Zachary Freitag Johnson

The 2025–26 late‐fall and winter climate pattern featured an extreme upper‐tropospheric zonal geopotential height gradient across North America, consisting of an amplified west ridge and an east trough, which produced record warmth and low snowpack across the western United States alongside severe cold and winter storms in the east. The dynamical origins of such extreme ridge‐trough patterns and whether their underlying circulation configurations are becoming more or less frequent under anthropogenic warming remain unresolved. Using ERA5 reanalysis and the CESM2 Large Ensemble, we find that the extreme ridge‐trough pattern was associated with convective suppression in the eastern tropical Pacific, consistent with a Gill‐type upper‐tropospheric response and a poleward‐propagating Rossby wave train that manifests as a zonal geopotential height gradient. We train a self‐organizing map on the joint 250 hPa streamfunction and outgoing longwave radiation anomalies and find that the convective‐circulation pattern over the eastern Pacific and North America sector, reminiscent of the 2025–26 extreme ridge‐trough pattern, increases in frequency by the end of the 21st century, despite a decrease in amplitude of the composite scalar defining the ridge‐trough pattern. The covariance between eastern tropical Pacific convection and the upper‐tropospheric circulation over the western United States also strengthens. This study suggests that the background state under anthropogenic warming increasingly favors the convective‐circulation pattern associated with the 2025–26 extreme ridge‐trough pattern.

The 2025-26 late-fall and winter climate pattern featured an extreme upper-tropospheric zonal geopotential height gradient across North America, consisting of an amplified west ridge and an east trough, which produced record warmth and low snowpack across the western US alongside severe cold and winter storms in the east. The dynamical origins of such extreme ridge-trough patterns and whether their underlying circulation configurations are becoming more or less frequent under anthropogenic warming remain unresolved. Using ERA5 reanalysis and the CESM2 Large Ensemble, we find that the primary physical mechanism driving the extreme ridge-trough pattern was convective suppression in the eastern tropical Pacific, inducing a Gill-type upper-tropospheric response and a poleward-propagating Rossby wave train that manifests as a zonal geopotential height gradient. We train a self-organizing map on the joint 250 hPa streamfunction and outgoing longwave radiation anomalies and find the convective-circulation pattern over the eastern Pacific and North America sector responsible for the 2025-26 extreme ridgetrough pattern approximately doubles in frequency by the end of the 21st century, despite a decrease in amplitude of the composite scalar defining the ridge-trough pattern. The coupling between eastern tropical Pacific convection and the upper-tropospheric circulation over the western US also strengthens. This study indicates that the background state under anthropogenic warming increasingly favors a tropically forced convective-circulation pattern associated with the 2025-26 extreme ridge-trough pattern.

Tropical cyclone (TC) seasonal landfall probability is challenging to forecast because of the limited seasonal predictability of steering flow patterns. Past studies mainly focus on the large-scale ocean and atmospheric conditions that lead to changes in seasonal tropical cyclone genesis frequency in a given basin, but less attention has focused on seasonal landfall probability inherent to changes in steering flow patterns linked to subtropical highs (STHs). Here, we examine SST anomaly patterns that control variability in summertime STH cells in the Northern and Southern Hemispheres. We link those ocean impacts to changes in early season TC landfall probability in the western North Pacific, North Atlantic, and south Indian Ocean Basins. STHs in the North Pacific, North Atlantic, and south Indian Ocean exhibit increased variability on their western peripheries linked to anomalous zonal SST gradients. In the Northern Hemisphere, an interbasin zonal contrast in SST anomalies fosters a westward extension in both North Pacific and North Atlantic STHs. As a result, TCs curve around STHs ∼6° in longitude farther west in the western North Pacific basin. In contrast, the Atlantic basin had the opposite effect due to minimal TC activity over the tropical Atlantic from inhibiting SST anomalies. The south Indian Ocean had a 9% increase in landfall probability for TCs that formed in the western half of the southern Indian Ocean during a positive localized SST dipole. The seasonal persistence of Southern Hemispheric STHs resembles aquaplanet simulations of STHs, in contrast to the seasonal evolution observed in their Northern Hemispheric counterparts.

Tropical cyclone (TC) seasonal landfall probability is challenging to forecast because of the limited seasonal predictability of steering flow patterns. Past studies mainly focus on the large-scale ocean and atmospheric conditions that lead to changes in seasonal tropical cyclone genesis frequency in a given basin, but less attention has focused on seasonal landfall probability inherent to changes in steering flow patterns linked to subtropical highs (STHs). Here, we examine SST anomaly patterns that control variability in summertime STH cells in the northern and southern hemispheres. We link those ocean impacts to changes in early-season TC landfall probability in the Western North Pacific, North Atlantic, and South Indian Ocean basins. STHs in the North Pacific, North Atlantic, and South Indian Ocean exhibit increased variability on their western peripheries linked to anomalous zonal SST gradients. In the northern hemisphere, an inter-basin zonal contrast in SST anomalies fosters a westward extension in both North Pacific and North Atlantic STHs. As a result, TCs curve around STHs 6 degrees in longitude farther west in the Western North Pacific basin. In contrast, the Atlantic basin had the opposite effect due to minimal TC activity over the tropical Atlantic from inhibiting SST anomalies. The South Indian Ocean had a 9% increase in landfall probability for TCs that formed in the western half of the Southern Indian Ocean during a positive localized SST dipole. The seasonal persistence of southern hemispheric STHs resembles aquaplanet simulations of STHs, in contrast to the seasonal evolution observed in their northern hemispheric counterparts.

A document by Zachary Johnson. Click on the document to view its contents.

The southwestern United States is highly sensitive to drought, prompting efforts to understand and predict its hydroclimate. Oftentimes, the emphasis is on wintertime precipitation variability, yet the southwestern United States exhibits a summertime monsoon where a significant portion of annual precipitation falls through daily convection activities manifested by a midtropospheric ridge of high pressure. Here, we examine synoptic patterns of the southwestern ridge through a k-means clustering analysis and assess how these synoptic patterns translate into streamflow changes in the upper Colorado River basin. A linear perspective suggests ∼17% of upper Colorado River discharge at the Lee’s Ferry, Arizona, gauge comes from summertime monsoon rains. The ridge of high pressure exhibits diversity in its intensity, structure, and position, inducing changes in moisture advection and precipitation. A ridge shifted north or east of its climatological center increases moisture and precipitation over the southwestern United States, while a ridge toward the south or northwest inhibits precipitation. A ridge east of its climatological center contributes to increased streamflow, whereas a ridge west or northwest of its climatological center decreases streamflow. Cooling in the central tropical Pacific and the Pacific meridional mode region favors an eastward shift of the ridge of high pressure corresponding to wet days. Eastern tropical Pacific warming favors a southward shift of the ridge corresponding to dry days. These results support an intermediate scale between climate forcing and summertime Colorado River discharge through changes in the intensity, structure, and position of the southwestern ridge of high pressure, integral to the U.S. Southwest hydroclimate.

Seasonal predictions of tropical cyclone (TC) landfalls are challenging because seasonal landfall count not only depends on the number and spatial distribution of TC genesis, but also whether those TCs are steered toward land or not. Past studies have separately examined genesis and landfall as a function of large-scale ocean and atmospheric environmental conditions. Here, we introduce a practical statistical framework for estimating the seasonal count of TC landfalls as the product of a Poisson model for seasonal TC genesis and a logistic model for landfall probability. We compute spatial variations in TC landfall and genesis by decomposing TC activity in the western North Pacific (WNP) basin into 10° × 10° bins, then identify coherent regions where El Niño–Southern Oscillation (ENSO) and the western extent of the Pacific subtropical high (WPSH) have significant influences on seasonal landfall count. Our framework shows that ENSO and the WPSH are weakly related to basinwide landfalls but strongly related to regional genesis and landfall probability. ENSO modulates the zonal distribution of TC genesis, consistent with past work, whereas the WPSH modulates the meridional distribution of landfall probability due to variations in steering flow associated with the Pacific subtropical high. These spatial patterns result in four coherent subregions of the WNP basin that define seasonal landfall variations: landfall count increases in the southwestern WNP during a positive WPSH and La Niña, the south-central WNP during a positive WPSH and El Niño, the eastern WNP during a negative WPSH and El Niño, and the northern WNP during a negative WPSH and La Niña.

Periods of water surplus and deficit in Northern California follow a pronounced quasi‐decadal cycle. This cycle is largely driven by the frequency of atmospheric rivers (ARs), affecting the region’s wet and dry periods. Our analyses demonstrate that the quasi‐decadal cycle of AR frequency relies on moisture transport associated with the position and intensity of the Aleutian Low. In observations, the Aleutian Low is shown to covary with tropical Pacific sea surface temperature anomalies. A modeling experiment, which incorporates ocean observations from the equatorial Pacific into the fully coupled climate model, provides support that the quasi‐decadal cycle of the Aleutian Low is forced by the tropical Pacific. Subsequently, the tropical Pacific modulates the wet season moisture transport toward California on decadal time scales, affecting AR frequency. These results provide metrics for improving interannual‐to‐decadal prediction of AR activity, which drives hydrological cycles in Northern California.

The El Niño–Southern Oscillation (ENSO) exerts a strong influence on tropical Atlantic variability, but it is also affected by Atlantic forcing. Previous research has proposed three Atlantic precursors for ENSO: the North tropical Atlantic, the equatorial Atlantic, and the entire tropical Atlantic. However, the relative importance of these Atlantic precursors for ENSO remains unclear. Here, we present evidence from a set of multimodel partial ocean assimilation experiments that equatorial Atlantic cooling is the main contributor for weakening equatorial zonal winds in the Indo‐Pacific sector and subsequent ocean warming in the tropical Pacific. Opposite tendencies occur for a warmer equatorial Atlantic. The equatorial Atlantic affects the interbasin climate seesaw between the Atlantic and Pacific through an atmospheric zonal Wavenumber 1 pattern. However, model mean state biases and systematic errors prevent a precise assessment of the response times for the equatorial Pacific trade winds to Atlantic forcing.

The Pacific Decadal Oscillation (PDO), the leading mode of Pacific decadal sea surface temperature variability, arises mainly from combinations of regional air-sea interaction within the North Pacific Ocean and remote forcing, such as from the tropical Pacific and the Atlantic. Because of such a combination of mechanisms, a question remains as to how much PDO variability originates from these regions. To better understand PDO variability, the equatorial Pacific and the Atlantic impacts on the PDO are examined using several 3-dimensional partial ocean data assimilation experiments conducted with two global climate models: the CESM1.0 and MIROC3.2m. In these partial assimilation experiments, the climate models are constrained by observed temperature and salinity anomalies, one solely in the Atlantic basin and the other solely in the equatorial Pacific basin, but are allowed to evolve freely in other regions. These experiments demonstrate that, in addition to the tropical Pacific’s role in driving PDO variability, the Atlantic can affect PDO variability by modulating the tropical Pacific climate through two proposed processes. One is the equatorial pathway, in which tropical Atlantic sea surface temperature (SST) variability causes an El Niño-like SST response in the equatorial Pacific through the reorganization of the global Walker circulation. The other is the north tropical pathway, where low-frequency SST variability associated with the Atlantic Multidecadal Oscillation induces a Matsuno-Gill type atmospheric response in the tropical Atlantic-Pacific sectors north of the equator. These results provide a quantitative assessment suggesting that 12–29% of PDO variance originates from the Atlantic Ocean and 40–44% from the tropical Pacific. The remaining 27–48% of the variance is inferred to arise from other processes such as regional ocean-atmosphere interactions in the North Pacific and possibly teleconnections from the Indian Ocean.