09/19/2007

The composite precipitation results for pure NAO(+) and pure NAO(-) show
an east-west dipole in the equatorial Pacific. The position of this
dipole is orthogonal to the counterpart of the pure ENSO mode. In
specifically, the zero line is collocated with the maximum center of the
precipitation anomalies in the composite pure ENSO mode. This may be
related to the second or higher order of the SST variability in the
equatorial pacific. In specific, although we eliminate the first EOF
mode, but it gives more opportunity to pull in the higher order of the
EOF modes . In this sense, the EOF analysis may not be a good way to
define the ENSO index. Because it is hard to be guaranteed to eliminate
higher order modes which may also be related to the ENSO variability in
the way of shifting the SST variability eastward or westward. Given
these consideration, we use area averaged SST anomalies over the NINO 3
region as the ENSO index, which is consistent with the Andrew(?). The
standard deviation of the SST also shows that the SST variability tends
to be strongest in this region.

North Pacific: NAO(+),NAO(-) El Nino

Now let's focus on the North Pacific session, and find out the different
impact in the North Pacific by the different phase of NAO when both are
combined with El Nino events. Negative and positive anomalies of
geopotenial height at 500hPa in the North Pacific are associated with
the positive and negative phase of pure ENSO, respectively. For the pure
NAO events, there are notable anomalies in the North Pacific besides the
north-south dipole in the North Atlantic. The polarity of the anomaly in
the North Pacific is the same as that in the North Atlantic, which makes
the NAO pattern more global in the North Hemisphere. This third center
in the North Pacific superimposed with the anomaly associated with the
pure El Nino mode make the geopotential height in the North Pacific
quite different in the combination modes between El Nino plus NAO (+)
and El Nino plus NAO (-). Specifically, in the combination of El Nino
and NAO(-), the negative anomaly is reinforced in the North Pacific, and
extended eastward. Thus the negative anomalies are around the
subtropical latitude circle. The stronger height gradient compared with
that associated with the combination of El Nino and NAO(-) results in
the stronger upper level westerlies. So that storm track tends to be
more frequent and stronger in the strengthened mean flow, and more
precipitations are expected to occur in the westerlies in the former
case. The maximum precipitation anomalies in the western coast of the
United States are 6 in the former case compared with 4 in the later
case.

The sea level pressure are in phase with the geopetential height at
500hPa, indicating the equivalent barotropical characteristics in the
North Pacific. The negative anomalies in the North Pacific are more
dominant in the former case. The southly winds the eastern flank of the
deep Aleutian low advect warmer and moister air along the west coast of
North American, so that SST increase more along the west coast of North
America, as well as changes in the coastal rainfall.

In summary, when El Nino events are combined with NAO+(NAO-), the signal
in the North Pacific are somewhat cancelled(reinforced) with each other,
and get a wavy (annular) pattern in the geopential field in 500hPa. In
the latter case, the westerlies are we strengthened in almost the whole
latitude circle, so that the storm track are more and frequent, giving
rise to more precipitation around the jet stream. So that we can see the
rainbelt starting from western coast of the United States extend
eastward to the inland of the Europe. However, in the former case, the
enhanced precipitation in the western coast of the United States is not
as strong as that in the latter case. The much warmer and wetter in the
western coast of the United States are also associated with stronger
southly surface winds, which advect warmer and moister air along the
west of North American.

North Atlantic: ENSO(+), ENSO(-) NAO(+)

I am not sure if I should take a look at the NAO in the spring season.

Because El Nino has very weak impact in the North Atlantic region, only
weak positive anomalies appear in the eastern subtropical Atlantic. Thus
when El Nino events are combined with NAO(+), the south dipole of the
NAO is eastward shift. However, when El Nino events are combined with
NAO(-), it shows the tripole pattern in the North Atlantic.

Changes in the surface temperature and rainfall in the North Atlantic
are related to the polarity of the NAO, because El Nino has very weak
influence in the North Atlantic. The

plot

1.  SD of SST(DJF)

2.  Polar stereo projection and 20-90N