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, precipitation, and the associated hydrological cycle. The evaluation reveals that precipitation exceeds evaporation climatologically within cyclonic domains while evaporation dominates within anticyclonic domains. These features as well as the net moisture transport from anticyclonic to cyclonic domains are all enhanced in the presence of the frontal zones. Oceanic frontal zones thus climatologically act to strengthen the hydrological cycle through increasing low-level storm-track activity and specific humidity. These findings aid our understanding of the relationship between midlatitude air-sea interactions on synoptic- and longer-time scales.

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 a meridionally uniform zonal background flow atypical to midlatitudes. We also demonstrate that the error in attributing eddy statistics to cyclones and anticyclones is significantly smaller than his estimation. Rather, we further demonstrate that the curvature-based methodology effectively eliminates the shear influence to identify cyclonic and anticyclonic regions, which is dismissed in his argument. We conclude that the curvature-based methodology is beneficial in evaluating distinct cyclonic and anticyclonic contributions to atmospheric energetics in realistic conditions.

The Northern Annular Mode (NAM) is the dominant pattern of atmospheric circulation variability in the wintertime Northern Hemisphere extratropics. This study utilizes a large ensemble atmospheric simulation dataset to examine the seasonality of the NAM variability and its modulations under global warming. We show an enhancement of the Aleutian Low anomaly associated with the NAM in a warmer climate. This enhancement is related to the emergence of the Aleutian-Icelandic Low seesaw (AIS) from early winter, which is in contrast prominent only in late winter in the historical climate. The large ensemble reveals a significant increase in the fraction of the NAM variance explained by sea surface temperature and sea ice variability, suggesting a higher potential predictability. In particular, the eastward extension of the El Niño-Southern Oscillation (ENSO) teleconnection under global warming contributes to the AIS formation even in early winter and a higher NAM-ENSO correlation.

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 midwinter minimum of the North Pacific transient eddy activity. Eddy energetics reveals that the net efficiency of the anticyclonic contribution in replenishing total transient eddy energy over the North Pacific exhibits a pronounced midwinter minimum, while that of the cyclonic counterpart does not in harmony with precipitation that peaks around midwinter. This study suggests that more attention should be paid to anticyclones in studying midlatitude storm-track dynamics.