The signature of sea surface temperature anomalies on the dynamics of semi-arid grassland productivity

Authors: CHEN, MAOSI - Colorado State University; PARTON, WILLIAM - Colorado State University; Del Grosso, Stephen - Steve; HARTMAN, MELANNIE - Colorado State University; DAY, KEN - Agricultural Technology Innovation Partnership (ATIP); TUCKER, COMPTON - National Aeronautics And Space Administration (NASA); Derner, Justin; KNAPP, ALAN K - Colorado State University; SMITH, WILLIAM - University Of Minnesota; OJIMA, DENNIS - Colorado State University; GAO, WEI - Colorado State University

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2017
Publication Date: 12/28/2017
Citation: Chen, M., Parton, W.J., Del Grosso, S.J., Hartman, M.D., Day, K., Tucker, C.J., Derner, J.D., Knapp, A., Smith, W.L., Ojima, D.S., Gao, W. 2017. The signature of sea surface temperature anomalies on the dynamics of semi-arid grassland productivity. Ecosphere. 8(12):e02069. https://doi.org/10.1002/ecs2.2069.
DOI: https://doi.org/10.1002/ecs2.2069

Interpretive Summary: We used long term observations of aboveground net primary productivity plant production (ANPP, 1939 to 2016), of grassland growing season remotely sensed normalized difference vegetation index (NDVI 1982 to 2016) and actual evapotranspiration (AET 1912 to 2016) to evaluate the impact of Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO) sea surface temperature (SST) anomalies on a semi-arid grassland in Northeastern Colorado. Because ANPP was well correlated (R2=0.58) to cumulative April to July actual evapotranspiration (iAET) and cumulative growing season NDVI (iNDVI) was well correlated to iAET and ANPP, we were able to quantify interactions between the long-duration (15-30 year) PDO temperature cycles and annual-duration ENSO SST phases on ANPP. We found that during cold phase SST PDO patterns, mean ANPP and iNDVI were lower, and the frequency of low ANPP years (drought years) were much higher, compared to warm phase PDO years. In addition, ANPP, iNDVI and iAET were highly variability during the cold phase PDOs. When ENSO values were negative there was a higher frequency of droughts and lower frequency of wet years regardless of the PDO phase. PDO and NINO-3 anomalies reinforced each other resulting in a high frequency of above normal iAET (52%) and low frequency of drought (20%) when both PDO and ENSO values were positive and the opposite pattern when both PDO and ENSO values were negative. Precipitation variability and subsequent ANPP dynamics in this grassland were dampened when PDO and ENSO SSTs had opposing signs. Thus, primary signatures of these SSTs in this semi-arid grassland are 1) increased inter-annual variability in ANPP during cold phase PDO’s, 2) drought with low ANPP occurring in almost half of those years with negative values of PDO and NINO-3, and 3) high precipitation and ANPP common in years with positive PDO and ENSO values.

Technical Abstract: We used long term observations of grassland plant production (ANPP, 1939 to 2016), growing seasonal AVHRR remote sensing NDVI data (1982 to 2016) and simulations of actual evapotranspiration (AET, 1912 to 2016) to evaluate the impact of Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomalies on a semi-arid grassland in Northeastern Colorado. Because ANPP was well correlated (R2=0.58) to cumulative April to July actual evapotranspiration (iAET) and cumulative growing season NDVI (iNDVI) was well correlated to iAET and ANPP [R2 = 0.62 (quadratic model) and 0.59, respectively], we were able to quantify interactions between the long-duration (15-30 year) PDO temperature cycles and annual-duration ENSO SST phases on ANPP. We found that during cold phase PDOs, mean ANPP and iNDVI were lower, and the frequency of low ANPP years (drought years) were much higher, compared to warm phase PDO years. In addition, ANPP, iNDVI and iAET were highly variability during the cold phase PDOs. When NINO-3 (ENSO index) values were negative there was a higher frequency of droughts and lower frequency of wet years regardless of the PDO phase. PDO and NINO-3 anomalies reinforced each other resulting in a high frequency of above normal iAET (52%) and low frequency of drought (20%) when both PDO and NINO-3 values were positive and the opposite pattern when both PDO and NINO-3 values were negative (24% frequency of above normal and 48% frequency of drought). Precipitation variability and subsequent ANPP dynamics in this grassland were dampened when PDO and NINO-3 SSTs had opposing signs. Thus, primary signatures of these SSTs in this semi-arid grassland are 1) increased interannual variability in ANPP during cold phase PDOs, 2) drought with low ANPP occurring in almost half of those years with negative values of PDO and NINO-3, and 3) high precipitation and ANPP common in years with positive PDO and NINO-3 values.