Precursors of ENSO Events from 27 Years of Satellite Data

Abstract:

The present study examines the role of different mechanisms as precursors for ENSO diversity by analyzing high-quality satellite observations of several key variables for the past 27 years, complemented by subsurface information from ocean reanalysis (GODAS, SODAsi). Our objective is to clarify the relative importance of large-scale atmospheric forcing from the Northern and Southern Hemispheres, as well as tropical atmospheric noise and ocean background conditions in triggering different ENSO types, thus providing insights in the event predictability and possible lead times.

This investigation uses an extensive suite of ocean satellite retrievals from microwave sensors, radiometers and scatterometers, all intercalibrated and consistently processed at Remote Sensing Systems. The satellite data record of wind speed, columnar water vapor, cloud water, and precipitation begins in 1988 with the first SSMI. Through-clouds SST observations started in 1998 with TMI. Wind vectors observations started with the scatterometer QuikSCAT in 1999. All of these variables have now been continuously monitored and processed to a high accuracy level, suitable for climate studies, and are available at high frequency, twice daily for each instrument, and at a fine spatial resolution of 0.25 degrees.

We first examine the large-scale extra-tropical precursors in both the Northern and Southern Hemispheres and their relative influence in triggering each El Niño/La Niña event during the satellite era. We then consider the role played by atmospheric noise in the form of Westerly Wind Bursts (WWBs) in triggering different event types, and the possible interplay of the WWB activity and extra-tropical influences. In particular, the relationship of the WWB amplitude and longitudinal evolution with the location of the eastern edge of the Warm Pool is examined over the total length of the satellite record. Special attention is devoted to the evolution of equatorial Pacific conditions during 2014/2015, which is examined in the context of previous ENSO events.