We use the GEOS-Chem model to interpret long-term measurements of tropospheric $ {O}_{3}$ and CO and to investigate the factors that contribute to their interannual variation (IAV) during the period from 1987 to 2005 with a focus on the northern mid-latitudes. We find that the model reproduces relatively well the observed IAV of $ {O}_{3}$ and CO in general, except the negative anomalies in $ {O}_{3}$ from 1991 to 1993 as well as the following upward trend in 1993-1996 observed at several sites in the northern mid-latitudes which are not represented by the model. We suggest that this likely results from a poor representation of stratospheric chemistry and dynamics in the model. Using a variety of sensitivity and tagged $ {O}_{3}$ simulations, we examine in details the period of 1998-1999, when a large anomaly in TOC is observed and simulated over Europe (maximum of +4.9 DU or +14% in February 1998). Three consecutive periods can be distinguished during this longlasting anomaly, during which different processes affected the $ {O}_{3}$ burden over Europe. The first period (spring 1998) is more largely influenced by the preceding 1997 El Nino that affects 1) stratosphere-troposphere exchange and 2) Asian pollution export and transport towards Europe by a change in convective activity in East Asia and a strengthening of the subtropical jet stream. An enhanced pollution export from North America is also noticed for this period. The second period (summer-fall 1998) shows a mixed influence from boreal wildfires occurring in summer 1998 and from Asian pollution. The third period is more significantly influenced by enhanced wildfires in Southeast Asia. Throughout the period from 1997 to 2005, positive anomalies in tropospheric $ {O}_{3}$ column and in surface $ {O}_{3}$ are found over Europe the following spring of an El Nino year.