Seesaw Fluctuations in Ozone between the North Pacific and North ...

Journal of the Meteorological Society of Japan, Vol. 82, No. 3, pp. 941--949, 2004

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NOTES AND CORRESPONDENCE Seesaw Fluctuations in Ozone between the North Pacific and North Atlantic

Yvan J. ORSOLINI Norwegian Institute for Air Research (NILU), Kjeller, Norway (Manuscript received 16 October 2003, in final form 18 February 2004)

Abstract Late winter surface pressure anomalies over the North Pacific and North Atlantic fluctuate from year to year in a seesaw. This Aleutian-Icelandic seesaw modulates the upward propagation of planetary waves into the stratosphere, thereby causing year-to-year fluctuations in the ozone layer . We first derive the Aleutian-Icelandic seesaw index from the 40-year re-analyses (ERA-40) compiled at the European Centre for Medium-Range Weather Forecast (ECMWF). February column ozone derived from two decades of satellite ozone observations is then regressed upon the Aleutian-Icelandic index to uncover the seesaw ozone signature. The regression map obtained is contrasted with the ozone regression map associated to the Arctic Oscillation. Both the quasi-stationary and the transient eddy components of ozone are influenced by the seesaw, in a manner consistent with the seesaw imprint upon upper-tropospheric meteorological fields. The year-to-year variations in the February-mean ozone over the Aleutian and Icelandic sectors, which are anti-correlated, are shown to be dominated by the seesaw.

1.

Introduction

There is recent evidence that climate variations over the North Pacific and Atlantic sectors are coupled in late winter. Honda et al. (2001), Honda and Nakamura (2001) and Nakamura and Honda (2002) analysed operational meteorological analyses from the National Center for Environmental Predictions (NCEP) over the years 1966 to 1997 and show that, in February and March, there exists an inter-annual seesaw between the strengths of the Aleutian Low (AL) and the Icelandic Low (IL). These two climatological features are the major wintertime, surface low-pressure cells in the northern hemisphere. Hence, they showed that the AL and the IL do not fluctuate independently, but

Corresponding author: Yvan J. ORSOLINI, Norwegian Institute for Air Research, PO Box 100, Instituttveien 18, N-2027 Kjeller, Norway. E-mail: [email protected]

rather show inter-annual out-of-phase variations. To characterise the observed negative correlation between the two anomalous strengths, they calculated an index which they termed the Aleutian-Icelandic Index (AII). The AII is defined as the normalised sea-level pressure anomaly over the AL minus the one over the IL. Late winter periods with anomalous strong IL and weak AL, correspond to the positive phase of the AII (or AIIþ). The leading hemispheric pattern of tropospheric variability, as determined using empirical orthogonal function analysis (EOF) and termed Arctic Oscillation (AO) by Thompson and Wallace (2000), also suggests out-of-phase variations between the Icelandic and the Aleutian sectors. Note that the AO is often thought of as associated with in-phase zonal or ‘‘annular’’ variations in midlatitudes. Deser (2000) however showed no statistically significant temporal correlation in surface pressure between the mid-latitudes of the Pacific and the Atlantic over the entire

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cold season. Rather than ‘‘annular’’, the AL-IL seesaw appears as a temporary linkage between the oceanic basins at high latitudes. Castanheira and Graf (2003) also found a coupling between tropospheric anomalies over the North Pacific and the North Atlantic, that appeared modulated by the winter-mean strength of the polar stratospheric vortex aloft, with a strong vortex enhancing the out-of-phase variations through the trapping of planetary waves. The AL-IL seesaw modulates the upward propagation of wintertime stationary planetary waves, and wave activity fluxes into the stratosphere (Nakamura and Honda 2002). The AIIþ phase, with its deeper than normal IL, corresponds to a more pronounced planetary wave-2, and to a diminished upward wave flux. Honda et al. (2001) demonstrated that the seesaw gives rise to marked changes not only in the stationary flow patterns, but also in the synoptic travelling weather systems in both oceanic basins. They further studied the seesaw lifecycle, and showed that North Atlantic anomalies linked to the seesaw originated over the North Pacific earlier in winter, as a downstream influence of the North Pacific variability. Because the AL-IL seesaw influences the circulation in the upper troposphere/lower stratosphere (UTLS), it can be expected to have a signature upon column ozone, as do other major climate patterns such as the NAO (Appenzeller et al. 2000), the AO (Thompson and Wallace 2000), the Scandinavian pattern, the East-Atlantic pattern or the European blocking patterns (Bronnimann et al. 2000; Orsolini and Doblas-Reyes 2003). Our aim is to characterise that signature, using global satellite ozone measurements spanning the last two decades. 2.

Data and method of analysis

Following Nakamura and Honda (2002), the AII is calculated using sea-level pressure, derived from meteorological analyses. The reanalyses ERA-40 recently produced at the European Centre for Medium-Range Weather Forecasts (ECMWF) are used, covering the years 1959–2002, at a horizontal resolution of 2.8 degrees, in both latitude and latitude. The ERA-40 climatological mean sea-level pressure for February is shown on Fig. 1, indicating the locations of two major low pressure cells over the North Atlantic and the North Pacific. The

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AII is defined as the difference between the February-mean normalised sea-level pressure anomaly over the Aleutian sector (50 N–60 N and 165 E–195 E), and the Atlantic sector (55 N–65 N and 30 W–60 W). The geographical definition of the sectors is only slightly different from Nakamura and Honda (2002), who chose sectors of similar size, but centered on the regions of maximum inter-annual variability of the low pressure cells. Fig. 1 and corresponding figures in Nakamura and Honda (2002), show that the AII is a relatively robust feature, provided that the AL and IL centres are sampled. Honda et al. (2001) and Nakamura and Honda (2002) had noted that the seesaw peaked in the period mid-February to mid-March. We have used February-mean analysed fields in our study. The two sectorial anomalies and their difference, i.e., the AII index, are shown on Fig. 2; the anti-correlation of the two anomalies is 0.39. Honda et al. (2001) found a more prominent AII signal in the years 1974–1993. The anti-correlation derived from ERA-40 for that sub-period increases to 0.67, and to 0.43 for the sub-period 1979–2000, covering years when ozone satellite observations are available (see below). Over the later period, the years 1983 and 1993 are examples of pronounced negative phase of the AII (or AII), and years 1982 and 1990 are examples of pronounced AIIþ phase. While the anomalous AL and IL are anticorrelated, the anomalies are not of opposite sign for every year. Such years, when the anomalies are of identical sign, are indicated with squares in Fig. 2, totalling 13 out of 44. February column ozone data is taken from the Total Ozone Mapping Spectrometer (TOMS) version 7.0 observations from 1979 to 2000. The longitude and latitude resolutions of the TOMS data are 1.25 degree and 1 degree, respectively. No observations were made in February 1995 and 1996, nor are observations available in the polar night. February-mean column ozone anomalies were calculated from daily ozone fields, and have been regressed upon the AII index. 3.

February-mean ozone in the two phases of the seesaw

Characteristic February-mean eddy ozone in the two phases are shown on Fig. 3. These maps

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Fig. 1. Climatological mean sea-level pressure (mb) for February derived from ERA-40 data for years 1959 to 2002, showing the two major low pressure centres over the North Pacific (Aleutian Low or AL) and the North Atlantic (Icelandic Low or IL).

represent a one standard deviation anomaly added to (or substracted from) the climatological February-mean ozone. In this context, eddy means departure from the zonal average. As the seesaw involves a modulation of the planetary-scale waves, it is the eddy component of ozone that is of relevance. In the AIIþ phase, the ozone maximum over the AL region of influence, i.e., eastern part of Eurasia and the northern Pacific, is less pronounced and more confined than in the AII phase. However, ozone is higher over the IL region of influence, i.e., over the eastern part of the Canadian Arctic, the Labrador and the North Atlantic in the AIIþ phase. There is more variation between the two phases however, than a modulation of the climatological wave-1: in the AIIþ phase, the low-ozone ridge over the North Atlantic and northern Europe

is weaker, but narrower and more elongated, stretching further to the north east. With the strengthening of ozone maximum over Labrador, a stronger wave-2 appears. These ozone signatures closely mirror upper-tropospheric geopotential eddy anomalies, as demonstrated by Nakamura and Honda (2002). On Fig. 4, the February-mean eddy geopotential at 250 mb characteristic of the two phases is shown, as a one-standard-deviation anomaly added to (or subtracted from) the climatology field. Again, the AII phase is characterised by a larger cyclonic anomaly over the Aleutian sector, and a prominent wave-1 structure. The AIIþ phase has a comparatively weaker and smaller Aleutian cyclonic anomaly, and an intensified cyclonic anomaly over Labrador. The ridge over Europe is also narrower and more elongated to the north-east. The overall agreement between

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Fig. 2. Aleutian-Icelandic Index (AII) for February 1958 to 2002, derived from the ECMWF ERA-40 analyses. February-mean normalised mean sea-level pressure anomalies over the Aleutian (AL) and Icelandic (IL) sectors are shown (full and dashed lines, respectively). Their difference is the AII. When the AII is positive, as in February 1990, the IL is anomalously deep and the AL anomalously weak.

the analysed geopotential anomalies, and the observed ozone field, is excellent, despite the shortness of the ozone record (20 years). While eddy quantities were shown in the preceding two figures, we show in Fig. 5 the ozone regression map associated to the AII index. For comparison, the AO-regressed ozone map is also shown as in Thompson and Wallace (2000), but for February1. The two regression

1

The NAO and AO indices are derived from empirical orthogonal function (EOF) analysis of the 40-year ERA data in winter (DFJ). The NAO is defined as the leading EOF of the 500 mb-geopotential over the Euro-Atlantic sector (Pavan et al. 2000), and the AO as the leading EOF of the global sea-level pressure.

maps can be constrasted: the AO-map shows a minimum over Eastern Eurasia (near 15DU), and a maximum (near 15DU) over the North Atlantic, while the AII-map shows more pronounced extrema over both the North Pacific (near 15DU) and the North Atlantic (near 25DU), and a broader influence over the Northern Atlantic and the Canadian Arctic. Consequently, the AII index is able to capture more of the year-to-year ozone variability in Feburary over the AL and IL sectors than the AO. This is demonstrated in Fig. 6, where eddy ozone averaged over both the IL and AL sectors is shown for February 1979 to 2000. Ozone is normally higher over the AL sector reflecting the background planetary wave-1 (see Fig. 3). In Fig. 6 (bold lines), strong year-to-year

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Fig. 3. Anomalous eddy ozone in February during the phases AIIþ (top) and AII (bottom). These maps represent a one standard deviation anomaly added to the climatological February-mean ozone. Eddy means departure from zonal average. Units are Dobson Units (DU).

seesaw variations between the two sectors are directly seen in the satellite ozone field without resorting to statistical regression against the AII. The anti-correlation of the two curves is 0.43, close to the SLP anti-correlation over the two sectors. The usefulness of the AII in capturing ozone year-to-year fluctuations over the AL and IL sectors is seen from the correlation of observed eddy ozone to an eddy ozone reconstructed by adding the AII-induced anomaly to the climatological mean (Fig. 6, thin full line). The correlation is high over both sectors:

Fig. 4. Anomalous eddy geopotential heights at 250 mb in February for the phases AIIþ (top) and AII (bottom). As in Fig. 3, these maps represent a one standard deviation anomaly (dam) added to the climatological February-mean.

0.73 and 0.78 respectively. Using NAO or AO indices, instead of the AII (Fig. 6, dashed and dot-dashed lines), one finds a correlation over the IL, but not over the AL sector (Table 1). The reconstructed curves based on NAO or AO

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maxima in the storm track regions, over both oceanic mid-latitude basins, akin to ‘‘ozone tracks’’ (Orsolini et al. 1998). In this study, the band-passed ozone M.A.D. is calculated for February. Figure 7 is a composite similar to Fig. 3 for the band-passed ozone M.A.D. In the AIIþ phase, the Atlantic ozone track is more active than in the AII phase, when it is more confined to the western Atlantic and does not penetrate into continental Europe. This result concurs with the storm track diagnostics in Honda et al. (2001). On the contrary, the Pacific ozone track is more zonally confined in the AII phase, but not noticeably stronger, unlike the Pacific storm track in Honda et al. (2001). 5.

Fig. 5. Ozone regression maps in February against the AII and the AO index (see text for definition). These maps represent an ozone anomaly associated with a one standard deviation anomaly of the pattern.

hence capture less of the observed year-to-year variability over both sectors. 4.

Ozone tracks in the two phases of the seesaw

Honda et al. (2001) demonstrated that anomalous activity of transient eddies migrating along the storm tracks exerts a forcing that contributes to the maintenance of stationary anomalies associated with the seesaw. Hence, the covariance between the AII index and ozone synoptic variability has also been investigated. Following how storm track variability is characterised by an Eulerian diagnostic, local ozone synoptic variability is estimated by the mean absolute deviation (M.A.D.) of the variance of ozone band-passed between 2 and 7 days. Climatological maps of the band-passed ozone M.A.D. over the extended winter season show

Discussion

In February, column ozone over the North Pacific and the North Atlantic, in the Aleutian and Icelandic sectors, fluctuates from yearto-year in a seesaw, in unison with surface pressure. The seesaw impacts not only monthlymean eddy ozone, but also the transient component of ozone. Low ozone over the North Atlantic, and high ozone over the North Pacific in February, are associated with the negative phase of the AII, and a stronger than normal AL. In the opposite phase, ozone is higher over the North Atlantic, as the weakened Euro-Atlantic ridge is elongated to the north-east. This ozone variability mirrors fluctuations in planetary waves in the UTLS region. For example, the AIIþ is characterised by a reinforced wave-2, and weaker wave-1. It is well known there exists a local correlation between geopotential height in the UTLS region, and column ozone on synoptic to seasonal time scales (e.g., Mote et al. 1991; Wirth 1993; Hood et al. 1997; Orsolini et al. 1998; Vigliarolo et al. 2001). Our results nevertheless suggest that such late winter ozone fluctuations can emerge from a planetary-scale teleconnection, between the distant geographical regions of the North Pacific and the North Atlantic. Using the AII, one can reconstruct late winter ozone time series over both the AL and IL sectors, that are well correlated with observations, while corresponding AO or NAObased time series were less correlated with observations over the IL sector, and not correlated over the AL sector.

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Fig. 6. February-mean eddy ozone averaged over the IL and AL sectors for the years 1979–2000 (Bold lines). There are no data in 1995–1996, nor in the polar night. The 3 thin lines are ozone reconstructed from the contributions of the AII (full line), the NAO (dashed line) or the AO (dotdashed line).

While the AII explains a significant amount of the ozone variability in February, it has no significant trend, and hence would not contribute directly to ozone decadal trends. Recent work (Yamane et al. 2003) indicates a decadal variability of the seesaw intensity and peak period, with a less active period in the 1950’s and 1960’s, and a more active period from the late 1970’s to the mid 1990’s. The later largely

Table 1. Correlation coefficients for the AL and IL sectors between an ozone time series based on TOMS observations and one reconstructed using the AII, NAO and AO indices (as in Fig. 3).

AII NAO AO

Aleutian sector

Icelandic sector

0.73 0.15 0.08

0.78 0.57 0.57

overlaps the ozone satellite record that we analysed here. The AII is highly correlated with the PacificNorth American (PNA) pattern, derived from mid-tropospheric teleconnection or EOF analysis (Wallace and Gutzler 1981; Pavan et al. 2000), as noted by Honda et al. (2001). In their analysis, the correlation was 0.84. Overland et al. (1999) also noted the high correlation between the AL intensity and the PNA, but remarked that the former is not entirely explained by a single pattern, and co-varies with other high latitude patterns. The PNA midtropospheric geopotential covariance clearly extends into the North Atlantic (Pavan et al. 2000; Ambaum et al. 2001), and the PNA is the second global mode of wintertime variability (Koide and Kodera 1999; Christianssen 2003). Reciprocally, Pavan et al. (2000) showed that the third Pacific sectorial EOF and its principal component matched with the leading Atlantic

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tentially suggests an interesting predictability capability for late winter ozone over Europe and the Atlantic, which remains to be tested. Acknowledgements The author was supported by the European Commission Climate and Environment Programmes through project CANDIDOZ. The author thanks Dr. D.B. Stephenson and Dr. F. Doblas-Reyes for comments on the manuscript, and Dr. Doblas-Reyes for providing the AO index. References

Fig. 7. February transient synoptic ozone variability, as expressed by the bandpassed ozone M.A.D. during the phases AIIþ (top) and AII (bottom). The M.A.D. is an Eulerian measure of the ozone transients, analogous to the one used to characterise storm tracks.

mode (the NAO), indicating that the NAO has a distinct planetary character. The AII appears in all likelihood, as a combination of several patterns. The canonical life-cycle of the seesaw, as demonstrated in Honda et al. (2001), implies that the Icelandic anomaly emanates from early to mid-winter anomalies in the North Pacific. The demonstrated influence on ozone po-

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