Seasonal diversity of IPWP volume expansion
Previous studies on the expansion of the IPWP have predominantly focused on changes in its surface size. However, the 3D structure of the IPWP should be considered when accessing the climatic implications of the IPWP expansion. Based on the Institute of Atmospheric Physics (IAP) global ocean temperature gridded product data from 1950–2020 (see Methods), we observe significant differences in IPWP across seasons (Hereafter, unless specified, we refer IPWP to as the entire 3D IPWP body, rather than solely the IPWP surface as in most publications) (Fig. 1). The IPWP tends to be located more towards southern (northern) latitudes in winter and spring (summer and autumn). The IPWP in spring has the largest volume (with the greatest surface area and depth) (Fig. 1b), while in autumn and winter its volume is smaller with a smaller surface size and shallower depth (Fig. 1a, d), especially in the Indian Ocean sector. Under the influence of a warming climate, the IPWP body exhibits significant expansion, particularly in the western Pacific sector where significant eastward expansion and deepening of the IPWP are observed in all four seasons (Fig. 2). However, there also exists some diversities in the IPWP expansion pattern among different seasons, especially in the Indian Ocean sector, for example, compared with spring and summer (Fig. 2b, c, f, g, j, k), the IPWP experiences a greater westward expansion and deepening during the autumn and winter (Fig. 2a, d, e, h, i, l). The warmer spring IPWP exhibits the largest volume (Fig. 2b), yet it demonstrates smaller expansion (Fig. 2b, f, j). Similarly, the summer IPWP experiences relatively limited expansion, particularly in the Indian Ocean sector (Fig. 2c, g, k), which could potentially be attributed to the influence of the Indian summer monsoon. Similar results are obtained based on the ocean temperature data from NCEP GODAS that spans from 1980–2020 (Figure S1).
The seasonal difference in IPWP volume expansion is accompanied by a reduction in the amplitude of its annual cycle. In Fig. 3a, a bimodal structure with peaks in spring and autumn is observed in the annual cycle of IPWP volume. Comparing the period of 1985–2020 (P2) to 1950–1984 (P1), we find that the main peak (in spring and summer) of annual cycle has slightly diminished, while the secondary peak (in late autumn and winter) has increased somewhat. This change indicates a decline in the amplitude of IPWP volume annual cycle during the P2. Calculating the change in the amplitude of IPWP volume annual cycle, we found a significant decreasing trend (-0.67×107 km3/decade) (Fig. 3b, red line).
Considering the substantial differences in the characteristics between the Indian Ocean sector and the Pacific Ocean sector, we further divided the IPWP into the IOWP and WPWP and examined the changes of their annual cycle (Fig. 3c, d). We find that the changes in the annual cycle of IOWP volume are the primary reason for IPWP seasonality change. In P2, the main peak (in spring and summer) of IOWP volume annual cycle slightly diminishes compared with P1, while in late autumn and winter it rises. Further calculation of the change in the amplitude of its annual cycle shows a significant trend of -0.34×107 km3/decade (Fig. 3b, yellow line). In contrast, we find the amplitude of the WPWP volume annual cycle increases in P2 compared with P1 (Fig. 3d), with an upward trend (0.12×107 km3/decade) (Fig. 3b, orange line).
The IPWP reaches a maximal depth of 100m. Given that the physical properties vary between upper-layer and lower-layer ocean, it is possible that the changes of IPWP volume differ between these two layers. It appears that the changes of IPWP annual cycle between the upper and lower layers are opposing (Figure S2). Namely, above 40m depth, greater IPWP expansion occurs in winter and autumn; in contrast, below 40m, increase in IPWP volume is faster in spring and summer. Based on these results, we further divide the IPWP into upper-layer and lower-layer at a depth boundary of 40m and analyze the changes of their annual cycles separately (Fig. 3e, f). The change in the annual cycle of the upper-layer IPWP volume appears to be consistent with that of the entire IPWP (Fig. 3e), with the main peak (in spring and summer) of annual cycle in P2 diminishes compared with P1, while the secondary peak (in late autumn and winter) in P2 rises. Therefore, the annual cycle amplitude of upper-layer IPWP volume shows a significant decreasing trend (-0.54×107 km3/decade) (Fig. 2b, light green line). However, the annual cycle of the lower-layer IPWP volume shows no apparent change (Fig. 3e) and exhibits an insignificant trend (Fig. 3b, dark green line).
These findings are interesting in terms that they highlight the IOWP, which accounts for only 25% of IPWP volume, dominates the change in IPWP volume seasonality. This result is consistent with Fig. 2, which shows that the seasonal differences of IPWP expansion are greatest in the Indian Ocean sector. It is precisely due to the huge seasonal diversity in the IOWP expansion that decreases the IOWP and IPWP volume seasonality. Besides, the opposite changes in the expansion of the IPWP volume between upper and lower layer (Fig. 3c, d) suggests that the variation of IPWP is not consistent between the upper and lower layer but rather exhibits as baroclinic variation. Similar results were obtained using ocean temperature data from NCEP-GODAS for the period 1980–2020 (Figure S3).
Cause of the seasonal diversity in IPWP expansion
In this section, we aim to explain the seasonal differences in IPWP volume expansion. Apparently, the IPWP volume expansion is directly influenced by the rapid warming of tropical Indo-Pacific Ocean. So, the most straightforward guess of the reason for the seasonal differences in IPWP expansion would be the seasonally uneven warming of Indo-Pacific Ocean temperature. However, our results suggest that this is not the case. The seasonal difference in tropical Indo-Pacific Ocean warming is relatively small (Fig. 4 and Figure S4), with a trend around 0.1°C /decade, while the expansion of IPWP varies greatly across seasons (Fig. 2). In the following, we try to dig into the problem by conducting a set of sensitivity experiments which assume different ideal conditions of topical Indo-Pacific Oceanic warming. Previous researches have defined the capacity for warm pool expansion22,54 and revealed its significant influence on the seasonal difference in surface IPWP expansion24. In this section, we find that the seasonal diversity in the capacity for IPWP volume expansion, which is determined by the spatial characteristics of climatological subsurface ocean temperature, is the primary driver of the seasonally different IPWP expansion trends.
The contradictory seasonal variations between Indo-Pacific Ocean warming and IPWP expansion (Fig. 2 and Fig. 4) implies that seasonal diversity in the IPWP expansion is not caused by the seasonally uneven warming of Indo-Pacific Ocean. Here, to further understand the reasons for the seasonal differences in IPWP expansion, we conducted three sensitivity experiments assuming that the IPWP warming is (a) seasonally and spatially uniform (seasonal and spatial uniform experiment; SSUE), (b) seasonally uniform only (seasonal uniform only experiment; SEUE), (c) spatially uniform only (spatial uniform only experiment; SPUE) to investigate the IPWP expansion under different ideal conditions (see Methods for detailed experiment design). By comparing the changes in the probability frequency distribution (PFD, see Methods) of IPWP oceanic temperature in the sensitivity experiments with observations, we can determine the primary reasons for the seasonal differences in IPWP expansion. Because the decrease in IPWP volume seasonality is mainly driven by changes in the upper layer IPWP (above 40m), which accounts for 66% of the total IPWP volume, thus the following analysis focuses on the upper-layer IPWP to discuss the reasons behind the seasonal diversity in the expansion of IPWP volume and its associated changes in seasonality.
Figure 5a-d presents a comparison between the IPWP temperature PFDs from observation and those obtained from the three sensitivity experiments in different seasons. We observe that the shapes of PFDs during P1 varied across seasons. For example, the PFD peaks in winter and autumn are close to 28°C (Fig. 5a, d, dark green lines), while those in spring and summer are close to 29°C (Fig. 5b, c, dark green lines). Additionally, the PFDs show a greater left skewness relative to 28°C (see Methods) in winter and autumn (Fig. 5e), indicating that there are more grids with temperature slightly below 28°C in P1 in these two seasons. This observation unveils a crucial information: the cooler subsurface temperature background in autumn and winter during P1 allows for a larger spatial extent for IPWP expansion in these two seasons during P2. In line with the definition from previous researches on such spatial extent for warm pool expansion, we adopt the concept of the capacity for warm pool volume expansion22,24,54 (see Methods). That is to say that the capacity for IPWP volume expansion may be greater in autumn and winter compared to spring and summer, as these seasons have more grids with temperatures around 28°C (Fig. 5b, c, e). To validate our hypothesis, we conducted sensitivity experiments under different warming conditions and calculated the PFDs, as well as the changes of IPWP volume (Fig. 5f).
We further compare the PFDs (Fig. 5a-d) and the IPWP expansion pattern (Figure S5) obtained from the sensitivity experiments and our observations. The results from the sensitivity experiments exhibits a high level of consistency with observations, suggesting that the sensitivity experiments can accurately reflect the overall observed change of IPWP volume. Figure 5f Consequently, the capacity for IPWP expansion reaches its maximum of 0.39 x 109 km3 (minimum of 0.31x109 km3) in winter (summer) (green shading in Fig. 5a-d and green bars in Fig. 5f). Comparing the capacity for IPWP volume expansion with the observed IPWP expansion, we find that the seasonal characteristics reflected by the two is generally consistent, except for an overall slight underestimation of IPWP volume change (~ 7%). The capacity for IPWP expansion is largest (smallest) in winter (summer), accordingly the observed expansion of IPWP is also greatest (smallest). The capacity for IPWP expansion is relatively equal in spring and autumn, which aligns with the close observed expansion speeds of the two seasons (Fig. 5f).
Further taking into account the spatial heterogeneity of IPWP warming (i.e., SEUE), the seasonal characteristics of IPWP expansion are consistent with those in the SSUE, but reduces the overall bias of the magnitude of IPWP expansion of all seasons (Fig. 5f). This suggest that spatially uneven warming of IPWP has limited effect on the seasonality of IPWP expansion, but contributes to the accuracy in the estimation of IPWP expansion. In contrast, if we consider the seasonal difference in IPWP warming while keeping the spatially uniform warming assumption (i.e., SPUE), no notable improvements in the estimation of IPWP expansion are observed compared with the SSUE (Fig. 5f). This confirms our previous claim that the small seasonal differences in IPWP warming (Fig. 4) does not contribute to the seasonal diversity of IPWP expansion and weakening seasonality of IPWP volume.
These results suggest that the seasonal differences in capacity for IPWP expansion, which is determined by spatial characteristics of subsurface Indo-Pacific Ocean temperature, is the main reason for seasonal diversity of IPWP volume expansion. Similar sensitivity experiments are also conducted on the lower-layer IPWP below 40m and comparable conclusions are obtained. However, in the case of the lower-layer IPWP, spatially uneven warming plays a more important role in the estimation of IPWP expansion (Figure S6). Another interesting observation is that the seasonal differences in skewness of lower-layer IPWP (Figure S6e) seems to be opposite to those of upper-layer IPWP (Fig. 5e), which corresponds to their inverse long-term changes in IPWP volume seasonality (Fig. 3b). For the Indian Ocean and Pacific Ocean sector, we also conduct similar sensitivity experiments, and the results support that the Indian Ocean sector contributes more to the seasonal difference of the capacity for IPWP expansion (Figure S7, S8).
Possible impacts on East Africa precipitation
The above analysis reveals the seasonal diversity in IPWP volume expansion and consequent weakening seasonality of IPWP volume. An important question is how the weakening seasonality of IPWP volume may affect the climate system. To address this, we take the East African rainfall as an example due to its close relationship with the IPWP 28,37,44. East Africa experiences two rainy seasons, named the long rains (March-April-May, MAM) and short rains (October-November-December, OND)55,56, and the rainfall during the rainy seasons play a crucial role in supporting vegetation growth and agricultural irrigation49,55–57. Previous studies reported a decreasing trend of precipitation in long rains, while precipitation during short rains increases significantly49–51,58, exhibiting an apparent seasonal difference in their long-term change. Is such seasonally different change of East Africa precipitation a result of the weakening IPWP seasonality? To explore this, we regress the atmospheric vertical motion and precipitation over Africa during different seasons onto the corresponding IOWP volume time series (Figure S9), especially focusing on the long rains and short rains (Fig. 6), aiming to examine the potential impact of seasonal differences in IOWP expansion on Africa East Africa precipitation across seasons.
Figure 6a and b show the warm pool volume (black lines) and its expansion trends (red lines) at each longitude during long rains and short rains. The results reveal that in Indian Ocean sector (west of 90°E), the warm pool volume is climatologically larger (smaller) in the long (short) rains. In contrast, the warm pool expansion is slower in the long rains, but faster in the short rains. By regressing Africa precipitation and vertical circulation over Africa onto the corresponding IOWP volume time series, we observe an intensified ascending motion associated with the increased IOWP volume observed over Africa in short rains (Fig. 6d). Considering the larger expansion trend of IOWP during short rains, the strengthening of ascending motion over Africa during short rains is likely a consequence of IOWP expansion. This enhanced ascending motion is favorable for East Africa precipitation. In Fig. 6f, we observe a significant increase in East Africa rainfall that is associated with increased IOWP volume during short rains. However, during long rains, we find the weakened ascending motion and precipitation over Africa related to IOWP volume (Fig. 6c, e). Given the relatively small expansion of IOWP volume during long rains, its impact on African circulation and precipitation may be limited. Precipitation in East African during long rains may also be influenced by the other systems such as western North Pacific SST related to the El Niño-Southern Oscillation44,49.
These results suggest that the different expansion rates of IOWP during the long rains and short rains have varying impacts on East African precipitation. The significant increase of East African precipitation in short rains can be attributed to the substantial expansion of IOWP volume. We conduct similar regression analyses on the vertical circulation and precipitation onto the corresponding IOWP volume for four different seasons (Figure S9), of which the results shows that the varied expansion patterns of IOWP across seasons also have diverse impacts on Africa precipitation in corresponding seasons. Specifically, the larger expansion of IOWP in autumn and winter (the short rains period) is related to enhanced ascending motion and increased precipitation over Africa, comparing with spring (the long rains period) and summer.