A LONGER, LESS VIGOROUS MID HOLOCENE CALIFORNIA CURRENT COASTAL UPWELLING SEASON?: DATA-MODEL COMPARISONS

Marine records clearly show activity in the California Current system to have been altered during the Mid Holocene. However, the specific nature of California Current activity during this period remains ambiguous, as proxies suggesting cooler than present sea surface temperatures co-occur in Mid Holocene marine sediments with multiple proxies suggesting decreased coastal upwelling. To test the sensitivity of wind driven upwelling in the California Current to Mid Holocene orbital forcing, we have employed a high resolution regional climate model. The seasonality of modern California Current wind driven coastal upwelling simulated by the regional climate model is in strong agreement with observational data. Further, we show that changes in the seasonality of solar insolation induced by Mid Holocene Milankovitch forcing decrease early and peak season coastal upwelling in the northern limb of the California Current, along with increasing late season coastal upwelling in that region. These results are in strong agreement with indirect proxies of coastal upwelling extracted from near shore marine sediments off of northern California. For instance, estimated opal percentages and coastal redwood pollen abundances indicate decreased coastal upwelling in the northern limb of the California Current during the Mid Holocene. Additionally, a Mid Holocene decrease of Pseudoeunotia doliolis, a warm gyre diatom, implies increased autumn upwelling during that time. Overall, our data-model comparisons suggest that, relative to present, California Current activity during the Mid Holocene was characterized by a longer and less vigorous coastal upwelling season, with decreased seasonal contrast.