NAO in the PaleoCSM Christine A. Shields, Bette Otto-Bliesner, and Esther Brady National Center for Atmospheric Research, Boulder, CO Pre-Industrial control case NCEP SLP Re-analysis data, 1958-1998 , SLP Normalized SLP differences between NAO index “High” years (index >1) and NAO index “Low” years (index <1). Normalized SLP differences between NAO index “High” years (index >1) and NAO index “Low” years (index < 1). Surface temperature anomalies regressed onto the NAO index for PaleoCSM control case (b058.02) and NCEP re-analysis data (1948-1997). Note that in the positive phase of the NAO, milder condtions are prevalent over northern Europe with colder conditons over the northwest Atlantic. Precipitation anomalies regressed onto the NAO index for PaleoCSM control case (b058.02) and Hulme precipitation observations /Hurrell slp observations (1900-1998). Note that during the positive phase of the NAO, wetter conditions exist over northern Europe with dry persisting over southern Europe. The boundary layer depth (meters) and mixed layer depth (not shown) also show a response to the NAO. |NAO >2| index values were used to compute the NAO “High” years minus the NAO “Low” years for this figure. Increased depth in the Labrador sea indicates an increase in oceanic convection. Note the decreased depths along the Greenland seas as well. This may show an NAO response to the thermohaline circulation. This is consistent with Dickson’s findings (1997, Nature, 386, 649-650) which show winter convective activity during the extreme phases Five Holocene PaleoCSM cases are represented in these figures (Pre-Industrial control, 3.5ka, 6ka 8.5ka, and 11ka). Orbital parameters varied for each experiment. The NAO index was computed for each case and is represented above with the standard deviations shown along the right axis. The “High-Low” plots are shown to the right. The distinctive feature in these plots is the lack of significant differences between the periods. Although the location of the Azores High and Icelandic Low do change slightly (not shown), the magnitude and orientation of NAO do not appear to alter significantly through the Holocene during the winter months. The North Atlantic Oscillation (NAO) is a dominant mode of atmospheric climate variability in the Atlantic Basin on decadal and interdecadal time scales. The changes in circulation between the Icelandic region in the North Atlantic and the Azores region in subtropical Atlantic can be represented by the NAO index. The NAO index is calculated using the difference in normalized sea level pressure between these two centers of action. The NAO index computed for this poster uses the winter season (December, January, February, March) pressure differences between Iceland (64.2N 22W) and Lisbon, Portugal, (38.75N 9W). PaleoCSM is CSM1.4 at model resolution T31x3p and modified for paleoclimate applications. The NAO signal is captured very well by this model. Evidence of NAO influence over the ocean circulations in PaleoCSM can be shown through the ocean surface wind stresses. NAO index years (|NAO| >1) correlated with these fluxes show the response to the increased surface westerlies across the North Atlantic and into Europe. Note that during the positive NAO phase, these model surface stresses become more meridional in nature as they move into Europe. This is consistent with observations. of the NAO during the late 1960s (negative) and early 1990s (positive).

NAO in the PaleoCSM · NAO in the PaleoCSM Christine A. Shields, Bette Otto-Bliesner, and Esther Brady Pre-Industrial National Center for Atmospheric Research, Boulder, CO control

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NAO in the PaleoCSM Christine A. Shields, Bette Otto-Bliesner, and Esther Brady

National Center for Atmospheric Research, Boulder, CO Pre-Industrial control case

NCEPSLP Re-analysisdata, 1958-1998

,SLP

Normalized SLP differences between NAO index “High” years (index>1) and NAO index “Low” years (index <1).

Normalized SLP differences between NAO index “High” years (index>1) and NAO index “Low” years (index < 1).

Surface temperature anomalies regressed onto the NAO index for PaleoCSM control case (b058.02) andNCEP re-analysis data (1948-1997). Note that in the positive phase of the NAO, milder condtions areprevalent over northern Europe with colder conditons over the northwest Atlantic.

Precipitation anomalies regressed onto the NAO index for PaleoCSM control case (b058.02) and Hulmeprecipitation observations /Hurrell slp observations (1900-1998). Note that during the positive phase ofthe NAO, wetter conditions exist over northern Europe with dry persisting over southern Europe.

The boundary layer depth (meters) and mixed layer depth (not shown) also show a response to the NAO. |NAO >2| index values were used to compute the NAO “High” years minus the NAO “Low” years for this figure. Increased depth in the Labrador sea indicates an increase in oceanic convection. Note the decreased depths along the Greenland seas as well. This may show anNAO response to the thermohaline circulation. This is consistent with Dickson’s findings

(1997, Nature, 386, 649-650) which show winter convective activity during the extreme phases

Five Holocene PaleoCSM cases are represented in these figures (Pre-Industrial control, 3.5ka, 6ka8.5ka, and 11ka). Orbital parameters varied for each experiment. The NAO index was computed foreach case and is represented above with the standard deviations shown along the right axis. The“High-Low” plots are shown to the right. The distinctive feature in these plots is the lack of significantdifferences between the periods. Although the location of the Azores High and Icelandic Low dochange slightly (not shown), the magnitude and orientation of NAO do not appear to alter significantlythrough the Holocene during the winter months.

The North Atlantic Oscillation (NAO) is a dominant mode ofatmospheric climate variability in the Atlantic Basin on decadaland interdecadal time scales.

The changes in circulation between the Icelandic region in theNorth Atlantic and the Azores region in subtropical Atlantic canbe represented by the NAO index. The NAO index is calculatedusing the difference in normalized sea level pressure betweenthese two centers of action.

The NAO index computed for this poster uses the winter season(December, January, February, March) pressure differencesbetween Iceland (64.2N 22W) and Lisbon, Portugal, (38.75N9W).

PaleoCSM is CSM1.4 at model resolution T31x3p and modifiedfor paleoclimate applications. The NAO signal is captured verywell by this model.

Evidence of NAO influence over the oceancirculations in PaleoCSM can be shownthrough the ocean surface wind stresses.

NAO index years (|NAO| >1) correlated withthese fluxes show the response to theincreased surface westerlies across the NorthAtlantic and into Europe.

Note that during the positive NAO phase,these model surface stresses become moremeridional in nature as they move intoEurope. This is consistent with observations.

of the NAO during the late 1960s (negative) and early 1990s (positive).