HR: 1340h
AN: A33D-0935    [Abstracts]
TI: Surface Atmospheric Temperature as Indicator of Climate Change in the Arctic
AU: * Esau, I
EM: igore@nersc.no
AF: Nansen Environmental and Remote Sensing Centre, Thormohlensgate 47, Bergen, 5006 Norway
AU: * Esau, I
EM: igore@nersc.no
AF: Bjerknes Centre for Climate Research, Thormohlensgate 47, Bergen, 5006 Norway
AB: Surface atmospheric temperature (SAT) measurements are the most reliable atmospheric temperature data characterizing the temperature at standard meteorological height 2 m. However, contrary to what has been implicitly assumed in the Climate Change debates, changes in the SAT often have loose relations with changes in the atmospheric heat content. These relations depend on regimes of turbulent mixing within the planetary boundary layer (PBL). The only source of turbulent energy in the wintertime Arctic - the wind shear - strongly depends on storm activity. Strong atmospheric temperature inversions (ATI) typical in the Arctic inhibit the mixing between a thin PBL and the adjacent troposphere. Physical reasoning suggests that a thin gas layer would quickly equilibrate its temperature with the boundaries. Capped by a very strong ATI, the SAT equilibrates with the surface temperature, which is typically of 10K (potentially) colder than the lower troposphere. A weak ATI in a storm-mixed troposphere permits the turbulence to penetrate deeper into the troposphere and to rise the SAT. Observations (e.g. SHEBA campaign) revealed that these two mixing regimes produce a bi-modal distribution in the SAT. The coldest temperature maximum in the distribution does not find its counterpart in the tropospheric temperatures. The mixing regimes in the Arctic PBL may have profound impact on the Arctic climate and climate change. Firstly, the observed SAT trends should be the combination of the trends due to changes in the atmospheric heat content and due to changes in the frequency of the mixing regimes. The latter changes reflect rather changes in storm activity than changes in a local heat budget. Using ERA-40 data, the impact of PBL regimes on the Arctic warming is quantified. Up to 70% of the SAT trends over the Atlantic could be attributed to changes in the PBL mixing. These changes account up to 50% of the SAT trends over East Europe, East Arctic and Alaska. Secondly, the warming tropospheric trends should result in ATI strengthening leading to a weaker warming or even cooling traced in the SAT. Only small changes in the PBL mixing have been found over Siberia and Canada. Strong warming trends in those areas in 70s - 80s has changed to small or negative trends in the recent years. Partially, it could be due to the PBL response on warmer troposphere and stronger temperature inversions. As any transitional process in a thin layer, this SAT trend inconsistency will be limited both in time and amplitude. Turbulence resolving simulations of responses in the Arctic PBL suggest that amplitude of the local changes could reach 10K, which means that global processes would need additional one-two decades to overcome the PBL changes. One should also expect temporal lingering of the SAT increase in central areas of continents.
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 3307 Boundary layer processes
DE: 3315 Data assimilation
DE: 3349 Polar meteorology
DE: 3379 Turbulence (4490)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005