A recent paper by AER Principal Scientist Rui M. Ponte and Staff Scientist Mengnan Zhao revisits the nature of the ocean bottom pressure seasonal cycle by leveraging the mounting GRACE‐based bottom pressure record and its assimilation in the ocean state estimates produced by the project for Estimating the Circulation and Climate of the Ocean (ECCO). The work focuses on the mean seasonal cycle from both data and ECCO estimates, examining their similarities and differences and exploring the underlying causes. Despite substantial year‐to‐year variability, the 21‐year period studied (2002–2022) provides a relatively robust estimate of the mean seasonal cycle. Results indicate that the bottom pressure annual harmonic tends to dominate but the semi‐annual harmonic can also be important (e.g., subpolar North Pacific, Bellingshausen Basin). Amplitudes and short‐scale phase variability are enhanced near coasts and continental shelves, emphasizing the importance of bottom topography in shaping the seasonal cycle in bottom pressure. Comparisons of GRACE and ECCO estimates indicate good qualitative agreement, but considerable quantitative differences remain in many areas. The GRACE amplitudes tend to be higher than those of ECCO typically by 10%–50%, and by more than 50% in extensive regions, particularly around continental boundaries. Phase differences of more than 1 (0.5) months for the annual (semiannual) harmonics are also apparent. Larger differences near coastal regions can be related to enhanced GRACE data uncertainties and also to the absence of gravitational attraction and loading effects in ECCO. Improvements in both data and model‐based estimates are still needed to narrow present uncertainties in ocean bottom pressure estimates.
Figure: Mean ocean bottom pressure anomalies in units of equivalent water thickness (cm) for seasons DJF (a, e), MAM (b, f), JJA (c, g) and SON (d, h), derived from GRACE observations (left) and an ECCO state estimate (right). The mean seasonal anomalies are calculated over the period 2002–2022.
Citation: How Well Do We Know the Seasonal Cycle in Ocean Bottom Pressure?
R. M. Ponte, M. Zhao, M. Schindelegger
Earth and Space Science, 11(17), e2024EA003661, 2024. https://doi.org/10.1029/2024EA003661