Marco Conceptual

In this Section we describe the variability in the northern hemisphere large- scale flow during the 1990-91 winter (defined as the period from 1 December 1990 to 28 February 1991) using some simple diagnostic indices. This season was characterized by a number of strong blocking episodes lasting for several days in both the Euro-Atlantic sector and the Pacific-north American (hereafter Pacific, for brevity) sector. Figure 4.1 shows a Hovmoller diagram of high- latitude planetary waves, computed from analyses of 50 kPa height by averaging the field in the 50°N-65°N latitude belt and Fourier-filtering the longitudinal profile to retain only waves with zonal wave number l < m ^ .

9 0 E 1 80 9QW

50 Q

L O N G I T U D E

Figure 4.1. H ovm oller diagram o f the 50 kPa height planetary-w aves in w inter 1990/91 (90 days with starting date 01.12.90), constructed using spectral com ponents with zonal w ave num ber 1 ^ ^ . The m eridional average has been com puted in the latitudinal belt 50°N<(p<65°N . C ontour interval 60 m starting from 30 m, shading from 330 m. (From Buizza and M olteni, 1996.)

In the Euro-Atlantic sector, the first half of December 1990 is characterized by a predominance of anticyclonic conditions at high latitudes. From the 10th of December, a strong ridge develops over the Atlantic, and by the 14th it has developed into a well defined blocking dipole over western Europe (Fig. 4.2a). The intense ridge centred on the Greenwich meridian corresponds to one of the three largest positive maxima in the Euro-Atlantic portion of the planetary wave Hovmoller diagram in the entire reason, and is also reflected in a relative maximum of hemispheric wave amplitude (Fig. 4.3a).

Over the Pacific sector, the flow is fairly zonal at the beginning of December, and does not show any persistent features until the middle of the month, when a ridge starts to develop over the eastern side of the ocean. During the following 15 days the circulation over this sector is characterized by a persistent ridge, which reaches its largest amplitude towards the end of the month. The 50 kPa map for 26 December (Fig. 4.2b) shows that the marked Pacific ridge is accompanied by another ridge over eastern Europe, giving a strong wavenumber-2 signature to the planetary wave pattern, corresponding to the second maximum in the hemispheric wave amplitude (Fig. 4.3a, dashed line for daily values and solid line for the 5-day running mean).

During the first ten days of January 1991, a zonal circulation characterizes the Euro-Atlantic sector. On the 11th (winter day 42), a ridge starts to develop over the eastern Atlantic, ahead o f a deep cut-off low. The rapid amplification o f this system leads to a blocked flow over Europe within a few days (see the map for 16 January in Fig. 4.2c). After this development, the European region remains characterized by strong ridges and blocking structures for almost 25 days. Towards the middle o f January, the flow over the eastern Pacific begins to evolve from zonal to blocked. The subsequent broadening and amplification of both the Atlantic ridge and the Pacific block leads to a hemispheric flow with exceptionally large wave amplitude by the 25th of January (see the 50 kPa map in Fig. 4.2d and the absolute maximum in the time series of Fig. 4.3a for winter day 56).

A n o i v s t î 9 G : - A n o i y s i i 9 0 1 2 2 6 e s A n a l y s i s . 9 1 0 1 1 6 A nalysis: 9 1 0 1 2 5 Anoivsis 9 1 0 2 0 5 A n a ly sis 9 1 0 2 2 6 5 7 c . 57.5'

Figure 4.2. Observed 50 kPa geopotential height du rin g strong blocking

developm ents: a) 14.12.90 (w inter day 14); b) 26.12.90 (w inter day 26): c) 16.01.91 (w inter day 47); d) 25.01.91 (w inter day 56); e) 5.02.91 (w inter day 67); f) 26.02.91 (w inter day 88). Contour interval 160 m. (From Buizza a n d M olteni, 1996.)

s < 20 UJ : 15 Z 10 < Z £ 1C 2C 30 DAY b) S e O c 20 0 60 7C DAY

Figure 4.3. a); planetary w ave rms amplitude {dam) com puted using 50 kPa height spectral com ponents with zonal w ave number l< m < 4. b): instability index D ashed lines refer to day values and solid lines to 5 -day running means. (From

In the following week, the Pacific block weakens and evolves into a zonal flow by the end o f January. In the Euro-Atlantic region the persistent anomalous ridge develops into yet another blocking dipole at the beginning of February, as seen in the map for the 5th of February (Fig. 4.2e).

This block decays about one week later, but the large-scale circulation remains moderately active throughout the rest of February, until another major amplification of planetary waves takes place in the Pacific in the last week of the month. A very intense ridge develops over the west coast of north America by the 26th o f February (Fig. 4.2f). During the successive days, the ridge extends further to the north-west, and on the 28th a geopotential maximum is located over Alaska. In the meanwhile, a less conspicuous block has become established over eastern Europe.

From the Hovmoller diagram in Fig. 4.1, one can see that substantial changes in the large-scale flow occurred on a time scale of a few days. In the following Sections, we will analyze tendencies and singular vectors computed over time periods comparable to the typical duration o f a transition to blocking. Specifically, optimization times o f 4 and 6 days have been tested. The results we obtained with these two time intervals were quite similar; for brevity we will not discuss the sensitivity o f our result to the time interval definition in further detail. Throughout the rest o f the paper, results for a 4- day period will be presented, since a shorter optimization time is more consistent with the linear approximation adopted in our study.