Youmin Chen

Association between winter temperature in China and upper air circulation over East Asia revealed by canonical correlation analysis

Canonical correlation analysis (CCA) was used to study the relationship between the winter temperature in China and the circulation at 500 hPa over East Asia. CCA identifies a number of paired patterns that depend on Principle Component (PC) truncation of the two fields. Retaining more PCs usually gives more physically meaningful CCA patterns, but the leading paired pattern has often less explained variance. When fewer PCs are adopted, the leading pattern possesses larger variance. However, it is often distorted in comparison with PC or rotated PC patterns. Various combinations with different PC and CCA patterns remained were tried, which shows that retaining as many PCs as possible and using the first several CCAs instead of only the first give most reasonable connection mechanisms. A statistical downscaling model based on CCA modes linking temperature with circulation was established to quantify the extent to which temperature variation can be explained by circulation. The model was optimised by varying numbers of the PCs and the CCA modes retained. With 13th PC truncation in the circulation and 8th PC truncation in the temperature as well as five CCA modes retained, the optimal CCA model is achieved based on cross-validation. The optimal downscaling model accounts for 47.5% temperature variance on average. However, there is a remarkable regional difference, which ranges from 10% to 70% in brier-based score (BBS). It is concluded that 500-hPa circulation is strongly linked to surface temperature in parts of the country, but it alone is not sufficient to achieve a successful statistical downscaling of the temperature for whole China. D 2003 Elsevier Science B.V. All rights reserved.

The sea-ice compactness in the Greenland and Barents Seas during 1979-2003: changes and links to the surface air flow

Abstract Annual and winter (December–April) sea-ice area and extent are calculated for the Greenland Sea (GS) and Barents Sea (BS) from daily ice concentrations obtained from space-borne microwave radiometry for 1979–2003. The ice extent decreases significantly, particularly during winter, by 65 000 km2 (decade)–1 in the GS and by 72 000 km2 (decade)–1 in the BS. Ice-extent fractions (of these total extents) occupied by ice of five different ice-concentration ranges are calculated and analyzed. Changes in these fractions are again significant and most pronounced during winter. In the GS, the fraction of close to very compact ice (65–95%) decreases by 17 000 km2 (decade)–1 and the fraction of very compact ice (>95%) increases by 29 000 km2 (decade)–1, corresponding to a loss of 19% and a gain of 58% relative to the 25 year mean, respectively. In the BS, the fraction of close to compact ice (65–85%) increases by 26 000km2 (decade)–1 and the fraction with compact to very compact ice (>85%) decreases by 66 000 km2 (decade)–1, corresponding to a gain of 30% and a loss of 67% relative to the 25 year mean, respectively. The changing surface wind pattern analyzed from ERA-40 data favours this increasing (decreasing) ice compactness in the GS (BS).