Tropical origins of the Pacific meridional mode associated with the nonlinear interaction of ENSO with the annual cycle

F. Jiang (1) , W. J. Zhang (2) , Julien Boucharel (3) , F. F. Jin

1 LPL - Lunar and Planetary Laboratory [Tucson]
2 Center for Atmospheric Environmental Study [Beijing]
3 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales

The Pacific Meridional Mode (PMM) has long been associated with extra-tropical air-sea coupling processes, which are thought to influence the development of El Nino-Southern Oscillation (ENSO). Here we show that the PMM on seasonal to interannual timescales is closely associated with a newly proposed tropical mode known as the ENSO Combination mode (C-mode), which arises from the nonlinear interaction between ENSO and the background annual cycle in the deep tropics. The PMM exhibits a remarkable resemblance with the C-mode in atmospheric patterns, spectral characteristics, and local impacts. Based on a simple Hasselmann-type model, we further demonstrate that the C-mode-related atmospheric anomalies can effectively drive PMM-like sea surface temperature anomalies. As the C-mode captures seasonally modulated ENSO characteristics, the seasonal-to-interannual PMM variability could naturally establish a connection with ENSO, thereby offering an alternative explanation for the observed relationship between PMM and ENSO. Previous studies demonstrated that the Pacific Meridional Mode (PMM) is driven by the extratropical climate systems, which can impact the development of El Nino-Southern Oscillation (ENSO) events in the tropics. In our study, we establish a close connection between the PMM on seasonal-to-interannual timescales and a tropical atmospheric mode arising from the nonlinear interaction between ENSO and the background annual cycle (the ENSO Combination mode, C-mode). The C-mode closely resembles the PMM in terms of atmospheric patterns and spectral characteristics. We further demonstrate that the C-mode could drive a PMM-like SST pattern in the tropical Pacific. Consequently, we suggest that the PMM on seasonal-to-interannual timescales has tropical origins, thereby offering an alternative explanation for the relationship between the conventional PMM index and the mature phase of ENSO. The spatiotemporal features of Pacific Meridional Mode (PMM) on seasonal-to-interannual timescales exhibit a remarkable resemblance with those of the tropical combination modeC-mode-related atmospheric anomalies are capable of inducing PMM-like sea surface temperature changes in the tropical PacificEl Nino-Southern Oscillation (ENSO)'s interaction with the annual cycle manifests in PMM-related air-sea conditions, which explains the observed ENSO-PMM linkage

Mots clés: PACIFIQUE

Domaines: Planète et Univers [physics]

F. Jiang, W. J. Zhang, Julien Boucharel, F. F. Jin. Tropical origins of the Pacific meridional mode associated with the nonlinear interaction of ENSO with the annual cycle. Geophysical Research Letters, 2023, 50 (24), e2023GL106225 [10 p.]. ⟨10.1029/2023gl106225⟩. ⟨hal-04430137⟩