An Indo-Pacic see-saw wobbles the Earth at intraseasonal timescales

Strong large-scale winds can relay their energy to the ocean bottom and elicit an almost immediate intraseasonal barotropic (depth independent) response in the ocean. The intense winds associated with the Madden-Julian Oscillation (MJO), over the tropical interface between the Indian Ocean and the Pacic Ocean (popularly known as Maritime Continent) generate signicant basin-wide intraseasonal barotropic sea level variability in the tropical Indian Ocean. Here we show, using an ocean general circulation model and a network of in-situ bottom pressure recorders, that the concerted barotropic response of the Indian and the Pacic Ocean to these winds leads to an intraseasonal see-saw of oceanic mass in the Indo-Pacic basin. This global-scale mass shift is unexpectedly fast, as we show that the mass eld of the entire Indo-Pacic basin is dynamically adjusted to MJO in a few days. We also explain how this near-global-scale MJO-induced oceanic phenomenon is the rst signature from a climate mode that can be isolated into the Earth polar axis motion, in particular during the strong see-saw of early 2013.

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 globalscale mass anomaly with a 45°-tilted ellipsoid 10 . 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 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 Paci c 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 Continent 3 . To determine to which extent the MJO winds also drive the large-scale variability in the Paci c 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-Paci c 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-Paci c 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 Paci c Ocean, most probably due to dominant local dynamics 13,14 .
Whereas the MC-EXP largely captures the variance in the TIO (with values >70%) 3 , its impact is also signi cant over the Paci c 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 Paci c Ocean. It is remarkable that the wind forcing from such a small region (~4% of global ocean coverage) casts such a large-scale in uence and excites ~15-30% of the intraseasonal oceanic mass uctuations over a large part of the tropical Paci c.
During a positive cycle of the index, the MJO winds drive ~2 Sv of Paci c waters into the Indian Ocean through the ITF. An equivalent ux is subsequently ushed out into the Southern Ocean after ~1-2 days. The Southern Ocean conveys it eastward and subsequently injects ~2 Sv into the Paci c 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 continent 15 . It is this intraseasonal reversing circulation that drives the Indo-Paci c 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. 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 Paci c 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). 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 -χ 1 for rotation around an axis at the Greenwich meridian (x-axis) and χ 2 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 χ 2 . Due to the Chandler wobble resonance that dominates the polar motion 16,17 , 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 sources 18 . 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 nonoceanic 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.

Summary
The MJO winds, acting over ~4% of the Earth's surface, induce a global-scale ocean mass redistribution, which in turn affects the Earth rotation. This entire phenomenon is schematically illustrated in Fig.4b. The strong boreal winter MJO winds over the Maritime Continent elicit an intraseasonal large-scale barotropic response from the Indian and the Paci c Ocean whose extent, unlike previously known oceanic manifestations of the MJO, is not only limited to the tropics but also reaches the extratropics. The winds induce a barotropic circulation around the Australian continent and its periodic reversal at intraseasonal timescales is manifested as a see-saw in the oceanic mass within the Indo-Paci c basin. The large-scale oceanic mass redistribution in the Indo-Paci c basin, accompanied by large-scale to-and-fro transports in the two basins associated with this see-saw, bene ts from a favourable geometry to excite polar motions, allowing the strong 2013 MJO event to be the rst climate mode signature detected in the polar motion signal through the ocean.
The magnitude of the see-saw reported here implies that the intraseasonal barotropic variability of the ocean needs to be carefully considered when interpreting the mass budget of the various ocean basins.