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

The Madden-Julian Oscillation (MJO) is the most energetic large-scale intraseasonal atmospheric disturbance, originating in tropical Africa and propagating eastward through the Indian and Pacific basins. Strong boreal winter MJO events force intense winds over the Maritime Continent that generate significant intraseasonal, basin-scale barotropic sea-level variability in the tropical Indian Ocean (4–6 cm), with mass redistribution achieved via fast-propagating barotropic waves adjusting the basin in ~2–3 days. Past studies on global ocean barotropic variability linked to MJO have been rare, and earlier understanding emphasized slower baroclinic adjustments (~2–3 months). The authors report that during boreal winter MJO, oceanic mass rises in the tropical Indian Ocean while simultaneously falling in the Pacific Ocean, and vice versa, producing an intraseasonal Indo-Pacific see-saw with a fulcrum near the Maritime Continent and a quasi-global oceanic response within days. Earth’s rotation varies; changes in angular velocity affect length-of-day, while rotation of the solid Earth around its axis corresponds to polar motion. Exchanges of angular momentum among the solid Earth, core, atmosphere, and ocean drive these signals. In the intraseasonal band, polar motion is mostly forced by the atmosphere, but ocean contributions are significant. Oceanic impacts mainly arise from mass redistribution, with some contribution from mass transport. The large-scale mass redistribution and currents associated with the MJO-induced see-saw are expected to leave a signature in polar motion. This study demonstrates that the MJO-induced see-saw leaves detectable oceanic footprints on polar motion, with oceanic excitations sometimes up to about half the atmospheric magnitude, particularly during early 2013.

Prior work established the dominance of atmospheric forcing on intraseasonal polar motion, with ocean contributions considered secondary, primarily via mass redistribution rather than motion terms. Earlier interpretations emphasized slow baroclinic ocean responses to intraseasonal forcing with adjustment times of months. Studies focusing on MJO-driven barotropic sea level variability documented coherent basin-wide signals in the tropical Indian Ocean and barotropic wave adjustments on timescales of a few days, but global-scale impacts and links to Earth’s rotation were rarely explored. Traditional findings suggested the mass term of oceanic excitation dominates the motion term by factors of 5–10 over many frequencies, including intraseasonal timescales. This paper revisits these views by highlighting a rapid, large-scale barotropic response across the Indo-Pacific and demonstrating comparable mass and motion contributions to polar motion excitation during a strong MJO event.

The study uses a high-resolution global ocean/sea-ice general circulation model (NEMO v3.6, ORCA12 configuration) with ~1/12° horizontal resolution (9 km at equator to 2 km near poles), 75 vertical levels with partial cells, a nonlinear free surface (split-explicit barotropic/baroclinic modes) and z* coordinates. Momentum advection uses a 3rd-order upstream-biased scheme with biharmonic-like dissipation; tracers use a TVD scheme; vertical mixing uses a k-ε GLS closure. Baroclinic and barotropic time steps are 360 s and 12 s, respectively. Forcings include wind, radiative fluxes, air temperature, precipitation, and humidity; atmospheric pressure gradients are omitted (negligible at >~3 days). River runoff and snow are monthly climatologies. Bathymetry is derived from ETOPO1 and GEBCO_08, minimum depth 12 m. Control run: The model is spun up for 30 years with ERA-Interim, then forced with 6-hourly NCMRWF fluxes from January 2009 to August 2019. SST and SSS are weakly restored (2-month timescale) to WOA13 climatologies. The control reproduces Indonesian Throughflow transports and intraseasonal bottom pressure variability. All analyses focus on December–April (boreal winter) and on intraseasonal timescales (30–80 days) isolated via a Lanczos filter. A See-saw Index is defined as the normalized difference of mean equivalent water depth anomaly (EWDA) between the Indian and Pacific basins. Basin net intraseasonal volume fluxes are computed across predefined basin boundaries. Spatial correlation maps are produced by correlating EWDA at a point in the Maritime Continent with EWDA across model grid points; significance is assessed with degrees of freedom estimated for the band-pass filtered series. Sensitivity experiment (MC-EXP): To isolate the role of Maritime Continent winds, a simulation applies observed 6-hourly NCMRWF winds only within 90°–140°E, 32°S–2°N (smoothed to zero at edges over 300 km), with other fluxes prescribed from CORE-II climatology globally. SST and SSS are strongly restored (12 h) to WOA13 to maintain realistic baroclinic structure outside the wind-forced region. Period: 2009–2019, same initial conditions as control. Observations: A network of 82 bottom pressure recorders (BPRs) from multiple programs provides bottom pressure in PSIA converted to equivalent water depth (670 mm water/PSIA). Hourly subsampled records (top of hour) are de-tided using TASK2000, then daily and band-pass filtered (30–80 days) via Lanczos filter. Correlations between a Maritime Continent BPR and all BPRs globally are tested for significance at 90%, with degrees of freedom estimated for band-passed series (DOF ≈ 60 for 10 winters of Dec–Apr daily data), implying |r| > 0.21 for 90% significance. Polar motion excitation: Daily oceanic excitation functions X1 and X2 (equatorial components) are computed from model output, each decomposed into mass (load) and motion (current) terms, by volume-integrating density and velocity fields, accounting for partial cells and ocean depth. Constants include Earth’s radius (R), rotation rate (Ω), and solid-Earth equatorial (A) and polar (C) moments of inertia; scaling factors for surface load yielding (1.44) and core decoupling (1.61) are applied. Intraseasonal components are isolated with a Lanczos filter. Observed polar motion excitations from IERS and non-oceanic contributions (atmospheric and hydrological angular momentum from ESMGFZ) are used to form residuals for comparison with modeled oceanic excitations, focusing on the strong 2012–2013 winter event.

• A coherent intraseasonal see-saw of oceanic mass exists between the Indian and Pacific Oceans during boreal winter MJO events. During positive See-saw Index peaks, the Indian Ocean gains ~1.5 Sv while the Pacific loses ~2.6 Sv, equivalent to a near-uniform basin rise of ~1.0 cm (Indian) and concurrent fall of ~0.8 cm (Pacific). The Southern Ocean closes the volume budget.
• The see-saw adjusts rapidly via barotropic dynamics: the tropical Indian Ocean adjusts within ~2–3 days, and a reversing anticlockwise barotropic circulation around Australia transmits ~2 Sv through the Indonesian straits, into the Southern Ocean (after 1–2 days), and back into the Pacific (after another ~1 day), completing the loop within a few days.
• Spatial correlations show basin-wide rise in EWDA across the tropical Indian coinciding with large-scale fall across much of the Pacific (tropics and southern extratropics) and parts of the Arctic, implying a quasi-global response; the Atlantic shows limited coherent participation.
• Forcing the model only with winds over the Maritime Continent (MC-EXP; ~4% of global ocean area) reproduces the Indo-Pacific see-saw pattern: it explains >70% of intraseasonal EWDA variance in the tropical Indian Ocean and ~15–20% over parts of the tropical and southern Pacific; Arctic and Atlantic impacts are minimal.
• Observations from a sparse BPR network corroborate the see-saw: a Maritime Continent BPR exhibits 4–6 cm peak-to-peak intraseasonal variability; a central Pacific BPR shows ~2–3 cm and is often out of phase, especially during 2011–2012 and 2012–2013. Nineteen of 45 Pacific BPRs (~42%) are significantly anticorrelated with the Maritime Continent BPR; Indian Ocean BPRs oscillate synchronously.
• The geometry of the see-saw preferentially excites the x2 polar motion component. During the strong 2012–2013 event, the oceanic intraseasonal excitation reached magnitudes comparable to the atmospheric contribution but was largely out of phase, damping the atmospheric excitation in observed residuals. The raw intraseasonal excitation standard deviation is ~16 mas, compared to an MJO-induced ocean signature of ~40 mas, necessitating subtraction of non-oceanic signals.
• In 2012–2013, MC-EXP accounts for ~70% of the variance of modeled oceanic excitation (control) and ~50% of the variance of the residual geodetic excitation in x2. Contrary to earlier expectations, the ocean mass and motion terms were similar in magnitude and synchronous during this event, constructively adding to produce a detectable signal up to about half the atmospheric magnitude. This synchronicity is weaker or absent in weaker MJO years.

The study demonstrates that strong boreal winter MJO winds over the Maritime Continent rapidly force a large-scale barotropic response that redistributes mass between the Indian and Pacific Oceans, creating an intraseasonal see-saw. This mechanism challenges the prior view that intraseasonal ocean adjustments occur predominantly via slower baroclinic processes. The near-optimal geometry of the associated circulation, particularly around Australia and across the Indo-Pacific, efficiently excites polar motion (x2 component). During the strong 2012–2013 MJO, the oceanic excitation had magnitude comparable to that of the atmosphere but was largely out of phase, thereby damping the net observed intraseasonal polar motion. The comparable and synchronous contributions of the mass and motion terms during this event contrast with earlier findings that emphasized mass dominance, highlighting that under specific, strong MJO conditions, ocean currents contribute as much as mass redistribution to the polar motion excitation. The findings underscore the importance of accounting for rapid barotropic variability in interpreting basin-scale mass budgets and in understanding short-term Earth rotation changes.

MJO winds acting over a small region (Maritime Continent) can induce rapid, basin-scale barotropic adjustments that produce an Indo-Pacific intraseasonal see-saw in ocean mass within days, extending impacts well beyond the tropics. The resulting large-scale mass redistribution and reversing circulation efficiently excite Earth’s polar motion, with detectable oceanic signatures evident during the strong 2012–2013 event. Oceanic excitation was comparable in magnitude and out of phase with atmospheric excitation, stabilizing polar motion changes. These results revise the prevailing view of intraseasonal ocean response, demonstrate occasions where ocean mass and motion terms contribute similarly, and imply that intraseasonal barotropic variability must be considered in basin mass budgets and Earth rotation studies. As MJO events are projected to intensify and become more erratic, detectable MJO-related polar motion signatures are likely to occur more frequently, motivating expanded observation and modeling of rapid barotropic processes.

• Observational constraints: the bottom pressure recorder network is sparse, limiting spatial coverage and detailed validation, especially in the Pacific and Arctic.
• Forcing simplifications: the model omits atmospheric pressure gradient forcing (assumed negligible beyond ~3 days), which could influence bottom pressure at shorter timescales or regionally.
• Sensitivity experiment design: MC-EXP applies winds only over the Maritime Continent with strong SST/SSS restoring elsewhere; while isolating wind impacts, this setup may suppress realistic coupled baroclinic adjustments and remote wind effects.
• Partial variance capture: MC-EXP explains only ~15–20% of Pacific intraseasonal EWDA variance in many regions; local dynamics and remote forcings outside the Maritime Continent also contribute.
• Event dependence: strong oceanic excitation and mass-motion synchronicity were evident primarily during the strong 2012–2013 MJO; in weaker MJO years, oceanic signals are subdued.
• Geodetic comparison: direct comparison to observed polar motion is complicated by Chandler wobble resonance; assessments rely on excitation functions and residuals after subtracting atmospheric and hydrological contributions, which carry model uncertainties.