Slab Ocean Research Articles

Small Sensitivity of the Simulated Climate of Tidally Locked Aquaplanets to Model Resolution

Abstract Tidally locked terrestrial planets around low-mass stars are the prime targets of finding potentially habitable exoplanets. Several atmospheric general circulation models have been employed to simulate their possible climates; however, model intercomparisons showed that there are large differences in the results of the models even when they are forced with the same boundary conditions. In this paper, we examine whether model resolution contributes to the differences. Using the atmospheric general circulation model ExoCAM coupled to a 50 m slab ocean, we examine three different horizontal resolutions (440 km × 550 km, 210 km × 280 km, and 50 km × 70 km in latitude and longitude) and three different vertical resolutions (26, 51, and 74 levels) under the same dynamical core and the same schemes of radiation, convection, and clouds. Among the experiments, the differences are within 5 K in global-mean surface temperature and within 0.007 in planetary albedo. These differences are from cloud feedback, water vapor feedback, and the decreasing trend of relative humidity with increasing resolution. Relatively small-scale downdrafts between upwelling columns over the substellar region are better resolved and the mixing between dry and wet air parcels and between anvil clouds and their environment are enhanced as the resolution is increased. These reduce atmospheric relative humidity and high-level cloud fraction, causing a lower clear-sky greenhouse effect, a weaker cloud longwave radiation effect, and subsequently a cooler climate with increasing model resolution. Overall, the sensitivity of the simulated climate of tidally locked aquaplanets to model resolution is small.

Time Scales and Mechanisms for the Tropical Pacific Response to Global Warming: A Tug of War between the Ocean Thermostat and Weaker Walker

AbstractDifferent oceanic and atmospheric mechanisms have been proposed to describe the response of the tropical Pacific to global warming, yet large uncertainties persist on their relative importance and potential interaction. Here, we use idealized experiments forced with a wide range of both abrupt and gradual CO2 increases in a coupled climate model (CESM) together with a simplified box model to explore the interaction between, and time scales of, different mechanisms driving Walker circulation changes. We find a robust transient response to CO2 forcing across all simulations, lasting between 20 and 100 years, depending on how abruptly the system is perturbed. This initial response is characterized by the strengthening of the Indo-Pacific zonal SST gradient and a westward shift of the Walker cell. In contrast, the equilibrium response, emerging after 50–100 years, is characterized by a warmer cold tongue, reduced zonal winds, and a weaker Walker cell. The magnitude of the equilibrium response in the fully coupled model is set primarily by enhanced extratropical warming and weaker oceanic subtropical cells, reducing the supply of cold water to equatorial upwelling. In contrast, in the slab ocean simulations, the weakening of the Walker cell is more modest and driven by differential evaporative cooling along the equator. The “weaker Walker” mechanism implied by atmospheric energetics is also observed for the midtroposphere vertical velocity, but its surface manifestation is not robust. Correctly diagnosing the balance between these transient and equilibrium responses will improve understanding of ongoing and future climate change in the tropical Pacific.

Improved methods for estimating equilibrium climate sensitivity from transient warming simulations

Equilibrium climate sensitivity (ECS) refers to the total global warming caused by an instantaneous doubling of atmospheric CO2 from the pre-industrial level in a climate system. ECS is commonly used to measure how sensitive a climate system is to CO2 forcing; but it is difficult to estimate for the real world and for fully coupled climate models because of the long response time in such a system. Earlier studies used a slab ocean coupled to an atmospheric general circulation model to estimate ECS, but such a setup is not the same as the fully coupled system. More recent studies used a linear fit between changes in global-mean surface air temperature (ΔT) and top-of-atmosphere net radiation (ΔN) to estimate ECS from relatively short simulations. Here we analyze 1000 years of simulation with abrupt quadrupling (4 × CO2) and another 500-year simulation with doubling (2 × CO2) of pre-industrial CO2 using the CESM1 model, and three other multi-millennium (~5000 year) abrupt 4 × CO2 simulations to show that the linear-fit method considerably underestimates ECS due to the flattening of the −dN/dT slope, as noticed previously. We develop and evaluate three other methods, and propose a new method that makes use of the realized warming near the end of the simulations and applies the −dN/dT slope calculated from a best fit of the ΔT and ΔN data series to a simple two-layer model to estimate the unrealized warming. Using synthetic data and the long model simulations, we show that the new method consistently outperforms the linear-fit method with small biases in the estimated ECS using 4 × CO2 simulations with at least 180 years of simulation. The new method was applied to 4 × CO2 experiments from 20 CMIP5 and 19 CMIP6 models, and the resulting ECS estimates are about 10% higher on average and up to 25% higher for models with medium–high ECS (> 3 K) than those reported in the IPCC AR5. Our new estimates suggest an ECS range of about 1.78–5.45 K with a mean of 3.61 K among the CMIP5 models and about 1.85–6.25 K with a mean of 3.60 K for the CMIP6 models. Furthermore, stable ECS estimates require at least 240 (180) years of simulation for using 2 × CO2 (4 × CO2) experiments, and using shorter simulations may underestimate the ECS substantially. Our results also suggest that it is the forced −dN/dT slope after year 40, not the internally-generated −dN/dT slope, that is crucial for an accurate estimate of the ECS, and this forced slope may be fairly stable.

Extratropical Influence on the Tropical Rainfall Distribution

This review focuses on recent progress in understanding the extratropical influence on the annual- and zonal-mean intertropical convergence zone (ITCZ) position using a hierarchy of model simulations and theory. Significant progress in our theoretical understanding of the zonal-mean ITCZ position has been made utilizing simulations with a slab ocean. Interhemispheric contrasts in the atmospheric heating (e.g., via an anomalous radiative forcing in one hemisphere) lead to a compensating cross-equatorial energy transport by Hadley circulation adjustments and corresponding meridional ITCZ shifts. In particular, high-latitude radiative perturbations have a strong influence on the ITCZ position. The effectiveness of extratropical forcing for resulting in ITCZ shifts is amplified by cloud radiative feedbacks in the midlatitudes and tropical water vapor feedback associated with the ITCZ displacement. However, more recently conducted fully coupled model simulations tend to show a less pronounced extratropical influence on the ITCZ position due to additional compensating effects from ocean dynamics. The oceanic damping effect on ITCZ shifts results from distinct ocean circulation components, including the Atlantic Meridional Overturning Circulation and the wind-driven subtropical cell. Both the relative importance of different ocean circulation components and the roles of different radiative feedbacks are sensitive to forcing location, making the tropical hydroclimate response to extratropical forcing sensitive to the geographical location of the forcing, for instance, in which ocean basin it occurs. The interaction between radiative feedbacks and ocean dynamical adjustment further confounds the determination of extratropical influence on the ITCZ position, which has motivated a recently initiated model intercomparison project. The zonal-mean energetics framework needs to be refined to explain beyond the time- and zonal-mean ITCZ position so as to incorporate transient propagation features and the spatial distribution of the tropical precipitation response.

Monsoon Low Pressure System–Like Variability in an Idealized Moist Model

AbstractIn this paper, it is shown that westward-propagating monsoon low pressure system–like disturbances in the South Asian monsoon region can be simulated in an idealized moist general circulation model through the addition of a simplified parameterization of land. Land is parameterized as having one-tenth the heat capacity of the surrounding slab ocean, with evaporation limited by a bucket hydrology model. In this model, the prominent topography of the Tibetan Plateau does not appear to be necessary for these storm systems to form or propagate; therefore, focus is placed on the simulation with land but no topography. The properties of the simulated storms are elucidated using regression analysis and compared to results from composites of storms from comprehensive GCMs in prior literature and reanalysis. The storms share a similar vertical profile in anomalous Ertel potential vorticity to those in reanalysis. Propagation, however, does not seem to be strongly dictated by beta drift. Rather, it seems to be more closely consistent with linear moisture vortex instability theory, with the exception of the importance of the vertical advection term in the Ertel potential vorticity budget toward the growth and maintenance of disturbances. The results presented here suggest that a simplified GCM configuration might be able to be used to gain a clearer understanding of the sensitivity of monsoon low pressure systems to changes in the mean state climate.

Global variability in radiative-convective equilibrium with a slab ocean under a wide range of CO2 concentrations

In radiative-convective equilibrium (RCE), radiative cooling of the troposphere is roughly balanced by the vaporization enthalpy set free by precipitating moist convection. Many earlier studies restricted the investigation of RCE to the dynamics of the atmosphere with constant boundary conditions including prescribed surface temperature. We investigate a GCM setup where a slab ocean is coupled to the atmosphere, and we explore a wide range of CO2 concentrations. We obtain reliable statistical quantities from thousand-year-long simulations. For moderate CO2 concentrations, we find unskewed temporal variations of 1–2 K in global mean surface temperature, with an almost constant climate sensitivity of 2 K. At CO2 concentrations beyond four times the preindustrial value, the climate sensitivity decreases to nearly zero as a result of episodic global cooling events as large as 10 K. The dynamics of these cooling events are investigated in detail and shown to be associated with an increase in large-scale low-level stratiform cloudiness in the subsiding region, which is a result of penetrative shallow convection being capped by an inversion and thus not ventilating the lower troposphere. These dynamics depend on the CO2 concentration: both through the effect of temperature on stratification and through the changing spatial scale of organization of the flow, which determines the spatial scale and temporal coherence of the stratiform cloud sheets.

A modelling exploration of the sensitivity of the India’s climate to irrigation

The Indian subcontinent is one of the most highly irrigated regions of the world where water use has intensified since the second half of the twentieth century. This agricultural intensification has resulted in changes in the water and energy balance between land surface and atmosphere is affected by these changes, which further influences climatic parameters that directly affect us. Over India, the practice of irrigation has changed in recent years resulting in an increased agricultural productivity as well as a depletion of groundwater. In this study, we use the Community Earth System Model CESM1.2 (Community Land Model (CLM4.5) with an active crop model coupled to the Community Atmospheric Model and a slab ocean) to examine the sensitivity of India’s climate to the amount of irrigation. We vary the amount of irrigation water by varying the irrigation factor in the CLM4.5 to produce a range of conditions from a bare minimum of no water stress in the crop to saturated soil. By holding all other forcings constant at year 2000 levels, we are able to examine the changes in temperature and precipitation over India due to varying amounts of irrigation. We find that irrigation decreases the maximum and minimum temperatures by nearly 3 °C and 4 °C respectively over the most heavily irrigated parts of the Indian subcontinent as a result of increased latent heat partitioning of energy. The resultant cooling (about 4% in annual mean temperature across India) impacts the monsoon circulation and reduces the moisture transport resulting in a statistically significant decrease of 1.46–4.17% in the all India summer monsoon rainfall with a decrease over the eastern part of the Gangetic basin and an increase over the Punjab region of Pakistan and India. While the large effect on temperature and precipitation may be a result of the model simulations having an irrigation seasonality that peaks in the pre-monsoon months, there is significant disagreement across different model based irrigation estimates on the timing and amount of the peak irrigation water added. This uncertainty in irrigation needs to be resolved to better understand its effects on India’s climate.

Atmospheric Dynamics in High Obliquity Planets

Ongoing discoveries of terrestrial exoplanets and the desire to determine their habitability have created an increasing demand for studies of a wide range of climatic regimes and atmospheric circulations. These studies have, in turn, challenged our understanding of our own planet’s atmospheric dynamics and provided new frameworks with which we can further our understanding of planetary atmospheres. In this work, we use an idealized moist general circulation model in aquaplanet configuration to study the atmospheric circulation of terrestrial planets with high obliquities. With seasonally varying insolation forcing and a shallow slab ocean as a lower boundary, we emphasize seasonal phenomena that might not be captured in simulations with annual mean forcing and that might involve nonlinear behaviors. By progressively increasing obliquity, we explore the response of the large-scale atmospheric circulation to more extreme seasonal cycles and a reversed annual mean equator-to-pole insolation distribution, and its impact on the energy and water cycles. We show that for high obliquities, the large-scale atmospheric circulation and the meridional energy transport are dominated by seasonally reversing broad cross-equatorial Hadley cells that transport energy from the summer to the winter hemisphere and significantly mitigate temperature extremes. These overturning cells also play a major role in shaping the planet’s hydrological cycle, with the associated ascending branches and precipitation convergence zones becoming progressively broader and more poleward shifted into the summer hemisphere with higher obliquities. While not embedded within the Hadley cell ascending branches, the hot summer poles of high obliquity planets experience nonnegligible precipitation during and at the end of the warm season: during the summer, lower-level moist static energy maxima at the summer poles force locally enhanced convective activity. As temperatures rapidly drop at the end of the summer and convective activity decreases, the water-holding capacity of the atmosphere decreases and water vapor stored in the atmospheric column rapidly condenses out, extending the duration of the summer pole rainy season into the corresponding autumn. Our study reveals novel understanding of how atmospheric dynamics might influence a planet’s overall climate and its variability.

An assessment of scale-dependent variability and bias in global prediction models

The paper presents a method for the scale-dependent validation of the spatio-temporal variability in global weather or climate models and for their bias quantification in relation to dynamics. The method provides a relationship between the bias and simulated spatial and temporal variance by a model in comparison with verifying reanalysis data. For the low resolution (T30L8) subset of ERA-20C data, it was found that 80–90% (depending on season) of the global interannual variance is at planetary scales (zonal wavenumbers k = 0−3), and only about 1% of the variance is at scales with $$k>7$$. The reanalysis is used to validate a T30L8 GCM in two configurations, one with the prescribed sea-surface temperature (SST) and another using a slab ocean model. Although the model with the prescribed SST represents the average properties of surface fields well, the interannual variability is underestimated at all scales. Similar to variability, model bias is strongly scale dependent. Biases found in the experiment with the prescribed SST are largely increased in the experiment using a slab ocean, especially in $$k=0$$, in scales with missing variability and in seasons with poorly simulated energy distribution. The perfect model scenario (a comparison between the GCM coupled to a slab ocean vs. the same model with prescribed SSTs) shows that the representation of the ocean is not critical for synoptic to subsynoptic variability, but essential for capturing the planetary scales.

The Role of Interactive SST in the Cloud‐Resolving Simulations of Aggregated Convection

AbstractThis study investigates the role of interactive sea surface temperature (SST) in the early development of aggregated convection using a vector vorticity equation cloud‐resolving model coupled to a slab ocean. The simulations are initialized by a mock Walker circulation driven by initial SST gradient in the elongated x axis, with an average of 300 K and sinusoidal variation of amplitude ranging from 1.5 to 3 K. According to large‐scale perturbation strength, which is caused by SST variation, the results can be divided into two groups. Under weak perturbation, convection‐SST feedback efficiently eliminates SST gradient and moisture anomaly. The large‐scale environment is homogenized within 2 days. Even though SST in the group with stronger perturbation undergoes a similar process, significant moist static energy (MSE) advection in the boundary produces enough moisture difference to introduce virtual temperature effect and aggregation is triggered. Once dry zone starts to expand, radiative and convective effects regenerate SST gradient, which intensifies circulation and accelerates the process. We further show that the evolution of aggregation or not is captured by the trend of MSE‐EIS (estimated inversion strength) variance. The results highlight the boundary layer processes on the formation of aggregated convection in the tropics.

Assessment of Indian summer monsoon variability in a regional climate model coupled to a slab ocean model

A suite of Regional Climate Model (RegCM) experiments are performed over south Asia to examine the skill of RegCM to simulate the seasonal and sub-seasonal mean features of Indian summer monsoon (ISM). Because of coupled nature of ISM, three model experiments with RegCM are conducted to examine the skill of coupled and uncoupled configurations of RegCM by forcing it with observed SST (Exp1), coupling it to a simple slab ocean model (SOM) with constant mixed layer depth (MLD; Exp2) and with climatology of MLD (Exp3). The coupled experiments show an overall improvement in several aspects of ISM variability at seasonal and intraseasonal time-scales, despite bias in simulated SSTs. Between coupled experiments; Exp3 reduces biases in SST distribution over the region to the north of Arabian Peninsula, eastern Arabian Sea (AS), and broadly over north Indian Ocean (NIO). Noteworthy is the improved precipitation over central India (CI), head Bay of Bengal (BoB), as well as the representation of easterly wind shear in coupled experiments. At intraseasonal time scales, Exp3 produces spectral peaks above red noise at 25–50-day and 15–20-day periods closely representing the northward propagating intraseasonal mode and quasi-biweekly oscillating mode as in observed precipitation. The improved representation of spatial distribution of intraseasonal activity over NIO as well as the SST and precipitation relationships over head BoB and eastern AS is attributed to better representation of air-sea interaction in Exp3. In brief, the coupling improves the model skill for the true representation of mean ISM variability during boreal summer, and the thorough evaluation of model for longer periods is required to employ it as a downscaling tool for regional climate change studies.

Processes and Timescales in Onset and Withdrawal of “Aquaplanet Monsoons”

Abstract Aquaplanets with low-heat-capacity slab-ocean boundary conditions can exhibit rapid changes in the regime of the overturning circulation over the seasonal cycle, which have been connected to the onset of Earth’s monsoons. In spring, as the ITCZ migrates off the equator, it jumps poleward and a sudden transition occurs from an eddy-driven, equinoctial regime with two weak Hadley cells, to a near-angular-momentum-conserving, solstitial regime with a strong, cross-equatorial winter-hemisphere cell. Here, the controls on the transition latitude and rate are explored in idealized moist aquaplanet simulations. It is found that the transition remains rapid relative to the solar forcing when year length and slab-ocean heat capacity are varied, and, at Earth’s rotation rate, always occurs when the ITCZ reaches approximately 7°. This transition latitude is, however, found to scale inversely with rotation rate. Interestingly, the transition rate varies nonmonotonically with rotation, with a maximum at Earth’s rotation rate, suggesting that Earth may be particularly disposed to a fast monsoon onset. The fast transition relates to feedbacks in both the atmosphere and the slab ocean. In particular, an evaporative feedback between the lower-level branch of the overturning circulation and the surface temperature is identified. This accelerates monsoon onset and slows withdrawal. Last, comparing eddy-permitting and axisymmetric experiments shows that, in contrast with results from dry models, in this fully moist model the presence of eddies slows the migration of the ITCZ between hemispheres.

A refined model for the Earth\u2019s global energy balance

A commonly-used model of the global radiative budget assumes that the radiative response to forcing, R, is proportional to global surface air temperature T, R=lambda T. Previous studies have highlighted two unresolved issues with this model: first, the feedback parameter lambda depends on the forcing agent; second, lambda varies with time. Here, we investigate the factors controlling R in two atmosphere–slab ocean climate models subjected to a wide range of abrupt climate forcings. It is found that R scales not only with T, but also with the large-scale tropospheric stability S (defined here as the estimated inversion strength area-averaged over ocean regions equatorward of 50^circ {}). Positive S promotes negative R, mainly through shortwave cloud and lapse-rate changes. A refined model of the global energy balance is proposed that accounts for both temperature and stability effects. This refined model quantitatively explains (1) the dependence of climate feedbacks on forcing agent (or equivalently, differences in forcing efficacy), and (2) the time evolution of feedbacks in coupled climate model experiments. Furthermore, a similar relationship between R and S is found in observations compared with models, lending confidence that the refined energy balance model is applicable to the real world.

Modulation of Monsoon Circulations by Cross-Equatorial Ocean Heat Transport

Abstract Motivated by observations of southward ocean heat transport (OHT) in the northern Indian Ocean during summer, the role of the ocean in modulating monsoon circulations is explored by coupling an atmospheric model to a slab ocean with an interactive representation of OHT and an idealized subtropical continent. Southward OHT by the cross-equatorial cells is caused by Ekman flow driven by southwesterly monsoon winds in the summer months, cooling sea surface temperatures (SSTs) south of the continent. This increases the reversed meridional surface gradient of moist static energy, shifting the precipitation maximum over the land and strengthening the monsoonal circulation, in the sense of enhancing the vertical wind shear. However, the atmosphere’s cross-equatorial meridional overturning circulation is also weakened by the presence of southward OHT, as the atmosphere is required to transport less energy across the equator. The sensitivity of these effects to varying the strength of the OHT, fixing the OHT at its annual-mean value, and to removing the land is explored. Comparisons with more realistic models suggest that the idealized model used in this study produces a reasonable representation of the effect of OHT on SSTs equatorward of subtropical continents, and hence can be used to study the role of OHT in shaping monsoon circulations on Earth.

Ural Blocking as a Driver of Early‐Winter Stratospheric Warmings

AbstractThis study explores the early‐winter atmospheric response to Ural blocking anomalies in November, using a nudging technique to constrain the temperature and dynamics in a high‐top atmospheric model. Persistent Ural blocking anomalies in November are associated with a warm Arctic/cold Siberia pattern and increased upward planetary waves entering the stratosphere, leading to a warming of the polar vortex. This stratospheric response then propagates in the troposphere, leading to increased occurrence of the negative North Atlantic Oscillation in December and January. In contrast, simulations with perturbed Barents‐Kara sea ice and Siberian snow in November do not reproduce a significant atmospheric response. In simulations including a slab ocean, the Ural blocking induces Barents‐Kara sea ice and Siberia snow anomalies that resemble composite analyses from observations. These results highlight Ural blocking variability in November as a robust driver of early‐winter stratospheric warming while questioning causality between sea ice/snow and Ural blocking anomalies.

Sub-cloud moist entropy curvature as a predictor for changes in the seasonal cycle of tropical precipitation

Convective Quasi-Equilibrium (CQE) may be a useful framework for understanding the precipitation minus evaporation (P − E) response to CO2-induced warming. To explore this proposition, a suite of aquaplanet simulations with a slab ocean from the Tropical Rain belts with an Annual cycle and a Continent Model Intercomparison Project (TRACMIP) is analyzed. A linear relationship between P − E and the curvature of sub-cloud moist entropy, a criterion for the onset of a tropical direct overturning circulation under CQE conditions, is shown to exist across many of the TRACMIP simulations. Furthermore, this linear relationship is a skillful predictor of changes in P − E in response to CO2-induced warming. The curvature metric also shows improvement in predicting P − E changes compared to the simpler method of relating P − E directly to the sub-cloud moist entropy field or a simple ‘wet-get-wetter’ type null hypothesis, especially on seasonal and shorter timescales. Using fixed relative humidity in the curvature metric and sub-cloud moist entropy degrades their ability to predict P − E changes, implying that both temperature and relative humidity changes in the boundary layer are important for characterizing future precipitation changes. To understand why the curvature metric is a skillful predictor of hydrological changes, a moist static energy (MSE) budget analysis is performed for a subset of the TRACMIP models. MSE divergence by transient eddies, which is well parameterized as a downgradient diffusive process, has a similar spatiotemporal structure to the curvature term, suggesting transient eddies are an important component to understanding the linear relationship between the curvature term and P − E.

Aquaplanet Models on Eccentric Orbits: Effects of the Rotation Rate on Observables

Abstract Rotation and orbital eccentricity both strongly influence planetary climate. Eccentricities can often be measured for exoplanets, but rotation rates are currently difficult or impossible to constrain. Here we examine how the combined effects of rotation and eccentricity on observed emission from ocean-rich terrestrial planets can be used to infer their rotation rates in circumstances where their eccentricities are known. We employ an Earth climate model with no land and a slab ocean, and consider two eccentricities (e = 0.3 and 0.6) and two rotation rates: a fast Earth-like period of 24 hr, and a slower pseudo-synchronous period that generalizes spin synchronization for eccentric orbits. We adopt bandpasses of the Mid-Infrared Instrument on the James Webb Space Telescope as a template for future photometry. At e = 0.3 the rotation rates can be distinguished if the planet transits near periastron, because slow rotation produces a strong day–night contrast and thus an emission minimum during periastron. However, light curves behave similarly if the planet is eclipsed near periastron, as well as for either viewing geometry at e = 0.6. Rotation rates can nevertheless be distinguished using ratios of emission in different bands, one in the water vapor window with another in a region of strong water absorption. These ratios vary over an orbit by ≲0.1 dex for Earth-like rotation, but by 0.3–0.5 dex for pseudo-synchronous rotation because of large day–night contrast in upper-tropospheric water. For planets with condensible atmospheric constituents in eccentric orbits, rotation regimes might thus be distinguished with infrared observations for a range of viewing geometries.

The Effect of Arctic Sea Ice Loss on the Hadley Circulation

AbstractOne of the most robust responses of the climate system to future greenhouse gas emissions is the melting of Arctic sea ice. It is thus essential to elucidate its impacts on other components of the climate system. Here we focus on the response of the annual mean Hadley cell (HC) to Arctic sea ice loss using a hierarchy of model configurations: atmosphere only, atmosphere coupled to a slab ocean, and atmosphere coupled to a full‐physics ocean. In response to Arctic sea ice loss, as projected by the end of the 21st century, the HC shows negligible changes in the absence of ocean‐atmosphere coupling. In contrast, by warming the Northern Hemisphere thermodynamic coupling weakens the HC and expands it northward. However, dynamic coupling acts to cool the Northern Hemisphere which cancels most of this weakening and narrows the HC, thus opposing its projected expansion in response to increasing greenhouse gases.

Estimation of sea ice parameters from sea ice model with assimilated ice concentration and SST

Abstract. A multi-category numerical sea ice model CICE was used along with data assimilation to derive sea ice parameters in the region of Baffin Bay and Labrador Sea. The assimilation of ice concentration was performed using the data derived from the Advanced Microwave Scanning Radiometer (AMSR-E and AMSR2). The model uses a mixed-layer slab ocean parameterization to compute the sea surface temperature (SST) and thereby to compute the freezing and melting potential of ice. The data from Advanced Very High Resolution Radiometer (AVHRR-only optimum interpolation analysis) were used to assimilate SST. The modelled ice parameters including concentration, ice thickness, freeboard and keel depth were compared with parameters estimated from remote-sensing data. The ice thickness estimated from the model was compared with the measurements derived from Soil Moisture Ocean Salinity – Microwave Imaging Radiometer using Aperture Synthesis (SMOS–MIRAS). The model freeboard estimates were compared with the freeboard measurements derived from CryoSat2. The ice concentration, thickness and freeboard estimates from the model assimilated with both ice concentration and SST were found to be within the uncertainty in the observation except during March. The model-estimated draft was compared with the measurements from an upward-looking sonar (ULS) deployed in the Labrador Sea (near Makkovik Bank). The difference between modelled draft and ULS measurements estimated from the model was found to be within 10 cm. The keel depth measurements from the ULS instruments were compared to the estimates from the model to retrieve a relationship between the ridge height and keel depth.

How Asymmetries Between Arctic and Antarctic Climate Sensitivity Are Modified by the Ocean

AbstractWe investigate how the ocean response to CO2 forcing affects hemispheric asymmetries in polar climate sensitivity. Intermodel comparison of Phase 5 of the Coupled Model Intercomparison Project CO2 quadrupling experiments shows that even in models where hemispheric ocean heat uptake differences are small, Arctic warming still exceeds Antarctic warming. The polar climate impact of this evolving ocean response to CO2 forcing is then isolated using slab ocean experiments in a state‐of‐the‐art climate model. Overall, feedbacks over the Southern Hemisphere more effectively dissipate top‐of‐atmosphere anomalies than those over the Northern Hemisphere. Furthermore, a poleward shift in ocean heat convergence in both hemispheres amplifies destabilizing ice albedo and lapse rate feedbacks over the Arctic much more so than over the Antarctic. These results suggest that the Arctic is intrinsically more sensitive to both CO2 and oceanic forcings than the Antarctic and that ocean‐driven climate sensitivity asymmetry arises from feedback destabilization over the Arctic rather than feedback stabilization over the Antarctic.