JETSTREAM AND SEASONAL ANOMALIES IN THE MEDITERRANEAN Marco Gaetani, Marina Baldi, Gianni Dalu 2nd ESF MedCLIVAR workshop Connections between Mediterranean.

JETSTREAMAND SEASONAL ANOMALIES

IN THE MEDITERRANEANMarco Gaetani,

Marina Baldi, Gianni Dalu

2nd ESF MedCLIVAR workshopConnections between Mediterranean

and Global Climate Variability La Londe les Maures , Toulon, FRANCE

8-10 October 2007

In the context of regional climate change, the Mediterranean climate variability constitutes an issue of particular concern:in winter, decrease in the number of cyclones and total rainfall;in summer, increase in the temperature and number of hot spells.

Trigo et al 2000

Xoplaki et al 2003

Xoplaki et al 2004

Baldi et al 2006

Atlantic jet

African jet

Heat waves Cool waves

Baldi et al. 2006, Int J Climatol, 26, 1477: relationship between the jetstram position and the occurrence of heat (cool) waves.Atlantic jet, generated by the thermal contrast between the cold air over the Polar ocean and the relatively warm air over the Atlantic ocean.African jet, generated by the thermal contrast between the African warm air and the Euro-Asian relatively cold air.

Since in the Mediterranean region the warm season is dry and the cold season is wet, we relate the temperature anomalies in the warm season, and the rainfall anomalies in the cold season of this region to the configuration of the westerly jetstream.

Since the longitudinal dimension of the Mediterranean basin is about 4 times its latitudinal dimension, usually, when it rains in the West basin, the East basin is relatively dry and viceversa: in fall and spring the rain is abundant in the West basin, while in winter it rains mainly in the East basin. Therefore, we analyze the jetstream configuration in fall, winter and spring to relate it to the east-west rainfall distribution.

EURO-ATLANTICJETSTREAM

POSITION

SUMMERTEMPERATURE

ANOMALIES(HOT – COOL)

RAINFALLANOMALIES

WINTER(WET – DRY)

FALLWINTERSPRING

(WEST – EAST)

We study the years 1979 - 2005, analyzing the temperature anomaly in summer, and the rainfall anomaly in winter in the Mediterranean basin.

In addition, we analyze the rainfall behavior in the West basin in fall (SO) and in spring (AM), and in the East basin in winter (DJF).

We use gridded rain gauge data merged with satellite estimates over land and oceans from the Global Precipitation Climatology Program (GPCP) at 2.5° Lat-Lon resolution.

We use the temperature, geopotential and the dynamical fields extracted from the NCEP – DOE reanalysis 2 at 2.5° Lat-Lon resolution.

In the Mediterranean region it rains mainly on the upstream side of the topographic features: the west coast of the Iberian peninsula, the Alps, the Carpathian and the Caucasian mountains, and the Turkish peninsula.

The rainiest months are November and December.

PRECIPITATION

WMB EMB

The jet is identified by the zonal component of the wind at 300hPa and located where its intensity exceeds 20 m/s in winter and 15 m/s in summer.

NOV – DEC CLIMATOLOGY

In November and December, the position of the stormtrack over the East Atlantic almost coincides with the position of the jet.

Uccellini & Kocin, 1987

cyclonic

cyclonicanticyc

anticyc

H

When the rainfall in the Mediterranean basin exceeds its climatological value of more than one standard deviation, the MB is wet, the intensity of the westerly jetstream in the Euro-Atlantic region is around its winter weakest value, and the distance between head of the Atlantic jet and the tail of the African jet is relatively small. The Atlantic jet has a small north-easterly tilt, and the African jet covers the southern part of the MB, in an almost zonal configuration: the storm track are driven in the MB.

WET NOV – DEC: 1980, 81, 95, 96, 97, 2002.

The African jet rules:cyclonic vorticity over the Mediterranean

When the rainfall in the Mediterranean basin is below its climatological value minus one standard deviation, the MB is dry, the westerly jetstream is more intense, the troposphere over the MB is less baroclinic, with a general tropospheric subsidence. The Atlantic jet, with its pronounced north-easterly tilt, diverts the storms towards North-West Europe. The African jet originates in the tropical Atlantic well off the west coast of North Africa.

DRY NOV – DEC: 1986, 91, 94.

H

H

direct circ.

direct circ.

indirect circ.

indirect circ.

The rainfall distribution between the West basin and the East basin has a well defined seasonal cycle: in September and October it rains more in the WMB, in winter it rains more in the EMB, in April and May the rain is more abundant in the WMB.

PRECIPITATION

We study the relation between the jetstream and the rainfall in the entire Mediterranean basin in November and December, in the West basin in fall and in spring, and in the East basin in winter.

This rainfall east-west distribution may well be due to larger scale patterns. The main differences between these patterns are an increase of wavelength, with a local change of phase, when the season changes from fall to winter, with a further change of phase when the season changes from winter to spring.

FALL, WINTER AND SPRING PATTERNS

In fall the North American continent and the Euro-Asian continent are still relatively warm, the westerly jetstream is still relatively weak, and the pattern of the geopotential over the Euro-Mediterranean region is more localized.

In winter, these two continents are very cold, the Atlantic jet and the African jet reach their maximum intensity and range, with the pattern of the geopotential organized on a larger scale.

WINTERFALL, WINTER AND SPRING PATTERNS

In late spring, when the continental masses start to warm up, the two jets weaken, and the phase of the geopotential over the two basins changes again.

The anomalous years (1985, 89, 96), when in fall it rained more in the EMB, the jetstream configuration and the general pattern were more winter like.

FALL

In fall when the WMB is wetter than the EMB, it rains over the north-west of Iberia and over Italy, mainly in the Alpine region; at 850 hPa the geopotential is low, with uprising winds from the west and from the south-west in the WMB, and subsiding north-westerly winds in the EMB.

In winter, when the EMB is wetter than the WMB, the westerly jetstream is more intense, and the Atlantic jet and the stormtrack have a large north-easterly tilt; the prevailing winds at 850hPa are rising south-westerlies in the East basin, and subsiding north-westerlies in the West basin.

WINTER

The anomalous years (1995, 89, 2000), when in winter it rained more in the WMB than in the EMB, the jets were less intense, and the general pattern was more fall like.

In spring, when the rain returns to the WMB, the Atlantic jet retreats over the eastern Atlantic ocean, while the African jet stays in its winter position.

SPRING

The anomalous years (1987, 91, 2001), when in spring it rained more in the EMB than in the WMB, the Atlantic jet and the African jet kept a winter configuration.

Having verified that the warmest months in the Mediterranean are July and August, we define that a summer is hot (cool) when the average temperature at 850hPa in these two months is one standard deviation above (below) the climatological average.

In summer, the westerly jetstream is less intense, more zonal, and runs at higher latitude than in winter. When the summer is hot, the distance between the head of the Atlantic jet and the tail of the African jet is at its minimum, with, in between them, a deep anticyclonic vortex which spans from the Algerian-Libyan desert to the Midi (South France) and the Alpine region. When the Libyan anticyclone invades the Mediterranean basin, a positive temperature anomaly is centered over the Italian peninsula, and a positive geopotential anomaly covers the WMB and the western part of the EMB, with a prevailing subsiding westerly north-westerly winds.

HOT JUL – AUG: 1988, 94, 98, 2001, 2003

When the summers are relatively cool, the two jets are more intense, they have a larger range and a north-easterly tilt, and the distance between the head of the Atlantic jet and the tail of the African jet is larger than when the summers are hot. To the north of the head of the African jet, there is a deep cyclonic vortex, centered over Turkey and the Black sea which spans from the Caspian sea to the Adriatic sea, while the Libyan anticyclone is back on Libyan-Algerian desert. At 850hPa a negative temperature and geopotential anomaly, centered over the Balkans, covers the entire Mediterranean region.

COOL JUL – AUG: 1979, 81, 84, 97

2. In the Euro-Mediterranean region, the wintertime perturbations of Atlantic origin follows a stormtrack which coincides with Atlantic jet. When the NAO index is weak, this jet is less intense and it crosses the Atlantic with a small north-easterly tilt, shortening its distance from the Mediterranean jet. This is when the rainfall is abundant, because the ageostrophic meridional circulation is frontogenetic, the baroclinicity is enhanced in the Mediterranean basin.

We have analyzed the jetstream configuration in the Euro-Atlantic region, in relation to the temperature anomaly in summer and the rainfall anomaly in winter over the Mediterranean region and the rainfall seasonal cycle between western basin and eastern basin.

1. Rainfall in the Mediterranean is related to the intensity of the jets, the orientation, the longitudinal position, and their relative position.

SUMMARY & CONCLUSIONS

3. In the warm season, when the distance between the Atlantic jet and the African jet is small, there is an anticyclonic anomaly over the Central Mediterranean. With this configuration there is a general subsidence over the Mediterranean, with a diffuse tropospheric adiabatic warming. The summers are cool when the Atlantic jet and the African jet are more intense and have a large north-easterly tilt. Finally, extreme hot summer in the WMB are related to the northward shift and to the intensification of the Libyan high, this occurs when the west flank of the African jet is weak.

2. The rainfall in the EMB basin lags one season behind the rainfall in the WMB: the West basin is wet in fall, while the East basin is wet in winter; in late spring the rain returns to the West basin. In years when falls and springs were winter like, it rained more in the EMB, and, when the winters were mild, it rained more in the WMB.

SUMMARY & CONCLUSIONS

Thanks for your attention.

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http://www.ibimet.cnr.it/Staff/gaetani