Contrasting Long-Term Trends in Near-Surface and Subsurface Indo-Pacific Warm Pool Volume Seasonality

Abstract The Indo-Pacific warm pool (IPWP) plays an important role in modulating the global climate system. Recent studies have discovered changes in the seasonality of surface IPWP under climate change. However, it remains unclear whether such findings extend to the three-dimensional IPWP body, which is of greater importance given the crucial role of subsurface ocean in climate systems. In this study, we find that the near-surface (0–60 m) IPWP expands faster in boreal winter than in boreal summer, while this pattern reverses as depth increases. This indicates vertically opposite responses of IPWP seasonality to climate change. While the near-surface IPWP seasonal cycle amplitude weakens significantly (−1.03 × 10 4 km 3 decade −1 ), that of the subsurface (60–100 m) IPWP exhibits a significant increasing trend (+0.4 × 10 4 km 3 decade −1 ). This vertically contrasting response of the IPWP seasonality to climate change is primarily due to the opposite seasonal cycle of the capacity for IPWP expansion between the near-surface and subsurface layers, which is determined by the seasonal variations of the spatial patterns of climatological Indo-Pacific temperature. The above results indicate the complex change in seasonal variation of the 3D IPWP body under greenhouse warming, which may have important implications for the seasonal tropical precipitation. These findings provide insights for understanding and predicting the seasonal migration of the climate system under future climate change. Significance Statement Most previous studies on Indo-Pacific warm pool (IPWP) have focused solely on its surface expansion, overlooking changes in the overall 3D IPWP body size. To address this gap, we dig into the impact of change on the 3D IPWP size, with a specific focus on the changes in IPWP volume seasonality. In this study, we discover that the seasonality of near-surface (0–60 m) and subsurface (60–100 m) IPWPs responds to climate change conversely. These opposite trends in near-surface and subsurface IPWP seasonality are primarily due to the vertically contrasting seasonal cycle of capacity for IPWP expansion. The study emphasizes the seasonality change in the 3D IPWP body, not just in the surface, which could exert diverse impacts on tropical precipitation in different seasons.