Abstract The distinction between eddy-driven and subtropical jets is conceptually important and well-founded based on different driving mechanisms and dominant types of variability. This climatological perspective may be augmented by considering instantaneous maxima in the wind field and linking these to the time-mean jets. Inspired by EOF and cluster analyses to explore the variability in jet occurrences, we propose a straightforward framework that naturally distinguishes subtropical from eddy-driven jets in instantaneous data. We document that for most ocean basins, there is a clear bimodality in instantaneous jet occurrences in potential temperature–wind speed space. The two types of jets in this phase space align well with the conceptual expectations for subtropical and eddy-driven jets regarding their vertical structure as well as their regional occurrence. Interestingly, the bimodality in phase space is most pronounced in the western North Pacific during winter. The climatological jet in this region is typically regarded as “merged,” resulting from a mixture of thermal driving and eddy driving. Our results clarify that the strongest instantaneous jets in this region are classified as subtropical, with eddy-driven jets occurring in close proximity to the climatological mean jet, though weaker and slightly more poleward. We also show that the regions of climatological transition from predominantly subtropical to predominantly eddy-driven jets are just downstream of the strongest climatological jets.
Abstract A one‐dimensional model of the atmosphere‐ice‐ocean column is used to study the effects of changing river runoff to the Arctic Ocean. River runoff is the largest contributor of freshwater to the Arctic and is expected to increase as the hydrological cycle accelerates due to global warming. The column model simulates the stratification of the Arctic Ocean reasonably well, capturing important features such as the fresh surface layer, the salty cold halocline, and the temperature maximum within the Atlantic Water layer. The model is run for 500 years with prescribed boundary conditions to reach steady state solutions. Increasing river runoff is found to strengthen the stratification and to produce a fresher and shallower surface mixed layer with warming (up to ∼1°C for a doubling of present‐day runoff) in the Atlantic Water layer below. An important consequence is that the effect of the larger vertical temperature gradient is able to balance that of the stronger stratification and yield a close to constant vertical heat flux toward the surface. As a result, the sea ice response is small, showing only slight increase (up to ∼15 cm for a doubling of present‐day runoff) in annual mean ice thickness. Limitations of the study include the idealized nature of the column model and uncertainties in the background vertical mixing within the Arctic Ocean.
Abstract. Blocking over Greenland is known to lead to strong surface impacts, such as ice sheet melting, and a change in its future frequency can have important consequences. However, as previous studies demonstrated, climate models underestimate the blocking frequency for the historical period. Even though some improvements have recently been made, the reasons for the model biases are still unclear. This study investigates whether models with realistic Greenland blocking frequency have a correct representation of its dynamical drivers, most importantly, cyclonic wave breaking (CWB). Because blocking is a rare event and its representation is model-dependent, we here use a multi-model large ensemble. All of the models underestimate CWB frequency and four out of five models underestimate the frequency of Greenland blocking. Nevertheless, they all show the typical Greenland blocking features, namely a ridge with anticyclonic anomaly over Greenland and an equatorward-shifted jet over the North Atlantic. However, we find that the model with the most realistic Greenland blocking frequency, MIROC5, has the most negative CWB frequency bias. While in reanalysis CWB is an important mechanism leading to blocking formation, the link between blocking and CWB is much weaker in MIROC5, suggesting that another mechanism leads to blocking in this model. Composites over Greenland blocking days show that the present and future experiments of each model are very similar to each other in both amplitude and pattern and that there is no significant change of Greenland blocking frequency in the future. However, this result must be taken with caution since the Greenland blocking driver is not well represented in all models. This highlights the need to accurately understand and represent the mechanisms leading to blocking formation and maintenance in models to get more reliable future projections.
Abstract. Blocking over Greenland is known to lead to strong surface impacts, such as ice sheet melting, and a change in its future frequency can have important consequences. However, as previous studies demonstrated, climate models underestimate the blocking frequency for the historical period. Even though some improvements have recently been made, the reasons for the model biases are still unclear. This study investigates whether models with realistic Greenland blocking frequency in winter have a correct representation of its dynamical drivers, most importantly, cyclonic wave breaking (CWB). Because blocking is a rare event and its representation is model-dependent, we use a multi-model large ensemble. We focus on two models that show typical Greenland blocking features, namely a ridge over Greenland and an equatorward-shifted jet over the North Atlantic. ECHAM6.3-LR has the best representation of CWB of the models investigated but only the second best representation of Greenland blocking frequency, which is underestimated by a factor of 2. While MIROC5 has the most realistic Greenland blocking frequency, it also has the largest (negative) CWB frequency bias, suggesting that another mechanism leads to blocking in this model. Composites over Greenland blocking days show that the present and future experiments of each model are very similar to each other in both amplitude and pattern and that there is no significant change in Greenland blocking frequency in the future. However, these projected changes in blocking frequency are highly uncertain as long as the mechanisms leading to blocking formation and maintenance in models remain poorly understood.
Abstract This study presents a detection scheme for upper-tropospheric jets. The scheme identifies locations on the dynamical tropopause where the wind shear perpendicular to the wind direction vanishes, and subsequently uses a masking criterion to filter out zero-shear locations that do not belong to jets. The scheme reliably detects jet axes in ERA-Interim data with instantaneous, weekly, or monthly averaged wind fields. The dynamical implications of the detected jet axes and their relation to objectively detected wave breaking and blocking are demonstrated for the synoptic evolution during the boreal winter 2013/14. This winter featured a remarkable episode with a stationary ridge–trough couplet over the American continent leading to anomalously cold conditions from central Canada to the eastern United States. The mean synoptic situation during this episode resembles the climatological winter mean, but featured a more spatially focused jet axis distribution in the northeastern Pacific. The tight distribution suggests that a sequence of similar weather events lead to the mean synoptic conditions. Although the distribution of jet axes and wave breaking events together with the persistence of the anomalous ridge over the northeastern Pacific indicate a blocked situation, the block is not detected with common conventional methods due to the lack of a persistent gradient reversal of potential temperature on the dynamical tropopause. In addition, the importance of subseasonal variations in this winter is demonstrated by pointing out a period in which the jet configuration deviated considerably from the seasonal mean.
<p>Extratropical cyclones are key players in the poleward transport of moisture and heat. This study investigates wintertime cyclone variability to better understand the large-scale controls on their frequency, path and impacts at higher latitudes. One of the main corridors for Arctic-bound cyclones is through the North Atlantic to the Barents Sea, a region that has experienced the greatest retreat of winter sea ice during the past decades. Large-scale atmospheric conditions are found to be decisive, with the strongest surface warming from cyclones originating south of 60N in the North Atlantic and steered northeastward by the upper-level flow. Atmospheric conditions also control cyclone variability in the Arctic proper: months with many cyclones are characterized by an absence of high-latitude blocking and enhanced local baroclinicity, due to the presence of strong upper-level winds and a southwest-northeast tilted jet stream more than changes in sea ice. Due to the large interannual variability in the number of Arctic-bound cyclones, no robust trends are observed over the last 40 years. Our results highlight the importance of accounting for internal variability of the large-scale atmospheric circulation in studies of long-term changes in extratropical cyclones.</p>
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