September 20, 2020 Seasonal Forecasts: Part II, NOAA Comparison with JAMSTEC

Written by Sig Silber

We published the NOAA Long-Term Forecast Part I  on September 18, 2020. Here we compare the NOAA forecast for Alaska and CONUS with the JAMSTEC forecast. It is easier to see the disagreements by comparing the maps which we show side by side in a table with a summary of the comparison. Obviously, the farther out you look, the less confidence you have in the forecasts and thus the differences in the forecasts. Also provided are the JAMSTEC World Forecasts. In Part I, we showed the differences in the assumptions with respect to ENSO (JAMSTEC is more convinced than NOAA that we will have more than a borderline La Nina) which may explain some of the differences in the forecasts. JAMSTEC sees the La Nina being shorter in duration. We have no way of knowing for sure but it seems like JAMSTEC is showing the Jet Steam to be farther north than NOAA has it.

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C. Comparison of the NOAA and JAMSTEC Forecasts

Below is the comparison of the NOAA and JAMSTEC temperature and precipitation forecast maps for three time-periods and from left to right the NOAA forecast for Alaska and CONUS (the contiguous mid-latitude U.S) and then JAMSTEC for North America (which includes Canada and Mexico). The NOAA forecast maps can be clicked on to enlarge. The JAMSTEC maps in the table are not set up to be clicked on to enlarge (because we have no larger version of them). We have concluded that these smaller images work fine for comparison purposes.  Later in the article, we show the World Forecasts.

JAMSTEC works with three-month seasons: Fall: SON, Winter: DJF and Spring: MAM. Out of each three months, there is one where the months in the two forecasts align perfectly for the first time period. This is not one of those months so for the first period we are comparing SON for JAMSTEC and OND for NOAA. It is not ideal but I doubt that it makes a big difference. For the two subsequent seasons, we are comparing the same months.

In addition to the value of comparing the JAMSTEC and NOAA forecasts, the JAMSTEC forecast by showing North America provides more context for the Alaska and CONUS Forecasts as the temperature and precipitation patterns cover North America, not just Alaska and CONUS.

Map Comparisons and our Comments

Temperature

	NOAA Alaska Plus CONUS	JAMSTEC North America
FALL OND 2020 for NOAA SON 2020/2021 for JAMSTEC
WINTER DJF 2020-2021
SPRING MAM  2021

Precipitation

	NOAA Alaska Plus CONUS	JAMSTEC North America
OND 2020 for NOAA SON 2020/2021 for JAMSTEC
WINTER DJF 2020-2021
SPRING MAM 2021

JAMSTEC World Forecasts

This month our comments are taken directly from the JAMSTEC discussion (the translation may be a bit stilted). Now that JAMSTEC has a number of models, we are not exactly sure of the best one to display and the forecasts vary somewhat model to model. We are using their newest model and the version which does not average the result with the original model. We assume that the JAMSTEC discussion considers their various models and provides their assessment having considered their full suite of models. The wording in the JAMSTEC discussion is stilted possibly because it is a translation from the Japanese version.

Fall which is SON 2020 for JAMSTEC

Temperature

Precipitation

Winter which is DJF 2000/2001

Precipitation

And Spring which is MAM 2021

Temperature

Precipitation

D. Conclusion

As usual, there is a disagreement between NOAA and JAMSTEC. If we were looking at North America as a whole, the disagreement would be less than when we look at only CONUS as some of the features shown by NOAA to be in CONUS are shown by JAMSTEC to be a bit farther north.

It is difficult to relate the differences in the forecast to differences in assumptions on ENSO. But JAMSTEC tends to consider other factors that may not be considered by NOAA. On the other hand, the new versions of the JAMSTEC model are early in their usage and may not have been fully calibrated. I do not think I have gone into it in detail but when it comes to models we need to recognize that there are limitations and NOAA and JAMSTEC use very different approaches which I have summarized in the below table.

So it is kind of a pick your poison choice. But both agencies have great skill at employing approaches that have inherent limitations.

JAMSTEC Discussion

We provided the full NOAA Discussion in Part I. The much shorter JAMSTEC Discussion was published on September 16, 2020 and we included it in Part I but we are repeating it here also.

Sep. 16, 2020 Prediction from 1st Sep., 2020

ENSO forecast:

Observation shows that the La Niña continues to develop. The SINTEX-F predicts that the La Niña-like condition will persist in this year with weak anomaly in December-February.

Indian Ocean forecast:

Observation shows a weak negative IOD-like condition in the tropical Indian Ocean at present. The ensemble mean prediction suggests that the present condition will return to a neutral state from mid-autumn.

Regional forecast:

On a seasonal scale, the SINTEX-F predicts that most part of the globe will experience a warmer-than-normal condition in boreal autumn, except for southwestern Australia, India, Thailand, and northern Africa. In boreal winter, the model also predicts that most part of the globe will experience a warmer-than-normal condition, except for western Canada, northern Brazil, southern Australia, and India.

As regards to the seasonally averaged rainfall in boreal autumn, a drier-than-normal condition is predicted for a southern/central part of U.S.A., Mexico, Chile, La Plata Basin, West Africa, southern China, some parts of the Russian Far East, most part of Southeast Asia, and most part of Europe. In contrast, western Canada, most part of the South American Continent, India, some parts of southern Africa, Philippines, Indonesia, northern Europe, and northern Russia will experience a wetter-than-normal condition. In boreal winter, a wetter-than-normal condition is predicted for Alaska, most part of U.S.A., northern part of the South American Continent, Philippines, and some parts of Indonesia. In contrast, southeastern U.S.A., Mexico, La Plata Basin, most of Australia, some part of southern Africa, most part of Europe, and some part of Southeast Asia will experience a drier-than-normal condition.

The model predicts most part of Japan will experience warmer-than-normal condition in autumn and winter as a seasonal average. As regards to the seasonally averaged rainfall, most part of Japan will experience drier-than-normal condition in autumn. In winter, southern part of Japan will experience drier-than-normal condition.

E. JAMSTEC, NOAA, and other Agency Indices

We showed these in Part I which can be accessed here.

But we are showing a few here that are especially important

NOAA has their own proprietary model which they rarely use. It is not exactly clear why they shun their own model.

In most cases, I freeze the models as of the date of publication but for this one, I am going to just let the above model run so if you refer to the article in the future, the values in the above may not relate well to the discussion. But I am doing that so if you refer to this article in say two weeks, you will see if there has been any change in the forecast from this model.

Comparison models would include JAMSTEC

And the Australian BOM

I am going to introduce another graphic which is the current view of the subsurface along the Equator.

This is static but without showing the earlier views I can tell you that the warm anomaly is undercutting the cool anomaly. So the issue is when will it displace the cool anomaly. Not an easy question to answer.

Now let us look at the SOI which is used to confirm the La Nina.

From the BOM Glossary

The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niño or La Niña events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin.

Sustained negative values of the SOI below −7 often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds, and a reduction in winter and spring rainfall over much of eastern Australia and the Top End. You can read more about historical El Niño events and their effect on Australia in the Detailed analysis of past El Niño events.

Sustainted positive values of the SOI above +7 are typical of a La Niña episode. They are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. Together these give an increased probability that eastern and northern Australia will be wetter than normal. You can read more about historical La Niña events and their effect on Australia in the Detailed analysis of past La Niña events.

I have not frozen the SOI graphic so we will be able to track the SOI in this article.