Response of the South Eastern Pacific Oxygen Minimum Zone to ENSO

While observations suggest a long-term expansion of the Oxygen Minimum Zone (OMZ) in the South Eastern Pacific (SEP), it also exhibits a large interannual to decadal variability in its upper and lower limits. The uncertainty of the fate of the SEP OMZ in a warmer climate as simulated by Earth system models also questions to which extent natural variability in the OMZ can obscure the detection of externally forced trends. Here we analyze long-term simulations from a hierarchy of models of the OMZ off Peru and Chile and show that a significant share of the variability is not linearly related to climate modes (including ENSO), suggesting that it originates from internal dynamics associated to both local non-linear physical and biogeochemical processes. Still the OMZ volume tends to shrink during strong Eastern Pacific El Niño while it expands during La Niña and Central Pacific El Niño events. It is shown in particular that La Niña and strong El Niño events significantly modulate the OMZ volume through the transport of water masses of equatorial origin by the Peru/Chile undercurrent and the propagation of extra-tropical Rossby waves. Implications of our results are discussed in terms of the challenge for developing an Eastern Pacific observing system in the frame of the TPOS2020 program, considering the time of emergence for climate-trend detection above background variability