Download influence of the north atlantic oscillation on winter equivalent

INFLUENCE OF THE NORTH ATLANTIC OSCILLATION ON
WINTER EQUIVALENT TEMPERATURE
J. Florencio Pérez (1), Luis Gimeno (1), Pedro Ribera (1), David Gallego (2), Ricardo García (2) and Emiliano Hernández (2)
(1) Universidade de Vigo
(2) Universidad Complutense de Madrid
Introduction
Overview
Data Analysis
Increases in both troposphere temperature and
water vapor concentrations are among the expected
climate changes due to variations in greenhouse gas
concentrations (Kattenberg et al., 1995). However
both increments could be due to changes in the
frecuencies of natural atmospheric circulation
regimes (Wallace et al. 1995; Corti et al. 1999).
We utilized temperature and humidity data
at 850 hPa level for the 41 yr from 1958 to
1998 from the National Centers for
Environmental Prediction–National Center for
Atmospheric
Research
(NCEP–NCAR)
reanalysis.
Changes in the long-wave patterns, dominant
airmass types, strength or position of climatological
“centers of action” should have important influences
on local humidity and temperatures regimes. It is
known that the recent upward trend in the NAO
accounts for much of the observed regional warming
in Europe and cooling over the northwest Atlantic
Hurrell (1995, 1996). However our knowledge about
the influence of NAO on humidity distribution is very
limited. In the three recent global humidity
climatologies (Peixoto and Oort 1996, Ross and
Elliot, 1996 and Randel et al., 1996) nothing is said
about the rol that NAO can play on humidity
distribution.
We calculated daily values of equivalent
temperature for every grid point according to
the expression in figure-1. The monthly,
seasonal and annual means were constructed
from daily means. Seasons were defined as
Winter
(January, February and March),
Spring (April, May and June), Summer (July,
August and September) and Fall (October,
November and December).
In a recent study about distribution and trends in
US surface humidity and temperature (Gaffen and
Ross, 1999), both increments were found. They
were consistent and also consistent with apparent
temperature, a measurement of human comfort that
combine temperature and humidity, but it was not
detected any influence of large-scale dynamics on
interannual humidity variations. Neither ENSO nor
NAO was significatively correlated with specific
humidity anomalies. However this study was limited
to US. The largest influence of changes of circulation
due to NAO are produced over Europe.
The objective of this study is to analyze the
influence on NAO on the pair temperature-humidity
at an hemispheric scale. A way of quantifing both
magnitudes in a single variable consists of using
equivalent temperature (Te), calculated in this study
at 850 hPa for the period 1958-1998.
For every season and for annual values,
anomalies from the period 1958-1998 were
calculated. Anomalies field was then used to
calculate:
•Composites for the 41 years
•41-year trends patterns.
•Composites for those years when NAO was
in a more positive or in a more negative
phases. To determine these years we use the
41 winter values of NAO index as the
normalized pressure difference between
Ponta Delgada (Azores) and Reykjavik
(Iceland) and 41 years mean +1SD and 41
years mean –1SD as thresholds. Seven years
were chosen as in positive NAO phase and
other seven as in negative.
•Regression of equivalent temperature on the
winter NAO index.
The equivalent Temperaturature is the temperature
that an air parcel would have if water vapor were
condensed out at constant preassure, the latent heat
released being used to heat the air.
The mixing rate (w) has been calculated from
temperature and relative humidity.
Te: equivalent temperature
L: latent heat
w: mixing ratio
Cpd: dry air specific heat
T: temperature
WINTER TEMPERATURE
TREND 1958-1998
WINTER AVERAGE EQUIVALENT
TEMPERATURE 1958-1998
The distribution shows a very zonal pattern.
Absolute maxima are located over continental
regions. One of them over the African
equatorial region, and other two in mid
lattitudes, one over Africa and the other over
Australia.
Rossby waves are easily detectable in
northen hemisphere mid and high lattitudes. In
the southern hemisphere those waves are not
so marked.
Absolute minimum values are detected over
Greenland.
In the Northern Hemisphere, a positive trend in the
temperature is observed over most Europe and
North America, and over the Atlantic in the 30ºN50ºN band. A negative trend is observed over
Iceland, Greenland and the Northeastern coast of
Canada, including Hudson Bay. To the south, over
the Northern Hemisphere South Atlantic and over
most of North Africa, the Middle East and Central
Asia, a negative trend in the temperature is
detected. Finally, over the Pacific, from 30ºN to the
north, a negative trend is detected as well.
In the Southern Hemisphere, a very consistent
positive trend is detected over most of the regions
south of 30ºS. The only negative observable trend
is detected at very high lattitudes and from 0ºE150ºE.
EQUIVALENT TEMPERATURE TREND 1958-1998
In the Northern Hemisphere Te shows a very negative
trend over Greenland, the Sahara, Middle East, and
Southeastern Asia is detected. Positive trends are
observed over Northwestern Canada, and from the
central United States to well into the Atlantic.
European western coast and most of central and
northern Asia also exhibit a positive trend.
In the Southern Hemisphere a positive trend is
detected over the Atlantic and Pacific oceans at about
30ºS, and over most of the Indian ocean. Over the
Antarctica, in a region similar to that observed for the
temperature, a negative trend is detected.
Figure 1
  L w 

T e = T 1 + 


  c pd T 
NAO Influence on Equivalent Temperature
CORRELATIONS BETWEEN NAO AND Te
COMPOSITE OF Te ANOMALIES FOR POSITIVE NAO YEARS
In this plot the correlation coefficients locate those
regions mostly influenced by this phenomenum. Very
high negative values are observed over Greenland and
the Northeastern Canadian coast, and over the central
Sahara. Additional negative values are detected over
west Australia and a small region in the Antarctica.
Maximum positive correlations are detected over
European western coast, mid-east USA, eastern China
and the south-eastern coast of South Africa.
COMPOSITE OF Te ANOMALIES FOR NEGATIVE NAO YEARS
In positive NAO years a high negative Te anomaly is
observed over Greenland and the central Sahara, while
positive anomalies are observed over a long belt that
goes from near Alaska to New England, and from there
crosses the Atlantic Ocean and intensifies the positive
anomaly over Europe and throghout most of central and
northern Asia.
DIFFERENCE BETWEEN Te FOR POSITIVE AND NEGATIVE NAO
YEARS
In this map the highest absolute values locate those
regions where the amplitude of the oscillations
originated by NAO reaches its maximum amplitude.
These regions are: Greenland, the Sahara, Europe
and most of Siberia and the western Canada.
All these regions are located in mid and high latitudes
in the Northern Hemisphere. This implies an almost
hemispheric-wide influence of NAO.
The negative NAO composites of Te show a pattern very
similar to that of the positive NAO, but as a negative
image. The highest positive anomalies are detected over
Greenland and the central Sahara, and the lowest
negative values over northwestaern America, Europe
and cenrtal Siberia.
ADDITION OF Te FOR POSITIVE AND NEGATIVE NAO YEARS
As it was expected, the range of the values obtained are
relatively low compared the the difference. This field is
representative of the out of phase phenomena related to
the North Atlantic Oscillation. There is not a very clear
distribution pattern. Ery low values are obtained to the
west of Greenland. And the highest positive values over
the Bering Strait and in the north of China.
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