Tropical Pacific Ocean in a warming climate

David Wang
Lamont-Doherty Earth Observatory


The tropical Pacific region is an essential part of global climate system. Recent studies suggest a number of robust global warming signatures in the tropical Pacific region, particularly the weakening of the Walker circulation. The present study further investigates the response of the tropical Pacific Ocean to future greenhouse warming in three aspects: the shallow meridional overturning circulation or subtropical cell (STC), the ventilation of the equatorial thermocline and undercurrent (EUC), and the regional sea level, in the framework of CMIP3 multi-model climate projection. First, it is found there is a contrasting tendency for the STC to be weakening (strengthening) in the northern (southern) hemisphere in the 21st century, consistent with large-scale surface wind change under global warming. The pycnocline transport convergence, as the equatorward branch of the STC, exhibits a robust weakening of about 3 Sv. The weakening mainly takes place in the ocean interior, consistent with the zonal slackening of the tropical pycnocline, whereas the change of the pycnocline transport along the western boundary is relatively small, affected by the shoaling of the pycnocline base. In addition, the Indonesian throughflow transport above the pycnocline base shows a robust weakening of about 1.5 Sv. Second, the source regions and transit time distributions of the EUC water are investigated using a simulated adjoint passive tracer for both the present-day climate and a warmer climate. Several major source regions are identified and associated with different transit times. In a warmer climate, the distribution of source waters are largely unchanged and the transit time becomes slightly younger. The warming of the EUC water can be traced back to the extra-tropical surface warming. Finally, it is found that the projected sea level change in the tropical Pacific region can be well reproduced by a linear, wind-driven model that embodies only baroclinic wave dynamics, especially when both the surface wind change and ocean stratification change are taken into account.