Laura de la Torre

Climatological features of cutoff low systems in the northern Hemisphere

This study presents the first multidecadal climatology of cutoff low systems in the Northern Hemisphere. The climatology was constructed by using 41 yr (1958-98) of NCEP-NCAR reanalysis data and identifying cutoff lows by means of an objective method based on imposing the three main physical characteristics of the conceptual model of cutoff low (the 200-hPa geopotential minimum, cutoff circulation, and the specific structure of both equivalent thickness and thermal front parameter fields). Several results were confirmed and climatologically validated: 1) the existence of three preferred areas of cutoff low occurrence (the first one extends through southern Europe and the eastern Atlantic coast, the second one is the eastern North Pacific, and the third one is the northern China-Siberian region extending to the northwestern Pacific coast; the European area is the most favored region); 2) the known seasonal cycle, with cutoff lows forming much more frequently in summer than in winter; 3) the short lifetime of cutoff lows, most cutoff lows lasted 2-3 days and very few lasted more than 5 days; and 4) the mobility of the system, with few cutoff lows being stationary. Furthermore, the long study period has made it possible (i) to find a bimodal distribution in the geographical density of cutoff lows for the European sector in all the seasons (with the exception of winter), a summer displacement to the ocean in the American region, and a summer extension to the continent in the Asian region, and (ii) to detect northward and westward motion especially in the transitions from the second to third day of occurrence and from the third to fourth day of occurrence. The long-term cutoff low database built in this study is appropriate to study the interannual variability of cutoff low occurrence and the links between cutoff lows and jet stream systems, blocking, or major modes of climate variability as well as the global importance of cutoff low in the stratosphere-troposphere exchange mechanism, which will be the focus of a subsequent paper.

Changes in the relationship NAO–Northern hemisphere temperature due to solar activity

Abstract The influence of the North Atlantic Oscillation (NAO) on wintertime Northern Hemisphere Temperature (NHT) is investigated. The results suggest that this relationship has different sign according to the phase of the solar cycle. For solar maximum phases NAO and NHT are positively correlated – a result assumed up to the moment – but for solar minimum phases correlations are not significant or even negative. This result is in agreement with the different extension of the NAO for solar cycle phases [Kodera, Geophys. Res. Lett. 29 (2002) 14557–14560] – almost hemispheric for maximum phases and confined to the eastern Atlantic for minimum phases.

The Deep Atmospheric Boundary Layer and Its Significance to the Stratosphere and Troposphere Exchange over the Tibetan Plateau

In this study the depth of the atmospheric boundary layer (ABL) over the Tibetan Plateau was measured during a regional radiosonde observation campaign in 2008 and found to be deeper than indicated by previously measurements. Results indicate that during fair weather conditions on winter days, the top of the mixed layers can be up to 5 km above the ground (9.4 km above sea level). Measurements also show that the depth of the ABL is quite distinct for three different periods (winter, monsoon-onset, and monsoon seasons). Turbulence at the top of a deep mixing layer can rise up to the upper troposphere. As a consequence, as confirmed by trajectory analysis, interaction occurs between deep ABLs and the low tropopause during winter over the Tibetan Plateau.

Global statistics of multiple tropopauses from the IGRA database

[1] We present statistics for the occurrence of multiple tropopauses for the entire globe, derived from the meteorological sounding data contained in the Integrated Global Radiosonde Archive (IGRA). The IGRA is the most comprehensive and largest radiosonde data set compiled to date, with more than 1500 stations and data starting from 1938. Statistics were derived from (a) the IGRA tropopause reports (reported tropopauses) and (b) tropopauses calculated from the sounding profiles reported in the IGRA (calculated tropopauses). This work constitutes a necessary precursor to conducting better research on the phenomena of multiple tropopauses and promotes understanding of the global structure of these events. Among other things, we calculated global counts and the latitudinal distribution of percentages of tropopauses with respect to the number of soundings, percentages of second and third tropopauses with respect to the first tropopause, and mean values of pressure and temperature of multiple

Changes in tropopause height for the Eurasian region determined from CARDS radiosonde data

Previous studies have identified the tropopause height (TH) as a promising fingerprint of climatic change. In the present paper, we report variations in TH for the Eurasian region over the period 1973–1998 and analyse the influence of the Northern Annular Mode (NAM) on these variations. As previous studies indicate that the greatest increases in TH occur in the extratropics, we focused our attention on this area. We applied a set of homogenization procedures to radiosonde data and considered three different scenarios that take into account change points and the main volcanic eruptions over the study period. Our results demonstrate that the number of stations with positive TH trends is very sensitive to the quality of data and the methods used to remove inhomogeneities. Consequently, when change points were included in the analysis, the number of stations with positive trends decreased markedly. Furthermore, stratospheric NAM appears to control TH in stations located at latitudes higher than 55°N.

On the Origin of the Air between Multiple Tropopauses at Midlatitudes

Multiple tropopauses are structures that regularly recur in the midlatitudes. Recent studies have relied on the notion of the excursion of tropical air from the upper troposphere into higher latitudes, thereby overlaying the tropopause of the midlatitudes. We herein analyse the origin and characteristics of the air at the Boulder radiosonde station, between the first and second tropopauses combining an analysis of radiosonde data with a Lagrangian approach based on the FlexPart model and ERA-40 analysis data. Our results show that the air between both tropopauses has its origin in midlatitudes.

A Climatology Based on Reanalysis of Baroclinic Developmental Regions in the Extratropical Northern Hemisphere

Regions of the occurrence of different phenomena related to the development of baroclinic disturbances are reviewed for the Northern Hemisphere extratropics, using National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data. The occurrence of height lows appears to be related to the orography near the earths surface and with surface‐ and upper‐air cyclogenesis in the upper troposphere. Over the cyclone tracks, the surface maxima appear to be trapped by land masses, whereas over the Mediterranean Sea they are located on the lee side of mountain ranges. The forcing terms of the geopotential tendency and omega equations mark the genesis (and, by the vorticity advection terms, the path) of the extratropical cyclones on the storm track. They occur mostly over the western coast of the oceans, beginning and having maxima on the lee side of the Rocky Mountains and the Tibetan Plateau. Their associated fronts form from the cold air coming from the continents and converging with the warm air over the Gulf and Kuroshio currents. Evident trends are found only for the Atlantic cyclone track (positive) and the Pacific cyclone track (negative) until the last decade when the tendency reverses. Over the southern Pacific, the number of fronts is lower during 1978–1997, coinciding with a period of strong El Niño Southern Oscillation episodes. This information is important for validating numerical models in order to predict changes associated with climate change and to study the behavior of extratropical cyclones and fronts.

Recovery of Meteorological Data for the Observatory of A Guarda, Spain

We herein describe the recovery of a series of data on temperature, humidity, precipitation, evaporation, wind, and local weather conditions from documentary sources obtained from the Jesuit observatory of A Guarda (Galicia, Spain) for the period 1881–1896. The data were digitized and made available in accessible electronic formats. Comparisons were made with present-day meteorological data obtained from two nearby stations. We further believe that the discovery of some new complementary documentary sources made during the present research could be a basis for future data recovery efforts. Among these new results, early ozone data from the period are of outstanding importance to meteorologists.

Imprints of the North Atlantic Oscillation on four unusual atmospheric parameters

Abstract Using data for 41 yr (1958–1998) from the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis the authors study the impact of the North Atlantic Oscillation (NAO) on four atmospheric parameters. These four parameters have three common characteristics: their previous limited use as climate diagnosis tools, their high dependence on changes in atmospheric circulation and their relationship with variables associated with the development of synoptic perturbances. They are: (1) relative angular momentum from 500 hPa to 200 hPa and from 55°N to 90°N, (2) advection of temperature at 500 hPa, (3) equivalent temperature at 850 hPa and (4) distribution of upper level low systems (ULL). The most significant results are: (1) a significant positive correlation between the relative angular momentum from 500 hPa to 200 hPa and from 55°N to 90 °N with the NAO index computed as the normalized pressure difference between Ponta Delgada (Azores) and Reykjavik (Iceland), (2) the NAO affects 500 hPa temperature advection from 45°N to 70°N and a positive NAO index is alternatively related to positive or negative anomalies of advection. The relationship is positive over the Atlantic, Asia and Western America and negative over Europe, the Pacific and Eastern America. (3) The correlation pattern between NAO index and the equivalent temperature at 850 hPa shows a semiannular structure with negative correlations over the Arctic and positive correlations over midlatitudes with the exception of the Pacific area, and (4) the distribution of ULL systems is only influenced by NAO over both sides of the Atlantic Ocean and for the latitude belt from 20°N to 50°N.

The use of atmospheric relative angular momentum to diagnose the Arctic oscillation

Abstract Recent studies suggest that the North Atlantic Oscillation is a regional expression of the Northern Hemisphere Annular Mode. However, the lack of an apparent link between Atlantic and Pacific sectors questions the annular paradigm. In any case, the phenomenon is the largest and most fundamental mode of variability in the Northern Hemisphere troposphere, being responsible for much of the warming in the Northern Hemisphere surface temperature. The main aim of this study is to diagnose the Northern Hemisphere Annular Mode using an annular magnitude, such as the relative angular momentum (RAM). RAM from 500 hPa to 200 hPa is computed for the polar cap, using 60° longitude sectors as well as the whole cap, from 55 to 90°N and for the period 1958–1998, using NCEP-NCAR reanalysis data. Annual values of RAM are then correlated to temperature and geopotential height at 200, 500 and 850 hPa. The Artic region is dominated by very negative correlations for the Northern Hemisphere RAM and for the six different RAM sectors. Midlatitudes are dominated by positive correlation, with intermittent or alternating significant regions not showing a regular annular pattern. The Pacific Ocean area is only significantly correlated with the Pacific RAM sector. Since an annular pattern is characterized by its hemispheric symmetry, two symmetric latitude rings are expected, but only the polar one is detected, the rest being incomplete. Due to the proximity of the grid points near the pole, which can lead to artificial zonal symmetry, all the calculations are repeated for RAM from 45 to 65°N, finding a similar pattern with lower correlations and less symmetry. So the results do not support the annular paradigm, understood as organized along latitude sectors, but a hemispheric dimension cannot be excluded.