Meteorologists have long noticed that the monthly and seasonal (particularly wintertime) averaged sea level pressure in stations in Iceland and the Azores, display an out-of-phase relationship with one another. This pressure pattern was called "North Atlantic Oscillation" (NAO); the NAO is the dominant mode of atmospheric variability in the North Atlantic sector throughout the year. A NAO index has been defined as the pressure difference between Azores and Iceland; the positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies. An extended version of the index can be derived for the winter half of the year (December to March) by using a station in the southwestern part of the Iberian peninsula (Hurrell, 1995a). For winter, the difference between the normalized sea level pressure over Gibraltar and the normalized sea level pressure over Southwest Iceland is a useful index of the strength of the NAO. Over the past 130 years, the NAO has exibited variability at quasi biennial and quasi decadal time scales; there have been several periods when the NAO index persisted in one phase over many winters (van Loon and Rogers, 1978; Barnett, 1985). The changes in the circulation pattern over the North Atlantic are accompanied by pronounced shifts in the storm tracks leading alteration in regional precipitations. Hurrell investigated the relationship between variations in the NAO index and decadal trends in northern hemisphere precipitation and temperature, successfully demonstrating the existence of a climate dipole (Hurrell, 1995a; Hurrell, 1995b; Hurrell, 1996). He used composited ECMWF analyses and showed that, during times of high NAO index, the axis of maximum moisture transport shifts to a more southwest-to-northeast orientation across the Atlantic and extends much farther to the north and east onto northern Europe and Scandinavia. A significant reduction of the total atmospheric moisture transport occurs over parts of southern Europe, the Mediterranean and North Africa. In conclusion he observed drier winter over much of central and southern Europe and Mediterranean when high NAO index winter occurred, on the contrary wetter than normal conditions affected the area from Iceland through Scandinavia. Lamb and Peppler (1987) provided the first focused regional investigation by connecting decreased Moroccan rainfall to a positive state of the NAO, after receiving an invitation from the government of Morocco, then concerned over the severe and persistent drought of 1979-1984. Sea surface temperature anomalies have been also identified as a significant quantity for long range climate evaluations and predictions. Folland et al. (1986) showed that interannual Sahel rainfall could be explained by sea surface temperature anomalies on a global scale. In this work we have studied the influence of North Atlantic Oscillation (NAO) and mediterranean sea surface temperature anomalies (SSTa) on average rainfalls variability between 1951-1991 in Tuscany (Italy). Correlations between monthly average rainfalls with simultaneous and previous patterns of NAO index and sea surface temperature anomalies in central-western Mediterraneanan were calculated for each month. In conclusion multiple linear regression analysis between three monthly cumulated rainfalls, NAO Index and sea surface temperature of the more correlated previous months, were developed for Arno's hydrological basin.
Regional scale seasonal rainfall anomalies in Italy predicted from large scale atmospheric and mediterranean sea quantities
Grifoni D;Meneguzzo F;
2000
Abstract
Meteorologists have long noticed that the monthly and seasonal (particularly wintertime) averaged sea level pressure in stations in Iceland and the Azores, display an out-of-phase relationship with one another. This pressure pattern was called "North Atlantic Oscillation" (NAO); the NAO is the dominant mode of atmospheric variability in the North Atlantic sector throughout the year. A NAO index has been defined as the pressure difference between Azores and Iceland; the positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies. An extended version of the index can be derived for the winter half of the year (December to March) by using a station in the southwestern part of the Iberian peninsula (Hurrell, 1995a). For winter, the difference between the normalized sea level pressure over Gibraltar and the normalized sea level pressure over Southwest Iceland is a useful index of the strength of the NAO. Over the past 130 years, the NAO has exibited variability at quasi biennial and quasi decadal time scales; there have been several periods when the NAO index persisted in one phase over many winters (van Loon and Rogers, 1978; Barnett, 1985). The changes in the circulation pattern over the North Atlantic are accompanied by pronounced shifts in the storm tracks leading alteration in regional precipitations. Hurrell investigated the relationship between variations in the NAO index and decadal trends in northern hemisphere precipitation and temperature, successfully demonstrating the existence of a climate dipole (Hurrell, 1995a; Hurrell, 1995b; Hurrell, 1996). He used composited ECMWF analyses and showed that, during times of high NAO index, the axis of maximum moisture transport shifts to a more southwest-to-northeast orientation across the Atlantic and extends much farther to the north and east onto northern Europe and Scandinavia. A significant reduction of the total atmospheric moisture transport occurs over parts of southern Europe, the Mediterranean and North Africa. In conclusion he observed drier winter over much of central and southern Europe and Mediterranean when high NAO index winter occurred, on the contrary wetter than normal conditions affected the area from Iceland through Scandinavia. Lamb and Peppler (1987) provided the first focused regional investigation by connecting decreased Moroccan rainfall to a positive state of the NAO, after receiving an invitation from the government of Morocco, then concerned over the severe and persistent drought of 1979-1984. Sea surface temperature anomalies have been also identified as a significant quantity for long range climate evaluations and predictions. Folland et al. (1986) showed that interannual Sahel rainfall could be explained by sea surface temperature anomalies on a global scale. In this work we have studied the influence of North Atlantic Oscillation (NAO) and mediterranean sea surface temperature anomalies (SSTa) on average rainfalls variability between 1951-1991 in Tuscany (Italy). Correlations between monthly average rainfalls with simultaneous and previous patterns of NAO index and sea surface temperature anomalies in central-western Mediterraneanan were calculated for each month. In conclusion multiple linear regression analysis between three monthly cumulated rainfalls, NAO Index and sea surface temperature of the more correlated previous months, were developed for Arno's hydrological basin.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
