The observed recent PDI decrease

The value of the typhoon-season (July–October) PDI in the recent pentad (2008–2012) was only ∼63% that of the 1993–1997 period (see Fig. 1a). To understand the cause of this decrease, we first examined the ocean environment because the ocean thermal conditions, including both the sea surface temperature (SST) and the upper ocean heat content (UOHC), are important factors for tropical cyclone intensity change and could impact PDI8,12,13,14,15,16,17,18. The UOHC (also called the tropical cyclone heat potential) is defined as the integrated heat content from the SST down to the 26 °C isotherm (D26, the measure of the subsurface warm ocean layer thickness) in the subsurface ocean14,16,17.

Figure 1: Time series of PDI and related parameters. Time evolution of the observed PDI and other parameters over the western North Pacific MDR in the past two decades. The trend line for each time series, based on linear regression is also depicted. Standard deviations are depicted by dotted curves. (a) PDI, (b) SST, (c) the depth of the 26 °C isotherm (D26) and TCHP (Tropical Cyclone Heat Potential or upper ocean heat content (UOHC)). (d) Typhoon case number in the typhoon season (July–October) of a year, (e) as in d, but for the averaged typhoon duration, (f) as in d, but for the averaged typhoon intensity. Full size image

Figure 1b,c depicts the typhoon season (July–October) SST and UOHC over the MDR. In the past two decades, although the SST exhibited little change, the UOHC and D26 increased considerably3,19 (Fig. 1c). In comparison to the 1990s, recent UOHC and D26 both exhibited clear increases of ∼10%, which opposed the decrease in PDI. The origin of these increases in UOHC and D26 were thought to be associated with the recent strengthening of the easterly trade winds, which pile up warm surface ocean water towards the western Pacific19,20. As a result, a thicker layer of warm water accumulates in the western North Pacific typhoon MDR3,19,20. Therefore, the objective of this study was to understand why typhoon PDI has actually decreased despite evident warming in the upper ocean.

The three PDI contributing factors

PDI is determined not only by typhoon intensity (that is, maximum surface wind speed) but also by duration (lifetime) and typhoon occurrence frequency (case number). The evolution of all of these parameters (Fig. 1d–f) shows that although the intensity increased somewhat, both the duration and number decreased considerably. Thus, the reduction in PDI was not due to the intensity but the duration and number.

This result is further quantified in Table 1, following the method proposed by Emanuel21, to separate the different contributions of the annual typhoon case number (N), annual-averaged weighted typhoon duration over ocean ( , see Methods section for details) and annual-averaged intensity (I, in wind speed cube) to PDI. Supplementary Fig. 2 shows these terms with their respective long-term mean (1993–2012) removed. The data were based on the annual typhoon season (July–October) over the MDR domain during 1993–2012.

Table 1 Contributing factors to annual PDI. Full size table

Table 1 summarizes the different contributions in each pentad. In the most recent pentad (2008–2012, 2nd-last row), PDI decreased by approximately −17% with respect to climatology (1993–2012 mean). Although the intensity contributed to an increase in PDI by ∼6–7%, the negative contributions of number and duration are approximately −16% and −7%, respectively (jointly approximately −23 %). Therefore, PDI still decreased because the joint contributions of number and duration were much larger than those of intensity. Relative to the pentad in 1993–1997, the −45% PDI decrease in the latest pentad (2008–2012) was also attributed to the −74% drop in number and duration, with a 29% offset from the positive contribution from the intensity (last row in Table 1; for more details, see Supplementary Table 1).

Atmospheric and ocean environments

It is important to understand the evident decreases in typhoon number and duration6,7 (Fig. 1d,e). Figure 2a,b shows that the decrease was accompanied by a strong increase in vertical wind shear (VWS, primarily contributed by the zonal VWS) and decrease in low-level relative vorticity in the typhoon genesis region (150–180°E, 10–17.5°N) (ref. 6, 7). Especially after 2008, the VWS reached 10–18 m s−1, which represented an environment unfavourable for formation6,7,22,23,24,25. Consistent with these developments, the typhoon genesis position also shifted north-westwards towards land26, reflecting the difficulty of formation at the usual genesis region (east of 150°E; Fig. 2b-right axis, Fig. 3 and Supplementary Figs 3–6). The weak VWS and high vorticity are the main atmospheric dynamic conditions necessary for formation22,23,24,25, and it becomes more difficult for typhoons to form further east. Because they form further to the northwest towards land26, their durations over the ocean are also shortened (Figs 1e, 2b and 3). This trend was confirmed by the strong correlations (r=0.56–0.88) found between the increase in the zonal VWS, decrease in vorticity, westward shift of the genesis longitude (1st lon), the reduction in typhoon duration ( ) and (case number times duration; boldface entries in Table 2 and Supplementary Table 2, see also Supplementary Note 1).

Figure 2: Vertical wind shear (VWS) and other parameters at the typhoon genesis region. (a) Time evolution of the typhoon-season averaged VWS and zonal VWS in the typhoon genesis region in the past two decades. (b) As in a, but for the 850 hPa relative vorticity (left axis) and the genesis longitude (right axis). (c) Coherent variability between PDI, the easterly wind at 850 hPa, SHAI (subtropical height area index) at 850 hPa and the zonal VWS. Note that the three y-axes at right are reversed, so as to show the reduction in PDI (left axis) with the increase in these three suppressive parameters. Full size image

Figure 3: Strengthening of the subtropical high. Strengthening of the subtropical high as depicted by the 1,560 and 1,530 geopotential lines at 850 hPa with anomalous wind vectors from the latest pentad (2008–2012) with respect to 20-year mean (1993–2012) overlaid. The averaged typhoon genesis positions (with 1 standard deviation) over three different periods in the past 20 years are also depicted. MDR=main development region. Full size image

Table 2 Correlations among 3 groups of parameters. Full size table

In fact, the above change in the atmospheric conditions is part of the large-scale environmental change over the Pacific in the recent decade27,28,29,30,31. Many recent studies27,28,29,32 have emphasized the dominance of this large-scale change by discussing the evident strengthening of the atmospheric circulation (including the enhanced easterly trade wind and Walker cell) and a La Nina-like decadal phenomenon27,28,29,32. This phenomenon is clearly visible in Fig. 3, which shows a strengthening of the subtropical high pressure system30,31,33,34 and the easterly wind anomaly at 850 hPa. In this research, we used three parameters to characterize the large-scale circulation condition: the 850 hPa easterly wind; the subtropical high area index (SHAI)6 and a newly proposed related parameter called subtropical high-intensity index (SHII; see Methods).

The strengthening of the large-scale circulation can cause an increase in the VWS. This effect occurs because the VWS is the difference between the winds in the upper (200 hPa) and lower (850 hPa) troposphere. Enhanced circulation29 can increase both the lower-tropospheric easterlies (trade wind) and upper-tropospheric westerlies, and thus the shear increases (see also Supplementary Figures 3 and 4). Climatologically, the low-level relative vorticity over the MDR is generally high24. With strengthening of the easterlies associated with large-scale circulation strengthening (Fig. 3), the magnitude of the relative vorticity decreases, as manifested in a weakened monsoon trough condition.

This control of the large-scale circulation on typhoon-related atmospheric parameters (that is, VWS and vorticity) can be observed in the high correlations (r=0.83–0.91) between the easterly wind, SHAI at 850 hPa and zonal VWS (see the boldface entries in Table 2 and Fig. 4). Furthermore, strong negative correlations (−0.77 to −0.83, see the boldface entries in Table 2 and Fig. 4) existed between these three parameters with PDI because an increase in these atmospheric parameters contributed to a reduction in PDI (Fig. 4). From an even broader perspective, the relative SST parameter (typhoon basin SST with respect to global, tropical mean SST)35,36 also showed a decreasing trend, supporting the decline in topical cyclone (TC) activity in the recent decade (other details see Supplementary Figs 7 and 8).

Figure 4: Flow diagram of key correlations. Flow diagram showing a summary of the key correlations in Table 2. Full size image

In addition to the correlations between the different parameter pairs, statistical analyses of each individual time series were also performed. As shown in Fig. 1d,e, the correlation was ∼0.7 for the typhoon count time series, with P=0.0008. For the typhoon duration time series, r∼0.5 and P=0.034. However, as the contributions from the typhoon intensity were opposite to the contributions from the duration and typhoon frequency (count), the P value for the PDI time series was larger (P=0.465). It should be noted that due to the competing and offsetting impacts between typhoon duration/frequency and intensity, and between ocean and atmosphere, it was not possible for PDI to exhibit a clear trend because it was the residual result from opposing and competing contributors.

Therefore, from the typhoon perspective, the recent strengthening of the large-scale circulation induced a ‘worsened’ atmosphere condition for typhoons and PDI, even though such strengthening also provided a ‘better’ ocean because the increase in the easterly wind piled up warm surface water to the western North Pacific and increased D26 and UOHC, as discussed earlier19,20,27,29. However, the impact of the ‘worsened’ atmosphere appeared to dominate over the ‘better’ ocean to reduce the typhoon destructive potential through the strong suppression of typhoon number and duration. This interesting interplay can be observed between the ocean and the atmosphere. Apparently, for the present epoch in the western North Pacific, the atmosphere dominates over the ocean in controlling typhoon PDI.

Although the possibility of an increase in PDI (and hence the destructiveness potential) due to ocean warming was suggested before7, our results provided new evidence to show that such a situation was not always applicable because the ocean was not always dominant. We found that the situation was more complex, and it was also possible for PDI to decrease despite ocean warming over the western North Pacific Ocean. A ‘worsened’ atmosphere can effectively dominate over a ‘warmer’ ocean to decrease the typhoon destructiveness potential.

Because PDI depends not only on the intensity but also on typhoon duration and number, even if the intensity increases due to ocean warming, it does not mean that typhoon duration and number will increase in tandem. This is especially true over the western North Pacific Ocean, in which these parameters are very much controlled by the dynamical factors in the atmosphere (for example, shear) than the thermal factors (for example, SST, UOHC and humidity). As in Table 1, contributions from duration and number to PDI were often opposite to the contribution from intensity (three out of four pentads). Therefore, the final PDI was mostly the residual after the offset.