Update on 2018 Blue Nile forecast

Update to the guest blog by the Ad hoc Blue Nile Forecast Group (listed alphabetically): Sarah Alexander (1), Paul Block (1), Annalise Blum (2), Shraddhanand Shukla (3), Shu Wu (1), Temesgen Yimane (2), Ben Zaitchik (2)*, and Ying Zhang (2).

  1. University of Wisconsin-Madison, Madison, WI, USA

2. Johns Hopkins University, Baltimore, MD USA

3. University of California Santa Barbara, Santa Barbara, CA, USA

*Correspondence can be addressed to zaitchik@jhu.edu

The end of July represents the midpoint of the Kiremt rainy season in the Blue Nile basin. On average, just over half of the June-September rainfall total is realized by the end of July. This is also true for the annual total rainfall, January-December (Figure 1; shown spatially in Figure S1). This makes the end of July an opportune time to provide an update on the seasonal forecasts of 2018 Blue Nile rainfall and river flow that we posted at the outset of the rainy season.

Figure 1: Cumulative Blue Nile basin rainfall, January-December, according to CHIRPSv2 1981-2017 climatology. On average, 54% of annual rainfall occurs by the end of July (month 7).
At the time of our first post, there were already reports that rains had come early to portions of the Blue Nile basin, and we found near unanimity across statistical and dynamically-based seasonal forecasts that the June-September Kiremt rainfall would be average to above average. Forecasts of Blue Nile flow showed the same tendency. Consistent with these forecasts, rainfall in the basin through the first dekad of July was well above average (Figure 2). Positive rainfall anomalies for early season rainfall exceeded 50 mm over the majority of the basin, with significant areas showing anomalies greater than 100 mm. For context: according to CHIRPS estimates, average June rainfall for the basin is 195 mm, and average June-September total rainfall is 942 mm. So anomalies on the order of 50-100 mm through only the first third of the rainy season are quite meaningful.
Figure 2: CHIRPS-prelim cumulative seasonal rainfall anomalies for June 1 – July 15, 2018. Information on the CHIRPS-prelim product is available at https://earlywarning.usgs.gov/fews/product/597.

Interestingly, however, the North American Multi-Model Ensemble (NMME) forecasts launched in July show a more mixed outlook than the same ensemble had offered in May. At the time of the May forecast, every NMME model had a mean prediction of average to above average rainfall for June-September (Figure 10 in our original post). Looking at forecast July-September rainfall in the July initialized NMME simulations, we see that there are now several models that predict below average rainfall, albeit with only a modest negative anomaly (Figure 3). This breaks the consensus that existed in May. This shift to drier forecasts might reflect the influence of a shifting El Niño outlook, as the predicted probability of an El Niño forming by the end of the season is higher now than it was in May (Figure 4; compare to Figure 9 in the original post).

Figure 3: Forecast of July-September rainfall anomaly for the Blue Nile basin (mm) in July initialized forecasts of NMME models. Mean is shown by (x), circles are individual members, and boxplots show quartiles. The “Average” boxplot consists of the average forecast of each participating model.
Figure 4: Oceanic Niño Index forecast, generated by NOAA CPC and the International Research Institute (IRI) and issued on July 23, 2018.

Notwithstanding this somewhat drier outlook from NMME, the strong rains observed in June and early July got the basin off to a wet start. This, combined with an NMME forecast that still, across all models, points to average rainfall conditions for the remainder of the season, suggests that the Kiremt rains will be average to above average overall. The probability of a dry year is low, though some NMME models now suggest that late season rains might be lower than average.

Recent Extreme Temperatures Enhanced by Climate Change

Chris Funk

As noted by a recent report by the World Meteorological Society (and many other news articles), July of 2018 has brought exceptionally warm air temperatures to many parts of the globe. Fires rage in Sweden and Greece, Japan experienced deadly torrential rains (1,800 mm at Shikoku) followed by temperatures reaching 106°F. In Algeria, Ouargla reported a maximum temperature of 124.3°F and Morocco set a new record at Bourfa at 110°F.  In Canada, as many as 70 people may have died due to an extreme heat wave in Quebec. Closer to home, the WMO report identifies an extreme of 125.6°F in Death Valley, while Chino, Burbank and Van Nuys set records at 120°F, 114°F and 117°F.

This blog posting is not intended to provide a formal climate attribution analysis, such as those provided by the Bulletin of American Meteorological Society. The goal here is just to look at the data, which is quite compelling on its own. We begin by looking at annual (July to June) air temperatures for Coastal Southern California (Figure 1). This data was obtained from the Earth Systems Research Laboratory. What is very concerning about this time series is how every year since 2014 has been very warm. I’ve circled these values. This type of persistent warmth can dry out vegetation and provide great background conditions for fires.

Annual (July-June) average tempetures for south coastal California.
Figure. 1. Annual (July-June) average temperatures for south coastal California.
Figure 2. Five year average air temperatures for south coastal California.
Figure 2. Five year average air temperatures for south coastal California.





We can present the same data as five-year averages to highlight the recent transition to warmer conditions (Figure 2). Now the past five-year average clearly stands out as way warmer than any value on record before 2013. The current five-year average temperature (~52.8°F) is about two degrees Fahrenheit warmer than the average from just a few years ago. This is a large change in a short period of time. Beginning with the last El Niño in 2014/16, we may have transitioned to a much warmer climate regime, and climate model projections indicate that this warming will continue. I have also plotted similar results for Central California where the Ferguson fire rages (Figure 3). We have seen an historically unprecedented and very rapid increase in air temperatures. Currently, about 20 fires are burning in California, and 2017 was clearly the most destructive fire year on record. While warm temperatures are just part of the recipe for fire disasters, this part of the puzzle has clearly been expanding rapidly.

Figure 3. Five year average temperatures for central California.
Figure 3. Five year average temperatures for central California.



If we produce a similar plot of GLOBAL five-year average air temperature anomalies, based on NASA estimates of land surface temperatures (Figure 4), we see that global temperatures have also jumped upwards over the past five years, reaching unprecedented heights. The magnitude of the jump in coastal (Figure 2) and central (Figure 3) California has been substantially greater in magnitude, however. Also shown in Figure 4 are completely independent predictions of global land air temperature anomalies based on the current state of the science collection of climate change simulations. The fit to the observations is extremely good (R2=0.97). Climate change has caused the recent increase in global temperatures.

Figure 4. Global NASA GISS five-year average air temperature anomalies for land areas.

The rise after 2018 is based on a pessimistic but realistic ‘business as usual’ climate change scenario in which the climate modelers have assumed a continued rapid increase in greenhouse gasses. I have annotated this time series with 10 year steps to emphasize what we may likely experience between 2019 and 2048. Between 2009 and 2018, we have already seen a problematic increase in global and California air temperatures related to numerous climatic hazards. Without dramatic efforts to reduce our greenhouse gas emissions, the models (which have been extremely accurate so far) tell us that we are likely to experience three more similar increases between now and mid-century. For poor people living in very warm regions (like India), such warming may lead to severe health impacts as described by Somini Sengupta. For California, a further intensification of droughts and fire risk seems likely as  temperatures continue to increase rapidly.

To visualize US temperature changes, we can use the cool ‘Climate Explorer’ website provided as part of the U.S. Climate Resilience toolkit. If you click HERE you should get a map of the continental U.S. that shows the number of days in a year with maximum air temperatures of greater than 95°F. The map is divided with a vertical bar identified with left-right arrows (<>). On the left hand side of the bar is a map of recent counts based on 1961-1990 observations. On the right is a map of estimates for 2090 based on a continued higher emissions trajectory. Grab the central bar and slide it back and forth, and you can see the predicted change – big increases in the frequency of very warm days.

We can also use the Climate Explorer to examine likely changes in a given location – like Santa Barbara county (HERE). This time series contrasts likely outcomes given a continuation of our current high emission pathway (shown in pink and red) and likely outcomes if we act to curb emissions (blue). As you can see, there is substantial uncertainty, but we see a substantial difference between the scenario averages (red and blue lines) by mid-century. If we do not curb emissions soon, by 2100 the models suggest that we could very well see annual average maximum temperatures increase by ~+7.3°F, according to the ensemble average.