FY 2020

Equatorial Pacific 1000-dbar velocity and isotherm displacements from Argo data: Beyond the mean and seasonal cycle

Zanowski, H., G.C. Johnson, and J.M. Lyman

J. Geophys. Res., 124(11), 7873-7882, doi: 10.1029/2019JC015032, View online (2019)

Equatorial Pacific zonal velocity at 1,000 dbar and vertical isotherm displacements from 0–2,000 dbar are analyzed using Argo data through February 2018. In agreement with previous studies, the mean 1,000‐dbar zonal velocity is characterized by alternating eastward and westward jets of amplitude 5–10 cm/s. Similarly, the seasonal cycle is dominated by an annual Rossby wave that is apparent in annual harmonic fits to both the isotherm displacement and velocity fields. Residual (mean, annual, and semiannual harmonics removed) zonal velocity and isotherm displacements are analyzed using Complex Empirical Orthogonal Functions (CEOFs) and their Principal Components. For the analysis at 1,000 dbar, the first velocity CEOF accounts for 28% of the variance and the first isotherm displacement CEOF 24%. Spatial patterns of the lead CEOFs are broadly consistent with Rossby waves, but only isotherm displacement CEOF1 can be clearly linked to known physical processes in the equatorial Pacific. This is true of the isotherm displacement CEOF1s across pressure surfaces, despite being estimated independently on each one. The Principal Component of isotherm displacement CEOF1 at 1,000 dbar is well correlated (r = –0.65) with the Niño3.4 index at a 12‐month lag, suggesting that El Niño signals propagate deep into the equatorial Pacific Ocean. The 3‐D structure of the isotherm displacement CEOF1s suggests that the deep isotherm displacement signals are consistent with a vertically propagating Rossby wave of vertical mode 3–4.

Plain Language Summary. The tropical Pacific Ocean is an important region for Earth's climate. It features a complex set of currents that exist at different depths and vary on a range of timescales from months to decades. The variations of these currents are influenced by planetary (Rossby) wave signals that travel both zonally and vertically along the equator. While it is well known that Rossby waves modulate these currents on yearly timescales, the data to observe deep current variability at longer timescales have only recently become available from the global array of Argo floats. Here we analyze velocities and isotherm displacements (vertical movements of constant temperature surfaces) from Argo float data to investigate their variations over timescales longer than a year. Using a statistical technique to find the most energetic large spatial scale and long‐timescale propagating signals in these data, we demonstrate that deep isotherm displacements are highly correlated with the El Niño‐Southern Oscillation (ENSO) across a large range of pressures but find that the velocities at 1,000 dbar are not. This mismatch is puzzling, but it may be due to the short record length and relatively small data set for velocity compared to the timescales of the examined velocity variations.