By Jacob Genuise and Guillaume Mauger
The past week’s cooler and wetter weather comes as a stark reminder that summer has begun to make room for fall here in the Pacific Northwest. With this seasonal shift, some of us are beginning to turn our attention to the upcoming winter, as a strong and potentially historic El Niño event continues to intensify in the tropical Pacific Ocean. In this month’s Climate Matters we take a look at what we know about this year’s El Niño event so far, and what it might mean for winter conditions across Washington.
This El Niño event will almost certainly be very strong.
Since late spring, it has become increasingly evident that this El Niño event was likely to be a strong one. The strength of El Niño is measured by how warm sea surface temperatures are in the tropical Pacific Ocean, relative to normal. Temperatures officially warmed above normal in late June and have continued to warm rapidly with some areas now exceeding +2.0-4.0°C above normal. These changes are large, and they emerged earlier than most El Niño events develop. It also means early forecasts were quite accurate in predicting this rapid strengthening of El Niño through the summer.
To understand the future strength of an El Niño event, we use the Climate Prediction Center’s (CPC) forecasts. They show that this event is likely to continue to intensify, and has a roughly 95% chance of becoming a very strong event this fall. The CPC has also said there is a 69% chance of a historic event that would exceed the strength of all El Niño events back to at least 1950. We are surely in for an event with very few historical examples as precedents. Importantly, the strength of the event does not necessarily mean we will experience the strongest impacts on record, but it does increase the likelihood we will experience impacts in Washington state.
What are the possible impacts of a very strong El Niño in Washington?
It is important to remember that all El Niño years are unique. El Niño is just one feature of our climate, and it is interacting with many other features of our climate system in complex ways. On average, about 30% of the variability in winter circulation patterns can be explained by El Niño/La Niña (Li et al., 2019). This means that El Niño events tilt the odds toward certain winter conditions, but we cannot say exactly how any single season will unfold.
Temperature
When we look across all El Niño events in the historical record, we see that El Niño typically brings increased chances of warmer than normal winter temperatures. As we saw this past winter, warm winter temperatures can cause low snowpack in the mountains, especially in the middle to lower elevations where temperatures are inherently warmer. This can occur even when precipitation is near normal for the season.
The map below shows the average November-March temperature anomalies for the seven strongest El Niño events since 1950. Oranges and reds represent areas that experience warmer than normal temperatures during these strong events, on average, and greens and blues show areas that experience cooler than normal temperatures. There is a clear pattern of warmer than normal temperatures across the northern portions of the West Coast and across the northern United States as a whole during strong El Niño events. This pattern also holds if we were to look at the much larger sample of all El Niño events, including weak, moderate, and strong events.
Map of temperature anomalies by climate division during the previous very strong El Niño events.
Precipitation
El Niño events also tend to slightly increase the odds of a drier than normal winter, on average in the Pacific Northwest, but this relationship is weaker than the relationship with temperature. Below is a similar map of the seven strongest El Niño events on record showing no clear pattern in precipitation across Washington. However, there is a dry signal across the interior northwest and northern plains for these strong events. Widening out to all past El Niño events, there tends to be a slightly higher chance of drier than normal winters across the Pacific Northwest during El Niño events.
Map of precipitation anomalies by climate division during the previous very strong El Niño events.
To give a sense of just how unique each El Niño is for precipitation in our region, the chart below shows precipitation anomalies, or differences from normal, for all El Niño winters since 1950 (Source). Each winter is sorted from the strongest El Niño event in the top left to the weakest El Niño event in the bottom right. You can see that the precipitation patterns for every year vary considerably across events. There is a general tendency for drier than normal conditions, on average, though there are plenty of exceptions. You can click on the image to expand it for easier viewing.
Maps of all past El Niño events on record in order of strength.
Interestingly, some of the strongest El Niño events on record actually brought above normal precipitation to the Pacific Northwest, including 1982-83, 2015-16, and 1957-58. Very strong El Niño events could very well behave differently than weaker El Niño events, but we do not have enough past examples of these strong events to understand how or whether they might be different.
Snowpack
Snowpack is dependent on both temperature and precipitation, so this year’s snowpack will depend on the interplay between these main factors as well as a variety of others. However, we do know that warm winter temperatures lead to lower snowpack, even if we receive normal precipitation. Although not an El Niño, this past year was a great example of the impact of warm temperatures on winter snowpack. The graph below shows total winter snowfall at Snoqualmie Pass every year since 1950 with El Niño and La Niña years marked with red and blue markers, respectively. Three main takeaways are clear:
Plot created by Nick Bond
Graph of annual snowfall at Snoqualmie Pass with shading indicating ENSO status.
- El Niño years tend to bring less snowfall and La Niña years tend to bring more snowfall, on average.
- There is also considerable variability in snowfall, with some El Niño years bringing similar snowfall as some La Niña years.
- There is an overall decreasing trend over time, a consequence of long-term warming.
Given what we’ve seen in the past, we can say that we’re likely to have lower than normal snowpack this winter. This past year of drought has left reservoirs below normal this fall which means that our water systems will not have carryover capacity to buffer another year of reduced snowpack as we head into next year’s growing season. And, as we noted in our March Climate Matters, the amount of water we can store in our reservoirs is much smaller than the amount in our snowpack when it peaks in spring. Time will tell, but this means the odds favor another drought year this coming spring.
El Niño increases our risk of coastal flooding
Aside from precipitation, temperatures, and snowpack, El Niño tends to cause a temporary increase in sea level along the West Coast of North America, increasing the chance of coastal flooding. This increase in sea level due to El Niño is caused by multiple factors including the expansion of water in the Pacific by increased ocean temperatures and a change in mean wind patterns that tends to pile water up on the West Coast. These temporarily elevated seas also compound with background sea level rise caused by climate change and storm surge from individual storms, adding to the rise and further exacerbating flooding impacts.
The graph below shows the steady, long-term rise in sea level at the Seattle tide gauge, as well as plenty of variations around that trend. Notable peaks in sea level, marked by red arrows, correspond to very strong El Niño winters as 1957-1958, 1982-1983, 1997-1998 and 2015-2016 which caused temporarily elevated sea levels.
Monthly mean sea level in Seattle Washington with very strong El Niño winters highlighted.
If we remove this long term sea level rise trend and plot just average high tide relative to the strength of El Niño and La Niña events according to the Relative Oceanic Niño Index (RONI), this relationship is also visible. The graph below shows an increase in mean high tide in Seattle during El Niño events, on average. These years are marked in red. This increase in average high tide is especially apparent for very strong events with a RONI value near +2.0. This year’s event is most likely to have a peak RONI value near +2.5.
Average high tide per year at the Seattle tide gauge. Figure created by Guillaume Mauger. Blue circles correspond to years with a La Niña in place, gray circles represent neutral conditions, and red circles represent winters with an El Niño in place. Year labels correspond to the end year of the winter period, i.e. a label of 2016 corresponds to the winter of 2015-2016.
It is important to be aware of the risk of coastal flooding during El Niño winters, and to prepare for elevated sea levels this winter, especially given the strength of this event. This is a hazard often overlooked during strong El Niño events, but can be quite impactful for coastal communities.
Although we cannot predict exactly how this El Niño event will unfold, we can prepare for possible impacts. Thanks to the hard work of volunteer observers (CoCoRaHS), scientists, and others who have collected and recorded information on the strength and impacts of El Niño events across many decades, we know that we have increased chances of warm winter conditions, lower than normal snowpack and higher sea levels during El Niño years. Following seasonal forecasts will be the best way to track how this El Niño event unfolds in real-time.
Reference:
Li, X., Hu, Z.-Z., Liang, P., & Zhu, J. (2019). Contrastive Influence of ENSO and PNA on Variability and Predictability of North American Winter Precipitation. Journal of Climate, 32(19), 6271–6284. https://doi.org/10.1175/JCLI-D-19-0033.1