Poleward Energy Flux Research Articles

Arctic warming induced by the Laurentide Ice Sheet topography

Abstract. It is well known that ice sheet–climate feedbacks are essential for realistically simulating the spatiotemporal evolution of continental ice sheets over glacial–interglacial cycles. However, many of these feedbacks are dependent on the ice sheet thickness, which is poorly constrained by proxy data records. For example, height estimates of the Laurentide Ice Sheet (LIS) topography at the Last Glacial Maximum (LGM; ∼ 21 000 years ago) vary by more than 1 km among different ice sheet reconstructions. In order to better constrain the LIS elevation it is therefore important to understand how the mean climate is influenced by elevation discrepancies of this magnitude. Here we use an atmospheric circulation model coupled to a slab-ocean model to analyze the LGM surface temperature response to a broad range of LIS elevations (from 0 to over 4 km). We find that raising the LIS topography induces a widespread surface warming in the Arctic region, amounting to approximately 1.5 ∘C per km of elevation increase, or about 6.5 ∘C for the highest LIS. The warming is attributed to an increased poleward energy flux by atmospheric stationary waves, amplified by surface albedo and water vapor feedbacks, which account for about two-thirds of the total temperature response. These results suggest a strong feedback between continental-scale ice sheets and the Arctic temperatures that may help constrain LIS elevation estimates for the LGM and explain differences in ice distribution between the LGM and earlier glacial periods.

Atmospheric Eddies Mediate Lapse Rate Feedback and Arctic Amplification

Projections of amplified climate change in the Arctic are attributed to positive feedbacks associated with the retreat of sea ice and changes in the lapse rate of the polar atmosphere. Here, a set of idealized aquaplanet experiments are performed to understand the coupling between high-latitude feedbacks, polar amplification, and the large-scale atmospheric circulation. Results are compared to CMIP5. Simulated climate responses are characterized by a wide range of polar amplification (from none to nearly 15-K warming, relative to the low latitudes) under CO2quadrupling. Notably, the high-latitude lapse rate feedback varies in sign among the experiments. The aquaplanet simulation with the greatest polar amplification, representing a transition from perennial to ice-free conditions, exhibits a marked decrease in dry static energy flux by transient eddies. Partly compensating for the reduced poleward energy flux is a contraction of the Ferrel cell and an increase in latent energy flux. Enhanced eddy energy flux is consistent with the upper-tropospheric warming that occurs in all experiments and provides a remote influence on the polar lapse rate feedback. The main conclusions are that (i) given a large, localized change in meridional surface temperature gradient, the midlatitude circulation exhibits strong compensation between changes in dry and latent energy fluxes, and (ii) atmospheric eddies mediate the nonlinear interaction between surface albedo and lapse rate feedbacks, rendering the high-latitude lapse rate feedback less positive than it would be otherwise. Consequently, the variability of the circulation response, and particularly the partitioning of energy fluxes, offers insights into understanding the magnitude of polar amplification.

The Role of Cloud Radiative Heating in Determining the Location of the ITCZ in Aquaplanet Simulations

Abstract The relationship between the tropical circulation and cloud radiative effect is investigated. Output from the Clouds On–Off Klimate Intercomparison Experiment (COOKIE) is used to examine the impact of cloud radiative effects on circulation and climate. In aquaplanet simulations with a fixed SST pattern, the cloud radiative effect leads to an equatorward contraction of the intertropical convergence zone (ITCZ) and a reduction of the double ITCZ problem. It is shown that the cloud radiative heating in the upper troposphere increases the temperature, weakens CAPE, and inhibits the onset of convection until it is closer to the equator, where SSTs are higher. Precipitation peaks at higher values in a narrower band when the cloud radiative effects are active, compared to when they are inactive, owing to the enhancement in moisture convergence. Additionally, cloud–radiation interactions strengthen the mean meridional circulation and consequently enhance the moisture convergence. Although the mean tropical precipitation decreases, the atmospheric cloud radiative effect has a strong meridional gradient, which supports stronger poleward energy flux and speeds up the Hadley circulation. Cloud radiative heating also enhances cloud water path (liquid plus ice), which, combined with the reduction in precipitation, suggests that the cloud radiative heating reduces precipitation efficiency in these models.

Climate Feedbacks and Their Implications for Poleward Energy Flux Changes in a Warming Climate

Abstract Feedbacks determine the efficiency with which the climate system comes back into equilibrium in response to a radiative perturbation. Although feedbacks are integrated quantities, the processes from which they arise have rich spatial structures that alter the distribution of top of atmosphere (TOA) net radiation. Here, the authors investigate the implications of the structure of climate feedbacks for the change in poleward energy transport as the planet warms over the twenty-first century in a suite of GCMs. Using radiative kernels that describe the TOA radiative response to small perturbations in temperature, water vapor, and surface albedo, the change in poleward energy flux is partitioned into the individual feedbacks that cause it. This study finds that latitudinal gradients in the sum of climate feedbacks reinforce the preexisting latitudinal gradient in TOA net radiation, requiring that the climate system transport more energy to the poles on a warming planet. This is primarily due to structure of the water vapor and cloud feedbacks, which are strongly positive at low latitudes and decrease dramatically with increasing latitude. Using the change in surface fluxes, the authors partition the anomalous poleward energy flux between the atmosphere and ocean and find that reduced heat flux from the high-latitude ocean further amplifies the equator-to-pole gradient in atmospheric energy loss. This implied reduction in oceanic poleward energy flux requires the atmosphere to increase its share of the total poleward energy transport. As is the case for climate sensitivity, the largest source of intermodel spread in the change in poleward energy transport can be attributed to the shortwave cloud feedback.

Recent and future changes of the Arctic sea‐ice cover

The present and future state of the Arctic sea ice cover is explored using new observations and a coupled one dimensional air–sea–ice model. Updated satellite observations of Fram Strait ice‐area export show an increase over the last four years, with ∼37% increase in winter 07–08. Atmospheric poleward energy flux declined since 1990, but advection of oceanic heat has recently increased. Simulations show that the ice area export is a stronger driver of thinning than the estimated ocean heat fluxes of 40 TW. Increased ocean heat transport will raise primarily Atlantic layer temperature. The ‘present 2007’ state of the Arctic ice could be a stable state given the recent high ice area export, but if ocean heat advection and ice export decrease, the ice cover will recover. A 2*CO2 scenario with export and oceanic heat flux remaining strong, forecasts a summer Arctic open ocean area of 95% around 2050.

Ice thickness variability in the Arctic Ocean between 1954–1990, results from a coupled ocean-ice-atmosphere column model

The interannual variation of the Arctic Ocean ice thickness during the period 1954–1990 is investigated by using a coupled ocean-ice-atmosphere column model. The model is forced by poleward energy flux in the atmosphere from NCEP/NCAR reanalysis data, ice export from satellite observations, cloudiness, and precipitation observed at the Russian North Pole drift stations. During the period 1977–1986 the model ice thickness decreased from 3.2 m to 2.0 m. The decrease is mainly caused by extra melting due to larger poleward energy flux in summer, and reduced ice growth in winter as a result of both increased cloudiness and energy flux. Precipitation and ice export are of less importance. A sensitivity study shows that the NCEP/NCAR data is accurate enough with respect to stochastic errors to ensure that the thinning is not caused by forcing errors. It is also shown that the poleward energy flux during summer is the dominant factor for regulating the ice thickness. The column model gives different results compared to other model studies using 2D ice models, especially towards the end of the period. Possible reasons for this disparity are discussed.

Dependence of the Arctic Ocean ice thickness distribution on the poleward energy flux in the atmosphere

The sensitivity of the Arctic Ocean ice cover on the atmospheric poleward energy flux, D, is studied using a coupled column model of the ocean, ice, and atmosphere. In the model, the ice cover is described by a thickness distribution and the atmosphere is a simple two‐stream gray body in radiative equilibrium. It is shown that the thickness distribution, in combination with the albedo function, gives a strong nonlinear response to positive perturbations of D. The response on D is sensitive to the albedo parameterization and the shape of the thickness distribution, controlled by ridging and divergence. An increase of about 9 W m−2 from a standard value of D = 103 W m−2 has a dramatic effect, reducing the ice thickness by more than 2 m, and generates a large open‐water fraction during summer. The reduction of ice thickness is characterized by a clear transition between two regimes, going from a regime where first‐year ice survives the next summer melt period to a seasonal ice regime where the first‐year ice melts completely. It is shown that the existence of seasonal ice regime is dependent on the surface mixed layer thickness. The model enters a completely ice‐free state if the thickness of the mixed layer is increased above a threshold value. The adjustment timescale for the ice cover is 6 years for small positive and negative perturbations in D. For larger positive perturbations of about 10 W m−2, the adjustment timescale is up to 20 years.

Estimating the effect of clouds on the arctic surface energy budget

The relationship between cloudiness and sea ice thickness over the Arctic Ocean is examined using an idealized equilibrium energy budget model and a single‐column model (SCM) forced by incoming solar radiation, atmospheric poleward energy flux, oceanic heat flux, and sea ice divergence. The sensitivity of ice thickness to cloud perturbations in the SCM is qualitatively different from previous modeling studies, which did not allow the atmospheric temperature profile to respond to perturbations in cloudiness. It is shown that this restriction implicitly changes the poleward energy flux in model experiments and obscures the intrinsic effect of clouds on the arctic climate. In the present model, equilibrium ice thickness has a strong negative correlation with cloud radiative forcing at the top of the atmosphere and no correlation with cloud radiative forcing at the surface. The results demonstrate how local processes, such as the surface albedo feedback, affect the sensitivity of ice thickness to cloud perturbations and reduce its predictability. Exploratory model experiments suggest that interactions between the Arctic and the lower latitudes (via the poleward energy flux) could strongly influence the dependence of ice thickness on cloudiness. The impact of these interactions in the model was predicted qualitatively by considering how the radiative effect of clouds is partitioned between the atmosphere and the surface. The results demonstrate the importance of using cloud radiative forcing at both the surface and the top of the atmosphere when estimating the climatic effect of perturbation in cloudiness.

Reflection of equatorial Kelvin waves at eastern ocean boundaries Part II: Pacific and Atlantic Oceans

Abstract. The effect of viscosity, non linearities, incident wave period and realistic eastern coastline geometry on energy fluxes are investigated using a shallow water model with a spatial resolution of 1/4 degree in both meridional and zonal directions. Equatorial and mid-latitude responses are considered. It is found that (1) the influence of the coastline geometry and the incident wave period is more important for the westward energy flux than for the poleward flux, and (2) the effect of the inclination of the eastern ocean boundary on the poleward energy flux, for the Pacific and Atlantic Oceans, decline as the period of the incident wave increases. Furthermore, the model simulations suggest that the poleward energy fluxes from meridional boundaries give plausible results for motions of seasonal and annual periods. For comparatively shorter periods, a realistic coastline geometry has to be included for more accurate results. It is recommended that any numerical model involving the reflection of baroclinic Rossby waves (of intraseasonal, seasonal or annual periods) on the eastern Pacific or Atlantic Oceans, should consider the effect of the coastline geometry in order to improve the accuracy of the results.Key words. Oceanography: general (climate and interannual variability; equatorial oceanography). Oceanography: physical (eastern boundary currents).

Annual mean meridional energy transport modelled by a general circulation model for present and 2 × CO2 equilibrium climates

The meridional energy flux modelled by the Bureau of Meteorology Research Centre general circulation model is examined. It is divided into atmospheric and oceanic components, and the resolved atmospheric components in turn into mean and eddy circulations. Comparison with observations shows the modelled total planetary meridional energy transport to be low, but shows better agreement for the resolved atmospheric component alone. The overall patterns of the individual circulation and energy components of the model also agree well, although strengths and locations do show some discrepancies. The doubled CO2 climate change is analyzed in terms of the changes in each of the circulation and energy components. It is found that the changes are the relatively small residual of larger, and generally opposing changes in sensible heat and potential energy fluxes. Despite the general decrease in poleward energy flux, the poleward latent heat flux is found to increase. The reduction in poleward transport is found to be dominated by changes in the mean meridional circulation at low southern latitudes, and changes in both mean circulations and eddy fluxes elsewhere.

Changes in the poleward energy flux by the atmosphere and ocean as a possible cause for ice ages

It is proposed that the two preferred modes of temperature and circulation of the atmosphere which occurred over the past 100,000 yr correspond to two modes of partitioning of the poleward energy flux between the atmosphere and ocean. At present the ocean carries an appreciable fraction of the transport, for example about three-eighths at 30°N. In the cold mode it is suggested that the ocean carries less, and the atmosphere more, than at present. During the formation of the ice, at 50,000 BP, for example, the overall flux is expected to be slightly lower than at present and during melting, at 16,000 BP, slightly higher. The transition between the modes is seen as a natural imbalance in the atmosphere-ocean energy budget with a gradual warming of the ocean during an Ice Age eventually cluminating in its termination. At the present the imbalance is thought to correspond to a natural cooling of the ocean, which will lead to the next Ice Age. The magnitude of temperature changes in the polar regions differ between the hemispheres in the same way as present seasonal changes, being larger in the northern than in the southern hemisphere. Overall the atmospheric energy cycle was more intense during the Ice Ages than now. Observational tests are proposed by which predictions from the present arguments may be compared with deductions about the environment of the past. Data used for the present state of the atmospheric general circulation are the latest global data available and contain no known major uncertainties. However, data for the oceanic circulation and energy budget are less well known for the present and almost unknown for the past. Hence the proposed imbalances must be treated as part of a speculative hypothesis, but one which eventually may be subject to observational test as no solar variability is invoked.

On the role of mean meridional circulations in the energy balance of the atmosphere

The poleward energy flux in the atmosphere over the Northern Hemisphere is first discussed. The flux associated with the mean meridional circulations is computed and compared with the calculations of the eddy flux. The results indicate a great difference between the tropics and the extratropics with respect to the mechanism of the flux: south of about 30° N the mean meridional circulations are the principal flux agency, whereas eddies dominate at higher latitudes. The total flux is compared with the mean meridional distribution of the heat sources and sinks of the atmosphere. Satisfactory agreement is found, particularly regarding the annual mean conditions. The role of mean meridional circulations in the hemispheric balance of kinetic energy is found to be small in summer but relatively large in winter, when they seem to be important, especially for the maintenance of the kinetic energy of the mean motion. DOI: 10.1111/j.2153-3490.1965.tb01420.x

The Meridional Flux of Sensible Heat over the Northern Hemisphere

The meridional eddy flux of sensible heat is examined by means of finite difference integration methods from northern hemisphere charts for the winter 1945–46. The eddy sensible heat flux is shown to occur principally in the lowest layers of the atmosphere and to decrease with elevation. The magnitude of the eddy sensible heat transports are compared with the total poleward energy flux required by the radiation balance, and are shown to comprise a very large fraction of this flux.