Thursday, November 5, 2015

Weekly SST indices

I've been looking at SST data that I could track on a sub-monthly timescale, in the way I use NCEP reanalysis. I initially tried the daily AVHRR data which I download and plot here. That worked, but oiSST V2 draws on a wider range of sources, and is widely used for monthly indices. NOAA produces weekly gridded data, and gives tables of weekly data for the various NINO regions.

But they don't seem to give weekly averages for the globe, nor for latitude bands, and I think that would be useful. So I have been experimenting with downloading and integrating the grids, as with NCEP, with a view to adding to the latest data page.

I tested the NINO region integrals against the NOAA figures here, and they match to the one decimal place that NOAA provides. So I think the anomaly formation, using the NOAA daily climatology here, and the spatial integration are OK.

In this post, I'll show an active plot for the weeks to date of 2015 for the globe, the main NINO regions (details below) and four latitude bands (SH 60°-tropic, SH tropics, and NH likewise). Later I hope to give longer duration plots, and WebGL global maps.

The NINO regions are (NOAA map here):
  • NINO4 (5S-5N, 160E-150W)
  • NINO3.4 (5S-5N, 170W-120W)
  • NINO3 (5S-5N, 150W-90W)
  • NINO1+2 (10S-0, 90W-80W)

The plot is a version of the active plot in the data page. I doubt that you'll need the x-axis drag facility, with only 43 weeks, and the trend won't be meaningful. The plot starts with everything showing, but you can toggle off data that you don't want. You may need to stretch the y-axis by draging (vertically) to the left of it. The anomaly base is 1981-2010. Here is is:


  1. Nice tooI. I have followed the OISST weekly for a while. It is interesting to compare the el Nino development 1997 and 2015 week by week. This year's el Nino is more central than that of 1997. Right now it is similar in Nino 3.4, stronger in Nino4, but weaker in Nino 1.2 and 3.

    A good place to follow SST in near real-time is here
    The common reanalyses does not provide SST (I believe) but CDAS does. If I get it right CDAS provide initiation conditions for weather models, and should thus be a kind of reanalysis.
    However, I trust OISST weekly much more than CDAS..

  2. By chance I was playing with the OISST daily grids, and also with the data in KNMI.

    For some reason I get a different (larger) trend integrating the daily grids using cdo compared to using KNMI. I haven't had time to look into why.

    1. More information: Apparently OISST monthly (on KNMI) is an older product without ship-buoy correction. OISST daily is a newer product including a correction. Hence the difference. So we should be using the daily product.


    2. Kevin,
      I believe I am using daily data, though it is published as a weekly average. The README file is here
      oisst_v2 seems to refer to the daily, the month data is tagged oimonth_v2.

  3. I noticed at NOAA/CPC that the latest OISST weekly Nino 3.4-anomaly is +2.8 ( Nov 4). Hence, the present el Nino has reached the peak level recorded in 1997-1998 (at Nov 26)
    The graph above show lower values, but I guess that it hasn't been baselined by the full 1981-2010 period, only from 1990.

    1. Olof,
      No, the baseline was correct - I made a mistake with the longitudes of region 3.4. I've corrected that now. I haven't updated the data here - I'm still planning to set up a regular mechanism.

  4. From what I read, one of the curious things about the ONI (Oceanic Niño Index) which is different from other indexes is that it is not all based on the same baseline. According to the NOAA site Description of Changes to ONI, each five year period is based on a different baseline. This is because the El Niño effect is based on a departure from average SST in the 3.4 region in that period, and that average has been changing (increasing) over the years.

    So, for example, the 1957 El Ninõ ONI is measured in terms of the anomaly from the 1941-1970 baseline, whereas the 1997 EL Niño ONI was measured in terms of the anomaly from the 1981-2010 baseline. This years ONI will eventually be recorded based on the 1995-2025 baseline, which we don't have yet. In the meantime it is calculated based on the most recent climatology (1981-2010), then updated every five years until the end of the target time period. That means that the current ONI will be adjusted a few times, likely downward, over the next 15 years. The next adjustment will be in 2016.

    You can see the effect in the NOAA data file

    The October 1957 3.4 index was 1.12 indicating that the SST of 27.39° was 1.12° above the baseline of 26.27°
    The October 2015 3.4 index was 1.85 indicating that the SST of 28.68° was 1.85° above the baseline of 26.83°

    More detail on the calculation is in this 2013 article on ONI at

    This is from my reading of the NOAA site. I'm not a climate scientist or expert but it seemed interesting and relevant.

    1. Yes, that's interesting. I think they are concerned with El Nino as an event - how different is it from the temperatures of the times? As they say, if you have a fixed reference value, then after decades of warming, the definition will create a permanent El Nino.

      It's relevant to the statements that because Nino3.4 is warmer than ever before, 2015 is the strongest ever El Nino. This thinking would say, maybe not, because the current climatology is warmer.

  5. Also of course you cannot fully detrend the data when you are only halfway through the (30 year) baseline period.

    If skeptics point out in 2026 that the 2014-2015 El Niño wasn't as strong as contemporary reports indicate, we'll know why.

  6. weekly and daily oisst are different products:

    nino 3.4 based on weekly oisst is running warmer than both ersst and daily oisst in recent months, see slide 10:

  7. OISSTv2 daily is now available (reduced to monthly resolution) on KNMI here:

    It's under monthly observations, labelled "1982-now: 1/4° NOAA OI v2 SST"

    1. Nice job over at Climate Etc with the Buoy only SST..

      I have understood that OISST daily is the most advanced satellite hybrid product, and show the largest trend of all SST indices during the alleged "hiatus" period.
      (It seems to be a frameshift error in OISST daily at Knmi, i e data should be shifted back one month, we can't be in December already)

      BTW, I'm looking forward to the Oct update of Hadcrut kriging. Will it be the warmest monthly anomaly ever? Hadcrut4 posted very early this month, already by 20 Nov,

    2. Oops - it hadn't even occurred to me to check this early! I've started it running. Final result tomorrow.

      I'll check the off-by-one error and notify Geert.