Saturday, 20 August 2011

Some external forcing factors with the ability to distrupt the general ocean circulation

As I have briefly discussed before, with reference to Heinrich events, the current ocean circulation pattern that exisits has not always do so and alterations to it in the past have resulted in huge shifts in the worlds climate. As well as those previously mentioned, there are some other external forcing factors that have the ability to affect the oceanic and atmospheric circulation. I think some of this stuff, like volcanic influence and Milankovich, comes up in the A2 syllabus although I don't know how much detail we go into - probably a bit more than I have done here, but hopefully the basics will still be useful!

At varying points in time, external forcing factors have and will continue to provoke variability in the ocean circulation. Understanding them is, again, crucial as some explain the past changes that occurred, whilst others endorse future predictions.

There are two types of Precession: this diagram illustrates Axial Precession
 and this alters the hemispheric distribution of insolation. For more on
Milankovich and the individual cycles see seperate blog post on
The race for a million year old ice sample 
Many proxy-climate records suggest that the Milankovich ‘pacemaker’ has driven alterations in the oceans, atmosphere and the cryosphere; due to the moderation in the hemispheric distribution of insolation provoked by the three cycles, precession (23ka and 19ka), obliquity (41ka) and eccentricity (100ka). The spectral analysis of d18O variations present in marine carbonates over the last 10Ma, contain signals of all three cycles moderating high northern latitude insolation variations.  Milankovich used this to explain the initiation and termination of ice ages and his theory states that the alterations in high-latitude insolation in the Northern Hemisphere are crucial in regulating the survival of snow through successive summers to warrant ice accumulation; a theory authenticated by the coincidence of glacial terminations and rapid increases in the solar insolation values of high latitudes in the Northern Hemisphere. The control Milankovich cycles exercise over hemispheric insolation distribution therefore dictates, although on a delayed timescale, the extension/shrinkage of ice masses (whether that be sea ice or ice-sheet/shelves) and it is because of this that they alter ocean circulation; although it is believed that precession has the largest influence. Generally speaking, the cycles range between two extremes, one favouring glaciation and the other deglaciation. When they favour glaciation, the THC is forced to downwell further south and thereby reduces northwards heat transfer and the associated ablation processes, to only further accelerate cooling. During glacial termination, the opposite is true and baseline increases in NADW intensity, beginning approximately 18ka, that parallel increasing Northern Hemisphere insolation, have been inferred from the RC11-83 South Atlantic Ocean floor sediment core. Many believe that it is the alterations provoked by this extrinsic forcing factor to the thermohaline circulation that drives, all atleast contributes, to the temperature changes that occur during transitions between glacial and interglacials.

Spectral analysis of variations of the chemical composition of ice cores have revealed that cycles shorter than those of Milankovich have occurred (periodicities of 11100, 6100 and 1470 years); with these sub-Milankovich cycles believed to be overtones generated within the climate system due to the influence longer cycles. The sub-Milankovich cycles can also be classed as an external forcing factor and can be attributed to shorter, abrupt alterations to the oceanic circulation. Precessional-driven climate alterations are believed to be linked to the 11100 year cyclicity which is being held responsible for temperature maximums being experienced every 11100ka in continents straddling the tropics. The 6100 year cyclicity seems to have a relationship with Heinrich events and other coolings such as the Little Ice Age and it is thought to only be amplified by ice-sheet presence. The shortest periodicity, because of the lethargic nature of ice-sheets, presumed to communicate readjustment of atmospheric circulation, is possibly due to solar output variations; something suggested by 14C records in tree-rings. When the sun is at its most ‘’energetic’, the Earth’s magnetic field is strengthened, blocking more cosmic rays. 14C is formed when cosmic rays hit plants, therefore measured in tree rings; with high levels of 14C suggesting an ‘inactive’ sun. Bond documented increases in icebergs and ice drift coinciding with the increase in 14C, indicating the sun was weaker at such times. Alterations in the volume of ice-rafted debris, in North Atlantic, also coincide with the 1470 year cyclicity, although are only 1/10 the size of the inconsistencies witnessed during the last glacial. Overall, there is agreement amongst scientists that these millennial-scale cycles, which have also been detected within ENSO, have a solar inception affiliated to the THC.

Volcanic eruptions, principally those that disperse ash and SO2 into the stratosphere which are most commonly high latitude (due to lower tropopause) explosive eruptions, can provoke a period of cooling that can last for a few years; like Pinatubo did after it erupted in 1991, releasing 20 million tonnes of SO2, cooling the Northern Hemisphere 0.5°C over  5 years.  Despite their apparent ability to affect the climate, the effect they have on ocean circulation is debatable due to the variability produced by GCM. Some models show a connection between volcanic eruptions and a reduction in MOC intensity. However, these reductions appear to be small and short term and, this fact combined with other models not picking up the link above, suggest that the impacts of volcanic eruptions on ocean circulation, if there is one at all, is of little consequence and do not seem to pose a threat to the stability of the ocean circulation.

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