In the western United States, we’ve experienced two warmer and drier than normal winters in a row, while the eastern United States has seen brutally cold, snowy winter weather during the same span. The proximate cause of this has been in a very persistent blocking pattern in the atmosphere with ridging over or just offshore of the U.S. west coast/Gulf of Alaska and a downstream trough from the Rockies eastward. This atmospheric ridging (high pressure) also contributed to warm weather in the west via a very large area of warmer than normal water over the northeast Pacific, which has been affectionately named “The Blob.” The Blob tempered the last couple of winters over the west and eventually spread into the coast, which helped to fuel the warmest summer ever recorded in many locations over the west.
With an El Nino already in progress and forecasts that indicate it will develop into a strong El Nino and persist into the spring of 2016, the questions are: what can we expect this winter? Will the blocking pattern/blob screw us over again? What effect will The Blob have on El Nino? Will it be cold in the east again?
Of course, there is no way to answer these questions with certainty, but looking at the latest data in the light of recent research can give us some clues.
First, the blocking ridge. Here is a chart showing the mean 500 mb height anomalies for last year and a half (Jan 2014 through Jul 2015), which includes the last two winters.
Figure 1: 500 millibar anomalies (meters) from Jan 2014 to Jul 2015
The strong positive anomaly (ridging) over west coast of North America jumps off the page, as does the strong negative anomalies (troughing) over eastern Canada and the eastern United States. Given that this pattern has been the norm for the last two years, it’s no wonder it’s been so cold in the east and warm in the west.
Recent research (referenced below) makes some important conclusions about the persistent ridging and associated abnormal weather patterns those of us in the west have experienced for the last two years, including: The blocking ridge itself is caused by abnormally warm water in the tropical west Pacific1, the ridge is the cause of The Blob2, and all of these things together are responsible for the dramatically cold winters in the east3.
Knowing these things, what does this mean in light of recent trends? Here are charts showing the 500 mb height anomalies for just the last month followed by the anomaly for the last week:
Figure 2: 500 mb height anomalies (decameters) for 01Aug-30Aug2015
Figure 3: 500 mb height anomalies (decameters) for 24-30Aug2015
Over the course of the last month, the blocking pattern weakened significantly, while in the last week it has disappeared, and now we have negative height anomalies (troughing) off the west coast. You will also notice that the area of strong positive anomalies has shifted way west out into the Aleutians. Of course, that doesn’t really mean anything, necessarily, as weather can certainly vary. But in the context of what else is occurring, there is reason to think this may not be just a transitory shift.
Before I get to that, let’s talk about The Blob. As I mentioned earlier, research suggests that the ridge is caused by positive temperature anomalies in the tropical western Pacific and the ridge then causes The Blob. Here are the SST anomalies as the blocking ridge began to form two winters ago:
Figure 4: SST anomalies (°C) on 02 Jan 2014
There were clearly positive SST anomalies in the tropical western Pacific, and you can see the blob developing off western Canada. The Blob morphed and grew over the last few years with peaks in the summer months of both 2014 and 2015. I would estimate from visual inspection of the SST charts (not scientific) that the blob peaked overall in late June or early July of this year (2015). But let’s see how the blob has changed over just the last month:
Figure 5: SST anomaly (°C) from 03 Aug 2015 (top) and 27 Aug 2015 (bottom)
As the ridging moved west over the last month, the blob has also shifted westward. Also notice that water temperatures over the tropical west Pacific have cooled dramatically in the last month. Was this the reason the ridge/blob shifted? Possibly. And will this continue? Also, possibly. If El Nino strengthens as forecast, I would think so, because as warm water continues to be drawn east away from the west Pacific (typical of El Ninos), upwelling replaces it with cooler water. And as we learned earlier, the blocking ridge is forced by warm SSTs in the west Pacific, thus cooler SSTs should at least no longer support it, if not result in its opposite.
For example, in early September 1997, the SST anomalies looked very similar to how they look now. But look at SST anomalies during the height of the 1997/98 El Nino (late Dec, 1997):
Figure 6: SST anomalies (°C) from 30 Dec 1997
The Blob had all but disappeared and water in the tropical west Pacific was cooler than normal. Does that mean it will happen this way again? No. But it shows that it can happen that way.
It is already apparent that water in the west Pacific is cooling and the atmospheric blocking pattern is diminishing. The Blob has already shifted west as a result, and this may lead to its eventual demise. There is certainly no guarantee these trends will continue, but if they do, it is likely that conditions across North America will be much different than the last few winters. Specifically, there is reason to think that the eastern U.S. will not experience another brutally cold and snowy winter like the last two. Furthermore, in the absence of The Blob, the west may experience an unfettered El Nino. What that brings is open to debate and was the subject of an earlier blog.
1 The pattern of SST across the Pacific (specifically, warm water in the west Pacific) is the cause of the blocking pattern we experienced over the last few years [Hartmann et al., 2015]. To quote Hartmann: “Although the pattern of SST anomaly is very strong in the North Pacific, it is virtually certain that the forcing for these anomalies originates with warm SST in the tropical west Pacific.” The mechanism for this is explained as Rossby wave energy that propogates northward from increased tropical convection owing to warmer SSTs [Wang et al., 2014]. In other words: because the ocean is warmer than normal, there is more convection than normal, and the energy from this convection is carried north and induces a ridge over the northeast Pacific.
2 The Blob owes its existence to the persistent atmospheric blocking ridge [Bond et al., 2015]. Persistent high pressure over the northeast Pacific blocks storms from the area, reduces upwelling, lowers the rate of heat loss, and results in relatively weak cold advection in the upper ocean. To quote Bond: “Based on a mixed layer temperature budget, these anomalies were caused by lower than normal rates of the loss of heat from the ocean to the atmosphere and of relatively weak cold advection in the upper ocean. Both of these mechanisms can be attributed to an unusually strong and persistent weather pattern featuring much higher than normal sea level pressure over the waters of interest.” The higher than normal sea level pressure was caused by the strong upper level blocking ridge.
3 It is fairly obvious that the blocking pattern was at least partially responsible for the very cold and snowy recent winters in the east. Nevertheless, both Hartmann and Wang make this abundantly clear. Hartmann states: “This pattern (the SST pattern, and by extension, the blocking ridge) is associated with high pressure in the northeast Pacific and low pressure and low surface temperatures over central North America.” And from Wang: “This persistent source of Rossby wave energy arguably contributed to the emergence of the anticyclone over the Gulf of Alaska that persisted and, subsequently, blocked the winter storms from reaching the West Coast. Also noteworthy is the Rossby wave energy dispersed downwind from this ridge, which amplified the trough stationed over northeast North America leading to the so-called ‘polar vortex.’”
Bond, N. A., M. F. Cronin, H. Freeland, and N. Mantua (2015), Causes and impacts of the 2014 warm anomaly in the NE Pacific. Geophys. Res. Lett., 42, 3414–3420. doi: 10.1002/2015GL063306.
Hartmann, D. L. (2015), Pacific sea surface temperature and the winter of 2014. Geophys. Res. Lett., 42, 1894–1902. doi: 10.1002/2015GL063083.
Wang, S. Y., L. Hipps, R. R. Gillies, and J.-H. Yoon(2014), Probable causes of the abnormal ridge accompanying the 2013–2014 California drought: ENSO precursor and anthropogenic warming footprint, Geophys. Res. Lett., 41, 3220–3226, doi:10.1002/2014GL059748.