Classifying and Forecasting Coastal Upwellings in Lake Michigan Using Satellite Derived Temperature Images and Buoy Data

Coastal upwellings are common in the Great Lakes but have lacked enumeration and systematic classification of spatial extent, frequency, duration, and magnitude. Near real-time sea surface temperature (SST) images derived from the Advanced Very High Resolution Radiometer (AVHRR) provide indices of upwelling events, but visual inspection of daily images can be tedious. Moreover, the definition of what constitutes an upwelling from AVHRR data is subjective. We developed a semi-automated method to classify upwellings during the period of thermal stratification using daily, cloud-free surface temperature charts from AVHRR SST data. Then we statistically evaluated the location, frequency, magnitude, extent, and duration of upwelling events in Lake Michigan from 1992–2000. Further, we analyzed meteorological data from the National Data Buoy Center buoys in an attempt to improve the reliability of the classification and to provide a means for future forecast of coastal upwelling. Although variable, upwelling events along the western shoreline were preceded by 4 days of southerly and west-to-northwesterly winds, while upwelling events occurring along the eastern shore were preceded by 4 days of northerly winds. Probability of an upwelling event occurring was a function of the direction-weighted wind speed, reaching a 100% probability at direction weighted wind speeds of 11 m s–1 for the western shore. Probability of an upwelling occurrence along the east coast reached 73% at 11 m s–1 and 100% at 13 m s–1. Continuous measurements of wind data with a sufficient temporal resolution are required during the entire upwelling season to improve the predictability of upwellings. INDEX WORDS: Upwelling, Lake Michigan, AVHRR, Great Lakes. *Corresponding author. E-mail: [email protected] Current address: NOAA Atlantic Oceanographic Meteorological Laboratory, 4301 Rickenbacker Causeway, Miami, FL 33149. †Current address: German Aerospace Center DLR, German Remote Sensing Data Center DFD, D-82234 Oberpfaffenhofen, Germany. INTRODUCTION Coastal upwelling, a phenomenon found in large stratified lakes, estuaries, and oceans, occurs when the continued forcing of offshore winds moves warm epilimnetic water away from the coast, transporting cold water from the hypolimnion to the surface. In the Laurentian Great Lakes, this exchange of water masses results in the short-term nutrient enrichment of surface waters (Haffner et al. 1984, Dunstall et al. 1990) and the redistribution or change in the nearshore plankton communities (Heufelder et al. 1982, Haffner et al. 1984, Dunstall et al. 1990, Megard et al. 1997). Moreover, upwellings may be responsible for reduction in species richness in benthic communities (Barton 1986, Kilgour et al. 2000), higher biomass and production of some benthic invertebrates (Nalepa et al. 2000), and changes in fish distributions through behavioral thermal regulation or passive advection (Heufelder et al. 1982, Fitzsimons et al. 2002). Upwellings may also cause mortality in adult fish due