The signature of sea surface temperature anomalies on the dynamics of semiarid grassland productivity

We used long‐term observations of grassland aboveground net plant production (ANPP, 1939–2016), growing seasonal advanced very‐high‐resolution radiometer remote sensing normalized difference vegetation index (NDVI) data (1982–2016), and simulations of actual evapotranspiration (1912–2016) to evaluate the impact of Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (ENSO) sea surface temperature (SST) anomalies on a semiarid grassland in northeastern Colorado. Because ANPP was well correlated ( R 2 = 0.58) to cumulative April to July actual evapotranspiration (iAET) and cumulative growing season NDVI (iNDVI) was well correlated to iAET and ANPP ( R 2 = 0.62 [quadratic model] and 0.59, respectively), we were able to quantify interactions between the long‐duration (15–30 yr) 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) was much higher, compared to warm‐phase PDO years. In addition, ANPP, iNDVI, and iAET were highly variable 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 semiarid 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.