Australia: The Land Where Time Began

A biography of the Australian continent 

Storm activity - the Medieval Warm Period and the Little Ice Age

The atmospheric pressure of the Earth at sea level is 1,013.25 millibars (mb). Differences in a number of factors cause departures from this sea level pressure (SLP), some of which are temperature, moisture content, dust content of the atmosphere, and these departures lead to areas of high and low pressure over the surface of the Earth.

In the Northern Hemisphere when the forces that result from the rotation of the Earth are added to these factors the winds rotate in a counterclockwise direction around areas of low atmospheric pressure and rotate in a clockwise direction around areas of high atmospheric pressure. In the Southern Hemisphere the wind direction around highs and lows move in the opposite direction to those in the Northern Hemisphere. According to the author1 it is expected that when the winds blow towards the ice core site on central Greenland from the ocean they transport chemicals such as sea salt to the ice core site, the lower the pressure of the low pressure system the stronger is this transport of chemicals. If the winds are associated with a high pressure system over land, then when the atmospheric pressure increases the transport of chemicals sourced from the land, such as dusts, also increases. If this wind is moving in the direction of Greenland it carries the dusts to the ice core site.

In winter sea salt (ss) sodium (Na) is transported from the ocean to the GSP2 site, and in spring non-sea salt (nss) potassium (K) is transported to the GISP2 site from the land. Where the mean pressure are the sea surface is 1.000 mb this change in SLP indicates changes in atmospheric circulation strength. The Icelandic Low, a low pressure field over the Atlantic in winter, is responsible for driving the winds counterclockwise off the North Atlantic towards Greenland and the SLP decreases. The Siberian High, SibHi, a regions of high pressure over the general area of Siberia, is responsible for driving winds towards Greenland in a clockwise direction as pressure increases in spring. When changes in ssNa and nssK are compared to the changes in SLP in winter and spring, respectively, over the Northern Hemisphere for the period for which there is a data overlap between the instrumental data and data from the GISP2 ice core, there are very strong associations between ssNa and the Icelandic Low and nssK and the Siberian High (Meeker & Mayewski, 2001).

When the 1,080 sites from which average monthly SLP in the Northern Hemisphere records over the period of instrumental records, from 1899-the present, are compared to the record in the GISP2 ice core the results allowed the autnor's1 team to forecast SLP changes as far back as part of the Medieval Warm Period (MWP) to the Little Ice Age (LIA).

It was revealed by the first set of figures that the sites with the highest correlation between the SLP and the ion chemistry, the strongest associations being between ssNa and the Icelandic Low (IceLow) and the nssK (dust) and the Siberian High; based on the comparison of data covering the period from 1899-1987, the year instrumental records of SLP began and the most recent ice core data point used in the comparison.

The Icelandic Low. In winter this is the dominant atmospheric surface pattern over the North Atlantic. It pushes oceanic air into central Greenland, and is also the source of the "Nor'easters" that often hit the coast of New England. Marine storms are at their most intense in winter, the season in which ssNa reaches its maximum in the snow on central Greenland. During spring in Siberia, March-May, nssK sourced from continental areas, that is most strongly associated with increased SLP changes over high pressure regions of Siberia. Most nssK is transported to central Greenland by winds originating in the Siberian High in spring.

In spring the strength of the Siberian High has a very close association with the nssK, the deposition of nssK at the GISP2 site results from stronger winds from Siberia to Greenland that are caused by the increased Siberian High SLP. Compared with the 1,000 mb of the mean SLP the changes in ssNa and nssK, though appearing to be small, up to 16 mb for the Icelandic Low and the Siberian High pressure of 3 mb, they are different enough from the mean SLP to cause major storms.

A statistical tool that is often used in meteorology, an empirical orthogonal factor (EOF) analysis, is made use of to compare the association from year to year, searching for associations, generating an approximation to both records, chemistry and SLP.

When ssNa is compared with the Icelandic Low, and nssK is compared with the Siberian High, matches were found to be close to 70 % of the signal. As some concentrations of chemicals are not related to SLP change the fit is not perfect. Atmospheric moisture content, topography, and temperature, e.g., are also factors controlling chemical concentrations at the drill site, though the fit is still very close, which allows the concentrations to be used as a proxy for the SLP behaviour back through time before the actual measurement of the SLP. The behaviour of these systems over the MWP and the LIA can be examined using these chemical proxies for the Icelandic Low and the Siberian High. Though they found variability for both systems, they found the the variability during the LIA was generally greater, suggesting that during the LIA the storminess over Siberia and the North Atlantic were more variable and intense. The SLP levels of the present still haven't returned to those of the MWP. According to the author1 they took this to indicate strongly that atmospheric circulation patterns that were established during the LIA are continuing at the present. Comparing the proxies for the Icelandic Low and the Siberian High against 14C residuals, a proxy for solar variability) has demonstrated how important the solar output is on the control of climate.

It has been found, by Karl Kreutz, University of Maine, that there is a similar situation in West Antarctica, finding that a proxy for the Amundsen Sea Low, described as an Antarctic version of the Icelandic Low. The feature was weaker during the MWP and has yet to return to the condition of the pre-LIA period. It has also been found that ssNa increased in Antarctica and Greenland sites at close to the same time in the early 1400s.

According to the author1 it is not known if the LIA started in all parts of the world at the same time, or in different regions, or whether it was the same in all regions or failed to occur in some regions. When this distribution of the LIA is known it will be possible to determine its cause. A very strong control factor, such as the output of energy from the Sun, will be indicated if it began at the same time in all parts of the world. If it turns out that the LIA started in a phased manner, researchers will need to visit the earliest site at which it occurred to determine the cause or causes. The author1 says that one thing is clear "the LIA is not dead!"

As early as the late 19th century to the early 20th century the temperatures of the LIA, that were relatively cooler, were beginning to dissipate, but the atmospheric circulation patterns of the LIA are still active, a full return to the types of patterns that were characteristic of the MWP having not yet occurred. According to the author1 if the LIA had ended, in terms of how it began, it would be the most abrupt RCCE (Rapid Climate Change Event), as well as one of the shortest of the last 110,000 years. Based on the record found in the GISP2 ice core the author1 suggests it would not be anomalous if it took another 200-500 years to complete, as the length of RCCEs is typically 1,000 or more years, and at this point, the LIA is only about 600 years old.

According to the findings from the GISP2 ice core and what is known about RCCEs the Earth should still be cooling, but it is actually warming. Part of the debate about the current bout of global warming is whether or not the present warming is the result of a natural cycle. It has been questioned whether the warming that has taken place over the last 100 years is abnormal, as the degree of warming that has occurred "is well within the bounds of natural variability" (Suplee, 2000). Some claim, correctly, that RCCEs move the climate dramatically towards cooling or warming, though variability can still occur within the period of the RCCE. While the climate is moving in the cooling direction by the LIA, there is the possibility that there would be warming periods within it. It is known that the LIA was/is a period of relative cooling, though of variable temperature, and it must be remembered that climatic variability operates on many scales.

Though the climate was not consistently colder during the LIA or warmer during the MWP. As the circulation patterns of the atmosphere are sill in the LIA state of activity, even though the temperatures have risen, supports the contention that the warming that occurred in the 20th century are, at least in part, not caused by natural climate controls.


Sources & Further reading

  1. Mayewski, Paul Andrew @ White, frank, 2002, The Ice Chronicles: The Quest to Understand Global Climate Change, University Press of New England. 


Author: M. H. Monroe
Last Updated 05/02/2013

Climate Change

Climate - multiple controls

Rapid Climate Change Events

Rapid Climate Change Events (RCCEs) "Rickies" in the Holocene  
Journey Back Through Time
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading