Intermodel Spread in the Impact of the Springtime Pacific Meridional Mode on Following-Winter ENSO Tied to Simulation of the ITCZ in CMIP5/CMIP6

The Pacific meridional mode (PMM) is the leading air-sea coupled mode over the subtropical northeastern Pacific and plays an important role in El Nino-Southern Oscillation (ENSO) occurrence. This study examined the performances of CMIP5/CMIP6 models in simulating the PMM and its association with ENSO. CMIP6 offers improvements in capturing the spatial structure and seasonality of PMM compared to CMIP5, but still considerably underestimates the air-sea coupling strength and coupling persistence time associated with the PMM. Additionally, large intermodel spread exists in the springtime PMM-wintertime ENSO relationship across the CMIP models, which is attributable to bias in the simulation of ITCZ. Models with a stronger climatological ITCZ provide favorable backgrounds for PMM-related sea surface temperature anomalies (SSTAs) to induce stronger atmospheric heating over the subtropical North Pacific. The enhanced air-sea coupling favors PMM-related SSTAs and atmospheric anomalies propagating southwestwards to the tropical central Pacific where they impact the following-winter ENSO.

Automated summary

The study found that CMIP6 models show improvements in simulating the Pacific meridional mode (PMM) compared to CMIP5, but still significantly underestimate the air-sea coupling strength and coupling persistence time associated with the PMM. There is a large intermodel spread in the relationship between springtime PMM and wintertime ENSO, attributed to bias in simulating the Intertropical Convergence Zone (ITCZ). Models with a stronger climatological ITCZ provide a favorable background for PMM-related sea surface temperature anomalies (SSTAs) to induce stronger atmospheric heating over the subtropical North Pacific, impacting the following-winter ENSO.