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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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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