Transitional Wave Climate Regions on Continental and Polar Coasts in a Warming World

We provide a comprehensive analysis of the spatial-temporal changes in the atmospheric-driven major wave climates (easterlies, southerlies, and westerlies) under two different Representative Concentration Pathways, the RCP2.6 and RCP8.5 scenarios for the end-of-the-century (2075–2099). By comparing the projected scenarios with historical conditions, we found that the easterly wave climates will be more frequents in the southwest basins (up to 15%) and the southerlies in the eastern basins (up to 20%). While the westerlies are projected to reduce their presence in the mid-latitudes and intensify for the high latitudes associated with the poleward extratropical circulation. As a result, coastal risk will be triggered in transitional wave climate regions, in addition to the risk induced by sea-level rise and storm wave generated, by spatial and frequency changes in the prevailing wave climates that will reach regions where up to now they have not, impacting future coastal environments.

Influence of Siberian Blocking on Long-Lived Cold Surges over the South China Sea

Cold surges occur frequently over the South China Sea (SCS) in winter, and most of them last only a few days. However, some cold surge events can persist longer, for instance, more than 5 days. This study focuses on these long-lived cold surge events and investigates the associated extratropical circulation anomalies. The results indicate that long-lived cold surges, characterized as strong northerlies over the SCS, can be triggered by a successive high anomaly center over East Asia. Accompanying this is an anomalously extensive and quasi-stationary anticyclone over Siberia in the midtroposphere, hinting at a more frequent occurrence of Siberian blocking. Further analyses reveal that the blocking frequency is indeed significantly high over 90°–150°E from day −4 to day +2 relative to the onset of long-lived cold surge events. Furthermore, there exist significant correlations between the leading occurrence of Siberian blocking and the sea level pressure (SLP) anomalies over East Asia, which are directly related to long-lived cold surges. The intensification of the high SLP anomaly over East Asia is found to mainly result from cold advection induced by the anomalous northerly winds along the southeastern edge of the Siberian blocking.

The Extratropical Linear Step Response to Tropical Precipitation Anomalies and Its Use in Constraining Projected Circulation Changes under Climate Warming

Rossby wave trains triggered by tropical convection strongly affect the atmospheric circulation in the extratropics. Using daily gridded observational and reanalysis data, we demonstrate that a technique based on linear response theory effectively captures the linear response in 250-hPa geopotential height anomalies in the Northern Hemisphere using examples of steplike changes in precipitation over selected tropical areas during boreal winter. Application of this method to six models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), using the same tropical forcing, reveals a large intermodel spread in the linear response associated with intermodel differences in Rossby waveguide structure. The technique is then applied to a projected tropicswide precipitation change in the HadGEM2-ES model during 2025–45 December–February, a period corresponding to a 2°C rise in the mean global temperature under the RCP8.5 scenario. The response is found to depend on whether the mean state underlying the technique is calculated using observations, the present-day simulation, or the future projection; indeed, the bias in extratropical response to tropical precipitation because of errors in the basic state is much larger than the projected change in extratropical circulation itself. We therefore propose the linear step response method as a semiempirical method of making near-term future projections of the extratropical circulation, which should assist in quantifying uncertainty in such projections.

ITCZ shift and extratropical teleconnections drive ENSO response to volcanic eruptions

The mechanisms through which volcanic eruptions affect the El Niño–Southern Oscillation (ENSO) state are still controversial. Previous studies have invoked direct radiative forcing, an ocean dynamical thermostat (ODT) mechanism, and shifts of the Intertropical Convergence Zone (ITCZ), among others, to explain the ENSO response to tropical eruptions. Here, these mechanisms are tested using ensemble simulations with an Earth system model in which volcanic aerosols from a Tambora-like eruption are confined either in the Northern or the Southern Hemisphere. We show that the primary drivers of the ENSO response are the shifts of the ITCZ together with extratropical circulation changes, which affect the tropics; the ODT mechanism does not operate in our simulations. Our study highlights the importance of initial conditions in the ENSO response to tropical volcanic eruptions and provides explanations for the predominance of posteruption El Niño events and for the occasional posteruption La Niña in observations and reconstructions.

An Eddy-Feedback Model for Propagating Annular Modes

<p>Some types of extreme events<span> in the extratropics are often associated with anomalous jet behaviors. A well-known example is the annular mode, wherein its variation e.g., the meandering in the north-south direction of the jet, disrupts the normal eastward migration of troughs and ridges.</span> <span>Since the seminal works of Lorenz and Hartmann, the annular mode has been mostly analyzed based on single EOF mode. However, a recent study showed that the first and second leading EOFs are strongly correlated at long lags and are manifestations of a single oscillatory decaying-mode. This means that the first and second leading EOF modes interact and exert feedbacks on each other. The purpose of this study is to develop an eddy-feedback model for the extratropical low-frequency variability that includes these cross-EOF feedbacks to better isolate the eddy momentum/heat flux changes with time- and/or zonal-mean flow. Our results show that, in the presence of the poleward-propagation regime, the first and second leading EOF modes interact and exert positive feedbacks at lags ~10 (~20) days about ~0.07 (~0.16) day</span><span><sup>-1</sup></span><span> in the reanalysis (idealized GCM). This feedback is often ignored in the previous studies, and in fact, the magnitude is nearly double the feedback exerted by the single EOF mode. We found that this apparent positive eddy feedback is a result of the effect of jet pulsation (strengthening and weakening) in zonal flow variability (z</span><span><sub>2</sub></span><span>) on the eddy momentum flux due to the meandering in the north-south direction of the jet (m</span><span><sub>1</sub></span><span>). A finite-amplitude eddy-mean flow interaction diagnostic has been performed to demonstrate the dynamics governing the positive feedback in the propagating regime of the annular modes. It is shown that the poleward propagation is caused by an orchestrated combination of equatorward propagation of wave activity (baroclinic process), nonlinear wave breaking (barotropic processes), and radiative relaxation. The latter two processes follow the first one, and as such, the meridional propagation of Rossby wave activity (likely generated by an enhanced baroclinic wave source at a low level) is the central mechanism. Finally, our model calculations suggest the rule of thumb that the propagating annular modes (i.e., when EOF1 and EOF2 together represent quasi-periodic poleward propagation of zonal-mean flow anomalies) exist if the ratio of the fractional variance and decorrelation time-scale of EOF2 to that of EOF1 exceeds 0.5 or the two leading PCs showing maximum correlations at larger lags. These criteria can be used to assess the predictability of preferred modes of extratropical circulation in GCMs. The present study advances and potentially transforms the state of our understanding of the low-frequency variability of the extratropical circulation.</span></p>

Connections between climate sensitivity and the extratropical circulation

<div>The dry dynamical core represents one of the simplest possible numerical models for studying the response of the extratropical circulation to climate change. In the model, the circulation is forced by relaxing temperature to a notional “equilibrium” using linear damping. The linear damping coefficient plays an essential role in governing the structure of the circulation. But despite decades of research with the dry dynamical core, the role of the damping coefficient in governing the circulation has received relatively little scrutiny.</div><div><br>In this work, we systematically vary the damping coefficient in a dry dynamical core in order to understand how the amplitude of the damping influences extratropical dynamics. Critically, we prove that the local climate feedback parameter is proportional to the damping coefficient – that is, the damping timescale is a measure of climate sensitivity for the dry atmosphere. The key finding is that the steady-state extratropical circulation responds to changes in this climate sensitivity.</div><div><br>Longer damping timescales (i.e. higher climate sensitivities) lead to a less dynamically active extratropical circulation, stronger and more persistent annular modes, and equatorward shifts in the jet. When perturbed with climate change-like forcings, changing the damping timescale can also change the dynamical response to the forcing. We argue that understanding the response of the circulation to climate change is critically dependent on understanding its climate sensitivity, and consider how climate sensitivity might be inferred from its effect on the circulation in the dry model and more complex general circulation models.</div>

ITCZ shift and extratropical teleconnections drive ENSO response to volcanic eruptions

<p>The mechanisms through which volcanic eruptions impact the El Niño-Southern Oscillation (ENSO) state are still controversial. Previous studies have invoked direct radiative forcing, an ocean dynamical thermostat (ODT) mechanism and shifts of the Intertropical Convergence Zone (ITCZ), among others, to explain the ENSO response to tropical eruptions. Here, these mechanisms are tested using ensemble simulations with an Earth System Model in which volcanic aerosols from a Tambora-like eruption are confined either in the Northern or the Southern Hemisphere. We show that the primary drivers of the ENSO response are the shifts of the ITCZ together with extratropical circulation changes, which affect the tropics; the ODT mechanism does not operate in our simulations. Our study highlights the importance of initial conditions in the ENSO response to tropical volcanic eruptions and provides explanations for the predominance of post-eruption El Niño events and for the occasional post-eruption La Niña in observations and reconstructions.</p>

Why is there a Zonal Wave 3 pattern in the Southern Hemisphere extratropical circulation?

<p>Studies of the Southern Hemisphere (SH) extratropical circulation are dominated by investigations of the zonally symmetric component of the Southern Annular Modular (SAM). However, there are significant asymmetries embedded in the zonal flow. In particular, a zonal wave 3 (ZW3) pattern is one of the dominant features of the SH circulation on daily, seasonal and interannual timescales. While the ZW3 circulation has had significant impacts on meridional heat transport and Antarctic sea ice extent in recent years, the physical mechanisms responsible for its presence still remain elusive. In this study, we use the Community Earth System Model (CESM) to understand the mechanisms that give rise to and modulate the ZW3 pattern in the SH extratropics. We examine, among other things, the popular belief that the ZW3 pattern is present due to the existence of three separate land-masses in the SH, namely Australia, Africa and South America, and whether it is modulated by both the land-ocean contrast and tropical forcing.</p>

Representation of the extratropical circulation in the MiKlip decadal prediction system - impacts of resolution and initialization

<p>This study analyzes the representation of the extratropical circulation over the North Atlantic (NA) region using the German decadal prediction system (MiKlip) of two different spatial resolutions. Four quantities are assessed, i.e. the storm track, blocking frequencies, cyclone frequencies, and windstorm frequencies. We investigate the effect of model initialization for the representation of the circulation in a lower resolution (LR, atm: T63L47, ocean: 1.5° L40) and higher resolution version (HR, atm: T127L95, ocean: 0.4° L40) of the decadal prediction system.</p><p>While LR shows common deficits in the climatological representation in both the initialized prediction system and the uninitialized historical projection, e.g. an overly zonal extratropical storm track and a deficit in blocking frequencies over the North Atlantic and Europe, the higher resolution version counteracts these biases. The initialized LR prediction system largely overestimates NA cyclone frequency, which is not the case for the uninitialized LR counterpart. This positive bias is mainly due to weak and short lived systems and is an effect of the initialization in the LR prediction system. Similar biases cannot be identified in the windstorm frequency which implies that the short lived cyclones are low of impact with respect to wind speed.</p><p>The initialization effect leading to an overestimation of weak and short lived cyclones cannot be found in the HR version. The overall better representation of the extratropical circulation in the HR version leads to an increased decadal prediction skill, which is measured in terms of anomaly correlation, with the increase in resolution for all four quantities. </p>

Improvement in the decadal prediction skill of the North Atlantic extratropical winter circulation through increased model resolution

<p>As the scientific and societal interest in skillful decadal predictions grows, a lot of effort is currently put into the development and advancement of such prediction systems worldwide. Studies evaluating the skill of basic atmospheric quantities, such as e.g. surface temperatures, in those prediction systems are numerous. However, dynamical quantities are discussed only rarely. Also, there is a lack of investigations which assess the exclusive impact of the model’s resolution on the forecast skill. </p><p>In this study, we address both these issues: we analyse a set of four quantities of the extratropical circulation (storm track, blocking frequencies, cyclone frequencies, windstorm frequencies) and compare the deterministic forecast skill for lead winters 2-5 within the German MiKlip prediction system of two different spatial resolutions. While the lower resolution (LR, atm: T63L47, ocean: 1.5° L40) shows common deficits in the climatological representation, e.g. an overly zonal extratropical storm track and a deficit in blocking frequencies over the North Atlantic and Europe, the higher resolution version (HR, atm: T127L95, ocean: 0.4° L40) counteracts these biases. In return, the deterministic decadal prediction skill, which is measured in terms of anomaly correlation, increases (statistically significant) with the increase in resolution for all four quantities. </p><p>The improvements found in our study for the different metrics follow a physically consistent line of argument, and the areas of improved forecast skill are crucial regions for the genesis and intensification of synoptic weather systems over the North Atlantic and for their impact on Europe. Thus, we identified a significant improvement of the storm track skill along the North Atlantic Current (i.e., the source region of synoptic eddies), a downstream improvement of the cyclone frequency skill over the central North Atlantic (where the synoptic systems intensify), and finally improved skill of the cyclone, windstorm and blocking frequencies over the European continent (i.e., the impact area).</p><p>Not only is the skill improved with the increase in resolution (HR vs. LR), but also the HR system itself offers significant deterministic decadal forecast skill for the extratropical circulation metrics in large regions over the North Atlantic and Europe (HR vs. ERA-Interim) for the considered lead time of two to five winters. </p><p>Our results are encouraging for the advancement of decadal prediction systems as they document that even small improvements in the bias of the model, through an increased spatial resolution and possibly a better representation of smaller scales, can have a substantial effect on the representation of dynamical processes and can ultimately lead to a significant improvement of the decadal prediction skill for extratropical features and extreme events.</p>