United States

Lets Blame the Heat Wave on the NAO

July 13, 2012; 8:37 AM

Commentary

We all love the blame game, so I think I found what to blame the heat wave on, the North Atlantic Oscillation (NAO). (See below for explanation of the NAO.) During the month of June, we saw the lowest recorded NAO value since 1950. I have to credit Ralph, a blog reader, for picking that out. Some studies have suggested that low values of NAO in the summer can lead to heat waves and certainly, the correlation between the lowest values in June and the extreme heat seems to be there. In the winter, the snow lovers in the East cheer on the negative NAO because that means blocking which in turns means, colder weather in the East and the potential for slow-moving East coast snowstorms.

So we have to blame the NAO for the heat wave, but I will go back to what I said in a previous post. The NAO is just one part the equation, and it's a chicken-and-egg situation in regards to what caused the lowest NAO. Could the El Nino that is developing have already caused the jet stream to change which in turn has caused blocking? Is the NAO/blocking a signal that the winter will be opposite of last winter where we had no blocking so this winter we have continuous blocking and snow up to our chins?

I do think the winter will more challenging then last winter in regards to snowstorms and figuring out the strength of the El Nino will be the critical factor in regards to the distribution of snow across the U.S.

In any case, we know that the NAO was low, so let's just blame blocking for the heatwave and an extreme record-setting June into early July.

What is the NAO?

One of the most prominent teleconnection patterns in all seasons is the North Atlantic Oscillation (NAO) (Barnston and Livezey 1987). The NOA combines parts of the East Atlantic and West Atlantic patterns originally identified by Wallace and Gutzler (1981) for the winter season. The NAO consists of a north-south dipole of anomalies, with one center located over Greenland and the other center of opposite sign spanning the central latitudes of the North Atlantic between 35°N and 40°N. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies over these regions. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track, and in large-scale modulations of the normal patterns of zonal and meridional heat and moisture transport (Hurrell 1995), which in turn results in changes in temperature and precipitation patterns often extending from eastern North America to western and central Europe (Walker and Bliss 1932, van Loon and Rogers 1978, Rogers and van Loon 1979).

Strong positive phases of the NAO tend to be associated with above-average temperatures in the eastern United States and across northern Europe and below-average temperatures in Greenland and oftentimes across southern Europe and the Middle East. They are also associated with above-average precipitation over northern Europe and Scandinavia in winter and below-average precipitation over southern and central Europe. Opposite patterns of temperature and precipitation anomalies are typically observed during strong negative phases of the NAO. During particularly prolonged periods dominated by one particular phase of the NAO, anomalous height and temperature patterns are also often seen extending well into central Russia and north-central Siberia.

The NAO exhibits considerable interseasonal and interannual variability, and prolonged periods (several months) of both positive and negative phases of the pattern are common. The wintertime NAO also exhibits significant multi-decadal variability (Hurrell 1995, Chelliah and Bell 2005). For example, the negative phase of the NAO dominated the circulation from the mid-1950s through the 1978/79 winter. During this approximately 24-year interval, there were four prominent periods of at least three years each in which the negative phase was dominant and the positive phase was notably absent. In fact, during the entire period, the positive phase was observed in the seasonal mean only three times, and it never appeared in two consecutive years.

An abrupt transition to recurring positive phases of the NAO then occurred during the 1979/80 winter, with the atmosphere remaining locked into this mode through the 1994/95 winter season. During this 15-year interval, a substantial negative phase of the pattern appeared only twice, in the winters of 1984/85 and 1985/ 86. However, November 1995 - February 1996 (NDJF 95/96) was characterized by a return to the strong negative phase of the NAO. Halpert and Bell (1997; their section 3.3) recently documented the conditions accompanying this transition to the negative phase of the NAO.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com