108
Chapter 6: Analysing the Boreal Summer Relationship
nental monsoon brought l~ss rain to the semi-arid region south of the Sahara
desert, the Sahel (see Section 6.5). So it appears that convergence into the
ITCZ is reduced and/or its latitude is furt her south over alllongitudes except for the western Pacific. Here there is enhanced near-surface convergence
near 0 - 20 0 N, with anomalous divergence to the south, extending into
the SPCZ. It is interesting that this tropic-wide atmospheric teleconnection
pattern has some similarities with the one associated with SST p 2 . Indeed,
it may be that there is a tropic-wide pattern of boreal summer atmospheric
variability that can be triggered by different forcings (Ward et al., 1994).
Extratropical circulation changes have also occurred in association with
SST p3. In the North Atlantic, the cooler waters have accompanied higher
SLP and more anticyclonic circulation over the western Atlantic and Western Europe. Over these longitudes there has been anomalous near-surface
divergence, while on the western side of the basin, anomalous near-surface
convergence has occurred south and southwest of Greenland.
6.4.4 Patterns in the Marine Atmosphere Associated
with Rotated EOF pA
SST PA is a more localised pattern of SST anomalies than EOFs p2 and p3.
The associated atmospheric anomalies (Figure 6.4) are also more localised.
The negative correlations with SLP in the tropical South Atlantic mirror the
SST pattern, and are consistent with the LN model. However, atmospheric
wind and divergence anomalies are strongest further north, probably reflecting strong inter action with the modified atmospheric circulation and rainfall
patterns that SST PA seems to force over West Africa (see Section 6.5). The
region of largest weights in SST PA is a region of very stable southeasterly
trades. Higher than normal SST in this region is likely to enhance the moisture content of the trades so that shallow convection will release more latent
heat and begin to erode the stability of the trades. This enhanced potential
instability of the air mass may lead to the enhanced rainfall on the Guinea
Coast that accompanies the positive phase of SST PA- Enhanced rainfall on
the Guinea Coast is·associated with reduced rainfall in the Sahel. The wind
and divergence anomalies in the northern and equatorial tropical Atlantic are
probably to a large part directly responding to the circulation and heating
anomalies over West Africa associated with these rainfall anomalies. Furthermore, the near-surface wind anomalies over the ocean may then feed-back on
the ocean to modify the SST anomalies. This interpretation requires verification with coupled ocean-atmosphere models. Nonetheless, the hypothesis
illustrates how statistical relationships between ocean and atmosphere can
result from a combination of local air-sea forcing that generates atmospheric
anomalies, remote teleconnections internal to the physics and dynamics of the
atmosphere, and remote teleconnections due to further air-sea interaction.
There is one important extratropical teleconnection with SST PA- Significant negative correlations with SLP are found in the central Mediterranean.
Chapter 6: Analysing the Boreal Summer Relationship
nental monsoon brought l~ss rain to the semi-arid region south of the Sahara
desert, the Sahel (see Section 6.5). So it appears that convergence into the
ITCZ is reduced and/or its latitude is furt her south over alllongitudes except for the western Pacific. Here there is enhanced near-surface convergence
near 0 - 20 0 N, with anomalous divergence to the south, extending into
the SPCZ. It is interesting that this tropic-wide atmospheric teleconnection
pattern has some similarities with the one associated with SST p 2 . Indeed,
it may be that there is a tropic-wide pattern of boreal summer atmospheric
variability that can be triggered by different forcings (Ward et al., 1994).
Extratropical circulation changes have also occurred in association with
SST p3. In the North Atlantic, the cooler waters have accompanied higher
SLP and more anticyclonic circulation over the western Atlantic and Western Europe. Over these longitudes there has been anomalous near-surface
divergence, while on the western side of the basin, anomalous near-surface
convergence has occurred south and southwest of Greenland.
6.4.4 Patterns in the Marine Atmosphere Associated
with Rotated EOF pA
SST PA is a more localised pattern of SST anomalies than EOFs p2 and p3.
The associated atmospheric anomalies (Figure 6.4) are also more localised.
The negative correlations with SLP in the tropical South Atlantic mirror the
SST pattern, and are consistent with the LN model. However, atmospheric
wind and divergence anomalies are strongest further north, probably reflecting strong inter action with the modified atmospheric circulation and rainfall
patterns that SST PA seems to force over West Africa (see Section 6.5). The
region of largest weights in SST PA is a region of very stable southeasterly
trades. Higher than normal SST in this region is likely to enhance the moisture content of the trades so that shallow convection will release more latent
heat and begin to erode the stability of the trades. This enhanced potential
instability of the air mass may lead to the enhanced rainfall on the Guinea
Coast that accompanies the positive phase of SST PA- Enhanced rainfall on
the Guinea Coast is·associated with reduced rainfall in the Sahel. The wind
and divergence anomalies in the northern and equatorial tropical Atlantic are
probably to a large part directly responding to the circulation and heating
anomalies over West Africa associated with these rainfall anomalies. Furthermore, the near-surface wind anomalies over the ocean may then feed-back on
the ocean to modify the SST anomalies. This interpretation requires verification with coupled ocean-atmosphere models. Nonetheless, the hypothesis
illustrates how statistical relationships between ocean and atmosphere can
result from a combination of local air-sea forcing that generates atmospheric
anomalies, remote teleconnections internal to the physics and dynamics of the
atmosphere, and remote teleconnections due to further air-sea interaction.
There is one important extratropical teleconnection with SST PA- Significant negative correlations with SLP are found in the central Mediterranean.