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We investigate the land–ocean warming contrast mechanisms, ϕ, defined as the land-mean surface air temperature (SAT) change divided by the ocean-mean SAT change, in a transient climate response (TCR) obtained from the Coupled Model Intercomparison Project Phase 6 (CMIP6) 1% per year CO2 increase experiments (1pctCO2). The energy budget framework devised in Part I is applied to 15 CMIP6 1pctCO2 simulations, and the climate response in year 140 when the CO2 concentration was quadrupled was compared with a near-equilibrium climate response (NEQ), defined as the last 30-year mean in the abrupt CO2 quadrupling (abrupt4×CO2) experiments. It is shown that ϕ is larger in TCR than in NEQ by approximately 4%, although the difference is not statistically significant. In TCR, effective radiative forcing is large over land compared to the ocean, and this is the main contributor to ϕ as in NEQ. The time evolution of ϕ in 1pctCO2 can be reconstructed by means of the fast and slow components of climate response in abrupt4×CO2, indicating that the essential mechanism for the land–ocean warming contrast shown in Part I applies to TCR. Furthermore, our analyses reveal a compensation between land-to-ocean atmospheric energy transport that decreases ϕ and ocean heat uptake that increases ϕ. Regardless of the time scale of the response, these two processes are linked by the change in atmospheric circulation, leading to the small combined effect. As a result, the multi-model mean ϕ in 1pctCO2 is roughly time-invariant at approximately 1.5 despite the continuous increase in CO2.

Modeling studies have shown that surface air temperature (SAT) increase in response to an increase in the atmospheric CO2 concentration is larger over land than over ocean. This so-called land–ocean warming contrast, φ, defined as the land–mean SAT change divided by the ocean-mean SAT change, is a striking feature of global warming. Small heat capacity over land is unlikely the sole cause because the land-ocean warming contrast is found in the equilibrium state of CO2 doubling experiments.Several different mechanisms have been proposed to explain the land–ocean warming contrast, but the comprehensive understanding has not yet been obtained. In Part I of this study, we propose a framework to diagnose φ based on energy budgets at the top of atmosphere and for the atmosphere, which enables the decomposition of contributions from effective radiative forcing (ERF), climate feedback, heat capacity, and atmospheric energy transport anomaly to φ. Using this framework, we analyzed the SAT response to an abrupt CO2 quadrupling using 15 Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system models. In the near-equilibrium state (years 121-150), φ is 1.49 ± 0.11, which is primarily induced by the land–ocean difference in ERF and heat capacity. We found that contributions from ERF, feedback, and energy transport anomaly tend to cancel each other, leading to a small inter-model spread of φ compared to the large spread of individual components. In the equilibrium state without heat capacity contribution, ERF and energy transport anomaly are the major contributors to φ, which shows a weak negative correlation with the equilibrium climate sensitivity.

Since the early twentieth century, observations have shown that the ocean surface has warmed almost globally, but the rate of sea surface temperature (SST) rise in the Kuroshio region (KR) has been twice that of the global-mean. This study focuses on the mechanisms driving long-term changes in SST in the KR during the period 1951–2010 using observations, reanalysis data, and 10-member ensembles of historical simulations based on a global climate model (GCM) called MIROC 5.2. The observational data indicates that SST in the KR slightly decreased until around 1980, but rapidly increased afterwards. The MIROC5.2 historical experiment reproduced the observed multidecadal changes in the KR SST, and a signal-to-noise analysis showed that one third to half of the changes were attributable to a forced component, represented by the ensemble mean. Comparison of historical runs with another ensemble in which sulfate aerosols were fixed at preindustrial levels, highlighted that multidecadal SST changes in the KR were primarily driven by sulfate aerosols, contributing to the warming for 1981–2010 as much as greenhouse gases. Analyses of the MIROC5.2 ensemble mean revealed that the key factor for enhanced warming after the 1980s in the KR was a high-pressure trend over the North Pacific. This is because it advects warm air to the northwestern Pacific in the lower troposphere and causes a northward shift in the Kuroshio current. The presence of this mechanism was supported by a significant correlation between SST trends in the KR and high-pressure trends over the North Pacific based on historical simulations from 27 GCMs, and a large ensemble of historical simulations using MIROC6. These results demonstrate the importance of an externally forced atmospheric circulation response over the North Pacific as it relates to enhanced ocean surface warming in the KR.