AND Is that the Reason Why This El Niño is Not Suppressing the California Drought as Expected?

Guest Post by Bob Tisdale

El Niño events come in different flavors, and those different flavors can have differing impacts on regional weather around the globe. There are East Pacific El Niño events like the 1997/98 El Niño, and there are Central Pacific El Niños (a.k.a. El Niño Modoki) like the one in 2009/10. See the maps shown in Figure 1. Where does the 2015/16 El Niño fall and why does it matter?

Figure 1

If the 2015/16 El Niño has the characteristics of an El Niño Modoki (Central Pacific El Niño), it may provide a possible answer for why some parts of California have not been flooded like they were during the 1997/98 El Niño…have not had the drought conditions lessened as much as had been hoped when the 2015/16 El Niño developed last year. See the ABC News article El Nino Leaves Much of Drought-Stricken California in Dust. (Yes, I understand there are forecasts for significant rainfall and snow in California this weekend. See California Drought: Big weekend storms coming with rain, snow, strong winds in the San Jose Mercury News.) Also see Is El Niño AWOL? at the NOAA ENSO Blog for other discussions.

ASHOK ET AL. (2009) – THE PAPER THAT DEFINED EL NIÑO MODOKI AND THE EL NIÑO MODOKI INDEX

One of the methods used to define a Central Pacific El Niño is the El Niño Modoki Index created in Ashok et al. (2007) El Niño Modoki and its possible teleconnection. The El Niño Modoki Index was prepared in that paper using HADISST sea surface temperature data (from the UK Met Office) for three regions in the tropical Pacific. See Figure 2. Ashok et al. write (their Figure numbers):

Based on the EOF2 pattern presented in Figure 2b and the PC2 time series shown in Figure 3, we derive an El Niño Modoki index (EMI). Because of the unique tripolar nature of the SSTA, the index is defined as follows: EMI= [SSTA]A-0.5*[SSTA]B-0.5*[SSTA]C (1) The square bracket in Equation (1) represents the area-averaged SSTA over each of the regions A (165E-140W, 10S-10N), B (110W-70W, 15S-5N), and C (125E-145E, 10S-20N), respectively.

Figure 2 – Corrected

UPDATE: Blogger tadchem on the cross post at WUWT noted that my original Figure 2 (Cell b of Figure 2 from Ashok et al. (2007)) showed the wrong coordinates on the map for Region B. I missed that error in the original Figure 2. I’ve replaced that map with one from the JAMSTEC El Niño Modoki webpage. Thanks tadchem. [End update.]

The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has an El Niño Modoki webpage, with links to El Niño Modoki Index data in monthly and weekly formats. Notes: The sea surface temperature data and El Niño Modoki Index data on those webpages have not been standardized, while Ashok et al. (2007) standardized the El Niño Modoki Index when they determined the threshold for an El Niño Modoki. The monthly El Niño Modoki Index data at JAMSTEC appears to be based on HADISST data and the weekly data should be based on the standard weekly Reynolds OI.v2 sea surface temperature data.

Back to precipitation…

Figure 3 is Figure 11 from Ashok et al. (2007). It includes correlation maps of December-January-February (DJF) precipitation with two ENSO indices. Cell b includes the correlation of precipitation with one of the commonly used ENSO indices, NINO3 region sea surface temperature anomalies. Of course, NINO3 sea surface temperature anomalies were quite high this El Niño. See the graph here, comparing the NINO3 sea surface temperature anomalies in 2015/16 to those in 1997/98. So, based on the positive precipitation correlations in California with NINO3 data, we might have expected a higher-than-normal chance of precipitation in California this boreal winter.

Figure 3

However, Cell a shows that there is no correlation of DJF California precipitation with the El Niño Modoki Index.

WHERE DOES THE CURRENT EL NIÑO FALL ON THE EL NIÑO MODOKI INDEX?

Unfortunately, being based on HADISST data, the monthly El Niño Modoki Index lags a few months. So I’ve included the weekly El Niño Modoki Index data for the 2-year periods of 1997/98, 2009/10 and 2015/2016 (year-to-date) in Figure 4. I’ve included a dashed line at 0.7 (Standardized EMI) as the threshold of an El Niño Modoki. It must be kept in mind, however, that Ashok et al. used that 0.7 value for the 4-month period of June-July-August-September (JJAS) and the 3-month period of December-January-February (DJF).

Figure 4

The 1997/98 El Niño was a strong East Pacific El Niño, so it registered a negative El Niño Modoki Index value. The 2009/10 El Niño rises well above the 0.7 threshold for an El Niño Modoki, and referring back to Figure 1, we can see that the 2009/10 El Niño was a Central Pacific El Nino.

The 2015/16 El Niño has cycled back and forth across the threshold of an El Niño Modoki for about the last 18 weeks.

Using a 12-week running-mean filter, Figure 5, we can see that the most recent 12-week average is above the 0.7 threshold of an El Niño Modoki. In other words, for the months of December-January-February (DJF), the 2015/16 El Niño should qualify as an El Niño Modoki.

Figure 5

BOTTOM LINE

There had been hope, and there continues to be hope, that the 2015/16 El Niño will help to lessen the drought conditions throughout California. If the precipitation in California does not meet expectations, I suspect some researchers will point to the El Niño Modoki Index as the basis for those shortcomings.

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