Monday 8 August 2011

Arctic Dipole: In the 20th Century.

The classic mode of variability in the Arctic atmosphere is the Arctic Oscillation. This is annular (circular) in form so moves air around the Arctic Basin. The AO is identified using a technique called Empirical Orthogonal Functions (EOFs), which is a technique used to identify patterns in data. Recently a pattern called the Arctic Dipole has been found, this has a key role in the recent atmospheric changes in the Arctic.

The earliest paper I've read that discusses an Arctic Dipole pattern is Wu et al (2006), the authors note that a 1995 study by Wang, Van der Baaren, and Mysak, found a dipole anomaly similair to the AD for the period 1953 to 1993 but those authors did not pursue the matter further. Wu et al examine the first 2 leading EOF modes during winter over the period 1960 to 2002. They find that the first (the Arctic Oscillation, AO) accounts for 61% of the SLP and is annular, as noted above, it moves air around the Arctic basin in a circular fashion. The second leading mode is meridional, i.e. it moves air across the Arctic basin, so can bring warmer sub-Arctic air into the region. This latter mode is the Arctic Dipole (AD), they find that it only accounts for 13% of the total variance during the period considered. Wu et al also present a time series of the interannual variations of the AD, this shows no trend in the AD over the period 1960 to 2002.

They note that the AD as found by them is a regional, not a hemispheric phenomenon, unlike the North Atlantic Oscillation, which is closely related to the AO (although see below regards Wu et al 2008). They also give reasons for their AD not to be confused with the Barents Oscillation. They define the AD index as follows:

1) Positive index when low pressure occupies most of the Arctic with a centre around the Laptev Sea, and high pressure over the Canadian Arctic Archipelago and Greenland, extending to the North Atlantic.

2) Negative index when there is high pressure over the Arctic Ocean with a centre around the Kara and Laptev Seas, and low pressure over the Canadian Arctic Archipelago and Greenland, but with the centre in the north east of the North Atlantic.

This broad definition has stuck in subsequent research, although the summer AD is found to have a different structure, and subsequently the structure has shifted.

Wu et al find that the positive phase of the AD is favourable for ice transport into the Greenland and Barents seas. The negative phase of the AD is not, it shows a strong anticyclonic circulation (high pressure over much of the Arctic), furthermore the Beaufort Gyre strengthens which accumulates sea-ice in the Arctic basin. This general finding fits with later papers on the AD. However with regards ice export through the Fram Strait, Wu et al find that the correlation between the AO index and Fram Strait sea-ice flux is 0.04, for the AD they find a correlation of 0.33, again this is for the period 1960 to 2002.

This finding of a low AO index/Fram Strait export correlation surprised me as from previous reading I'd thought it was a higher correlation. Wu et al cite two studies that they say back up this finding of a low AO/Fram Strait flux correlation. One study covers the period 1950 to 2000, the other of the cited studies is by Jung & Hilmer, in an earlier study they found that between 1958 and 1977 the correlation between the NAO and Fram Strait ice fluxes was essentially zero, but from 1978 to 1997 it was about 70%. They propose that the last two decades of the last century may be unusual in terms of spatial patterns of NAO variability, hence this relationship between the NAO and Fram Strait fluxes may be unusual in the longer term context, (ref). As Wu et al's study is for the period 1960 to 2002 they may have understated the higher correlation between the AO and Fram Strait fluxes that only occurs for part of that period.

I've been looking for modelling studies that replicate the AD, with little success. However, also in 2006, Watanabe et al published a study finding the same AD structure as in Wu et al but for the period 1900 to 2100. They use the MIROC medium resolution model, in 2009 Wang and Overland used MIROC in a study examining the prospects for a sea-ice free summer because it was one of the models that reproduced the seasonal sea ice cycle well. The models that reproduce the seasonal cycle well also show the greatest rates of sea-ice loss.

Watanabe et al find aspects of the modelled AD to reproduce the AD found by Wu et al. This included the AO (first EOF mode) accounting for 59% of the variance and the AD accounting for 19%. Results that are remarkably close to Wu et al. Furthermore Wu et al noted that the AD was 'quasi-barotropic', being detectable in both sea level pressure and at 500hPa, Watanabe et al find the same in their model study. Barotropic means that pressure varies with density, in meteorlogical terms it is a non-frontal type atmospheric state. This fits in with research I'll cover in a later post that suggests shallow convective processes in the Arctic atmosphere due to open ocean are responsible for the changes in Arctic circulation. Unfortunately Watanabe et al do not use the model to analyse the processes involved in the formation of the AD. It would be of great use to see such a study done to cover the changes of AD during this century as compared to the 20th century.

Another paper, Wu et al 2008, again states that the AD is Barotropic and asserts that the dipolar nature of the AD is due to alternating locations of the polar vortex centre. When the AD is in it's positive phase the centre of the polar vortex is over the Canadian Arctic, in the negative phase it's over the Siberian Arctic Basin. In other words the vortex is where the high pressure region of the AD is. Wu et al 2008 also find that the spectrum of the AD has a predominant peak of 6 years, with a secondary 3 year periodicity peak, and that from 1978 to 1987 positive ADs prevailed, with 1965 to 1977 and 1988 to 2005 being mainly negative. Wu et al 2008 also identify lower lattitude weather impacts of the AD which I will address in a future post.

So the AD isn't a new atmospheric mode in the Arctic, it has been present in both observations and simulations throughout the Arctic 20th Century. However, as I'll discuss shortly, its secondary role has recently changed.

Eiji Watanabe et al, 2006, "Arctic dipole anomaly and its contribution to sea ice export from the Arctic Ocean in the 20th century."
Bingyi Wu et al, 2006, "Dipole Anomaly in the Winter Arctic Atmosphere and its Association with Sea Ice Motion."
Bingyi Wu et al, 2008, "Arctic dipole anomaly and summer rainfall in Northeast China."

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