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

Abstract

Seasonal‐to‐interannual variability of intermediate circulations in the southwest tropical Pacific is key to interhemisphere and interbasin water mass exchange and is shown to be interconnected with wind forcing and the El Niño–Southern Oscillation (ENSO) based on numerical model results validated by mooring observations. The Lower Equatorial Intermediate Current and the intermediate part of the New Guinea Coastal Undercurrent show clear seasonal variability and a significant interannual response to ENSO at a 7–12‐month lag, both of which are induced by Rossby waves. Baroclinic modes 1 and 2 of the direct wind‐forced Rossby waves contribute mainly to the seasonal variability. In contrast, the Rossby waves from baroclinic modes 2 and 4 and the reflection of Kelvin waves are the main contributors to the variability on ENSO time scales. The presence of the southwest tropical Pacific coastline enhances the intermediate seasonal variance and the interannual relationship between intermediate currents and ENSO.

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

“…The lead‐lag correlations reach a peak value of 0.63 in ORAS4 and 0.65 in GECCO3 when ENSO leads Tr a by about 10–12 months. The 10‐ to 12‐month lag is generally in agreement with the previous studies (Ma et al., 2020; J. Wang et al., 2021). Note that Tr a also shows a significant negative peak correlation with ENSO when Tr a leads ENSO by about 1–3 months.…”

“…The enhanced and eastward-displaced westerly τ x a during the warm PDO can generate Kelvin waves with an intensified amplitude and eastward shift (Figures S3a and S3b in Supporting Information S1). These waves are reflected into Rossby waves at the eastern boundary (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). The EICs exhibit a strong seasonality, which is attributed to the vertical propagation of the first meridional mode Rossby waves forced by the westward‐propagating annual zonal wind (Kessler & McCreary, 1993; Marin et al., 2010; J. 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).…”

“…. The EICs exhibit a strong seasonality, which is attributed to the vertical propagation of the first meridional mode Rossby waves forced by the westward-propagating annual zonal wind (Kessler & McCreary, 1993;Marin et al, 2010;J. Wang et al, 2021).…”

“…Significant progress has been made in past decades in understanding the interdecadal modulation of PDO on the ENSO-related atmospheric and upper oceanic variables, such as the sea level over the tropical Pacific Ocean (e.g., Moon et al, 2015), the summer monsoon over Asia (e.g., Xue et al, 2018), and the precipitation over the United States (e.g., Kurtzman & Scanlon, 2007). However, whether the PDO modulation effects can extend to the ocean's intermediate depths remains unclear.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).…”