Satoru Okajima

Migratory cyclones and anticyclones account for most of the day-to-day weather variability in the extratropics. These transient eddies act to maintain the midlatitude jet streams by systematically transporting westerly momentum and heat. Yet, little is known about the separate contributions of cyclones and anticyclones to their interaction with the westerlies. Here, using a novel methodology for identifying cyclonic and anticyclonic vortices based on curvature, we quantify their separate contrib

Abstract Storm-track activity over the North Pacific (NP) climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM),” has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energet

Abstract Mechanisms for the maintenance of a large-scale wintertime atmospheric response to warm sea surface temperature (SST) anomalies associated with decadal-scale poleward displacement of the North Pacific subarctic frontal zone (SAFZ) are investigated through the following two ensemble experiments with an atmospheric general circulation model (AGCM): one with climatological-mean SST and the other with positive SST anomalies along the SAFZ prescribed on top of the climatological-mean SST. As

Abstract Sets of atmospheric general circulation model (AGCM) experiments are conducted to assess the importance of prominent positive anomalies in sea surface temperature (SST) observed over the midlatitude North Pacific in forcing a persistent basin-scale anticyclonic circulation anomaly and its downstream influence in 2011 summer and autumn. The anticyclonic anomaly observed in October is well reproduced as a robust response of an AGCM forced only with the warm SST anomaly associated with the

Abstract The North Pacific storm-track activity is suppressed substantially under the excessively strong westerlies to form a distinct minimum in midwinter, which seems inconsistent with linear baroclinic instability theory. This “midwinter minimum” of the storm-track activity has been intensively investigated for decades as a test case for storm-track dynamics. However, the mechanisms controlling it are yet to be fully unveiled and are still under debate. Here we investigate the detailed season

Abstract The Kuroshio‐Oyashio Extension and Gulf Stream oceanic frontal zones are characterized by enhanced activity of synoptic‐scale cyclones and anticyclones and vigorous air‐sea heat and moisture exchange in the cold season. However, the time‐mean air‐sea exchange attributed separately to cyclones and anticyclones has not been assessed. Here we quantify cyclonic and anticyclonic contributions around the frontal zones to surface turbulent heat fluxes, precipitation, and the associated hydrolo

Abstract Dynamical understandings of midlatitude transient eddy activity, especially its midwinter minimum over the North Pacific, are still limited, partly because conventional Eulerian eddy statistics are incapable of separating cyclonic and anticyclonic contributions. Here we evaluate the two contributions separately based on local curvature of instantaneous flow fields to compare their seasonality between the North Pacific and North Atlantic storm‐tracks. The anticyclonic contribution is fou

Abstract The frequency of extratropical cyclones in East Asia, including those traveling along the Kuroshio off the south coast of Japan, maximizes climatologically in early spring in harmony with local precipitation enhancement. Springtime cyclone activity is of great socioeconomic importance for East Asian countries. However, mechanisms for the early spring peak in the cyclone activity in East Asia have been poorly understood. This study aims to unravel the mechanisms, focusing particularly on

Abstract Chang (2024, https://doi.org/10.1029/2024gl110011 ) challenged the methodology proposed recently by Okajima et al. for evaluating cyclonic and anticyclonic contributions to Eulerian eddy statistics and atmospheric energetics based on the local flow curvature. He argued that using the local wind curvature to separate energetic contributions from cyclones and anticyclones is not physically meaningful. Here we argue that his claims are based on an unrealistic assumption of monopolar relati

Dynamical understandings of midlatitude transient eddy activity, especially its midwinter minimum over the North Pacific, are still limited, partly because Eulerian eddy statistics are incapable of separating cyclonic and anticyclonic contributions. Here we evaluate the two contributions separately based on local curvature of instantaneous flow fields to compare their seasonality between the North Pacific and North Atlantic storm-tracks. The anticyclonic contribution is found crucial for the mid

Abstract The 2023 East Asian summer experienced a record‐breaking compound hot‐humid extreme and an unprecedented concurrent marine heatwave (MHW) in surrounding oceans. Understanding and quantifying the impacts of such MHWs on land heatwaves can enhance seasonal prediction skills for these extremes. Here we evaluate the impact of the 2023 MHW on the record‐breaking atmospheric heatwave in East Asia during the summer. Through a set of regional atmospheric model experiments, we demonstrate that t

Chang (2024) challenged the methodology proposed recently by Okajima et al. for evaluating cyclonic and anticyclonic contributions to Eulerian eddy statistics and atmospheric energetics based on the local flow curvature. He argued that using the local wind curvature to separate energetic contributions from cyclones and anticyclones is not physically meaningful. Here we argue that his claims are based on an unrealistic assumption of monopolar relative vorticity in an entire storm-track domain and

The Kuroshio-Oyashio Extension and Gulf Stream oceanic frontal zones with sharp sea-surface temperature gradients are characterized by enhanced activity of synoptic-scale cyclones and anticyclones and vigorous air-sea exchange of heat and moisture in the cold season. However, the air-sea exchanges attributed separately to cyclones and anticyclones have not been assessed. Here we quantify cyclonic and anticyclonic contributions around the oceanic frontal zones to surface turbulent heat fluxes, pr

The 2023 East Asian summer experienced a record-breaking compound hot-humid extreme and an unprecedented concurrent marine heatwave (MHW) in surrounding oceans. Understanding and quantifying the impacts of such MHWs on land heatwaves can enhance seaso