Dynamics of Upwelling Annual Cycle in the Equatorial Pacific Ocean

Abstract

A recent work proposed a simple theory based on the framework of Zebiak–Cane (ZC) ocean model, and successfully characterized the equatorial Atlantic upwelling annual cycle as a combination of the local wind-driven Ekman upwelling and nonlocal wind-driven wave upwelling. In the present work, utilizing the same simple framework, we examined the fidelity of the upwelling Pacific annual cycle using observations and simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We demonstrated that the theoretical upwelling annual cycles generally match the original upwelling annual cycles in the equatorial Pacific in both observations and CMIP5 simulations. Therefore, this simple formulation can be used to represent the upwelling annual cycle in the equatorial Pacific. Observationally, the equatorial Pacific upwelling annual cycle is dominated by the local wind-driven Ekman upwelling, while the remote wave upwelling is confined near the eastern boundary with little contribution. In CMIP5 simulations, though the theoretical-reconstructed upwelling well-reproduces the original upwelling, the contribution is totally different compared to the observation. The wave upwelling serves as the main contributor instead of the Ekman upwelling. We further demonstrated that such discrepancy is attributable to the bias of the central to eastern equatorial thermocline depth patterns. This amplified, westward-shift wave upwelling weakened the impacts of the Ekman upwelling, and contributes to the entire Pacific equatorial upwelling annual cycle substantially. This implies that a realistic simulation of the equatorial Pacific upwelling annual cycle in models is very sensitive to the careful simulation of the equatorial thermocline depth annual evolutions.

Continued weakening of the equatorial Pacific upwelling annual cycle in CMIP5 future projections

“…In the central basin, the downwelling (negative value) peaks around May–June, while the upwelling (positive value) occurs at the beginning and the end of year. Despite of different used period, these findings are consistent with Wang and Yu 15 , indicating the robustness of the results from ORA-S3 observational data. In the present stage of the CMIP5 simulations, the amplitudes of the upwelling annual cycles simulated by the 19 models appeared to be slightly uneven.…”

“…Despite some amplitude discrepancies, the simulated original and theoretical upwellings still reproduce key features of the observed ones. In addition, the weighted theory works well in identifying the equatorial Pacific upwelling annual cycle in observations as well as CMIP5 simulations, which is consistent with Wang and Yu 15 . Thus, this study continued to utilize CMIP5 models to investigate the potential changes of equatorial Pacific upwelling annual cycle under global warming.…”

“…Wang et al 13 showed that CMIP5 simulations can represent well the key features of equatorial upwelling annual cycle in the Atlantic Ocean compared to ORA-S3 observations. Wang and Yu 15 also pointed out that the theoretical and original upwelling annual cycles in the equatorial Pacific are generally consistent in observations as well as CMIP5 simulations. Because the data period and the number of selected CMIP5 models in this study are slightly different from those in Wang and Yu 15 , the performance of CMIP5 was re-examined in supplementary material (Fig.…”

“…For instance, some models (including (a) ACCESS1-0, (b) ACCESS1-3, (l) GISS-E2-R, (m) HadGem2-CC and (n) HadGEM2-ES) simulated relatively strong upwellings, and some models simulated relatively weak values in the eastern Pacific (including (c) BCC-CSM1-1 and (h) CMCC-CM). Such inter-model discrepancy in amplitudes of upwelling annual cycles is most likely resulted from the biased equatorial wind stress forcing or thermocline annual variations 13 – 15 . Previous studies have discussed similar dynamics that the weaker equatorial wind stress forcing might result in a flanker thermocline, weaker upwelling and SST variability 20 , 21 .…”

“…A recent study developed a linear-weighted theory to characterize the annual cycle of equatorial upwelling based on the Zebiak–Cane (ZC) ocean model 12 . They demonstrated that two important mechanisms jointly contributed to the equatorial upwelling annual cycle: (1) the Ekman upwelling directly driven by winds; and (2) the wave upwelling associated with vertical displacements of thermocline remotely induced by winds 13 – 15 .…”