A mechanism is proposed that can sustain unstable equatorially trapped westward-propagating low-frequency modes in the tropics. The roles of evaporation–wind feedback and wave–CISK (convergence feedback) in modifying the n = 0 equatorially trapped modes are studied. It is shown that the convergence feedback by itself cannot make the waves unstable but can modulate the instability introduced by evaporation–wind feedback. We show that the evaporation–wind feedback introduces a new westward-propagating n = 0 mode in addition to dramatically modifying the dry mixed Rossby–gravity (MRG) mode. The new mode is generally damped for mean background easterlies but can he nearly neutral for moderate strength of evaporation–wind feedback and strong convergence feedback. The evaporation–wind feedback makes the MRG mode unstable in a westward-propagating low-frequency regime and in an eastward-propagating high-frequency regime. If the background mean winds are easterlies, the gravest low-frequency mode resembles the westward-propagating mode with period of about 4 days and wavelength of about 7000 km observed over the central and western Pacific. The evaporation-wind feedback also makes the meridional structure of the eigenfunctions of these modes frequency dependent. The low-frequency waves are more tightly trapped around the equator as compared to their high-frequency counterparts. It also introduces a meridional propagation for the mode. The sensitivity of the characteristics of the gravest low-frequency mode to variations of the strength of the two feedbacks is discussed.

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