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The Lower Arctic Stratosphere in Winter since 1952: an Update

Karin Labitzke, Freie Universität Berlin, Germany (labitzke@strat01.met.fu-berlin.de)

Barbara Naujokat, Freie Universität Berlin, Germany

Markus Kunze, Freie Universität Berlin, Germany

Four years ago we published a table with monthly mean temperatures (ºC) at 30 hPa over the North Pole ([Labitzke and Naujokat, 2000], hereafter LN). These data started with the winter 1955/56, i.e. 45 winters were available. One point of interest in the paper was the fact that there was a period of 7 winters without Major Warmings: Northern Hemisphere winter 1991/92 to 1997/1998. We pointed out that during low solar activity the winters in the west phase of the Quasi-Biennial Oscillation (QBO) tended to be cold and stable (Labitzke and van Loon, 2000) and this combination existed during 4 out of the 7 winters.

Here, we would like to give a follow up on the North Pole temperatures and on the characteristics of the last four Arctic winters by supplementing Table 1 of LN. After the extremely cold winter of 1999/2000 (LN), we observed four highly disturbed winters with a total of five Major Warmings (Table 1). Two of the recent winters belong to the winters in the west phase of the QBO and high solar activity, and they are expected to be connected with Major Warmings.

Table 1. RJ is the monthly mean of the sunspot numbers in January; in the column marked QBO the phase of the QBO is given (determined using the equatorial winds between 50 and 40 hPa in January-February); FW gives an indication of the timing of the Final Warmings which are the transitions from the winter to the summer circulation. CW stands for Canadian Warmings and * indicates the occurrence of a Major Mid-Winter Warming. The long-term mean and the standard deviation are based on the period 1955/56 to 2003/04. The values of the linear trend are for the full data set, n=49 years. C stands for a cold monthly mean (about half a standard deviation or more below the long term average; see discussion in LN). [Data: Free University Berlin (FUB) until 2000/01, ECMWF afterwards] For better resolution, please contact the SPARC Office

Particularly interesting was the winter of 2001/2002 when two Major Warmings occurred in December and in February (Naujokat et al., 2000).  This has been observed only once before, during the winter of 1998/99.

Compared with our earlier results (Table 1 in LN) the overall trend has not changed much during early and mid-winter. The trend over the Arctic lower stratosphere is clearly negative in November and practically zero from December till February.

Major Midwinter Warmings are connected with the breakdown of the polar vortex, much reduced activity of planetary waves, and weak transport of energy.  This leads often in late winter to the re-establishment of a persistent cold polar vortex and to the so-called late winter cooling, which is clearly visible in the Arctic temperatures during March and April, especially in the 2003/04 winter (Table 1). In all four winters the Final Warmings were late.

Figure 1 shows the time series of the 30 hPa North Pole temperatures (ºC) for March, an update of Figure 1 in LN.  The overall trend for 49 years is weakly negative, but depending on how one divides the data, the trend can be positive, as in the first half of the data set or negative, as in the second half of the data. The change in the sign of the trend between the two different periods is, however, confirmed by the re-analyses of NCEP/NCAR and by the ECMWF-ERA-40 data (Labitzke  and Kunze, submitted).

Figure 1. Time series of the monthly mean temperatures (°C) at 30 hPa over the North Pole in March, 1956 to 2004. Linear trends are given for three different periods: 1956-1979, 1979-2004 and 1956-2004 (Labitzke and van Loon, 1999, updated). Data: Free University Berlin (FUB) (open dots), ECMXF (full dots).     For better resolution, please contact the SPARC Office

Acknowledgements

FU-Berlin: http://strat-www.met.fu-berlin.de/products/cdrom

NCEP/NCAR: http://wesley.wwb.noaa.gov/reanalysis.html

ERA40: http://www.ecmwf.int/research/era

References

Labitzke, K. and H. van Loon, The Stratosphere: Phenomena, History, and Relevance. New York: Springer, Berlin Heidelberg, 1999.

Labitzke, K. and H. van Loon, The QBO effect on the global stratosphere in northern winter. J. Atmos. Sol. Terrest. Phys., 62, 621-628, 2000.

Labitzke, K. and B. Naujokat, The lower arctic stratosphere in winter since 1952. SPARC Newsletter, 15, 11-14, 2000.

Labitzke, K. and M. Kunze, Stratospheric temperatures over the Arctic: Comparison of three data sets. Submitted to Meteorolog. Zeitschrift, 2004.

Naujokat, B., et al., 2002: The early major warming in December 2001—exceptional?. Geophys. Res. Lett., 29 (21), 2023, doi:10.1029/2002GL015316, 2

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