Decadal changes of El Niño - Southern Oscillation persistence barrier in 3 SST and ocean heat content indices : 1958 – 2001

6 [1] Decadal changes of persistence barriers in various indices of sea surface temperature 7 (SST) and ocean heat content (OHC) are examined in this study using observations and 8 ocean data assimilation products for the period 1958–2001. It is found that the SST 9 indices in the eastern and central equatorial Pacific exhibit very different decadal 10 barrier variability. The variability is large for the eastern Pacific SST indices (NINO1+2 11 and NINO3) whose persistence barriers shifted abruptly in 1976/1977 and 1989/1990. 12 In contrast, the central Pacific SST indices (NINO3.4 and NINO4) experienced little 13 decadal barrier variability and have had their persistence barriers fixed in spring in the past 14 four decades. The zonal mean OHC index averaged over the equatorial Pacific shows 15 decadal barrier changes similar to those in the eastern Pacific SST indices and always 16 leads the NINO3 SST barrier by about one season. It is noticed that the SST persistence 17 barrier appeared first in the eastern Pacific before 1976/1977, first in the central Pacific 18 between 1976/1977 and 1989/1990, and almost simultaneous in both the eastern and 19 central Pacific after 1989/1990. These timings coincide with the westward propagating, 20 eastward propagating, and standing pattern of El Niño-Southern Oscillation (ENSO) SST 21 anomalies observed in these three periods. These results suggest that ENSO SST 22 anomalies in the equatorial Pacific can be considered to consist of two different processes: 23 a central Pacific process whose phase transition (such as onset) and barrier always 24 happen in spring, and an eastern Pacific process whose phase transition and barrier change 25 from decade to decade and are influenced by changes in the mean state of the ocean 26 and atmosphere.