How Does El Niño Affect Predictability Barrier of Its Accompanied Positive Indian Ocean Dipole Event?

Da Liu,Rong Feng,Wansuo Duan

doi:10.3390/jmse9111169

Abstract

The effects of El Niño on the predictability of positive Indian Ocean dipole (pIOD) events are investigated by using the GFDL CM2p1 coupled model from the perspective of error growth. The results show that, under the influence of El Niño, the summer predictability barrier (SPB) for pIOD tends to intensify and the winter predictability barrier (WPB) is weakened. Since the reason for the weakening of WPB has been explained in a previous study, the present study attempts to explore why the SPB is enhanced. The results demonstrate that the initial sea temperature errors, which are most likely to induce SPB for pIOD with El Niño, possess patterns similar to those for pIOD without El Niño, whose dominant errors concentrate in the tropical Pacific Ocean (PO), with a pattern of negative SST errors occurring in the eastern and central PO and subsurface sea temperature errors being negative in the eastern PO and positive in the western PO. By tracking the development of such initial errors, it is found that the initial errors over PO lead to anomalous westerlies in the southeastern Indian Ocean (IO) through the effect of double-cell Walker circulation. Such westerly anomalies are inhibited by the strongest climatological easterly wind and the southeasterlies related to the pIOD event itself in summer, while they are enhanced by El Niño. This competing effect causes the intensified seasonal variation in latent heat flux, with much less loss in summer under the effect of El Niño. The greater suppression of the loss of latent heat flux favors the positive sea surface temperature (SST) errors developing much faster in the eastern Indian Ocean in summer, and eventually induces an enhanced SPB for pIOD due to El Niño.

Highlights

The Indian Ocean dipole (IOD) is a dominant interannual variability in the tropical Indian Ocean (IO), which is characterized by a west–east dipole pattern in the sea surface temperature anomalies (SSTAs) [1,2,3]
BSPyBc.omBypacorimngptahreindgevtheelodpemveenlotpomf SeSnTtAosf fSoSrTpAIOs DfoervpeInOtsDocecvuern-ts occurring with El Niño and pure IOD events (Figure 13), we found, under the effects of anomalous southeasterlies, that the negative SSTAs appear over southeastern IO (SEIO) in spring, and the negative SSTAs develop along with the wind anomalies, which intensify for both types of IOD events
The Geophysical Fluid Dynamics Laboratory (GFDL) CM2p1 coupled model is used to investigate the effects of El Niño on the summer predictability barrier (SPB) of the Indian Ocean dipole (IOD), under the assumption that the prediction uncertainties are totally induced by the initial errors

Summary

Introduction

The Indian Ocean dipole (IOD) is a dominant interannual variability in the tropical Indian Ocean (IO), which is characterized by a west–east dipole pattern in the sea surface temperature anomalies (SSTAs) [1,2,3]. In the present study, we demonstrate that, when considering the influence of El Niño, which is the warm phase of ENSO, the strength of WPB is decreased, while the intensity of SPB is enhanced (the details can be seen in Section 3), which makes it challenging to determine whether the effects of ENSO on IOD predictability are positive or negative. Under such a situation, further investigations are necessary to understand the dynamical mechanisms of how ENSO affects the IOD predictability.

Model and Experimental Strategy

Dynamic Mechanism Related to SPB Enhanced by El Niño

Summary and Discussion