Andrea Marcheggiani

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

Midlatitude sea surface temperature (SST) fronts exert a strong influence on storm tracks and westerly jets by modulating lower‑tropospheric baroclinicity, moisture availability, and diabatic heating. Yet the sensitivity of the large‑scale atmospheric circulation to the detailed structure of these fronts remains difficult to isolate in realistic model configurations. We introduce an idealised SST framework for aquaplanet experiments that allows to prescribe the strength, width, and latitude of a midlatitude SST front. The SST profile is constructed from analytically derived meridional gradients, ensuring that changes to the front generate physically consistent moisture distributions through the Clausius–Clapeyron relation. Using this framework, we conduct a suite of sensitivity experiments to quantify the influence from various aspects in the structure of the SST front on the climatological-mean energy and water cycles as well as the associated atmospheric circulation. Preliminary results demonstrate that increasing the frontal SST gradient or narrowing the frontal width systematically intensifies storm‑track activity and strengthens the midlatitude jet, whereas shifting the front meridionally alters the latitudinal position of the storm track in a non-linear way. Warm‑ and cold‑climate experiments further show that imposing a fixed freezing threshold in atmosphere‑only setups artificially reduces high‑latitude baroclinicity, whereas allowing SSTs to go below freezing produces a more realistic polar thermal structure. Overall, our results demonstrate that the experimental framework developed in this study provides a useful basis for targeted process studies of how midlatitude SST‑front characteristics shape the atmospheric circulation, energy budget, and hydrological cycle.

When decomposing the atmospheric flow into a basic state and perturbations, the perturbations are generally interpreted as the contribution from chaotic non-linear weather. We explore the link between day-to-day weather and the climatological zonal mean perspective on zonal momentum in more detail by systematically linking eddy momentum fluxes to weather events. Specifically, we first decompose the full momentum flux divergence into contributions from mean flow and perturbations both in the time and zonal direction as well as their combinations, and then systematically relate synoptic jets, cyclones, and Rossby wave breaking events to the instantaneous momentum fluxes. We thus construct a step-by-step link between the time-zonal mean perspective on momentum flux convergence and the synoptic perspective.With this approach, we show that both the time and zonal averaging are a residual of a large compensation of momentum flux convergence and divergence. In both dimensions, the mean must be regarded as a residual that is at least an order of magnitude smaller than the original signal. Further, a large fraction of eddy momentum flux convergence and divergence occurs in association with weather features, with synoptic jets alone accounting for 60-80% of the convergence from the subtropics throughout the mid-latitudes. Rossby wave breaking, on the other hand, only features less than 30% of the momentum flux convergence in the midlatitudes.Finally, the attribution of the full-field momentum flux convergence is nearly indistinguishable from the attribution of eddy-momentum flux convergence, irrespective of whether the eddies are defined as perturbations in time, zonal direction, or the combination of both. The effect of stationary waves to the momentum fluxes is thus implicitly included in the selected transient weather events.

The storm tracks along the two main western boundary currents, the Kuroshio-Oyashio and Gulf Stream, are an integral feature of the Northern Hemisphere climate. Even though diabatic processes play a fundamental role in the evolution of storm tracks, especially related to the enhanced water cycle along sea surface temperature fronts, our theoretical understanding of the impact of moist dynamic processes is still incomplete. To shed light on the relative importance of diabatic effects on storm tracks, we quantify diabatic and adiabatic contributions to variations in baroclinicity using a framework based on isentropic slope tendencies.We reveal a dichotomy in the maintenance of baroclinicity between the near-surface and free troposphere. Specifically, changes in baroclinicity due to adiabatic and diabatic processes have opposite phases with adiabatic depletion preceding diabatic generation of baroclinicity in the near-surface, while diabatic generation precedes adiabatic depletion in the free troposphere.In the near-surface troposphere, cold air outbreaks (CAOs) are the primary contributors to variability in baroclinicity, while outside of CAOs variability is significantly weaker and largely incoherent with the overall near-surface variability. In the free troposphere, on the other hand, most of the variability in baroclinicity is attributable to extra-tropical cyclones and fronts. Despite their limited areal extent, they explain more than half the total variance in baroclinicity. The contribution to total variability from atmospheric rivers is small, indicating that the presence of moisture alone does not necessarily translate into diabatic production of baroclinicity in the absence of a mechanism for ascent.

Moist diabatic processes significantly impact storm track variability, position, and intensity. The distribution of atmospheric moisture is closely linked to sea surface temperatures (SSTs) through the Clausius-Clapeyron relation. Therefore, midlatitude atmospheric circulation is affected by the spatial distribution of SSTs, especially midlatitude SST fronts associated with oceanic western boundary currents.We quantify the storm track’s response to moisture availability by performing idealised aqua-planet simulations where we modify the distribution of SST by changing the position, intensity, and width of midlatitude SST fronts. We assess the sensitivity of atmospheric circulation by comparing the water cycle and climatological mean energy cycle resulting from each simulation. Specifically, we find that storm tracks tend to align with SST fronts when these are located in midlatitudes, and that stronger SST gradients enhance storm track activity by increasing baroclinicity and moisture fluxes. The storm track’s latitudinal variability is strongly dependent on the latitude of the SST front, while its amplitude and maximum gradient primarily affect storm track intensity. Two additional experiments where we uniformly increase and decrease absolute temperature highlight the response of storm tracks to climate change: the water cycle intensifies in a warmer climate, but storm track activity appears more sensitive to the total meridional temperature contrast than to absolute temperature. Finally, we present preliminary results from ongoing work exploring the synoptic drivers of storm track response, including changes in cyclone distribution, baroclinicity, and the role of moist diabatic processes, which significantly impact storm track variability, position, and intensity.

Cold air outbreaks (CAOs) are key drivers of near-surface baroclinicity in midlatitude oceanic regions, where cold continental air masses interact with warm sea surface temperatures, giving rise to strong surface heat fluxes. Despite their relatively limited spatio-temporal extent, CAOs exert a disproportionate influence on the variability of near-surface baroclinicity, particularly in the entrance regions of the North Atlantic and North Pacific storm tracks. To further clarify this relationship, we use the isentropic slope framework to distinguish between diabatic and adiabatic changes in baroclinicity and quantify the contribution of CAOs to near-surface baroclinicity variability in the Gulf Stream and Kuroshio-Oyashio extension regions. Moderate-intensity CAOs account for up to 40 % of the total near-surface baroclinicity variability in the Gulf Stream Extension, while occupying less than 15 % of the region. In the Kuroshio-Oyashio Extension, CAOs explain a smaller fraction of variability despite their broader spatial extent. We employ phase space analysis to diagnose the typical phasing between adiabatic depletion and diabatic restoration of baroclinicity, with the former leading in time on the latter. Phase portraits and synoptic composites focused on CAO-related variability show that this characteristic phasing is predominantly linked to CAOs, whereas background variability contributes weakly and incoherently. These findings highlight the central role of CAOs in shaping near-surface baroclinicity and suggest that they are essential to the evolution of midlatitude storm tracks.

Cold air outbreaks (CAOs) are key drivers of near-surface baroclinicity in midlatitude oceanic regions, where cold continental air masses interact with warm sea surface temperatures, giving rise to strong surface heat fluxes. Despite their relatively limited spatio-temporal extent, CAOs exert a disproportionate influence on the variability of near-surface baroclinicity, particularly in the entrance regions of the North Atlantic and North Pacific storm tracks. To further clarify this relationship, we use the isentropic slope framework to distinguish between diabatic and adiabatic changes in baroclinicity and quantify the contribution of CAOs to near-surface baroclinicity variability in the Gulf Stream and Kuroshio-Oyashio extension regions. Moderate-intensity CAOs account for up to 40 % of the total near-surface baroclinicity variability in the Gulf Stream Extension, while occupying less than 15 % of the region. In the Kuroshio-Oyashio Extension, CAOs explain a smaller fraction of variability despite their broader spatial extent. We employ phase space analysis to diagnose the typical phasing between adiabatic depletion and diabatic restoration of baroclinicity, with the former leading in time on the latter. Phase portraits and synoptic composites focused on CAO-related variability show that this characteristic phasing is predominantly linked to CAOs, whereas background variability contributes weakly and incoherently. These findings highlight the central role of CAOs in shaping near-surface baroclinicity and suggest that they are essential to the evolution of midlatitude storm tracks.