Abstract
Recent Arctic sea ice loss in fall has been posited to drive midlatitude circulation changes into winter and even spring. Past work has shown that sea ice loss can indeed trigger a weakening ...of the stratospheric polar vortex, which can lead to delayed surface weather changes. But the mechanisms of such changes and their relevant time scales have remained unclear. This study uses large ensembles of idealized GCM simulations to identify how and over what time scales the atmospheric circulation responds to short-term surface heat flux changes in high latitudes. The ensemble-mean response of the atmospheric circulation is approximately linear in the amplitude of the surface forcing. It is also insensitive to whether the forcing is zonally asymmetric or symmetric, that is, whether stationary waves are generated or not. The circulation response can be decomposed into a rapid thermal response and a slower dynamic adjustment. The adjustment arises through weakening of vertical wave activity fluxes from the troposphere into the stratosphere in response to polar warming, a mechanism that differs from sudden stratospheric warmings yet still results in a weakened stratospheric circulation. The stratospheric response is delayed and persists for about 2 months because the thermal response of the stratosphere is slow compared with that of the troposphere. The delayed stratospheric response feeds back onto the troposphere, but the tropospheric effects are weak compared with natural variability. The general pathway for the delayed response appears to be relatively independent of the atmospheric background state at the time of the anomalous surface forcing.
Abstract
Annular modes are the leading mode of variability in extratropical atmospheres, and a key source of predictability at midlatitudes. Previous studies of annular modes have primarily used dry ...atmospheric models, so that moisture’s role in annular mode dynamics is still unclear. In this study, a moist two-layer quasigeostrophic channel model is used to study the effects of moisture on annular mode persistence. Using a channel model allows moisture’s direct effects to be studied, rather than changes in persistence due to geometric effects associated with shifts in jet latitude on the sphere. Simulations are performed in which the strength of latent heat release is varied to investigate how annular mode persistence responds as precipitation becomes a leading term in the thermodynamic budget. At short lags (<20 model days, ≈4 Earth days), moisture increases annular mode persistence, reflecting weaker eddy activity that is less effective at disrupting zonal-mean wind anomalies. Comparisons to dry simulations with weaker mean flows demonstrate that moisture is particularly effective at damping high-frequency eddies, further enhancing short-lag persistence. At long lags (>20 model days), moisture weakly increases persistence, though it decreases the amplitudes of low-frequency annular mode anomalies. In the most realistic simulation, the greater short-lag persistence increases the
e
-folding time of the zonal index by 21 model days (≈4 Earth days). Moisture also causes a transition to propagating variability, though this does not seem to affect the leading mode’s persistence.
Sea ice is important for Earth's energy budget as it influences surface albedo and air-sea fluxes in polar regions. On its margins, waves heavily impact sea ice. Routine and repeat observations of ...waves in sea ice are currently lacking, and therefore a comprehensive understanding of how waves interact with sea ice and are attenuated by it is elusive.
Southern Ocean (SO) surface winds are essential for ventilating the upper ocean by bringing heat and CO₂ to the ocean interior. The relationships between mixed layer ventilation, the southern annular ...mode (SAM), and the storm tracks remain unclear because processes can be governed by short-term wind events as well as long-term means. In this study, observed time-varying 5-day probability density functions (PDFs) of ERA5 surface winds and stresses over the SO are used in a singular value decomposition to derive a linearly independent set of empirical basis functions. The first modes of wind (72% of the total wind variance) and stress (74% of the total stress variance) are highly correlated with a standard SAM index (r = 0.82) and reflect the SAM’s role in driving cyclone intensity and, in turn, extreme westerly winds. The joint PDFs of zonal and meridional wind show that southerly and less westerly winds associated with strong mixed layer ventilation are more frequent during short and distinct negative SAM phases. The probability of these short-term events might be related to midlatitude atmospheric circulation. The second mode describes seasonal changes in the wind variance (16%of the total variance) that are uncorrelated with the first mode. The analysis produces similar results when repeated using 5-day PDFs from a suite of scatterometer products. Differences between wind product PDFs resemble the first mode of the PDFs. Together, these results show a strong correlation between surface stress PDFs and the leading modes of atmospheric variability, suggesting that empirical modes can serve as a novel pathway for understanding differences and variability of surface stress PDFs.
Weather and climate variations on subseasonal to decadal time scales can have enormous social, economic, and environmental impacts, making skillful predictions on these time scales a valuable tool ...for decision-makers. As such, there is a growing interest in the scientific, operational, and applications communities in developing forecasts to improve our foreknowledge of extreme events. On subseasonal to seasonal (S2S) time scales, these include high-impact meteorological events such as tropical cyclones, extratropical storms, floods, droughts, and heat and cold waves. On seasonal to decadal (S2D) time scales, while the focus broadly remains similar (e.g., on precipitation, surface and upper-ocean temperatures, and their effects on the probabilities of high-impact meteorological events), understanding the roles of internal variability and externally forced variability such as anthropogenic warming in forecasts also becomes important. The S2S and S2D communities share common scientific and technical challenges. These include forecast initialization and ensemble generation; initialization shock and drift; understanding the onset of model systematic errors; bias correction, calibration, and forecast quality assessment; model resolution; atmosphere–ocean coupling; sources and expectations for predictability; and linking research, operational forecasting, and end-user needs. In September 2018 a coordinated pair of international conferences, framed by the above challenges, was organized jointly by the World Climate Research Programme (WCRP) and the World Weather Research Programme (WWRP). These conferences surveyed the state of S2S and S2D prediction, ongoing research, and future needs, providing an ideal basis for synthesizing current and emerging developments in these areas that promise to enhance future operational services. This article provides such a synthesis.
Abstract
Strong surface winds under extratropical cyclones exert intense surface stresses on the ocean that lead to upper-ocean mixing, intensified heat fluxes, and the generation of waves, that, ...over time, lead to swell waves (longer than 10-s period) that travel long distances. Because low-frequency swell propagates faster than high-frequency swell, the frequency dependence of swell arrival times at a measurement site can be used to infer the distance and time that the wave has traveled from its generation site. This study presents a methodology that employs spectrograms of ocean swell from point observations on the Ross Ice Shelf (RIS) to verify the position of high wind speed areas over the Southern Ocean, and therefore of extratropical cyclones. The focus here is on the implementation and robustness of the methodology in order to lay the groundwork for future broad application to verify Southern Ocean storm positions from atmospheric reanalysis data. The method developed here combines linear swell dispersion with a parametric wave model to construct a time- and frequency-dependent model of the dispersed swell arrivals in spectrograms of seismic observations on the RIS. A two-step optimization procedure (deep learning) of gradient descent and Monte Carlo sampling allows detailed estimates of the parameter distributions, with robust estimates of swell origins. Median uncertainties of swell source locations are 110 km in radial distance and 2 h in time. The uncertainties are derived from RIS observations and the model, rather than an assumed distribution. This method is an example of supervised machine learning informed by physical first principles in order to facilitate parameter interpretation in the physical domain.
Surface winds from Southern Ocean cyclones generate large waves that travel over long distances (>1,000 km). Wave generation regions are often colocated with enhanced air‐sea fluxes and upper ocean ...mixing. Ocean wave spectra contain information about storm wind speed, fetch size, and intensity at their generation site. Two years of seismic observations on the Ross Ice shelf, combined with modern optimization (machine learning) techniques, are used to trace the origins of wave events in the Southern Ocean with an accuracy of ±110 km and ±2 hr from a hypothetical point source. The observed spectral energy attenuated within sea ice and in the ice shelf but retains characteristics that can be compared to parametric wave models. Comparison with the Modern‐Era Retrospective Analysis for Research and Applications, Version 2, and ERA5 reanalyses suggests that less than 45% of ocean swell events can be associated with individual Southern Ocean storms, while the majority of the observed wave events cannot be matched with Southern Ocean high wind events. Reanalysis cyclones and winds are often displaced by about 350 km or 10 hr in Modern‐Era Retrospective Analysis for Research and Applications, Version 2, and ERA5 compared to the most likely positions inferred from the seismic spectra. This high fraction of displaced storms in reanalysis products over the South Pacific can be explained by the limited availability of remote sensing observations, primarily caused by the presence of sea ice. Deviation of wave rays from their great circle path by wave‐current interaction plays a minor role.
Plain Language Summary
Surface winds under storms over the Southern Ocean make large ocean waves that travel over long distances (>1,000 km). Regions of wave generation coincide with regions where ocean uptake of heat and CO2 is large, so knowledge about wave generation regions helps us to understand the role of the Southern Ocean in the climate system. A 2‐year field campaign made new observations of ocean wave arrivals at the Ross Ice Shelf. These observations are used to trace the origins of the wave events in the Southern Ocean. Even though the waves observed in the sea ice are much smaller than in the open ocean, the observations are good enough to identify ocean waves. The wave arrivals can be used to infer a most likely time and location of the storm that generated the waves. Comparison with two reanalysis products (Modern‐Era Retrospective Analysis for Research and Applications, Version 2, and ERA5) suggests that more than half of the observed ocean wave events cannot be matched to individual Southern Ocean storms. This high percentage of displaced storms in the reanalysis products can be explained by the limited availability of satellite observations caused by the presence of sea ice.
Key Points
Ocean swell events are detected in the Ross Ice shelf year‐round, but 45% or less of the swell can be linked with individual storms
About 55% or more of the observed swell can only be associated with reanalysis winds whose positions are displaced
Reanalysis winds are often displaced when sparse or no observations of surface winds are available, as for example, over or close to sea ice
Abstract
Southern Ocean (SO) surface winds are essential for ventilating the upper ocean by bringing heat and CO
2
to the ocean interior. The relationships between mixed-layer ventilation, the ...Southern Annular Mode (SAM), and the storm tracks remain unclear because processes can be governed by short-term wind events as well as long-term means.
In this study, observed time-varying 5-day probability density functions (PDFs) of ERA5 surface winds and stresses over the SO are used in a singular value decomposition to derive a linearly independent set of empirical basis functions. The first modes of wind (72% of the total wind variance) and stress (74% of the total stress variance) are highly correlated with a standard SAM index (
r
= 0.82) and reflect SAM’s role in driving cyclone intensity and, in turn, extreme westerly winds. This Joint PDFs of zonal and meridional wind show that southerly and less westerly winds associated with strong mixed-layer ventilation are more frequent during short and distinct negative SAM phases. The probability of these short-term events might be related to mid-latitude atmospheric circulation. The second mode describes seasonal changes in the wind variance (16% of the total variance) that are uncorrelated with the first mode.
The analysis produces similar results when repeated using 5-day PDFs from a suite of scatterometer products. Differences between wind product PDFs resemble the first mode of the PDFs. Together, these results show a strong correlation between surface stress PDFs and the leading modes of atmospheric variability, suggesting that empirical modes can serve as a novel pathway for understanding differences and variability of surface stress PDFs.
Subseasonal to Decadal Prediction Merryfield, William J.; Baehr, Johanna; Batté, Lauriane ...
Bulletin of the American Meteorological Society,
09/2020, Letnik:
101, Številka:
9
Journal Article
Ocean surface winds, currents, and waves play a crucial role in exchanges of momentum, energy, heat, freshwater, gases, and other tracers between the ocean, atmosphere, and ice. Despite surface waves ...being strongly coupled to the upper ocean circulation and the overlying atmosphere, efforts to improve ocean, atmospheric, and wave observations and models have evolved somewhat independently. From an observational point of view, community efforts to bridge this gap have led to proposals for satellite Doppler oceanography mission concepts, which could provide unprecedented measurements of absolute surface velocity and directional wave spectrum at global scales. This paper reviews the present state of observations of surface winds, currents, and waves, and it outlines observational gaps that limit our current understanding of coupled processes that happen at the air-sea-ice interface. A significant challenge for the coming decade of wind, current, and wave observations will come in combining and interpreting measurements from (a) wave-buoys and high-frequency radars in coastal regions, (b) surface drifters and wave-enabled drifters in the open-ocean, marginal ice zones, and wave-current interaction ``hot-spots'', and (c) simultaneous measurements of absolute surface currents, ocean surface wind vector, and directional wave spectrum from Doppler satellite sensors.
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