IDENTIFICACIÓN, CARACTERIZACIÓN CLIMÁTICA Y PREDECIBILIDAD DE CUT- OFF LOW SYSTEMS Raquel Nieto Universidad de Vigo - Campus de Ourense

Introducción Vórtices ciclónicos aislados en la media y alta troposfera que se forman a partir de vaguadas profundas en la circulación general del oeste. Fácilmente reconocibles en mapas isobáricos como contornos cerrados de geopotencial que en su fase final se aíslan de la circulación principal del oeste, con una persistencia de unos pocos días. En términos dinámicos las DANAs constituyen centros aislados de alta vorticidad potencial (VP) en superficies isoentrópicas. Se caracterizan por el hundimiento de la tropopausa en su parte central. Una tropopausa anómalamente baja ayuda a que se produzca intercambio entre troposfera y estratosfera (STE).

Geopotencial en 200 hPa Geopotencial en 1000 hPa PARÁMETROS DE DIAGNÓSTICO Geopotencial en 200hPa Geopotencial en 1000hPa Geopotencial en 200hPa Geopotencial en 1000hPa Geopotencial en 1000hPa Geopotencial en 200hPa Espesor equivalente hPa Parámetro Frontal Térmico (PFT) hPa Temperatura en 500 hPa Vorticidad potencial (VP)

 Bajo las DANAs la troposfera es habitualmente inestable y pueden ocurrir fenómenos convectivos severos  Pueden originar precipitaciones de intensidad moderada o fuerte sobre áreas extensas, particularmente bajo el flanco situado al sudoeste.  Son unos de los sistemas más importantes que afectan al sur de Europa y norte de África, responsables de eventos catastróficos. TIEMPO ASOCIADO A UNA DANA -- NUBOSIDAD Y PRECIPITACIÓN ASOCIADAS -- situaciones de predicción compleja. sobre océano sobre tierra

COLs The work has been divided into three main objectives: 1st  to develop long-term comprehensive NH climatologies of cutoff lows for a 41-yr period (1958–98). 2nd  to examine interannual variability in the climatological character of cutoff lows associated with ENSO, NAO, PNA and NAM, as well as trends in the number of cutoff lows and their relationship with blocking events. 3rd  to check the expected weather events according to the conceptual model in an area, such as the Iberian Peninsula, where precipitation due to cutoff lows is relevant.

Trabajo hecho  Climatology over the whole NH (J. Climate, 2005)  Analysis of the precipitation and cloudiness associated with COLs occurrence in the Iberian Peninsula (MAP 2007)  Interannual variability of COLs over the European sector: the role of blocking and the Northern Hemisphere circulation modes (MAP 2007)  Influence of the Stratospheric Circulation in Tropospheric Synoptic Systems (MAP 2007) UPDATE Extend the Dataset from 1948 to 2006 with data from NCAR-NCEP REFORECAST Predictability of COL systems occurrence using reforecasts ERA-40 COL Dataset from 1978 to 2005 with reanalisys ERA-40 Compare with the previous dataset PRESENTE

 Climatology over the whole NH [a] [a] Nieto R et al. (2005) Climatological features of COLs in the Northern Hemisphere. J Climate, 18, 2805–2823. Objective method: daily data (Z, u and T) at multiple levels from the reanalysis NCEP-NCAR (horizontal resolution 2.5º x 2.5º) for the period from 1958 to 1998 (41 years) in a band 20ºN-70ºN We identify COLs using an approach based in imposing the 3 main physical characteristics of the conceptual model of COLs

Distribución Espacial NºP1P2 P17946 P % P %78.66% CLIMATOLOGÍA DE DANAS EN EL HEMISFERIO NORTE versión suavizada calculada para cajas de 5º latitud x 10º longitud.

Several results were climatologically validated 3 preferred areas of occurrence seasonal cycle short life time (Top) Total nº of COLs by grid point (2.5ºx2.5º) for the period without splitting in seasons. (Bottom) a smoothed version calculated for boxes of 5ºlat x 10º lon Nº of COL as a function of the duration for the three main sector of occurrence. The long studied period has permitted to find more specifically characteristics: Number of COLs for each season and main sector of occurrence. Coloured areas include at least the 95% of the total nº of COLs for each sector and season. Numbers denote places where more than one COL was present in that period.  a bimodal distribution in the geographical density of COLs for the European area  a summer displacement to the ocean in the American region  a summer extension to the continent in the Asian region [a] Nieto R et al. (2005) Climatological features of COLs in the Northern Hemisphere. J Climate, 18, 2805–2823.

The long-term COL database built in this study was used to study: A. the local importance of COLs in the weather events caused over one of the preferred area of occurrence (the Iberian Peninsula) [b] B. the interannual variability of COL occurrence [c] C. the link between COLs and blocking [c] D. the link between COLs and major modes of climate variability [c] E. the influence of stratospheric circulation in COL systems [d] [b] Nieto R et al. (2006) Analysis of the precipitation and cloudiness associated with COLs occurrence in the Iberian Peninsula. MAP Volume 96, Numbers 1-2 / April, 2007 ; DOI /s [c] Nieto R et al. (2006) Interannual variability of cut-off low systems over the European sector: the role of blocking and the northern hemisphere circulation modes. MAP Volume 96, Numbers 1-2 / April, 2007 ; DOI /s [d] Gimeno L, Nieto R & Trigo R (2006) Decay of the Northern Hemisphere stratospheric polar vortex and the occurrence of cut-off low systems. MAP Volume 96, Numbers 1-2 / April, 2007 ; DOI /s

 Analysis of the precipitation and cloudiness associated with COLs occurrence in the Iberian Peninsula The objective is to analyse the weather events (rainfall and cloudiness layer) associated to COLs in Iberia with tools not previously used: the use of the new multidecadal COLs database to characterize the amounted weather events (rainfall and cloudiness layer from the main stations of the INM -Spanish National Meteorology Institute-) associated with COLs and the extensive use of radiosoundings to analyse convective instability in areas inside and close to the COL (radiosounding data from IGRA) Two criteria were used:  1 st criterion is a source oriented method, when a particular COL was followed and its associated precipitation and cloudiness analysed directly over four quadrants in which Iberia was divided  2 nd is a receptor oriented method, when the precipitation associated to COLs was analysed in given areas. These areas were classified by using precipitation climatological patterns. PRECIPITATION TYPES OVER IBERIAN PENINSULA Blue boxes: % of COLs that produce precipitation over: the Northern region (2); the South-centre region (3); the East- Mediterranean region (4); for null precipitation (1) and when the precipitation is generalized over the whole Peninsula (5). Orange boxes: number and percentage of COLs situated in each quadrant. Scheme of precipitation types for that COLs attending to their position

RESUMEN Y CONCLUSIONES Existen 3 áreas de ocurrencia preferentes: el sur de Europa y la costa este del océano Atlántico, el noreste del océano Pacífico y el norte de China y las regiones de Siberia extendiéndose hasta la costa noroeste del Pacífico. El área europea es la de mayor frecuencia de DANAs. Se observa una distribución bimodal en la región de Europa -región este del océano Atlántico y la parte sur del continente, incluyendo la región mediterránea-. En la región Americana se observa un desplazamiento de los sistemas hacia el océano durante el verano. En la región Asiática se observa una expansión hacia el continente también durante el verano. Las DANAs se forman mayoritariamente durante el verano - 50% de los casos-. Las mayoría de las DANAs tienen un tiempo de vida entre 2 y 3 días Aunque con limitaciones, se puede decir que las DANAs son más duraderas cuando ocurren a latitudes mayores.

No todas las DANAs conllevan precipitación intensa de tipo convectivo, como habitualmente se le asocia a sistemas históricamente denominados “gotas frías”. Las precipitaciones sobre la zona mediterránea se debe principalmente (>60%) a precipitación propia de las DANAs localizadas sobre esa región. En la región mediterránea las precipitaciones asociadas a DANAs, por su cuantía sí, son debidas a fenómenos convectivos producidos en el centro de las DANAs. La mayor parte de la precipitación asociada a DANAs procede del escudo baroclino, sobre todo en los casos de DANAs localizadas en la mitad oeste peninsular. Existe un gran número de sistemas que no provocan precipitación alguna (≈30%), localizándose su mayor parte en la mitad sur peninsular, y sobre todo durante el otoño. Existe un porcentaje muy importante de DANAs (30%) que provocan precipitación generalizada en todo el territorio -DANAs sobre la mitad oeste peninsular-. CONCLUSIONES

ACTIONANALYSIS TOOLOBJECTIVES Interannual variability of COLs occurrence §Linear regression (trends) §MTM analysis (significant oscillations in the whole studied period) §Wavelet analysis (significant oscillations in any part of the studied period) ØTo know if COLs occurrence has increased (decreased) in the last decades ØTo check if COLs occurrence exhibits trend or oscillations similar to those of the main modes of climate variability (possible relation) Association of COLs with modes of climate variability and teleconnections §Correlations between COLs occurrence and teleconnection indices §Composites of geopotential height fields associated with COL occurrence. ØTo explore possible associations, rejecting those in which correlations are not significant. ØTo check the physical mechanism, testing if the signs of significant correlations match the expected ones, according to the relation of the modes with the jet structure. ØTo give physical sense to the statistical connection, looking for height patterns similar to those that characterize the different phases of the modes and teleconnections Association with blocking events §Objective identification of blocking events linked to COLs §Comparison of monthly frequencies §Comparison of geographical distributions ØTo estimate how many COLs occur linked to blocking events. ØTo estimate when and where is more probable this linked occurrence Summary of the research methodology  Interannual variability of COLs over the European sector: the role of blocking and the NH circulation modes COL data: from Nieto et al. (2005) Blocking data: dataset by Barriopedro et al. (2006) Indices of climate variability modes from NOAA/CPC

Annual and seasonal series of COLs occurrence in the European sector and their trends (red line notes significant trend at 95 %). Proposed physical mechanism that links the COLs with larger- scale phenomena. The blue lines indicate the relationships found in previous studies. The relations analysed are indicated with red arrows. Dashed black line denotes other possible influences on COL occurrence. Months with signal COLs PERIOD WINTERSPRINGSUMMERAUTUMN NAODJFMAMJJASON EADJFMA----SON XX0.25 EA-JET----AMJJA X X EA-WRDJFMAM---SON X-0.04 SCANDJFMAM--ASON QBODJFMAMJJASON NIÑO 3.4DJFMAMJJASON NIÑO 4DJFMAMJJASON Correlation coefficients between major modes of climate variability and COLs number Values in red are statistically significant at a 95% confidence level. Blue ones at a 90%  The annual occurrence of COLs presents a large interannual variability that is also observed for seasonal occurrence; however there is only a significant and negative trend for spring series  The interannual variability is partially modulated by larger scale phenomena. Correlation analyses results are in moderate agreement with those expected according to the structure of the jet stream. ENSO negative (positive) phases are related to weaker (stronger) jets and higher (lower) probability of COL occurrence. Correlation analysis suggests that the NAO has the highest influence in COLs occurrence  positive phases of NAO favour the annual occurrence of COL

Link between COLs and blocking in the European area Monthly mean distribution of COLs (blue) and COLs-B (red) for the European sector. Monthly distribution of the mean latitude (left) and of the mean longitude (right) for the all the COLs (blue) and for the subset of these that are associated with situations of blocking (red). The negative (positive) values for the longitude indicate positions towards the W (E) of the Greenwich meridian.  More than 50% of the COLs found in the European sector are associated to blocking events  higher in spring and winter when blocking events are more frequent.  Almost half of the COLs associated to blocking occur simultaneously with blocking events.  Most of the COLs were associated with blocking, especially during the life cycle of the block, suggesting that COLs may also play a significant role in blocking maintenance Frequency of COLs-B attending to if they are ‘previous’, ‘simultaneous’ or ‘posterior’ to blocking events.

BUT !! There are 3 important reasons that suggest that NAO is not a major influence factor in the development of winter COLs: the low correlation between COL occurrence and NAO (0.11), the lack of trend in COL occurrence whereas NAO has shown a significant positive trend during the analysed period and the lack of coincidence in the geographical position of the height anomalies dipole associated to COLs and NAO.  The occurrence of winter COLs seems to be closely related to blocking events and there is a solid theoretical study that shows that blocking events must be influenced by NAO (Luo, 2005).  An explanation for this paradox could be the difference found in the NAO dipole for strong and weak polar vortex  Walter and Graf (2005) have recently found a two dipole structure for weak vortex and only one for strong vortex. Only the negative upper tropospheric ‘NAO’ during strong polar vortex has similar structure to blocking height situations over Europe and could be important in explaining the occurrence of both blocking and COLs.  During spring the connection between COLs and NAO is not as clear as during autumn, and the correlation analysis supports the relationship, but the negative trend in COLs occurrence is in clear disagreement with the positive trend found in NAO indices.  In addition, the dipole structure in height fields associated with COLs continues being displaced to North. These results suggest that COLs occurrence during spring could be more influenced by variations in the occurrence of the spring onset (Black et al. 2005), which has a similar spatial structure, than by NAO  

 Influence of the Stratospheric Circulation in COL Systems Effect of the timing of the stratospheric vortex breakup in the occurrence of COL systems in the NH There are two dynamical reasons to think that COLs occurrence could be influenced by the persistence of the stratospheric vortex: a) the quasi-steady summerlike state after the vortex decay with strong weakening of the westerlies favors COL development and b) the fact that remnants of the vortex survive as coherent potential vorticity structures for around two months for early breakup years Among the most representative coherent potential vorticity structures are the COLs, which dynamically are isolated regions of high potential vorticity that affect both the stratosphere and the troposphere. [d] Gimeno L, Nieto R & Trigo R (2007) Decay of the Northern Hemisphere stratospheric polar vortex and the occurrence of COLs (MAP) DATA used: Persistence of the Arctic vortex, conbined B05-K05. N(COLs)Five later breakup years Five earlier breakup years NH 20ºN-70ºN (48.9%)286 (51.1%) NH 20ºN-45ºN (46.4%)181 (53.6%) NH 45ºN-70ºN (52.7%)105 (47.3%) European sector (25ºN-47.5ºN, 50ºW-40ºE) (42.4%)87 (57.6%) Pacific sector (25ºN-45ºN, 100ºW-150ºW) 6530 (46.2%)35 (53.8%) Asian sector (35.7ºN-62.5ºN, 100ºE, 180ºE) 8143 (53.1%)38 (46.9%)  Nº of COLs for the 10 analyzed extreme years (N) and for the 5 later (earlier) breakup years (in brackets, %).

Seasonal and Monthly COLs occurrence for the 5 earlier (red) and later (blue) breakup years according to the combined B05-K05 for two latitude bands: 20ºN-45ºN included (left) and 45ºN excluded-70ºN(right) Perceptible transition during May. JFMA: no significant differences in the COLs frequency according to the vortex persistence. From May onwards COLs are more frequent when the stratospheric vortex decays earlier. This higher frequency persists until September. The t-student test showed that the monthly frequency distributions for earlier and later breakup years, during the period that spans from May to September, were different at p<0.10. For the main sectors of COLs occurrence and the monthly occurrence limited only to the European sector. COLs are more frequent at lat. lower than 45ºN during spring and summer for earlier vortex years. T-student test: the differences in the occurrence from May to September were statistically significant at p<0.05. During spring and summer for earlier breakup years the higher occurrence of COLs is observed for the European and the Pacific sectors. The results are very similar to those obtained for latitudes lower than 45ºN, although the differences started in April.

Concluding remarks  In the European sector COLs are more frequent for earlier breakup years.  For latitudes lower than 45ºN: * Spring and summer COLs occurrence is partially controlled by the timing of the stratospheric vortex breakup, so in those years when the vortex decays earlier, there are more COLs in both seasons. The differences start in May and last until September, which coincides with the period when more COLs occur in the NH. These differences are statistically significant.  For latitudes higher than 45ºN: * There are not significant differences in the COL occurrence according to the stratospheric vortex persistence, although during July and August more COLs occurred for later breakup years. ** These latitudinal discrepancies can reside in the different nature of the formation of lower latitudes COLs vs higher latitudes ones.  These results confirm that the earlier transition to summerlike dynamical state and the persistence of remnants of the vortex during earlier breakup years favor the conditions of lower latitudes COLs development since about one month of the breakup and that this influence persists for the whole summer.  The timing of the stratospheric vortex breakup is easily calculated with only a few days of delay, therefore we believe that results here obtained have some predictive skill, by anticipating low occurrence of COLs, at lower latitudes, for late vortex decays and high occurrence for early decays.

MAP Special Issue on Cut-off Low Systems (COL) Volume 96, Numbers 1-2 / April, 2007

PRESENT and FUTURE  UPDATE the dataset extending the period from 1948 to 2005 with data from NCAR-NCEP with a resolution of 2.5x2.5º.  A comparison between ERA-40 and NCAR/NCEP reanalyses: Climatology of COLs Application of the objective methodology used to ERA 40 reanalysis to build a new COLs dataset. The technique will use the full available temporal (6-hourly) and spatial resolutions (1.125º regular grids). A comparison between NCEP/NCAR and ERA-40 will be performed for spatial resolution, duration, tracks and main characteristics.  Predictability of COLs occurrence using Reforecasts Based in the larger scale climatology of these systems over the whole NH and using the CDC reforecast dataset (a dataset of 15 ensemble forecasts from 1979 to version of the NCEP MRF 1998 model-), we will attempt to shed light on the prediction of COLs for a period valid in climatological time. Another objective is to find the most predictable seasonal distribution pattern for the main areas of occurrence of COLs using the 15 members of the ensemble.

 UPDATE the dataset extending the period from 1948 to 2006 with data from NCAR-NCEP with a resolution of 2.5x2.5º.  A comparison between ERA-40 and NCAR/NCEP reanalyses: Climatology of COLs

 Predictability of COLs occurrence using Reforecasts CDC Comparison between different data COLs In order to demonstrate the ability of reforecast data we compare the geophysical distribution of COL points over the NH. NCAR-NCEP 00UTC hPa (as Nieto et.al 2005) CDC Reforecast Ensemble Mean hPa 12 h36 h24 h Data Used: The larger scale climatology of these systems by Nieto et al. (2005) over the whole Northern hemisphere and the CDC reforecast dataset (a dataset of 15 ensemble forecasts from 1979 to 2005 with a 2.5° by 2.5° resolution -version of the NCEP MRF 1998 model-). What is the idea? We will attempt to shed light on the prediction of COLs for a period valid in climatological time. Another objective is to find the most predictable seasonal distribution pattern for the main areas of occurrence of COLs using the 15 members of the ensemble for five years. Method: The same automated procedure by Nieto et al. (2005) was used to identify grid points that fitted the COL criteria. Cutoff systems northward of 70° or southward of 20° were not included in the study. Additional Data: For this study we use geopotential, zonal wind, and T daily data from 250 and 500 hPa. NCAR-NCEP 00UTC hPa

As in Nieto et al. (2005) there are three preferred areas of cutoff low occurrence for the six first ensemble forecast time (12 to 72h): southern Europe and the eastern Atlantic coast, the eastern North Pacific, and the North China–Siberian region extending to the northwestern Pacific coast. But very few systems was detected over 60ºN. 12 h Decay of the Number of COL Points with Reforecast The total number of COLs, when the ensemble mean forecast is used, decreases exponentially in the medium range, beyond two to three days of forecasting. COLs are only predictable at most 3 days before. Seasonal Cycle Ensenble Mean It is during summer (JAS) when the reforecast data has lower predictability of COLs. In the analysis the maximum occurs during summer while in the forecast occurs during spring.

In general, COLs are detected at higher latitudes in the forecast than in the analysis, specially between two to three days of forecasting. The latitudinal bands between 30ºN-40ºN and 40ºN-50ºN seem to be the most constants after 24h and COLs are detected at higher latitude as reforecast time increases. Between 50ºN-60ºN COLs are detected at lower latitude as reforecast time increases. There are not COL points for reforecast data between 60º-70º, and very few between 20º-30º. In summay, the frequency distribution as a function of latitude for the ensemble mean seems higher between a band of 30º-50ºN (more than 70% of the systems), and very few systems was detected over 60ºN. Frequency distribution of COL as a function of Latitude (2pts.mov.aver.) Frequency distribution of COL as a function of Longitude (10pts.mov.aver.) Mean Latitude Frequency of Mean Latitude by Bands of Ocurrence

Coincidence between Reforecast COLs vs. NCAR COLs COL prediction performance as a function of forecasting time for CDC ensemble prediction system. Dashed lines show the frequency of correctly forecast COLs, dotted lines show the COLs forecast by the model which did not appear in the analysis, and the solid lines show the total COLs frequency. All values are normalized by the initial COL frequency. Results show THE POOR PREDICTABILITY OF THE COLs. Coincidence of COLs between reforecasting and analysis by different boxes of longitude and latitude. The bigger is the box the higher is the coincidence.