The deep ocean has long been regarded as a quiet realm, largely isolated from the dynamic processes that shape Earth's climate. However, new observations from the western equatorial Pacific reveal robust intraseasonal variability at depths of 1500–3000 m, with kinetic energy levels reaching up to 10 cm2s-2.
A research team led by Prof. WANG Jianing from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) analyzed five consecutive years of mooring data collected at the equator (142°E). Their analysis uncovered significant intraseasonal kinetic energy (IKE) in the deep ocean, primarily concentrated in the 40–90-day period and dominated by meridional currents.
The study was published in Journal of Physical Oceanography on Oct. 1.
Spectral analysis shows that ~81% of the deep IKE is attributable to equatorial Yanai waves—also known as mixed Rossby–gravity waves—with energy concentrated in vertical modes 2~6. "This finding offers a new perspective on how energy and signals from the upper ocean are transmitted into the deep ocean," said Dr. ZHANG Hang, first author of the study.
By integrating reanalysis data, the researchers traced the source of this deep IKE to the intermediate layer (1,000–1,750 m) between 134°E and 137°E—a region near the western boundary. There, strong intermediate western boundary currents generate barotropic instability through horizontal shear, exciting wave energy that propagates eastward and downward along the equator, closely following the theoretical ray paths of Yanai waves.
Moreover, the study identified a clear seasonal modulation in the dominant frequency of deep intraseasonal variability. At around 2,500 m depth, this frequency exhibits a significant negative correlation with the zonal velocity of the upper deep branch of the Pacific Meridional Overturning Circulation (U-PMOC). This anti-correlation is explained by a Doppler shift effect, confirming the westward phase propagation of the observed Yanai waves.
Previous studies of oceanic intraseasonal variability have largely focused on the upper ocean, where energy is typically thought to originate from surface forcing (e.g., the Madden–Julian Oscillation). In contrast, this work reveals a previously underappreciated energy pathway: Yanai waves efficiently channel intraseasonal kinetic energy directly from the intermediate ocean into the deep and even abyssal layers.
Understanding how energy propagates from the upper ocean into the deep interior is crucial for improving the vertical energy budget in ocean models. "Our results demonstrate that equatorial wave dynamics—particularly Yanai waves—serve as a critical link between upper- and deep-ocean dynamical systems," added Prof. WANG, the corresponding author.
Time-depth variation of intraseasonal kinetic energy observed by moorings, and vertical profiles of the time-averaged intraseasonal kinetic energy for the zonal and meridional velocity components. (Image by IOCAS)
(Text by ZHANG Hang)
Media Contact:
ZHANG Yiyi
Institute of Oceanology
E-mail: zhangyiyi@qdio.ac.cn
(Editor: ZHANG Yiyi)