Madden-Julian oscillation winds excite an intraseasonal see-saw of ocean mass that affects Earth's polar motion

The Madden-Julian Oscillation (MJO) is a dominant intraseasonal atmospheric phenomenon originating in tropical Africa and propagating eastward across the Indian and Pacific Oceans. Its influence extends beyond the tropics, impacting various aspects of the Earth system. Previous research has focused on the MJO's effects on atmospheric circulation and precipitation, but its influence on the ocean, especially at intraseasonal timescales, remains less understood. This study investigates the MJO's impact on ocean mass redistribution and its subsequent effect on Earth's polar motion. The research question centers on determining the extent to which the MJO's intense winds over the Maritime Continent drive a large-scale see-saw of oceanic mass between the Indian and Pacific Oceans and whether this see-saw has a detectable impact on Earth's polar motion. Understanding this interaction is crucial for improving our understanding of Earth's climate system and improving weather forecasting. The importance of this study lies in its potential to refine our comprehension of the complex interactions between the atmosphere and ocean at intraseasonal timescales and the effects of these interactions on Earth's rotation. This will lead to a more precise representation of climate processes and improve the accuracy of geophysical predictions. The novelty of this research lies in directly linking the MJO, a known atmospheric driver, to intraseasonal changes in global-scale ocean mass, ultimately leading to observable impacts on Earth's polar motion. This expands our understanding of the coupled atmosphere-ocean system beyond traditional, longer-timescale interactions.

Past research on the MJO has primarily focused on its atmospheric characteristics, including its impact on weather patterns and climate variability. Studies have detailed the MJO's eastward propagation, its associated convective activity, and its influence on regional rainfall patterns. However, investigations into its direct influence on large-scale ocean dynamics, particularly on intraseasonal timescales, have been limited. While some research has explored the MJO's interaction with the ocean through slower baroclinic processes (taking months to manifest), the immediate and significant barotropic response highlighted in this study represents a notable advancement. Studies focusing on the MJO's impact on sea level have shown localized effects in the tropical Indian Ocean, but this study extends that understanding to a basin-wide and even global scale. The connection between oceanic mass redistribution and Earth's polar motion has been established through past research, but the specific mechanism linking MJO winds to this phenomenon via rapid barotropic ocean response has not been previously demonstrated. This study fills a critical gap in our understanding by illustrating a direct link between a prominent atmospheric phenomenon and global-scale oceanic mass fluctuations with relatively rapid response times.

This research employed a combination of numerical modeling and observational data analysis to investigate the MJO's influence on ocean mass and Earth's polar motion. The primary tool was the Nucleus for European Modelling of the Ocean (NEMO) version 3.6, a state-of-the-art ocean general circulation model (OGCM). The model's ORCA12 configuration provided high spatial resolution, resolving crucial features such as the Indonesian straits, which play a vital role in the exchange of water between the Indian and Pacific Oceans. A control run of NEMO covering the period 2009-2019 was conducted, using high-resolution atmospheric forcing data from the National Centre for Medium Range Weather Forecasting (NCMRWF) to drive the ocean model. Sea surface temperature (SST) and salinity (SSS) were weakly restored to climatological values to maintain realistic ocean conditions. To isolate the effects of MJO winds over the Maritime Continent, a sensitivity experiment (MC-EXP) was performed, restricting wind forcing to this specific region while maintaining climatological forcing for other variables. The model outputs were used to analyze intraseasonal variability in equivalent water depth, volume fluxes between ocean basins, and barotropic circulation patterns. To validate the model findings, observational data from a network of in-situ bottom pressure recorders (BPRs) deployed across the global oceans were analyzed. These data, after correcting for tidal effects, provided independent measurements of sea level variability, allowing for comparison with the model's predictions. The influence of the MJO on Earth's polar motion was assessed by calculating polar motion excitation functions from both the model output and International Earth Rotation and Reference Systems Service (IERS) observations. These functions quantify the contributions of atmospheric angular momentum (AAM), hydrological angular momentum (HAM), and oceanic angular momentum to polar motion. Subtracting AAM and HAM from the IERS data allowed for isolating the oceanic contribution to polar motion changes and comparing it to the model-derived estimates. Statistical methods, such as correlation analysis and variance decomposition, were employed to analyze the relationships between various variables and assess the significance of observed patterns. The degree of freedom (DOF) for band-passed time series was carefully estimated to determine the significance levels for correlation coefficients.

The study revealed a robust intraseasonal see-saw of oceanic mass in the Indo-Pacific basin, driven by MJO winds over the Maritime Continent. This see-saw is characterized by a periodic rise in equivalent water depth in the tropical Indian Ocean concurrent with a fall in the Pacific Ocean, and vice versa. The model simulations demonstrated that this see-saw is a basin-wide phenomenon, not limited to the tropics, extending into the extratropics and even influencing the Arctic Ocean. Surprisingly, the model showed that the mass field adjusts to the MJO forcing within just a few days, a much faster response than previously assumed based on baroclinic wave propagation. The sensitivity experiment (MC-EXP) confirmed that winds over the Maritime Continent were the primary drivers of this intraseasonal see-saw, even though this area covers only a small fraction of the global ocean surface. Analysis of the bottom pressure recorder data corroborated the model results, showing an anticorrelation of sea level variability between the Indian and Pacific Oceans. The study's most significant finding was the detection of the MJO-induced oceanic see-saw's impact on Earth's polar motion. Analysis revealed a detectable oceanic signal in the polar motion excitation during the strong 2013 MJO event, which was comparable in magnitude to the atmospheric contribution. Importantly, the oceanic signal was largely out of phase with the atmospheric signal, suggesting the ocean can partially counteract the atmospheric influence on polar motion. The MC-EXP showed that the MJO winds over the Maritime Continent alone could account for a substantial portion of the observed oceanic signal in polar motion during 2013. This was notable because the MC-EXP only captured a relatively small percentage of the total variance in equivalent water depth in certain regions of the Pacific Ocean. This suggests that the mass and motion components of the oceanic excitation function acted in concert to produce a stronger signal during this event.

The findings of this study provide compelling evidence for a previously unrecognized mechanism linking the MJO to global-scale ocean dynamics and Earth's polar motion. The rapid barotropic response of the ocean to MJO winds highlights the crucial role of fast-propagating waves in redistributing oceanic mass at intraseasonal timescales. The fact that a relatively small region of intense wind forcing (Maritime Continent) can induce such a significant large-scale oceanic response has significant implications for our understanding of global ocean dynamics. The observation that this oceanic see-saw demonstrably affects Earth's polar motion underscores the importance of considering ocean dynamics when interpreting Earth's rotation variations. The finding that the oceanic influence can be comparable to, and even partially offset, the atmospheric contribution to polar motion suggests that accurate modeling of Earth orientation requires a comprehensive representation of both atmospheric and oceanic processes. The strong 2013 MJO event provided an ideal case study for detecting this subtle effect, highlighting the need to further investigate the interplay between atmospheric forcing and ocean dynamics under various MJO conditions. Future research should focus on validating these findings using more extensive observational datasets and exploring the sensitivity of the observed effects to model parameters and different atmospheric forcing scenarios.

This study demonstrates that MJO winds over the Maritime Continent induce a rapid, global-scale see-saw in oceanic mass, significantly influencing Earth's rotation. The findings highlight the crucial role of fast barotropic ocean dynamics in the coupled atmosphere-ocean system and emphasize the importance of considering these processes when analyzing Earth's rotation variations. Future research should focus on investigating this phenomenon under different MJO conditions and with enhanced observational datasets. Further exploration of the interplay between atmospheric forcing and ocean dynamics is crucial for improving weather forecasting and climate modeling.

The relatively sparse network of bottom pressure recorders used for validation limits the spatial coverage and potentially affects the accuracy of the observational analyses. The model's reliance on atmospheric forcing data (NCMRWF) introduces uncertainties associated with the accuracy of the atmospheric data itself. The strong restoration of SST and SSS in the MC-EXP might have slightly altered the baroclinic structure of the ocean, which could affect the results. Further studies using more extensive observational data and improved atmospheric forcing would strengthen the robustness of the findings.