Modification of n = 0 equatorial waves due to interaction between convection and dynamics

Abstract

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.