INFLUENCE OF THE NORTH ATLANTIC OSCILLATION ON WINTER EQUIVALENT

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 Data Analysis Overview

Increases in both troposphere temperature and We utilized temperature and data WINTER TEMPERATURE concentrations are among the expected at 850 hPa level for the 41 yr from 1958 to WINTER AVERAGE EQUIVALENT climate changes due to variations in greenhouse gas 1998 from the National Centers for TEMPERATURE 1958-1998 TREND 1958-1998 concentrations (Kattenberg et al., 1995). However Environmental Prediction–National Center for both increments could be due to changes in the frecuencies of natural atmospheric circulation Atmospheric Research (NCEP–NCAR) regimes (Wallace et al. 1995; Corti et al. 1999). reanalysis. Changes in the long-wave patterns, dominant We calculated daily values of equivalent airmass types, strength or position of climatological “centers of action” should have important influences temperature for every grid point according to on local humidity and regimes. It is the expression in figure-1. The monthly, known that the recent upward trend in the NAO seasonal and annual means were constructed accounts for much of the observed regional warming from daily means. Seasons were defined as in Europe and cooling over the northwest Atlantic Winter (January, February and March), Hurrell (1995, 1996). However our knowledge about Spring (April, May and June), Summer (July, the influence of NAO on humidity distribution is very August and September) and Fall (October, limited. In the three recent global humidity November and December). climatologies (Peixoto and Oort 1996, Ross and In the Northern Hemisphere, a positive trend in the Elliot, 1996 and Randel et al., 1996) nothing is said The distribution shows a very zonal pattern. temperature is observed over most Europe and about the rol that NAO can play on humidity For every season and for annual values, Absolute maxima are located over continental anomalies from the period 1958-1998 were North America, and over the Atlantic in the 30ºN- distribution. regions. One of them over the African calculated. Anomalies field was then used to 50ºN band. A negative trend is observed over In a recent study about distribution and trends in calculate: equatorial region, and other two in mid Iceland, Greenland and the Northeastern coast of US surface humidity and temperature (Gaffen and lattitudes, one over Africa and the other over Canada, including Hudson Bay. To the south, over Ross, 1999), both increments were found. They were consistent and also consistent with apparent ·Composites for the 41 years Australia. the Northern Hemisphere South Atlantic and over temperature, a measurement of human comfort that Rossby waves are easily detectable in most of North Africa, the Middle East and Central combine temperature and humidity, but it was not ·41-year trends patterns. northen hemisphere mid and high lattitudes. In Asia, a negative trend in the temperature is detected any influence of large-scale dynamics on the southern hemisphere those waves are not detected. Finally, over the Pacific, from 30ºN to the interannual humidity variations. Neither ENSO nor ·Composites for those years when NAO was NAO was significatively correlated with specific so marked. north, a negative trend is detected as well. in a more positive or in a more negative Absolute minimum values are detected over humidity anomalies. However this study was limited phases. To determine these years we use the In the Southern Hemisphere, a very consistent to US. The largest influence of changes of circulation Greenland. positive trend is detected over most of the regions due to NAO are produced over Europe. 41 winter values of NAO index as the normalized difference between south of 30ºS. The only negative observable trend The objective of this study is to analyze the Ponta Delgada (Azores) and Reykjavik is detected at very high lattitudes and from 0ºE- influence on NAO on the pair temperature-humidity (Iceland) and 41 years mean +1SD and 41 150ºE. at an hemispheric scale. A way of quantifing both magnitudes in a single variable consists of using years mean –1SD as thresholds. Seven years were chosen as in positive NAO phase and equivalent temperature (Te), calculated in this study at 850 hPa for the period 1958-1998. other seven as in negative. ·Regression of equivalent temperature on the EQUIVALENT TEMPERATURE TREND 1958-1998 winter NAO index. 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 The equivalent Temperaturature is the temperature Figure 1 central United States to well into the Atlantic. that an air parcel would have if water vapor were European western coast and most of central and condensed out at constant preassure, the northern Asia also exhibit a positive trend. released being used to heat the air. é æ L w öù T = T ê1+ ç ÷ú In the Southern Hemisphere a positive trend is The mixing rate (w) has been calculated from e ç ÷ detected over the Atlantic and Pacific oceans at about ê è c pd T øú temperature and relative humidity. ë û 30ºS, and over most of the Indian ocean. Over the Te: equivalent temperature Antarctica, in a region similar to that observed for the L: latent heat w: mixing ratio temperature, a negative trend is detected. Cpd: dry air specific heat T: temperature

NAO Influence on Equivalent Temperature

CORRELATIONS BETWEEN NAO AND T e COMPOSITE OF Te ANOMALIES FOR POSITIVE NAO YEARS In this plot the correlation coefficients locate those regions mostly influenced by this phenomenum. Very In positive NAO years a high negative T anomaly is high negative values are observed over Greenland and e observed over Greenland and the central Sahara, while the Northeastern Canadian coast, and over the central positive anomalies are observed over a long belt that Sahara. Additional negative values are detected over goes from near Alaska to New England, and from there west Australia and a small region in the Antarctica. crosses the Atlantic Ocean and intensifies the positive Maximum positive correlations are detected over anomaly over Europe and throghout most of central and European western coast, mid-east USA, eastern China northern Asia. and the south-eastern coast of South Africa.

COMPOSITE OF T ANOMALIES FOR NEGATIVE NAO YEARS e DIFFERENCE BETWEEN Te FOR POSITIVE AND NEGATIVE NAO YEARS

The negative NAO composites of Te show a pattern very In this map the highest absolute values locate those similar to that of the positive NAO, but as a negative regions where the amplitude of the oscillations image. The highest positive anomalies are detected over originated by NAO reaches its maximum amplitude. Greenland and the central Sahara, and the lowest These regions are: Greenland, the Sahara, Europe negative values over northwestaern America, Europe and most of Siberia and the western Canada. and cenrtal Siberia. All these regions are located in mid and high latitudes in the Northern Hemisphere. This implies an almost hemispheric-wide influence of NAO.

References: ADDITION OF Te FOR POSITIVE AND NEGATIVE NAO YEARS

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