Reciprocity in the Indian Ocean: Intraseasonal Oscillation and Ocean Planetary Waves

• Published in: Journal of geophysical research. Oceans Vol. 126; no. 9
• Main Authors: Rydbeck, Adam V., Jensen, Tommy G., Flatau, Maria K.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-09-2021
• Subjects: air‐sea interaction; Anomalies; Atmosphere; Atmospheric convection; Baroclinic mode; Clouds; Downwelling; Easterlies; Enthalpy; Equator; Feedback loops; Geophysics; Heat; Heat content; Heat transfer; Intraseasonal oscillation; intraseasonal variability; Motivation; Ocean dynamics; Ocean surface; Ocean waves; Oceanic convection; Oceans; physical oceanography; Planetary waves; Pressure gradients; Reciprocity; Rossby waves; Satellite observation; Strong winds; Surface water waves; Synchronism; Synchronization; Tropical atmosphere; Tropical climate; Upper ocean; Warm water; Water temperature; Wind; Wind stress; Winds; Indian Ocean
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC017546

During the intraseasonal oscillation (ISO) in the tropical Indian Ocean, a recent study observed an interesting relationship between atmospheric convection and ocean heat content anomalies. Ocean heat content anomalies maximize during periods of enhanced convection. Using that study as motivation, the processes responsible for this behavior are further investigated using satellite observations and a state‐of‐the‐art ocean reanalysis. These data show that ocean dynamics linked to first baroclinic mode equatorial Rossby (ER) waves are responsible for the routine synchronization of anomalous ocean heat content and atmospheric convection during the ISO. Using a novel index to identify oceanic ER waves in the Indian Ocean, we observe the following sequence of events indicative of two‐way feedbacks between large‐scale intraseasonal modes in the atmosphere and ocean: (a) Westerly wind stress associated with the enhanced convective phase of the ISO piles mass in the eastern Indian Ocean forcing a westward oriented pressure gradient along the equator. (b) The easterly wind stress that immediately follows combines with the pressure gradient forcing to generate downwelling ER waves that slowly propagate westward. (c) The waves maximize in the central Indian Ocean where they depress the isotherms and increase the ocean heat content as the next enhanced convective phase of the ISO passes and intensifies. The entire feedback loop transpires over 90 days.
Plain Language Summary
A large collection of precipitating clouds and strong winds in the tropical atmosphere, known as the intraseasonal oscillation (ISO), generates planetary waves in the ocean. These waves are only several centimeters tall at the ocean surface, but are capable of transporting large amounts of heat. The planetary waves, also referred to as Rossby waves, travel very slowly and take several months to cross the Indian Ocean from east to west. We observe that these ocean waves are initially generated by the eastward winds within the ISO and subsequently intensified by the westward winds that are in front of the next ISO. The waves increase the amount of warm water in the upper ocean so that when the next ISO arrives, the waves support the development and intensification of clouds and precipitation within the ISO. In short, a portion of the energy from the ISO is stored in the ocean in the form of ocean Rossby waves, which then warm the upper ocean and strengthen the following ISO.
Key Points
Rossby waves in the tropical Indian Ocean increase ocean heat content prior to the intensification of intraseasonal oscillation (ISO) convection
The increase of ocean heat content is mainly due to ocean dynamics including vertical and meridional advection
The oceanic Rossby waves link successive atmospheric ISOs