the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Chaotic oceanic excitation of low-frequency polar motion variability
Abstract. Studies of Earth rotation variations generally assume that changes in non-tidal oceanic angular momentum (OAM) manifest the ocean's direct response to atmospheric forces. However, fluctuations in OAM may also arise from chaotic intrinsic ocean processes that originate in local nonlinear (e.g., mesoscale) dynamics and can map into motions and mass variations at basin scales. To examine whether such random mass redistributions effectively excite polar motion, we compute monthly OAM anomalies from a 50-member ensemble of eddy-permitting global ocean/sea-ice simulations that sample intrinsic variability through a perturbation approach on model initial conditions. The resulting OAM (i.e., excitation) functions, χ̂O, are examined for their spread, spectral content, and role in the polar motion excitation budget from 1995 to 2015. We find that intrinsic χ̂O signals are comparable in magnitude to the forced component at all resolved periods except the seasonal band, amounting to ∼46 % of the total oceanic excitation (in terms of standard deviation) on interannual time scales. More than half of the variance in the intrinsic mass term contribution to χ̂O is associated with a single, global mode of random bottom pressure variability, likely generated by nonlinear dynamics in the Drake Passage. Comparisons of observed interannual polar motion excitation against the sum of known surficial mass redistribution effects are sensitive to the representation of intrinsic χ̂O signals: Reductions in the observed excitation variance can be as high as 68 %, or as low as 50 % depending on the choice of the ensemble member. Chaotic oceanic excitation thus emerges as a new factor to consider when interpreting low-frequency polar motion changes in terms of core-mantle interactions or employing forward-modeled OAM estimates for Earth rotation predictions.
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RC1: 'Review Report on esd-2024-21', Anonymous Referee #1, 28 Aug 2024
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I read with interest the manuscript "Chaotic oceanic excitation of low-frequency polar motion variability" submitted by Börger et al. for possible publication in Earth System Dynamics. The paper utilizes the OCCIPUT large ensemble with 50 realizations of time-variable eddy-permitting ocean mass and flow fields to calculate effective ocean angular momentum functions characterizing the excitation of changes in the solid Earth's orientation with respect to inertial space. The paper is very well written und certainly fits into the scope of the journal. I recommend this work for publication as soon as a number of comments have been reasonably well addressed.
(1) The analysis presented in this paper is based on the SPACE2018 series of Earth Orientation Parameters as processed at the JPL. Authors should explain in more detail why SPACE2018 is used here instead of the associated COMB2018 series, or a more recent reprocessing of the same data (i.e., COMB2019). Authors should also consider to use the newly published EOP series from the ITRF2020 computation that are operationally updated as EOP 20 C04 by scientists from the Paris Observatory. In any case, it needs to be discussed in the article how a particular choice of the EOP series might affect the interpretations of the results presented here.
(2) Time-variable gravity field representations from GRACE that are additionally augmentend by SLR and DORIS observations to extent the time-series have been used only rarely in Earth Orientation Parameter research. In view of the cautious comments provided by the authors in line 240, I propopse to explicitly show the hydrological angular momentum functions derived from GRACE+SLR+DORIS for the whole time-period 1995-2015, and compare it with GRACE-based excitation functions -- ideally derived from publicly available Level-3 products, like the Cost-G combination solution available via gravis.gfz-potsdam.de to make results traceable -- and an independent model-based hydrologic excitation function published elsewhere. Please note that a detailed discussion of the contributions from Greenland and Antarctica is not necessary at this point.
(3) It is quite suprising to see that the largest interannual surface mass variations outside Greenland and Antarctica are found on the Malakka peninsula in South-East Asia. This is not really intuitive from a hydrometeorological perspective and calls for further investigations. In particular, it should be thoroughly checked if poorly treated tectonic signals associated with the 2004 Sumatra-Andaman earthquake (and later events in neigboring areas) are responsible for this feature. Please report in detail about any modifications made to the GRACE+SLR+DORIS processing, which is not yet really well covered in the scientific literature.
(4) Authors speculate in both abstract and conclusions about possible implications of this work for EOP prediction, but fail to elaborate it further in the article. I suggest to remove this comment from the abstract in order to avoid raising unrealistic expectations with the reader. In any case, rigorously assessing the potential consequences for EOP predictions should be left for future study.
Citation: https://doi.org/10.5194/esd-2024-21-RC1
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