Pacific International Center for High Technology Research

The importance of sea-ice loss on the Arctic amplification of near-surface warming remains contentious, as Arctic amplification emerges even in model experiments with disabled surface-albedo feedback. Here we show that the characteristics and underlying dynamics of Arctic amplification may change greatly in a future ice-free climate using a series of climate model experiments. Our analysis indicates that although Arctic amplification continues over the 22nd century, it weakens markedly with a less distinct seasonality in a future ice-free climate. These changes are found to occur because the strength and seasonality of Arctic amplification in the current climate are attributed mainly to a tight coupling between cold-season lapse-rate feedback and sunlit-season surface-albedo feedback. The substantial differences in the characteristics of simulated Arctic amplification between the current and future ice-free climate therefore suggest that the presence of Arctic sea ice is an essential component of the current Arctic amplification regime. Arctic amplification persists even in a year-round sea ice-free state due to greenhouse gas forcing, but weakens in a future year-round sea ice-free climate, implying Arctic sea ice’s importance, according to analysis of climate model simulations.

<strong class="journal-contentHeaderColor">Abstract.</strong> Large-scale interaction among the three tropical ocean basins is an area of intense research that is often conducted through experimentation with numerical models. A common problem is that modelling groups use different experimental setups, which makes it difficult to compare results and to delineate the role of model biases from differences in experimental setups. To address this issue, an experimental protocol for examining interaction among the tropical basins is introduced. The tropical basin interaction model intercomparison project (TBIMIP) consists of experiments in which sea surface temperatures (SSTs) are prescribed to follow observed values in selected basins. There are two types of experiments. One type, called standard pacemaker, consists of simulations in which SSTs are restored to observations in selected basins during a historical simulation. The other type, called pacemaker hindcast, consists of seasonal hindcast simulations in which SSTs are restored to observations during the forecast. TBIMIP is coordinated by the Climate and Ocean - Variability, Predictability, and Change (CLIVAR) Research Focus on Tropical Basin Interaction. The datasets from the model simulations will be made available to the community to facilitate and stimulate research on tropical basin interaction and its role in seasonal-to-decadal variability and climate change.

Abstract. Stratosphere-troposphere Processes And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) project has conducted new experiments to explore the modulation of the QBO by El Niño-Southern Oscillation (ENSO). This paper provides an overview of the experiment design and investigates the modulation of the QBO by ENSO using nine climate models from the QBOi. A key finding is a consistent lengthening of the QBO period during La Niña compared to El Niño across all models, aligning with observational evidence. However, the magnitude of this lengthening shows large intermodel differences. By contrast, even the sign of the ENSO effect on QBO amplitude varies among models -models employing variable parameterized gravity wave sources generally exhibit greater sensitivity of the QBO amplitude to the presence of ENSO than those models using fixed sources. The models capture key observed ENSO-related characteristics, including a weaker Walker circulation and increased equatorial precipitation during El Niño compared to La Niña, as well as a characteristic response in zonal mean zonal wind and temperature. These modulations influence the propagation and filtering of gravity waves. Notably, models with variable parameterized gravity wave sources show stronger wave forcing during El Niño, potentially explaining the shorter QBO period modulation in these models. Further investigation into the complex interplay between ENSO, gravity waves, and the QBO can contribute to improved model formulations.

The observed rate of global warming since the 1970s has been proposed as a strong constraint on equilibrium climate sensitivity (ECS) and transient climate response (TCR)-key metrics of the global climate response to greenhouse-gas forcing. Using CMIP5/6 models, we show that the inter-model relationship between warming and these climate sensitivity metrics (the basis for the constraint) arises from a similarity in transient and equilibrium warming patterns within the models, producing an effective climate sensitivity (EffCS) governing recent warming that is comparable to the value of ECS governing long-term warming under CO[Formula: see text] forcing. However, CMIP5/6 historical simulations do not reproduce observed warming patterns. When driven by observed patterns, even high ECS models produce low EffCS values consistent with the observed global warming rate. The inability of CMIP5/6 models to reproduce observed warming patterns thus results in a bias in the modeled relationship between recent global warming and climate sensitivity. Correcting for this bias means that observed warming is consistent with wide ranges of ECS and TCR extending to higher values than previously recognized. These findings are corroborated by energy balance model simulations and coupled model (CESM1-CAM5) simulations that better replicate observed patterns via tropospheric wind nudging or Antarctic meltwater fluxes. Because CMIP5/6 models fail to simulate observed warming patterns, proposed warming-based constraints on ECS, TCR, and projected global warming are biased low. The results reinforce recent findings that the unique pattern of observed warming has slowed global-mean warming over recent decades and that how the pattern will evolve in the future represents a major source of uncertainty in climate projections.

Abstract Most El Niño events decay after a peak in boreal winter, but some persist and strengthen again in the following year. Several mechanisms for regulating its decay pace have been proposed; however, their relative contributions have not been thoroughly examined yet. By analyzing the fast-decaying and persistent types of the events in a 1200-yr coupled simulation, we quantify the key dynamic and thermodynamic processes in the decaying spring that are critical to determining the decay pace of El Niño. The zonal advection due to upwelling Kelvin wave accounts for twice as much the cooling difference as evaporation or meridional advection does. The upwelling Kelvin wave is much stronger in the fast-decaying events than the others, and its strength is equally attributed to the reflected equatorial Rossby wave and the equatorial easterly wind forcing over the western Pacific in the preceding 2–3 months. Relative to the fast-decaying events, the evaporative cooling is weaker but the meridional warm advection is stronger in the persistent events. The former is due to more meridionally asymmetric wind and sea surface temperature anomalies (SSTA) signaling positive Pacific meridional mode. The latter results from the advection of equatorial warm SSTA by climatological divergent flow, and the warmer SSTA persists from the mature stage subject to weaker cloud-radiative cooling in response to the central-Pacific-type SSTA distribution in the persistent events relative to the fast-decaying events. Our result consolidates the existing knowledge and provides a more comprehensive and physical pathway for the causality of El Niño’s diverse duration.

Earth's global climate system is vast, complex, and intricately interrelated. Many areas are influenced by global-scale phenomena, including the "triple dip" La Niña conditions that

Abstract Arctic summer sea ice decline accelerated from the mid‐2000s to 2012, with the 2012 record low remaining unbroken. While frequent La Niña events during this period have been suggested as a driver of this trend acceleration, no convincing evidence has been presented. Here, using a climate model nudged to observed pan‐tropical sea surface temperatures (SST), we show that the back‐to‐back La Niña events during 2010–2011, followed by a North Pacific cooling and a marginal El Niño, were a key contribution to the 2012 record low. Specifically, the La Niña events in 2010–2011 warmed the Arctic Pacific sector, whereas tropical SST anomalies in 2012 strengthened the Greenland high pressure, leading to an Arctic dipole‐like pressure pattern and strengthening of transpolar ice drift. These Arctic temperature and circulation anomalies led to the record low sea ice extent in 2012, highlighting the strong influence of tropical SSTs on Arctic climate.

Abstract This study examines regional characteristics of atmospheric and oceanic feedback processes in the western and eastern equatorial Pacific, by applying a localized surface heating in the respective region in a hierarchy of climate models. A western Pacific forcing is largely offset by a negative shortwave cloud radiative effect and damping via wind‐evaporation‐SST feedback. In contrast, an eastern Pacific forcing, while being partially compensated for by ocean dynamical adjustments, induces an amplified warming extending to the central Pacific due to weak local damping mechanisms. As for the inter‐model spread of the future tropical Pacific surface warming pattern, the ocean heat uptake response in the east can explain much of the spread both on fast (&lt;5 years) and slow (&gt;100 years) timescales. Our results suggest that an “El Niño‐like” warming pattern is probable in response to increasing greenhouse gases owing to the strong negative feedback intrinsic to the western Pacific.