Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 <em>&eta;</em>_{<em>nlti</em>}, 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 <em>&eta;</em>_{<em>nlti</em>} and its correlation with sea surface temperature, are focused on the Ni&ntilde;o4 region. Furthermore, <em>&eta;</em>_{<em>nlti</em>} averaged over the Ni&ntilde;o4 region is highly correlated with indices of Central Pacific El Ni&ntilde;o 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 <em>&eta;</em>_{<em>nlti</em>} can be estimated from observations, using satellite altimeter data and a linear multi-mode model. The time series of <em>&eta;</em>_{<em>nlti</em>}, 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 Ni&ntilde;o4 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.