Climatic Shift of the Tropical Cyclone Activity Affecting Vietnam’s Coastal Region

Duc Tran-Quang Department of Meteorology and Climate Change, Hanoi University of Science, Vietnam National University, Hanoi, Vietnam

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Ha Pham-Thanh Department of Meteorology and Climate Change, Hanoi University of Science, Vietnam National University, Hanoi, Vietnam

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The-Anh Vu Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, Indiana

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Chanh Kieu Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, Indiana

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Tan Phan-Van Department of Meteorology and Climate Change, Hanoi University of Science, Vietnam National University, Hanoi, Vietnam

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Abstract

This study examines the climatic shift of the tropical cyclone (TC) frequency affecting Vietnam’s coastal region during 1975–2014. By separating TC databases into two different 20-yr epochs, it is found that there is a consistent increase in both the number of strong TCs and the number of TC occurrences during the recent epoch (1995–2014) as compared with the reference epoch (1975–94) across different TC databases. This finding suggests that not only the number of strong TCs but also the lifetime of strong TCs affecting Vietnam’s coastal region has been recently increasing as compared with the reference epoch from 1975 to 1994. To understand the physical connection of these shifts in the TC frequency and duration, large-scale conditions obtained from reanalysis data are analyzed. Results show that meridional surface temperature gradient (STG) during the recent epoch is substantially larger than that during 1975–94. Such an increase in the meridional STG is important because it is potentially linked to the increase in large-scale vertical wind shear as well as the reduced intensity of summer monsoon in the South China Sea between the two epochs.

Corresponding authors: Duc Tran-Quang, tranquangduc@hus.edu.vn; Tan Phan-Van, phanvantan@hus.edu.vn

Abstract

This study examines the climatic shift of the tropical cyclone (TC) frequency affecting Vietnam’s coastal region during 1975–2014. By separating TC databases into two different 20-yr epochs, it is found that there is a consistent increase in both the number of strong TCs and the number of TC occurrences during the recent epoch (1995–2014) as compared with the reference epoch (1975–94) across different TC databases. This finding suggests that not only the number of strong TCs but also the lifetime of strong TCs affecting Vietnam’s coastal region has been recently increasing as compared with the reference epoch from 1975 to 1994. To understand the physical connection of these shifts in the TC frequency and duration, large-scale conditions obtained from reanalysis data are analyzed. Results show that meridional surface temperature gradient (STG) during the recent epoch is substantially larger than that during 1975–94. Such an increase in the meridional STG is important because it is potentially linked to the increase in large-scale vertical wind shear as well as the reduced intensity of summer monsoon in the South China Sea between the two epochs.

Corresponding authors: Duc Tran-Quang, tranquangduc@hus.edu.vn; Tan Phan-Van, phanvantan@hus.edu.vn

1. Introduction

Tropical cyclones (TCs), also known as typhoons in the northwestern Pacific (WPAC) basin, are major extreme weather events in the atmosphere. Climatologically, the WPAC basin is the most active area with an average of ~28–30 TCs each year, about one-quarter of which are supertyphoons that have large impacts on economy and loss of properties and life in countries with long coastlines (e.g., Feser and von Storch 2008; Tan et al. 2016). In the context of global climate change, various studies have projected that the number of intense TCs, defined as category 4 and above according to the Saffir–Simpson scale, tends to increase in the future warmer climate (Bengtsson et al. 2007; Knutson et al. 1998, 2007, 2010, 2013; Oouchi et al. 2006; Murakami et al. 2011). While this scenario of TC activity is expected for the long-term future, a recent record of TC activity from 2013 to 2017 in the Vietnam East Sea (VES), also called the South China Sea, which is one of the main regions affected by TCs, could provide some preliminary signals to validate the projection by the IPCC (2013). Given the fact that Vietnam is one of the countries that is most vulnerable to climate change [Hijioka et al. 2014, Working Group II contribution to the IPCC’s Fifth Assessment Report (WGII AR5), section 24.4.4; Wang et al. 2017], such an assessment of the TC projection for Vietnam’s coastal region is therefore critical.

Among numerous factors that can affect TC frequency such as sea surface temperature (SST), vertical wind shear, midlevel moisture, tropical tropopause layer temperature, or tropospheric stratification (see, e.g., Gray 1968; Nolan et al. 2007; Goldenberg et al. 2001; Wang et al. 2010; Murakami et al. 2011; Emanuel et al. 2013; Wang et al. 2014; Lin and Chan 2015; Ferrara et al. 2017; Moon and Kieu 2017; Kieu and Zhang 2018; Ryglicki et al. 2018), SST is often considered the first-order factor that can influence the formation, intensity, and the frequency of storms. Many observational and modeling studies have shown that future warmer SSTs in the WPAC basin could lead to not only more intense TCs but also a shift in the track pattern that could directly affect the TC landfalls in this basin (Chan and Xu 2009; Murakami et al. 2011; Wang et al. 2010).

As a result of its unique geographic location, the VES is located over a tropical region with warm SST that accounts for roughly one-third of the total number of TCs (10–12 in average) in the WPAC basin. Geographically, the VES is on the northwestern edge of the WPAC basin, where SST is often higher than the other parts of the WPAC (e.g., Y. Yang et al. 2015; Tan et al. 2016). As a result, it is expected that the SST variations will strongly affect TC development in this region. On average, there are about 5–7 TCs in the VES region that can either directly or indirectly impact Vietnam’s coastline every year (Phan et al. 2015), ranging from heavy rainfall, strong wind, inundation, to extreme storm surges that are of great threats to the nation.

Unlike the North Atlantic basin where the impacts of SST are most apparent (e.g., Goldenberg et al. 2001; Wang et al. 2010; Murakami et al. 2011; Emanuel et al. 2013; Lin and Chan 2015; Ferrara et al. 2017), the role of SST in the WPAC basin is, however, strongly interfered by other factors such as vertical wind shear, or other large-scale circulations (Wang and Chan 2002; Murakami et al. 2011; Wang et al. 2010; Lin and Chan 2015). Specifically, TC activity in the VES is complicated by summer monsoon system, which is characterized by the dominance of southwesterly wind in the southern part of the VES (Lander 1994; Carr and Elsberry 1995; Wang and Chan 2002; Wu et al. 2013). Thus, the lower troposphere experiences a frequent competition between the westerly/southwesterly wind from the Bay of Bengal during the TC main season (i.e., from July to October) and the northeasterly wind from the WPAC subtropical high and cold air from higher latitudes (e.g., Zeng and Li 2002; Li and Zeng 2002, 2003; Wang et al. 2004; Feng et al. 2010). During the active phase of summer monsoon in the VES, the southwest wind could extend from surface up to 500 hPa (e.g., Li et al. 2010, 2012). At the upper levels, the large-scale flow is, however, mainly easterly or northeasterly, which is opposite to the lower tropospheric winds and result in strong vertical wind shear (>10 m s−1). Because of these large-scale factors, the VES is under the strong influences of vertical wind shear and monsoonal circulations, leading to higher variability of TC activities in the WPAC basin beyond the direct SST influence (Wang et al. 2010; Lin and Chan 2015). These complicated interactions of TCs with other large-scale systems result in a real challenge of projecting TC activity affecting Vietnam’s coastal region.

Given the unique properties of environmental conditions in the VES, the main focuses of this study are 1) to determine the epochal shift of the TC frequency in this area during a 40-yr period from 1975 to 2014 and 2) to examine the main large-scale factors that are responsible for such a climatic shift in the TC frequency.

The rest of this work is organized as follows. In the next section, detailed descriptions of the data and methodology are provided. Section 3 shows the climatology of TCs in the VES from several different angles to demonstrate the climatic shift in the TC frequency between two epochal periods 1975–94 and 1995–2014. In section 4, analyses of the statistical relationship between TC activities and other large-scale factors are presented. Discussions and conclusions are given in the final section.

2. Data and methods

a. Data

In this study, TC data from 1975 to 2014 are retrieved from three different databases maintained by the U.S. Joint Typhoon Warning Center (JTWC), the Japan Regional Specialized Meteorological Center (RSMC), and the Unisys Weather.1 These data sources contain all TC information such as the TC classification, the maximum 10-m sustained wind speed, the minimum central pressure as well as the latitude and longitude of the TC centers. Because of the difference in the definition of the wind average between the RSMC and Unisys Weather/JTWC, a conversion factor of 1.08 is applied to the RSMC dataset to convert its 10-min sustained wind average to the 1-min average in the Unisys Weather/JTWC dataset (see, e.g., Knapp and Kruk 2010; Harper et al. 2010).

Although some TC databases such as JTWC contain a much longer record back to 1945, the high uncertainties in TC information, especially TC intensity, prior to the satellite era make it difficult to derive any meaningful climatic changes. In addition, different operational centers have different techniques of categorizing and calculating TC intensity. Thus, TC databases in the WPAC basin are often different in terms of TC intensity and storm center locations. For example, JTWC tends to overestimate the storm intensity, while the Japan Meteorological Agency and the Shanghai Typhoon Institute appear to underestimate intensity in this basin (see, e.g., Song et al. 2010; Kamahori et al. 2006). Given such uncertainties in the TC records in the WPAC basin, it is important to not only examine the change in TC activity but also assess the consistency and/or difference among the different datasets that one has to consider in any study of climate change. Limiting all TC databases after 1975 is therefore essential to reduce the large TC bias among different databases prior to the satellite era.

With regard to surface temperature and the atmospheric data required for our analyses of surface temperature gradient and monsoon indices, the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis with a horizontal resolution of 2.5° × 2.5° is employed (Kalnay et al. 1996). We note that both the NCEP atmospheric and surface temperature data are chosen herein instead of the more commonly used Hadley Center SST data. This is because it is important to maintain the consistency between the surface temperature and the atmospheric structure required for calculating vertical wind shear and monsoon indices. Such an internal consistency between the surface data and the atmospheric structure is critical because a different SST data source could result in conflicted atmospheric response, which affects the interpretation of physical mechanisms. Although the NCEP–NCAR reanalysis data have some issues with the resolution that cannot capture properly TC intensity as discussed in Murakami (2014) and Hodges et al. (2017), the use of the NCEP–NCAR reanalysis in this study is only confined to the large-scale environment where observed TCs locate. As such, the underestimation of TC intensity detected from the reanalysis data is expected to have minimum effects on our analysis herein.

b. Method

To examine the climatic shift in the TC frequency, the three TC datasets are first divided into two 20-yr epochs: one is from 1975 to 1994 (hereinafter referred to as a reference epoch), and the other from 1995 to 2014 (hereinafter a recent epoch). This 20-yr period is sufficiently long to capture the decadal climate signal, which is consistent with the 20-yr period previously used in the IPCC AR4 (2007) (i.e., 1980–99) and IPCC AR5 (2013) (i.e., 1986–2005). One could in principle choose a longer TC record to ensure the statistical significance of climatic shifts. However, it should be noted that, except for the JTWC and the Unisys database, the data record in the RSMC dataset is available only after 1975. As such, the 1975–2014 period with two 20-yr periods is chosen here to maximize the common data among the three datasets. We note that because the RSMC data with complete information about TC intensity are only available after 1976, the missing data points in the RSMC dataset for the years 1975 and 1976 must be cautioned when evaluating the total changes in TC counts between two epochs with the RSMC data.

Given the above TC datasets, all TCs of at least category 8 and above (i.e., TCs with recorded names or specific numbers) that can directly or indirectly influence Vietnam’s coastline were then selected. For this purpose, any TC that tracks inside a domain of [105°–120°E] × [5°–25°N] in the VES (Fig. 1) are chosen for our analyses, regardless of genesis location inside the VES or elsewhere. Given this definition of TCs affecting Vietnam’s coastal region, each TC database is searched through such that all TCs as well as their occurrences (at a 6-h interval) inside the domain shown in Fig. 1 will be counted. Three different metrics are introduced next to quantify the change of TC activity in the VES, which include 1) the number of TCs, 2) the number of TC occurrences, and 3) TC lifetime inside the VES. Here, the TC lifetime in the VES is defined as a duration of a TC when it first enters and last exits the domain shown in Fig. 1, and so it is not necessary to be its whole lifetime in the western Pacific (WP) basin. Unlike the TC number, TC occurrences and lifetime also encode information about the duration of TCs within the domain of interest, thus providing additional insight into TC activity beyond simple TC counts.

Fig. 1.
Fig. 1.

The domain in the VES where the TCs affecting Vietnam’s coastal region are counted in this study (solid-outlined box), and the domain for calculating the monsoon index (dash-outlined box). The hatched stripes denote the latitudinal bands for which the surface temperature gradients are computed (see the text and Table 1 for more discussion). Light-shaded curves denote the observed tracks of all TCs, whose initial locations are marked by filled black circles for the 1975–94 period and filled black triangles for the 1995–2014 period. The crosses denote the ends of TCs recorded in the JTWC best-track dataset.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

To shed more light into the climatology of TCs with different strength, further stratification of TCs into different intensity groups is also conducted in this study. Specifically, the TCs in the VES are divided into three different groups, based on their maximum 10-m sustained wind (VMAX) reported in the TC dataset. Using the Beaufort scale (m s−1), all TCs are categorized into three TC groups, which include (i) weak TCs whose intensity is in the range of category 8–11 (17–32 m s−1), (ii) moderate TCs whose VMAX is in the range of category 12–13 (33–41 m s−1), and (iii) strong TCs whose intensity is of category 14 and above (>41 m s−1). This TC group classification on the basis of their strength suffices to distinguish TC characteristics while still ensuring an adequate statistical sample size during the 1975–2014 period.

Figure 2 shows the distributions of the three TC groups for three different datasets. Overall, all datasets display a consistent TC distribution, with a majority of the TC data in the weak and moderate TC groups for both epochs. The consistently larger percentage of weak TCs in all of these datasets apparently indicates the dominance of weak storms in the VES during the entire 1974–2014 period. Note, however, that, except for the RSMC dataset that has a smaller number of strong TC counts, both JTWC and Unisys datasets show a relatively larger number of TCs in the strong TC group than that in the moderate group during the recent epoch.

Fig. 2.
Fig. 2.

Distribution of the three TC groups including the weak (light shaded), the moderate (medium shaded), and the strong (dark shaded) TCs from all three TC datasets used in this study for (a) the reference period from 1975 to 1994 and (b) the recent period from 1995 to 2014.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

To relate the epochal shifts of TC climatology to the north–south surface temperature gradient, a domain between 105° and 120°E are partitioned into a pair of two latitudinal bands: one in the northern part of the VES and the other band is in the southern part of the VES region (see Fig. 1 and Table 1). The average surface temperature (TSK; including both the SST over the ocean and the skin surface temperature over land) in each band is calculated and then subtracted from each other to obtain the north–south surface temperature gradient. As a way to evaluate the sensitivity of this surface temperature gradient calculation to different latitudinal bands, two pairs of latitudinal bands are also used. One pair is defined for two latitudinal bands between 25° and 30° and between 5° and 10°N, the other pair is defined for two bands between 20° and 30° and between 5° and 15°N. These latitudinal bands have different distances among them such that the effects of the north–south surface temperature gradient magnitudes can be evaluated.

Table 1.

The average surface temperature (TSK; °C) and SST (°C), and the corresponding gradients for each pair of the latitudinal bands, shown in Fig. 1, along with their difference between the two epochs 1975–94 and 1995–2014 for the ST group and the WM group.

Table 1.

With regard to the monsoon index, the wind fields in the NCEP–NCAR reanalysis are used to calculate the averaged zonal wind on a given isobaric surface that can best characterize the summer monsoon activities in the VES. Specifically, the South China Sea summer monsoon (SCSSM) index is defined as an average of the zonal wind component in the region (5°–15°N, 110°–120°E) as shown in Fig. 1 at the 850-hPa level (Wang et al. 2004). To better capture the southwest monsoon in the summer and partly reflect the effects of the off-season wind direction, the monsoon index calculation is also carried out for two different seasons including a period of seven months from May to November, and another 4-month period from June to September as proposed in Li and Zeng (2002), Wang et al. (2004), and Li et al. (2010, 2012).

3. TC variation in Vietnam’s coastal region

To begin the analyses, a broad perspective of the change in TC frequency, Fig. 3 compares the annually averaged number of TCs between the reference and the recent epochs as obtained from the three TC databases for the domain shown in Fig. 1. As seen in Fig. 3, the total number of TCs for all categories during the recent epoch decreases as compared to the reference epoch across all TC databases. The JTWC dataset shows an average number of ~9.8 TCs during the reference epoch, and it is reduced to ~9.1 during the 1995–2014 epoch. These numbers for the two epochal periods are, respectively, 9.8 and 8.6 for the Unisys data and 10.4 and 8.3 for the RSMC data, which confirm the consistency in the reduction of the overall TC number in the VES. Among the three datasets, we note that the decrease in the overall TC number is statistically significant (at 95% confidence interval based on the t test) only for the JTWC dataset. Similar to the overall TC counts, the average numbers of both the weak and moderate TC groups during the recent epoch are also reduced as compared to the reference epoch, albeit the statistical significance is realized again for the JTWC data only. Despite such a lack of the statistical significance in the other two databases, the consistent reduction in the number of TCs in the weak and moderate TC groups as well as the total TC numbers could at least suggest no increasing trend in the average number of TCs in the VES, similar to the projection by IPCC (WGI AR5, sections 2.6.3–2.6.4; Wang et al. 2017).

Fig. 3.
Fig. 3.

The annual average number of TCs for three different TC groups inside the domain shown in Fig. 1 during the 1975–94 period (dark-shaded columns) and 1995–2014 period (light-shaded columns) as obtained from the (a) Unisys, (b) JTWC, and (c) RSMC datasets.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

It is of particular interest to note, however, that, unlike the statistics of the moderate and weak TC groups, the average number of TCs in the strong TC group during the recent epoch increases relative to the reference epoch (Fig. 3). This increase of strong TC numbers is also true for the percentage of the strong TCs in each epochal period as shown in Figs. 2 and 3. Specifically, the average number of TCs in the strong TC group increases from 1.9 (20%) for the reference epoch to 2.2 (~27%) for the 1995–2014 period with the JTWC database. Likewise, the same increase in the percentage of strong TCs is also observed in the Unisys database, which are 1.9 (20%) and 2.3 (29%), respectively (Fig. 3). These changes in the average number of strong TCs are very robust, regardless of how the dataset from 1975 to 2014 is partitioned. In fact, our sensitivity analyses in which the 1975–2014 period is split at different years capture similar increase in the number of strong TCs between two epochs (not shown).

It can be also seen in Fig. 3 that the increase of the TC count in the strong TC group is not only realized in terms of the relative percentage or average numbers but also in terms of the absolute number of strong TCs as well. Indeed, the total number of TCs in the strong TC group during the reference epoch is 39, and it increases to 49 during 1995–2014 as obtained from both the Unisys and JTWC datasets. A similar result is obtained from the RSMC data, even though the RSMC data are only available after 1976 (cf. Fig. 3). While the increase of strong TCs in the recent epoch as compared to the reference epoch is only statistically significant for the JTWC data, the consistency of this increase among all three datasets could help support the results from the previous modeling studies and IPCC/AR5 that the total number of strong TCs tends to increase in the VES, a result that also accords with the TC projection in the entire WPAC basin (see, e.g., Knutson et al. 2010; Wang et al. 2017).

Along with the shift in the average number of TC counts between the two epochs, another metric that is also of equal importance in determining the long-term variation of TC activity is TC occurrence and TC lifetime, which contain not only the information about the total TC number but also their duration. This metric is often combined in the form of the accumulated energy (ACE) or power dissipative index (PDI) when examining TC intensity variability in previous studies (e.g., Emanuel 2005; Ferrara et al. 2017). Because main focus of this study is on the overall change in TC frequency, a simple count of TC occurrences and annual average of TC lifetime is used instead of the more complex intensity-related ACE or PDI metric.

In this regard, Fig. 4 compares the total number of TC occurrences between the two epochs, similar to the total TC counts shown in Figs. 2 and 3. For the Unisys dataset, the overall changes in storm occurrences possess very similar behaviors as compared to the changes in the average number of TCs between the reference epoch and the recent epoch (cf. Figs. 2 and 3). That is, the average number of TC occurrences for the weak and moderate TC groups as well as the overall category all show a decrease from 69.1, 21.1, and 98.8 during the reference epoch to roughly 50.4, 18.2, and 81.4 during the recent epoch, respectively. The average number of TC occurrences for the strong TC group exhibits, however, again an increase from ~8.7 during the recent epoch, in accordance with the change in the TC count shown in Figs. 2 and 3. For the JTWC and RSMC datasets, a similar change in the number of strong TC occurrences is also obtained, although none are statistically significant.

Fig. 4.
Fig. 4.

As in Fig. 3, but for the annual total number of TC occurrences that are counted within the domain in Fig. 1.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

Consistent with the change in TC occurrence between the two epochs, one notices from Fig. 5 again that the annual average lifetime of TCs affecting Vietnam’s coastal region tends to decrease for the weak and moderate TC groups. For the strong TC group, the average TC lifetime appears to increase as obtained from the JTWC and RSMC dataset, similar to the increase in the TC occurrence frequency or the average number of strong TCs shown in Figs. 3b and 4b. It is observed also that the change in the annual average TC lifetime is only statistical significance for the JTWC (Fig. 5b). Despite the difference in the exact number of TC occurrences and lifetime among different datasets due to different records of TC intensity and classification, the uptrend tendency of strong TCs and the decreasing tendency of weak and moderate TCs is very persistent. In this regard, our results reiterate the projection of an increase in the activities of strong TCs near Vietnam’s coastal region in the future.

Fig. 5.
Fig. 5.

As in Fig. 4, but for the annual average lifetime of all TCs inside the domain shown in Fig. 1.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

4. Large-scale environmental effects

Given the observed changes in the TC numbers and the TC occurrences between the reference and the recent epochs as presented section 3, how these changes are connected to the variability in environmental conditions in the VES will be next examined. There are three specific large-scale factors in the VES that will be focused on in this section, which include the surface temperature, vertical wind shear, and monsoonal activity. These factors are known to be among the dominant controls to TC activities not only in the VES but also in the entire WPAC basin (e.g., Gray 1968; Lander 1994; Carr and Elsberry 1995; Wang et al. 2010; Murakami et al. 2011). As demonstrated in the previous section, the contrast in climatic shifts is most apparent between the strong TC group and the weak and moderate TC groups consisting of TCs whose intensity is category 13 and below. Thus, we will combine the weak and moderate TCs into a single group (WM group) to allow for a more statistical comparison with the strong TC group (ST group) between the reference and recent epochs.

a. Surface temperature gradient effects

To examine the change in surface temperature between the two epochs, Fig. 6 shows the averaged surface temperature distribution for these two epochs, along with their difference. Note that these surface temperature distributions are not annual means, but averaged only for the days when there exists at least one TC (195 TCs during 1975–94 and 171 TCs during the 1995–2014 period as indicated in the domain shown in Fig. 1). These storm-following averages are carried out such that the large-scale surface temperature could represent the actual condition that a TC was recorded, rather than the annual or seasonal surface temperature average. In addition, this storm-following average can help separate the specific days with strong or weak TCs, thus allowing us to quantify the ambient environment of the exact day when TCs exist for evaluating epochal changes.

Fig. 6.
Fig. 6.

Distribution of the averaged surface temperature (°C) for (a),(d) the reference epoch from 1975 to 1994 and (b),(e) the recent epoch from 1995 to 2014, along with (c),(f) the difference between the recent period and the reference period, for (left) the WM group (category 13 and below) and (right) the ST group (category 14 and above).

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

As seen in Fig. 6, the overall surface temperature distributions during the reference and the recent epochs are fairly similar, with a warmer SST to the south of the VES. The three warmest pools of SST can be seen in the Gulf of Thailand, south of the Philippine Sea, and the VES, which are apparent in both the reference and the recent epochs (Fig. 6). A closer comparison shows, however, that the SST in the VES is slightly warmer in the recent epoch than that in the reference epoch (in the range of 0.2° to 0.4°C, Fig. 6). Moreover, the surface temperature change in the recent epoch is not homogenous but possesses two important differences between the ST and WM groups. First, the region of the largest SST change for the WM group is at the central region of the VES (Fig. 6c), but it is shifted to the south of the VES for the ST group (Fig. 6f). Second, there appears a negative SST difference in the northern part of the VES in the ST group, which is evident not only for SST but also for the skin surface temperature as well.

While the overall magnitude of the SST difference is similar between these two TC groups, the southward shift of the largest SST difference along with a larger negative surface temperature gradient in the north of the VES for the ST group as shown in Fig. 6 leads to an enhanced north–south surface temperature gradient (STG). As a result, the changes in the north–south STG between the reference and the recent epochs are very different between the WM and ST groups, with the ST group having the STG about 50% larger than that in the WM group.

For more quantitative estimation of the change in the north–south STG, Table 1 compares the averaged surface temperature for the two pairs of latitudinal bands at the north and south bounds of the VES as described in the previous section (cf. Fig. 1). It can be seen from Table 1 that the STG for the WM group is −9.6°C between the 25°–30° and 5°–10°N bands (pair P1), and −7.0°C between the 20°–30° and 5°–15°N bands (pair P2) for the reference epoch. These gradient values are, respectively, −1.5° and −7.7°C for pairs P1 and P2 for the recent 1995–2014 epoch, which result in a change of ~−0.9° and −0.7°C between the two epochs for the WM group.

In contrast, the change in the STG between the two epochs is much larger for the ST group, which are −1.6° and −1.3°C for pairs P1 and P2, respectively (Table 1). Similar calculations of the meridional gradient using SST instead of the surface temperature also captures the same epochal shift of SST gradient for the ST group. Such change in the north–south surface temperature gradient presents an important clue for the shift in TC activity in the VES, particularly the increase of strong TCs as well as the strong TC occurrences during the recent epoch as shown in Figs. 24. Specifically, this result suggests that the STG change is more important for a climatic shift of the strong TC frequency than the change in the absolute SST value.

From a physical perspective, the less important role of absolute SST in TC activity in the VES is not unusual, because the SST in the VES is always warm (>28°C as seen in Fig. 6). As such, an increase of 0.4°–0.8°C would have a smaller impact on the overall TC intensity relative to other internal variability. In fact, Ramsay (2013) showed that an increase of 1°C in the absolute value of SST may lead to an increase of 2%–3% in the TC potential intensity. Such a small increase of TC intensity due to warmer SST is well within the marginal variability of TC intensity (Kieu and Moon 2016; Kieu et al. 2016), which is hard to be statistically detected from a limited sample. Beyond such a direct response of TC potential intensity to the absolute SST, it should be noted that the actual intensity that a TC can attain is generally far from the potential intensity limit that TCs can achieve theoretically. As such, a 0.5°C increase in the absolute value of SST may not directly translate to an increase in the overall number of intense TCs or TC occurrences, as compared to changes in the large-scale environment caused by the north–south STG.

On the other hand, changes in the meridional STG could imply variations of other large-scale atmospheric conditions such as vertical wind shear, moisture content, or tropospheric stratification that TCs reside in. These large-scale factors may or may not collaborate with each other and can therefore enhance or depress TC activity. For example, a weaker surface temperature gradient would promote stronger wind shear via reduced thermal wind, thus affecting TC formation (e.g., Na et al. 2018). This can be seen from Fig. 7, which shows indeed that STG and domain-averaged wind shear has a significant negative correlation during 1995–2014, with a correlation of about −0.54 for pair P1 and −0.46 for pair P2. Likewise, a larger surface temperature gradient could enhance monsoon activity, resulting in changes in TC development. These effects related to STG variation will be further examined in the next two sections.

Fig. 7.
Fig. 7.

Time series of the detrended area-averaged 1000–100-hPa vertical wind shear (solid curve) and the surface temperature gradient for pair P1 (dotted curve) and pair P2 (dashed curve) obtained from the NCEP–NCAR reanalysis during 1975–2014. The correlation between TSK and STG for each pair is also included.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

b. Vertical wind shear effects

To a first-order impact, a change in the meridional STG can modulate large-scale vertical wind shear via a thermal wind relation, which is known to be inimical to TC formation (e.g., Na et al. 2018; White and Staniforth 2008). To examine this indirect consequence of STG change on the vertical wind shear during the two epochs, Fig. 8 shows the total vertical wind shear averaged for the reference and the recent epochs, along with their difference for the WM and ST groups. Note that the average wind shear is computed between the 1000- and 100-hPa levels only for the days with at least one TC occurrence to ensure the consistency with STG calculations.

Fig. 8.
Fig. 8.

Horizontal distribution of the vertical wind shear (m s−1) for (a),(d) the reference epoch 1975–94 and (b),(e) the recent epoch 1995–2014, along with (c),(f) the difference between the reference and recent epochs, for (left) the WM group (category 13 and below) and (right) the ST group (category 14 and above).

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

As seen in Fig. 8, noticeable change in the overall wind shear is captured between the reference and the recent epochs. Specifically, for the WM TC group, the difference of the zonal shear between the two epochs is mostly in the range from −3 to 7 m s−1. However, the ST group captures a more apparent difference in the zonal wind shear with a strong positive change in the Southeast of VES and a negative shear change in the VES (the negative change indicates that the easterly wind shear is strengthened), with a larger range from −7 to 9 m s−1. This increase in the easterly shear is especially clear in the VES for the ST group (Fig. 8f) as compared to the WM group (Fig. 8c).

Further examination of the average wind shear within the domain shown in Fig. 1 confirms the different role of wind shear in the climatic shift of TC activity between the ST and WM groups (Fig. 9). Unlike the negative correlation between the vertical shear and WM development, one notices a higher number of strong TCs when the wind shear in the VES increases with time. While none of these correlations is statistically significant due to strong fluctuation of the TC number from year to year, the difference in mean shear between the two epochs is significant at 90% confidence interval (using the standard pairwise t test). In combination with the significant difference in the numbers of TCs between two epochs obtained from the JTWC data shown in Fig. 3, this result suggests a relation between the large-scale shear flow and TC activity in the VES that responds to change in the STG shown in Fig. 6.

Fig. 9.
Fig. 9.

Time series of the area-averaged vertical wind shear (dashed curves) and the number of TCs (black solid curves) for (a) the WM group (category 13 and below) and (b) the ST group (category 14 and above) obtained from the JTWC dataset during the 1975–2014 period. Dashed lines denote the linear best fit for each corresponding time series, and thick solid gray lines denote the epochal averages of TC numbers for the 1975–94 and 1995–2014 periods.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

Although the increase of the easterly shear could explain the overall decrease of weak/moderate TCs in the recent epoch, the larger increase of mean shear in the ST group is harder to understand if one recalls that the number of strong TCs increases in the recent epoch as shown in Fig. 2 (see also Table 1). Such a larger number of strong TCs for higher shear in the recent epoch seems to contradict our current understanding of TC formation according to which fewer TCs are generally formed in a strong shear environment. Among several possible explanations for this contradicting result, a justification may lie in the complex dependence of TC dynamics on environment as well as the TC intrinsic dynamics (e.g., Jones 1995; Zhang and Kieu 2005, 2006; Reasor and Eastin 2012; Chen and Gopalakrishnan 2015). Unlike the general statistics of tropical cyclogenesis that is negatively dependent on vertical wind shear, the fact that a TC could grow and approach a very high intensity in the ST group indicates that other environmental conditions beyond wind shear must be very favorable for TC development. For these cases, the adverse impacts of increased wind shear are alone insufficient to prevent TC development. Such resilience of strong TCs to vertical wind shear has been reported in previous studies (see Zhang and Kieu 2005, 2006) and is quite robust for a range of strong storms. Thus, strong easterly wind shear may play a smaller role in the development of strong TCs as compared to weak TCs and explain the larger number of strong TCs even in stronger easterly shear during the recent epoch.

c. Monsoonal effects

As the meridional gradient of surface temperature in the VES increases, another possible side effect on large-scale conditions beyond the increase in the vertical shear is the change in monsoon activity, which could affect TC formation and intensity (e.g., Krishnamurti et al. 2007; Chen et al. 2017; Na et al. 2018; Zhong et al. 2020). While vertical wind shear can be considered as a first-order response of the atmosphere to STG change, the resulting variation in monsoon activity could either augment or offset the impact of wind shear on TC development, which is much elusive at present.

To examine the relationship between TC climatology and monsoon activities in the VES, we follow Wang et al. (2004) and use the SCSSM index defined from May to November for both the reference and the recent epochs in our analyses herein. Figure 10 compares the SCSSM for two epochs, which captures a monsoon index of 1.95 m s−1 for the recent epoch and a larger value in the reference epoch (~2.02 m s−1). Note that the decrease of the SCSSM index between the two epochs does not depend on the summer months used to define the index. In fact, the same result is obtained when calculating the intensity of the southwest monsoon in the VES from June to September as shown in Fig. 8, indicating the systematic changes of monsoon during the two epochs.

Fig. 10.
Fig. 10.

The SCSSM index as obtained from the NCEP–NCAR reanalysis data for the domain shown in Fig. 1 for the reference period 1975–94 and the recent period 1995–2014. The gray columns are for the calculation from May to November, and the striped columns are for June–September months.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

The decrease in monsoon intensity (~0.07 m s−1 or 3.5%) as shown in Fig. 10 is small and not statistically significant (at 95% confidence interval). Nonetheless, this monsoon activity reduction is consistent with previous studies (Li and Zeng 2002, 2003, 2005; Wang et al. 2008), which showed an overall increase in surface pressure in the WPAC basin. The decrease of the epochal average monsoon index shown in Fig. 10 is also consistent with the overall downtrend of the SCSSM time series during the 1975–2014 period (Fig. 11). One notices in Fig. 11 an overall downtrend of the SCSSM time series during 1975–2014, which is opposite to the uptrend of the number of strong TCs. A negative correlation of −0.18 is obtained from these time series, albeit the correlation is again not statistically significant for all three TC datasets.

Fig. 11.
Fig. 11.

As in Fig. 7, but for the monsoon index SCSSM obtained from the NCEP–NCAR reanalysis from May to November.

Citation: Journal of Applied Meteorology and Climatology 59, 10; 10.1175/JAMC-D-20-0021.1

Physically, weakened monsoon activity often implies (i) a shift in the atmosphere mass such that the sea level pressure increases over most of the WPAC and (ii) a reduction in the large-scale precipitation. As argued in Li and Zeng (2005), such changes in large-scale precipitation could reduce the troposphere static stability, thus allowing for TCs to reach higher intensity and affecting the climatology of strong TCs (Kieu and Zhang 2018). In this regard, the changes in the monsoon activity can impact not only TC frequency (via barotropic instability; see, e.g., H. Yang et al. 2015; Govardhan et al. 2017; Zheng and Huang 2019) but also TC intensity (via change in the tropospheric stratification; see, e.g., Krishnamurti et al. 2007; Fan et al. 2012; Kieu and Zhang 2018).

It is of interest to note that the effects of summer monsoons on TC frequency are not only reflected in the VES but also captured in higher latitudes. For example, summer monsoon has been shown to be closely related to the variability in TC frequency affecting South Korea (Choi et al. 2017). The correlation between the TC number and the East Asia summer monsoon index is very persistent, even when excluding the years of ENSO influence as pointed out in Choi et al. (2017). In this regard, these previous findings as well as our results in this study suggest that the variation in summer monsoon can affect the TC activity in the VES.

5. Conclusions

In this study, the climatic shifts of TC activity in the VES that affect Vietnam’s coastal region during the 40-yr period from 1975 to 2014 were examined. Using several available TC databases including the JTWC, the RSMC, and Unisys Weather datasets, it was found that there is a noticeable shift in TC frequency as well as TC occurrences in the VES between a reference epoch from 1975 to 1994 and a recent epoch from 1995 to 2014. Specifically, there is a consistent increase in the number of strong TCs during the recent epoch in all TC datasets, defined to be TCs with category 13 and above according to the Beaufort scale, as compared to the reference epoch. Unlike the strong TC statistics, the group of weak and moderate TCs (categories 8–12) showed a decrease in the number of TCs during the recent epoch. Although the change in the TC numbers is statistically significant with the JTWC data only, the consistency in the increase of strong TCs and the decrease of weak TCs in the VES is noteworthy, as it supports the recent projections of more intense TCs in the WPAC basin.

Similar to the changes in the number of TCs, it was found that TC occurrences also displayed an increase for the strong TC group and a decrease for the weak/moderate TC group. Because TC occurrences include not only TC frequency but also their life cycle, these findings suggest that both the number of strong TCs affecting Vietnam’s coastal region and their corresponding life cycle tend to increase during the last 20 years.

To understand the physical connection of these shifts in TC frequency and life cycle, large-scale conditions including surface temperature, vertical wind shear, and monsoonal strength were analyzed for the two epochs. Comparison of the north–south surface temperature gradient (STG) between the two epochs revealed that the STG during the recent epoch is substantially larger than that in the 1975–94 period, especially for the strong TC group. This increase in the meridional STG is persistent regardless of how the STG is defined, suggesting a larger pressure difference between the northern and the southern parts of the VES that provides more favorable conditions for TC disturbances to develop.

On the other hand, the larger meridional STG could produce stronger vertical wind shear and prohibit TC development. This inimical impact of the STG was observed in our vertical wind shear analysis, which showed that the easterly zonal shear is indeed stronger in the recent epoch. Such an increase in the easterly zonal shear justifies why the number of weak TCs decreases during the recent epoch relative to the reference epoch. For strong TCs, it is of interest to note also that stronger wind shear does not seem to be detrimental to their activity. Such a resilience of strong TCs to vertical shear is likely because other favorable conditions for TC development could dominate such that strong vertical wind shear alone would not cause much of an impact on their development.

Along with stronger vertical wind shear due to the enhanced north–south STG, it was found that the intensity of summer monsoon in the VES is also weaker during the recent epoch. This explains why tropical cyclogenesis decreases for weaker monsoon activity as found in previous studies (e.g., Chen et al. 2017; Zhong et al. 2020), despite overall warmer SST in the VES over the last several decades. The combined effects of vertical wind shear and monsoon activity in the VES may explain why TC climatology in this region could not display a clear signal of variability as compared with other basins. In particular, the roles of SST are less prominent and are strongly interfered with by other factors such that SST cannot be used as a single proxy to examine the TC climatology in this region.

Regardless of the competing large-scale factors in the VES region, the fact that both TC frequency and occurrences captured a consistent increasing trend in the recent epoch across all TC datasets is significant. On the one hand, it supports the projection of TC activity in the WPAC basin issued by IPCC/AR5. On the other hand, these results highlight the complex nature of TCs in the VES whose underlying mechanisms are strongly influenced by competing effects of vertical wind shear, monsoon, and SST gradient. Therefore, more in-depth research on TCs to isolate the roles of each large-scale factor in the VES region will be needed to help better capture the future TC activity in this part of the WPAC basin.

Acknowledgments

This research was funded by the Vietnam Ministry of Science and Technology Foundation (KC.09.15/16-20). Author Kieu was partially supported by the Indiana University Grand Challenge Initiative and ONR funding (N000141812588). The authors thank three anonymous reviewers whose constructive comments and suggestions have helped to improve this work substantially.

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1

JTWC and RSMC databases are freely accessible to the public, whereas Unisys Weather requires a service subscription to access their TC database (for an archived version, see http://web.archive.org/web/20190613003758/http://50.206.172.193/hurricane/index.php).

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