Journal Article

The Influence of Extratropical Ocean on the PNA Teleconnection: Role of Atmosphere‐Ocean Coupling

Hiroaki Tatebe, Hiroki Tokinaga, Hisashi Nakamura, Yu Kosaka, Masato Mori, Bunmei Taguchi

2024

Overview

The Pacific/North American (PNA) pattern is a major low‐frequency variability in boreal winter. A recent modeling study suggested that PNA variability increases through extratropical atmosphere‐ocean coupling, but the effect was not fully extracted due to a particular experimental design. By comparing coupled and two sets of uncoupled large‐ensemble global model simulations, here we show that the PNA‐induced horseshoe‐shaped sea‐surface temperature (SST) anomaly in the North Pacific returns a non‐negligible influence on the PNA itself. Its magnitude depends on the presence or absence of atmosphere‐ocean coupling. The coupling accounts for ∼16% of the PNA variance, while the horseshoe‐shaped SST anomaly explains only 5% under the uncoupled condition. The coupling reduces the damping of available potential energy by modulating turbulent heat fluxes and precipitation, magnifying the PNA variance. Precipitation processes in the extratropics as well as tropics are therefore important for realistically representing PNA variability and thereby regional weather and climate. Plain Language Summary Atmospheric flow is not entirely random; patterns of circulation variability appear recurrently in the same regions, known as teleconnection patterns. A major wintertime teleconnection pattern over the North Pacific‐North American sector is called the Pacific/North American (PNA) pattern. It causes strong fluctuations in precipitation, air temperature, and pressure over North America through persistent strengthening or meandering of the jet stream. While the influence of tropical ocean variability, such as El Niño/La Niña, on the formation and persistence of the PNA has been known, the role of the extratropical ocean remains unclear. Here we perform a vast number of numerical model simulations to detect the influence of the extratropical ocean on PNA. We show that the atmosphere‐ocean coupling (two‐way interaction between the ocean and atmosphere) enhances the PNA variability (i.e., the magnitude of meandering and strengthening of the westerlies) compared to the uncoupled condition. Furthermore, we propose possible mechanisms behind this enhancement. The findings of this study are expected to contribute to improving the accuracy of long‐term forecasts, such as one‐month predictions, and reducing uncertainty in future climate change projections through the improvement of numerical models. Key Points Extratropical air‐sea coupling enhances the variance of the Pacific/North American (PNA) pattern, which explains 16% of the total variance The enhancement is due to the reduced damping of available potential energy through modulations of turbulent heat fluxes and precipitation Atmosphere‐only simulation is likely to underestimate the impact of extratropical sea surface temperature anomalies on the PNA variability

• Publisher: American Geophysical Union (AGU),John Wiley & Sons, Inc,Wiley
• Subject: Air temperature / air‐sea coupling / air‐sea interaction / Atmosphere / atmosphere‐ocean coupling / Atmospheric turbulence / Climate change / Climate models / Climate prediction / Coupling / Damping / Design of experiments / El Nino / El Nino phenomena / Experimental design / Future climates / Geophysics. Cosmic physics / Heat flux / Heat transfer / Jet stream / La Nina / Mathematical models / Meandering / Numerical models / Numerical simulations / Ocean models / Oceans / Pacific-North American (PNA) pattern / PNA / Potential energy / Precipitation / Precipitation processes / QC801-809 / Sea surface temperature / Sea surface temperature anomalies / Strengthening / Surface temperature / teleconnection / Teleconnection patterns / Teleconnections / Tropical environments / Variability / Variance / Westerlies