Modeling the 1998–2003 summer circulation and thermal structure in Lake Michigan

[1] A three-dimensional primitive equation numerical model was applied to Lake Michigan on a 2 km grid for 6 consecutive years to study interannual variability of summer circulation and thermal structure in 1998–2003. The model results were compared to long-term observations of currents and temperature at seven moorings and two NOAA buoys. The accuracy of modeled currents improved considerably relative to previous summer circulation modeling done on a 5 km grid, while the accuracy of temperature simulations remained the same. Particle trajectory model results were also compared with satellite-tracked surface drifter observations. Large-scale circulation patterns tend to be more cyclonic (counterclockwise) toward the end of summer as the thermocline deepens and density effects become more important. Circulation in southern Lake Michigan appears to be more variable than circulation in northern Lake Michigan. An important new feature not previously seen in observations was found in southern Lake Michigan: an anticyclonic gyre extending northward from the southern shore of Lake Michigan, sometimes occupying the entire southern basin.