The impact of Arctic warming on the midlatitude jet stream

127 Pages·2015·40.32 MB·English

The influence of Arctic warming on the midlatitude jet-stream:
Can it? Has it? Will it?
Elizabeth A. Barnes
Colorado State University
with much help and input from my collaborators:
Marie McGraw, CSU
James Screen, U. of Exeter
Lorenzo Polvani, Columbia U.
Etienne Dunn-Sigouin, Columbia U.
Giacomo Masato, U. of Reading
Tim Woollings, Oxford
NOAA Climate Diagnostics and Prediction Workshop Oct 29, 2015
The Arctic is warming
NASA GISS
CSU Elizabeth A. Barnes
The Arctic is warming
NASA GISS
CSU Elizabeth A. Barnes
Some suggested effects of Arctic warming
- slower jet-stream
Arctic warming
- equatorward jet-stream (-NAO) temperature
jet variability
variations
- slower wave propagation ?
- higher amplitude Rossby waves
blocking
extremes
- more frequent blocking

… just to name a few
Globe image from NASA/Goddard Space Flight Center
Scientific Visualization Studio
CSU Elizabeth A. Barnes
3 distinct questions
CSU Elizabeth A. Barnes
3 distinct questions
(1) Can Arctic warming influence the midlatitude jetstream?
CSU Elizabeth A. Barnes
3 distinct questions
(1) Can Arctic warming influence the midlatitude jetstream?
(2) Has Arctic warming significantly influenced the midlatitude jetstream?
CSU Elizabeth A. Barnes
3 distinct questions
(1) Can Arctic warming influence the midlatitude jetstream?
(2) Has Arctic warming significantly influenced the midlatitude jetstream?
(3) Will Arctic warming significantly influence the midlatitude jetstream?
CSU Elizabeth A. Barnes
Can Arctic warming influence the midlatitude jetstream?
Model simulations & theory
CSU Elizabeth A. Barnes
344
Modeling evidence J O U R N A L O F C L I M A T E V 23
OLUME
500 hPa geopotential height
response in Jan.-Feb.
336 JOURNAL OF CLIMATE VOLUME 23
-NAO pattern
(equatorward jet shift)
dozens of atmosphere-only GCM studies have demonstrated that
removing Arctic sea ice can influence the midlatitude circulation
atmosphere-only CAM3 simulations
Deser, Tomas, et al. (2010; JCLI)
FIG.1.BimonthlydistributionsofArctic(a)seaiceconcentration(%)and(b)seaicethickness(m)during1980–99and
CSU Elizabeth A. Barnes
2080–99fromCCSM3.
F . 12. Bimonthly geopotential height responses at 1000 and 500 hPa. The contour interval is 10 m, with positive (negative) values
IG
occurs year-round as the ice edge retreats from the pe- ice thinning is relatively uniform throughout the year,
ripheral Arctic seas. The areal reduction in Arctic with maximum values ;2.5–3.5 m in the central Arctic
seaiceisaccompaniedbyathinningoftheicepack.SIT Ocean.Incontrast,thereductionsinSICareseasonally in red (blue) and the zero contours omitted. Shading indicates values that exceed the 5% confidence level based on a two-sided
in the central Arctic Ocean decreases from 3–4 m to dependent, with the largest decreases (;80%–90%)
0.5–1 m in winter and from 2.5–3.5 m to ,0.5 m in withinthecentralArcticOceaninsummer(September–
Student’s t test.
summer. The late-twentieth-century SIC and SIT dis- October) and smaller decreases (;50%–60%) within
tributions are generally realistic compared to the avail- the marginal seas in winter.
able observations (Holland et al. 2006).
b. Surface energy flux response
The bimonthly changes in SIT and SIC between the
latetwentiethandtwenty-firstcenturiesareshowninthe ThechangesinArcticseaicearecommunicatedtothe
toptworowsofFig.2.Themagnitudeandpatternofsea atmosphereviachangesinthenetsurfaceenergyfluxes.
height responses at 1000 and 500 hPa, are shown in Arctic baroclinic response is also evident in midwinter,
Fig. 12. The circulation responses are weak (generally but it competes with the equivalent barotropic ridge
,10 m and not statistically significant) during the warm aloft that weakens the surface trough compared to that
season (June–September), in accord with the small re- in early winter.
sponse of the net surface energy fluxes. Although the The shallow baroclinic atmospheric circulation re-
circulation responses are larger and statistically signifi- sponse over the Arctic in early (and late) winter may be
cant during the cold season (October–May), they exhibit understood as a linear dynamical response to enhanced
considerable variation in pattern and amplitude. The boundary layer heating induced by the underlying loss of
response in November–December (and in each month sea ice (Hoskins and Karoly 1981). On the other hand,
individually; not shown) exhibits a baroclinic vertical the equivalent barotropic component of the circulation
structure over the Arctic consisting of negative values response in midwinter (e.g., the NAO) and the ridge
(220 to 230 m) at 1000 hPa and positive (10–20 m) response over Eurasia in early and late winter represent
values at 500 hPa, and an equivalent barotropic (e.g., a nonlinear dynamical response to enhanced boundary
amplifying with height) ridge over central and eastern layer heating in which transient eddy momentum flux
Russia and trough over the Bering Sea. Similar fea- feedbacks associated with perturbations in the storm
tures are found in March–April with weaker amplitudes. track play a dominant role (Lau and Holopainen 1984;
A different circulation response is seen in midwinter Peng et al. 1997; Deser et al. 2007; among others). We
(January–February), which resembles the negative po- conjecture that the lack of a surface circulation response
larity of the NAO (although this occurs mainly in over the Arctic in midwinter is due to the near cancellation
February; not shown). In this season, the Arctic is dom- between the competing effects of the linear and nonlinear
inated by an upper-level ridge response (maximum am- dynamical components of the response. A quantitative
plitude ;50 m at 500 hPa) and negligible response at the analysis of the momentum balances of the circulation re-
surface accompanied by equivalent barotropic troughs sponses in CAM3 is beyond the scope of this paper.
over the Atlantic and northeast Pacific. Internal modes of atmospheric circulation variability
More detail on the vertical structure of the circulation have been shown to play a role in shaping the structure
responses is given in Fig. 13, which shows transects of of the atmospheric response to different types of exter-
the temperature and geopotential height changes along nal forcing, for example SST changes, sea ice anomalies,
908E in early (November–December) and mid-(January– or orbital variations (Peng et al. 1997; Deser et al. 2004;
February) winter. In early winter, a shallow baroclinic Hall et al. 2001; among others). In the case of our CAM3
geopotential height response with a nodal point near experiments, however, there is little correspondence
925 hPa develops over the Arctic in association with the between the dominant patterns of internal circulation
ice-induced near-surface warming. Farther south, the variability and the patterns of geopotential height re-
response consists of an equivalent barotropic ridge with sponse to Arctic sea ice loss, with the notable exception
maximum values ;40 m at 250 hPa near 658N. The of the month of February (not shown).