David Gallego

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.

A Review of 2000 Years of Paleoclimatic Evidence in the Mediterranean

The integration of climate information from instrumental data and documentary and natural archives; evidence of past human activity derived from historical, paleoecological, and archaeological records; and new climate modeling techniques promises major breakthroughs for our understanding of climate sensitivity, ecological processes, environmental response, and human impact. In this chapter, we review the availability and potential of instrumental data, less well-known written records, and terrestrial and marine natural proxy archives for climate in the Mediterranean region over the last 2000 years. We highlight the need to integrate these different proxy archives and the importance for multiproxy studies of disentangling complex relationships among climate, sea-level changes, fire, vegetation, and forests, as well as land use and other human impacts. Focusing on dating uncertainties, we address seasonality effects and other uncertainties in the different proxy records. We describe known and anticipated challenges posed by integrating multiple diverse proxies in high-resolution climate-variation reconstructions, including proxy limitations to robust reconstruction of the natural range of climate variability and problems specific to temporal scales from interannual to multicentennial. Finally, we highlight the potential of paleo models to contribute to climate reconstructions in the Mediterranean, by narrowing the range of climate-sensitivity estimates and by assimilating multiple proxies.

Dynamic identification of moisture sources in the Orinoco basin in equatorial South America

Abstract The main areas of net moisture uptake are examined in air masses over the Orinoco River basin, located in equatorial South America, north of the Amazon basin. Although the Orinoco River has the third largest average annual discharge in the world (with 5.4 × 1011 m3 year−1 draining into the Atlantic Ocean), the sources of moisture that feed it have not previously been studied in any detail. The results are presented from analyses of back-tracking of all the air masses over the Orinoco basin over a period of five years (2000–2004) using the diagnostic Lagrangian tool FLEXPART. The input data for the model were obtained from the European Centre for Medium-range Weather Forecasts (ECMWF). Air transported into the Orinoco basin experiences a large uptake of water over the tropical North Atlantic within the three days prior to its arrival over the basin. The Tropical South Atlantic and the eastern coast of the Pacific become significant moisture sources for about 5–10 days before arriving over the Orinoco basin. Contrary to what might be expected, large areas of the Amazon basin, along with the Gulf of Mexico, do not provide significant moisture to the study area. Interestingly, over these zones the air experiences net moisture loss. Preliminary analysis of the processes that occur leads to the conclusion that most of the water observed over the Orinoco basin derives from advective fluxes into the area, while recycling of moisture is negligible.

Contributions to the moisture budget of airmasses over Iceland

Motivated by the excellent skills of a new Lagrangian diagnosis method to identify the contributions to the moisture budget over a region (STOHL and JAMES, 2004, 2005), this study examines the main areas where there is net uptake of moisture in airmasses over Iceland. The method computes budgets of evaporation minus precipitation by calculating changes in the specific humidi ty along back-trajectories for the previous 10 days. We tracked the origin of all air-masses, including precipit ating airmasses, residing over Iceland during a period of five years (2000‐2004). Air transported into the Ic elandic waters has a large uptake of water over the Norwegian Sea in the preceding first three days and from th e Western-North Atlantic in the range of 3‐10 days. Concerning the days with observed precipitation in SW-Iceland, it was found that the major net uptake of moisture was the final northward segment of the Gulf Stream and the Atlantic waters immediately surrounding Iceland. Zusammenfassung Motiviert durch die ausgezeichneten Fahigkeiten einer neuen Lagrangeschen-Diagnosemethode zur Identifizierung der Beitrage zum Feuchtigkeitshaushalt uber ei ner Region (STOHL und JAMES, 2004, 2005), analysiert diese Studie die Hauptgebiete in denen eine Netto-Feuchtigkeitsaufnahme der Luftmassen uber Island stattfindet. Diese Methode berechnet den Nettobetra g der Evaporation abzuglich des Niederschlags, indem sie Anderungen in der spezifischen Feuchte uber Ruckwa rtstrajektorien fur die vorhergehenden 10 Tage errechnet. Hiermit haben wir den Ursprung aller Luftmassen, einschlieslich regentrachtiger Luftmassen, die sich uber Island wahrend einer Periode von funf Jahren (2000‐2004) befanden, zuruckverfolgt. Die Luftmassen, die in die islandischen Gewasser transportiert werden, nehmen eine grose Menge Feuchtigkeit uber dem norwegischen Meer in den vorhergehenden drei Tagen auf, sowie uber dem Nordwestatlantik uber einen Zeitraum von 3‐10 Tagen. Hinsichtlich der Tage mit beobachtetem Niederschlag uber Sudwest-Island wurde die hochste Nettofeuchtigkeitsaufnahme uber dem nordlichsten Abschnitt des Golfstroms und den atlantischen Gewassern, die Island direkt umgeben, gefunden.

Witnessing North Atlantic westerlies variability from ships’ logbooks (1685–2008)

Abstract A monthly index based on the persistence of the westerly winds over the English Chanel is constructed for 1685–2008 using daily data from ships’ logbooks and comprehensive marine meteorological datasets. The so-called Westerly Index (WI) provides the longest instrumental record of atmospheric circulation currently available. Anomalous WI values are associated with spatially coherent climatic signals in temperature and precipitation over large areas of Europe, which are stronger for precipitation than for temperature and in winter and summer than in transitional seasons. Overall, the WI series accord with the known European climatic history, and reveal that the frequency of the westerlies in the eastern Atlantic during the twentieth century and the Late Maunder Minimum was not exceptional in the context of the last three centuries. It is shown that the WI provides additional and complementary information to the North Atlantic Oscillation (NAO) indices. The analysis of WI series during the industrial era indicates an overall good agreement with the winter and high-summer NAO, with the exception of several multidecadal periods of weakened correlation. These decoupled periods between the frequency and the intensity of the zonal flow are interpreted on the basis of several sources of non-stationarity affecting the centres of the variability of the North Atlantic and their teleconnections. Comparisons with NAO reconstructions and long instrumental indices extending back to the seventeenth century suggest that similar situations have occurred in the past, which call for caution when reconstructing the past atmospheric circulation from climatic proxies. The robustness and extension of its climatic signal, the length of the series and its instrumental nature make the WI an excellent benchmark for proxy calibration in Europe and Greenland.

The stratospheric QBO signal in the NCEP reanalysis, 1958–2001

The spatiotemporal evolution of the zonal wind in the stratosphere is analyzed based on the use of the NCEP reanalysis (1958-2001). MultiTaper Method-Singular Value Decomposition (MTM-SVD), a frequency-domain analysis method, is applied to isolate significant spatially-coherent variability with narrowband oscillatory character. A quasibiennial oscillation is detected as the most intense coherent signal in the stratosphere, the signal being less intense in the lower levels. There is a clear downward propagation of the signal with time at low latitudes, not evident at mid and high latitudes. There are differences in the behavior of the signal over both hemispheres, being much weaker over the SH. In the NH an anomaly in the zonal wind field, in phase with the equatorial signal, is detected at approximately 60degreesN. Two different areas at subtropical latitudes are detected to be characterized by wind anomalies opposed to that of the equator.

An ENSO signal in the North Atlantic subtropical area

Precipitation in the Canary Islands is studied from a synoptic point of view for the 1955–1998 period, using data from the NCEP-NCAR reanalysis. The use of automatic objective techniques allows the identification of five patterns leading to precipitation in the area. The influence of intense El Nino and La Nina events on these patterns is then studied. The results show that La Nina conditions are characterized by more intense and frequent precipitation episodes as result of deeper and more frequent geopotential anomalies at the 500 hPa level.

Tracking the Indian Summer Monsoon Onset Back to the Preinstrument Period

AbstractThe Indian summer monsoon onset is one of the most expected meteorological events of the world, affecting the lives of hundreds of millions of people. The India Meteorological Department has dated the monsoon onset since 1901, but its original methodology was considered subjective and it was updated in 2006. Unfortunately, the new method relies on OLR measurements, which impedes the construction of an objective onset series before the 1970s. An alternative approach is the use of the wind field, but the development of such an index is limited to the period covered by reanalysis products. In this paper historical wind records taken on board ships are used to develop a new onset series using only wind direction measurements, providing an objective record of the onset since the late nineteenth century. The new series captures the rapid precipitation increase associated with the onset, correlates well with previous approaches, and is robust against anomalous (bogus) onsets. A tendency for later-than-av...

Reconstructing the Western North Pacific Summer Monsoon since the Late Nineteenth Century

AbstractA new index, the western North Pacific directional index (WNPDI), based on historical wind direction observations taken aboard sailing ships, has been developed to characterize the western North Pacific summer monsoon (WNPSM) since 1898. The WNPDI measures the persistence of the surface westerly winds in the region 5°–15°N, 100°–130°E and easterly winds in the region 20°–30°N, 110°–140°E, exhibiting a consistent relationship with the summer precipitation in the areas affected by the WNPSM throughout the entire twentieth century. Its length doubles that of the previous WNPSM index (1948–2014) based on reanalysis products, which allows uncovering different relevant features of the WNPSM variability. The WNPSM had a significant interdecadal variability throughout the twentieth century. In particular, the period 1918–48 was characterized by less variable and stronger monsoons than in recent decades. Additionally, the relationship between the WNPSM and ENSO or El Nino Modoki has been evaluated during t...

The Use of Equivalent Temperature to Analyse Climate Variability

Equivalent temperature based in the NCEP/NCAR reanalysis database has been used as a simultaneous measure of temperature and humidity. Its variations during the 1958-1998 added to the effect of the inclusion of satellite data during the late seventies have been analyzed. An increase of the globally averaged equivalent temperature has been detected, the trend has been considerably greater during the first half of the study period and significant differences can be found between continental and oceanic areas. The relation of the trend with four of the main modes of climate variability has been assessed. The North Atlantic Oscillation and the Artic Oscillations are closely related to the equivalent temperature over the North Atlantic basin, extending toward Northern Asia in the second case. El Niño/Southern Oscillation and the Antarctic Oscillation seem to have a more global effect.