Spatial patterns and recent trends in cloud fraction and cloud-related diffuse radiation in Amazonia

As the climate of tropical forest regions changes, there are likely to be concurrent changes in cloud cover and in the light regime experienced by tropical forest canopies. We utilize data from the International Satellite Cloud Climatology Project to examine spatial patterns and trends in cloud cover over Amazonia during the period 1984-2006. Cloud cover seasonality appears to be increasing in Amazonia, with a significant decline in dry season cloud fraction (0.3% yr(-1)) and increase in wet season cloud fraction (0.1% yr(-1)) over the last two decades. A novel cloud-related diffuse radiation (CRDR) climatology for Amazonia was derived from satellite cloud data. There is a clear decreasing gradient from the northwest to the southeast: annual CRDR proportion (CRDRP) varies by about 15% across the region. Analysis of trends over time indicates a 1-2% decline in CRDRP in Amazonia over the last two decades, particularly in the east of the region. This is particularly marked in the dry season in the east where CRDRP declined at a rate of 0.3% yr(-1), and the wet season decline was 0.1% yr(-1). In the west of the region a 1% increase in CRDRP is indicated. Changes in forest composition and productivity may be linked to changes in CRDRP in that decreases in cloud cover in sunny regions or dry seasons may cause a decline in productivity, whereas declines in cloud cover in cloudy regions, or during cloudy seasons, may cause an increase in productivity.