North Atlantic climate variability: phenomena, impacts and mechanisms

127

This paper analyses the spatial and temporal variability of winter droughts in a semi-arid geographic gradient in Northeast Spain, from the Pyrenees in the north to the Mediterranean coastland in the south. Droughts that occurred between 1952 and 1999 were analysed by means of the Standardised Precipitation Index (SPI). The influence of the weather-type frequency and of the general North Atlantic atmospheric circulation patterns was analysed. The results indicate that winter droughts show an important spatial variability in the study area, differentiating three well-defined patterns. These correspond to the Pyrenees, the centre of the Ebro Valley, and the Mediterranean coastland. General negative trends in winter SPI have been found, which are indicative of the increase in winter drought conditions in the study area. Nevertheless, important spatial differences have also been recorded. Dominant north-south gradients in the influence of weather types are shown. Moreover, the negative trends in winter-SPI values agree with the negative trend in the frequency of the weather types prone to cause precipitation, such as the C, SW and W weather types and the increase in the frequency of A weather types. Nevertheless, in the Mediterranean coastland, the positive trend in SPI values agrees with the increase in the frequency of weather types of the east (E, SE), which are prone to cause precipitation in this area. Interannual variations in the frequency of the different weather types have been highly determined by different general atmospheric circulation patterns, mainly the North Atlantic Oscillation (NAO). Nevertheless, the correlation between the time series of weather-type frequency and the winter SPI is higher than that found between the SPI and the NAO. Thus, although the interannual NAO variability explains a high percentage of the interannual differences in the frequency of different weather types, it is not sufficient to explain the spatial and temporal variability of droughts, which respond better to atmospheric variability at more detailed (synoptic) spatial scales.

Section: General Atmospheric Circulation Patternsmentioning

confidence: 99%

The influence of atmospheric circulation at different spatial scales on winter drought variability through a semi‐arid climatic gradient in Northeast Spain

Vicente‐Serrano

López‐Moreno

2006

127

“…The most important phenomena on interannual to decadal timescale in this region are the Pacific El Niño-Southern Oscillation (ENSO), the Atlantic zonal equatorial mode (or the 'tropical Atlantic Niño') and the tropical Atlantic meridional gradient (TAMG) (or the 'tropical Atlantic dipole index') (Berlage, 1957;Giannini et al, 2000;Marshall et al, 2001;Martis et al, 2002;Rajagopolan et al, 1997;Robertson and Mechoso, 2002;Villwock, 1998;Wang, 2001Wang, , 2005. The above phenomena are caused by the variability in sea-surface temperatures (SSTs).…”

Section: Introductionmentioning

confidence: 99%

“…The different mechanisms are schematically shown in Figure 1. Garzoli et al (1999); Giannini et al (2000); Marshall et al (2001) and Villwock (1998) have shown that the rainfall in northern South America is more related to the sea-surface temperature anomalies (SSTAs) in the tropical North Atlantic (TNA) than to the tropical South Atlantic (TSA) and the tropical Pacific ENSO.…”

Section: Introductionmentioning

confidence: 99%

“…This reduces sea-level pressure and northeast trade winds in the TNA region, while southeasterly trade winds are enhanced. Evaporation is reduced and, as a result, precipitation increases on land during August-September (Marshall et al, 2001;Robertson and Mechoso, 2002;Wang, 2001). and weaker, respectively, leading to weakened northeast trade winds in the TNA and decreased surface evaporation (Ambrizzi et al, 2005;Marshall et al, 2001).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Rainfall variability in suriname and its relationship with the tropical Pacific ENSO SST anomalies and the Atlantic SST anomalies

Nurmohamed

Naipal

Becker

2006

Abstract:Spatial correlations in the annual rainfall anomalies are analyzed using principal component analysis (PCA). Cross correlation analysis and composites are used to measure the influence of sea-surface temperature anomalies (SSTAs) in the tropical Atlantic (TA) and the tropical Pacific Ocean on the seasonal rainfall in Suriname. It is shown that the spatial and time variability in rainfall is mainly determined by the meridional movement of the inter-tropical convergence zone (ITCZ). The rainfall anomalies are fairly uniform over the whole country. The strongest correlation in the December-January rainfall (short wet season) at station Cultuurtuin is found to occur with the SSTAs in the Pacific region and is about c k Nino 1+2 = 0.59 at lag 1 month. In the March-May rainfall (beginning of the long wet season), there is a lagged correlation with the SSTAs in the Pacific region (c lag 3 Nino 1+2 = 0.59). The June-August rainfall (end of the long wet season) shows the highest correlation with SSTAs in the TSA region and is about c = −0.52 for lag 0. In the September-November long dry season there is also a lagged correlation with the TSA SSTAs of about c lag 3 = 0.66. These different correlations and predictors can be used for seasonal rainfall predictions.

“…These extend throughout the troposphere but peak around The NAO is expressed in tropical Atlantic SSTs and air pressure (Venzke et al, 1999;Marshall et al, 2001), and relationships with southern Angola river flows can be tested. The Kavango time series exhibits spectral energy at both interannual and decadal periods, consistent with SST fluctuations in the tropical North Atlantic (Melice and Servain, 2003;Andreoli and Kayano, 2004).…”

Section: Synthesis Of Seasonal Patterns and Their Statisticsmentioning

confidence: 99%

Climate and weather factors modulating river flows in southern Angola

Jury

2009

ABSTRACT:The large-scale circulation and thermodynamic fields that modulate the flow of the Kavango River in southern Angola during austral summer are studied. According to composite patterns constructed from National Center for Environmental Prediction (NCEP) data, enhanced river flow is associated with an anomalous wave train of upper level winds that emanate from the North Atlantic. Sea-surface temperatures (SST) show a warm-north/cool-south Atlantic dipole condition as the Kavango River rises. Correlations are analysed with respect to the Kavango River flow and a 0 lag value of −0.41 is found for the upper zonal wind over the North Atlantic. Rainfall over the African Sahel region is positively linked with Kavango River flow at 6 month lead time (r = +0.34). The work is extended to the event scale and it is found that winds draw tropical moisture over southern Angola in response to an anomalous low-high pair and bifurcated subtropical jet stream. A continental scale sea breeze circulation amplifies the convection during afternoons. Knowledge of run-off into Namibia is critical to the management and planning of water resources in the northern Kalahari savanna.