Master´s Thesis Defense by Rune Gjermundbo – Niels Bohr Institutet - Københavns Universitet

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Master´s Thesis Defense by Rune Gjermundbo

The amplified temperature response of the Arctic relative to the rest of the globe during a shift in the climate towards higher temperatures, is known as Arctic amplification. The amplified temperature response is caused by a range of climatic feedbacks that respond to an initial perturbation by amplifying the temperature increase. This thesis studies the relative importance of the initial perturbation caused by local greenhouse forcing, and that caused by climate changes in regions far from the Arctic. This is done using the Community Atmosphere Model version 3, coupled to the Community Land Model, and a slab ocean with fixed, spatially uniform, mixed layer depth, incorporating a hermodynamic sea ice model.

The model is forced in two different ways: (i) by increasing low latitude sea surface temperatures (SSTs) to their 2CO2 equivalent level, while the atmospheric CO2
content is kept at the reference level (355ppm) and (ii) by doubling the atmospheric CO2 content globally while keeping low latitude SSTs from responding to the forcing. These two setups are used to investigate both the change in equilibrium- and transient climate, the latter through an ensemble experiment. The equilibrium surface temperature of the Arctic (defined as north of 70N) increases as much in one setup as in the other, while estimates of changes in energy transports, are compromised by large uncertainties. Further, a surface based peak in temperature increase is caused by greater greenhouse trapping in (ii), and greater cloud cover in (i). Importantly, neither of these experiments produce Arctic climate change, comparable to that of the full 2CO2 experiment, implying that both local and remote forcing is partial to the Arctic warming seen in the full 2CO2 run. The analysis of the transient climate shows largely the same response, however, the change in the energy budget of the Arctic surface, along with changes in cloud cover, reacts so quickly to the forcing, that the temporal resolution of the output of one month, is too low to investigate how the low latitude warming is communicated to the high latitudes, in great detail.

These experiments do, however, suggest that the remote forcing causes warm and humid air to protrude into the Arctic atmosphere with subsequent increase in greenhouse trapping and downwelling LW radiation at the surface, starting a range of locally amplifying feedbacks, and cloud cover increases. Further studies are needed to confirm this claim with greater certainty.