Intellicast - This Week in Weather

By Joe D'Aleo

Monday, October 20, 2008

The PDO continues to be strongly negative and the ENSO measures are trending back towards La Nina. Should that surprise us? No, because most La Ninas have a tendency to persist more than one year and the negative PDO states favors more and stronger and longer lasting La Ninas.

THE PACIFIC DECADAL OSCILLATION

The first hint of a basin wide cycle was the recognition of a major regime change in the Pacific in 1977 among climatologists that became known as the Great Pacific Climate Shift. Later on, this shift was shown to be part of a cyclical regime change given the name Pacific Decadal Oscillation (PDO) by fisheries scientist Steven Hare in 1996 while researching connections between Alaska salmon production cycles and Pacific climate. This followed research first showing decadal like ENSO variability by Zhang in 1993.

In a paper in 1997, Mantua et al found the "Pacific Decadal Oscillation" (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time. Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics - the opposite is true for ENSO.

A study by Gershunov and Barnett (1998) shows that the PDO has a modulating effect on the climate patterns resulting from ENSO. The climate signal of El Niño is likely to be stronger when the PDO is highly positive; conversely the climate signal of La Niña will be stronger when the PDO is highly negative. This does not mean that the PDO physically controls ENSO, but rather that the resulting climate patterns interact with each other.

You can see the similarity of the El Nino to the positive PDO and La Nina to the negative PDO ocean temperature anomaly pool configurations above. Not surprisingly as the following shows, El Ninos as shown by the positive MEI red spikes are more common during the warm phases (1977-1998) with La Nina (blue spikes down) more common in the cold phases as we found from 1947 to 1977 and after 1998.

There is a clear distinction in both strength and frequency.

The following shows how they follow in lock step with El Ninos and La Ninas augmenting the strength of the accompanying PDO state when they are synchronous and weakening it when they oppose each other.

You can see from the global anomalies how El Ninos favor more global warmth and La Ninas cooling.

Since El Ninos lead to more global warmth than La Ninas, periods during which the PDO is positive and there are more and stronger El Ninos are not surprisingly periods of increasing global warmth (what we observed from 1979 to around 2000). Conversely cold PDO phases with more cool La Ninas lead to global cooling as we observed from the 1940s to 1970s and again after 2002.

WHERE TO FROM HERE

Expect PDO to stay negative and at least a weak La Nina to return this winter. The early October record snows in the northern Rockies are consistent with this thinking. More on the winter upcoming.

References:

Gershunov, A. and T.P. Barnett. Interdecadal modulation of ENSO teleconnections. Bulletin of the American Meteorological Society, 79(12): 2715-2725.

Mantua, N.J. and S.R. Hare, Y. Zhang, J.M. Wallace, and R.C. Francis,1997: Pacific interdecadal climate oscillation with impacts on salmon production. Bulletin of the American Meteorological Society, 78, pp. 1069-1079.

McPhaden, M.J and D. Zhang (2002), "Slowdown of the meridional overturning circulation in the upper Pacific Ocean", Nature, 415(7), 603–608 (2002).

Miller, A.J., D.R. Cayan, T.P. Barnett, N.E. Graham and J.M. Oberhuber, 1994: The 1976-77 climate shift of the Pacific Ocean. Oceanography 7, 21-26.

Minobe, S. 1997: A 50-70 year climatic oscillation over the North Pacific and North America. Geophysical Research Letters, Vol 24, pp 683-686.

Power, S.B. and I.N. Smith (2007), "Weakening of the Walker Circulation and apparent dominance of El Niño both reach record levels, but has ENSO really changed?" Geophysical Research Letters, vol. 34, L18702, doi:10.1029/2007GL030854, 2007

Vecchi, G.A., B Soden, A.T. Wittenberg, I. M. Held, A. Leetmaa, and M. J. Harrison (2006), "Weakening of Tropical Pacific Atmospheric Circulation Due to Anthropogenic Forcing", Nature, 441, 73–76 (4 May 2006).

Vecchi, G.A. and B Soden (2007), "Global Warming and the Weakening of the Tropical Circulation", Journal of Climate, Vol. 20, (Sep 2007), pp 4316-4340

Wolter, K., and M.S. Timlin, 1993: Monitoring ENSO in COADS with a seasonally adjusted principal component index. Proc. of the 17th Climate Diagnostics Workshop, Norman, OK, NOAA/N MC/CAC, NSSL, Oklahoma Clim. Survey, CIMMS and the School of Meteor., Univ. of Oklahoma, 52-57.

Zhang, Y., J.M. Wallace, D.S. Battisti, 1997: ENSO-like interdecadal variability: 1900-93. Journal of Climate, 10, 1004-1020

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