Coupled Atmosphere Ocean General Circulation Model Research Articles

Role of the tropical Indian Ocean in orbitally induced mid-Holocene precipitation variation in northwest China

Investigating long-term natural precipitation variability in Northwest China is crucial for our understanding of the recent wetting trend in this region. This study explores the mechanisms of precipitation variations in Northwest China during the mid-Holocene (MH) using the Community Earth System Model (CESM). We emphasize the role of ocean feedbacks in the response of precipitation changes in the summer half-year (April–September; the rainy season of Northwest China) to orbital forcing, via comparing simulation results from the single atmosphere general circulation model (AGCM) and fully coupled atmosphere–ocean general circulation model (AOGCM) of the CESM. The AOGCM simulations reveal a decrease of precipitation in Northwest China during the MH compared to the pre-Industrial (PI) era, matching well with proxy records. The drier MH is reproduced by the AGCM with orbital forcing when the tropical Indian Ocean cooling is taken into account. The tropical Indian Ocean cooling is affected by the decrease of spring insolation, contrasting to Eurasian warming caused by the increase of summer insolation during the MH. The tropical Indian Ocean cooling contributes to a reduction of the upper-level westerlies to the south of the westerly jet axis and hence a northward shift of the westerly jet during the MH relative to the PI, via amplifying the decrease of meridional thermal gradient from the tropical Indian Ocean to northern Eurasia and weakening the strength of the South Asian high. The northward migration of the westerly jet benefits anomalous downward motion and ultimately weakens precipitation in Northwest China during the MH. Therefore, the tropical Indian Ocean plays an important role in orbital forcing of MH precipitation variations in Northwest China. We further infer that abnormal warming of the tropical Indian Ocean is probably a key factor for the recent wetting trend in Northwest China.

The simulation of mineral dust in the United Kingdom Earth System Model UKESM1

Abstract. Mineral dust plays an important role in Earth system models and is linked to many components, including atmospheric wind speed, precipitation and radiation, surface vegetation cover and soil properties and oceanic biogeochemical systems. In this paper, the dust scheme in the first configuration of the United Kingdom Earth System Model UKESM1 is described, and simulations of dust and its radiative effects are presented and compared with results from the parallel coupled atmosphere–ocean general circulation model (GCM) HadGEM3-GC3.1. Not only changes in the driving model fields but also changes in the dust size distribution are shown to lead to considerable differences to the present-day dust simulations and to projected future changes. UKESM1 simulations produce a present-day, top-of-the-atmosphere (ToA) dust direct radiative effect (DRE – defined as the change in downward net flux directly due to the presence of dust) of 0.086 W m−2 from a dust load of 19.5 Tg. Under climate change pathways these values decrease considerably. In the 2081–2100 mean of the Shared Socioeconomic Pathway SSP5–8.45 ToA DRE reaches 0.048 W m−2 from a load of 15.1 Tg. In contrast, in HadGEM3-GC3.1 the present-day values of −0.296 W m−2 and 15.0 Tg are almost unchanged at −0.289 W m−2 and 14.5 Tg in the 2081–2100 mean. The primary mechanism causing the differences in future dust projections is shown to be the vegetation response, which dominates over the direct effects of warming in our models. Though there are considerable uncertainties associated with any such estimates, the results presented demonstrate both the importance of the size distribution for dust modelling and also the necessity of including Earth system processes such as interactive vegetation in dust simulations for climate change studies.

Climate and ocean circulation in the aftermath of a Marinoan snowball Earth

Abstract. When a snowball Earth deglaciates through a very high atmospheric CO2 concentration, the resulting inflow of freshwater leads to a stably stratified ocean, and the strong greenhouse conditions drive the climate into a very warm state. Here, we use a coupled atmosphere–ocean general circulation model, applying different scenarios for the evolution of atmospheric CO2, to conduct the first simulation of the climate and the three-dimensional ocean circulation in the aftermath of the Marinoan snowball Earth. The simulations show that the strong freshwater stratification breaks up on a timescale of the order of 103 years, mostly independent of the applied CO2 scenario. This is driven by the upwelling of salty waters in high latitudes, mainly the Northern Hemisphere, where a strong circumpolar current dominates the circulation. In the warmest CO2 scenario, the simulated Marinoan supergreenhouse climate reaches a global mean surface temperature of about 30 ∘C under an atmospheric CO2 concentration of 15×103 parts per million by volume, which is a moderate temperature compared to previous estimates. Consequently, the thermal expansion of seawater causes a sea-level rise of only 8 m, with most of it occurring during the first 3000 years. Our results imply that the surface temperatures of that time were potentially not as threatening for early metazoa as previously assumed. Furthermore, the short destratification timescale found in this study implies that Marinoan cap dolostones accumulated during the deglacial period, given that they were deposited under the influence of a freshwater environment.

Planetary climate under extremely high vertical diffusivity

Aims. Planets with large moon(s) or those in the habitable zone of low-mass stars may experience much stronger tidal force and tide-induced ocean mixing than that on Earth. Thus, the vertical diffusivity (or, more precisely, diapycnal diffusivity) on such planets, which represents the strength of vertical mixing in the ocean, would be greater than that on Earth. In this study, we explore the effects of extremely high diffusivity on the ocean circulation and surface climate of Earth-like planets in one asynchronous rotation orbit. Methods. The response of planetary climate to 10 and 100 times greater vertical diffusivity than that found on Earth is investigated using a fully coupled atmosphere–ocean general circulation model. In order to perform a clear comparison with the climate of modern Earth, Earth’s orbit, land–sea configuration, and present levels of greenhouse gases are included in the simulations. Results. We find that a larger vertical diffusivity intensifies the meridional overturning circulation (MOC) in the ocean, which transports more heat to polar regions and melts sea ice there. Feedback associated with sea ice, clouds, and water vapor act to further amplify surface warming. When the vertical diffusivity is 10 (100) times the present-day value, the magnitude of MOC increases by ≈3 (18) times, and the global-mean surface temperature increases by ≈4 °C (10 °C). This study quantifies the climatic effect of an extremely strong vertical diffusivity and confirms an indirect link between planetary orbit, tidal mixing, ocean circulation, and surface climate. Our results suggest a moderate effect of varying vertical ocean mixing on planetary climate.

Response of the Upper‐Level Monsoon Anticyclones and Ozone to Abrupt CO2 Changes

AbstractThe summer monsoon anticyclones are the dominant climatological features of the Northern Hemispheric (NH) summertime circulation in the upper troposphere and lower stratosphere (UTLS). However, the response of these anticyclones to the increased levels of remains highly uncertain, as does the impact on the distribution of UTLS ozone and other tracers. This study examines the response of the NH summertime monsoon anticyclones and UTLS ozone to the abrupt increase in forcing using output from a suite of coupled ocean–atmosphere general circulation model simulations. These models show an equatorward shift of the Asian summer monsoon anticyclone, a weakening of the North American summer monsoon anticyclone, and a stronger westerly flow penetrating deep into the tropics above the Pacific Ocean and North America. We use additional idealized experiments from atmosphere‐only general circulation models with prescribed SSTs and sea ice concentration to isolate the direct atmospheric radiative effects from the indirect effect of SST warming on the UTLS monsoon anticyclones. Comparison between atmosphere‐only and coupled ocean–atmosphere experiments shows that SST warming is the principal mechanism producing UTLS monsoonal circulation changes. The experiments result in a significant reduction up to 40%–50% of the UTLS ozone in the northern tropics, which could have an impact on radiative balance near the surface.

Mitigation of the double ITCZ syndrome in BCC-CSM2-MR through improving parameterizations of boundary-layer turbulence and shallow convection

Abstract. The spurious double Intertropical Convergence Zone (ITCZ) is one of the most prominent systematic biases in coupled atmosphere–ocean general circulation models (CGCMs), and the underestimated marine stratus over eastern subtropical oceans has been recognized as a possible contributor. Rather than modifying the cloud scheme itself, this study significantly ameliorates the marine stratus simulation through improving parameterizations of boundary-layer turbulence and shallow convection in the medium-resolution Beijing Climate Center Climate System Model version 2 (BCC-CSM2-MR). The University of Washington moist turbulence scheme is implemented in BCC-CSM2-MR to better represent the stratocumulus, and a decoupling criterion is also introduced to the shallow convection scheme for improving the simulation of the stratocumulus-to-cumulus transition. Results show that the simulated precipitation in the eastern Pacific south of the Equator is largely reduced, alleviating the double ITCZ problem. The tropical precipitation asymmetry index increases from −0.024 in the original BCC-CSM2-MR to 0.147 in the revised BCC-CSM2-MR, which is much closer to the observation. The study suggests that improving parameterizations of boundary-layer turbulence and shallow convection is effective for mitigating the double ITCZ syndrome in CGCMs.

AMOC and Climate Responses to Dust Reduction and Greening of the Sahara during the Mid-Holocene

AbstractNorth Africa was green during the mid-Holocene [about 6000 years ago (6 ka)] and emitted much less dust to the atmosphere than in the present day. Here we use a fully coupled atmosphere–ocean general circulation model, CESM1.2.2, to test the impact of dust reduction and greening of the Sahara on the Atlantic meridional overturning circulation (AMOC) during this period. Results show that dust removal leads to a decrease of AMOC by 6.2% while greening of the Sahara with 100% shrub (100% grass) cover causes an enhancement of the AMOC by 6.1% (4.8%). The AMOC is increased by 5.3% (2.3%) when both the dust reduction and green Sahara with 100% shrub (100% grass) are considered. The AMOC changes are primarily due to the precipitation change over the west subtropical North Atlantic, from where the salinity anomaly is advected to the deep-water formation region. Global-mean surface temperature increases by 0.09° and 0.40°C (0.25°C) when global dust is removed and when North Africa and the Arabian region are covered by shrub (grass), respectively, showing a dominating effect of vegetation over dust. The comparison between modeled and reconstructed sea surface temperature is improved when the effect of vegetation is considered. The results may have implications for climate impact of future wetting over North Africa, either through global warming or through building of solar farms and wind farms.

Responses of global monsoon and seasonal cycle of precipitation to precession and obliquity forcing

In this paper, the response of global monsoon to changes in orbital forcing is investigated using a coupled atmosphere–ocean general circulation model with an emphasis on relative roles of precession and obliquity changes. When precession decreases, there are inter-hemispheric asymmetric responses in monsoonal precipitation, featuring a significant increase over most parts of the Northern Hemisphere (NH) monsoon regions and a decrease over the Southern Hemisphere (SH) monsoon regions. In contrast, when obliquity increases, global monsoon is enhanced except for the American monsoon. Dynamic effects (caused by changes in winds with humidity unchanged) dominate the monsoonal precipitation response to both precession and obliquity forcing, while thermodynamic effects (caused by changes in humidity with winds unchanged) is related to the northward extension of the North African summer monsoon. During minimum precession, the seasonal cycle of tropical precipitation is advanced with respect to the maximum precession. The rainfall increase in the transitional season (April-June in the NH and October-December in the SH) is dominated by the dynamic component. From an energetics perspective, the southward (northward) cross-equatorial energy transport during April-June (October-December) corresponds to a northward (southward) shift of tropical precipitation, which results in a seasonal advance in the migration of tropical precipitation. Nonetheless, there is no significant change in the seasonal cycle in response to obliquity forcing.

FORTE 2.0: a fast, parallel and flexible coupled climate model

Abstract. FORTE 2.0 is an intermediate-resolution coupled atmosphere–ocean general circulation model (AOGCM) consisting of the Intermediate General Circulation Model 4 (IGCM4), a T42 spectral atmosphere with 35σ layers, coupled to Modular Ocean Model – Array (MOMA), a 2∘ × 2∘ ocean with 15 z-layer depth levels. Sea ice is represented by a simple flux barrier. Both the atmosphere and ocean components are coded in Fortran. It is capable of producing a stable climate for long integrations without the need for flux adjustments. One flexibility afforded by the IGCM4 atmosphere is the ability to configure the atmosphere with either 35σ layers (troposphere and stratosphere) or 20σ layers (troposphere only). This enables experimental designs for exploring the roles of the troposphere and stratosphere, and the faster integration of the 20σ layer configuration enables longer duration studies on modest hardware. A description of FORTE 2.0 is given, followed by the analysis of two 2000-year control integrations, one using the 35σ configuration of IGCM4 and one using the 20σ configuration.

Improvements in tropical precipitation and sea surface air temperature fields in a coupled atmosphere–ocean data assimilation system

AbstractA coupled atmosphere–ocean data assimilation system, the Meteorological Research Institute‐Coupled Data Assimilation System Version 1 (MRI‐CDA1), was developed based on the coupled atmosphere–ocean general circulation model and separate atmosphere and ocean analysis routines operated by the Japan Meteorological Agency (JMA). To implement coupled atmosphere–ocean data assimilation, 6‐hr data assimilation cycles with the coupled model in the outer loop are adopted in atmospheric data assimilation, whereas incremental analysis updates with 10‐day data assimilation cycles are adopted in ocean data assimilation. A coupled data assimilation (reanalysis) experiment (CDA‐Exp) is conducted using MRI‐CDA1 along with an uncoupled reanalysis experiment (UCPL‐Exp) in which the same atmospheric component of the coupled model is forced by prescribed sea surface temperature (SST) similarly to the JMA reanalysis, JRA‐55. Climatological precipitation and variations in precipitation and sea surface air temperature (SAT) are better represented in CDA‐Exp than in JRA‐55, which is brought about by the modification of the atmosphere model physics of the coupled model to better represent climate states. In CDA‐Exp, the SST adjustment to the atmosphere amplifies the lead/lag correlations between subseasonal variations of SST and precipitation. The atmosphere–ocean coupling generates SST variations associated with tropical instability waves, and the SAT field responds to SST variations. The SST–precipitation and SST–SAT relationships on the weather timescale are also recovered in CDA‐Exp, although they are hardly seen in UCPL‐Exp. The coupled model physics generates weather‐timescale SST variations consistent with the atmospheric state, and the atmospheric parameters respond to the SST variations through the coupled model physics. This study suggests that the benefits of coupled data assimilation would be more evident if the ability of MRI‐CDA1 to represent SST variations and its interaction with the atmosphere are improved.

Tropical Pacific climate variability under solar geoengineering: impacts on ENSO extremes

Abstract. Many modelling studies suggest that the El Niño–Southern Oscillation (ENSO), in interaction with the tropical Pacific background climate, will change with rising atmospheric greenhouse gas concentrations. Solar geoengineering (reducing the solar flux from outer space) has been proposed as a means to counteract anthropogenic climate change. However, the effectiveness of solar geoengineering concerning a variety of aspects of Earth's climate is uncertain. Robust results are particularly challenging to obtain for ENSO because existing geoengineering simulations are too short (typically ∼ 50 years) to detect statistically significant changes in the highly variable tropical Pacific background climate. We here present results from a 1000-year-long solar-geoengineering simulation, G1, carried out with the coupled atmosphere–ocean general circulation model HadCM3L. In agreement with previous studies, reducing the solar irradiance (4 %) to offset global mean surface warming in the model more than compensates the warming in the tropical Pacific that develops in the 4 × CO2 scenario. We see an overcooling of 0.3 ∘C and a 0.23 mm d−1 (5 %) reduction in mean rainfall over the tropical Pacific relative to preindustrial conditions in the G1 simulation, owing to the different latitudinal distributions of the shortwave (solar) and longwave (CO2) forcings. The location of the Intertropical Convergence Zone (ITCZ) in the tropical Pacific, which moved 7.5∘ southwards under 4 × CO2, is restored to its preindustrial position. However, other aspects of the tropical Pacific mean climate are not reset as effectively. Relative to preindustrial conditions, in G1 the time-averaged zonal wind stress, zonal sea surface temperature (SST) gradient, and meridional SST gradient are each statistically significantly reduced by around 10 %, and the Pacific Walker Circulation (PWC) is consistently weakened, resulting in conditions conducive to increased frequency of El Niño events. The overall amplitude of ENSO strengthens by 9 %–10 % in G1, but there is a 65 % reduction in the asymmetry between cold and warm events: cold events intensify more than warm events. Notably, the frequency of extreme El Niño and La Niña events increases by ca. 60 % and 30 %, respectively, while the total number of El Niño events increases by around 10 %. All of these changes are statistically significant at either 95 or 99 % confidence level. Somewhat paradoxically, while the number of total and extreme events increases, the extreme El Niño events become weaker relative to the preindustrial state, while the extreme La Niña events become even stronger. That is, such extreme El Niño events in G1 become less intense than under preindustrial conditions but also more frequent. In contrast, extreme La Niña events become stronger in G1, which is in agreement with the general overcooling of the tropical Pacific in G1 relative to preindustrial conditions.

Pliocene Model Intercomparison Project (PlioMIP2) simulations using the Model for Interdisciplinary Research on Climate (MIROC4m)

Abstract. The second phase of the Pliocene Model Intercomparison Project (PlioMIP2) has attracted many climate modelling groups in its continuing efforts to better understand the climate of the mid-Piacenzian warm period (mPWP) when atmospheric CO2 was last closest to present-day levels. Like the first phase, PlioMIP1, it is an internationally coordinated initiative that allows for a systematic comparison of various models in a similar manner to the Paleoclimate Modelling Intercomparison Project (PMIP). Model intercomparison and model–data comparison now focus specifically on the interglacial at marine isotope stage KM5c (3.205 Ma), and experimental design is not only based on new boundary conditions but includes various sensitivity experiments. In this study, we present results from long-term model integrations using the MIROC4m (Model for Interdisciplinary Research on Climate) atmosphere–ocean coupled general circulation model, developed at the institutes CCSR, NIES and FRCGC in Japan. The core experiment, with CO2 levels set to 400 ppm, shows a warming of 3.1 ∘C compared to the pre-industrial period, with two-thirds of the warming being attributed to the increase in CO2. Although this level of warming is less than that in the equivalent PlioMIP1 experiment, there is slightly better agreement with proxy sea surface temperature (SST) data at PRISM3 (PRISM – Pliocene Research Interpretation and Synoptic Mapping) locations, especially in the northern North Atlantic where there were large model–data discrepancies in PlioMIP1. Similar spatial changes in precipitation and sea ice are seen and the Arctic remains ice-free in the summer in the core experiments of both phases. Comparisons with both the proxy SST data and proxy surface air temperature data from paleobotanical sites indicate a weaker polar amplification in model results. Unlike PlioMIP1, the Atlantic Meridional Overturning Circulation (AMOC) is now stronger than that of the pre-industrial period, even though increasing CO2 tends to weaken it. This stronger AMOC is a consequence of a closed Bering Strait in the PlioMIP2 paleogeography. Also, when present-day boundary conditions are replaced by those of the Pliocene, the dependency of the AMOC strength on CO2 is significantly weakened. Sensitivity tests show that lower values of CO2 give a global SST which is overall more consistent with the PRISM3 SST field presented in PlioMIP1, while SSTs at many of the PRISM4 sites are still too high to be reconciled with any of the model results. On the other hand, tropical Pacific SST in the core experiment agrees well with more recent proxy data, which suggested that PRISM3 SST there was overestimated. Future availability of climate reconstructions from proxy data will continue to help evaluate model results. The inclusion of dynamical vegetation and the effects of all possible extreme orbital configurations outside KM5c should be considered in future experiments using MIROC4m for the mPWP.

Temperature and precipitation responses to El Niño-Southern Oscillation in a hierarchy of datasets with different levels of observational constraints

El Nino-Southern Oscillation (ENSO) is the dominant mode of climate variability, affecting climate conditions over large areas of the globe. There are, however, substantial differences in how ENSO teleconnections with regional climate variability are represented in different datasets such as gridded observations and climate models. Here we examine the global responses of temperature and precipitation over land to ENSO within a hierarchy of datasets with different levels of observational constraints. Anomaly maps of observed El Nino and La Nina responses are compared to reanalysis, atmospheric models driven by observed sea surface temperature (SST), and coupled atmosphere–ocean general circulation models. There is a gradual decline in anomaly pattern agreement relative to observations moving down the dataset hierarchy to lower observational constraints. We find a positive relationship between the models’ fidelity in representing ENSO temperature and precipitation, though the relationship is stronger for El Nino teleconnections than La Nina. The models also reproduce El Nino response patterns with greater fidelity than La Nina patterns. Additionally, the fidelity of model-simulated responses to El Nino is related to the magnitude of SST variability in the tropical Pacific, but no such relationship could be found for La Nina responses. This comprehensive evaluation highlights the importance of realistically simulating atmospheric circulation and SST variability to better capturing the patterns of regional climate variability in response to ENSO in climate models.

Estimating the Effect of Radiative Feedback Uncertainties on Climate Response to Changes in the Concentration of Stratospheric Aerosols

Using the two-box energy balance model (EBM), we explore the climate system response to radiative forcing generated by variations in the concentrations of stratospheric aerosols and estimate the effect of uncertainties in radiative feedbacks on changes in global mean surface temperature anomaly used as an indicator of the response of the climate system to external radiative perturbations. Radiative forcing generated by stratospheric sulfate aerosols from the second-largest volcanic eruption in the 20th century, the Mount Pinatubo eruption in June 1991, was chosen for this research. The global mean surface temperature response to a specified change in radiative forcing is estimated as a convolution of the derived impulse response function corresponding to EBM with a function that describes the temporal change in radiative forcing. The influence of radiative feedback uncertainties on changes in the global mean surface temperature is estimated using several “versions” of the EBM. The parameters for different “versions” were identified by applying a specific procedure for calibrating the two-box EBM parameters using the results of climate change simulations conducted with coupled atmosphere–ocean general circulation models from the Coupled Model Intercomparison Project phase 5 (CMIP5). Changes in the global mean surface temperature caused by stratospheric aerosol forcing are found to be highly sensitive not only to radiative feedbacks but also to climate system inertia defined by the effective heat capacity of the atmosphere–land–ocean mixed layer system, as well as to deep-ocean heat uptake. The results obtained have direct implications for a better understanding of how uncertainties in climate feedbacks, climate system inertia and deep-ocean heat uptake affect climate change modelling.

Oceanographic Considerations for Exoplanet Life Detection

Abstract Liquid water oceans are at the center of our search for life on exoplanets because water is a strict requirement for life as we know it. However, oceans are dynamic habitats—and some oceans may be better hosts for life than others. In Earth’s ocean, circulation transports essential nutrients such as phosphate and is a first-order control on the distribution and productivity of life. Of particular importance is upward flow from the dark depths of the ocean in response to wind-driven divergence in surface layers. This “upwelling” returns essential nutrients that tend to accumulate at depth via sinking of organic particulates back to the sunlit regions where photosynthetic life thrives. Ocean dynamics are likely to impose constraints on the activity and atmospheric expression of photosynthetic life in exo-oceans as well, but we lack an understanding of how ocean dynamics may differ on other planets. We address this issue by exploring the sensitivity of ocean dynamics to a suite of planetary parameters using ROCKE-3D, a fully coupled ocean–atmosphere general circulation model. Our results suggest that planets that rotate slower and have higher surface pressure than Earth may be the most attractive targets for remote life detection because upwelling is enhanced under these conditions, resulting in greater nutrient supply to the surface biosphere. Seasonal deepening of the mixed layer on high-obliquity planets may also enhance nutrient replenishment from depth into the surface mixed layer. Efficient nutrient recycling favors greater biological activity, more biosignature production, and thus more detectable life. More generally, our results demonstrate the importance of considering oceanographic phenomena for exoplanet life detection and motivate future interdisciplinary contributions to the emerging field of exo-oceanography.

Spatial Radiative Feedbacks from Internal Variability Using Multiple Regression

AbstractThe sensitivity of the climate to CO2 forcing depends on spatially varying radiative feedbacks that act both locally and nonlocally. We assess whether a method employing multiple regression can be used to estimate local and nonlocal radiative feedbacks from internal variability. We test this method on millennial-length simulations performed with six coupled atmosphere–ocean general circulation models (AOGCMs). Given the spatial pattern of warming, the method does quite well at recreating the top-of-atmosphere flux response for most regions of Earth, except over the Southern Ocean where it consistently overestimates the change, leading to an overestimate of the sensitivity. For five of the six models, the method finds that local feedbacks are positive due to cloud processes, balanced by negative nonlocal shortwave cloud feedbacks associated with regions of tropical convection. For four of these models, the magnitudes of both are comparable to the Planck feedback, so that changes in the ratio between them could lead to large changes in climate sensitivity. The positive local feedback explains why observational studies that estimate spatial feedbacks using only local regressions predict an unstable climate. The method implies that sensitivity in these AOGCMs increases over time due to a reduction in the share of warming occurring in tropical convecting regions and the resulting weakening of associated shortwave cloud and longwave clear-sky feedbacks. Our results provide a step toward an observational estimate of time-varying climate sensitivity by demonstrating that many aspects of spatial feedbacks appear to be the same between internal variability and the forced response.

Roles of Sea Ice–Surface Wind Feedback in Maintaining the Glacial Atlantic Meridional Overturning Circulation and Climate

AbstractDuring glacial periods, climate varies between two contrasting modes, the interstadials and stadials. These climate changes are often associated with drastic reorganizations of the Atlantic meridional overturning circulation (AMOC). Previous studies highlight the important role of sea ice in maintaining contrasting modes of the AMOC through its insulating effect on the oceanic heat flux and the buoyancy flux (sea ice–buoyancy flux feedback); however, the effect of feedback from the atmosphere remains unclear. Here, the effect of sea ice–surface wind interactions over the North Atlantic Ocean on the AMOC is explored. For this purpose, results from comprehensive atmosphere–ocean coupled general circulation models (AOGCMs) are analyzed. Then, sensitivity experiments are conducted with the atmospheric component of the AOGCM. Last, to explore the impact of modifications in surface winds induced by sea ice on the maintenance of the AMOC, partially coupled experiments are conducted with the AOGCMs. Experiments show that the expansion of sea ice associated with a weakening of the AMOC reduces surface winds by suppressing the oceanic heat flux and increasing the atmospheric static stability. This wind anomaly then causes a weakening of the wind-driven ocean salt transport to the northern North Atlantic and maintains the weak AMOC, therefore working as a positive feedback. It is shown that, together with the sea ice–buoyancy flux and sea ice–surface wind feedback, changes in sea ice and oceanic heat flux sustain the contrasting modes of the AMOC. These results may provide useful information for interpreting the differences in the self-sustained internal oscillations of the AMOC produced by recent AOGCMs.

Modeling a modern-like <i>p</i>CO<sub>2</sub> warm period (Marine Isotope Stage KM5c) with two versions of an Institut Pierre Simon Laplace atmosphere–ocean coupled general circulation model

Abstract. The mid-Piacenzian warm period (3.264 to 3.025 Ma) is the most recent geological period with present-like atmospheric pCO2 and is thus expected to have exhibited a warm climate similar to or warmer than the present day. On the basis of understanding that has been gathered on the climate variability of this interval, a specific interglacial (Marine Isotope Stage KM5c, MIS KM5c; 3.205 Ma) has been selected for the Pliocene Model Intercomparison Project phase 2 (PlioMIP 2). We carried out a series of experiments according to the design of PlioMIP2 with two versions of the Institut Pierre Simon Laplace (IPSL) atmosphere–ocean coupled general circulation model (AOGCM): IPSL-CM5A and IPSL-CM5A2. Compared to the PlioMIP 1 experiment, run with IPSL-CM5A, our results show that the simulated MIS KM5c climate presents enhanced warming in mid- to high latitudes, especially over oceanic regions. This warming can be largely attributed to the enhanced Atlantic Meridional Overturning Circulation caused by the high-latitude seaway changes. The sensitivity experiments, conducted with IPSL-CM5A2, show that besides the increased pCO2, both modified orography and reduced ice sheets contribute substantially to mid- to high latitude warming in MIS KM5c. When considering the pCO2 uncertainties (+/-50 ppmv) during the Pliocene, the response of the modeled mean annual surface air temperature to changes to pCO2 (+/-50 ppmv) is not symmetric, which is likely due to the nonlinear response of the cryosphere (snow cover and sea ice extent). By analyzing the Greenland Ice Sheet surface mass balance, we also demonstrate its vulnerability under both MIS KM5c and modern warm climate.

Role of cloud microphysics in improved simulation of the Asian monsoon quasi-biweekly mode (QBM)

A major sub-seasonal variability of the tropics and sub-tropics, the quasi-biweekly mode (QBM), is known to have significant influence on the seasonal mean of the south Asian monsoon rainfall. A coupled Atmosphere–Ocean General Circulation Model (AOGCM) being essential for seasonal prediction, the ability of the AOGCMs in simulating the space–time characteristics with fidelity is critical for successful seasonal prediction of the south Asian monsoon in particular and seasonal prediction in the tropics in general. However, strength and weaknesses in simulating the QBM by AOGCMs have remained poorly investigated so far. Here, we examine the simulation of the QBM in AOGCM and show that improvement of parameterizations of both convection and microphysics is required to improve the simulation of the QBM. While the standard version of the model overestimates the variance of QBM and simulates a smaller scale Rossby waves (n = 1), the modified version of the model where the simple Arakawa–Schubert (SAS) convection parameterization is combined with a new improved microphysics parameterization (MCMv.1) proposed by us, simulates a more realistic space–time characteristics of the QBM. In yet another version of the model, we combine the new SAS with the new improved microphysics parameterization. Interestingly, this version of the model also simulates the space–time structure poorly with poor westward propagation and fragmented organization, but it simulates a reasonable variance. These results indicate that a synergy among the convective parameterization and microphysics parameterizations is critical in simulating the QBM in particular and equatorial waves in general. We show that most the biases in simulating the QBM may be related to the biases of the model in simulating the stratiform fraction of precipitation. While the simulation of the space–time characteristics of QBM is better simulated in the MCMv.1, the convective coupling is still too strong as compared to observations, an area for future improvement of the model.

How accurately can the climate sensitivity to hbox {CO}_{2} be estimated from historical climate change?

The equilibrium climate sensitivity (ECS, in K) to hbox {CO}_{2} doubling is a large source of uncertainty in projections of future anthropogenic climate change. Estimates of ECS made from non-equilibrium states or in response to radiative forcings other than hbox {2}times hbox {CO}_{2} are called “effective climate sensitivity” (EffCS, in K). Taking a “perfect-model” approach, using coupled atmosphere–ocean general circulation model (AOGCM) experiments, we evaluate the accuracy with which hbox {CO}_{2} EffCS can be estimated from climate change in the “historical” period (since about 1860). We find that (1) for statistical reasons, unforced variability makes the estimate of historical EffCS both uncertain and biased; it is overestimated by about 10% if the energy balance is applied to the entire historical period, 20% for 30-year periods, and larger factors for interannual variability, (2) systematic uncertainty in historical radiative forcing translates into an uncertainty of {pm },30, {rm to} ,45% (standard deviation) in historical EffCS, (3) the response to the changing relative importance of the forcing agents, principally hbox {CO}_{2} and volcanic aerosol, causes historical EffCS to vary over multidecadal timescales by a factor of two. In recent decades it reached its maximum in the AOGCM historical experiment (similar to the multimodel-mean hbox {CO}_{2} EffCS of 3.6 K from idealised experiments), but its minimum in the real world (1.6 K for an observational estimate for 1985–2011, similar to the multimodel-mean value for volcanic forcing). The real-world variations mean that historical EffCS underestimates hbox {CO}_{2} EffCS by 30% when considering the entire historical period. The difference for recent decades implies that either unforced variability or the response to volcanic forcing causes a much stronger regional pattern of sea surface temperature change in the real world than in AOGCMs. We speculate that this could be explained by a deficiency in simulated coupled atmosphere–ocean feedbacks which reinforce the pattern (resembling the Interdecadal Pacific Oscillation in some respects) that causes the low EffCS. We conclude that energy-balance estimates of hbox {CO}_{2} EffCS are most accurate from periods unaffected by volcanic forcing. Atmosphere GCMs provided with observed sea surface temperature for the 1920s to the 1950s, which was such a period, give a range of about 2.0–4.5 K, agreeing with idealised hbox {CO}_{2} AOGCM experiments; the consistency is a reason for confidence in this range as an estimate of hbox {CO}_{2} EffCS. Unless another explosive volcanic eruption occurs, the first 30 years of the present century may give a more accurate energy-balance historical estimate of this quantity.