Cian Woods
Department of Meteorology, Stockholm University


The role of high-latitude circulation and moisture transport in Arctic climate variability and change during winter


This thesis examines the connections between atmospheric circulation in the high-latitudes, northward moisture transport, and Arctic climate variability and change during winter. An event based approach is taken by objectively defining phenomena termed ``moisture intrusions'' -- filamentary flows of anomalously moist air which originate at 70°N and cross the entire Arctic basin. They typically emanate from within the poleward advecting branches of mid-latitude cyclones held in place by blocking patterns to the east. Moisture intrusions contribute only a minority of the total northward moisture transport at 70°N, yet drive a significant proportion of the inter annual variability in surface temperature and downward longwave radiation over the entire polar cap. A positive trend in the frequency of these events, in response to a moistening of the atmosphere, is shown to have driven approximately 45% and 35% of the observed warming and sea ice decline in the Barents Sea during Dec-Jan over the past two decades. Moisture intrusions act to erode the temperature inversion and thus contribute to bottom amplified warming even in the absence of sea ice loss. Negative sea ice anomalies induced by intrusions persist for up to weeks at a time -- promoting upward turbulent heat fluxes and further bottom amplified warming. Systematic biases in the statistics of moisture intrusions are discovered in the CMIP5 models. The biases are predominantly a result of misrepresentation of the intense moisture fluxes and are almost entirely due to biases in the meridional velocity. Moisture intrusion biases explain only about 17% of the temperature bias in the Atlantic sector. The predicted biases, while small in amplitude, are very highly correlated with the true biases in the models however, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback. A analysis of the uncertainties in computed turbulent air-sea flux (TASF) climatologies arising due to the parameterisation of bulk formulae is also presented. TASF climatologies are computed over a series of sensitivity experiments using surface state variables from ERA-Interim. The largest source of uncertainty is related to the computation of the transfer coefficients and hence the choice of bulk algorithm itself. The majority of parameter approximations have small impacts when tested individually, but can lead to large disagreements when implemented in tandem.

Time and place
Friday 2 December 2016, 10.00
Högbomssalen, Geoscience Building (3rd floor)