The region east of the Philippine coast has energetic mesoscale eddies and high eddy kinetic energy (EKE) in the subsurface layer. These eddies play essential roles in the intraseasonal variation of western boundary currents and the northward intrusion of South Pacific water.

However, due to the lack of long-term continuous observations focusing on subthermocline eddies in this region, most of the previous studies tend to concentrate on the statistical characteristics, and only a few studies pay attention to the temporal variation of subthermocline eddies and their associated EKE.

Based on long-term subsurface mooring observations and model outputs, the research team led by Prof. HU Dunxin from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) recently revealed the interannual modulation of subthermocline EKE east of the Philippines and its connection with the El Nino and Southern Oscillation (ENSO) events.

The study was published in Journal of Geophysical Research: Oceans on May 5. 

Researchers analyzed two mooring Acoustic Doppler Current Profilers (ADCP) measurements with 2-4 years of records at 130°E section. They found that the EKE east of the Philippines in the subsurface layer (300-700 m) shows distinguished interannual variation, which is associated with ENSO events.

Further analysis based on Oceanic General Circulation Model (OGCM) for the Earth Simulator (OFES) model outputs indicates that almost every El Nino is followed by an elevation of subthermocline EKE, and the variation of EKE generally lags the Nino3.4 index by 14 months. The modulations of both baroclinic and barotropic instability of background flows are the main cause for the interannual variation of subthermocline EKE, and the barotropic instability is dominant.

Barotropic instability is related to different background currents. In the southern and northern part of the Philippine coast, it is related to the subsurface component of the anticyclonic Halmahera Eddy (HE) and the Mindanao Undercurrent (MUC) respectively. Both MUC and HE are modulated by the westward-propagating first-mode baroclinic Rossby wave excited by the wind stress curl anomalies west of the dateline during ENSO events. Therefore, MUC/HE and the corresponding subthermocline EKE lag the Nino3.4 index.

"Traditionally, baroclinic instability is considered as the primary energy source of subthermocline eddies east of the Philippine coast, we show that barotropic instability is also important to the generation of subthermocline eddies, especially in the local region east of the Mindanao island," said HUI Yuchao, first author of the study.

"This work has significantly promoted our understanding of the multi-scale variability of the subsurface circulation in the western tropical Pacific," said ZHANG Linlin, the corresponding author.

(a) All-time mean subthermocline EKE derived from OFES outputs. (b) and (c) are the Nino3.4 index and observed EKE time series at 8.5°N, 130°E between Sep. 2016 to Sep. 2019. (d) The blue and red curves are the regional (5°-14°N, 125°-130°E) mean time series of subthermocline EKE and barotropic energy conversion rate calculated with OFES during 1995-2017, and the gray shading is Nino3.4 index.

Hui Y., L. Zhang*, Z. Wang, F. Wang and D. Hu. (2022). Interannual modulation of subthermocline eddy kinetic energy east of the Philippines, Journal of Geophysical Research: Oceans, 127, e2022JC018452. 

Zhang L.*, Y. Hui, T. Qu, and D. Hu. (2021). Seasonal variability of subthermocline eddy kinetic energy east of the Philippines, Journal of Physical Oceanography, 51, 3, 685-699, https://doi.org/10.1175/JPO-D-20-0101.1.

ZHANG Linlin

Institute of Oceanology

E-mail: zhanglinlin@qdio.ac.cn 

(Editor: ZHANG Yiyi)