Sea Surface Temperature Errors Push the North Pacific Westerly Jet Southward

Climate models are the workhorses of modern climate science, and their credibility rests on how faithfully they reproduce the present-day world. Every projection of future warming, every estimate of regional rainfall change, and every assessment of shifting storm tracks ultimately depends on simulations that begin with an accurate picture of today’s climate. Yet no model is perfect. When researchers compare simulated present-day conditions against decades of observations, systematic deviations known as biases appear in nearly every field the models produce, from cloud cover to ocean heat content. A new study published in Geophysical Research Letters by researchers at Ehime University examines one such bias and traces its consequences deep into the atmospheric circulation of the North Pacific, showing that errors in sea surface temperature can physically displace one of the most important wind systems in the Northern Hemisphere.

The focus of the study is the North Pacific westerly jet, a narrow band of strong winds that blows from west to east in the upper troposphere at midlatitudes. The jet is not merely a curiosity of atmospheric dynamics; it acts as a guiding current for weather systems, steering storm tracks, shaping the position of surface pressure patterns, and influencing temperature and precipitation across East Asia, including Japan. When the jet sits in its correct position, the models that depend on it can represent regional climate with reasonable skill. When it drifts, the errors propagate. A jet displaced to the south or north of its observed position can shift storm tracks, alter the transport of heat and moisture, and distort the simulated response of East Asian climate to greenhouse gas forcing. Understanding what pushes the jet out of place is therefore a matter of practical consequence for anyone who relies on model projections in this densely populated region.

The research team approached the problem by exploiting a distinctive feature of the Coupled Model Intercomparison Project Phase 6, or CMIP6, the international framework through which research institutions around the world contribute coordinated climate model experiments. CMIP6 includes two families of simulations that are directly relevant here. In the first, fully coupled simulations, the atmosphere and ocean evolve together, and the sea surface temperature emerges from the model’s own physics. In the second, atmosphere-only simulations, the ocean is not free to evolve; instead, the atmosphere is driven by observed sea surface temperatures prescribed from historical records. By comparing the position of the westerly jet across these two types of experiments, the researchers could isolate the influence of the sea surface itself, because the atmospheric models and their physics remain the same while the treatment of the ocean surface changes.

The comparison produced a strikingly consistent result. In 34 of the 35 models examined, the westerly jet over the North Pacific sat farther south in the coupled simulations, where sea surface temperature was generated by the models themselves, than in the atmosphere-only simulations, where observed sea surface temperatures were prescribed. This near-unanimous pattern across a diverse international ensemble is significant because it suggests a shared, structural problem rather than a quirk of any individual model. If only a handful of models showed the displacement, it might be attributed to idiosyncratic parameter choices or numerical schemes. The fact that virtually the entire ensemble shifts the jet southward when the ocean is simulated points to a common source, and the most obvious candidate is the systematic sea surface temperature bias that coupled models share.

Establishing a correlation between simulated sea surface temperatures and jet position was only the first step. To demonstrate that the temperature biases actually cause the southward displacement, the researchers turned to targeted atmospheric model experiments. In these experiments, the biases in sea surface temperature diagnosed from the coupled climate models were added directly onto the observed sea surface temperatures, and the atmospheric model was then run with this artificially biased ocean surface. The result was decisive: the experiments reproduced the southward shift of the westerly jet. This is a clean causal demonstration. The atmospheric model, its physics, and its initialization were held fixed, and the only change was the pattern of sea surface temperature. When the biases were imposed, the jet moved. When they were absent, it stayed where observations say it should be.

The team then refined the analysis by asking where, geographically, the responsible biases reside. The sea surface temperature errors in climate models are not uniform across the basin; they vary in sign and magnitude from one region to another, with some areas running too cold relative to observations and others too warm. By imposing the biases separately in different regions, the researchers could test the contribution of each. These regional experiments revealed that biases in the northwestern Pacific play a particularly important role in shifting the westerly jet southward. This finding is physically meaningful, because the northwestern Pacific lies upstream of and adjacent to the jet’s entrance region over East Asia, where the atmosphere is especially sensitive to the thermal contrast between the ocean surface and the continent. Errors in the surface temperature field in this sensitive zone can reshape the large-scale temperature gradients that the jet depends on for its position and strength.

The mechanism connecting sea surface temperature to jet position runs through the atmosphere’s fundamental dynamics. The midlatitude westerly jets are thermally driven winds, sustained by the meridional temperature gradient between the warm tropics and the cold pole. Anything that alters the distribution of temperature in the lower atmosphere alters that gradient, and with it the vertical wind shear and the latitude at which the jet core settles. A sea surface temperature bias in the northwestern Pacific changes the heating of the air column above it, modifies the temperature gradient, and through the response of the atmospheric circulation nudges the jet away from its observed latitude. Because the jet then organizes storm tracks and eddy activity around itself, the displacement is not a transient perturbation but a self-consistent rearrangement of the circulation that persists for as long as the underlying bias does.

The implications extend well beyond the North Pacific. The westerly jet is closely tied to weather and climate in East Asia, including Japan, where it modulates the East Asian winter monsoon, the passage of extratropical cyclones, and the distribution of precipitation through the year. A model whose jet is systematically displaced southward will tend to place weather systems in the wrong places, and those placement errors will be baked into projections of how the region’s climate changes over the coming decades. The study’s central conclusion is that sea surface temperature biases play an important role in biases in the position of the North Pacific westerly jet, which means that a substantial fraction of the circulation error in these models traces back to the ocean surface rather than to the atmospheric physics itself.

This attribution carries a practical message for the modeling community. Improving the representation of sea surface temperature in climate models, whether through better simulation of ocean currents, mixed-layer processes, air-sea heat exchange, or the cloud fields that control the surface energy balance, could therefore lead to a more accurate representation of the atmospheric circulation affecting Japan and East Asia. In an era when regional climate information is increasingly demanded by governments, insurers, and infrastructure planners, narrowing the gap between simulated and observed circulation is one of the most direct routes to more reliable projections. The study also illustrates the value of the CMIP6 experimental design itself: the pairing of coupled and atmosphere-only simulations, combined with targeted sensitivity experiments, gives scientists a powerful diagnostic toolkit for tracing model errors to their sources.

More broadly, the work is a reminder that the reliability of climate projections is built from the ground up, one bias at a time. A temperature error of a degree or two at the ocean surface may seem trivial against the scale of the Pacific, but the atmosphere integrates such errors, amplifies them through circulation dynamics, and expresses them as displaced jet streams and shifted storm tracks. By demonstrating that 34 of 35 CMIP6 models share this vulnerability, and by pinpointing the northwestern Pacific as the critical region, the Ehime University team has given model developers a concrete target and given users of climate projections a clearer sense of where uncertainty in East Asian circulation originates. The path to trustworthy future climate scenarios runs through the present-day simulation, and studies of this kind map that path with increasing precision.

Subject of Research: The influence of sea surface temperature biases on the position of the North Pacific westerly jet in CMIP6 climate models

Article Title: Can errors in sea surface temperature shift the westerly jet?

Article References: Can errors in sea surface temperature shift the westerly jet?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: sea surface temperature, climate models, westerly jet, North Pacific, CMIP6, model bias, atmospheric circulation, East Asia, Japan, Geophysical Research Letters, Ehime University, storm tracks