Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
22 August 2020

Budgets for Decadal Variability in Pacific Ocean Heat Content

Science

By comparing the relative contributions of surface heat flux and ocean dynamics to changes in OHC for different phases of IPO, we try to identify the underlying physical processes involved. Our results suggest that during IPO phase transitions, changes of 0-300-meter OHC across the northern extra-tropical Pacific are positively contributed by both surface heat flux and oceanic heat transport. By contrast, oceanic heat transport appears to drive the OHC changes in equatorial Pacific whereas surface heat flux acts as a damping term. During a positive phase of IPO, weakened wind-driven circulation acts to increase the OHC in the equatorial Pacific while the enhanced evaporation acts to damp OHC anomalies. In the Kuroshio-Oyashio Extension region, a dipole anomaly of zonal heat advection amplifies an OHC dipole anomaly that moves eastward, while strong turbulent heat fluxes act to dampen this OHC anomaly. In the northern subtropical Pacific, both the weakened evaporation and the downstream advection of the OHC anomaly near Kuroshio extension contributes to the OHC increase from a positive to negative IPO phase. For the northern subpolar Pacific, both surface heat flux and enhanced meridional advection contribute to the positive OHC anomalies during the positive IPO phase.

Impact

The mechanisms mentioned here may vary for other models and may also differ from those in observations. Due to the lack of systematic observational data, it is nearly impossible to do a similar type of analysis using observed data. Therefore it is important to investigate whether the same mechanism is at work for other coupled models, such as the models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5) or phase 6 (CMIP6). Potentially similar analysis can be done to the ocean data simulation products, such as ECCO reanalysis data.

Summary

We investigate the influence of the IPO on the time evolution of OHC and the related heat budget in the Pacific Ocean using a CESM1 preindustrial control simulation. Based on the model-simulated IPO frequency, we construct a 25-year non-overlapping ensemble composite for both OHC distribution and heat budget analysis around the composite positive IPO peak in order to examine the underlying physical processes affecting the OHC changes during IPO phase transitions and at IPO positive/negative peaks. Specifically, we assess the relative role of surface heat flux and oceanic heat transport in the temporal evolution of OHC distribution in the Pacific Ocean.

Around the positive IPO peak year, the pattern of OHC anomalies in the upper 300-meter layer shows a similar structure to SST anomalies in a typical climatological positive IPO mean state, with positive OHC anomalies over the equatorial Pacific and subpolar North Pacific, and negative anomalies over the subtropical North/South Pacific. In the Kuroshio-Oyashio Extension region, a dipole pattern of OHC anomalies (negative in the north and positive in the south) may represent a shift in the position of the Kuroshio Current under different phases of the IPO. During the IPO phase transition from positive to negative, this dipole pattern enhances and move eastward. However, during the IPO phase transition from negative to positive, the enhancement and eastward movement of this dipole pattern are not as clear.

Contact
Aixue Hu
NCAR
Publications
Hu, Z, A Hu, Y Hu, and N Rosenbloom.  2020.  "Budgets for Decadal Variability in Pacific Ocean Heat Content."  Journal of Climate 33(17): 7663-7678.  https://doi.org/10.1175/jcli-d-19-0360.1.