Positive forcing over land cools the Eastern Pacific Ocean

We set out to disentangle the impacts of forcing over land vs. forcing over ocean on the sea surface temperature (SST) pattern. Based on previous research showing that forcings over land and ocean have distinct impacts on the circulation, we hypothesize that they would also affect the pattern of sea surface temperatures in different ways. We investigate the research question by quadrupling the CO₂ concentration either only over ocean or only over land in the coupled global climate model MPI-ESM-1.2.

Our main results are:

• the climate response to 4 x CO₂ forcing only over land surface and forcing only over ocean adds up surprisingly linearly to the climate response to forcing everywhere.

• 4 x CO₂ forcing over land causes a cooling of up to 1.4 K in the equatorial, Eastern, and Southeastern Pacific Ocean within two years. In contrast, positive forcing over the ocean does not produce such a cooling on any time scale

• Two main mechanisms contribute to the Pacific cooling in response to positive forcing over land:

  • (a) a northward ITCZ shift originating from the fact that there is more land in the Northern than the Southern hemisphere, enhancing equatorial upwelling and cooling from strengthened trade winds

  • (b) the monsoon-desert mechanism (Rodwell & Hoskins 1996), which strengthens the subtropical highs in response to atmospheric heating over the Americas, increasing the equatorward advection of cold air and initiating a wind-evaporation-SST feedback.

 

We find an equatorial and Eastern Pacific cooling not only in the abrupt land-forced simulation, but also in a transient simulation forced with a 1% / year CO₂ increase over land, on a time scale of 20 years. The surprising finding that a positive forcing can cause a cooling in the Eastern Pacific, along with the mechanisms we describe, may contribute to better understanding the recent cooling of the Eastern Pacific Ocean as well as the long-standing model bias in simulating Eastern Pacific sea surface temperature patterns.