<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 3.0//EN" "html.dtd"> <HTML> </HEAD> <BODY BGCOLOR="#FFFFFF"> <br> <CENTER> <TITLE> ABSTRACT</TITLE> <!-- Changed by: Jeff Whitaker, 23-Feb-1996 --> <H3> Mechanisms for Sub-Seasonal Variations in Extratropical ENSO Signals </H3> <b>Jeffrey S. Whitaker and Joseph J. Barsugli</b><BR><br> NOAA-CIRES Climate Diagnostics Center, Boulder, CO<br><br> </CENTER> <P>
</HEAD> <center><h3>Abstract</h3></center>
How do the effects of El Niņo- Southern Oscillation (ENSO) vary during the course of a winter? This question is addressed quantitatively using ensembles of medium-range weather forecasts made with and without tropical sea surface temperature anomalies. The National Centers for Environmental Prediction operational medium-range forecast model is used. Results will presented for the 1997-98 warm event and the 1998-99 cold event. It is found that anomalous tropical forcing substantially affects forecast skill in midlatitudes as early as the fifth day of the forecast. The effect of the anomalous sea surface temperatures in the medium range is defined as the ENSO signal. The ENSO signal over North America is found to vary significantly durint the course of a season, and sometimes can depart dramatically from the midlatitude pattern classically associated with ENSO. <p>
Our method provides more information than conventional seasonal forecasts based on long-term observed correlations or based on simulations with climate general circulation models (GCMs), because it can detect non-standard or "unexpected" effects of ENSO, and because it provides information about the changing impacts of ENSO on time scales of a few days to a two weeks. Because our method is based on forecasts made with a skillful weather prediction model with observed initial conditions, we can also make deductions about individual weather events that occurred in nature. The statistics generated by the ensemble forecasting method enable statistical testing of the robustness of our conclusions. There is, however, one major limitation to our study that is inherent in our use of forecasts made with observed initial conditions: we are only able to deduce the effects of ENSO during the forecast period. The effects of ENSO on the initial conditions themselves are not accounted for. Therefore, it is likely that our method provides a conservative assessment of the effects of ENSO. <p>
Possible dynamical mechanisms for sub-seasonal variations in the ENSO signal include; 1) variations in the strength and location of the anomalous tropical convection, and 2) variations in the mid-latitude flow, particularly the Pacific Jet. Both of these processes can affect the propagation of baroclinic Rossby waves from the tropics into the mid-latitudes. The relative importance of these two mechanisms will be assessed, and their implications for our understanding of the impact of ENSO on extratropical climate will be discussed.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics