Abstract:Equilibrium climate sensitivity (ECS) to doubling of atmospheric CO 2 concentration is a key index for understanding the Earth's climate history and prediction of future climate changes. Tropical low cloud feedback, the predominant factor for uncertainty in modeled ECS, diverges both in sign and magnitude among climate models. Despite its importance, the uncertainty in ECS and low cloud feedback remains a challenge. Recently, researches based on observations and climate models have demonstrated a possibility t… Show more
“…As reviewed by Kamae et al (2016) and Klein et al (2017), near-surface temperature advection is recognized as an important cloud controlling factor that represents how strongly large-scale atmospheric circulation enhances upward SHF in favor of the formation of lowlevel clouds. As shown in Figs.…”
The south Indian Ocean is characterized by enhanced midlatitude storm-track activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene high. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the storm-track activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced storm-track activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although its zonally extended distribution is correspondent with that of LCF, EIS alone cannot explain the wintertime LCF enhancement, which precedes the EIS maximum under continuous lowering of SST and enhanced SHF in winter. Basinwide cold advection associated with the wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 158-258S, while seasonally enhanced storm-track activity augments SHF around 308S. The analysis highlights the significance of large-scale controls, particularly through SHF, on the seasonality of the climatological LCF distribution over the south Indian Ocean, which reflect the seasonality of the Mascarene high and storm-track activity.
“…As reviewed by Kamae et al (2016) and Klein et al (2017), near-surface temperature advection is recognized as an important cloud controlling factor that represents how strongly large-scale atmospheric circulation enhances upward SHF in favor of the formation of lowlevel clouds. As shown in Figs.…”
The south Indian Ocean is characterized by enhanced midlatitude storm-track activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene high. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the storm-track activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced storm-track activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although its zonally extended distribution is correspondent with that of LCF, EIS alone cannot explain the wintertime LCF enhancement, which precedes the EIS maximum under continuous lowering of SST and enhanced SHF in winter. Basinwide cold advection associated with the wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 158-258S, while seasonally enhanced storm-track activity augments SHF around 308S. The analysis highlights the significance of large-scale controls, particularly through SHF, on the seasonality of the climatological LCF distribution over the south Indian Ocean, which reflect the seasonality of the Mascarene high and storm-track activity.
“…In the ascending branch of the tropical hydrological cycle some observational studies concluded on weak positive correlations between both upper-tropospheric cloudiness and SST, as well as precipitation efficiency and SST (e.g., Lin et al 2006;Su et al 2008), while others observed a narrowing and strengthening of the Hadley cell with smaller average cloud cover (Su et al 2017). Low liquid water clouds in subsidence regions are another key uncertainty for climate sensitivity prediction (Bony et al 2004;Bony and Dufresne 2005;Zhai et al 2015;Kamae et al 2016;Ceppi et al 2017;Klein et al 2017). This study quantifies the cloud cover evolution with SST under both strong ascent and strong descent in instantaneous observations.…”
Better understanding of how moisture, clouds, and precipitation covary under climate warming lacks a comprehensive observational view. This paper analyzes the tropical atmospheric water cycle’s evolution with sea surface temperature (SST), using for the first time, the synergistic dataset of instantaneous observations of the relative humidity profile from the Megha-Tropiques satellite, clouds from the CALIPSO satellite, and near-surface precipitation from the CloudSat satellite, and quantifies their rates of change with SST warming. The dataset is partitioned into three vertical velocity regimes, with cloudy grid boxes categorized by phase (ice or liquid), opacity (opaque or thin), and the presence of near-surface precipitation. Opaque cloud cover is always larger in the presence of near-surface precipitation (high ice clouds especially). Low liquid water clouds in the descending regime dominate for SSTs < 299.25 K, where the free troposphere is dry (~20%), and opaque liquid water cloud cover decreases with SST warming (−8% K−1) and thin liquid water cloud cover stays constant (~20%). High ice clouds dominate the ascending regime in which, for 299.25 < SST < 301.75 K, humidity increases with SST in the lower free troposphere and peaks around 302 K. Over the warm SST range (>301.75 K), in the ascending regime, opaque high ice cloud cover decreases with SST (−13% K−1), while thin ice cloud cover increases (+6% K−1). Over the warm SST range, total cloudiness decreases with warming in all regimes. This paper characterizes fundamental relationships between aspects of the tropical atmospheric water cycle and SST.
“…Given that the instantaneous radiative forcing and the stratospheric adjustment are spatially uniform, the tropospheric adjustment associated with clouds is likely the key to this mechanism (Fig. 9b; Andrews et al 2012;Sherwood et al 2015;Kamae et al 2015). The horizontal distribution of all-sky and cloud-sky ERFs and the corresponding change in cloud cover (not mediated by the surface temperature change) are illustrated in Fig.…”
Section: B Reinterpretation Of Existing Mechanismsmentioning
confidence: 99%
“…This happens within several days after the initial perturbation but remains as long as the radiative forcing exists (Dong et al 2009;Doutriaux-Boucher et al 2009). At longer time scales, land clouds may decrease with the drying of land associated with the global-mean SAT increase (Fasullo 2010;J08;Joshi et al 2013;Kamae et al 2016), thus enhancing land warming by reducing shortwave reflection.…”
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.
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