CHANGES IN EAST ASIAN WINTER ATMOSPHERIC CIRCULATION WU, M C YEUNG, K H LEUNG, Y K

We attempt to find out If there has been any change in the East Asian winter (December-February) atmospheric circulation in the last several decades ( )? –Trends of the characteristics of its major components? their possible influences to Hong Kong’s winter temperature –Changes in the interrelationships among the components? –Influences of some dominate climate patterns, namely the Arctic Oscillation (AO) and the Pacific Decadal Oscillation (PDO)?

+-+- Overall Changes in Northern Hemisphere (NH) winter Linear trend ( ) therefore a weakening of the East Asian winter monsoon there should also be some changes in the circulation against the background of global warming Weakening of N-S SLP gradient + ve - ve

Aloft (200 hPa) Mid-troposphere (500 hPa) Lower troposphere (surface, 1000 hPa) 8 major components JET EMTWMIALPISMH EAW SH_I SH_W SH_R EAT Index:

Trends in the components? To compare the trends of different indices Convert the trends to a common scale expressed in terms of a fraction of the standard deviation Trend/stdev Mann-Kendall test is used to test the significance of trend in the time series of these indices Significant trends (at 5% level) are found in the indices of the 7 components in the lower and mid-troposphere a remarkable change in the East Asian winter atmospheric circulation The most prominent trend in lower troposphere is the weakening of the Siberia-Mongolia high whereas in mid-troposphere, it is the eastwards shift of the East Asian trough No significant trend is noted in the strength of the subtropical jet aloft Index Change corresponding to the trend Lower troposphere WMIWeaker monsoon surge SMHWeaker Siberia-Mongolia High ALPIDeepening of the Aleutian low EMTWeakening of the Equatorial monsoon trough Mid- troposphere SH_IStronger subtropical high SH_RSubtropical high shift northwards SH_WSubtropical high shift westwards EAWZonal flow dominate in Eurasia sector EATEast Asian trough shift eastwards AloftJETNo significant trend Magnitude of trend is obtained by Sen’s slope Trend (/decade) Trend/stdev

Strength of the Siberia-Mongolia High Longitudinal position of the East Asian Trough Strength (hPa) Longitude ( o E) Time series of SMH and EAT Decrease in strength Shift eastward

The influences of the Aleutian low and the equatorial monsoon trough are apparently slight as the difference in HK's winter temperature for large and small values of ALPI and of EMT are statistically not significant + the trend of the index would likely to bring a warmer winter to HK with the exception of the intensifying Aleutian low, the trends of all indices are found to be consistent with a warmer winter in Hong Kong IndexDifference ( o C)Change corresponding to the trend WMI1.48*Weaker monsoon surge SMH1.49*Weaker Siberia-Mongolia High ALPI-0.13Deepening of the Aleutian low EMT0.66Weakening of the Equatorial monsoon trough SH_I1.04*Stronger subtropical high SH_R1.47*Subtropical high shift northwards SH_W1.14*Subtropical high shift westwards EAW1.26*Zonal flow dominate in Eurasia sector EAT0.81*East Asian trough shift eastwards Change in Hong Kong’s winter temperature accompanied with the trends The change in HK should be similar to that in southern China *:the difference is significant at the 5% level. The significance of the difference is tested using the KS test the difference in HK’s winter temperature between the 10 highest values and 10 lowest values of an index, from which the expected change in HK’s winter temperature accompanied with the trend of the index will be inferred

Temporal variation in the relationship (correlation) between any two indices in the mid-troposphere Moving correlation analysis with a 29-year sliding window is used –Spearman’s rank correlation Any change in covariability? Mid- troposphere SH_ISubtropical high intensity SH_RSubtropical high ridge line position SH_WSubtropical high western position EAWMid-latitude westerlies EATLongitudinal position of the East Asian trough Lower latitudes Mid-latitude reflect if there any change in the covariability between lower latitudes and mid-latitude circulation

EAW vs SH_I (intensity) Correlation coefficient Correlation in the period ( ) EAW vs SH_R (ridge-line position) EAW vs SH_W (western position) Moving correlation analysis SignificantInsignificant SignificantInsignificant SignificantInsignificant Correlation in the period ( ) 1% significant level 5% significant level (between the westerlies and subtropical high intensity) (between the westerlies and western position)(between the westerlies and the ridge position)

The degree of correlations between the Westerlies and the three subtropical high indices are declining Indicates a decreasing covariability between lower latitudes circulation and mid-latitude circulation On the other hand the interrelationships among the subtropical high indices are ratherl robust –for example, Moving correlation analysis (between the intensity & western position of subtropical high) 1 % significant level 5 % significant level Therefore,

Decline in correlation from significant to insigificant Significant correlation Schematic diagram showing the interrelationships among the components of the winter atmospheric circulation in mid-troposphere Another indication of the change in the East Asian winter atmospheric circulation in the last several decades Lower latitudes circulation

Question Could the decline in covariability observed be related to global warming in terms of the rising trend of NH winter temperature? The purpose of detrending is to ensure that one looks at the relationship between temperature and the index, not just the time variation of the index in view of the increase in temperature with time under global warming Series A Series B 29 years selected as colder group (Group C) 29 years selected as warmer group (Group W) Correlation between series A and B for Group C Correlation between series A and B for Group W Detrend ALL time series (NH winter temperature, EAW, SH_I, etc) rank according to the detrended NH winter temperature increase in the detrended NH winter temperature Winter temperature year 1985,1972, 1969…… warmest coldest Runs-test suggests this sequence is random (p-value= 0.38) in terms of YEAR e.g. EAW e.g. SH_I

IndicesCorrelation for group CCorrelation for group W EAW vs SH_I 0.47*0.07 EAW vs SH_R EAW vs SH_W -0.50*-0.12 Comparison of the correlations between Group C and Group W * Correlation significant at 1% level. With a warmer NH winter, a decline in the correlation between the Westerlies and the subtropical high indices is suggested ( most clear for the intensity and western position of the subtropical high ) Decline in correlation

Distribution of correlation 10,000 correlation Consider the covariability between the Westerlies (EAW) and the Subtropical high intensity (SH_I) as an example How significant the relation between NH winter temperature and the change in covariabiltiy is? Detrend time series of EAW and SH_I Correlation for the 29 pairs Repeat 10,000 times Randomly select 29 pairs of EAW and SH_I

IndicesCorrelation for group CCorrelation for group W EAW vs SH_I 0.47*0.07 Rank correlation between detrended EAW and SH_I % (out of 10,000 samples) Distribution of correlation <3.1%<5.4% the relationship between the covariability and NH winter temperature could be rather pronounced

Influences by some dominant modes of oscillation? PDO, AO Cumulative summation (CUSUM) –defined as the accumulating sum of anomalies (from the overall mean) of all preceding values. aid to locate, in a qualitative sense, the change points in a time series to see whether changes tend to match up across indices

PDO1976 AO1988 CUSUM charts for PDO & AO PDO change phase by mid-1970s AO change phase by mid- to late1980s Nino 3.4 SST ENSO varies according to the background of PDO

Western-positionIntensity Ridge-line position CUSUM charts for Subtropical High Changes coincide with the climatic shift in the PDO in the mid-1970s can be located

East Asian Trough position (EAT) Eurasian Westerlies (EAW) CUSUM charts for mid-latitude’s components Changes are noted in the westerlies and trough position by mid- to late 1980s AO ~concurrent with the phase change in AO

Changes in the East Asian winter atmospheric circulation (in terms of the changes in EAW, EAT, SH_I etc) could also be a manifestation of some natural climate variability (if AO and PDO as natural oscillation) Subtropical high is more strongly influenced by PDO Components in mid-latitude are likely to be influenced by AO Therefore

Conclusion Changes in the East Asian winter atmospheric circulation are apparent

Thank You 謝謝

Conclusion Trends are noted in the components studied except the strength of the subtropical jet aloft –indicates an apparent change in the atmospheric circulation pattern –with the exception of the intensifying Aleutian low, all trends found are consistent with a rising winter temperatures in HK and southern China The covariability between the lower latitudes circulation and the mid-latitude circulation is declining which is another indication of the climatic change in the East Asian winter atmospheric circulation

Indices’ definition and Data sources

Discussion The possibility of anthropogenic forcing of the observed changes cannot be ruled out because natural climate oscillations could have been affected Difficult to distinguish between natural variability and change due to anthropogenic forcing anthropogenic forcing natural climate oscillations observed changes

EAT EAW NH winter temp 1985 HK winter temp CUSUM Changes can be seen in the time series of the winter temperature of NH and winter temperature of Hong Kong in the mid- to late 1980s.

Western-position vs Intensity Western-position vs Ridge-line position Intensity vs Ridge-line position Why SH_I, SH_R and SH_W (they are different features of the SH and do have different influences on the weather) e.g., TC track, rainbelt (south China vs central China)

Trend vs Interrelationship (correlation)

EAW vs SH_W IndicesCorrelation for group CCorrelation for group W EAW vs SH_W -0.50*-0.12

CUSUM for detrended time series AOPDO SH_W EAW, EAT

Distribution of years Ranking (from coldest to warmest NH winter temperature after detrending) year

Changes in some components of the East Asian winter atmospheric circulation have been suggested to be related to global warming based on the results of coupled GCMs simulations. These include for examples, the weakening of the Siberia-Mongolia high [BRANDEFLT, 2006], the deepening of Aleutian low [HU et al, 2000] and the weakening of the East Asian winter monsoon [HORI et al, 2006].

The two-sample KS test is one of the most useful and general nonparametric methods for comparing two samples, as it is sensitive to differences in both location and shape of the empirical cumulative distribution functions of the two samples For a single sample of data, the Kolmogorov-Smirnov test is used to test whether or not the sample of data is consistent with a specified distribution function. When there are two samples of data, it is used to test whether or not these two samples may reasonably be assumed to come from the same distribution

Rank correlation (pearson correlation) Hong Kong vs 28 stations in SC = (0.936) = (0.933)

PDO1976 AO1988 NH winter temp 1985

Western-position Intensity Ridge-line position Sub-tropical high SH CUSUM Charts (2) Mid-1970s PDO

The relationship between the Pacific Decadal Oscillation (PDO) and the Arctic Oscillation (AO) on decadal timescale in the extended winter (November-March) is investigated in this study. The results indicate that AO plays an important role in the low frequency variability of PDO. When AO leads PDO by 7-8 years, the lagging correlation between them becomes the strongest with correlation coefficient The leading decadal variability of AO provides a valuably precursory signal for predicting the variability of PDO. The results of regression and lagging correlation reveal the possible mechanism for the AO-PDO coupling: A strong AO would lead to an enhanced Aleutian Low that is linked to PDO by ocean-atmosphere interaction in the North Pacific, and vice versa.

(1)Intensity index (SH_I): The sum of the code of geopotential height  588 dagpm (that is 588 dagpm as 1, 589 as 2, 590 as 3, and so on). Region considered is ( oE) (2)Western position index (SH_W): The western most longitude of the contour of 588 dagpm in the region oE. (3)Ridge-line index (SH_R): The mean latitude of the ridge-line of 588 dagpm contour in the region oE. It is calculated by averaging the values of latitude of the ridge-line at longitudes 110oE, 115oE, 120oE, 125oE, 130oE, 135oE, 140oE, 145oE and 150oE. The Eurasia westerlies index (Iw) is defined as: where the zonal (Iz) and meridional (Im) indices represent the mid-latitude circulation conditions in the Eurasia sector (0-150oE, 45-65oN) and the ￣ denotes the average. Detailed definition of Iz and Im can be found in Zhang (1999). Basically, these indices measure zonal and meridional pressure (or geopotential height) gradient aloft, respectively. Positive I corresponds to the dominance of zonal flow pattern in Eurasia.

Northern Annular Mode (Arctic Oscillation) Available format: HTML Table The NAM (or Arctic Oscillation) is defined as the first EOF of NH (20-90N) winter SLP data (see below). It explains 23% of the extended winter mean (December-March) variance, and it is clearly dominated by the NAO structure in the Atlantic sector. Although there are some subtle differences from the regional pattern over the Atlantic and Arctic, the main difference is larger amplitude anomalies over the North Pacific of the same sign as those over the Atlantic. This feature gives the NAM a more annular (or zonally-symmetric) structure. For more information, see

1958/ /86: +0.24/decade (not sig.) 1986/ /05: -0.17/decade (not sig.) Winter temperatures in Hong Kong

1986/87 Regime shifts in winter temperature in southern China (average of the 28 stations) >0.8 o C

AO (+ve phase) AO (-ve phase) Weaker winter monsoon Stronger winter monsoon EAWMI WMI late 1980s Relationship between Arctic Oscillation and winter monsoon

Strength of the Siberia-Mongolia High Longitudinal position of the East Asian Trough

Change point Trend magnitude Result in trend