Changes in high-altitude winds over the South Pacific produce long-term effects

Posted: November 5, 2019 by oldbrew in Cycles, Ice ages, Natural Variation, Ocean dynamics, research, wind
Tags: ,


‘Long-term’ here means really long-term. The 21k year precession period quoted looks like that of the perihelion.

In the past million years, the high-altitude winds of the southern westerly wind belt, which spans nearly half the globe, didn’t behave as uniformly over the Southern Pacific as previously assumed.

Instead, they varied cyclically over periods of ca. 21,000 years, reports ScienceDaily.

A new study has now confirmed close ties between the climate of the mid and high latitudes and that of the tropics in the South Pacific, which has consequences for the carbon budget of the Pacific Southern Ocean and the stability of the West Antarctic Ice Sheet.

The study was prepared by Dr Frank Lamy, a geoscientist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, together with researchers from Chile, the Netherlands, the USA and Germany, and has just been released in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

Changes in the southern westerly wind belt (SWW) produce fundamental effects on the intensity and position of the Antarctic Circumpolar Current, which is the world’s largest ocean current and shapes ocean circulation worldwide.

In this regard, one key factor is the wind-driven upwelling of CO2-rich deep-water masses, which, due to their comparative warmth, influences both the stability of the West Antarctic Ice Sheet and the carbon budget of the Southern Ocean.

On the basis of sediment cores, the team of researchers investigated precipitation-driven changes in sediment input in the Pacific off the coast of Chile.

Assessing the past 1 million years, they identified what are known as precession cycles: changes caused by natural variations in the Earth’s orbital parameters; in this case, cyclical changes in the rotation of its axis that occurred roughly every 21,000 years.

Changes in these and other orbital cycles are generally considered to be a major driver for the alternation between extended glacials and interglacials over the past million years.

“At first it was hard to explain why the changes in the composition of continental sediments up to the southern margin of the Atacama Desert in northern Chile indicated pronounced precipitation variations over the 21,000 years, but less pronounced variations throughout the longer-term cycles of interglacials and glacials,” recalls sedimentologist Helge Arz (Leibniz Institute for Baltic Sea Research Warnemuende).

The nature of the phenomenon was ultimately deciphered with the aid of American climate modeller John Chiang (University of California, Berkeley): “Our climate models show that the precipitation changes recorded in the sediment cores are connected to the configuration of high-altitude winds over the subtropical Pacific. In this regard, the division of the high-altitude wind changes into a northern, subtropical branch, a middle branch, and a subpolar branch in the course of the 21,000-year cycle.”

Full article here.

Comments
  1. Ian Wilson says:

    Modified to get around a known bug with WordPress.

    The reason why it is important to use a reference frame that is fixed with respect to the Earth’s orbit (i.e. anomalistic year), rather than one which is fixed with respect to seasons (i.e. tropical year) or the stars when studying the full effects of, the long term alignments between climate forcing factors and the seasons, upon climate

    The most noticeable short-term variation that is observed in the Earth’s weather, other than the diurnal variation, is the annual seasonal cycle. The progression of this cycle is marked by the apparent change in the declination of the Sun as seen from the Earth from 0° at the Vernal Equinox to +23 ½° at the Summer Solstice, back to 0° at the Autumnal Equinox and then onto –23 ½° at the Winter Solstice, before returning to 0° at the Vernal Equinox [N.B. northern hemisphere conventions are used throughout this post]. This apparent change in declination of the Sun is a direct result of the combined effects of the 23 ½° tilt of the Earth’s axis to the ecliptic plane and the annual motion of the Earth about the Sun.

    Currently, the Sun passes through the Winter Solstice around December 21st of each year, which is close to the time when the Earth is closest to the Sun at Perihelion on January 3rd. What this means is that the Earth is closest to the Sun (147.1 million km) when the land dominated northern hemisphere is experiencing winter and furthest from the Sun (152.1 million km) when it is experiencing summer. Consequently, the northern hemisphere receives 1.0 ̶ (147.1/152.1)^2 = 6.5% less solar flux in summer than it does in winter because of the variation in Earth-Sun distance between the seasons. Hence, the net effect of the current proximity of the Winter Solstice to Perihelion is to moderate temperature differences between summer and winter in the northern hemisphere.

    In contrast, the Earth is closest to the Sun when the southern hemisphere is experiencing summer and furthest from the Sun when it is experiencing winter. Therefore, the net effect of the current proximity of the Winter Solstice (i.e., the Southern summer) to Perihelion is to exaggerate temperature differences between summer and winter in the southern hemisphere. Fortunately, the southern hemisphere is dominated by oceans rather than continents, which has the effect of moderating the more extreme seasonal temperature differences between summer and winter.

    Hence, the overall net effect of the current proximity of the Winter Solstice to Perihelion, combined with the imbalance of land and water between two hemispheres, produces an overall planetary-wide moderation of temperature differences between summer and winter.

    This is not always the case, however, because both the tilt of the Earth’s rotation axis and the Perihelion of the Earth’s orbit slowly drift with respect to the stars. The combined effect of these two motions means that in ~ 10,500 years from now, the Winter Solstice will occur on a date that is close to Aphelion. Under these specific conditions, the Earth will be closest to the Sun when the northern hemisphere experiences summer and furthest from the Sun when it experiences winter. This will increase temperature differences between summer and winter in the land-dominated northern hemisphere – significantly changing the Earth’s climate.

    Hence, the Earth should have a long-term climate cycle whose length is determined by the time it takes for the date of the Perihelion of the Earth’s Orbit to return to the date of either the Winter or Summer Solstice. The length of this climate cycle is determined by:

    a) The general precession of the Earth’s rotation axis with respect to the stars which currently progresses at a rate of 50.28796 arc-seconds per year in a retrograde direction.

    b) The apparent period of precession of the perihelion of the Earth’s orbit with respect to the stars which currently progresses at a rate of 11.615 arc-seconds per year in a prograde direction.

    The rate of general precession of the Earth’s rotation axis is such that it takes roughly 25,770 years for it to complete one full cycle with respect to the stars (in a retrograde direction), while the apparent period of precession of the perihelion of the Earth’s orbit takes roughly 111,580 years to complete one cycle with respect to the stars (in a prograde direction).

    In effect, this means that time required for the Earth’s seasons (as expressed by the tilt of the Earth’s rotation axis with respect to the ecliptic) to complete one full cycle of the Earth’s orbit (e.g. to move from Perihelion to Perihelion) is approximately equal to:

    (111,580 * 25,770) / (111,580 + 25,770) = 20,935 ~ 21,000 years

    An important consequence of the existence of this 21,000-year climate cycle is that if you want to study the full effects of, the long term alignments between climate forcing factors and the seasons, upon climate, it is important that you do so in reference frame that is fixed with respect to the Earth’s orbit (i.e. anomalistic year), rather than one which is fixed with respect to seasons (i.e. tropical year) or the stars.

  2. Ian Wilson says:

    Sorry, The equation should be:

    (111,580 * 25,770) / (111,580 + 25,770) = 20,935 ~ 21,000 years

    [mod] fixed

  3. Ian Wilson says:

    And, of course, the intricacies of the Earth’s orbit sometimes splits the 21,000-year climate cycle into a 23,000-year cycle and a weaker 19,000-year cycle.

    e.g. Variations in the strength of the North African Monsoon have been found to be strongly related to the stronger 23,000-year precessional cycle.

  4. oldbrew says:

    Ian Wilson says: November 6, 2019 at 2:11 pm

    The ratio of these three cycles is 3:13:16 or very close.
    https://tallbloke.wordpress.com/2016/02/01/why-phi-a-unified-precession-model/

    Obliquity being variable of course, within a limited range.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s