engleski

Ensemble analysis of the Northern Hemisphere atmosphere response to observed SST anomalies 1855-2002

The January-March (JFM) climate response of the Northern Hemisphere atmosphere to observed sea surface temperature (SST) anomalies for the period 1855-2002 is analysed from a 35-member ensemble made with SPEEDY, an atmospheric general circulation model (AGCM) of intermediate complexity. The model was run at the T30-L8 resolution, and initial conditions and the early stage of model runs differ among ensemble members in the definition of tropical diabatic heating (Ctrl experiment). The ENSO impact on the Northern Hemisphere atmosphere is examined by composite analysis based on a categorisation of SST anomalies in the Ni&ntilde ; o3.4 region into classes extending from strong cold to strong warm. As expected, the strongest signal is detected over the Pacific-North America (PNA) region. Over the North Atlantic-Europe region (NAE) the signal is much weaker than that over the PNA region, but same similarities with the PNA are found (spatial symmetry between the action centres ; substantial anti-symmetry in amplitude – atmospheric response is stronger for warm composite than for cold composite of the same category ; linear component of the response dominates over nonlinear component). The relative contribution of the SSTs in the tropical Pacific is analysed by an independent 35-member ensemble of model realisations in which observed SST anomalies within the 1855-2002 period were prescribed within the tropical Pacific only (TroPac experiment). Results show that a considerable qualitative resemblance exists between fields of atmospheric variables obtained by the Ctrl and TroPac experiments indicating that atmospheric response to ENSO phenomena in Ctrl experiment may be attributed mainly to the influence of SST anomalies in the tropical Pacific. Results presented in this study indicate that SST anomalies outside the tropical Pacific do not have significant impact on atmospheric response over the PNA region. Over the NAE region, they reduce the amplitude of the atmospheric response.

ensemble analysis; tropical-extratropical teleconnections; ENSO forcing

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