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Comparison between 1997 and 2002 El Niño Events: Role of Initial State versus Forcing

Antonio Busalacchi(1) , Eric Hackert(1) , Joaquim Ballabrera-Poy(1) , and Rong-Hua Zhang(1)

(1) University of Maryland, 224 Computer and Space Science Bldg, 20742 College Park, MD, United States


Within the past 15 years, a series of ocean and atmosphere remote sensing satellites (e. g., TOPEX/Poseidon and now Jason 1) have been launched that serve to supplement and enhance the coverage and latitudinal extent of the in situ observations being taken at the surface, and at depth, in the equatorial Pacific Ocean. The 1997 "El Niño Event of the Century" and the weaker 2002 event were the best monitored El Niños on record from the perspective of in situ and space-based observations. The 1997 El Niño was the first time a major El Niño event was observed from start to finish with a combination of remotely-sensed measurements of sea surface temperature, sea surface topography, sea surface winds, ocean color, and precipitation. This presentation will analyze the evolution of the 1997 and 2002 El Niño events from the perspective offered by several of these remotely sensed observations, data assimilation, modeling and coupled models. In particular we will isolate the differing roles of initial conditions and forcing that gave rise to the distinct nature of these two events.

Long time series of high quality ocean surface topography information allows a thorough comparison between two distinct El Niño events. The 1997 El Niño showed a strong ocean-atmopshere coupled system which propagated from west to east. The weaker 2002 event developed later in the calendar year and showed strongest anomalies near the dateline. In this study, observational data of sea level and sea surface temperature (SST) from satellite, and subsurface temperature and salinity (from all available sources including ARGO) are used along with data assimilation to improve the initial state of the ocean. Ocean model experiments are used to isolate differences between initial states of the system and forcing in the development of the two events. A statistical atmospheric model is utilized to highlight the atmospheric response to anomalous SST fields.

Different initial conditions (IC) for Nov 96, Nov 01 and Nov Climatology are used to initiate model experiments with climatological ECMWF forcing for the two El Niño periods (Nov 96-Dec 97, Nov 01-Dec 02). By differencing two experiments with the same forcing, but different ICs, the role of the ocean state at the start of the two events (Nov 96 versus Nov 01) can be examined. These results show that both initial conditions are predisposed to induce a subsequent El Niño. However for the first half of the 1997 event (Nov 96- Jun 97) the initial state of the ocean induces an eastward propagating wind field. For the 2002 event, the initial state served to limit the eastward propagation of the wind field by inducing destructive interference near the dateline. For the second half of the event (Jul 97 - Dec 97) the initial state contributed little. However, the role of initial conditions contributed nearly half the total signal for the 2002 event.

In the same way that the role of the ICs can be isolated, the role of the different forcing fields can be identified. These results show that for both events forcing played a very weak role over the first half of the year. (In fact the statistical wind anomalies for these periods showed anomalous easterlies.) However, for the second half of the 1997 event the air/sea coupled forcing accounts for nearly all the wind anomaly. For the 2002 event, ICs and forcing contribute approximately equally to the wind anomaly field.


Workshop presentation


                 Last modified: 07.10.03