The Madden-Julian Oscillation (MJO) is the most energetic large-scale intraseasonal atmospheric disturbance1,2. It originates in tropical Africa, travels eastward towards Indian and Pacific basins and returns to Africa every 30-80 days. Strong boreal winter MJO is associated with intense winds over the Maritime Continent, the tropical interface between the Indian and the Pacific Ocean, generating significant intraseasonal barotropic sea level variability in the entire tropical Indian Ocean (TIO)3. It amounts to a basin-scale signal of 4-6 cm in the entire TIO, with the mass redistribution achieved through fast-propagating barotropic waves adjusting the TIO in ~2-3 days3. We report that during boreal winter MJO, the rise of oceanic mass in the TIO is concurrent with a fall of oceanic mass in the Pacific Ocean, and vice-versa at intraseasonal timescales. The periodic reversal of MJO winds therefore leads to an intraseasonal see-saw in the oceanic mass in the Indo-Pacific basin, with the fulcrum stationed over the Maritime Continent. Due to the extent of Pacific Ocean – well beyond the tropics – this fast barotropic dynamics incites a far-reaching, quasi-global oceanic response to the MJO encompassing tens of thousands of kilometers within a few days. This discovery challenges the earlier understanding of a response only via slow-propagating baroclinic waves thereby adjusting the density field of the basins in ~2-3 months4,5 .
The see-saw generation involves large-scale mass redistribution and currents across and within the Pacific and Indian basins. The associated global-scale angular momentum, coming both from a near-global rotation of the ocean or a global-scale mass redistribution, is expected to leave its signature in the Earth’s rotation6,7. The Earth’s rotation is not constant, and presents fluctuations in a broad range of frequencies8,9. Most of those signals come from the variability in the angular momentum of the Earth’s core and the atmosphere8. The signature from the ocean is more elusive, because the geometry of the ocean, interspersed with continental landmasses, is not optimal to generate a strong global angular momentum fluctuation associated with an observable impact on the Earth’s rotation. The polar motion, i.e. the wobble of the solid Earth around its rotation axis, has an even less favorable geometry: it can be excited by either a global current corresponding to a rotation around an axis at the equator or a global-scale mass anomaly with a 45°-tilted ellipsoid10. An ocean impact over polar motion can thus only be generated by strong global-scale dynamics, with a very particular geometry. As a consequence, though the ocean is believed to be a major driver of the polar motion, there is hitherto only little evidence of climate modes excitation of the polar motion through the ocean. Nevertheless, our study demonstrates that the MJO-induced see-saw, being a near-global process, does leave oceanic footprints on the polar motion.
Revelation of the see-saw. The see-saw is evidenced through a high-resolution global ocean circulation model, Nucleus for European Modelling of the Ocean (NEMO)11, which can resolve adequately the narrow Indonesian straits between the Indian and Pacific basins. The control run (Methods) covers the 2009-2019 period and captures fairly well the Indonesian Throughflow (ITF) transports (Supplementary Fig.S1) and the observed ocean mass variability at intraseasonal timescales (Supplementary Fig.S2). This whole study focuses on intraseasonal timescales (30-80 days band) during boreal winter (December-April) months. We define a See-saw Index as the normalized difference of mean Equivalent Water Depth Anomaly (EWDA) from the control run between two equatorial boxes ((65°E:75°E, 5°S:5°N) and (155°W:165°W, 5°S:5°N)) in the Indian and Pacific Ocean. A positive index indicates a high water level in the Indian Ocean and a low water level in the Pacific Ocean. In Fig.1a, the time-series of the See-saw Index (shaded grey) and the net volume flux into the Indian (red line) and the Pacific (blue line) Ocean are plotted for the period 2009-2019. The See-saw Index variability is strongest during 2011-12 and 2012-13 - two winters of strong MJO activity (Fig.1c) over the Maritime Continent. Most of the time, and in particular during intense MJO events, the two volume fluxes are in opposition of phase, i.e., a net inflow of water in the Indian Ocean is accompanied by a net outflow of water from the Pacific Ocean and vice versa, thereby displaying characteristics of a see-saw in the Indo-Pacific basin. As expected, the maximum transports occur at the times of sign change of the See-saw Index (Fig.1b). During a strong positive (negative) See-saw Index peak, ~1.5 Sv (2.6 Sv) [1 Sv = 106 m3s-1] of water is gained (lost) in the Indian (Pacific) Ocean, which is equivalent to a spatially uniform basin rise (fall) of ~1.0 cm (0.8 cm) of EWD in the Indian (Pacific) Ocean (Supplementary Fig.S3).
To explore the spatial extent of this see-saw, the EWDA over the Maritime Continent (at the location of the yellow star on Fig.2a) is correlated with the EWDA at all model grid locations during December-April. Interestingly, a basin-wide rise in EWD in the TIO is accompanied by a large-scale fall in EWD extending over the tropics, the southern extratropics and the northern edge of the Pacific Ocean and over most parts of the Arctic Ocean (Fig.2a). In contrast, only isolated small patches in the Atlantic Ocean participate in this coherent dance of oceanic mass. It is striking to discover that a large-scale see-saw of oceanic mass encompasses the vast majority of the world ocean.
Role of MJO. The large-scale EWDA variability in TIO is driven by MJO winds over the Maritime Continent3. To determine to which extent the MJO winds also drive the large-scale variability in the Pacific Ocean, a sensitivity model experiment (MC-EXP; Methods) is run for the 2009-2019 period, with wind forcing restricted to the Maritime Continent (black box in Fig.2b). The spatial correlation of EWDA over the Maritime Continent from MC-EXP during each December-April with the same at all model grid points is plotted in Fig.2b. The correlation pattern in the Indo-Pacific basin is, to a large extent, similar to the correlation pattern obtained from the control run (Fig.2a). So, the intraseasonal see-saw in the Indo-Pacific basin persists even if the model is forced only by winds over the Maritime Continent. Note that the signature is not consistent with what is observed in the EWDA in the Arctic and North Pacific Ocean, most probably due to dominant local dynamics13,14.
Whereas the MC-EXP largely captures the variance in the TIO (with values >70%)3, its impact is also significant over the Pacific Ocean (Fig.2c). The winds over the Maritime Continent alone can generate as much as ~30% of the variance in EWDA over the tropical and southern Pacific Ocean. It is remarkable that the wind forcing from such a small region (~4% of global ocean coverage) casts such a large-scale influence and excites ~15-30% of the intraseasonal oceanic mass fluctuations over a large part of the tropical Pacific.
During a positive cycle of the index, the MJO winds drive ~2 Sv of Pacific waters into the Indian Ocean through the ITF. An equivalent flux is subsequently flushed out into the Southern Ocean after ~1-2 days. The Southern Ocean conveys it eastward and subsequently injects ~2 Sv into the Pacific Ocean after another ~1 day, thereby closing this anti-clockwise circulation around the Australian continent (Supplementary Fig.S4). This barotropic circulation is schematically illustrated in Fig.2d. As expected, the circulation reverses its direction during the negative phase of the see-saw. This intraseasonal circulation occurs over and above a permanent anti-clockwise barotropic circulation around the Australian continent15. It is this intraseasonal reversing circulation that drives the Indo-Pacific see-saw.
Observational imprint of the see-saw. We investigated the imprint of the intraseasonal see-saw through the Bottom Pressure Recorder (BPR) network, although this network is very sparse. Fig.3a shows the evolution of intraseasonal EWDA from two BPRs, one located in the Maritime Continent (BPR-MC; red line) and another one in the central Pacific Ocean (BPR-PAC; blue line) during boreal winters of 2009-2019 (see Methods for data processing). BPR-PAC is often out-of-phase with BPR-MC, particularly in 2011-12 and 2012-13 when the MJO wind stress was strong over the Maritime Continent. The variability in the Indian Ocean amounts to 4-6 cm peak-to-peak, that of the Pacific Ocean is ~2-3 cm - half compared to the Indian Ocean. EWDA at BPR-MC was correlated with EWDA from all available BPRs globally (Fig.3b). All the Indian Ocean BPRs synchronously oscillate, whereas many of the Pacific BPRs (~42% of all analyzed Pacific BPRs) and all BPRs in the Arctic Ocean are anti-correlated with the Indian Ocean BPRs. The observed features are in line with the model results, and consistent with what is expected from the existence of a large-scale see-saw between the Indian and the Pacific basins.
See-saw impact on polar motions.
The geometry of the see-saw circulation (Fig.2d) is near optimal for generating a large signature in the polar motion excitation. In addition, the North-South asymmetry of the global-scale mass distribution anomaly also impacts the polar motion. The excitation of the polar motion is classically estimated using excitation functions – χ1 for rotation around an axis at the Greenwich meridian (x-axis) and χ2 for rotation around an axis that passes through the Indian Ocean at 90ºE (y-axis) – representing the amount of additional rotation provided by the ocean (Methods). Considering the geometry of the currents shown on Fig.2d, the see-saw motion mostly impacts the polar motion through χ2. Due to the Chandler wobble resonance that dominates the polar motion16,17, it is not possible to directly compare our model-derived estimates with polar motion observation. However, we can compute the excitation functions required to generate the observed polar motion during the strong see-saw of 2012-13.
Detection of 2012-13 event. The ocean is only one of the contributors to intraseasonal polar motion excitation - the atmosphere and the hydrology being the other sources18. An oceanic signal can only be separated from the climate noise if it is sensibly larger in the excitation than the non-oceanic contributions, or if we can correct the observed excitation with such precision that the residuals are notably smaller than the oceanic contribution. When dealing with intraseasonal excitation, the standard deviation, estimated over the last 10 years, is at the level of ~16 milliarcseconds (mas), to be compared with the 40 mas of the MJO-induced ocean signature, which makes it necessary to subtract the non-oceanic signal. The raw observed excitation during 2012-13 is plotted in Fig.4a, together with the residuals when the non-oceanic signals are subtracted. We observe a strong oceanic signal in early 2013. The MC-EXP captures about half of this signal, but the agreement in amplitude and phase between the residual excitation and the ocean excitation demonstrates that the small region of the Maritime Continent is indeed able to excite the Earth wobble to a detectable level during the strong MJO events.