There are several reasons why this has been occurring; however one should NOT automatically assume that they are in some way related to the controversial issue of global climate change, b/c they ARE NOT related to it.
Lets start w/ the ENSO signal, which right now is statistically neutral, however continuing to warm.
http://www.nhc.noaa.gov/gifs/pac_anom.gif
http://www.cpc.noaa.gov/products/analys ... week_c.gif
You can CLEARLY see the warm plume forming in the equatorial PAC between roughly 120W and the Dateline (180)
14JUL2004 20.9-1.0 25.5 -.1 27.7 .6 29.4 .8
21JUL2004 20.7 -.9 25.2 -.2 27.7 .7 29.4 .9
28JUL2004 20.3-1.1 25.4 .0 27.9 .9 29.4 .9
04AUG2004 19.9-1.3 25.2 .0 27.8 .9 29.4 .9
11AUG2004 19.7-1.2 25.1 .1 27.6 .8 29.2 .8
Furthermore, the ENSO region 3.4 has not been below +.5 DEG C since the week of JUL 14. Meanwhile both the NINO 1+2 region (closest to S America) has remained anomalously cooler than average---leading to a pronounced west to east split in the ENSO regions w/ la Nina like cold in the EPAC and El Nino like warmth West of 120W.
Graphical representation of the ENSO regions:
http://www.cpc.noaa.gov/products/analys ... reas_c.jpg
Normally the COLDEST summers over the EUS WRT the correlation between ONLY the observed ENSO signal and US temps---have occurred in the face of a decaying la Nina, weak la Nina, or well-established ENSO. We will use the summers of 1950, 1951, 1962, 1965, 1967, 1968, 1976, 1982, 1992, and 1993 as examples. The HOTTEST summers (e.g., 1952, 1959, 1977, 1980, 1988, 1991, 1994, 1998, 2000, 2001) in correlation w/ the ENSO signal seem to occur in years where El Nino made an attempt at developing in the Early to mid spring and then failed to establish its self during the summer, years w/ a moderate to strong La Nina, or an El Nino centered in the 3.4 or 4.0 regions.
This summer of course, we have seen SSTA > +.5 C over the Central and WRN Equatorial Pacific, with La Nina Like cold in the 1+2 region of between -1.0 to -1.5 C. So we would NOT fit into the Moderate or strong La Nina category (obviously), we would also not fit into the weak or decaying La Nina categories either since there was No la Nina present to begin with. The initial attempt at warming over the EQ-PAC did not take place this past spring, and certainly did NOT begin in the 1+2 region then weaken, so that correlation is out also. The only one left here that COULD fit the current profile would be an EL NINO centered in the 3.4 and 4.0 region for the JUN-AUG period. Using the correlation between the ENSO signal with the warm SSTA in the 3.4 and 4.0- region, the majority of the nation SHOULD have seen above average temperatures. That as you know has NOT been the case east of the Rockies.
In all actuality it is MOST likely that the EPAC signal/EPO (Eastern Pacific Oscillation) is playing a significant role in deciding this summer’s outcome, and the unusual pattern that has been going along with it.
For those unfamiliar---the EPO is normally characterized by a TRIPOLE (or three primary centers) of anomalous heights, where those of similar sign are located in the EPAC near Hawaii, and over Hudson Bay, with the dissimilar anomaly in the vicinity of AK.
In the +EPO phase, above normal heights (at the 500H level) are found Near Hawaii and over Hudson Bay, with below average heights near Alaska. This is a WARM signal for the EUS during all seasons. The negative phase is the reverse WRT the tripole---and implies below normal temperatures over the EUS, pronounced blocking/split flow over the EPAC in addition to stronger than average and displaced polar vortex (near Hudson Bay).
The EPO has been clearly and overwhelmingly negative since the beginning of the period, which would strongly support a cold signal for the EUS and a warm Signal for NW Canada and AK. One must remember that the blocking is consistent with warmer-than-average temperatures underneath it. A persistently or strongly negative EPO can lead to record breaking heat over WRN Canada and AK.
The persistence of this pattern is the result of the North pacific SSTA configuration Since JUN
http://www.osdpd.noaa.gov/PSB/EPS/SST/d ... 5.2004.gif
http://www.osdpd.noaa.gov/PSB/EPS/SST/d ... 7.2004.gif
http://www.osdpd.noaa.gov/PSB/EPS/SST/d ... 7.2004.gif
We find MUCH warmer than average SSTA surrounding the west coast of North America, and in the Gulf of AK. Normally this type of SSTA configuration is consistent with the aforementioned –EPO pattern and blocking/split flow over the North pacific which we have seen quite a bit of. The reverse of this pattern (cold SSTA surrounding NA, and warm SSTA near Hawaii) would support the opposite.
But it does not end there, the PNA (Pacific North America) pattern, which is under normal circumstances non-prevalent during the heart of the summer, has been showing it’s self quite frequently also. A positive PNA pattern (Western Ridge and downstream eastern trough configuration) is common during periods when we have a similar SSTA configuration over the North pacific, and during El Nino years.
The eastern US trough can reach extreme amplitudes during periods when the Positive phase of the PNA coincides with a –NAO.
This condition has been satisfied many times since the Middle of JUL.
Looking down the road for changes: it looks as if the latest run of the GFS ensembles try to bring the trough back into the west around the 25th as the large block along the north Shore of AK gets kicked out and retrogrades toward Siberia while weakening, and ridging develops over the EUS in response to the pulling back of the mean trough position to the WRN US, however with the current SSTA configuration over the NE PAC, I cant find many reasons why such a pattern would set in for an appreciable amount of time. It will most likely be a transient adjustment.
For you winter wx enthusiasts, IF this SSTA configuration over the NPAC can hold for the next 4-6 months w/ the onset of Weak El Nino conditions, there is the POTENTIAL for an early---and very exciting start to the winter.
