Interannual Modulation of Subthermocline Eddy Kinetic Energy East of the Philippines. Issue 5 (13th May 2022)

• Record Type: Journal Article
• Authors: Hui, Yuchao; Zhang, Linlin; Wang, Zhenxiao; Wang, Fan; Hu, Dunxin
• Is Part Of: Journal of geophysical research. Volume 127:Issue 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-13
• Subjects: subthermocline EKE -- interannual variation -- western tropical Pacific -- baroclinic/barotropic instability; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC018452
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Interannual variation of subthermocline eddy kinetic energy (EKE) east of the Philippines is investigated based on mooring measurements during 2015–2019 and ocean‐state estimates during 1995–2017 from the Oceanic General Circulation Model for the Earth Simulator (OFES). Prominent interannual variation of EKE is detected below the thermocline east of the Philippine coast, which is closely related to the El Niño and Southern Oscillation (ENSO) events and generally lags the Nino3.4 index by 14 months. Further energy diagnostic analysis indicates that the interannual variation of subthermocline EKE is controlled by both baroclinic and barotropic instability of the background flows and dominated by the barotropic instability especially. Barotropic instability in the southern part of the Philippine coast is associated with the subsurface component of the quasi‐permanent anticyclonic Halmahera Eddy (HE), while that in the northern part is closely related to the Mindanao Undercurrent (MUC). Both HE and MUC are modulated by the ENSO events. When El Niño occurs, negative sea surface height anomalies appear near the dateline and propagate westward in the form of the first‐mode baroclinic Rossby wave, exerting delayed impacts upon the western boundary currents east of the Philippine coast and further modulating the interannual variation of subthermocline EKE. Moreover, the barotropic energy conversion rate and its corresponding subthermocline EKE at lower latitudes respond relatively faster to ENSO due to the higher Rossby wave group velocity there. Plain Language Summary: Subthermocline mesoscale eddies that have velocity core below the thermocline with almost no signal at the sea surface are distributed widely over the world oceans and play a crucial role in the transport of subsurface waters. The region east of the Philippine coast features energetic eddies and high eddy kinetic energy (EKE) in the subsurface layer. These eddies are intrinsic to the ocean and extract energy from the background currents. Based on 4‐year mooring measurements and numerical model outputs during 1995–2017, strong interannual variation of subthermocline EKE is detected in this region. Further analysis demonstrates that this interannual variation is mainly modulated by the evolution of barotropic instability, which represents the energy flux between the mean kinetic energy and EKE associated with the background flows. Additionally, the interannual variation of subthermocline EKE seems to be influenced by El Niño and Southern Oscillation. The wind anomalies associated with El Niño events excite negative sea surface height anomalies near the dateline, which propagate westward in the form of Rossby wave, producing delayed impacts on the western boundary currents and then on the variation of subthermocline EKE. This work enriches our knowledge of the low‐frequency modulation of subthermocline EKE in this region. Key Points: Significant interannual variation of subthermocline eddy kinetic energy (EKE) related to ENSO is detected east of the Philippines with mooring and model Model shows that both barotropic and baroclinic instability are responsible for interannual EKE variation, and barotropic factor is dominant Interannual modulation of barotropic instability is associated with the variation of Mindanao Undercurrent and Halmahera Eddy