Investigation of the Summer Climate of the Contiguous United States and Mexico Using the Regional Atmospheric Modeling System (RAMS). Part II: Model Climate Variability

Abstract: Summer simulations over the contiguous United States and Mexico with the Regional Atmospheric Modeling System (RAMS) dynamically downscaling the NCEP–NCAR Reanalysis I for the period 1950– 2002 (described in Part I of the study) are evaluated with respect to the three dominant modes of global SST. Two of these modes are associated with the statistically significant, naturally occurring interannual and interdecadal variability in the Pacific. The remaining mode corresponds to the recent warming of tropical sea surface temperatures. Time-evolving teleconnections associated with Pacific SSTs delay or accelerate the evolution of the North American monsoon. At the period of maximum teleconnectivity in late June and early July, there is an opposite relationship between precipitation in the core monsoon region and the central United States. Use of a regional climate model (RCM) is essential to capture this variability because of its representation of the diurnal cycle of convective rainfall. The RCM also captures the observed long-term changes in Mexican summer rainfall and suggests that these changes are due in part to the recent increase in eastern Pacific SST off the Mexican coast. To establish the physical linkage to remote SST forcing, additional RAMS seasonal weather prediction mode simulations were performed and these results are briefly discussed. In order for RCMs to be successful in a seasonal weather prediction mode for the summer season, it is required that the GCM provide a reasonable representation of the teleconnections and have a climatology that is comparable to a global atmospheric reanalysis.

The full text is freely available at the link provided above. I’m going to extract the first two figures and ignore the rest of the paper. There are 22 pages in this paper, and the two figures are on pages 3 and 4. So there is a lot more to this paper than what I will talk about. They show the first 3 rotated EOFs (empirical orthogonal functions) as an extension of two previous papers to the global sea surface temperature data from 1950-2000.

The EOFs show the dominant spatial patterns in the data.

FIG. 1. The first three REOFs of global SST for the period 1950–2000. For REOF patterns units are arbitrary and contour interval is one unit. Values greater (less) than 2 (-2) are shaded dark (light).

As can be seen, the first EOF looks like an ENSO signal. For more information about ENSO, see this page from ESRL. In brief, ENSO is a semi-regular pattern affecting the temperatures of the tropical Pacific.

The second EOF is almost entirely positive, except for a small portion of the northern tropical Pacific. This pattern will be discussed below.

They note, “These REOFs are not statistically significant by standard tests of eigenvalue separation (e.g., North et al. 1982). To establish that these patterns reflect significant time variation in global SST, a multitaper frequency domain, singular value decomposition (MTM–SVD) approach is used.” [Not shown here.]

FIG. 2. Normalized PC time series (1950–2000) for boreal summer SST modes in Fig. 1 (solid line). [Ignore the dashed line.]

These are the time series, or principle components (PC), corresponding to the three EOFs above. The first PC has a pattern of about 7-10 years, which is consistent with ENSO. Since the SST map looks like ENSO, and the time series looks like ENSO, I believe that they can interprete this mode of variability to ENSO.

The third PC has a decadal to multi-decadal variability, which they do not interprete as the PDO, but it sure looks like the PDO to me. From The Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington, “[t]he Pacific Decadal Oscillation (PDO) Index is defined as the leading principal component of North Pacific monthly sea surface temperature variability (poleward of 20N for the 1900-93 period).” If that didn’t make any sense to you, don’t worry. It’s not important for this post.

The second PC is the interesting one. We can see that from 1950 to 2000, this time series has almost monotonically increased. There are several steps and level periods, but for the most part it shows a continual warming. They even refer to this mode as the “tropical SST warming mode”. Actually, this shouldn’t be interesting at all. The globe is warming, we would expect the oceans to warm as well. But Dr. Pielke has continually the Lyman ocean heat content paper to show that the oceans are cooling. [See the correction to Lyman.] But this paper, on which he’s second author, does show the oceans to be warming, albeit only until 2000. It would be interesting to see this approach extended until the present. I would expect the dominant EOFs and PCs to remain about the same.

References:
Castro, C.L., R.A. Pielke, J.O. Adegoke, S.D. Schubert, and P.J. Pegion, 2007: Investigation of the Summer Climate of the Contiguous United States and Mexico Using the Regional Atmospheric Modeling System (RAMS). Part II: Model Climate Variability. J. Climate, 20, 3866–3887.

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