Climate change projection for the western tropical Pacific Ocean using a high-resolution ocean model: Implications for tuna fisheries

The Western Pacific Warm Pool is a region of high tuna catch, and how future climate change might impact the tuna fisheries is an important regional issue. By using a high-resolution ocean model forced by the simulated climate of the 2060s, we investigate whether enhanced spatial resolution and bias correction of the mean state could alter the climate change projection for the western tropical Pacific and examine the consequences this might have for tropical tuna distributions. For most of the physical environmental variables, enhanced resolution and bias correction had only a minor impact on the projected changes. The climate projections showed a maximum surface warming east of the Warm Pool, a shoaling of the thermocline in the Warm Pool, and an eastward expansion of the Warm Pool. In the Warm Pool, the shoaling of the thermocline raises the nutricline into the photic zone and increases phytoplankton and primary productivity, a feature that is most evident in the high-resolution model projection but also weakly present in the coarse-resolution projection. The phytoplankton and primary productivity response to climate change was where ocean model resolution produced a clear difference. With enhanced resolution, the simulation had stronger and better-defined zonal currents, which were more consistent with observations. Along the equator, the high-resolution model enabled vertical current shear mixing to generate a sub-surface phytoplankton maximum both inside and outside the Warm Pool, which is an observed phenomenon. With climate change, the enhanced-resolution model projected enhanced vertical shear mixing, increased vertical supply of nutrients to the photic zone, and increased sub-surface phytoplankton concentrations. The increase in sub-surface phytoplankton concentrations helps to offset the decline in surface phytoplankton concentrations and results in a projection of almost no change in the western tropical Pacific primary productivity. In contrast, the low-resolution model projected a substantial reduction in phytoplankton concentrations and primary productivity; such a response is typical of climate change projections for the region. Importantly, enhanced resolution dramatically altered the projected response of phytoplankton and primary productivity to climate change. Using the enhanced-resolution model, the projected increase in the size of the Warm Pool with little change in primary productivity and in suitable habitat for skipjack tuna suggest that by the 2060s climate change will not have a large impact on skipjack tuna fisheries.