Observed and modeled teleconnections along the equatorial waveguide during IOD-ENSO interactions

In this paper we examine teleconnections along the equatorial wave guide during IOD-ENSO interactions, with a focus on the year 2006. A marked feature of 2006 is that it is the year of the lowest seasonal prediction skill in the modern record, despite the appearance of a moderately strong El Nino event. The low prediction skill is marked in global and regional rainfall, but is also clear in other dynamic variables such as geopotential height. Another interesting feature of the 2006 El Nino was the low predictability for the event itself. Analysis of multi-model predictions collected and provided by the International Research Institute for Climate and Society, reveals that multiple phases of the 2006 event was poorly predicted, including the initiation and termination.

A detailed analysis of multiple variables representing key oceanic and atmospheric variability is presented to discuss the evolution of an Indian Ocean Dipole (IOD) and El Nino during this year. In particular we focus on the robust and large-scale development of intense sea surface height and temperature anomalies at the eastern Indian Ocean during early June. Coupled convective variability rapidly develops leading to a strong IOD event that lasts from late May to December. Temporal analysis of the evolving El Nino shows that it lags the IOD by several months. Further analysis shows that strong and persistent westerly winds extending into the western Pacific warm pool were initiated and sustained by the anomalously high pressure region over the eastern Indian Ocean and the maritime continent in response to the abnormally weak convective activity in the region. These winds are noted to be steady on an interannual timescale and marked by the absence of notable intraseasonal activity. We argue that the initiation of the El Nino event was driven by these abnormal westerly winds which triggered the displacement of warm subsurface waters into the eastern Pacific through free oceanic Kelvin waves. As coupled convective variability is weak over the tropical Pacific during the El Nino episode, we argue that the event was by and large controlled by the equatorial teleconnections generated and sustained by the IOD event.

The termination of the 2006 El Nino was preceded by reversing winds that changed from westerly to easterly at the eastern Indian and the western Pacific warm pool region from mid-November. We show that the reversal of winds are related to the development of phase-lagged anomalous positive/strong convective activity at the western Indian Ocean, and may be interpreted as a Gill-Matsuno type response to convection at the equator. Linear Baroclinic modeling as well as numerical experiments using the Weather Research and Forecasting (WRF) model in a tropical channel configuration are used to further clarify and substantiate this hypothesis. Finally we carry out a multi-event analysis spanning the last 60 years of observations to further examine teleconnections along the equatorial wave guide during IOD-ENSO interactions.