Midlatitudes and Arctic Amplification

Question 1

orographic features on vorticity

Answer 1

lead to changes in depth (compression of air) -> produced an anti-cyclonic spin as absolute vorticity is reduced -> leads to deceleration but as the air passes over the mountain again it increases in speed = mountain lee waves = standing eddies
e.g. rockies -> North Atlantic Storm Track (SW-NE deflection) (Brayshaw et al., 2009)

Question 2

Mid-latitude Jetstream -> form in the NH and SH

Answer 2

strength is dependent on temp gradient
within them are rossby waves produced by changes in vorticity (Brayshaw et al., 2009)

Question 3

storm tracks -> persistent eddies = located under the jetstream (Hall et al., 2015)

Answer 3

only form at latitudes 20N and S as no vorticity is present below these latitudes since temp gradient not present
NH has more seasonal variability and is less zonally symmetric than SH

Question 4

transient eddies -> form in regions of baroclinicity lasting 3-5 days (Hoskins and Valdes, 1990)

Answer 4

formation in storm tracks = induced by changes in vorticity
changes in temperature influence vorticity by causing the column of air to expand -> conservation of vorticity (relative vorticity is linked with depth of flow and changes in vorticity mean depth of flow is conserved) means convergence of warm and cold air at the surface = low pressure = convective uplift = warm conveyor (Harold, 1973) belt as warm air from equatorial regions drawn in (Catto et al., 2010) -> leads to upper air divergence -> causes advection resulting in the system moving in a west to east direction -> intensification of the temp gradient as coupling of the lower and upper atmosphere (Willison et al., 2013) -> storm ceases when the coupling reduces

Question 5

eddy-driven jet -> idea that cold air advection on the west and warm air advection on the right reinforces baroclinicity strengthening the storms themselves

Answer 5

latent heat also has a positive feedback mechanism -> altering vorticity as air parcels rise intensifying the system as a result (Wilson et al., 2013)

Question 6

External forcing on the Jetstream

Answer 6

cryosphere -> sea-ice extent and snow -> influence meridional temp gradients.
oceanic -> North Atlantic SSTs and ENSO = El Niño causes NAO-
Stratosphere -> volcanic eruptions, solar variability, Quasi-Biennial Oscillation (Hall et al., 2015)

Question 7

Variability in the mid-latitudes -> seasonality

Answer 7

Stronger temp gradients in winter = alters position of storm tracks
Coriolis stronger in winter as jet is stronger = more standing waves

Question 8

Variability in the mid-latitudes -> NAO

Answer 8

NAO -> index from Azores High to Icelandic Low = determine the intensity of the jet -> alters the region of transient eddy formation

Question 9

NAO+

Answer 9

 Pressure gradient is stronger: the high is higher, and the low is lower.
 Stronger jet and more storms, storms are stronger.
 Jet is pushed further north, which brings warm air northwards.
 Warmer and wetter winters in UK/Northern Europe but drier Mediterranean/Southern Europe (Gerber and Vallis, 2009).
 E.g. December of 2015/16, NAO index highly positive (2.24 in Dec) (also some influence of El Niño), wettest calendar month in UK record, receiving 182% of long-term average rainfall

Question 10

NAO-

Answer 10

 Pressure gradient weaker: high and low are weakened.
 Weaker jet, with fewer and weaker storms.
 Jet meanders south, as winds are not as strong.
 Colder and drier winter in UK, cooler and wetter Mediterranean.
 E.g. winter of 2009/10, NAO index was very negative (-1.85 in Dec), UK had coldest winter in over 100 years, average temp 5C below 1971-2000 mean (Seager et al., 2010)

Question 11

Controls on the NAO

Answer 11

ENSO controls NAO state (Bronnimann et al., 2007) -> easier to predict NAO state from ENSO from November using GloSea5 (Scaife et al., 2014)
Internal Variability -> stochastic forcing in SSTs, temperatures etc..
SSTs -> transition into certain states (Omrani et al., 2021)
Stratosphere -> Stratospheric Polar Vortex and the production of blocking events
Other -> Pacific North American Oscillation

Question 12

Stratospheric Changes and the NAO

Answer 12

Arctic Oscillation -> air pressure and winds over the Arctic move through two core phases = strong mode during winter with no solar radiation = leads to an intensification of the SPV which moves east to west (westerlies)
Outside of winter -> the SPV is not strong so there is nothing to disrupt = no blocking events

Question 13

Formation of a blocking event

Answer 13

When the SPV weakens = weakening of Rossby waves below in the form of breaking waves = alteration in vorticity = weakening of the SPV and sinking which can cause warming = production of break off blobs due to momentum driven differences which can sit over the North Atlantic Storm Track/Jetstream leading to blocking highs.
- Blocking highs = last for a few weeks -> only removed through solar radiation as transient eddies cannot form as the Jetstream is diverted.
- E.g. Blocking winters in 1992/93 (Woollings et al., 2010).
- Ozone depletion -> cooled the stratospheric polar vortex = strengthening -> will lead to more stratospheric polar vortex breakdowns

Question 14

C.C and position of jets -> contestation

Answer 14

 CMIP5 -> Storm tracks expected to shift poleward in a warmer climate (Barnes and Polvani, 2013) -> jet speed will increase in the SH but will remain the same in the NH (Barnes and Polvani, 2013)
 IPCC (2013) has “medium confidence” in poleward shift

Question 15

C.C. and a poleward shift in jetstreams

Answer 15

 Hadley cell expansion (Seidel et al., 2007)
 Reduction in surface temperature gradient (Lu et al., 2010)
 Increased midlatitude tropopause height (Williams, 2006)
 Increased Rossby phase speeds (Chen, 2008)

Question 16

C.C. and a equatorward shift in jetstreams

Answer 16

 Changes is subtropical stability -> stronger temperature gradient (O’Gorman and Singh, 2013) -> enhanced in the upper troposphere (cooling in poles) (Catto et al., 2019).
 Possible weakening meridional overturning circulation (Woollings et al., 2012)

Question 17

C.C. and regional projections

Answer 17

 Increase in precipitation and latent heating – more intense ETCs = Clausius-Clapeyron (Catto et al., 2019) -> also found to be a trend in CMIP modelling of cyclogenesis = cyclone occurrence leads to higher precipitation rates (Zappa et al., 2013)
 Poleward shift = increased winter storms in UK and NW Europe, and decreased winter storms in Mediterranean
 CMIP5/6 drying in summer over N. Europe -> due to storm track decreasing (Chang et al., 2022)

Question 18

C.C. and individual ETCs

Answer 18

 Projected increase in precipitation intensity and volume due to more available moisture and latent heating (Trenberth and Stepaniak, 2003).
 weaker eddies able to transport more energy due to increased atmospheric moisture
-> higher intensity
-> likely increases in flooding
 More eddies -> reduced temp gradient = less ETCs -> complex (Catto et al., 2019)

Question 19

C.C and NAO

Answer 19

 NAO under C.C. shifting poleward -> more intense in N.Europe = more eddy/storm formation over the region (Fereday et al., 2012)

Question 20

Models and the mid-latitude climate

Answer 20

o Persistent mean-state biases in jet stream position: too zonal and too far south (Woollings, 2010)
o Need finer scale resolution to understand latent heating impacts
o Blocking underestimated in models (Scaife et al., 2010)
o SST gradients underrepresented (Woollings et al., 2009)
o high model variability -> low confidence e.g. NAST often placed too far equatorward (Catto et al., 2019)
o CMIP -> ensemble mean biases influence the poleward shifts (Change et al., 2022) and CMIP5 jetstream too equatorward (Zappa et al., 2013)

Question 21

UK ETC storms and floods in winter 2013/24

Answer 21

o December-January 2013/14 was the wettest 2-month period since 1910 (372.2mm).
o N Atlantic jet stream was unusually strong (30% stronger than normal), due to extreme cold in N America giving a persistent pattern of perturbations in the jet stream (Huntingford et al., 2014).
o Stronger than usually temperature gradient into jet.
o Positive phase of NAO (Slingo et al., 2014)

Question 22

Arctic Amplification -> 2x faster warming than the rest of the world = ice-albedo feedback mechanism (Held, 1993; Screen and Simmonds, 2010)

Answer 22

polar warming -> greatest through the winter than summer as storms form in the winter = greater ice loss

Question 23

Ice sheet loss

Answer 23

Greenland Ice Sheet -> lost 4890Gt (1992-2020).
Arctic Sea Ice Extent -> peak loss occurs in September -> levels are the lowest they have been for the last 1kyr (Kasp and Schneider, 2013).
Thwaites Glacier -> warming underneath the glacier -> lead to a massive ice loss

Question 24

Positive lapse rate in the Arctic

Answer 24

heating at the top of the atmosphere is less than at the lower atmosphere = heat buildup at the surface -> a result of inhibited convective uplift since the atmosphere in the arctic is stable -> inversion layer (Held, 1993)
= leads to rapid warming at the poles identified in CMIP6 -> reduces the equator-pole pressure gradient (Taylor et al., 2022)
CONTRAST to the tropics = negative lapse rate as cooling at the surface due to convective uplift and warming in the high latitude

Question 25

Modelling Arctic Amplification

Answer 25

CMIP5-6 -> warming due to arctic amplification will worsen e.g. some scenarios of forcing = ice free Arctic by 2050.

Question 26

Arctic Amplification feedback mechanisms

Answer 26

surface temp rise = sea ice/snow melt = more biological activity, lower albedo = more warming, decreased ice stability, more convective uplift, release of aerosols from ice etc…
-> removal of ice = exposure of the sea -> dark and therefore absorbs heat -> increased sensible heat flux within the ocean = warming -> leads to amplified Arctic warming via the water vapour triple effect since poleward heat flux leads to more melting (Taylor et al., 2022)
= made worse since arctic cannot lose heat via convection