Effects of Tropical Cyclone Activity on the Boundary Moisture Budget over the Eastern China Monsoon Region

Over the past few decades, many studies have focused on regional or global water vapor transport. (Knippertz and Wernli, 2010) analyzed water vapor transport channels related to subtropical rainfall and storms using global reanalysis data. (Zhou et al., 1999) calculated the global air-sea flux of freshwater and analyzed the characteristics of global water vapor transport.

In summer, the net meridional atmospheric moisture fluxes are much stronger than zonal transport over eastern China, and moisture is advected mainly by the monsoon flow (Huang et al., 1998). The sources of moisture for rainfall over China mainly come from the western North Pacific (WNP) and the Indian Ocean, and anomalous water vapor supply from these regions directly affects rainfall in southern China (Xie and Dai, 1959; Zhang et al., 2009; Chen et al., 2012a). In wet years, abundant moisture in southeastern China is associated with transportation from the WNP (Simmonds et al., 1999). (Zhou and Yu, 2005) found that tropical water vapor transport related to typical rainfall anomalies over China originates from the tropical western Pacific Ocean. (Li and Zhou, 2013a) investigated spatial and temporal variations in summer moisture circulation over East Asia (EA) and revealed that the variation in regional precipitation depends strongly on externally imported moisture, rather than on local evaporation. (Wei et al., 2012) estimated the evaporative moisture sources for Yangtze River valley rainfall with a water vapor back-trajectory method and showed that the moisture from the Bay of Bengal and the western Pacific compensates each other for the rainfall in the Yangtze River valley during April-September.

The WNP is the only ocean basin in the world where tropical cyclones (TCs) form throughout the year. Nearly one-third of the TCs in the world occur in The WNP and 80% of them become tropical storms, or typhoons (the maximum wind speed exceeds 32 m s^-1). Climate studies on the TC activity over the WNP constitute an important branch of TC research. Many works have focused on the effects of the El Niño-Southern Oscillation (ENSO) (e.g., Zhang et al., 1990; Wang and Chan, 2002; Camargo and Sobel, 2005; Li and Zhou, 2012a), the subsurface temperature of the western Pacific warm pool (Bender and Ginis, 2000; Shay et al., 2000; Lin et al., 2014), the subtropical northwestern Pacific high (SNPH) (Gong and Ho, 2002; Ho et al., 2004), the Madden-Julian Oscillation (MJO), and the quasi-biweekly oscillation (QBWO) (e.g., Kim et al., 2008; Li and Zhou, 2012a; Li and Zhou, 2013a, 2013b), on TC activity.

TCs are important weather systems in EA, where they have significant impacts on precipitation and may cause widespread damage to infrastructure (Rodgers et al., 2000; Kubota and Wang, 2009). TCs bring substantial rainfall to southern China almost every year, accounting for 20%-40% of the total annual precipitation in most of the southeastern coastal regions (Ren et al., 2002, 2006). (Kwon et al., 2007) indicated that a remarkable increase in the number of typhoons passing through southeastern China might be partly responsible for the increased precipitation, based on observations since the mid-1990s. (Chen et al., 2012b) quantified the contribution of TCs that form in the South China Sea to the increase in summer rainfall over southern China around 1993. Strong peripheral cyclonic circulation and heavy rainfall events associated with TCs can modulate atmospheric circulations and precipitation over EA (Schenkel and Hart, 2015). Hence, it is important to link TC activity with water vapor transport over EA.

There have been a few studies on the relationship between TCs and moisture transport, but they have focused mainly on moisture changes within the TCs (DiMego and Bosart, 1982; Deng et al., 2005; Kung and Zhao, 2007). The present study documents the impacts of TCs on the boundary moisture budgets over EA.

The rest of this paper is organized as follows: section 2 describes the datasets used and the area of study; section 3 investigates the background of water vapor transport over EA and the moisture budget in the humid region, and shows their relationships with rainfall; section 4 reveals the coupling between TCs and moisture circulation; and section 5 discusses the study's findings.

The datasets used in this study consist of: (1) 6-h interval TC data from the China Meteorological Administration Tropical Cyclone Database (http://tcdata.typhoon.gov.cn) (Ying et al., 2014); (2) atmospheric data, including wind and specific humidity, from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis (Kalnay et al., 1996); and (3) version 4 of the Extended Reconstructed Sea Surface Temperature (ERSST.v4) dataset from the National Oceanic and Atmospheric Administration (Smith and Reynolds, 2003).

The vertically integrated moisture flux can be expressed as $$ {Q}=\dfrac{1}{g}\int_{P_t}^{P_s}q{V}dp ,(1)$$

where g is the acceleration of gravity, q is the specific humidity, V is the horizontal wind vector, P _s is the surface pressure, and P _t is set at 300 hPa as the moisture content above this level is negligible (Trenberth, 1991).

The atmospheric moisture transport via each boundary (N) is calculated by $$ N=\dfrac{1}{g}\int_L{Q}\cdot {n}dl ,(2)$$ where L is the length of the boundary and n is the inward-pointing normal vector of the boundaries of the target region (Schmitz and Mullen, 1996).

Warm and cold ENSO events are based on a threshold of 0.5^°C for the Oceanic Niño Index [three-month running mean of ERSST.v4 SST anomalies in the Niño 3.4 region (5^°N-5^°S, 170^°-120^°W)] and the above thresholds are exceeded for a period of at least 5 consecutive overlapping 3-month seasons (available at: \hrefhttp://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml; Trenberth, 1997). According to the definition, there were eight El Niño years (1982, 1986, 1987, 1991, 1994, 1997, 2002, and 2009) and six La Niña years (1988, 1998, 1999, 2000, 2007, and 2010) during 1979-2010.

The domain of the analysis in this study is roughly (22^°-35^°N, 105^°-122^°E) (Fig. 1a), in accordance with (Liao and Zhao, 2010). This region is under the influence of the monsoon, and is thus denoted hereafter as the eastern China monsoon region (ECMR). The ECMR is located to the east of the Tibetan Plateau and has a regional average annual precipitation above 400 mm. For the ECMR, the southern and eastern boundaries (S and E in Fig. 1a) act as two major moisture inflow boundaries in summer. The moisture budgets of the two boundaries are influenced by TC activity and correspond well to precipitation anomalies over the ECMR. Hence, the variations of moisture fluxes via the eastern and southern boundaries are the focus of our research. Most TCs have a radius of about 3^°-6^° (Kubota and Wang, 2009). In this study, we focus on TCs in EA that enter the region north of 17^°N and west of 127^°E (dashed line in Fig. 1a), and affect the moisture transport over the ECMR directly. July-September (JAS) is taken to be the TC season during which the summer monsoon circulation dominates the ECMR. And about 70% of the TCs in EA occur during this time period.

Figure 1.  (a) Vertically integrated climate mean (1979-2010 average) JAS water vapor flux (vectors; units: kg m^-1 s^-1) and its divergence (shading; units: 10^-4 kg m^-2 s^-1). Red shading denotes divergence over 2× 10^-4 kg m^-2 s^-1 and blue denotes divergence less than -2× 10^-4 kg m^-2 s^-1 (blue). (b) Time series of the moisture budget (units: 10⁷ kg s^-1) for all boundaries (black), and the southern (red), eastern (blue) and "other" (orange) boundaries. The red polygon indicates the ECMR (22^°-35^°N, 105^°-122^°E, the same below) and the dotted lines mark the scope of TC activity in EA (north of 17^°N and west of 127^°E, the same below).

Climatologically, the ECMR is influenced by three main branches of water vapor transport from the lower latitudes (Fig. 1a): (1) the cross-equatorial flow around 105^°E; (2) the southeasterly flow from southwest of the SNPH; and (3) the southwesterly flow from the Bay of Bengal. The moisture convergence zone extends from east of the Philippines to the southeast of the ECMR. Figure 1b shows time series of the moisture budget via each boundary.

In summer, the moisture flows into the ECMR mainly from the eastern and southern boundaries. On average, the values of the moisture inflow via these two boundaries are about the same: 20.1× 10⁷ kg s^-1 for the eastern boundary and 22.5× 10⁷ kg s^-1 for the southern boundary. The moisture leaves the region at other boundaries, but the moisture outflow is less than the total inflow via the eastern and southern boundaries. In general, the net boundary water vapor flux of the ECMR is positive during JAS. The moisture inflow via the eastern and southern boundaries displays pronounced year-to-year variations. Plus, there is a significant inverse correlation between them, with the correlation coefficient reaching -0.68.

The boundary moisture budget is closely related to the water vapor convergence (divergence) over the WNP and corresponds well to the precipitation anomalies over the ECMR (Fig. 2). An abnormal increase in water vapor transport via the southern boundary corresponds to an anticyclonic anomaly over the tropical and subtropical WNP-EA region (Fig. 2a), with more rainfall appearing in the Yangtze-Huaihe river basin (northwest of the ECMR) (Fig. 2c). When more water vapor flows into the ECMR via the eastern boundary, strong convergence and cyclonic moisture circulation anomalies dominate the subtropical WNP-EA region, with two anticyclonic anomaly zones lying to its south and north. Under these conditions, there is more rainfall in the southeast part of the ECMR and less rainfall in the Yangtze River basin (Figs. 2b and d). The moisture budgets of the eastern and southern boundaries are affected by the moisture circulation over the WNP-EA region. During the TC season (JAS), the peripheral cyclonic circulation of TCs could impact the atmospheric background circulation over EA. A significant increase in TC activity in EA may lead to changes in the moisture budgets and precipitation in the ECMR.

Figure 2.  Regressed vertical integrals of JAS water vapor flux anomalies (vectors; units: kg m^-1 s^-1) and their divergence (shading; units: 10^-6 kg m^-2 s^-1) based on the moisture budget of the (a) southern and (b) eastern boundaries. Anomalies of precipitation (shading; units: mm d^-1) regressed upon the moisture budget of the (c) southern and (d) eastern boundaries.

Figure 3.  The average TC frequencies (a) and the number of TC days (c) in each month, and the total TC (b) and the number of days (d) during JAS each year from 1979 to 2010. Data shown in red are for EA and those shown in blue are the total for the WNP.

About 30 TCs form in the WNP each year. There is an obvious seasonal difference in the number of TCs in the WNP; about 80% occur in July-November (Fig. 3a). No TCs occur in EA during January-March, while almost 70% of TCs in EA occur in JAS, reaching a maximum frequency in August. On average, the number of TC days in EA exceeds six in July, August and September (Fig. 3c). For the rest of the year, the total number of TC days in EA is only eight. We note that, in EA, a higher frequency of TCs occurs in July than in September, but more TC days occur in September than in July. In other words, the lifetimes of TCs in September are longer than those of TCs in July, in most cases.

During 1979-2010 about 6.5 TCs per year affect the ECMR. The average number of TC days per year is 20, accounting for almost a quarter of the total days in JAS. The time series of TC frequencies and TC days in JAS are shown in Figs. 3b and d. There is a downward trend of the yearly frequencies of TCs in the WNP, but no obvious trend for TC activity in EA (Fig. 3b). TCs in EA are near land and have a direct and severe influence on moisture circulation over the ECMR. The amplitude of the interannual variability in the number of TC days in EA displays an increase after the 1990s, probably in relation to the Pacific Decadal Oscillation and ENSO (He and Jiang, 2011).

The relationship between TC activity and the net moisture fluxes across all boundaries is weak. However, TC activity (frequency and number of TC days) in EA has a significant positive correlation with the boundary moisture flux via the eastern boundary, and a negative correlation with the moisture flux via the southern boundary (Table 1). Thus, the TC activity is an important factor that leads to the reversal in moisture fluxes between the eastern and southern boundaries, as mentioned in section 3. In JAS, many TCs occur in EA and move around, or across the eastern and southern boundaries of the ECMR. Because of accompanying strong cyclonic circulation, TC activity could affect atmospheric moisture conditions over the ECMR by modulating the atmospheric fluxes at the eastern and southern boundaries.

Figure 4.  Composites of the water vapor flux (vectors; units: kg m-1 s-1) and its divergence (shading; units: 10-4 kg m-2 s-1) during (a) TC days and (b) non-TC days in JAS (1979-2010).

Composite analysis is used to investigate the relationship between water vapor transport and TC activity. The composite water vapor flux during the days with TCs (TC days) shows a strong moisture convergence over the subtropical WNP-EA region (Fig. 4a). The moisture inflow via the eastern boundary is nearly twice the climatological mean, while the moisture inflow via the southern boundary is an order of magnitude smaller than the climatological mean. In contrast, the composite water vapor flux during non-TC days shows a weak moisture convergence over the ECMR. The moisture inflow via the southern boundary on non-TC days is much higher than average, but the inflow via the eastern boundary is very small (Fig. 4b).

In general, TC activity can change the direction of water vapor transport over the ECMR and strengthen atmospheric moisture convergence in southeastern China. Combined with Figs. 2c and d, the moisture convergence zone corresponds to the areas of rainfall. If there are no TCs in summer, the moisture supply into the ECMR occurs mainly along the southern boundary, and rainfall appears in the Yangtze-Huaihe river basin. In years with more TCs, the peripheral cyclonic circulation of TCs may block water vapor transport at the southern boundary and contribute to moisture inflow via the eastern boundary by changing the direction of the winds. Under these conditions, a strong moisture convergence zone extends from the WNP to the southeast of the ECMR, with a significant rainfall anomaly there.

The contribution of TC days and non-TC days to the moisture budget at the eastern and southern boundaries is shown in Fig. 5. At the southern boundary, almost all water vapor inflow occurs on non-TC days and the contribution of TC days can be neglected except for a few years (Figs. 5a and b). For the eastern boundary, nearly 80% of water vapor inflow occurs on TC days and the contribution of TC days is larger than on non-TC days in most years (Figs. 5a and c). If we consider the eastern and southern boundaries together, less moisture inflow occurs on TC days than that on non-TC days, apart from during some active TC years (not shown). On average, the proportion of moisture inflow on TC days to non-TC days is about 4:6 (Fig. 5a).

Figure 5.  (a) Percentage contribution of water vapor inflow at each boundary [southern (S), eastern (E), and southern plus eastern (S+E)] for TC and non-TC days during JAS (1979-2010). (b, c) Time series of the (b) southern and (c) eastern boundary moisture budget (units: 10⁷ kg s^-1) on all days (black line), TC days (red line), and non-TC days (blue line), during JAS.

Figure 6.  Analyses during the eight El Niño years of the (a) percentage contribution of water vapor inflow at each boundary [southern (S), eastern (E), and southern plus eastern (S+E)] for TC and non-TC days, (b) water vapor fluxes (vectors; units: kg m^-1 s^-1) and their divergence anomalies (shading; units: 10^-4 kg m^-2 s^-1), and (c, d) water vapor fluxes (vectors; units: kg m^-1 s^-1) and their divergence (shading; units: 10^-4 kg m^-2 s^-1) for (c) TC days and (d) non-TC days.

Moisture circulation anomalies over EA and TC activity are modulated by the status of the large-scale background. As discussed in the introduction, ENSO is one of the most important factors that affects TC frequency, track, and intensity. To clearly identify the effects of TC activity on water vapor transport associated with ENSO, composite analyses based on the oceanic Niño Index are applied for the two types of ENSO events.

During the eight El Niño years, the total number of TC days and non-TC days is 290 and 446, respectively. There are westerly zonal transport anomalies near 10^°N, and an anomalous cyclonic moisture circulation dominates the subtropical WNP-EA region (Fig. 6b). The moisture convergence zone is located to the south of the ECMR, and the total moisture inflow to the ECMR is less than the climatological mean during 1979-2010 (Table 2). Figure 6d reflects this information, indicating that moisture transport from lower latitudes to the ECMR is weak, due to the impact of El Niño, during non-TC days. However, TC activity during El Niño years can change the moisture circulation over the ECMR and strengthen the moisture inflow via the eastern and southern boundaries. A strong moisture convergence appears in the southeastern part of the ECMR on TC days (Fig. 6c). Compared with the climatological mean, moisture transport on TC days is more important in El Niño years. The proportions of moisture inflow on TC days to non-TC days are almost equal (Fig. 6a).

During La Niña years, there are, on average, 188 TC days and 364 non-TC days. The spatial features of the water vapor fluxes are opposite to those during El Niño years. There are easterly zonal transport anomalies to the south of the ECMR and an anomalous anticyclonic moisture circulation over the subtropical WNP-EA (Fig. 7b). In contrast to El Niño years, the contribution of moisture inflow via the southern boundary is lower than the contribution of moisture inflow via the eastern boundary higher for TC days during La Niña years, due to the different patterns of moisture circulation (Figs. 6a and 7a). There is abundant moisture flowing into the ECMR at the southern boundary, and the ECMR is a wet area in La Niña years (Table 2). The locations of the moisture convergence in the ECMR are different on TC and non-TC days (Figs. 7c and d). Because of the high moisture inflow on non-TC days, the proportion of imported moisture on TC days is lower, and the ratio of TC days to non-TC days is 3:7 (Fig. 7a).

The above composite analyses may overlook some details in moisture transport. In the next section, a more detailed analysis is conducted to consider the impacts of different TC tracks on the boundary moisture budget, and the moisture inflow via the eastern and southern boundaries at each layer.

Figure 7.  As in Fig. 6, but for La Niña years.

Figure 8.  TC tracks in EA divided into four types: (a) westward-moving, Type 1; (b) northwestward-moving and entering the ECMR from the southern boundary, Type 2; (c) northwestward-moving and entering the ECMR from the eastern boundary, Type 3; (d) recurving, Type 4.

To investigate the detailed effects of TCs on the boundary moisture budget, the amount of moisture transported across the eastern and southern boundaries during different TC tracks is calculated. TCs in EA are categorized into four types based on their tracks (Fig. 8): (1) westward-moving and not moving into the ECMR; (2) northwestward-moving and entering the ECMR at the southern boundary; (3) northwestward-moving and entering the ECMR at the eastern boundary; and (4) recurving and not moving into the ECMR. In the analysis, three different time points for each TC are considered: Time0, before the TC enters EA; Time1, when the TC is in EA, but has not entered the ECMR; and Time2, when the TC is in the ECMR.

Type 1 TCs, the westward-moving ones, occur to the south of the ECMR (Fig. 8a). Compared with the results before the Type 1 TCs enter EA, TCs hinder moisture inflow via the southern boundary (Fig. 9b) and increase moisture inflow via the eastern boundary significantly (Fig. 9c), because of their peripheral cyclonic circulation after they move into EA. Differences in the water vapor fluxes occur mainly below 500 hPa, and the lower the level, the greater the difference. Overall, the total imported moisture via the southern and eastern boundaries likely increases slightly under the influence of Type 1 TCs (Fig. 9a).

Most of the Type 2 TCs move in a straight line and then enter the ECMR at the southern boundary (Fig. 8b). The TCs have little effect on moisture transport over the ECMR before they enter EA (Figs. 10a and b). After the TCs move into EA, the moisture inflow via the southern boundary decreases and the moisture inflow via the eastern boundary increases, similar to Type 1 TCs (Figs. 10c and d). The water vapor flux anomalies for Type 2 TCs are larger than those for Type 1 TCs (Fig. 11), because Type 2 TCs have more time and distance to intensify over the open ocean. However, when the ECMR is controlled by cyclonic circulation, the inhibition of water vapor inflow at the southern boundary and increasing moisture inflow at the eastern boundary are weakened (Figs. 10e and f). Generally, when Type 2 TCs move into EA [but not the ECMR; Time1], the total moisture inflow via the eastern and southern boundaries increases at higher levels, but decreases at lower levels. In fact, strong convergence and updraft occur at the lower levels and moisture rises to the higher levels, resulting in the different water vapor transport at the lower and higher levels. The opposite change appears between the lower and higher levels for all four TC types when they move into EA (but not the ECMR), especially for Type 2 TCs. After the TCs enter the ECMR, a large amount of water vapor is brought into the ECMR via the southern and eastern boundaries, which is significant below 800 hPa because the water vapor is concentrated mainly in the lower troposphere. When the TCs make landfall, their effect on the water vapor transport gradually weakens (Figs. 11b and c).

Type 3 TCs enter the ECMR at the eastern boundary and follow both straight-moving and recurving tracks (Fig. 8c). Their impact on the moisture inflow is similar to Type 2 TCs. When the Type 3 TCs move into EA [but not the ECMR; Time1], the anomalous moisture inflow via the eastern boundary is nearly offset by the anomalous outflow via the southern boundary. When the TCs enter the ECMR [Time2], the total moisture inflow via the two boundaries increases significantly. The increase in total imported moisture at Time2 for the Type 3 TCs occurs because of anomalous water vapor inflows at the eastern boundary (Fig. 12c). At the southern boundary, water vapor outflows still occur during Time2 (Fig. 12b), in contrast to the conditions for Type 2 TCs (Fig. 11b). Because of the differences in TC tracks and locations when they influence the moisture levels, there are some other differences between Type 2 and 3 TCs. For instance, Type 3 TCs appear to cause stronger anomalous moisture inflow via the eastern boundary, and outflow via the southern boundary, during Time1 (Figs. 12b and c).

Type 4 TCs start by moving northwestwards, and then turn to move northwards on the east side of the ECMR (Fig. 8d). The impacts of these TCs on atmospheric circulation are similar to the other three TC types when they are at relatively low latitudes: small opposite anomalies at the two boundaries, with a slight decrease in the total moisture inflow (Fig. 13). When the Type 4 TCs move to relatively high latitudes, but still in the area of influence, their impacts are complex and vary widely among different events.

Figure 9.  Vertical distributions of water vapor flux anomalies (units: 10⁵ kg s^-1) via (a) both the southern and eastern boundaries, (b) the southern boundary only, and (c) the eastern boundary only, of the ECMR, before [Time0, black line] and after [Time1, red line] Type 1 TCs enter EA.

Figure 10.  Composites of (a, c, e) 850-hPa water vapor flux anomalies (vectors; units: 10^-2 kg m^-1 s^-1) and (b, d, f) 500-hPa water vapor flux anomalies (vectors; units: 10^-2 kg m^-1 s^-1), during Type 2 TCs, at (a, b) Time0, before the TCs enter EA; (c, d) Time1, after the TCs enter EA; and (e, f) Time2, after the TCs enter the ECMR.

Figure 11.  As in Fig. 9, but for Type 2 TCs. Green lines denote flux anomalies at Time2, representing when the TCs enter the ECMR.

Figure 12.  As in Fig. 11, but for Type 3 TCs.

Figure 13.  As in Fig. 9, but for Type 4 TCs.

In this study, the characteristics of water vapor transport on TC days and non-TC days in EA, and the effects of TC activity on the boundary moisture budgets of the ECMR, are investigated. For the ECMR, major moisture inflows exist along its eastern and southern boundaries during JAS. According to the relationship between the water vapor transport field and the boundary moisture budget, it is clear that the moisture circulation over the WNP-EA region affects the boundary moisture budget.

JAS is the period with most frequent TC activities in the ECMR. TCs in EA have a direct and large influence on moisture circulation over the ECMR. Their peripheral cyclonic circulation could affect the air flow over the WNP-EA region and change the moisture transport routes. TC activity in EA can modulate atmospheric fluxes via the eastern and southern boundaries of the ECMR. The composite analyses show very different circulation patterns in moisture transport throughout EA on TC and non-TC days. When there is no TC affecting the ECMR, water vapor flows into the ECMR at the southern boundary and more rainfall appears in the northeast of the ECMR. But in years with more TCs, moisture inflow via the southern boundary may be weakened and the inflows at the eastern boundary increase significantly. This results in strong moisture convergence and rainfall in the southeast of the ECMR.

The contributions of TC activity to boundary moisture budgets over the ECMR are calculated. From the perspective of the climatological mean, almost 80% of the water vapor transport via the eastern boundary occurs on TC days; but for the southern boundary, most of the inflows of water vapor occur on non-TC days. If these two main inflow boundaries are regarded as a whole, the total imported moisture increases slightly on TC days. Because the number of non-TC days is much greater than the number of TC days, the ratio of contribution of total water vapor inflow to the ECMR between TC and non-TC days is 4:6.

The moisture transport and boundary moisture budget of the ECMR are significantly modulated by ENSO. In El Niño years, the moisture convergence zone is located to the south of the ECMR, and moisture transport from the lower latitudes to the ECMR is weak on non-TC days. However, strong moisture convergence appears in the southeastern part of the ECMR and the moisture inflow via the eastern and southern boundaries increases on TC days. This means that the effects of TC activity on moisture transport are more important in El Niño years for the ECMR. In La Niña years, there is a large supply of moisture for the ECMR on non-TC days due to the anomalous anticyclonic moisture circulation. The ratio of contribution of moisture inflow is lower on TC days.

Further analysis is conducted to understand the detailed effects of different types of TCs (classified based on their tracks) in different time points on the boundary moisture budget. For westward-moving TCs that cannot move into the ECMR, the moisture inflow via the southern boundary is hindered, and that at the eastern boundary increases. Type 2 and 3 TCs enter the ECMR and have greater intensities than Type 1 TCs. The changes caused by Type 2 and 3 TCs when they enter EA are similar to those of Type 1, but the magnitudes are much larger. The anomalies of the total imported moisture (i.e., the eastern plus the southern boundary) are different at low and high levels; at higher levels, inflows increase, and at lower levels inflows decrease. This phenomenon is associated with the dynamic conditions for the development of TCs, including convergence at lower levels, divergence at higher levels, and ascending motion. When Type 2 and 3 TCs move into the ECMR, their influence gradually weakens, and the net atmospheric fluxes via the two boundaries increase, especially at lower levels. For TCs with a northward-turning track that remain outside of the ECMR, their impacts are similar to the other types at relatively low latitudes, but are complex at high latitudes. Further research is needed to explain this. We consider different effective TC radii and the definition of the ECMR in order to test the sensitivity of the results, but the results are independent of the definitions chosen for analysis.

Progress in the understanding of water vapor transport can greatly advance our knowledge of regional rainfall (Simmonds et al., 1999; Zhou and Yu, 2005). (Chen et al., 2012b) suggested that TCs in the South China Sea contribute a large amount to the summer rainfall in southern China. Hence, the relationships among TC activity, moisture transport, and rainfall in the ECMR will be the focus of future work. ENSO is the most important predictor of TC activity in the WNP (Chan et al., 2001; Liu and Chan, 2003), and can effectively influence the summer moisture circulation and precipitation anomalies over EA in a quasi-four-year period (Li and Zhou, 2012b).

In recent years, many studies have found that changes in WNP TCs closely follow the evolution of the ENSO cycle, and the two types of El Niño events have different impacts on TCs over the WNP (Chen and Tam, 2010; Kim et al., 2011; Wang et al., 2014). For central Pacific and eastern Pacific El Niño events, the atmospheric response in EA is also different (Weng et al., 2007; Chen et al., 2014). Thus, the impact of TCs on the moisture transport over EA associated with ENSO still requires further investigation.