according to the 11-yr solar cycle. The decadal coupled responses of the tropical Pacific climate system are 2 years more lag in the SOL experiment than in the observation/reanalysis. Significance Statement Here, we propose a new mechanism that the heating effect of the accumulative solar irradiation during the 11-yr solar cycle can be “integrated” into the tropical Pacific OHC and then provide a bottom-up effect on the atmosphere at decadal time scales. The strongly coupled processes in this region amplify the decadal phase-locking of the covariations to the 11-yr solar cycle. Our study demonstrates the role of the 11-yr solar cycle in the tropical Pacific decadal variability and provides a new explanation for the “bottom-up” mechanism of the solar cycle forcing. Our results update the understanding of the tropical Pacific decadal variability and may help to improve climate predictions at decadal time scales.

Description: El Niño–Southern Oscillation (ENSO) is a major source for teleconnections, including towards the tropical North Atlantic (TNA) region, whereby TNA sea surface temperatures (SSTs) are positively correlated with ENSO in boreal spring following an ENSO event. However, the Pacific–Atlantic connection can be impacted by different ENSO characteristics, such as the amplitude, location, and timing of Pacific SST anomalies (SSTAs). Indeed, the TNA SSTAs may respond nonlinearly to strong and extreme El Niño events. However, observational data for the number of extreme ENSO events remain limited, restricting our ability to investigate the influence of observed extreme ENSO events. To overcome this issue and to further evaluate the nonlinearity of the TNA SSTA response, two coupled climate models are used, namely the Community Earth System Model version 1 – Whole Atmosphere Community Climate Model (CESM-WACCM) and the Flexible Ocean and Climate Infrastructure version 1 (FOCI). In both models the TNA SSTAs respond linearly to ENSO during extreme El Niño events but nonlinearly to extreme La Niña events for CESM-WACCM. We investigate differences by using indices for all major mechanisms that connect ENSO to the TNA and compare them with reanalysis. CESM-WACCM and FOCI overall represent the teleconnection well, including that the tropical and extratropical pathways are similar to observations. Our results also show that a large portion of the nonlinearity during La Niña is explained by the interaction between Pacific SSTAs and the overlying upper-level divergence.

Description: Despite several studies on decadal-scale solar influence on climate, a systematic analysis of the Sun's contribution to decadal surface climate predictability is still missing. Here, we disentangle the solar-cycle-induced climate response from internal variability and from other external forcings such as greenhouse gases. We utilize two 10-member ensemble simulations with a state-of-the-art chemistry–climate model, to date a unique dataset in chemistry–climate modeling. Using these model simulations, we quantify the potential predictability related to the solar cycle and demonstrate that the detectability of the solar influence on surface climate depends on the magnitude of the solar cycle. Further, we show that a strong solar cycle forcing organizes and synchronizes the decadal-scale component of the North Atlantic Oscillation, the dominant mode of climate variability in the North Atlantic region.

Description: This paper uses two subsets of ensemble historical-Nat simulations and pi-Control simulations from CMIP5 as well as observational/reanalysis datasets to investigate responses of the tropical Pacific to the 11-yr solar cycle. A statistically significant 11-yr solar signal is found in the upper-ocean layers above the thermocline and tropospheric circulations. A warming response initially appears in the upper layers of the central equatorial Pacific in the solar maximum years in observations, then increases and shifts into the eastern Pacific at lagged 1–3 yr. Meanwhile, an anomalous updraft arises over the western equatorial Pacific and shifts eastwards in the following years with anomalous subsidence over the Maritime Continent. These lagged responses are confirmed by the historical-Nat simulations, except that the initial signal is located more to the west and all the responses are weaker than the observed. A simplified mixed-layer heat budget analysis based on the historical-Nat simulations suggests that the atmospheric forcing, especially the shortwave radiation, is the major contributor to the initial warming response, and the ocean heat transport effect is responsible for the eastward displacement of the lagged warming responses. In the solar maximum years, the zonal ocean temperature gradient in the western-central Pacific is reduced by the initial warming, and anomalous westerly winds appear over the western equatorial Pacific and extend into the eastern Pacific during the lagged years. These anomalous westerly winds reduce the wind-driven ocean dynamical transport, resulting in the initial warming in the central equatorial Pacific being amplified and the surface warming shifting eastward during the lagged 1–3 yr

Description: A La Niña condition in the equatorial Pacific began in the early summer of 2020 and has lasted more than two and a half years (referred to as the 2020 La Niña hereafter). Predicting its temporal evolution had attracted a lot of attention. Considering the possible phase-locked impact of the 11-year solar cycle on the tropical Pacific variability, in this study the authors present the possible modulations by the solar cycle 25 (SC25) started from December 2019, on the future temporal evolution of the 2020 La Niña. Based on statistical features of historical solar cycles, the authors propose three possible scenarios of the timing of the SC25 maximum year and discuss its possible impacts on the temporal evolution of the 2020 La Niña in the next two years. The ongoing ascending phase of SC25 dampens the development of a super El Niño condition to some extent in 2023.

Description: Key Points: - North Atlantic biases are alleviated by an eddying nested ocean configuration embedded in a global climate model, FOCI-VIKING10 - It is indicated that reduction of the North Atlantic biases could improve the representation of NAO sub-decadal (8 years) variability - For detecting weak external imprints with limited computational resources, an ensemble with a coarse-resolution model is favorable Increasing the horizontal resolution of an ocean model is frequently seen as a way to reduce the model biases in the North Atlantic, but we are often limited by computational resources. Here, a two-way nested ocean model configuration (VIKING10) that consists of a high-resolution (1/10°) component and covers the northern North Atlantic, is embedded in a 1/2° ocean grid as part of the global chemistry-climate model, FOCI (called FOCI-VIKING10). This configuration yields a significantly improved path of the North Atlantic current (NAC), which here reduces the North Atlantic cold bias by ∼50%. Compared with the coarse-resolution, non-eddying model, the improved thermal state of upper ocean layers and surface heat fluxes in a historical simulation based on FOCI-VIKING10 are beneficial for simulating the subdecadal North Atlantic Oscillation (NAO) variability (i.e., a period of 8 years). A northward drift of the NAO-forced ocean thermal anomalies as seen in observations and the eddying FOCI-VIKING10, provide a lagged ocean feedback to the NAO via changes in the net surface heat flux, leading to the NAO periodicity of 8 years. This lagged feedback and the 8 years variability of the NAO cannot be captured by the non-eddying standard FOCI historical simulation. Furthermore, the argumentative responses of the North Atlantic to the 11-year solar cycle are re-examined in this study. The reported solar-induced NAO-like responses are confirmed in the 9-member ensemble mean based on FOCI but with low robustness among individual members. A lagged NAO-like response is only found in the nested eddying simulation but absent from the non-eddying reference simulation, suggesting North Atlantic biases importantly limit climate model capability to realistically solar imprints in North Atlantic climate.

Description: Previous studies indicated that the North Tropical Atlantic (NTA) SST can serve as a precursor for the El Niño–Southern Oscillation (ENSO) predictability and the connection of NTA-ENSO is modulated by the mid-high latitude atmospheric variability. Despite significant solar footprints being found in the North Atlantic and tropical Pacific separately, their role in the two basins’ connection is still missing. In this study, we systematically examined this point by using observational/reanalysis datasets and outputs of a pair of sensitivity experiments with and without solar forcings (SOL and NOSOL). In observations, DJF-mean NAO-like SLP anomalies have a linear covariation with the subsequent JJA-mean El Niño Modoki-like SST anomalies in the tropical Pacific in the following 1 year. This observed SLP-SST covariation shows up in the high solar activity (HS) subset and disappears in the low solar activity (LS) subset. In the HS years, positive NAO-like SLP anomalies are produced by the stronger solar-UV radiation through a “top-down” mechanism. These atmospheric anomalies can enhance the influence of the NTA on the tropical Pacific SST by triggering significant and more persistent subtropical teleconnections. Here we proposed an indirect possible mechanism that the solar-UV forcing can modulate the tropical Pacific SST variability via its impacts on the atmospheric anomalies over the North Atlantic region. However, based on the same analysis method, we found a different coupled mode of the SLP and SST anomalies in the modeling outputs. The SLP anomalies in the North Atlantic, with a triple pattern (negative SLP anomalies in the Pole and the NTA, positive SLP anomalies in the mid-latitude), have “lead-lag” covariations with the Eastern Pacific El Niño-like SST anomalies in both the SOL and NOSOL. Although the impact of the solar activity is found in the North Atlantic and the tropical Pacific respectively in the SOL, no solar effect is involved in the simulated SLP-SST coupled mode.