Interannual Variability of Lower Equatorial Intermediate Current Response to ENSO in the Western Pacific

Abstract

The ocean currents of the tropical Pacific Ocean vary with El Niño‐Southern Oscillation (ENSO) cycles. A mooring time series obtained during 2014–2018 in the western Pacific reveal that interannual variability extends to Lower Equatorial Intermediate Current (LEIC). The LEIC velocity anomalies are significantly correlated with the Niño‐3.4 index at an 11‐month lag. Monthly velocity data from the global ocean physical reanalysis product and from a linear continuously stratified ocean model during 1993–2018 capture the 2015–2016 signal and are used to identify the underlying mechanism. During El Niño (La Niña) events, the direct wind forcing in the central tropical Pacific and the reflection of Kelvin waves in the eastern Pacific during the first autumn‐winter period contribute comparably to an eastward (westward) current anomaly in the western Pacific during the second autumn‐winter period. This process is achieved mainly through the generation of the second baroclinic mode and off‐equatorial westward‐propagating downwelling (upwelling) Rossby waves.

Linking Seasonal‐to‐Interannual Variability of Intermediate Currents in the Southwest Tropical Pacific to Wind Forcing and ENSO

“…Our companion paper (Ma et al, 2020) found that the interannual anomaly of LEIC velocity at 142°E is mainly related to the second baroclinic mode of Rossby waves triggered by wind anomalies during the El Niño-Southern Oscillation (ENSO) mature stage. When the variability anomalies at seasonal, interannual, and intraseasonal time scales were larger than the mean currents on which they superimposed, the disappearance of the LEIC was observed at different longitudes (Cravatte et al, 2017;Firing et al, 1998;Ma et al, 2020;Marin et al, 2010). The seasonal variation of NGCUC-I is inversely correlated with that of the LEIC (Kawabe et al, 2008;Zenk et al, 2005).…”

“…The seasonal variability in the LEIC (denoting both EIC and LEIC for convenience hereafter) velocity and temperature is induced by the vertical propagation of the first meridional Rossby wave, which is directly forced by the westward-propagating seasonal zonal wind fields (Kessler & McCreary, 1993;Marin et al, 2010). Our companion paper (Ma et al, 2020) found that the interannual anomaly of LEIC velocity at 142°E is mainly related to the second baroclinic mode of Rossby waves triggered by wind anomalies during the El Niño-Southern Oscillation (ENSO) mature stage. When the variability anomalies at seasonal, interannual, and intraseasonal time scales were larger than the mean currents on which they superimposed, the disappearance of the LEIC was observed at different longitudes (Cravatte et al, 2017;Firing et al, 1998;Ma et al, 2020;Marin et al, 2010).…”

Interdecadal Modulation of ENSO‐Related Anomalous Equatorial Intermediate Currents in the Western Pacific by the PDO

“…The westerly zonal wind stress anomaly ( τ x a ) in the above key regions during the mature stage of WPDO‐EN is more than twice as large as that of CPDO‐EN (Figure 4a). The previous study has demonstrated that the wind forcing during the El Niño mature stage (from Lag = −4 to Lag = 4) plays a critical role in the presence of the eastward U a (Ma et al., 2020). Furthermore, the full longitudinal range of westerly τ x a during the El Niño mature stage in the warm PDO is displaced eastward approximately 15° compared to that in the cold PDO (Figures 4a–4e).…”

“…Several works about the ENSO's impact on the equatorial intermediate currents (EICs) in the western Pacific have been done in recent years (e.g., Ma et al., 2020; J. Wang et al., 2021). The EICs here are defined as the zonal currents at the depths of 400–1,000 m. Three intermediate currents are located between 2°S and 2°N: the westward mean Equatorial Intermediate Current (EIC) at the depths of approximately 400–600 m, the westward mean Lower Equatorial Intermediate Current (LEIC) at the depths below approximately 800 m, and the eastward mean zonal current between the EIC and the LEIC (e.g., Cravatte et al., 2017; Firing et al., 1998; Johnson et al., 2002; Marin et al., 2010).…”

“…Wang et al, 2021). Under the dominant influence from the ENSO, the EICs show a significant interannual variation, also resulting from the vertical propagation of the first meridional mode Rossby waves (Ma et al, 2020;J. Wang et al, 2021).…”