Diagnosing the ocean response to tropical Pacific westerly wind events in CMIP6 models

In this study, we analyze the spatiotemporal distribution of equatorial Pacific westerly wind events (WWEs) and oceanic Kelvin waves in a subset of CMIP6 models that provide daily-resolved 20°C isotherm depth (a proxy for thermocline depth). WWEs that trail western Pacific convection associated with the Madden-Julian oscillation (MJO) can lead to the initiation of oceanic Kelvin waves that propagate along the Equator at roughly 2.6 m/s, reaching the eastern Pacific approximately two months after their initiation in the western Pacific. The suppression of the eastern equatorial Pacific thermocline by these Kelvin waves is associated with the development and maintenance of El Niño conditions, which in turn influence the eastward propagation of MJO convection.

The wavelength, amplitude, and propagation speed of the WWE-forced Kelvin wave are sensitive to combinations of WWE strength and fetch and to ocean stability and thermocline depth. We therefore analyze biases in Kelvin wave variance in relation to biases in WWE forcing and to biases in ocean stratification. We find that most models produce too weak and too infrequent WWE forcing in the western Pacific, while biases in thermocline depth (either too deep or too shallow) affect Kelvin wave propagation speed. We present evidence that these biases are consistent with model biases in the seasonally-dependent evolution of El Niño.

An additional outcome of this work is the demonstrated value for process understanding enabled by daily ocean output variables. Diagnostics developed as part of this work are being incorporated into the NOAA Model Diagnostics Task Force Process-Oriented Diagnostics package.