•They are instabilities of the
background flow
•They dominate the analysis
and forecast errors
•They are not taken into
account in data assimilation except for 4D-Var and Kalman Filtering (very expensive methods)
•Their shape can be estimated with breeding
•Their shape is frequently simple (low dimensionality, Patil et al, 2001)
•A coupled ENSO system contains “errors of the month”