north atlantic climatology report

7/31/2019 North Atlantic Climatology Report

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Climatology Report: The North Atlantic Basin

Introduction

The Atlantic Ocean is the second largest ocean on Earth with unique complex and interlinked oceanic

and atmospheric processes. The Atlantic Ocean is climatically the most sensitive ocean due to the

strong cyclogenesis related to the Gulf Stream and the formation of deep water in the north (Bigg,

2003: 217). The processes that occur within the ocean and above it cause unique climates for its

neighbours including the east coast of the United States to the west and western Europe and Africa to

its east, as shown in figure one, which vary on a wide range of time scales ranging from years to

millennia (Lorenzo, Taboada & Iglesias, 2009). This creates a great importance in examining the

effects on the climate in order to mitigate negative impacts that can occur to human environments.

Due to the complexity of the Atlantic Ocean, this report will focus on the North Atlantic; its processes,

how these create unique climates for the continents on its boundaries and how these processes may

change due to anthropogenic forcing creating different climates in the future. The report will also look at

the data sets available for the different processes as a long data record gives confidence in the ability

to draw conclusions and identify trends.

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Figure 1: The Atlantic Ocean and neighbouring countries (Encyclopdia Britannica Online, 2012)


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North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is one of the most prominent patterns of atmospheric circulation

dictating the climate from the eastern United States to Siberia and from the Arctic to the sub-tropical

Atlantic (Hurrell et al. 2003). It is defined as the alternation of atmospheric mass between the Icelandic

low and Azores high (Chaudhuri, Gangopadhyay & Bisagni, 2011) during the winter months.

The index has two states, positive and negative, and can alternate with variable frequency as shown in

figure 2. The positive phase occurs when the Icelandic low pressure system is lower than average and

the Azores high pressure system is high than average causing a stronger pressure gradient

(Sarafanov, 2009). This causes strong westerly winds and the jet stream to cross the eastern Atlantic,

causing less severe winters on the east coast of the U.S whilst bringing wet, warm storms to northern

Europe and dryness to the Mediterranean (Christopherson, 2006). Alternatively, the negative phase of

NAO is caused by a weaker pressure gradient than normal between the two pressure cells, reducing

westerly wind and jet stream strength causing storm tracks to move south in Europe bringing dry

conditions to the north and wet, warm storms to the Mediterranean with the eastern United States

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Figure 2: Standardised 3 monthly mean for NAO showing positive and negative cycles from 1980 Jan 2012 (NOAA, 2012)


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experiencing cold, snowy winters

(Christopherson, 2006). The

phenomena resulting from NAO to

specific regions is shown in table 1.

Monthly data is available online for

the NAO from NOAA from 1950 which

supports that there is no trend for the

variation between the two phases

though figure 2 shows that post-1995,

the NOA has mainly been in a

negative phase.

The NAO is concurrent with the Arctic

Oscillation (AO), which is

characterised by a redistribution of

atmospheric mass between the higher

latitudes and mid-latitudes, with a

positive phase corresponding to

reduced sea level pressure over the

Arctic causing increased westerlywind strength (Delworth & Dixon,

2000). The NAO is classified as the

largest changes that occur from AO in the mid-latitudes over the Atlantic in the Northern hemisphere

during winter months (Delworth & Dixon, 2000).

Thermohaline Circulation

Thermohaline circulation (THC), in the North Atlantic is the system of deep water currents driven by

density gradients created through sea surface temperature (SST) and salinity as part of the greater

oceanic circulation system (Lorenzo, Taboada & Iglesias, 2009) as shown in figure 3.

It begins with the creation of North Atlantic Deep Water, due to cold sea surface temperatures in the

North Atlantic, the density increases and the water body begins to sink at a rate of 15-20 Sverdrups

(Marotzke, 2000) and move southwards to 60S (Smithson, Addison & Atkinson, 2008), as shown in

figure 3, and Antarctic Bottom Water. As this water moves south, warmer surface waters move North

due to pressure gradients, with the gyres rotating anticyclonically attributable to the Coriolis force, with

the western margins of each gyre having a strong poleward current (Bigg, 2003: 17).

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Table 1: NAO related climate impacts on Atlantic Basin regions

(Marshall et al. 2001)


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Figure 3: Simplified sketch of the global overturning circulation system (Kuhlbrodt et al. 2007)

Figure 5: AMO Index: 10 year running mean of de-trended Atlantic SSTs north of the equator (Enfield,

Mestas-Nuez & Trimble, 2001).

THC is intrinsically linked with the NAO with changes in oceanic circulation under positive and negative

NAO cycles shown in figure 4. This is due to the NAO changing the intensity of THC by deviating from

normal wind stress relating to the positive and negative cycles (Lorenzo et al. 2008). Also, theweakening of THC causes a decrease in SSTs producing a weakening of pressure in the Icelandic Low

affecting the behaviour of the NAO (Lorenzo et al. 2008). Fluctuations found in THC can cause abrupt

climate change and is monitored as Atlantic meridional overturning circulation (MOC) (Marshall et al.

2001).

THC plays a significant role in determining the climate in countries surrounding the Atlantic Basin as it

transports heat and salt in large quantities towards the poles (Laurian et al. 2010) allowing Europe to

be abnormally mild for its latitude compared to other areas on the same latitude such as Canada,

though it has been found that poleward heat transport is more profound in the atmosphere than the

ocean outside of the tropics (Trenberth & Caron, 2001). THC also regulates the formation of sea ice in

the north Atlantic (Trenberth & Caron, 2001).

Atlantic Multidecadal Oscillation

The Atlantic Multidecadal Oscillation (AMO) is the 65 - 80 year cycle of North Atlantic SST variation,

varying with a 0.4C range (Enfield, Mestas-Nuez & Trimble, 2001).

Monthly data sets for AMO are available online through NOAA from 1856 to present, these are used to

show trends and are shown in figure 5.

The AMO affects the climate of the USA severely; during AMO warming periods, the U.S will see less

than average rainfall and it accounted for the Midwest droughts of the 1930s and 1950s (Enfield,

Mestas-Nuez & Trimble, 2001). Between AMO warm and cool phases, outflow from the Mississippi

River can vary by 10% whilst inflow to Lake Okeechobee can vary up to 40% (Enfield, Mestas-Nuez &

Trimble, 2001). AMO also affects the frequency and ferocity of Atlantic hurricanes as during warm

phases of AMO SSTs are higher and therefore the number of tropical storms maturing into severe

hurricanes is much greater than during cool phases (NOAA, 2009).

Future Predictions

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Figure 4: THC circulation changes in positive and negative North Atlantic Oscillation phases (Chaudhuri et al.2012).


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Anthropogenic forcing of the enhanced greenhouse effect causing warming of global temperatures, as

is currently occurring and predicted to increase, will have a large impact on the climate of the North

Atlantic basin.

Literature tends to agree that anthropogenic forcing, in particular greenhouse gases, ozone, aerosolsand solar radiance changes are likely to be responsible for a long-term trend in the increase of the NAO

index, with a positive correlation between greenhouse gas forcing and winter NAO index increasing

(Gillet, Graf & Osborn, 2003). This would cause climate impacts as described earlier.

Zhang & Delworth (2005) conducted experiments that found that a significantly weakened THC led to

environmental responses outside of the Atlantic; discovering that the Intertropical Convergence Zone

(ITCZ) shifted south in the tropical Pacific causing El Nio like conditions and a weakened Walker

circulation in the southern tropical Pacific whilst in the northern Pacific La Nia like conditions occurred

and a stronger Walker circulation.

The AMO Index became positive circa 1995, however due to the increase of greenhouse gases in the

atmosphere, SSTs are expected to be greater than previous positive cycles thus causing the U.S to

show a further decrease in annual rainfall particularly in the eastern Mississippi basin (Enfield, Mestas-

Nuez & Trimble, 2001). AMO will also raise the bar for coupled climate models as they will not predict

accurately regional rainfall if they do not include the AMO variability and its impacts (Enfield, Mestas-

Nuez & Trimble, 2001).

Conclusion

The oceanic and atmospheric processes occurring within the North Atlantic basin are vast, complex

and all inter-related. Together they create the climates we know in the U.S, Europe and the

Mediterranean. They create conditions that can cause fluctuating floods, droughts and hurricanes for

the United States and milder conditions and fluctuating dry and wet winters for Europe and the

Mediterranean. With anthropogenic forcing of enhanced global warming these processes will be

affected; due to the complexity global climate models are unable to reproduce the effects that show

how regional areas will be affected though literature agrees changes will be seen with the NAO Index

and AMO Index trending towards positive phases and a weakening of THC.

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References

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Christopherson, R.W. (2006) Geosystems: An Introduction to Physical Geography. 6th edn.

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http://www.aoml.noaa.gov/phod/amo_faq.phphttp://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/month_nao_index.shtmlhttp://www.aoml.noaa.gov/phod/amo_faq.phphttp://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/month_nao_index.shtml

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