Results and Discussion

To investigate the temporal degree of association among the seven different TCIs and Indian summer monsoon rainfall anomalies viz., AIR, NWR, and PIR, concurrent and lagged correlations (CCs) were computed. Thus, AIR, NWR, and PIR seasonal monsoon rainfall anomalies have been correlated with various monthly/seasonal TCIs series. These months and seasons are (i) the months from January to May and two seasons DJF (December, January, and February) and MAM (March, April, and May), antecedent to summer monsoon season, (ii) the months from June to September and a season JJA (June, July, and August), concurrent to the summer monsoon, and (iii) the months October, November, December and two seasons SON+ (September, October, and November), and DJF+ (December, January, and February), the succeeding periods with respect to the summer monsoon season.

The analysis shows several significant correlations on which our discussion will be restricted. Table 1 shows the CCs between Indian summer monsoon rainfall and TCI series for the period 1953-1982 along with their levels of significance.

As far as concurrent relationships are considered, Table 1 shows significant negative CC of -0.37 (at 5% level) between TCI (A-M) JJA and NWR. In addition, significant negative CCs are also obtained between concurrent August TCI (A-M) and AIR, NWR, and PIR (CC = -0.51, -0.51, and -0.48, all at 1% level). September TCI (A-M) and PIR show

Table 1. Correlation coefficients (CCs X 100) between Indian summer monsoon rainfall and monthly/seasonal TCIs.

Months/Season

AIR

NWR

PIR

Months/Season

AIR

NWR

PIR

TCI(A-M) Aug(0)

—51(1)

—51(1)

—48(1)

TCI

C-N) Feb( —)

—50(1)

—42(2)

—38(5)

TCI(A-N) Sep(0)

-33

—24

—42(2)

TCI

C-N) Apr( —)

—39(5)

— 17

—38(5)

TCI(A-M) JJA(0)

— 22

—37(5)

— 15

TCI

C-N) Jun(0)

—42(2)

—37(5)

—44(2)

TCI(A-N) May(-)

36(5)

33

20

TCI

C-N) Jul(0)

—37(5)

—38(5)

—28

TCI(A-N) Jul(0)

—47(1)

—47(1)

—42(2)

TCI

C-N) JJA(0)

—44(2)

—43(2)

—38(5)

TCI(A-N) JJA(0)

—32

—42(2)

—33

TCI

C-N) Nov(+)

—56(1)

—34

—48(1)

TCI(A-N) DJF(+)

—46(1)

—32

—52(1)

TCI

C-N) Dec(+)

—42(2)

—26

—37(5)

TCI(A-C) Feb(-)

36(5)

46(1)

23

TCI

C-N) SON(+)

—38(5)

—22

—27

TCI(A-C) Oct(+)

37(5)

36(5)

18

TCI

C-N) DJF(+)

—41(5)

— 15

—39(5)

TCI(A-C) Nov(+)

49(1)

28

40(5)

TCI

I-N) Jan( —)

—31

—39(5)

—27

TCI(A-C) SON(+)

41(5)

23

23

TCI

I-N) May( —)

46(1)

31

39(5)

TCI(C-M) Aug(0)

— 70(1)

— 66('1)

—58('1)

TCI

I-N) Jul(0)

—43(2)

—23

—39(5)

TCI(C-M) JJA(0)

—51(1)

—49(1)

—41(5)

TCI

I-N) Sep(0)

29

14

36(5)

TCI(C-M) Oct(+)

—46(1)

—39(5)

—35

TCI

I-N) JJA(0)

—39(5)

—33

—37(5)

TCI(C-M) Nov(+)

—41(5)

—24

—34

TCI

I-N) DJF(+)

— 61('1)

— 61('1)

—57('1

TCI(C-M) SON(+)

—54(1)

—41(5)

—43(2)

TCI

I-M) Jan( —)

—37(5)

—50(1)

—30

TCI

I-M) DJF(—)

—16

—40(5)

— 16

TCI

I-M) Aug(0)

—51(1)

—49(1)

—41(5)

TCI

I-M) JJA(0)

—36(5)

—28

—28

TCI

I-M) DJF(+)

-61Î'1)

_64<-i>

-ST«-1

Notes: (-1), (2) and (B) stand for 0.1%, 1%, 2%, and 5% significant level; (-), (0), (+) stand for antecedent, concurrent, and succeeding periods.

Notes: (-1), (2) and (B) stand for 0.1%, 1%, 2%, and 5% significant level; (-), (0), (+) stand for antecedent, concurrent, and succeeding periods.

significant negative CC of —0.42 (at 2% level). But Table 1 does not show any significant relationship between antecedent and succeeding TCI (A-M) and AIR, NWR, and PIR.

For TCI (A-N), Table 1 shows significant negative CC of —0.42 (2% level) between concurrent TCI (A-N) JJA and NWR. The CCs between July TCI (A-N) and AIR, NWR, and PIR are —0.47 and —0.47 (both at 1% level) and —0.42 (2% level), respectively, while succeeding relationships show significant negative CCs of —0.46 (1% level) and —0.52 (1% level) between TCI (A-N) of DJF and AIR and PIR, respectively. However, the antecedent relationship shows significant positive CC of 0.36 (5% level) between TCI (A-N) of May and AIR only.

Concurrent TCI (A-C) does not show any significant relationship with AIR, NWR and PIR (Table 1). However, succeeding October, November, and SON show significant positive CCs of 0.37 (5% level), 0.49 (1% level), and 0.41 (5% level) with AIR; succeeding October TCI (A-C) with NWR (CC = 0.36 at 5% level), and succeeding November TCI (A-C) with PIR (CC = 0.40 at 5% level). As far as antecedent relationships are concerned, Table 1 shows that only February TCI (A-C) have significant positive CCs of 0.36 (5% level) with AIR and 0.46 (1% level) with NWR. Thus, it appears that February TCI (A-C) has the potential to be used as a predictor for Indian summer monsoon rainfall all over India and the northwest India.

For TCI (C-M), Table 1 shows the strong and inverse CCs of —0.70, —0.66, and —0.58 (all are significant at 0.1% level) between concurrent August TCI (C-M) and AIR, NWR, and PIR. Significant negative CCs are also obtained between concurrent JJA TCI (C-M) and AIR, NWR, and PIR (CC = —0.51 at 1% level), —0.49 (at 1% level), and —0.41 (at 5% level), respectively. Figure 1 shows the year to year relationships between the anomalies of TCI (C-M) of concurrent August, and JJA and AIR showing the inverse association between them, while significant negative CCs are obtained between succeeding October TCI (C-M) and AIR and NWR (CC = —0.46 at 1% level, —0.39 at 5% level), respectively. November TCI (C-M) with AIR shows negative CC of —0.41 at 5% level and succeeding SON with AIR, NWR, and PIR (CC = —0.54 at 1% level with AIR, —0.41 at 5% level with NWR, and —0.43 at 2% level with PIR). However, there is no significant relationship between antecedent TCI (C-M) and AIR, NWR, and PIR.

Considering TCI (C-N), Table 1 shows significant negative CCs of —0.44, —0.43, and —0.38 at 2% and 5% levels between concurrent JJA and AIR, NWR and PIR, respectively. In addition, significant negative

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YEARS

■M H—i—i—i—i—i—i—i—m—i • |—)— i" i-r~i—i—i—i—i—i—i—! ■ i 11—i—i—i—

YEARS

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YEARS

Fig. 1. Time series of standardized rainfall anomaly AIR (marked by square) and TCI (C—M) of concurrent (a) August (marked by plus) and (b) JJA (marked by plus).

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YEARS

Fig. 1. Time series of standardized rainfall anomaly AIR (marked by square) and TCI (C—M) of concurrent (a) August (marked by plus) and (b) JJA (marked by plus).

CCs are also obtained between concurrent June and July TCI (C-N) and AIR (CC = -0.42 at 2% level and -0.37 at 5% level), and with NWR (CC = -0.37 and -0.38), both significant at 5% level. Concurrent June TCI (C-N) and PIR show CC of -0.44 at 2% level, while succeeding November, December, SON, and DJF show significant and negative CCs of -0.56

(1% level), -0.42 (2% level), -0.38 (5% level), and -0.41 (5% level) with AIR and PIR. Succeeding November, December, and DJF TCI (C-N) show negative CCs of -0.48 (1% level), -0.37 (5% level), and -0.39 (5% level). As far as antecedent relationships are concerned, TCI (C-N) of February shows strong and significant negative CCs of -0.50 (1% level) with AIR, -0.42 (2% level) with NWR, and -0.38 (5% level) with PIR, while April TCI (C-N) shows significant negative CCs of -0.39 and -0.38 both at 5% level with AIR and PIR. Thus, February TCI (C-N) appears to be a potential predictor for AIR, NWR, and PIR.

For TCI (I-N), it is found that concurrent JJA shows significant negative CCs of -0.39 at 5% level with AIR and -0.37 (5% level) with PIR. In addition, significant negative CCs are also obtained between concurrent July TCI (I-N) and AIR (CC = -0.43 at 2% level) and PIR (CC = -0.39 at 5% level) and September TCI (I-N) with PIR (CC = -0.36 at 5% level). Succeeding DJF TCI (I-N) shows significant negative CCs of -0.61 (0.1% level) with AIR and NWR and -0.57 (0.1% level) with PIR. For antecedent relationship, Table 1 shows significant negative CCs of -0.39 (5% level) with January TCI (I-N) and NWR. The antecedent May TCI (I-N) shows significant CCs of 0.46 at 1% level and 0.39 at 5% level with AIR and PIR.

From the above analysis, it appears that May TCI (I-N) has some potential as a predictor for long-range forecasting of Indian summer monsoon rainfall over AIR and PIR. The strong and significant inverse CCs between TCI (I-N) of succeeding DJF suggest that a strong (weak) summer monsoon rainfall activity is followed by unusually high (low) pressure over northwest India and unusually low (high) pressure over Mascarene High during the following winter months.

Considering TCI (I-M), Table 1 shows significant negative CC of -0.36 (5% level) between concurrent TCI (I-M) JJA and AIR. TCI (I-M) of August shows significant negative CCs of -0.51 (1% level) with AIR, -0.49 (1% level) with NWR, and -0.41 (5% level) with PIR. Succeeding DJF shows negative CCs of -0.61 (0.1% level), -0.64 (0.1% level), and -0.57 (0.1% level) with AIR, NWR, and PIR, respectively. Significant relationship (CC = -0.40 at 5% level) is also found between antecedent TCI (I-M) DJF and NWR. In addition, significant negative CCs are also obtained between antecedent TCI (I-M) of January and AIR (CC = -0.37 at 5% level) and NWR (CC = -0.50 at 1% level).

To examine the consistency of significant TCI relationships, the CCs of different months for the sliding 25-year periods have been recomputed during the period 1953-1982. Stability analysis has been done only for those TCIs which display strong correlation (mostly for antecedent period). Stability analysis of antecedent February TCI (A-C) shows significant correlation over all the different periods of study with PIR, except for the first 25-year period viz., 1953-1982. It is interesting to note that the correlation is not significant with PIR during the full period, but it becomes significant during different periods of study except the earliest period. Hence, it can be used as a predictor for PIR.

Considering concurrent TCI (C-M) of August, the analysis shows that the correlation is highly significant over various periods of study from 1953 to 1982 for AIR, NWR, and PIR. For antecedent TCI (C-N) of February, correlation remains significant for AIR and PIR over different sliding periods of study. For TCI (C-N) of antecedent April, correlation is significant during 1953-1977 and 1954-1978 for AIR and NWR, and in 1955-1979 for AIR only. While TCI (I-N) of antecedent May shows that the correlation is significant over all the periods of study for AIR only and for NWR, correlation is significant during the period 1953-1977 only. However, for antecedent TCI (I-M) of January, no significant correlation is found during different sliding periods (although, it shows significant correlation during the full period of study). So it may not be a good predictor.

From the stability analysis of different TCIs, it appears that among all the above TCIs, TCI (C-N) of antecedent February shows the strongest negative CCs with the following all India and regional peninsular Indian summer monsoon rainfall, and it can be used as a potential for long-range forecasting of summer monsoon over AIR and PIR.

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