Weakening seasonality of Indo-Pacific warm pool size in a warming world since 1950

The long-term changes in IPWP seasonality could provide important clues about how anthropogenic climate change might affect the global climate through the IPWP. Here, we first present a comparative analysis of the size of the IPWP and its long-term change in different seasons from 1950–2020, which shows an obvious seasonal diversity of the expansion trends of the IPWP.

The long-term changes in IPWP size differ greatly between seasons, with the largest expansion trend in winter [December–January–February (DJF), 0.28 $\times$ 10⁷ km²/decade], followed by autumn [September–October–November (SON), 0.26 $\times$ 10⁷ km²/decade]. The increases in IPWP size are relatively smaller in summer [June–July–August (JJA), 0.18 $\times$ 10⁷ km²/decade] and spring [March–April–May (MAM), 0.17 $\times$10⁷km²/decade] (figure 1(a)). It is worth noting that the rising trend of IPWP size in winter is almost twice as large as that in spring and summer. The seasonal differences in IPWP expansion exist not only in the long-term trends but also in the spatial patterns. We find that the winter and autumn IPWP expand greatly to the west in the Indian Ocean sector in addition to the apparent eastward expansion in the Pacific sector (figures 1(c) and (f)), but in the spring and summer the IPWP mainly expands eastward in the Pacific sector (figures 1(d) and (e)). The different expansion patterns in Indian Ocean sectors cause diverse forcing effects on the Walker circulation, with the fast expanding winter and autumn IPWP in the Indian Ocean sector coupling with the reinforced and westward-shifted Walker circulation in the Indian ocean sector (figure S2). Interestingly, despite the great differences in IPWP size changes in different seasons, the increasing trends of the Indo-Pacific SST are rather constant across seasons (∼0.09 °C/decade; figure 1(b)) (Xie et al 2010, Huang et al 2013, Geng et al 2020). These results imply a significant seasonal diversity in the IPWP expansion trend under the seasonally homogeneous ocean warming background.

The large seasonal diversity in the IPWP expansion rate indicates that the seasonality of the IPWP has been changing in the past few decades. By dividing the period 1950–2020 into two halves, figure 2(a) compares the annual cycle of the IPWP in the pre-1985 period (P1, 1950–1984) and the post-1985 period (P2, 1985–2020). The seasonal cycle of IPWP size, with a bimodal pattern peaking in spring and autumn, has generally shifted upward under the background of global warming. In particular, the expansion trends in winter and autumn (secondary peak) were larger than those in spring and summer (primary peak) (bars in figure 2(a)). This non-uniform change made the secondary peak of the seasonal cycle rise to a larger extent compared with the primary peak. As a result, a decrease in the amplitude of the seasonal cycle of IPWP size is observed in P2 relative to P1. Quantitatively, the seasonal cycle amplitude (defined as $\frac{{{\text{Peak}} - {\text{Trough}}}}{{{\text{Average}}}} \times 100\%$) reduces from 43% to 29% from P1 to P2. The decrease in the seasonal cycle amplitude can also be clearly demonstrated from the 4–15 months band-pass filtered IPWP size series, which mainly captures the signal of the seasonal cycle (figure 2(b)), with its annual standard deviation decreasing at a rate of −4.0 $\times$ 10⁵ km²/decade (figure 2(c)). Similar conclusions can be obtained from observations from 1979 to 2020 (figures S3(a)–(d)) and those based on the ERSSTv5 dataset (figures S4 and S5). Besides, the weakening seasonality is observed not only for the size of the IPWP but also for its volume (figures S6(a) and (b)). An interesting phenomenon is that there seems to be a reverse change in the seasonal cycle amplitude of the shallow IPWP and deep IPWP size (figures S6(c)–(f)).

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According to previous research, the variabilities and characteristics of the IOWP differ largely from those of the WPWP. Schneider et al (1996) concluded that SWR affects the vertical temperature structure more in the Pacific than in the Indian Ocean. Fasullo and Webster (1999) found that the IOWP SST is less sensitive to surface thermal forcing than that in the Pacific, and is influenced by the Indian summer monsoon (Kim et al 2012, Wang et al 2019). Thus, we further divide the IPWP into the IOWP and WPWP (figure S1), and examine their contributions to the weakening IPWP seasonality. Surprisingly, we find that the seasonal diversity of the IPWP expansion trend is almost solely contributed by the IOWP.

As shown in figures 2(d)–(i), the IOWP and WPWP exhibit different seasonality, although both are primarily dominated by a 12-month cycle. For the IOWP, the peak (trough) of its seasonal cycle is in spring (late summer), with a large seasonal cycle amplitude (104%) in the period 1950–2020 (figure 2(d)). In contrast, the WPWP seasonal cycle has its peak (trough) in autumn (winter) and its amplitude (36%) is much smaller than that of the IOWP (figure 2(g)). This result indicates a stronger fluctuation of the IOWP seasonal cycle than that of the WPWP, which is consistent with previous findings (Kim et al 2012, Wang et al 2019).

Similar to the IPWP, the change in size of the IOWP from P1 to P2 also exhibits obvious seasonal diversity, showing that the largest expansion trend in winter (0.17 $\times$ 10⁷ km²/decade) is more than twice as large as that in spring (0.07 $\times$ 10⁷ km²/decade) (figure 2(d)). This indicates a long-term change in IOWP seasonality. Because the speed of IOWP expansion in winter and autumn is twice that in spring and summer (figure 2(d)), the seasonal variation in IOWP size has been diminished since 1950, with its seasonal cycle amplitude dropping from 127% to 88% from P1 to P2. This is also evident in the 4–15 months bandpass filtered IOWP size series and its annual standard deviation, which depicts a downward trend of −2.0 $\times$ 10⁵ km²/decade (figures 2(e) and (f)). These changing patterns in IOWP size are found to be consistent with those of the IPWP.

On the contrary, although the variation in WPWP size does exhibit obvious seasonality, the WPWP expands almost identically in all seasons from P1 to P2, with its size increase of each month ranging from 0.09 × 10⁷ to 0.13 $\times$ 10⁷ km² (figure 2(g)). Consequently, the seasonal cycle of WPWP size shifts upward approximately uniformly (figure 2(g)). The seasonal cycle amplitude stays on almost the same level (38% in P1 and 34% in P2), and the standard deviation of 4–15 months band-pass filtered WPWP size shows a statistically insignificant long-term trend (−4.0 $\times$ 10⁴ km²/decade; figures 2(h) and (i)). Based on the comparison of the changes in the seasonality of the IOWP and WPWP sizes (figures 2(d) and (g)) as well as the spatial patterns of IPWP expansion in four seasons (figures 1(c)–(f)), we conclude that the seasonal difference in IOWP expansion weakens the amplitude of IPWP seasonality. Consistent results are also obtained from ERSSTv5 (figure S7).

The seasonal diversity of IPWP expansion somehow contradicts the seasonally homogeneous SST warming over the Indo-Pacific region. The overall regional-mean SST trend (figure 1(b)) and the spatial pattern of SST change show no obvious seasonal differences (figures S8 and S9). In this section, based on the image histogram approach (see supplementary materials), we show that the seasonally diverse IPWP expansion speed is mainly due to the seasonality of the climatological Indo-Pacific SST pattern.

Comparing periods P1 and P2, the SST PFD of each season (left column of figures 3(a)–(d)) shifts to the right, to the first order, without obvious changes in shape and skewness (figure 3(e)), which increases the area where SST ⩾ 28 °C. The change in IPWP size largely depends on the PFD shape. For example, in P1, the Indo-Pacific basin SST is relatively cool and the PFD peak is near 28 °C in winter, indicating a large number of grids (area) whose SSTs were slightly lower than 28 °C (figure 3(a)). These grids will be identified as part of the IPWP in P2 as the ocean warms. On the other hand, there are relatively fewer grids (area) whose SSTs are slightly lower than 28 °C in spring (figure 3(b)). Hence, the change in IPWP size in spring is significantly smaller, although the increase in SST is rather similar in spring and winter (figures 1(b), S8 and S9). In other words, the SST PFD shape, which is associated with the climatological SST pattern, indicates the potential for IPWP expansion, namely the capacity for change in IPWP size (Kajtar et al 2021, Park et al 2022). The seasonal difference in the capacity for IPWP size change may be the main cause of the weakening of IPWP seasonality in the past decades.

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Two ideal experiments (see supplementary materials) which assume (a) seasonal uniform warming only (SUE) and (b) seasonally and spatially homogeneous warming (SSUE) were further carried out. Results show that both SUE (purple line) and SSUE (green line) generally capture the observed PFD changes of different seasons (figures 3(a)–(d)), although the SST PFD shapes under SSUE are slightly different from reality because the observed spatially uneven warming pattern is neglected. The amplitude of the IPWP seasonal cycle under SSUE (SUE) drops from 43% to 28% (43% to 29%) from P1 to P2, being consistent with the observed change (14%). The IPWP size changes under both SSUE (1.02 $\times$ 10⁷, 0.67 $\times$ 10⁷, 0.70 $\times$ 10⁷ and 0.86 $\times$ 10⁷ km² in winter, spring, summer and autumn, respectively) and SUE (1.10 $\times$ 10⁷, 0.66 $\times$ 10⁷, 0.68 $\times$ 10⁷ and 0.86 $\times$ 10⁷, respectively), consistent with the observed changes (1.03 $\times$ 10⁷, 0.60 $\times$ 10⁷, 0.65 $\times$ 10⁷ and 0.89 $\times$ 10⁷, respectively) (figure 3(f)).

Besides the small differences [(0.01–0.07) $\times$ 10⁷ km²] in IPWP size change between SSUE (i.e. the capacity for IPWP size change) and observation, consistent results are also evident in the seasonal diversity (figure 3(f)). In addition, the spatial patterns of the capacity for IPWP change and the IPWP change in SUE from P1 to P2 are also consistent with observation (figures 3(g)–(j)). The presented ideal experiments prove that the seasonality of the capacity for IPWP size change plays a dominant role in changing the seasonal variability of the IPWP, and emphasizes the importance of the intrinsic properties of the climatological SST pattern over the Indo-Pacific Ocean in analyzing seasonal IPWP expansion. Meanwhile, the contribution of spatially inhomogeneous SST change is fairly small when assessing the seasonality of the capacity for IPWP size change.

The ideal experiments further confirm that that the seasonality change in IPWP size is mainly determined by the capacity for IOWP size change (0.60 $\times$ 10⁷, 0.27 $\times$ 10⁷, 0.22$\times$ 10⁷ and 0.39 $\times$ 10⁷ km² in winter, spring, summer and autumn, respectively) (figure 3(f)). The seasonal cycle amplitude of the capacity for IOWP size change decreases from 127% to 89%, which is highly consistent with observations (127% to 88%) and consistent with that of the IPWP. On the contrary, the capacity for WPWP size change does not show obvious seasonal differences (figure 3(f)). Consequently, the amplitude of the seasonal cycle of WPWP size shows no significant change (from 38% to 33%).

The above analysis emphasizes the crucial impacts of seasonality in the climatological Indian Ocean SST pattern that determines the capacity for IOWP expansion in weakening the IPWP seasonality. Based on heat budget analyses (see supplementary materials), we further show that that the SWR and LHF primarily determine the climatological SST pattern, with LHF playing a key role in the tropical Indian Ocean because of the strong Indian winter and summer monsoons (figures 4(a) and (b)). In winter in the Bay of Bengal and Arabian Sea, the negative LHF anomaly caused by the strong Indian winter monsoon and negative SWR anomaly leads to a negative net surface heat flux anomaly, which further results in a negative anomaly in SST tendency (figures 4(c), (d), (f) and (g)). The warm advection in the Arabian Sea weakens the negative anomaly in SST tendency to some extent (figure 4(e)). The negative anomaly in SST tendency couples with the cool SST pattern over the tropical Indian Ocean and may provide a large capacity for IPWP expansion in winter. In the tropical Indian Ocean in spring, the positive LHF and SWR anomalies lead to a positive net surface heat flux anomaly (figures 4(c), (d), (f) and (g)), which results in the warm SST pattern over the tropical Indian Ocean and a smaller capacity for IPWP expansion. The heat budget analysis reveals that it is the seasonal differences in atmospheric forcing (especially SWR and LHF) that determine the seasonality of the climatological SST pattern over the tropical Indian Ocean, which further influences the seasonal diversity of IPWP expansion.

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