Not just ice albedo Ce n’est pas qu’une histoire d’albédo de la glace…

Another interesting question is: How does the loss of sea-ice affect the ‘planetary heat engine’, when more heat escapes from high northern latitudes? If heat loss in the Arctic is enhanced as sea-ice retreats and uncovers a sea surface with higher skin temperature? This may be more than compensated for by a reduction in the albedo locally, as well as increased heat transport in the ocean/atmosphere, or enhanced AGW. It is plausible that a ‘cloud-enhanced greenhouse effect’ in the Arctic may cancel part of the oceanic heat loss, and this effect is consistent with the findings of Francis & Hunter. Thus, enhanced cloudiness and associated increase in the downward long-wave radiation may act as an additional positive feed-back for the Arctic surface, in addition to the albedo-effect, whereas the additional heat loss acts like a negative feed-back. Although subtle, Francis & Hunter’s findings may have quite important implications for the planetary heat budget.

Sea-ice area from NSIDCPositive feed-back processes taking place on a local scale, may give rise to larger local variations. In addition to the effect of wind-driven sea-ice, such feed-backs may explain the large local temperature anomalies and large natural variations in the Arctic. Another aspect, that sometimes also gets neglected, is the fact that the polar regions represent fairly ‘small’ regions in terms of degrees of freedom. Furthermore, temperature fluctuations tend to be fairly coherent over large parts (there is an anti-correlation in the see-saw structure associated with the NAO, however). Thus, profiles of zonal mean values (the mean values of the measurements taken at a constant latitude ring around the planet) involves comparing long stretched areas near the equator to broad short regions near the poles (if the surface area is compensated for), but due to spatial correlation, this implies that the bands near the equator involve many more degrees of freedom than those near the poles. Such latitudinal profiles of zonal means are analogous to comparing mean values of different sample sizes (a bit like comparing daily values to monthly and annual means). The implication is that events such as the early century warming are not as significant as when the a warming of similar magnitude is seen in the zonal mean profile at lower latitudes.

Traduit par Valérie Masson Delmotte

A partir de nouvelles observations satellitales, un article récent de Francis & Hunterfournit une discussion intéressante sur les raisons de la diminution actuelle de l’extension de glace de mer (banquise) dans l’Arctique. On invoque souvent à propos de la glace de mer une rétroaction positive parce que la glace modifie l’albédo planétaire (la manière dont notre planète réfléchit le rayonnement solaire vers l’espace avant que cette énergie n’entre dans “le système climatique”). Pourtant, l’histoire est plus complexe, car la glace de mer agit également comme un couvercle isolant posé sur la surface de la mer.

Certains effets sont subils, ainsi, la planète perd davantage de chaleur depuis les mers libres que depuis les zones couvertes de banquise (une partie de cette chaleur est absorbée par l’atmosphère, mais les régions englacées ont un climat d’hiver plus continental : sec et froid). La perte de chaleur par l’océan dépend bien sur de la température de surface de la mer (SST, acronyme de “Sea surface temperature”). Les mers libres sont aussi une source d’humidité, par contraste avec les les zones de banquise (parce qu’elles sont froides, et non parce qu’elles sont sèches); mais l’humidité atmosphérique est aussi influencée par le transport d’humidité lié aux vents (advection de vapeur d’eau).

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