Influence of Cumulus Convection Schemes on the ENSO-like Phenomena Simulated in a CGCM

[摘要] The features of ENSO-like phenomena simulated in a coupled ocean-atmosphere model (CGCM) incorporated with two different convection schemes, the Kuo scheme (KUO) and the prognostic Arakawa-Schubert scheme (PAS), are compared. There are the significant differences in the model ENSO properties between KUO and PAS. The sea surface temperature (SST) anomaly in KUO shows a standing oscillation pattern in the eastern equatorial Pacific, while it propagates westward in the western equatorial Pacific. In contrast, the SST anomaly propagates eastward slowly across the entire equatorial Pacific basin in PAS. The amplitude of Niño3 SST anomaly is larger in PAS and smaller in KUO than observations. The strongest equatorial westerly wind anomalies during the mature phase of the ENSO are displaced slightly eastward in PAS but westward in KUO compared to observations, associated with the difference in the mean equatorial SST. The examination of heat balance shows that the SST anomaly is strongly influenced by the thermocline depth anomaly in PAS, which is favorable for the eastward phase propagation of SST anomalies. In KUO, on the other hand, the westward phase propagation of SST anomalies in the western equatorial Pacific is related to the anomalous horizontal advection. Comparing PAS with KUO, the mean thermocline depth in the western equatorial Pacific is shallower in PAS than KUO, while the mean zonal SST contrast in the western equatorial Pacific is stronger in KUO than PAS. They are responsible for the difference in the phase propagation of SST anomalies between KUO and PAS. In KUO, the Kelvin waves reflected at the western boundary seem to contribute to the reversal of the model ENSO. In PAS, on the other hand, the western equatorial wind anomalies appear during the mature phase of the model ENSO, and then induce the forced-Kelvin waves playing an important role in the termination of the model ENSO. These Kelvin waves cross the equatorial Pacific as an air-sea coupled mode with a slow phase speed, compared to the reflected Kelvin waves in KUO. As a result, the time needed for Kelvin waves acting as a negative feedback to propagate in the eastern equatorial Pacific is longer in PAS than KUO, leading to the amplification of Niño3 SST anomalies. These differences in ENSO properties in turn are related to the difference in the mean states that result from the two different convection schemes. A comparison of the mean wind fields suggests that the difference in ENSO characteristics can be attributed to the difference in the mean wind fields over the western Pacific.

[学科分类] 大气科学