New paper finds the natural Pacific Decadal Oscillation controls North American temperatures via changes in cloud cover

A paper published today in Atmospheric Research describes a mechanism by which changes in the natural Pacific Decadal Oscillation [PDO] affect air temperatures and water vapor, which subsequently affects cloudiness over North America [planetary albedo], and therefore land surface temperatures. As noted by Dr. Roy Spencer in his book, 

"The most obvious way for warming to be caused naturally is for small, natural fluctuations in the circulation patterns of the atmosphere and ocean to result in a 1% or 2% decrease in global cloud cover. Clouds are the Earth’s sunshade, and if cloud cover changes for any reason, you have global warming — or global cooling."

According to the authors, "The TCM [thermodynamic model] generates a kind of atmospheric bridge by which the SST [sea surface temperature] produces a T7A [change in air temperature], the consequent condensation of water vapour anomaly and the corresponding εA [cloudiness] over the continent, affecting the planetary albedo and therefore the LST [land surface temperature]."

Fig. 1. PDO index for the summers (average of June, July and August) based on the CAF for SST over the North Pacific region during a period of 105 years, from 1900 to 2004 (ftp://ftp.atmos.washington.edu/mantua/pnw_impacts/INDICES/PDO). The global warming signal has been removed from the data. The upward bars indicate years with positive index, and the downward bars indicate years with negative index.

Simulation of the PDO effect on the North America summer climate with emphasis on Mexico

• Víctor M. Mendoza^a, , 
• Berta Oda^a, 
• René Garduño^a, 
• Elba E. Villanueva^a, 
• Julián Adem^a, b

• ^a Centro de Ciencias de la Atmósfera, UNAM, Ciudad Universitaria 04510, México DF, México
• ^b Member of El Colegio Nacional, México

Highlights

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A thermodynamic model simulates the effect of the PDO phases on summer variables of Mexico.

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Combined effect of cloudiness and evaporation according to the soil moisture during the y phases.

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Cloudiness anomalies form an atmospheric bridge.

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Abstract

Five composite anomaly fields (CAF) are built for the summer of each Pacific Decadal Oscillation (PDO) phase: skin temperature; air temperature (T₇), zonal (u₇) and meridional (v₇) wind at the 700 mb level; and precipitation (R).

An energy balance model, named Thermodynamic Climate Model (TCM), is integrated on the NH to compute the summer anomalies (sub-index A) of the land surface temperature (LST),T₇, u₇, v₇, R  and cloudiness (ε). To study the effect of the PDO phases on Mexico’s climate, the CAF of the sea surface temperature (SST) is used in the TCM as an input. The output fields are objectively compared with their respective CAF (except SST_A) using an index of agreement, and the six variables are mainly discussed on the north Pacific and adjacent continents (NPAC), with emphasis on Mexico.

The TCM [thermodynamic model] generates a kind of atmospheric bridge by which the SST_A [sea surface temperature] produces a T_7A [change in air temperature], the consequent condensation of water vapour anomaly and the corresponding ε_A  [cloudiness] over the continent, affecting the planetary albedo and therefore the LST [land surface temperature].

The u_7A [zonal wind] forms a large meridional wave train over the NPAC [North Pacific and adjacent continents] centre, which is part of the Pacific/North American pattern in both PDO phases and is more intense in winter than in summer. In the PDO warm phase and over the eastern half of the NPAC [North Pacific and adjacent continents], the v_7A  [meridional wind] is positive, so that the moisture flux from the Pacific Ocean toward North America (NA) increases the precipitation during NA [North American] monsoon. These results have an acceptable agreement with the CAF.

We also analysed the combined effect of cloudiness and evaporation according to the soil moisture, over the eastern NA and the Gobi Desert for both PDO phases, showing its thermal moderator effect.