The annual cycle of Northern Hemisphere storm-tracks. Part 2: regional detail

In Part 1 of this study, the annual cycle of the Northern Hemisphere storm-tracks was investigated using feature tracking and Eulerian variance based diagnostics applied on both vorticity and meridional wind. Results were presented and discussed for the four seasons at both upper (250hPa) and lower (850hPa) tropospheric levels. Here, using the meridional wind diagnostics, the annual cycles of the North Pacific and North Atlantic storm-tracks are examined in detail. This is done using monthly and 20° longitudinal sector averages. Many sectors have been considered, but the focus is on sectors equally spaced in the two main oceanic storm-tracks situated at their western, central and eastern regions, the western ones being mainly over the upstream continents. The annual cycles of the upper and lower tropospheric storm-tracks in the central and eastern Pacific, and western and central Atlantic sectors all have rather similar structures. In amplitude, each sector at both levels has a summer minimum and a relatively uniform strength from October to April, despite the strong winter maxima in the westerly jets. However, high intensity storms occur over a much wider latitudinal band in winter. The storm-track in each sector moves poleward from May to August and returns equatorward from October to December, and there is a marked asymmetry between spring and autumn. There are many differences between the North Pacific and North Atlantic storm-tracks, and some of these seem to have their origin in the behaviour over the upstream East Asian and North American continents, suggesting the importance of seeding from these regions. The East Asian storm-track near 48°N has marked spring and autumn maxima and weak amplitude in winter and summer. The 33°N track is strong only in the first half of the year. In contrast, the eastern North American storm-track is well-organised all year, around the baroclinicity that moves latitudinally with the seasons. The signatures associated with these features are found to gradually decrease downstream in each case. In particular, there is very little latitudinal movement in the storm-track in the Eastern Atlantic

Prediction errors of tropical cyclones in the western north Pacific in the Met Office global forecast model

The prediction of Tropical Cyclones (TCs) in the Western North Pacific (WNP) and the Philippines Area of Responsibility (PAR) has been explored in the UK Met Office (UKMO) global forecasting system over a 10 year period at 0-7 day lead times. Both the high resolution deterministic and lower resolution ensemble systems have been considered. Location errors for verification against the observations are comparable for the deterministic, control and ensemble mean forecasts, however, the ensemble spread indicates the ensemble is under-dispersive. Intensity error metrics, for pressure and surface winds, show large biases relative to the observations, with the smallest biases for the deterministic system. For the intensity metrics the ensemble spread shows the ensemble is severely under-dispersive primarily due to the large errors relative to the observations. Verification against the analyses show similar results to verification against the observations for location. This is also the case for the intensities albeit with smaller errors and less under-dispersion. The PAR region has larger intensity errors and biases and larger intensity ensemble spread compared with the broader WNP region. Forecast errors for location and intensity have reduced significantly with system upgrades over the period studied (2008-2017) for the deterministic and ensemble systems. Intensity errors for the latest configuration of the deterministic system at day 4 are smaller than the initial errors of all the earlier configurations for both pressure and winds. The Madden-–Julian oscillation (MJO) and Boreal Summer Intra-Seasonal Oscillation (BSISO) significantly affect the intensity forecast errors, but not the location errors. Intensity errors are lower at the initiation and for early lead times of the forecasts started in phases 6--7 and 7--8, when the MJO and BSISO are active in the WNP. These reduced errors appear to result mainly from the variations in intensity of the observed storms with MJO and BSISO phases, though the initial states of the forecasts are also affected. Over the studied period, the European Centre for Medium-Range Weather Forecasts (ECMWF) deterministic and ensemble systems have lower errors and biases for both location and intensity than the UKMO forecast systems

The annual cycle of Northern Hemisphere storm-tracks. Part 1: seasons

In this paper and Part 2 a comprehensive picture of the annual cycle of the Northern Hemisphere storm-tracks is presented and discussed for the first time. It is based on both feature tracking and Eulerian based diagnostics, applied to vorticity and meridional wind in the upper and lower troposphere. Here, the storm-tracks, as diagnosed using both variables and both diagnostic techniques, are presented for the four seasons for each of the two levels. The oceanic storm-tracks retain much of their winter mean intensity in spring with only a small change in their latitude. In the summer they are much weaker, particularly in the Pacific and are generally further poleward. In autumn the intensities are larger again, comparable with those in spring, but the latitude is still nearer to that of summer. However, in the lower troposphere in the eastern ocean basins the tracking metrics show northern and southern tracks that change little with latitude through the year. The Pacific mid-winter minimum is seen in upper troposphere standard deviation diagnostics, but a richer picture is obtained using tracking. In winter there are high intensities over a wide range of latitudes in the central and eastern Pacific, and the west Pacific has high track density but weak intensity. In the lower troposphere all the diagnostics show that the strength of the Pacific and Atlantic storm-tracks are generally quite uniform over the autumn-winter-spring period. There is a close relationship between the upper tropospheric storm-track, particularly that based on vorticity, and tropopause level winds and temperature gradients. In the lower troposphere, in winter the oceanic storm-tracks are in the region of the strong meridional SST gradients, but in summer they are located in regions of small or even reversed SST gradients. However, over North America the lower tropospheric baroclinicity and the upstream portion of the Atlantic storm-track stay together throughout the year

The role of serial European windstorm clustering for extreme seasonal losses as determined from multi-centennial simulations of high resolution global climate model data

Extratropical cyclones are the most damaging natural hazard to affect western Europe. Serial clustering occurs when many intense cyclones affect one specific geographic region in a short period of time which can potentially lead to very large seasonal losses. Previous studies have shown that intense cyclones may be more likely to cluster than less intense cyclones. We revisit this topic using a high resolution climate model with the aim to determine how important clustering is for windstorm related losses. The role of windstorm clustering is investigated using a quantifiable metric (storm severity index, SSI) that is based on near surface meteorological variables (10-metre wind speed) and is a good proxy for losses. The SSI is used to convert a wind footprint into losses for individual windstorms or seasons. 918 years of a present-day ensemble of coupled climate model simulations from the High-Resolution Global Environment Model (HiGEM) are compared to ERA-Interim re-analysis. HiGEM is able to successfully reproduce the wintertime North Atlantic/European circulation, and represent the large-scale circulation associated with the serial clustering of European windstorms. We use two measures to identify any changes in the contribution of clustering to the seasonal windstorm loss as a function of return period. Above a return period of 3 years, the accumulated seasonal loss from HiGEM is up to 20% larger than the accumulated seasonal loss from a set of random resamples of the HiGEM data. Seasonal losses are increased by 10-20% relative to randomised seasonal losses at a return period of 200 years. The contribution of the single largest event in a season to the accumulated seasonal loss does not change with return period, generally ranging between 25-50%. Given the realistic dynamical representation of cyclone clustering in HiGEM, and comparable statistics to ERA-Interim, we conclude that our estimation of clustering and its dependence on the return period will be useful for informing the development of risk models for European windstorms, particularly for longer return periods

The properties and genesis environments of South Atlantic cyclones

A new climatology of South Atlantic cyclones is produced to provide new insights into the conditions leading to genesis in different regions of the domain. Cyclones are identifed and tracked based on the relative vorticity at 850 hPa computed from the winds. The characteristics of the cyclones are obtained by diagnostic variables sampled within a radial distance from the center to produce a spatial distribution of cyclone properties at the time of genesis. Also, cyclone centered composites are used to analyze the cyclone structure and evolution of cyclones during their genesis. There are four main cyclogenesis region in the South Atlantic Ocean: the Southern Brazilian coast (SE-BR, 30◦S), over the continent near the La Plata river discharge region (LA PLATA, 35◦S), the southeastern coast of Argentina (ARG, 40◦S-55◦S) and the Southeastern Atlantic (SE-SAO, centered at 45◦S and 10◦W). We found that cyclogenesis northward of 35◦S occurs mainly due to low-level forcing associated with moisture transport in the summer, and is associated with upper-level forcing in the winter due to a strong baroclinic environment and potential vorticity intrusions. Southward of 35◦S, cyclones develop in a high baroclinic environment throughout the year with only a small influence from moist processes. The cyclone composites reveal that SE-BR and SE-SAO cyclones are associated with secondary development, the LA PLATA cyclones development is influenced by an orographic low in their early stages, and ARG cyclones are influenced by thermal advection as an essential mechanism in the reduction of static stability

Storyline description of Southern Hemisphere midlatitude circulation and precipitation response to greenhouse gas forcing

As evidence of climate change strengthens, knowledge of its regional implications becomes an urgent need for decision making. Current understanding of regional precipitation changes is substantially limited by our understanding of the atmospheric circulation response to climate change, which to a high degree remains uncertain. This uncertainty is reflected in the wide spread in atmospheric circulation changes projected in multimodel ensembles, which cannot be directly interpreted in a probabilistic sense. The uncertainty can instead be represented by studying a discrete set of physically plausible storylines of atmospheric circulation changes. By mining CMIP5 model output, here we take this broader perspective and develop storylines for Southern Hemisphere (SH) midlatitude circulation changes, conditioned on the degree of global-mean warming, based on the climate responses of two remote drivers: the enhanced warming of the tropical upper troposphere and the strengthening of the stratospheric polar vortex. For the three continental domains in the SH, we analyse the precipitation changes under each storyline. To allow comparison with previous studies, we also link both circulation and precipitation changes with those of the Southern Annular Mode. Our results show that the response to tropical warming leads to a strengthening of the midlatitude westerly winds, whilst the response to a delayed breakdown (for DJF) or strengthening (for JJA) of the stratospheric vortex leads to a poleward shift of the westerly winds and the storm tracks. However, the circulation response is not zonally symmetric and the regional precipitation storylines for South America, South Africa, South Australia and New Zealand exhibit quite specific dependencies on the two remote drivers, which are not well represented by changes in the Southern Annular Mode

The effects of springtime mid-latitude storms on trace gas composition determined from the MACC reanalysis

The relationship between springtime air pollution transport of ozone (O3) and carbon monoxide (CO) and mid-latitude cyclones is explored for the first time using the Monitoring Atmospheric Composition and Climate (MACC) reanalysis for the period 2003–2012. In this study, the most intense spring storms (95th percentile) are selected for two regions, the North Pacific (NP) and the North Atlantic (NA). These storms (∼60 storms over each region) often track over the major emission sources of East Asia and eastern North America. By compositing the storms, the distributions of O3 and CO within a "typical" intense storm are examined. We compare the storm-centered composite to background composites of "average conditions" created by sampling the reanalysis data of the previous year to the storm locations. Mid-latitude storms are found to redistribute concentrations of O3 and CO horizontally and vertically throughout the storm. This is clearly shown to occur through two main mechanisms: (1) vertical lifting of CO-rich and O3-poor air isentropically, from near the surface to the mid- to upper-troposphere in the region of the warm conveyor belt; and (2) descent of O3-rich and CO-poor air isentropically in the vicinity of the dry intrusion, from the stratosphere toward the mid-troposphere. This can be seen in the composite storm's life cycle as the storm intensifies, with area-averaged O3 (CO) increasing (decreasing) between 200 and 500 hPa. The influence of the storm dynamics compared to the background environment on the composition within an area around the storm center at the time of maximum intensity is as follows. Area-averaged O3 at 300 hPa is enhanced by 50 and 36% and by 11 and 7.6% at 500 hPa for the NP and NA regions, respectively. In contrast, area-averaged CO at 300 hPa decreases by 12% for NP and 5.5% for NA, and area-averaged CO at 500 hPa decreases by 2.4% for NP while there is little change over the NA region. From the mid-troposphere, O3-rich air is clearly seen to be transported toward the surface, but the downward transport of CO-poor air is not discernible due to the high levels of CO in the lower troposphere. Area-averaged O3 is slightly higher at 1000 hPa (3.5 and 1.8% for the NP and NA regions, respectively). There is an increase of CO at 1000 hPa for the NP region (3.3%) relative to the background composite and a~slight decrease in area-averaged CO for the NA region at 1000 hPa (-2.7%)

Objective determination of the extratropical transition of tropical cyclones in the Northern Hemisphere

Extratropical transition (ET) has eluded objective identification since the realisation of its existence in the 1970s. Recent advances in numerical, computational models have provided data of higher resolution than previously available. In conjunction with this, an objective characterisation of the structure of a storm has now become widely accepted in the literature. Here we present a method of combining these two advances to provide an objective method for defining ET. The approach involves applying K-means clustering to isolate different life-cycle stages of cyclones and then analysing the progression through these stages. This methodology is then tested by applying it to five recent years from the European Centre of Medium-Range Weather Forecasting operational analyses. It is found that this method is able to determine the general characteristics for ET in the Northern Hemisphere. Between 2008 and 2012, 54% (±7, 32 of 59) of Northern Hemisphere tropical storms are estimated to undergo ET. There is great variability across basins and time of year. To fully capture all the instances of ET is necessary to introduce and characterise multiple pathways through transition. Only one of the three transition types needed has been previously well-studied. A brief description of the alternate types of transitions is given, along with illustrative storms, to assist with further stud

The influence of mid-latitude cyclones on European background surface ozone

The relationship between springtime mid-latitude cyclones and background ozone (O3) is explored using a combination of observational and reanalysis data sets. First, the relationship between surface O3 observations at two rural monitoring sites on the west coast of Europe – Mace Head, Ireland and Monte Velho, Portugal – and cyclone track frequency in the surrounding regions is examined. Second, detailed case study examination of four individual mid-latitude cyclones and the influence of the associated frontal passage on surface O3 is performed. Cyclone tracks have a greater influence on the O3 measurements at the more northern coastal European station, Mace Head, located within the main North Atlantic (NA) storm track. In particular, when cyclones track north of 53° N, there is a significant relationship with high levels of surface O3 (> 75th percentile). The further away a cyclone is from the NA storm track, the more likely it will be associated with both high and low (< 25th percentile) levels of O3 at the observation site during the cyclone's life cycle. The results of the four case studies demonstrate a) the importance of the passage of a cyclone's cold front in relation to surface O3 measurements, b) the ability of mid-latitude cyclones to bring down high levels of O3 from the stratosphere and c) that accompanying surface high pressure systems and their associated transport pathways play an important role in the temporal variability of surface O3. The main source of high O3 to these two sites is from the stratosphere, either from direct injection into the cyclone or associated with aged airstreams from decaying downstream cyclones that can become entrained and descend toward the surface within new cyclones over the NA region