Seasonal Sea Ice Prediction with the CICE Model and Positive Impact of CryoSat-2 Ice Thickness Initialization

The Los Alamos sea ice model (CICE) is being tested in standalone mode for its suitability for seasonal time scale prediction. The prescribed atmospheric forcings to drive the model are from the NCEP Climate Forecast System Reanalysis (CFSR). A built-in mixed layer ocean model in CICE is used. Initial conditions for the sea ice and the mixed layer ocean in the control experiments are also from CFSR. The simulated sea ice extent in the Arctic in control experiments is generally in good agreement with observations in the warm season at all lead times up to 12 months, suggesting that CICE is able to provide useful ice edge information for seasonal prediction. However, the ice thickness forecast has a positive bias stemming from the initial conditions and often persists for more than a season, limiting the model’s seasonal forecast skill. In addition, thicker ice has a lower melting rate in the warm season, both at the bottom and top, contributing to this positive bias. When this bias is removed by initializing the model using ice thickness data from satellite observations while keeping all other initial fields unchanged, both simulated ice edge and thickness improve. This indicates the important role of ice thickness initialization in sea ice seasonal prediction.

Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry–climate model with mixed-layer ocean

In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH4) mixing ratios have been analysed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation-driven temperature feedback. Therefore, we perform sensitivity simulations with doubled and quintupled present-day (year 2010) CH4 mixing ratios with the chemistry–climate model EMAC (European Centre for Medium-Range Weather Forecasts, Hamburg version – Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry) and include in a novel set-up a mixed-layer ocean model to account for tropospheric warming. Strong increases in CH4 lead to a reduction in the hydroxyl radical in the troposphere, thereby extending the CH4 lifetime. Slow climate feedbacks counteract this reduction in the hydroxyl radical through increases in tropospheric water vapour and ozone, thereby dampening the extension of CH4 lifetime in comparison with the quasi-instantaneous response. Changes in the stratospheric circulation evolve clearly with the warming of the troposphere. The Brewer–Dobson circulation strengthens, affecting the response of trace gases, such as ozone, water vapour and CH4 in the stratosphere, and also causing stratospheric temperature changes. In the middle and upper stratosphere, the increase in stratospheric water vapour is reduced with respect to the quasi-instantaneous response. We find that this difference cannot be explained by the response of the cold point and the associated water vapour entry values but by a weaker strengthening of the in situ source of water vapour through CH4 oxidation. However, in the lower stratosphere water vapour increases more strongly when tropospheric warming is accounted for, enlarging its overall radiative impact. The response of the stratosphere adjusted temperatures driven by slow climate feedbacks is dominated by these increases in stratospheric water vapour as well as strongly decreased ozone mixing ratios above the tropical tropopause, which result from enhanced tropical upwelling. While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH4 radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH4 changes in this chemistry–climate model set-up is not significantly different from the climate sensitivity in carbon-dioxide-driven simulations, provided that the CH4 effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes.

Dependence of Climate Response on Meridional Structure of External Thermal Forcing

This study shows that the magnitude of global surface warming greatly depends on the meridional distribution of surface thermal forcing. An atmospheric model coupled to an aquaplanet slab mixed layer ocean is perturbed by prescribing heating to the ocean mixed layer. The heating is distributed uniformly globally or confined to narrow tropical or polar bands, and the amplitude is adjusted to ensure that the global mean remains the same for all cases. Since the tropical temperature is close to a moist adiabat, the prescribed heating leads to a maximized warming near the tropopause, whereas the polar warming is trapped near the surface because of strong atmospheric stability. Hence, the surface warming is more effectively damped by radiation in the tropics than in the polar region. As a result, the global surface temperature increase is weak (strong) when the given amount of heating is confined to the tropical (polar) band. The degree of this contrast is shown to depend on water vapor– and cloud–radiative feedbacks that alter the effective strength of prescribed thermal forcing.

Sensitivity of Intertropical Convergence Zone Movement to the Latitudinal Position of Thermal Forcing

A variety of recent studies have shown that extratropical heating anomalies can be remarkably effective at causing meridional shifts in the intertropical convergence zone (ITCZ). But what latitudinal location of forcing is most effective at shifting the ITCZ? In a series of aquaplanet simulations with the GFDL Atmospheric Model, version 2 (AM2), coupled to a slab mixed layer ocean, it is shown that high-latitude forcing actually causes a larger shift in the ITCZ than when equivalent surface forcing is applied in the tropics. Equivalent simulations are run with an idealized general circulation model (GCM) without cloud and water vapor feedbacks, also coupled to an aquaplanet slab ocean, where the ITCZ response instead becomes weaker the farther the forcing is from the equator, indicating that radiative feedbacks must be important in AM2. In the absence of radiative feedbacks, the tendency for anomalies to decrease in importance the farther away they are from the equator is due to the quasi-diffusive nature of energy transports. Cloud shortwave responses in AM2 act to strengthen the ITCZ response to extratropical forcing, amplifying the response as it propagates toward the equator. These results emphasize the great importance of the extratropics in determining the position of the ITCZ.

The Importance of the Montreal Protocol in Protecting Earth’s Hydroclimate

The 1987 Montreal Protocol regulating emissions of chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODSs) was motivated primarily by the harm to human health and ecosystems arising from increased exposure to ultraviolet-B (UV-B) radiation associated with depletion of the ozone layer. It is now known that the Montreal Protocol has helped reduce radiative forcing of the climate system since CFCs are greenhouse gases (GHGs), and that ozone depletion (which is now on the verge of reversing) has been the dominant driver of atmospheric circulation changes in the Southern Hemisphere in the last half century. This paper demonstrates that the Montreal Protocol also significantly protects Earth’s hydroclimate. Using the Community Atmospheric Model, version 3 (CAM3), coupled to a simple mixed layer ocean, it is shown that in the “world avoided” (i.e., with CFC emissions not regulated), the subtropical dry zones would be substantially drier, and the middle- and high-latitude regions considerably wetter in the coming decade (2020–29) than in a world without ozone depletion. Surprisingly, these changes are very similar, in both pattern and magnitude, to those caused by projected increases in GHG concentrations over the same period. It is further shown that, by dynamical and thermodynamical mechanisms, both the stratospheric ozone depletion and increased CFCs contribute to these changes. The results herein imply that, as a consequence of the Montreal Protocol, changes in the hydrological cycle in the coming decade will be only half as strong as what they otherwise would be.

Adjustment to Radiative Forcing in a Simple Coupled Ocean–Atmosphere Model

This study calculates the adjustment to radiative forcing in a simple model of a mixed layer ocean coupled to the overlying atmosphere. One application of the model is to calculate how dust aerosols perturb the temperature of the atmosphere and ocean, which in turn influence tropical cyclone development. Forcing at the top of the atmosphere (TOA) is the primary control upon both the atmospheric and ocean temperature anomalies, both at equilibrium and during most of the adjustment to the forcing. Ocean temperature is directly influenced by forcing only at the surface, but is indirectly related to forcing at TOA due to heat exchange with the atmosphere. Within a few days of the forcing onset, the atmospheric temperature adjusts to heating within the aerosol layer, reducing the net transfer of heat from the ocean to the atmosphere. For realistic levels of aerosol radiative forcing, the perturbed net surface heating strongly opposes forcing at the surface. This means that surface forcing dominates the ocean response only within the first few days following a dust outbreak, before the atmosphere has responded. This suggests that, to calculate the effect of dust upon the ocean temperature, the atmospheric adjustment must be taken into account explicitly and forcing at TOA must be considered in addition to the surface forcing. The importance of TOA forcing should be investigated in a model where vertical and lateral mixing of heat are calculated with fewer assumptions than in the simple model presented here. Nonetheless, the fundamental influence of TOA forcing appears to be only weakly sensitive to the model assumptions.