Bottom pressure interannual variability in the Southern Ocean

1. Bottom pressure interannual variability in the Southern Ocean Rui M. Ponte, Christopher G. Piecuch @ AER* and the ECCO group ECCO Meeting @ CalTech (Oct 31 - Nov 2, 2012 *Atmospheric and Environmental Research, Inc. Lexington, Massachusetts, USA

2. Motivation and main points • Is there more to large-scale interannual sea level variability than steric height changes? • Enhanced ocean bottom pressure (OBP) fluctuations can now be observed from space in the Southern Ocean • OBP is well correlated with sea level and needs to be considered when interpreting large-scale sea level in terms of heat content, steric height changes • What is the nature of the dynamics and forcing in regions with enhanced OBP changes? • Main forcing provided by changes in wind stress curl associated with climate modes such as the Antarctic Oscillation • Motions along grad(H/f) balance for the most part the wind vorticity input, with baroclinic processes also involved, in contrast with intraseasonal dynamics

3. Sea level vs. bottom pressure Standard deviations based on ECCO and data fields smoothed over 750 km and low-pass filtered to include periods > 1 year • Large-scale OBP and sea level variability similar in magnitude and spatial patterns • Remarkable agreement between ECCO and GRACE estimates of OBP variability

4. ECCO vs. data comparison • RMS differences (ECCO – data) smaller than variability in both OBP and SSH • Significant correlations for both OBP and SSH, particularly in regions of stronger variability

5. Time series AAB and BB regions • AAO: Antarctic Oscillation corr(AAO, OBP) = 0.49 corr(AAO, SSH) = 0.62 corr(AAO, OBP) = −0.66 corr(AAO, SSH) = −0.67 • Good correspondence between OBP and SSH • Variability correlated with Antarctic Oscillation index and roughly out of phase between AAB and BB regions

6. Basic theory Bottom pressure equation (Gill and Niiler 1973): J(OBP, H/f) + J(1/f, B) ≈ − curl (wind stress / f) 0 B = ∫ (gρz) dz −H J denotes Jacobian • Main vorticity balance between wind input and motions along gradients of H/f • J(1/f, B) explicitly involves effects of baroclinicity

7. Forcing and topography • Enhanced forcing in some of the regions of largest OBP • Gradients of H/f important in modulating response • Other factors, including dependence of ocean transfer function on spatial scale, also contribute (c) Normalized ratio wind curl/grad(H/f)

8. Main vorticity balance • Good balance between wind and Jacobian terms, with residual typically much smaller than variability in either term • Anticorrelation between wind curl in two regions

9. Topography, β and baroclinicity • Importance of topographic gradients, particularly for BB • Non-negligible baroclinic effects, especially at longer timescales

10. Dependences on frequency (ω) • Topographic influence strongest in BB, somewhat compensated by other J terms • Red character of baroclinic term consistent with theory • Main balance holds at most ω, hints of missing terms at the highest ω

11. Final remarks • Great progress made in ability to observe and estimate OBP low-frequency variability on the global scale • OBP variability important at mid and high latitudes for quantitative understanding of relation between sea level, heat content and steric height at interannual timescales • Knowledge of winds and topography can provide useful diagnostics of large-scale variability with a main balance between vorticity input by winds and motions along grad(H/f) • Baroclinic effects become more important with decreasing frequency, consistent with basic theory, and pointing to the need for 3-D description of the stratification …moving towards using GRACE data to constrain the ECCO estimates