OBSERVATIONAL RELATIONSHIPS BETWEEN SUMMER AND WINTER MONSOONS OVER EAST ASIA. PART II: RESULTS

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 25: (05) Published online in Wiley InterScience ( DOI: /joc.1153 OBSERVATIONAL RELATIONSHIPS BETWEEN SUMMER AND WINTER MONSOONS OVER EAST ASIA. PART II: RESULTS M. C. WU and J. C. L. CHAN* Laboratory for Atmospheric Research, Department of Physics and Materials Science, City University of Hong Kong, Kowloon Hong Kong, People s Republic of China Received 10 March 04 Revised 23 September 04 Accepted 23 September 04 ABSTRACT Using the framework presented in part I of this study, three possible summer-to-winter monsoon and four possible winterto-summer monsoon relationships are identified. A generalized relationship between summer and winter monsoons is virtually non-existent, and some of the possible relationships are in fact tied to the influence of the El Niño southern oscillation (ENSO). Indeed, relationships between summer and winter monsoons are specific in terms of both the winter monsoon strength and the ENSO conditions. It is found that the strength of winter monsoon is unlikely to be an important forcing regarding the possible winter-to-summer monsoon relationships, since the summer monsoon is unlikely to be weak following a non-enso-coupled winter monsoon, regardless of the winter monsoon strength. On the other hand, possible summer-to-winter relationships are noted only when the summer monsoon is not weak, regardless of the ENSO condition. An alternation or opposite tendency in the summer monsoon strength is noted between the onset year (tends to be unlikely weak) and the following year (tends to be unlikely strong) of an El Niño. Therefore, certain possible relationships between summer and winter monsoons are obvious when the winter monsoon tends to be weaker during the mature phase of an El Niño. For a La Niña, the signature in the summer monsoon strength is less clear, as indicated from the assessment of summer monsoon indices. Nevertheless, when the winter monsoon tends to be strong when coupled with a La Niña, the following summer monsoon also tends to be weaker. A biennial alternation of the summer and winter monsoons is noted, i.e. that a stronger summer monsoon precedes a weaker winter monsoon and a weaker winter monsoon is followed by a stronger summer monsoon. This biennial alternation is associated with a transition of ENSO warm phase to ENSO cold phase, representing the biennial signal in the interannual variability of the monsoons as well as in ENSO. Concurrent with this biennial alternation is an evident variation in the subtropical-high strength. It appears that the commonly recognized tropical biennial oscillation (TBO) is not tied to the biennial signal in the interannual variability of the East Asian monsoons, because the TBO is constituted by a strong (weak) summer monsoon followed by strong (weak) winter monsoon process. Furthermore, it is suggested that a complete biennial oscillation in the interannual variability of the monsoons is not observed because of the breakdown of a cycle (or oscillation) in the summer monsoon following a La Niña onset. Copyright 05 Royal Meteorological Society. KEY WORDS: winter monsoon; summer monsoon; East Asia; ENSO 1. INTRODUCTION This is the second of a two-part study investigating the relationships between summer monsoons (SMs) and winter monsoons (WMs) over East Asia in terms of the strength of the monsoons, using the framework presented in part I of the study (Wu and Chan, 05). In part I, six WM categories are defined. The strength of the WM is determined by the unified monsoon index (UMI; see Lu and Chan (1999) and part I). Strong/weak WM is stratified into El Niño-southern oscillation (ENSO)-coupled and non-enso-coupled groups because of the apparent coupling between the WM and * Correspondence to: J. C. L. Chan, Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China; Johnny.chan@cityu.edu.hk Copyright 05 Royal Meteorological Society

2 454 M. C. WU AND J. C. L. CHAN ENSO. Also incorporated in the WM classification is the ENSO condition in the following year. Analogous to the principle of falsification (Popper, 1959), the SM condition is classified as unlikely strong (not-strong or nots for short) or unlikely weak (not-weak or notw for short). The SM condition (and also the relationship) is primarily determined by considering the contingency table showing the relative occurrence of positive and negative anomalies for a total of five SM indices. It is referred to as unlikely weak (unlikely strong) or notw (nots) if a perceptible signal for a higher tendency of positive (negative) anomaly based on an integrated consideration among the five SM indices is noted, using the definitions of apparent bias and definite bias introduced in part I. As in the case of preparing a statistical seasonal categorical forecast, the problem of using contingency tables is the unreliability given a small sample size (e.g. Goddard and Mason, 01). Therefore, the SM conditions following and before each of the six WM categories are also considered by referring to the rainfall condition over China and the characteristics of the subtropical high (SH). These will be used for consolidating and cross-validating the assessment based on SM indices to discuss the reliability of the possible relationships identified. In this part, possible winter-to-summer monsoon (summer-to-winter monsoon) relationships are considered by assessing the SM conditions following (preceding) various WM categories. If such relationships do exist, then the nature and the physical causes of these relationships are explored. When no signal is noted, it can be interpreted as the relationship being too weak (in terms of the indicator considered) or being affected by climatic factors other than the monsoons or ENSO, or due to the chaotic nature, such that the nature of the relationship cannot be determined based on the available information. Note that the contrast (i.e. the opposite tendency), if any, in the possible relationships under different conditions (such as different WM categories or different ENSO conditions) could be more profound and useful than the individual relationship. The paper is arranged as follows. Since the same datasets as in part I are used, they will not be described here again. The winter-to-summer monsoon relationship is first considered in Section 2 and the summer-towinter monsoon relationship will be discussed in Section 3. In both Sections 2 and 3, the assessment of the SM is first described by the various monsoon indices, followed by that based on the rainfall conditions over China and the subtropical high. To highlight the ENSO influence, a review of the SM conditions during the summers following and preceding El Niño (EN) and La Niña (LN) onset years is given in Section 4. Section 5 describes the winter-to-summer-to-winter monsoon linkage as a whole. Finally, the summary and some concluding remarks are given in Section WINTER-TO-SUMMER MONSOON RELATIONSHIPS 2.1. Monsoon indices Assessments of the SM conditions following different categories of strong WMs (SWMs) weak WMs (WWMs) suggest that an apparent bias in the SM strength in the following summer can be found for all WM categories except for SWME (Table I). Using the criterion of two or more indices showing similar apparent bias, four possible winter-to-summer monsoon relationships can be identified, namely SWMN-tonotW SM, LSWM-to-notS SM, WWMN-to-notW SM and EWWML-to-notS SM. However, the reliability for the SWMN-to-notW SM relationship is unlikely to be high because the result is obtained from two cases only. Among the five SM indices (SMI, EASMI, LSTD, RM2 and WNPMI), LSTD and WNPMI appear to be the most sensitive in reflecting the winter-to-summer monsoon relationship. The strongest signal is observed for WWMN and EWWML, such that three out the five SM indices revealed similar apparent bias. According to Sun and Chen (1996), the SM in the year following a strong (weak) Asian WM is relatively weak (strong). Although this is consistent with two of the four possible relationships found here (SM following WWMN unlikely to be weak and that following LSWM unlikely to be strong), the SM following an ENSOcoupled WWM is unlikely to be strong. This suggests that the existence of a generalized winter-to-summer monsoon relationship is unlikely. In fact, the need to stratify SWMs/WWMs into the ENSO-coupled and non-enso-coupled groups in studying the winter-to-summer monsoon relationship is clearly illustrated from the different relationships in the two groups (Table II). The contrasts between the ENSO-coupled and non- ENSO-coupled conditions for both WWMs and SWMs are particularly evident, such that the SM is unlikely Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

3 EAST ASIAN WINTER AND SUMMER MONSOON RELATIONSHIPS II 455 Table I. Number of cases of different SM conditions after a WM of various categories. nots (negative anomaly) and notw (positive anomaly) denotes not-strong and not-weak SMs respectively. Bold highlights those cases with an apparent bias. Because of the availability of some indices, the total number of cases in each SM index category may not be equal to the number of cases of each WM category WM category a (no. of cases) Strength SM conditions Monsoon index b SMI EASMI LSTD RM2 WNPMI SWM SWMN (2) nots notw SWME (5) nots notw LSWM (5) nots notw WWM WWMN (5) nots notw EWWM (3) nots notw EWWML (4) nots notw a See WM and Chan (05; table VI) for WM category definitions. b See WM and Chan (05; section 3.3.1) for monsoon index definitions. Table II. Conditions of SM as assessed from monsoon indices for non-enso-coupled and ENSO-coupled WMs WM (category) SM (indices showing bias) Non-ENSO coupled SWM (SWMN, SWME) notw (LSTD, WNPMI) ENSO-coupled SWM (LSWM) nots (SMI, LSTD) Non-ENSO-coupled WWM (WWMN) notw (SMI, LSTD, RM2) ENSO-coupled WWM (EWWM, EWWML) nots (EASMI, LSTD, WNPMI) to be strong for the ENSO-coupled case whereas it is unlikely to be weak for the non-enso-coupled case. In fact, both the SMs following the ENSO-coupled SWM (WWM) and ENSO-coupled WWM (SWM) are unlikely to be strong (weak). That is, similar SM conditions are observed even though the WM strengths are quite different. In addition, the contrast between the ENSO-coupled and non-enso-coupled SWM/WWM is particularly clear using the LSTD, in that apparent bias is noted for all four of the ENSO-coupled and non-enso-coupled WM categories. These results suggest that the WM strength may not be the cause of the winter-to-summer monsoon relationships for the ENSO-coupled cases. It should be noted that a result with no definite or apparent bias (i.e. no possible relationship in the context of the present study) does not necessarily imply the non-existence of a relationship in reality. This could be due to the small sample size and/or the relationship not being strong, so that no evidence for a possible winter-to-summer monsoon relationship (as for the cases of SWME and EWWM) is observed or the nature of the relationship cannot be retrieved or established using monsoon indices. For example, whereas the SM is likely to be notw during the onset year of an EN (as will be shown in Section 4), virtually no winterto-summer monsoon relationship is noted from Table I when the SM following SWME corresponds to the onset year of an EN. In fact, the results are consistent in terms of the WNPMI. Similarly, it seems that the situations for EWWM and EWWML should be consistent with the condition of the year following an EN onset year. Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

4 456 M. C. WU AND J. C. L. CHAN 2.2. Rainfall condition As discussed in part I, the rainfall condition over China can provide supplementary inference to the SM strength. Consensus regarding the SM strength between monsoon indices and rainfall condition enhances the reliability of the SM condition determined. The rainfall condition over China will be considered in terms of the rainfall pattern and rainfall bias distribution. For completeness and reference purposes (because relationship can alternatively be formulated in terms of rainfall condition), the situations following all six WM categories are considered, although no relationship has been noted for some categories (i.e. the SWME and EWWM) Rainfall pattern. In matching the rainfall pattern with different preceding WM situations, it is found that each SM category is associated with two rainfall patterns except for SWME (Table III), which suggests that the (unlikely) pattern should be considered. Based on the usual relationship between rainfall pattern and SM strength (e.g. Shi and Zhu (1996); also see part I, section 3.3), the non-occurrence of rainfall pattern I (III) for LSWM (SWMN and WWMN) is consistent with the assessment according to the monsoon indices that the SM is unlikely to be weak (strong). On the other hand, when the conditions for EWWM and EWWML are not followed by the occurrence of pattern II, it is difficult to interpret the SM strength directly in terms of unlikely strong or unlikely weak. For SWME, although pattern I is the most unlikely, the small difference suggests no apparent signal, which is consistent with the result from the monsoon indices. Overall, the assessment of SM condition using monsoon indices is consistent with the rainfall pattern condition for all the possible winter-to-summer monsoon relationships identified except for the EWWML-to-notS SM, where no further implication is obtained from the rainfall pattern (from the non-occurrence of pattern II) Rainfall bias distribution. Overall, the summer rainfall bias (see part I for the method of identification) distribution following each WM category shows no regular pattern in terms of geographical boundary (Figure 1). In other words, the consideration of rainfall variations through area-averaging over a predefined geographical region (by taking the average of some selected stations) could lead to unclear or different conclusions. Likewise, the geographical reference in the discussion of regional SM rainfall variations should be defined loosely. The distribution for SWMN is rather scattered, probably because only two cases are involved. For SWME, most parts of southern China are unlikely to see a summer drought, whereas a bias of not-wet is noted in northern China. The latter, to a certain extent, corresponds to the fact that pattern I is most unlikely for SWME. Following an LSWM, a wetter condition is most likely over the region between the Yangtze River and Yellow River (including the Huaihe River valley). In addition, the chances of seeing abundant rainfall are smaller over the southeastern part of China and most parts of northern China. As a first approximation, Table III. Rainfall pattern condition in June August (JJA) (0). Bold indicates the rare situation for the six SWM/WWM categories. Also shown are the expected conditions of the SM according to the general relationship between rainfall pattern and the SM strength (NCC, 1999; Shi and Zhu, 1996) and the assessment of the SM based on monsoon indices (two or more indices showing the bias; see Table I). Situation not considered or cannot be deduced directly in NCC (1999) and Shi and Zhu (1996) is stated as not specified WM Rainfall pattern SM condition according to I II III Rainfall pattern Monsoon index SWM SWMN Strong notw SWME Not specified No signal LSWM Weak nots WWM WWMN Strong notw EWWM Not specified No signal EWWML Not specified nots Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

5 EAST ASIAN WINTER AND SUMMER MONSOON RELATIONSHIPS II 457 SWMN WWMN SWME EWWM LSWM EWWML Figure 1. Bias distribution of rainfall in the summer (JJA) following different WM categories. Black circle (open square) denotes the bias of not-dry (not-wet). See text for the definition of bias the bias distribution resembles pattern II, which is described as a + (i.e. less more less) rainfall variation pattern. Most parts south of the Yellow River have a drier condition in the summer following a WWMN. On the other hand, an apparent bias of wetter conditions over the southeastern coastal areas is suggested. A drier condition south of the Yangtze River and wetter condition north of the Yellow River corresponds to the non-occurrence of pattern III. In terms of the rainfall pattern condition, a + + rainfall variation coincides with a typical pattern I. A noticeable difference is observed between EWWM and EWWML over the Huaihe River valley, such that a drier condition is shown with the former and no apparent condition is suggested with the latter, which probably signifies the different ENSO evolutions in the year following the onset year (i.e. development or Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

6 458 M. C. WU AND J. C. L. CHAN non-development of LN). Nevertheless, no considerable bias towards a wetter condition over the Huaihe River valley for either EWWM or EWWML is found, consistent with the non-occurrence of pattern II. A similar feature is also noted for the SM before LSWM (see Figure 2 in Section 3.2), which also sees the non-occurrence of pattern II (see Section 3.2). The bias towards a wetter condition over regions south of the Yangtze River is particularly clear for EWWML. Further, no apparent bias of not-dry along the southeastern coastal areas is observed. Overall, consistent results are noted between rainfall conditions over China (in terms of rainfall pattern and rainfall bias distribution) and SM strength (in terms of monsoon indices) for SWMN, LSWM, WWMN and EWWML. Although no signal is indicated from SM indices, patterns in terms of rainfall bias can be highlighted for SWME and EWWM. In particular, it seems that the SM strength for EWWM should be consistent with that of the EWWML (see Table I). SWMN WWMN SWME EWWM LSWM EWWML Figure 2. As Figure 1, but for the preceding SM Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

7 EAST ASIAN WINTER AND SUMMER MONSOON RELATIONSHIPS II SH To understand the reasons for the possible relationships obtained in Section 2.2.2, the characteristics of the summertime SH associated with those possible relationships are examined. As expected, the SH strength is strongly correlated with the WM during the previous winter, except for the SWMN (which had only two cases) (Table IV). For SWME and LSWM, the summertime SH tends to be weaker and shifts further eastward. On the other hand, the summertime SH tends to be stronger, located southward and extends westward in the summer following the ENSO-coupled WWM, especially for the EWWML. The only signal noted for SWMN is that the SH tends to be located further northward. In general, the SH during the wintertime is weaker for SWM and stronger for WWM (not shown). Conditions of the summertime SH strength following SWME, LSWM, EWWM and EWWML are consistent with the fact that the strength of the SH tends to persist from winter to summer (not shown). Note that the persistence of the SH strength from winter to summer for the ENSO-coupled conditions (i.e. LSWM, EWWM and EWWML) could be a remarkable clue concerning the ENSO evolution. However for WWMN, whereas a weaker SH is observed in the following summer, no evident bias in winter is noted (not shown). Comparing the results in Tables I and IV concerning the status of SM and the summertime SH strength, a stronger summertime SH is associated with a nots (weaker) SM for the WWM cases (both ENSO-coupled and non-enso-coupled groups). However, the opposite is suggested from the result of LSWM, so that the relationship between the summertime SH strength and the SM is not strictly robust. In fact, Jiang and Winston (1989) argued that the strength of the summertime SH is not the determining factor for flooding and drought over the Yangtze River valley (and thus SM). Nevertheless, the relationship between the ridge line and the SM strength is consistent with the results of SWMN and EWWML, such that a northward (southward) displaced SH corresponds to a notw (nots) SM. Overall, certain well-described features of the summertime SH (including its strength, and/or western point and/or ridge line) are observed when possible relationships are found based on monsoon indices, as for the cases of SWMN, LSWM, WWMN and EWWML. This provides additional evidence for the possible relationships identified using SM indices and rainfall condition over China. On the other hand, bias in the SH Table IV. Conditions of the summertime (JJA) SH as referred to the strength index, western position and ridge axis using data provided by the NCC. The number gives the number of cases with positive or negative anomalies. East (West) represents a tendency for the SH to displace eastward (westward) with respect to the normal longitude. Similarly, North (South) represents the northward (southward) displacement of the ridge axis relative to the normal position. Note that the western position index refers to the longitude (dege) of the westernmost position of the SH, so that a negative anomaly denotes a more westward extent of the SH and vice versa. Bold numbers indicate cases with an apparent bias WM category Anomaly Strength Western position Ridge line SWM SWMN (2) SWME (5) Weaker 1 East 3 LSWM (5) WWM WWMN (5) Weaker EWWM (3) Stronger 2 1 EWWML (4) West North South Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

8 460 M. C. WU AND J. C. L. CHAN alone does not necessarily imply an evident signal in terms of monsoon indices, as can be highlighted from the cases of SWME and EWWM. As the summer following the ENSO-coupled WM is in the mature phase of an ENSO event, the SM conditions in EN + 1andLN+ 1 (i.e. years following the onset years of an EN and LN respectively) are worthwhile considering to highlight the ENSO association with the possible winter-to-summer monsoon relationships. A comparison of the results for ENSO could also, to a certain extent, offer a review to the ability of the assessment methods used in the present study, because the ENSO signatures are better defined in general. This, together with the situation for summer-to-winter monsoon relationship, will be considered in Sections 4 and SUMMER-TO-WINTER MONSOON RELATIONSHIPS 3.1. Monsoon indices Three possible summer-to-winter monsoon relationships are identified, namely notw SM-to-SWMN, notw SM-to-SWME and notw SM-to-EWWML (Table V). The possible nots SM-to-LSWM, as indicated by EASMI and WNPMI, is discarded to avoid the potential problem of redundancy, because both the EASMI and the WNPMI consider the 8 hpa horizontal zonal wind shear (see part I, section 3.3.1). Thus, it appears that the summer-to-winter monsoon relationship is relatively more evident when the SM is unlikely to be weak. In other words, no obvious summer-to-winter monsoon relationship exists when the SM is relatively weak. Moreover, the notw SM is unlikely to be a unique precursor signal to the strength of the following WM, as the WM can be strong or weak following a notw SM. The LSTD is the most sensitive index in reflecting the summer-to-winter monsoon relationship, such that apparent bias is noted for four WM categories. And the signals are very pronounced for SWME and EWWML, such that apparent biases of notw are suggested by four monsoon indices. The situation for EWWM is unusual, in that although both the preceding summers of EWWM and EWWML are the onset year of an EN, the result for EWWM seems to be different from that of the EWWML. Nevertheless, although the SM condition preceding the EWWM is not evident, a notw bias is obtained if EWWM and EWWML are considered collectively as an ENSO-coupled WWM. This suggests the evident ENSO influence in the summer-to-winter monsoon during the development of an EN event (Table VI) and will be considered in Section 4. Note that the same biases (notw) are obtained for the non-enso-coupled SWM and the ENSO-coupled WWM. Table V. As Table I, but for the preceding SM WM category (no. of cases) Strength SM conditions Monsoon index SMI EASMI LSTD RM2 WNPMI SWM SWMN (2) nots notw SWME (5) nots notw LSWM (5) nots notw WWM WWMN (5) nots notw EWWM (3) nots notw EWWML (4) nots notw Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

9 EAST ASIAN WINTER AND SUMMER MONSOON RELATIONSHIPS II 461 Table VI. As in Table II, but for the preceding SM. Note that (1) no bias is observed for SWMN and EWWM, (2) only LSTD shows apparent bias for WWMN, and (3) the case for LSWM is discarded (see text) WM (category) SM (indices showing bias) Non-ENSO-coupled SWM (SWMN, SWME) notw (SMI, LSTD, RM2, WNPMI) ENSO-coupled WWM (EWWM, EWWML) notw (EASMI, LSTD, RM2, WNPMI) 3.2. Rainfall condition Rainfall pattern. Similar to the consideration of the winter-to-summer monsoon relationship (Section 2), non-occurrence of certain patterns is noted (not shown). Regarding the cases showing possible relationships (i.e. SWMN, SWME and EWWML), no contradicting result is noted between the SM strength indicated from monsoon indices and the rainfall pattern. As observed, pattern I is most likely for WWMN, which suggests that the SM would be less likely to be weak. This is different from the result according to the monsoon index LSTD, which reflects a nots SM. This inconsistency concerning WWMN can be accounted for by: (1) the nots SM is only indicated by the LSTD monsoon index; (2) pattern III (an indication for a weaker SM to a first approximation) is also observed. For (1), this does not mean that the result obtained by referring to a single monsoon index is misleading, but that the level of uncertainty (significance) is higher (lower). Apparent bias reflected from a single monsoon index could be significant or representative to a certain extent when similar bias is reflected from other aspects, like rainfall condition. For example, the result for EWWM is in line with a nots SM reflected from the monsoon index RM2. However, although a notw SM is evident for EWWML based on monsoon indices (see Table V), a deficiency in using the rainfall pattern to represent the SM strength is also apparent because no clear conclusion is obtained using the rainfall pattern. Therefore, the rainfall pattern is more useful in verifying than in defining the SM strength. On the whole, pattern I dominates in the summer preceding the SWM, whereas no evident bias is noted for the case of WWM as a whole. The former agrees with the general relationship between rainfall pattern and the SM strength, such that the rain belt associated with a stronger SM is likely to be displaced northward Rainfall bias distribution. The rainfall bias distribution for SWMN resembles a typical pattern I configuration (Figure 2). For SWME when non-occurrence of pattern III is observed (see Section 3.2.1), the bias distribution reflects a tendency towards the dry side south of the Yangtze River. Therefore, there is a good agreement between rainfall condition (from rainfall pattern and rainfall bias distribution) and the SM strength based on monsoon indices for both SWMN and SWME. A feature that characterizes the non-occurrence of pattern II for LSWM is the not-wet bias over the region between the Yellow River and Yangtze River or the Huaihe River valley (also see Section 2.2.2). Although the not-dry bias south of the Yangtze River is consistent with the nots SM, no definite signal is assigned to the SM condition for LSWM (see Section 3.1). An implication is that the pattern of rainfall variation is not necessarily attributed to the variation in the SM strength. For WWMN, a bias towards a wetter condition is observed over the lower Yellow River valley, whereas most parts south of the Huaihe River show a tendency towards a drier condition. That the lower Yellow River valley sees a wetter condition is consistent with the fact that pattern I is most likely for WWMN. For the case of EWWM, non-occurrence of pattern I is associated with a not-wet condition over northern China, whereas the Huaihe River valley is unlikely to see a drier condition. For EWWML, the most evident feature is the not-dry bias south of the middle lower Yangtze River valley. Although no bias suggests a wetter condition over northern China, the condition over the Huaihe River valley is unclear. Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

10 462 M. C. WU AND J. C. L. CHAN 3.3. SH Apparent biases in terms of the summertime SH strength are noted for all of the six WM categories except SWMN and EWWM (not shown). Although the signal is not apparent for EWWM, it is also in line with EWWML. Therefore, similar to the winter-to-summer monsoon relationship (Section 2), the strength of the summertime SH appears to play an essential role in the summer-to-winter monsoon relationship as well. Apparently, the summertime SH tends to be weaker preceding a WWM, such that 10 out of 12 cases show negative anomalies in the SH intensity index. No evident signal for a SWM as a whole is obtained because the conditions are different for SWME and LSWM. The observation for the western position of summertime SH can be explained by the negative correlation with the strength of SH (not shown). In addition, the summertime SH tends to be located further north for SWMN and SWME. As a whole, the SH in the summer before a SWM is likely to be located northward, although the relationship is not evident for the case of LSWM. 4. ENSO INFLUENCE As can be expected from the relationships regarding the ENSO-coupled WM, the evolution (developing and decaying phases) associated with ENSO may play a substantial role in linking the SMs and WMs. Therefore, the ENSO influence in the relationships between SMs and WMs is examined in this section. Although the influences could be different for a different onset timing (i.e. the SP- and SU-types; see part I), the consideration here is to examine the general ENSO influence to identify the dominant signatures. Thus, altogether there are ten (seven) cases of EN (LN) under consideration. The EN (LN) onset year will be abbreviated as E0 (L0), whereas the year following an EN (LN) onset will be abbreviated as EN + 1(LN + 1). Note that the summer situations following the SWME, the ENSO-coupled SWM (i.e. LSWM) and the ENSO-coupled WWM (i.e. EWWM and EWWML) can be compared to that of E0, LN + 1andEN+ 1 respectively. Similarly, comparison can be made for the summer conditions preceding the ENSO-coupled SWM (i.e. LSWM) and the ENSO-coupled WWM (i.e. EWWM and EWWML) with that of L0 and E0 respectively Monsoon indices Table VII summarizes the SM conditions during ENSO onset years (E0 and L0) and the years following onset (EN + 1andLN+ 1). Clearly, an alternation in the SM strength is suggested from E0 to EN + 1, where the SM tends to be stronger in E0 and tends to be weaker in EN + 1. The results here are consistent with the possible relationships between SMs and WMs for the cases of ENSO-coupled WMs regarding the SM following EWWML and the SM preceding SWME and EWWML (see Tables I and V). Based on WNPMI alone, an alternation of SM (in terms of a stronger and weaker SM) is noted from onset year to the following year for both the EN and LN, which is the result of Wang et al. (01). However, the deficiency in using a single monsoon indicator is clear, as can be observed from the different SM conditions reflected from LSTD and WNPMI in LN + 1. Although a notw is indicated from WNPMI (consistent with Wang et al. (01) using WNPMI where the SM tends to be strong after the mature phase of a cold ENSO (or LN + 1)), the rainfall pattern condition (to be shown in Section 4.2) indicates a likelihood of a weaker SM that supports the assessment of a weaker (i.e. nots) SM based on LSTD (note: rainfall pattern condition does Table VII. Assessment of the SM condition using monsoon indices during ENSO onset year and the year following onset. Monsoon indices showing an apparent bias are given ENSO category Onset year Year following onset EN notw (LSTD, WNPMI) nots (EASMI, LSTD, WNPMI) LN nots (EASMI, WNPMI) nots (LSTD), notw (WNPMI) Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

11 EAST ASIAN WINTER AND SUMMER MONSOON RELATIONSHIPS II 463 not imply a weaker SM, but it provides further evidence if a weaker SM is indicated from other indicators). Furthermore, the sensitivity to ENSO influence could be different for different monsoon indices (which are not supposed to be the ENSO indicators simultaneously). For example, whereas the WNPMI is sensitive to the ENSO condition (as indicated from the apparent bias for all the ENSO conditions), the relationships of RM2 with basin-scale sea-surface temperature (SST) anomalies (SSTAs) and the Walker circulation are insignificant and marginal at best (Lau et al., 00). Overall, no evident signal in L0 (because the apparent bias of nots is reflected from EASMI and WNPMI only) and LN + 1 is defined based on the present analysis. Although this does not contradict the result of a nots SM following an EWWML (see Table I), it shows that the SM condition is likely to be better defined in L0 when a transition from EN is involved. In addition, the asymmetry in the result between EN and LN seems to be a corollary of the fact that, in terms of the pattern of the SST distribution, an EN is the reverse of the normal condition (i.e. warm in the east and cool in the west), whereas a similar SST pattern (i.e. cool in the east and warm in the west) is associated with an LN. Finally, although no evidence from monsoon indices suggests that the possible relationship LSWM-to-notS SM is attached to the ENSO influence, the ENSO link for the relationship (also true for other possible relationships) is noted from the rainfall condition, as shown below Rainfall conditions Rainfall pattern. Evidently, the rainfall pattern conditions for the summer following and preceding various WM categories (see Table III and Section 3.2.1) are consistent with that of the consideration of ENSO (Table VIII). That EN + 1(LN + 1) is unlikely to see rainfall pattern II (I) shows that the ENSO influence evident in EN + 1andLN+ 1 can be described in terms of the difference in the relative occurrence of patterns I and II. In addition, the similarity during L0 and EN + 1, where both are characterized by the non-occurrence of pattern II, highlights the possible transition from an EN to LN. As has been discussed, the SM strength is difficult to interpret from the non-occurrence of pattern II (see Section 2.2). However, it seems that the non-occurrence of pattern II is likely to be associated with a stronger SM when comparing the results for EN + 1 and L0 using monsoon indices (see Table VII). On the whole, using the general relationship between rainfall pattern and monsoon strength (e.g. Shi and Zhu, 1996), the SM strength cannot be inferred clearly and directly from the rainfall pattern condition during different ENSO conditions except in LN + 1. Actually, the rainfall pattern condition in LN + 1 (a preference for patterns II and III) helps to evaluate the SM condition as assessed from monsoon indices, as has been discussed above Rainfall bias distribution. In E0, northern China is unlikely to see wetter conditions and abundant rainfall is more likely over the Huaihe River valley (Figure 3), which is consistent with the result of the rainfall pattern (see Table VIII). Similarly, an unlikely abundant rainfall condition in the Huaihe River valley noted in EN + 1 and L0 is consistent with the non-occurrence of pattern II. No signal is noted south of the Yangtze River, including southeastern China for both E0 and L0. The result for EN + 1 is consistent with the finding by Liu and Ding (1995) and Huang et al. (1990), that the Huaihe River Valley is unlikely to see an abundant rainfall condition whereas the area south of the Yangtze River valley tends to be on the wetter side. In LN + 1, abundant rainfall is observed over the Huaihe River Valley and the lower Yangtze River Valley. The largest contrast between the configurations in EN + 1and Table VIII. Rainfall pattern conditions for ENSO during onset year and the following year. Zero represents non-occurrence of the rainfall pattern. The value gives the proportion for each rainfall pattern ENSO category Onset year Year following onset I II III I II III EN 2/10 5/10 3/10 6/10 0 4/10 LN 4/7 0 3/7 0 4/7 3/7 Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

12 464 M. C. WU AND J. C. L. CHAN E0 L EN LN Figure 3. As Figure 1, but for different ENSO conditions LN + 1 is in the Huaihe River valley and the area south of the Yangtze River, which depicts the difference in the relative occurrence of patterns I and II (see Table VIII). However, no contrast in terms of the SM strength is noted between EN + 1andLN+ 1 (refer to Section 4.1). Therefore, the SM strength is unlikely to be characterized simply by the rainfall variation over the Huaihe River valley and the area south of the Yangtze River SH The summertime SH conditions in E0 and L0 are opposite, such that it is weaker for the former and stronger for the latter (Table IX). Similar, but reversed, situations occur for EN + 1 and LN + 1. Apparently, the westward extent of the summertime SH in L0 is in line with a stronger summertime SH, as noted. On the other hand, no signal is noted for the ridge line. These results are consistent with Fu and Teng (1993), who showed that the main atmospheric circulation characteristics of anomalous summer climate in East Asia under the influence of a developing EN event are the changes in intensity and the degree of the westward extension of the SH. The strength and the western position of the SH in the summer of EN + 1andLN+ 1 are also consistent with the findings by Ai and Chen (00). Furthermore, a weaker (stronger) SM is likely to be associated with a stronger (weaker) SH as suggested from the situations in E0 and EN + 1. On the whole, an apparent variation in the SM strength is noted for EN, whereas it is unclear for LN from consideration of the monsoon indices. This is consistent with the fact that the climatic variation associated with EN is, in general, more profound than that of the LN. On the other hand, certain characteristics associated with different ENSO conditions in terms of the rainfall condition over China and also the SH are noted. Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

13 EAST ASIAN WINTER AND SUMMER MONSOON RELATIONSHIPS II 465 Table IX. As Table VII, but for the conditions of the subtropical high ENSO category Onset year Year following onset Strength Western position Ridge line Strength Western position Ridge line EN Weaker Stronger West LN Stronger West Weaker East 5. BIENNIAL ALTERNATION OF THE SMS AND WMS Based on the results in Sections 2.1 and 3.1, three possible summer-to-winter monsoon and four winter-tosummer monsoon relationships are identified (Table X). Clearly, the possible relationships between SMs and WMs for the ENSO-coupled groups (relationships 3, 6 and 7) and also for the case of SWME (relationship 2) are tied to the influence of ENSO, because similar SM conditions are noted for different ENSO conditions (see Figure 4). Possible summer-to-winter monsoon relationships are noted only when the SM is unlikely to be weak (relationships 1 and 2). However, the WM strength following a notw SM also depends on the ENSO conditions. The WM following a notw SM would be likely to be stronger if not coupled with a maturing ENSO phase (relationship 3). This constitutes a persistence of a stronger monsoon situation from summer to winter. On the other hand, the relationship of notw SM-to-EWWML is related to the fact that the SM during an EN onset year would be likely to be weaker, whereas the WM would be likely to be weak during the mature phase of an EN. It appears that the strength of the WM does not play an important role in the winter-to-summer relationship, because similar SM conditions (notw for relationships 4, 5 and nots for relationships 6, 7) are suggested when the WM strength is completely opposite. The relationship WWMN-to-notW SM, which describes a weaker WM followed by a stronger SM, seems to complement the relationship of a stronger non-enso-coupled SWM preceded by a stronger SM (relationship 2). A similar SM condition (nots) following an ENSO-coupled WM (EWWNL and LSWM) can be related to the fact that the differences in the SSTA conditions during L0 and LN + 1 are not large, such that the negative Niño-3.4 SSTA persists to the summer of LN + 1 (six out of the Table X. Summary of the possible relationships between SMs and WMs Summer-to-winter 1. notw SM non-enso-coupled SWM (SWMN) 2. notw SM non-enso-coupled SWM (SWME) 3. notw SM ENSO-coupled WWM (EWWML) Winter-to-summer 4. (SWMN) non-esno-coupled SWM notw SM 5. (WWMN) non-esno-coupled WWM notw SM 6. (EWWML) ENSO-coupled WWM nots SM 7. (LSWM) ENSO-coupled SWM nots SM ENSO status Onset year Year following onset EN: SH: weaker SM: notw SH: stronger, westward-extended SM: nots LN: SH: stronger, westward-extended SM: non-unique SH: weaker, eastward-retreated SM: non-unique Figure 4. Characteristics of the SH during different ENSO conditions. Also given is the SM condition based on monsoon indices Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

14 466 M. C. WU AND J. C. L. CHAN seven LN cases have negative Niño-3.4 SSTAs in the summer following onset year, actually, all cases for those LN cases in EWWML). On the other hand, an evident alternation of the SM occurs for the case of EWWML that a stronger SM precedes a weaker WM and a weaker SM follows the weaker WM, which is then followed by a stronger SM (Figure 5). These alternations of the SMs and WMs will be referred to as a biennial alternation (BA). This BA is different from the tropical biennial oscillation (TBO) relationship (e.g. Meehl, 1994) that basically describes a linkage of a SWM preceded (followed) by a strong (weak) SM or a WWM preceding (followed) by a weak (strong) SM. In particular, the TBO, when tied to ENSO, implies a strong (weak) SM after an EN-like (LN-like) condition in the preceding winter (e.g. Chen and Lau, 1995). However, one should be aware of the different meanings of strong/weak SM over China. For example, Shen and Lau (1995) equated a strong (weak) SM to a wet (dry) condition over China. On the other hand, Sun et al. (01) stated that a strong (weak) SM described by LSTD corresponds to dry (wet) conditions over the Yangtze River and Huaihe River valleys. In any case, the transition from EN to LN involved in the BA of the monsoons is consistent with the fact that an SM ENSO connection tends to occur during the transition phases of EN and LN (Lau and Wu, 01). The nots SM following an LSWM means a breaking down of the biennial cycle (relationship 7 in Table X). In fact, the breaking down also represents that no detectable (in the sense of the statistical analysis of the present study) bias or signal is noted, as the SM condition in LN + 1 (and also L0) is also unclear in light of the monsoon indices (see Figure 4). Meanwhile, it is not common to see an EN onset in LN + 1 (only one case, , in the last years). Recall that the WM preceding an EN onset is unlikely to be weak (either strong or neutral in a tercile categorization; see section 4 in part I), such that half of the 10 EN cases are preceded by an SWM (i.e. those SWME cases) and the other five cases are preceded by neutral WMs (not shown). The cycle in terms of SWM/WWM can also break down in the WM preceding the EWWML, because three of the four EWWML cases are preceded by a neutral WM (and thus the WM preceding EWWML in Figure 5 is unlikely to be an SWME). On the other hand, the WM is still stronger relative to the conditions of a WM coinciding with 1 st year 2 nd year 3 rd year DJF JJA DJF JJA DJ F JJA ENSO EN onset EN mature LN onset LN mature Monsoon WM SM WM SM WM SM neutral notw weak (EWWML) nots strong (LSWM) nots Stronger Stronger Weaker Weaker Stronger Weaker SH Weaker Stronger Stronger Weaker Biennial Alternation Figure 5. Schematic evolution showing the ENSO-coupled monsoon process involving a transition of warm to cold phase. It shows the evolutions of (1) an ENSO cycle from EN to LN, (2) transitions between SMs and WMs and (3) the SH strength. A notw (nots) SM is described as a stronger (weaker) SM condition to highlight the alternation of the SM strength. A biennial alternation of the monsoons strength is noted from the first year to the second year Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

15 EAST ASIAN WINTER AND SUMMER MONSOON RELATIONSHIPS II 467 and succeeding EWWML. As such, it is suggested that the breakdown of a complete biennial oscillation is mainly associated with the non-unique SM condition in LN + 1. Again, this could be related to the fact that the differences in the SSTA conditions during L0 and LN + 1 are not large, as has been discussed above. In summary, a BA of the monsoons is noted when coupled with an ENSO warm-to-cold transition. A complete cycle of biennial oscillation is unlikely, but the BA describes a coupling between the biennial process of the monsoon and ENSO. Whereas the BA depicted in Figure 5 is specified for the region of eastern Asia (i.e. the East Asian SM), it is different from the TBO process described by Meehl (1994), among others, that link the South Asian summer monsoon and the Australian summer monsoon, for which the TBO is constituted by a strong (weak) SM followed by a SWM (WWM) process. As such, the commonly recognized TBO is not tied to the biennial signal in the interannual variability of the East Asian (summer and winter) monsoon. Finally, an alternation of the strength of the SH accompanies the BA of the monsoon. 6. CONCLUSION AND REMARKS The present study, using the framework presented in part I, investigates the relationships between SMs and WMs. The SM conditions preceding and following each of the six WM categories are examined and classified into unlikely strong (nots) or unlikely weak (notw) based on an aggregated assessment of five SM indices. Among the five SM indices, LSTD (representing the land sea temperature contrast) appears to be the most sensitive in reflecting both the summer-to-winter monsoon and winter-to-summer monsoon relationships. Three possible summer-to-winter monsoon and four winter-to-summer monsoon relationships are identified. Overall, the assessment of the SM condition using monsoon indices is consistent with that based on rainfall condition (in terms of rainfall bias distribution and/or rainfall pattern condition) for these possible relationships. In addition, variations in the SH are also noted. A generalized relationship between SMs and WMs is virtually non-existent because of the asymmetry in the relationships and the difference between ENSO-coupled and non-enso-coupled groups. This highlights the importance in stratifying the WM into various categories in terms of the WM strength and ENSO condition in the studying of the relationships between SMs and WMs. By noting the SM condition during different ENSO conditions, it is shown that some possible relationships are, in fact, tied to the influence of ENSO. From the nature of the possible winter-to-summer monsoon relationships, it is noted that the strength of the WM is unlikely to be an important forcing regarding the relationships. In addition, possible summer-to-winter monsoon relationships are noted only when the SM is not weak, probably indicating a corresponding stronger forcing when the SM is stronger. A BA of the SMs and WMs is noted, in which a stronger SM precedes a weaker WM and the weaker WM is followed by a stronger SM. This BA is associated with a transition from an ENSO warm phase to an ENSO cold phase, representing the biennial signal in the interannual variability of the monsoons and ENSO. Concurrent with this BA is an evident variation in the SH strength. It appears that the commonly recognized TBO is not tied to the biennial signal in the interannual variability of the East Asian monsoons, because the TBO is constituted by a strong (weak) SM followed by SWM (WWM) process. Furthermore, it is suggested that a complete biennial oscillation in the interannual variability of the monsoons is not observed because of the breaking down of a cycle (or oscillation) in the SM following an LN onset. In the present study, a certain degree of reliability in the assessment of SM strength (i.e. nots or notw) and, thus, the relationships is provided by the consensus or, more precisely, the absence of apparent inconsistency among different SM strength indicators. Note that the contrasts in the tendency of the SM strengths (and, thus, the relationships) are more profound. In order to obtain a more definite result, the atmospheric (in addition to the SH characteristics) and oceanic conditions, as well as the physical processes responsible, have to be investigated. In an exploratory study, it is suggested that the Eurasian winter snow cover plays an important role in linking one of the possible winter-to-summer monsoon relationships. It is obvious that processes related to the evolution of ENSO will also play a substantial role in linking the possible relationships between SMs and WMs. Finally, the results presented can provide statistical guidance for dynamical seasonal forecasting. Copyright 05 Royal Meteorological Society Int. J. Climatol. 25: (05)

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