El Nino-Southern Oscillation : A New Atmospheric Perspective via GPS Radio Occultation

We present initial results of a study on El-Nino-Southern Oscillation (ENSO) focusing on free tropospheric water vapor derived from Global Positioning System Radio Occultation (GPSRO) observations. The globally- distributed, high vertical-resolution, cloud penetrating profiles used in this study are derived from two sources: the CHAllening Minisatellite Payload (CHAMP) and the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC). The CHAMP data for the 2002-2008 period, allowed us to discover the signature of El Nino episodes, as indicated by the wettest portions of the free troposphere over the central Pacific at the peak of the El Nino. The location of the wettest profiles shift with the ENSO cycle, suggestive of a new atmospheric ENSO index. Additionally, the observed dryness of the free troposphere during January followed by very wet profiles in the Southern Pacific convergence zone in March/April preceding La Nina episodes, suggests a predictive skill a year in advance of the La Nina cold phase. To emphasize the importance of the results from GPSRO we contrast them to the results from the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, which contain a much weaker signal in the wettest profiles. A more in-depth study is possible with data from the six-satellite COSMIC constellation, which provides far denser coverage than previous GPSRO datasets. While far too short for a climatology (mid-2006 to the present), the tropical system has experienced both warm (fall 2006- spring 2007) and cold (fall 2007-spring 2008) ENSO phases over this short interval. In order to establish correlations between the 4D water fields and other key ENSO variables such as SST and OLR, we have created a new vertically-resolved, gridded water vapor dataset via a cluster analysis. We will summarize our approach and some of key spatio-temporal patterns that we have found in this study.