The research team, led by Taufan Wiguna of the Bandung Institute of Technology and the National Research and Innovation Agency of Indonesia, together with colleagues from IPB University, focused on two elegant diagnostic variables. The first is the depth of the 26.5 degree Celsius isotherm, abbreviated T26.5, which marks the top of the thermocline where warm surface water gives way to cooler water below. The second is the depth of the 34.4 practical salinity unit isohaline, or S34.4, a boundary in the salinity profile that separates fresher near-surface water from saltier water masses of Pacific origin. When these boundaries shoal, or move closer to the surface, cooler and saltier water is brought within reach of wind-driven mixing and upwelling; when they deepen, that supply is cut off.

To track these boundaries through time, the team used the GLORYS12V1 global ocean physics reanalysis from the Copernicus Marine Service, a state-of-the-art dataset that blends ocean model simulations with observations at a horizontal resolution of one-twelfth of a degree. Monthly anomalies of T26.5 and S34.4 were computed relative to the corresponding monthly climatological means over the full 1993 to 2025 analysis period, a step that removes the seasonal cycle and isolates the low-frequency signal of interest. The resulting anomaly fields were then passed through a 13-year low-pass filter, a mathematical treatment that suppresses anything oscillating faster than the interdecadal band and allows the slow IPO signal to emerge cleanly from the noise of El Niño, the monsoon, and other shorter-lived phenomena.

The IPO itself was quantified using the Tripole Index, derived from the NOAA Extended Reconstructed Sea Surface Temperature version 5 dataset and maintained by the NOAA Physical Sciences Laboratory. This index captures the characteristic seesaw pattern of the IPO, in which sea surface temperature anomalies in the central tropical Pacific vary out of phase with those in the northwestern and southwestern Pacific. By correlating the filtered IPO index with the filtered isotherm and isohaline depth anomalies, and by examining lagged cross-correlations to test whether the ocean response follows the climate index in time, the researchers could map where and how strongly the interdecadal pulse penetrates into the Maluku Sea.

The results are strikingly consistent. Across the regions analyzed, the correlation between the IPO and the depth of the 26.5 degree Celsius isotherm ranged from negative 0.73 to negative 0.83, while the correlation with the 34.4 PSU isohaline depth ranged from negative 0.57 to negative 0.72. These are strong relationships by the standards of ocean-climate teleconnection studies, and the negative sign carries a clear physical meaning: when the IPO is in a positive phase, the thermocline and halocline in the Maluku Sea tend to shoal, bringing cooler, saltier water upward and creating conditions favorable for upwelling. In the negative phase, the boundaries deepen and the upwelling-favorable configuration relaxes. Notably, the temperature boundary responded more coherently than the salinity boundary, suggesting that heat content in the upper ocean is the more faithful recorder of the interdecadal signal in this basin.

What makes the study particularly valuable is its honesty about what the IPO does not control. The team also examined horizontal divergence, a dynamical quantity related to the vertical stretching of surface waters, and satellite-derived chlorophyll-a, a proxy for phytoplankton biomass that often blooms where nutrient-rich water upwells. Here the correlations were weaker and spatially patchy, and phase composites, which average conditions during positive and negative IPO states separately, revealed far less consistent differences. In other words, the clearest expression of low-frequency IPO variability in the Maluku Sea is written in the thermohaline structure itself, not necessarily in the biological response at the surface. This distinction matters for anyone hoping to predict fisheries productivity from climate indices alone, because a shoaling thermocline does not automatically translate into a chlorophyll bloom.

To probe the vertical dimension of the response, the researchers turned to monthly vertical velocity fields from the ECCO2 Cube92 data synthesis, accessed through the Asia-Pacific Data-Research Center. Composites of vertical motion during different IPO phases revealed phase-dependent patterns of upward and downward movement that differed among subregions of the sea and among depth ranges. The upwelling system of the Maluku Sea, it turns out, does not respond to the interdecadal forcing as a single coherent block. Instead, the vertical-motion response is modulated by local circulation features, including the intermediate western boundary current that recent moored observations and numerical modeling have shown connects Pacific circulation to the Indonesian Throughflow through this very basin.

The Maluku Sea occupies a special position in the global ocean conveyor. It is one of the eastern gateways of the Indonesian Throughflow, receiving North Pacific water masses through passages between Halmahera and Sulawesi and exporting them toward the Banda Sea and ultimately the Indian Ocean. Because the thermocline waters carried by the Throughflow help set the temperature and salinity of the Indian Ocean’s upper layers, decadal changes in the Maluku Sea’s stratification can ripple far beyond Indonesian waters. Previous work has documented warming trends in the Indonesian seas and interannual to decadal shifts in the Throughflow’s vertical profile, but the specific link between the IPO and the Maluku Sea’s thermohaline boundaries had remained largely unquantified until now.

Methodologically, the study is a careful exercise in teleconnection analysis. The choice of a 13-year low-pass filter is well matched to the dominant IPO timescale, and the use of lagged cross-correlation acknowledges a well-known pitfall in climate science: correlated signals do not by themselves establish causation, and spurious correlations can arise when two filtered time series share energy in overlapping frequency bands. By testing multiple lags and by comparing the strong thermohaline response against the weaker divergence and chlorophyll responses, the authors build a case that the IPO-isotherm relationship is physically meaningful rather than a statistical artifact. The phase composites add a complementary line of evidence, showing that the distribution of T26.5 and S34.4 anomalies genuinely differs between IPO regimes rather than merely correlating with them in a linear sense.

The practical implications extend from climate science into resource management. The Banggai upwelling region in the southern Maluku Sea supports locally important fisheries, and interdecadal shifts in thermocline depth could modulate the nutrient supply that underpins the marine food web over timescales relevant to management planning. Because the IPO swings slowly, its state provides a degree of predictability that faster modes such as ENSO cannot offer at decadal horizons. The datasets underpinning the study, including the GLORYS12 reanalysis, the Copernicus biogeochemistry hindcast, the NOAA Tripole Index, and the ECCO2 vertical velocities, are all publicly available, and the derived regional time series can be requested from the corresponding author. Funded by Indonesia’s National Research and Innovation Agency and Institut Teknologi Bandung, the work exemplifies how open reanalysis products can illuminate climate dynamics in data-sparse seas. As the Pacific potentially transitions between IPO phases in the coming decades, the thermohaline boundaries of the Maluku Sea will continue to rise and fall in response, a slow tide of climate change written not in sea level but in the depth of an isotherm.

Subject of Research: The influence of the Interdecadal Pacific Oscillation on upper-ocean thermohaline structure and upwelling variability in the Maluku Sea, Indonesia

Article Title: Interdecadal pacific oscillation and upwelling in the Maluku Sea, Indonesia: Evidence from isotherm and isohaline variability

Article References: Wiguna, T., Putri, M. R., Rachmayani, R., Zuraida, R., & Atmadipoera, A. S. (2026). Interdecadal pacific oscillation and upwelling in the Maluku Sea, Indonesia: Evidence from isotherm and isohaline variability. Ocean Dynamics, 76(10), Article 100. https://doi.org/10.1007/s10236-026-01854-z

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01854-z

Keywords: Maluku Sea, Interdecadal Pacific Oscillation, upwelling, thermocline, isotherm depth, isohaline depth, Indonesian Throughflow, GLORYS12 reanalysis, ocean dynamics, climate variability, chlorophyll-a, Indonesia