Roaring Forties Multidecadal Oscillation Explained

The Roaring Forties is the area of ocean that is notorious for being very dangerous. The seas are very rough due to persistent westerlies, which are winds blowing from the west. The lack land area in the Southern Hemisphere makes it more favorable for persistent westerlies. The Northern Hemisphere has more land mass, so oceans tend to be less rough as a result.

The Roaring Forties I define is between 40°S and 60°S. It is the entire ocean. It goes back to 1870 and anomaly is based on monthly 1880-2010 average. Keep in mind, any data before 1950 should be taken with a grain of salt as they are not directly measured. Data tends to be sparse as well. Here is the Roaring Forties Table.

Roaring Forties Multidecadal Oscillation Anomaly Value
Strong Cool = 0.5

I created a chart with smooth moving average of 120 months or 10 years. I notice that there is a cycle as there are warm and cool phases. There is even a period of neutrality, where it fluctuates between warm and cool.

RFMO_1870-2016_MonthlyValue

From the chart, there is a warm period in the 1870s to 1880s. Here are the cycles.

Warm
Prior To 1887

Cool
1887 to 1947

Neutrality
1947 to 1968

Warm
1969 to ?

Looking at the past, the cool phase lasts about 60 years, while neutrality is about 20 to 21 years. The warm phase started in 1969 and still going on, which is 47 years. Assuming about the cool phase, than it could last about 50 to 60 years. The last warm phase, which ended in 1887 probably went for 50 to 60 years.

What effects does Roaring Forties have? Let’s start with upper air pattern at 500 millibar level or 18,000 feet. Here is a spatial correlation charts. I used Linear Correlations in Atmospheric Seasonal/Monthly Averages

roaring-forties-correlation-500mb-geopotential-height-sh

roaring-forties-correlation-500mb-geopotential-height-nh

roaring-forties-correlation-500mb-geopotential-height-worldwide

Starting with the Southern Hemisphere, there is a strong negative correlation, especially over Antarctica. Cooler Roaring Forties favors ridging over Antarctica. If Roaring Forties are warmer, it favors troughing. It is opposite outside of Antarctica in which warmer Roaring Forties favors ridging over most of the Southern Hemisphere. Troughing usually favors a cooler Antarctica as it is a very cold place.

The Northern Hemisphere is affected by Roaring Forties. It is generally positive correlation for more ridging at 18,000 feet. The only negative correlation is over Aleutian Islands and North Central Siberia. Higher geopotential height or ridging favors warmer, while lower geopotential height or troughing favors cooler weather. So, what does Roaring Forties have an affect on temperature?

roaring-forties-correlation-surface-air-temperature-sh

roaring-forties-correlation-surface-air-temperature-nh

roaring-forties-correlation-surface-air-temperature-worldwide

For most of the Southern Hemisphere, there is a positive correlation between Roaring Forties and surface air temperature. The cooler the Roaring Forties are, the cooler it is. The cooler the Roaring Forties are, the cooler it is. Interestingly, there are pockets of negative correlation over Antarctica, Botswana, South Africa, Northern New Zealand, and Northern Australia. In that case, the warmer the Roaring Forties, the cooler it is. It is vice versa with a cooler Roaring Forties. Interestingly, Roaring Forties and air temperature over Antarctica is not as strongly correlated as in some parts of the Southern Hemisphere. As mentioned, cooler Roaring forties favors ridging, which should mean it is warmer.

A warmer Roaring Forties favors warmer temperatures for the Northern Hemisphere. The exceptions are North Pacific, Eastern Russia, Japan, Northern Africa, Nepal, India, and Southwest China. A warmer Roaring Forties favors cooler temperatures for them. Let’s look at Roaring Forties and zonal winds, better known as westerlies.

roaring-forties-correlation-surface-zonal-wind-worldwide

A warmer Roaring Forties is more favorable for westerlies for that part of the region. It is most favorable over the South Indian Ocean and Equatorial Pacific. It is no wonder why the Southern Ocean is rough even on clear days. Even the most hardened sailors can get seasick. Let’s look at sea surface temperature.

roaring-forties-correlation-sst-worldwide

This gets rather interesting. A warmer Equatorial Indian Ocean, Southeast Africa, and off the coast of South America are more favorable for warmer Roaring Forties. Ironically, parts of the Roaring Forties region have pockets of negative correlation in which one region is cooler than the others. The Roaring Forties feeds off the tropical waters. This is nature’s way to keep temperature in balance from being too hot or too cold as there are less tropical cyclones in the Southern Hemisphere. The Northern Hemisphere has more land mass and as a result the waters are much warmer. Nature compensates with the lack of westerlies at 40 degrees north and greater by tropical cyclones. It is no wonder that the Northern Hemisphere sees more tropical cyclones, especially in Western Pacific and Atlantic. Tropical cyclones remove warm air from the tropical areas to further north. The lack of tropical cyclones in Southern Hemisphere leads to strong westerlies and rough oceans in the Roaring Forties and southward to Antarctica.

Advertisements