Amazonian drought – does the SeaWinds scatterometer active microwave data contain a signal?

Abstract - A strong drought occurred in southwestern Amazonia in 2005 (Maerngo et al. 2008), and the passive optical/NIR record of this drought’s impact on the forest, as observed by MODIS, has been interpreted in different ways (Saleska et al. 2007; Samanta et al. 2010-in press). Interpretation of the optical/NIR signal is compromised by variable atmospheric aerosol depths and frequently obscured by clouds. The sensitivity of neotropical evergreen broadleaf forests to drought is still not well-known. We analyze nine years (2000-2008) of active microwave backscatter collected by SeaWinds on QuikScat (Ku-band; f = 13.4 GHz, l = 2.24 cm) over northern South America, using data from the NASA Scatterometer Climate Record Pathfinder project at Brigham Young University (http://www.scp.byu.edu/). We analyzed both horizontally (HH) and vertically (VV) polarized backscatter, developing a 9-year climatologies of each and of the difference between ascending (morning overpass) and descending (eveningoverpass) at a minimum spatial resolution of ~ 4.5 km, and a minimum temporal resolution of 4 days. In parallel to the MODIS analyses of Saleska et al. (2007) and Samanta et al. (2010-in press), we compute a ‘full-record’ backscatter climatology (2000-2008) and then look at spatial and temporal patterns in seasonal anomalies from this climatology for a signal of the 2005 drought relative to other years.

Anomalies and Landcover (MCD12C1):

ReferencesMarengo, J. A., C. A. Nobre, J. Tomasella, M. D. Oyama, G. S. De Oliveira, R. De Oliveira, H. Camargo, L. M. Alves, and I. F. Brown (2008), The drought of Amazonia in 2005, J. Clim., 21(3), 495–516.Saleska, S. R., K. Didan, A. R. Huete, and H. R. da Rocha (2007), Amazon forests green‐up during 2005 drought, Science, 318(5850), 612.Samanta, A., S. Ganguly, H. Hashimoto, S. Devadiga, E. Vermote, Y. Knyazikhin, R. R. Nemani, and R. B. Myneni (2010), Amazon forests did not green‐up during the 2005 drought, Geophys. Res. Lett., 37, L05401, doi:10.1029/2009GL042154.

Backscatter trends – land cover change?

JAS Monthly Mean Backscatter Anomalies (V-V Ascending)Backscatter – EVI correlation:

Correlation between MODIS EVI and SeaWinds backscatter over 9 years (2000-2008) is generally low or negative in broadleaf forest..

Trend in backscatter at eastern site (lower panel) leads to increased frequency of negative anomalies late in time series. Is this evidence of land use land cover change (see google earth landscape shot of this 4.5 km pixel in blue panel above)? Western site (upper panel) has no significant trend in backscatte. This site has no sign of disturbance in google earth landscape shot (see blue panel above).

evergreen broadleaf forest

other

Significant negative anomaly in backscatter in August and September 2005 in western Amazon.

Conclusions so far:• there is a strong signal (reduced backscatter) in 2005

in drought region - not the only signal in the record.

Still to be determined:• what does the signal represent? NOT a record of greening/browning or productivity; probably a record of total canopy water content.• anomaly relationship with land cover and change.• anomaly relationship with precipitation anomalies. • spatial & temporal correlations with MODIS EVI.

July August September

4.5 km pixel