Figure 1 shows the time series of the standardized WNPCI (bars) and TC days (line). A TC day was counted if there was at least one TC appearing to the west of 160°E over the WNP on a certain day during summer. Both time series show clear interannual variations and have no distinct decadal variations or long-term trends. The correlation coefficient between the WNPCI and TC days is −0.41 during 1979–2018, significant at the 99% confidence level. That is, when convection is enhanced in the tropical WNP, there are more TCs over the WNP.
Figure 1. Time series of the standardized WNPCI (bars) and the number of TC days (line) during 1979–2018. The black bars indicate the strong-convection years.
Enhanced-convection years were selected based on the WNPCI, and six years were chosen according to the criterion of the WNPCI being lower than −1.0 standard deviation (1981, 1985, 2001, 2004, 2012, and 2018). The number of TC days during these six summers is 46, 53, 48, 63, 57, and 69, respectively (Table 1), and the ratios of TC days to total summer days range from 50% to 75%, suggesting that TCs appear frequently over the WNP, at least for these summers of strong convection. The following analyses are mainly concentrated on these six summers, on the basis that their relatively greater frequencies of TCs should make the contribution of TCs to the EAP pattern clearer. The contribution of TCs was estimated by comparing the results for TC days and for total days in summer.
Strong-convection summers 1981 1985 2001 2004 2012 2018 Mean TC 46 53 48 63 57 69 56 Westward-moving TC 15 6 16 15 16 17 14 Northward-moving TC 14 16 27 44 27 39 28 Extratropical TC 8 9 13 19 10 26 14
Table 1. The number of TC days, westward-moving TC days, northward-moving TC days, and extratropical TC days during the strong-convection summers, and the average number of days for these six years.
Figure 2 shows the composite OLR anomalies for TC days (Fig. 2a) and total days (Fig. 2b) for the strong-convection summers. Negative OLR anomalies appear over the tropical WNP for both TC days and total days (JJA), indicating strong convection over that region. In addition, stronger convection occurs on TC days, as compared to the JJA mean. The WNPCI for TC days and total days is −12.17 and −9.23 W m−2, respectively (Table 2), indicating that convection is enhanced significantly over the tropical WNP during TC days. The WNPCI on TC days decreases by 32% relative to the JJA mean. In addition, there are positive OLR anomalies over the rainy belt of subtropical East Asia, as a component of the well-known seesaw pattern of rainfall between the tropics and subtropics in summer.
Figure 2. Composite maps of OLR anomalies for (a) TC days and (b) the JJA mean during the strong-convection summers. The contour interval is 5 W m−2. Zero contours are omitted. Shading indicates that the correlation coefficient between the WNPCI and OLR anomalies is significant at the 95% confidence level.
JJA mean TC days Westward-moving TC days Extratropical TC days WNPCI −9.23 −12.17 (+32%) −15.01 (+63%) −7.44 (−19%) WNPSMI 3.24 4.70 (+45%) 6.00 (+85%) 3.25 (0%) EAPI 1.22 1.86 (+52%) 2.07 (+70%) 1.32 (+8%) EAJI 3.23 4.40 (+36%) 2.53 (−22%) 5.66 (+75%)
Table 2. The WNPCI (units: W m−2), WNPSMI (units: m s−1), EAPI and EAJI (units: m s−1), for the JJA mean, TC days, westward-moving TC days, and extratropical TC days during the strong-convection summers. Values in parentheses are the changes in these values relative to the JJA mean.
Figure 3 shows the composite circulation anomalies for TC days and total days for the strong-convection summers. These circulation anomalies, including 850- and 200-hPa wind anomalies and 500-hPa geopotential height anomalies, can be used to illustrate the meridional teleconnection pattern over WNP–EA. In the lower troposphere, there is a cyclonic anomaly over the subtropical WNP, and a relatively weak anticyclonic anomaly centered over southern Japan, for both TC days and total days (Figs. 3a and 3b). However, the cyclonic anomaly on TC days (Fig. 3a) is obviously stronger than that in the JJA mean (Fig. 3b), which is confirmed by the WNPSMI being larger for TC days than in the JJA mean (4.70 m s−1 versus 3.24 m s−1, equivalent to a 45% increase; Table 2).
Figure 3. Composite maps of (a, b) 850-hPa horizontal wind anomalies (units: m s−1), (c, d) 500-hPa geopotential height anomalies (units: m), and (e, f) 200-hPa zonal wind anomalies (units: m s−1) during the strong-convection summers. The left-hand panels denote TC days, and the right-hand panels denote the JJA mean. Small values of horizontal wind (magnitude less than 0.7 m s−1) are omitted in (a, b). The contour interval is 4 m in (c, d) and 1 m s−1 in (e, f). Zero contours are omitted. The thick solid lines in (e, f) show the axis of climatological mean westerly jet. Shading in (b) indicates that the correlation coefficient between the WNPCI and zonal or meridional wind anomalies is significant at the 95% confidence level, and shading in (d, f) is the same as in (a) but for geopotential height and zonal wind anomalies.
There are negative and positive 500-hPa geopotential height anomalies over the subtropical WNP and midlatitude East Asia, respectively, for both TC days and JJA (Figs. 3c and 3d). There are also negative anomalies over the Okhotsk Sea and adjacent Far East, but with a moderate statistical significance, as indicated by Fig. 3d. Thus, we defined an EAP index by the subtropical and midlatitude components of this pattern, as mentioned in section 2, without considering the high-latitude anomalies. It was found that both the subtropical and midlatitude anomalies are remarkably stronger on TC days (Fig. 3c) than in the JJA mean (Fig. 3d). The EAPI for TC days is 1.86, stronger than that for the JJA mean (1.22). Compared with the JJA mean, the EAPI on TC days increases by 52% (Table 2).
In the upper troposphere, zonal wind anomalies are zonally elongated and alternately negative and positive from the tropical western Pacific to East Asia, for both TC days and JJA (Figs. 3e and 3f). In particular, there are positive (negative) anomalies to the north (south) of the climatological jet axis, which is defined by where the first derivative of the 200-hPa zonal winds is zero and represented by the thick solid lines, suggesting a northward displacement of the EAJ. On the other hand, both the positive and negative anomalies to the north and south of the jet axis are stronger on TC days than in JJA, further indicating a more remarkable northward displacement of the EAJ on TC days. This can be confirmed by the composite EAJI results, i.e., 4.40 m s−1 and 3.23 m s−1 on TC days and in JJA, respectively. This difference is equivalent to a 36% increase (Table 2).
The results presented in this section indicate that this meridional teleconnection, illustrated by the cyclonic anomaly in the lower troposphere, EAP pattern in the middle troposphere, and EAJ meridional displacement in the upper troposphere, is stronger on TC days than on a JJA-mean basis. Therefore, it can be concluded that TCs strengthen rather than weaken this meridional teleconnection.