Taher A. Sharif Department of Atmospheric science, University of AlFateh, Tripoli, Libya [email protected].

Taher A. Sharif

Department of Atmospheric science, University of AlFateh, Tripoli, Libya

[email protected]

Outline

North Atlantic oscillation OverviewCorrelation of NAO index with

seasonal rainfall over LibyaSummary

2MedCLIVAR Wokshop Trieste Oct 2008

Definition of North Atlantic Oscillation

The North Atlantic Oscillation (NAO) establishes a link between the intensity of the Icelandic low pressure and Azores high pressure.

It is traditionally quantified by an index defined as the difference between normalized SLP anomalies in the Azores and Icelandic regions, respectively.



North Atlantic Variability

Climate variability in the North Atlantic sector, comprise three primary interrelated phenomena:

Tropical Atlantic Variability (TAV)North Atlantic Oscillation (NAO)Atlantic Meridional Overturning Circulation

(MOC)


Other Important Actors

Arctic Oscillation (AO)Blocking High (a persistent

anomalous high pressure over Northern Europe)

Atlantic Ridge (Northward extension of Azores High pressure)


Normal Sea-level pressure for January


Normal sea-level pressure for July


NAO exhibits two main anomalies of pressure cores of opposite signs, Extending respectively from Iceland to Spitzberg and from center of the Atlantic to the Iberian peninsula.


Sea Level Pressure

Sea Level Pressure


Positive NAO

Deepening of the Icelandic Low (negative SLP anomalies over the Icelandic region throughout the Arctic).

Strengthening and eastward extension of the Azores High (positive SLP anomalies over the Azores region)



Features Associated with Positive NAO

Stronger than average westerlies across middle latitudes in the Atlantic and western Europe.

Colder winter conditions dominate the northwest Atlantic.

Warmer weather over Europe.Storm tracks are shifted to the north

with wetter conditions from Iceland through Scandinavia.

Dryer conditions over south-western Europe and the Mediterranean Basin.


Weakening of the Icelandic Low ( positive SLP anomalies over the Icelandic region throughout the Arctic).

Weakening the Azores High (negative SLP anomalies over the Azores region).

MedCLIVAR Wokshop Trieste Oct 2008 14

Negative NAO


Weaker than average westerlies across middle latitudes in the Atlantic and western Europe.

Warmer winter conditions dominate the northwest Atlantic.

cooler weather over Europe.Storm tracks are shifted to the south

with dryer conditions from Iceland through Scandinavia.

Wetter conditions over south-western Europe and the Mediterranean Basin.

Features Associated with Negative NAO





Fig 1: The Correlation Between August NAO Index and October to February Rainfall


Fig 2: The Correlation Between August NAO Index and November to February Rainfall


Fig 3: The Correlation Between August NAO Index and December to February Rainfall


Fig 4: The Correlation Between August NAO Index and January to February Rainfall


On-going work

Nowadays scientists are involved in a major research program focusing on Atlantic climate variability in the context of CLIVAR. The main objectives of the program are:

To describe and model coupled atmosphere-ocean-land interactions in the Atlantic sector, quantify there influence on the regional and global climate system, and determine their predictability.

To assemble quantitative historical and real time data sets that may be used to test, improve and initialize models of coupled Atlantic climate variability.

To investigate the sensitivity of the MOC to changes in the surface forcing and assess the likelihood of abrupt climate change.


Summary

NAO is a very important synoptic feature in predicting climate over Europe and Mediterranean regions.

Rainfall over Libya is significantly correlated with NAO index. It increases when the NAO is in negative phase.

NAO variability seems to be a possible predictor for statistical prediction of climate over Europe and Mediterranean regions.

The Standing mean of Icelandic low, Azores high and Siberian high pressure are natural synoptic controller systems That should be preserved for any climate prediction modelling over Europe and Mediterranean.



Taher A. Sharif Department of Atmospheric science, University of AlFateh, Tripoli, Libya [email protected].

Documents

medclivar wokshop trieste

negative nao slide

nao variability

summary nao

nao exhibits

atlantic climate variability

monthly nao index

sea level pressure slide