13 Jan 2023
13 Jan 2023
Status: this preprint is open for discussion.

A simple diagnostic based on sea surface height with application to Central Pacific ENSO

Jufen Lai1,2,4, Richard John Greatbatch2,3, and Martin Claus2,3 Jufen Lai et al.
  • 1State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
  • 3Faculty of Mathematics and Natural Sciences, Christian Albrechts Universität, Kiel, Germany
  • 4College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China

Abstract. We use output from a freely-running NEMO model simulation for the equatorial Pacific to investigate the utility of linearly removing the local influence of vertical displacements of the thermocline from variations in sea surface height. We show that the resulting time series of residual sea surface height, denoted ηnlti, measures variations in near-surface heat content that are independent of the local vertical displacement of the thermocline and can arise from horizontal advection, surface heat flux and diapycnal mixing processes. We find that the variance of ηnlti and its correlation with sea surface temperature, are focused on the Niño4 region. Furthermore, ηnlti averaged over the Niño4 region is highly correlated with indices of Central Pacific El Niño Southern Oscillation (CP ENSO), and its variance in 21 year running windows shows a strong upward trend over the past 50 years, corresponding to the emergence of CP ENSO following the 1976/77 climate shift. We show that ηnlti can be estimated from observations, using satellite altimeter data and a linear multi-mode model. The time series of ηnlti, especially when estimated using the linear model, show pronounced westward propagation in the western equatorial Pacific, arguing an important role for zonal advective feedback in the dynamics of CP ENSO, in particular for cold events. We also present evidence that the role of the thermocline displacement in influencing sea surface height increased strongly after 2000 in the eastern part of the Niño4 region, at a time when CP ENSO was particularly active. Finally, the diagnostic is easy to compute and can be easily applied to mooring data or couple climate models.

Jufen Lai et al.

Status: open (until 10 Mar 2023)

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Jufen Lai et al.

Jufen Lai et al.


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Short summary
The El Niño Southern Oscillation (ENSO) has a global influence on weather and climate. Over most of the equatorial Pacific, where ENSO is focused, variations in sea surface height, such as measured by satellite, are strongly influenced by vertical displacements of the ocean thermocline. We show that linearly removing this influence leads to a time series of sea surface height that capture ENSO dynamics in the central Pacific, where ENSO variability has become more active in recent decades.