A Survey of Statistical Relationships between Tropical Cyclone Genesis and Convectively Coupled Equatorial Rossby Waves

Convectively coupled equatorial Rossby waves(ERW)modulate tropical cyclone activities over tropical oceans.This study presents a survey of the statistical relationship between intraseasonal ERWs and tropical cyclone genesis(TCG)over major global TC basins using four-decade-long outgoing longwave radiation(OLR)and TC best-track datasets.Intraseasonal ERWs are identified from the OLR anomalies using an empirical orthogonal function(EOF)analysis method without imposing equatorial symmetry.We find that westward-propagating ERWs are most significant in four tropical ocean basins over the summer hemisphere and that ERWs exhibit similar northeast-southwest(southeast-northwest)tilted phase lines in the northern(southern)hemisphere,with an appreciable poleward advance of wave energy in most TC basins.The EOF-based ERW indices quantitatively show that ERWs significantly modulate TC genesis.The convectively active(suppressed)phases of ERWs coincide with increased(reduced)TCG occurrences.The TCG modulation by ERWs achieves the maximum where the ERWs propagate through the climatological TCG hotspots.As a result,the total number of TCG occurrences in the TC basins varies significantly according to the ERW phase.The ERW-TCG relationship is significant over the northwestern Pacific Ocean,northeastern Pacific Ocean,and the northern Indian Ocean during the northern summer seasons.In the southern summer season,the ERW-TCG relationship is significant over the southern Indian Ocean,Indonesian-Australia basin,and the southwestern Pacific Ocean.However,ERW activities are weak in the main TC development region of the Atlantic Ocean;and the impact on Atlantic TCG appears to be insignificant.

Nonlinearly Coupled KdV Equations Describing the Interaction of Equatorial and Midlatitude Rossby Waves

Amplitude equations governing the nonlinear resonant interaction of equatorial baroclinic and barotropic Rossby waves were derived by Majda and Biello and used as a model for long range interactions （teleconnections） between the tropical and midlatitude troposphere. An overview of that derivation is nonlinear wave theory, but not in atmospheric presented and geared to readers versed in sciences. In the course of the derivation, two other sets of asymptotic equations are presented： the long equatorial wave equations and the weakly nonlinear, long equatorial wave equations. A linear transformation recasts the amplitude equations as nonlinear and linearly coupled KdV equations governing the amplitude of two types of modes, each of which consists of a coupled tropical/midlatitude flow. In the limit of Rossby waves with equal dispersion, the transformed amplitude equations become two KdV equations coupled only through nonlinear fluxes. Four numerical integrations are presented which show （i） the interaction of two solitons, one from either mode, （ii） and （iii） the interaction of a soliton in the presence of different mean wind shears, and （iv） the interaction of two solitons mediated by the presence of a mean wind shear.

NUMERICAL EXPERIMENTS ON THE TROPICAL AIR-SEA INTERACTION WAVES

By means of the numerical method,the tropical air-sea interaction waves are studied.The results show that when the Kelvin waves are filtered out and only the equatorial Rossby waves are reserved both in the atmosphere and in the ocean,the disturbances can also propagate eastward because of the air-sea interaction.The critical wavelength of the eastward propagating waves is related to the intensity of the air-sea interaction.The stronger the air-sea interaction,the larger the eastward propagating components of the air-sea interaction waves.The results of the numerical experiments are in good agreement with those of the theoretical analysis(Chao and Zhang,1988).

Dynamical effect of the zonal wind anomalies over the tropical western Pacific on ENSO cycles

The circulation and zonal wind anomalies in the lower troposphere over the equatorial western Pacific and their roles in the developing and decaying processes of the 1982–1983, 1986 –1987, 1991–1992 and 1997–1998 El Ni?o events and the occurrence of La Ni?a events are analyzed by using the observed data in this paper. The results show that before the developing stage of these El Ni?o events, there were cyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies brought the westerly anomalies over the Indonesia and the tropical western Pacific. However, when the El Ni?o events developed to their mature phase, there were anticyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies made the easterly anomalies appear over the tropical western Pacific. A simple, dynamical model of tropical ocean is used to calculate the response of the equatorial oceanic waves to the observed anomalies of wind stress near the sea surface of the equatorial Pacific during the 1997/98 ENSO cycle, which was the strongest one in the 20th century. It is shown that the zonal wind stress anomalies have an important dynamical effect on the devel-opment and decay of this El Ni?o event and the occurrence of the following La Ni?a event.