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Process-oriented evaluation of oceanic equatorial waves in the Indian and west Pacific Ocean forced by intraseasonal westerly wind events

Equatorial shallow water ocean wave modes (OWMs), such as eastward-propagating Kelvin
waves and westward-propagating equatorial Rossby (ER) waves, help regulate the depth of the thermocline, ocean heat content (OHC), ocean currents, sea surface height (SSH), and sea surface temperature (SST). Their modification of upper-ocean thermal characteristics influences the evolution of important coupled air-sea phenomena including the Madden-Julian oscillation (MJO), the Indian Ocean dipole (IOD), and the El Niño Southern Oscillation (ENSO). In the tropical Indian and Pacific Oceans, OWMs are frequently forced by strong, but short lived intraseasonal westerly wind events (WWEs; occurring every 30-70 days and lasting 3-21 days) acting on the ocean surface. The strength and meridional structure of the WWE forcing and the ocean mean state (including the depth of the thermocline and the stability of the upper ocean) help determine the amplitude and propagation characteristics of the OWMs.
For the first time, diagnosis of intraseasonal WWE-forced OWMs in CMIP models is possible
with daily output of the depth of the thermocline in several CMIP6-member models, which was
not available in previous CMIP archives. Our main goal is to diagnose the fidelity of
intraseasonal WWE-forced OWMs in CMIP6 and other model databases relative to observations and link OWM biases to biases in the WWE forcing, or to biases in the ocean mean state. We will also examine changes to WWEs, tropical OWMs, and the ocean mean state under climate change. Our work plan is to:
1. Diagnose the fidelity of tropical OWM spectra and spatial variance patterns in CMIP6
models and other climate model databases relative to observations.
2. Characterize the frequency, intensity, and meridional structure of intraseasonal WWEs in
models and observations.
3. Assess the realism of intraseasonal WWE-forced OWMs as a function of WWE intensity
and meridional structure in models relative to observations.
4. Evaluate the stability of the ocean mean state in models relative to observations and its
relationship to OWM amplitude and phase speed.
5. Quantify changes in OWM climatology, WWE statistics, and ocean stability under climate
change and relate OWM changes to changes in WWE characteristics and ocean stability.
This work will result in a tropical OWM process-oriented diagnostic (POD) with several
diagnostic components that will be added to the Model Diagnostic Task Force software package.
The OWM POD fills “clearly-identified gaps in the existing MDTF software package” including
“open- and coastal ocean systems” and advances the evaluation of coupled processes in climate models. Our objectives are highly relevant to one of the main goals of the MAPP Process-Oriented Diagnostics call to “better understand and benchmark process-level deficiencies that result in model performance biases for simulated Earth system and climate phenomena.” More broadly, this work advances NOAA’s long-term goal to “advance [the] understanding of the Earth’s climate system.” Ultimately, understanding the processes that lead to OWM biases is needed to improve OWM representation in models and obtain better predictions of climate modes influenced by OWMs, such as the MJO, the IOD, and ENSO.

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