1. Tropical/extratropical forcing on wintertime variability of the extratropical temperature and circulation
Bin Yu1 and Hai Lin2
1. Climate Research Division, Environment Canada, Toronto
2. RPN-A, Environment Canada, Dorval, QC
1. Introduction
2. Methodology
3. Results
a. secular trends
b. interannual variability
4. Summary
Hai, please update your address.

2. Trends of Ts over 1951-1999 NAO index Global SSTs Trop. SSTs
Pattern corr = 0.93
Global SSTs
Trop. SSTs
Shin & Sardeshmukh (2011)
NAO index
Full forcing
Trop. forcing
Extra-trop.
forcing
1. Climate variability and changes in global atmospheric circulations and their generation mechanisms have been investigated extensively.
However, relatively few have focused on temperature variations, especially on sub-continental scale temperature trends and variability
and their generation mechanisms.
2. Recently, Shin and Sardeshmukh reported the critical influence of tropical warming on annual mean Ts trends over the landmasses
surrounding the North Atlantic.
3. In general, atmospheric temperature anomalies are closely related to atmospheric circulation anomalies. Our previous studies indicated
that extratropical forcing (including SST, sea ice and other processes that are not resolved by the model dynamics) dominates tropical forcing
for the interannual variability of the observed NAO index, while the tropical forcing drives the upward trend of the NAO in the second half
of the 20th Century.
Greatbatch etal. (2003)

3. Purposes of this study
a. examine the trends and interannual variability
of extratropical temperatures over northern lands
and circulations over the northern hemisphere;
b. numerically explore the extent to which extratropical
and tropical forcings affect the temperature and
circulation variability;
c. examine the influences of forcings on stationary
wave anomalies and the synoptic eddy activity
and feedback to help understand the generation
and maintenance mechanisms associated with
the temperature and circulation variability.
purpose

4. Model and simulations a. Model b. Simulations
Primitive equation AGCM (Hall 2000) – similar configuration of model forcing as the Marshall-Molteni model, but not Q-G.
T31, 10 levels
Time-independent forcing to maintain the winter climate
No moisture equation, no interactive convection
Hai, would you please add some info that briefly describes the model we used and the forcing diagnosed?

5. a. Stationary wave anomalies
Diagnostic methods
a. Stationary wave anomalies
● quasi-stationary wave activity (Plumb,1985)
● Rossby wave source (Sardeshmukh & Hoskins,1988)
b. Eddy activity and eddy feedback
● synoptic eddy vorticity forcing (e.g., Trenberth & Hurrell,1994)
● synoptic eddy heat flux forcing
● eddy feedback strengths (e.g., Choi etal.,2010)
and
We examine the influences of forcings on stationary wave anomalies and the synoptic eddy activity and feedback to help
understand the generation and maintenance mechanisms associated with the temperature and circulation variability.

6. Wintertime surface air temperature from 1961-2010
There is a trend shift of surface temperatures around 1990 in the time series for NCEP and model simulations.
The positive Ts trend over Canada enhances in the last two decades, compared to the temperature evolution
before Ts trends change sign around 1990 over Greenland and the East Asian mid-latitudes.

7. Linear trends of Ts (colors) and Ф500 (contours)
Over the NH, positive temperature trends appear over Canada, eastern U.S., Mexico, and Eurasian mid-high latitudes for
In association with the temperature changes, 500-hPa height trends are lower over the North Atlantic high latitudes and the central
North Pacific. The exchange of midlatitude and polar air, implying the trend in temperature advection, supports the temperature changes.
b. In contrast, opposite temperature trends prevail over relative to , with the exception of western-central parts
of North America.
c. The control simulation reasonably well captures the temperature and circulation trends and interannual variability, particularly
the signals over the North Atlantic sector.

8. Linear trends of Ts (colors) and Ф500 (contours)
Model experiments indicate that the extratropical forcing contributes to the temperature and circulation trends in both periods,
while the contribution of the tropical forcing is somewhat sensitive to the period considered.
b. For , temperature trend patterns in both Ext and Trp runs are similar to those in NCEP and Ctr. So the spatial
correlations and covariances of temperature trends between NCEP and the model runs are comparable in the Ext and Trp
forcing simulations, consistent with the corresponding circulation trends.
c. For , temperature trends in Ext are also similar to those in NCEP and Ctr, but not for the Trp run.

9. Ts, Ф500
SCF=62%
SCF=28%
Next, let’s look at the temp and circulation variability. This slide show the two leading patterns of maximum covariance analysis
between Ts and Phi500 anomalies and the corresponding MCA expansion coefficients for NCEP from
The two leading MCA modes explain comparable fractions of total interannual temperature variance and
are also the two principal modes of interannual temperature variability over the northern lands, characterized
by an EOF analysis.
b. The leading MCA modes are closely associated with the NAO and PNA teleconnections, with corrs. of 0.95
with the NAO and PNA indices.

10. Ts, Ф500
T850, Ф250
The NAO and PNA associated teleconnection patterns are well defined at middle and upper troposphere levels,
and are characterized by equivalent barotropic structures.

11. Ts projected time series
To make a quantitative comparison between NCEP and the model simulations, we project the model results onto the leading MCA patterns of NCEP.
The control simulation captures much of the NAO and PNA associated temperature and geopotential variability, especially for the NAO related mode.
Also the spread of the results among 10 Ctr members is small.
b. The extratropical forcing accounts for a significant amount of the interannual temperature and circulation variability, while the tropical forcing
only accounts for a small fraction of the total variability, especially for the NAO related variance.

12. We then examine the stationary wave anomalies associated with the two MCA modes by analyzing the stationary wave activity fluxes
and the Rossby wave sources. Here the background colors indicate the climatological means, and the contours are the NAO and PNA
associated anomalies for NCEP.
In association with the NAO mode, larger wave activity fluxes originate from the North Atlantic and Greenland and flowing southward
towards the subtropical North Atlantic. The climatological Rossby wave sources generally enhance over the North Atlantic, with the
strongest anticyclonic wave forcing that is related to large wave activity fluxes originating from the North Atlantic.
b. Likewise, the PNA associated Fs fluxes originate mainly from the North Pacific and flowing downstream toward North America and then
the subtropical western North Atlantic. S250 enhances over the PNA sector, with the strongest forcing anomalies originating from
the North Pacific.

13. The variability of 250-hPa geopotentials and Rossby wave sources in association with the two MCA modes are well simulated
in the control simulation. Time series of Ф250 and S250 anomalies projected on their corresponding spatial patterns for NCEP
and the control simulation are highly correlated.
b. Results from the forcing experiments further indicate that the extratropical forcing accounts for more of the interannual Ф250
and S250 variability than the tropical forcing, especially for the NAO associated variability.

14. Synoptic eddy feedbacks
Finally, let’s look at the synoptic eddy feedbacks on the temperature and circulation anomalies.
This slide shows the dynamic and thermodynamic eddy feedback strengths for NCEP and the ensemble mean of the control simulation.
Positive values mean the synoptic eddies play positive roles in reinforcing the anomalous circulation and temperature via the eddy fluxes.
250-hPa EFSd exhibits mostly positive values over the extratropical region, indicating that the synoptic eddies are reinforcing and
maintaining the upper tropospheric climate flow. EFSd is strong in mid-latitudes with two strongest centers over the North Pacific
and North Atlantic, collocating with the dominant centers of action of the MCA modes, indicating the role of synoptic eddies in
reinforcing the PNA and NAO related variability.
b. 850-hPa EFSt is dominated by negative values over the extratropical region, indicating that the synoptic eddies destroy
the mean temperature perturbation. Strong feedback values also appear over the North Pacific and North Atlantic.
c. The patterns of 250-hPa EFSd and 850-hPa EFSt are broadly similar in NCEP and Ctr. The main difference between them
is seen in the feedback strength, especially over the North Pacific.

15. Synoptic eddy feedbacks
This slide shows the ensemble mean differences of the feedbacks between Ctr and the two experiments.
EFSd differences are dominated by positive values in the mid-latitudes and negative values in its north. The differences
between the two are comparable over the North Pacific, suggesting that both tropical and extratropical forcings contribute to
maintaining the upper tropospheric circulation over the North Pacific. In contrast, the differences of EFSd are stronger
between Ctr and Trp than those between Ctr and Ext over the North Atlantic, indicating the more significant contribution of
the extratropical forcing on the circulation anomalies over the North Atlantic. This further supports the results obtained above.
b. The differences of 850-hPa EFSt are less well organized. However, the distinction of EFSt is also evident over
the North Atlantic, again consistent with the results discussed above.

16. Summary ● Distinct Ts trends over 1961-1990 and 1991-2010 are found
over most of the extratropical lands.
Both the diagnosed extratropical and tropical forcings
contribute to the temperature and circulation trends
over , while the extratropical forcing dominates
tropical forcing for the trends over
● Covariances between the interannual Ts and Ф500 anomalies
for NCEP over are dominated by two leading modes
associated with the NAO and PNA teleconnections.
The extratropical forcing accounts for a significant part of
the NAO and PNA associated variability.
The tropical forcing contributes to the PNA related variability,
but has a small contribution to the NAO associated variability.
conclusions