Friday map discussion, 9 April 2010
Ross A. Lazear

The focus of Friday Map Discussion on 9 April was on the Northern Hemisphere pattern over the past two months, with an emphasis on the transition from a strongly negative phase of the Arctic Oscillation (AO) to a more neutral phase in late February.  The negative phase of the AO was a dominant feature throughout the boreal winter, and as will be discussed, the transition to a more neutral state was likely brought on by both changes in the North Pacific flow, and the distribution of sea-surface temperature (SST) anomalies in the equatorial Pacific.

(Note:  Visit the link below to Heather Archambault's map discussion page, which features loops of many of the plots discussed below)
http://www.atmos.albany.edu/student/heathera/mapdisc_040910/

Though the AO was in a strongly negative phase throughout most of the winter (from CPC), there was a brief period in late January during which it became more neutral.  This was followed by a return to a negative phase, with the AO Index falling to -5 standard deviations (SD).  During the sixteen-day period from 7-22 February during which the AO Index averaged approximately -4.5 SD, a striking similarity is apparent between the sea level pressure (SLP) anomalies (ESRL) and the AO loading pattern.  In this period, a time-mean SLP below 988 mb is noted east of Newfoundland, associated with a negative SLP anomaly of 24 hPa.  The high-latitude flow pattern is dominated by a large anomalous anticyclone, with a positive SLP anomaly of over 20 hPa.  Also noteworthy is a region of negative SLP anomalies over the North Pacific.

Highly meridional flow is noted in both the North Pacific and North Atlantic in the same time period.  In the time-mean 300-hPa geopotential height, a pattern resembling a cyclonic wave break is seen over the North Atlantic, with a high-amplitude ridge in the northeast Pacific and western North America.  In the 300-hPa height anomaly field, an anomaly dipole in the North Atlantic allows us to infer impressively weak midlatitude westerlies from the Great Lakes east to Ireland, a feature associated with high-latitude blocking and a negative AO (and negative NAO).  Finally, an examination of 850-hPa temperature anomalies in the Northern Hemisphere during the strongly negative phase of the AO in mid-February shows anomalous warmth in eastern Canada associated with high-amplitude ridging in the North Atlantic, as well as anomalously cool temperatures in the southeast U.S.

After the transition to a more neutral phase of the AO (7-22 March), SLP and 300-hPa height anomalies returned to a more climatological state.  Though positive 300-hPa height and SLP anomalies remained over portions of the North Atlantic (centered near Iceland), the anomalies were of a lower magnitude, and their centers were located slightly farther east.

During and just before the strongly negative phase of the AO in mid-February, several strong cyclogenesis events occurred over the Northwest Atlantic.  These events include cyclones discussed in recent map discussions, and each were associated with high-amplitude cyclonic wave break events and rapid meridional transport of low upper-tropospheric potential vorticity (PV) air into high latitudes.  Some of these events can be seen in Heather Archambault’s maps of tropopause potential temperature, winds, and lower-tropospheric relative vorticity (visit the main link page for a description of the figure) over the North Atlantic (2 Feb, 5 Feb, 12 Feb, 15 Feb, 17 Feb).  It is noteworthy that many of these cyclones were of a large spatial scale and intensified rapidly, thus likely reducing the upper-tropospheric PV through diabatic heating as well.

One way in which the reduction of the poleward transport of low PV (helping to maintain a strongly negative AO) may occur is to lessen the frequency of these strong cyclogenesis events in the northwest Atlantic.  Indeed, the number of strong cyclogenesis events does decrease by the end of February, with a few exceptions (most notably on 26 Feb), but these events are more infrequent, and occur farther west, thus preventing low PV air to be transported to high latitudes.  Again, full loops of these figures in multiple domains can be found here.

In an analysis of the dynamic tropopause over the full Northern Hemisphere in this same period, it is evident that each of these cyclogenesis events are brought on through an interaction between “seed” disturbances originating downstream of the high-amplitude ridge over the North Pacific (seen in the time-mean 300-hPa height from 7-22 February), and the enhanced subtropical jet over the southeast United States.  Multiple examples of these “seed” disturbances originating over northwest Canada can be seen throughout early and mid-February.  Examples of such events can be seen on 7 Feb (Saskatchewan/Manitoba border) and 14 Feb (South Dakota).  Not surprisingly, the time-mean 300-hPa height from 7-22 March (neutral AO) shows a more zonal flow across the North Pacific, reducing the number of Arctic-originating disturbances in the eastern United States.

Another means of lessening the frequency of northwest Atlantic cyclonic wave break events may also be through a shift in the tropical east Pacific SST anomalies.  A loop of SST anomalies from CPC shows sudden warming of the eastern tropical Pacific in late February, coincident with the rapid transition into a neutral AO.  A Hovmöller diagram of OLR anomalies from CPC in a 10° latitude band straddling the equator shows a sudden increase in convection over the same region (Niño 1-2 and Niño 3 regions, especially 100W-120W) beginning in mid-February.  GOES-12 water vapor satellite imagery shows suppressed convection in the east Tropical Pacific in early February, and a marked increase in convection by the end of February.  Associated with this increase in equatorial heating, a stronger subtropical jet in both hemispheres would be expected.  In the Northern Hemisphere, this appears to result in a more elongated subtropical jet across the North Atlantic.  Thus, it is hypothesized that after the peak equatorial heating moves into the far east Pacific, a more favorable region for North Atlantic cyclogenesis also becomes displaced farther east.  This lessens the occurrence of high-latitude blocking in the northwest Atlantic (near Greenland), and thus potentially aids in weakening the strongly negative AO.