Class Notes April 17

Class Review for Tuesday, April 17, 2001

During this class we continued our discussion of ENSO observations

1. We compared three different ocean general circulation models, none of
which contained heat flux parameterizations. The cold pool for the
simulations were much too narrow compared to the width of the observed
cold pool. Are heat fluxes responsible for this difference?

2. We looked at ways to measure seasonal variances in ENSO.
Interestingly we saw, by the lagged autocorrelations of SST through the
calendar year that SST is not correlated across March! Furthermore,
there are not a lot of SST anomalies in March. We wondered: Is this a
time when we are switching from a warm pool to a cold pool or vice
versa? (and "unstable" idea) Or is it simply difficult to create
anomalies at this time even when tweaked, especially if the seasonal
cycle is very strong (a "stable" idea).

3. Next we looked at a time series (from 1985 to 1995) of the 20 degree
depth of the thermocline, SST anomalies and rainfall anomalies in the
tropical Pacific. We made three observations: One, that SST anomalies
behave like a standing oscillations - they do not apparently move in
waves west to east and back. Two, that rainfall anomalies are maximum at
the dateline with the general observation that cold SST anomalies bring
much less rain and warm SST anomalies bring more rain. Three, there DOES
seem to be propagation in the depth of the thermocline across the
equatorial Pacific which take about one year to traverse the entire
distance. This is interesting because it is something of a smoking gun
for the physic of ENSO. Unfortunately, our data on the thermocline does
not extend beyond 1985. The anomalies in the thermocline are greatest in
the east and the west=20

4. We reviewed that there is a large annual cycle / seasonal forcing in
the tropics, with September SST in the east Pacific being the coldest.
David offered the self described "lame" argument that this could be
triggered by the monsoon at the western boundary of the Walker
circulation.

5. We compared various measures of ENSO (Nino 1, Nino 2, Nino3 and SOI).
The "Ninos" measure SST anomalies at various locations and the SOI is a
measure of air pressure anomalies between Tahiti and Darwin. The two
major points here were that, again, temperature anomalies were greatest
in the east and that the SOI is very strongly correlated to SST
anomalies.

6. Next we compared the leading EOFs of both SST variance and the
variance of heat content in the thermocline. We did this with the caveat
that EOF analysis may not be the best way to study this since this
mathematical analysis forces the pictures of explained variance to be
orthogonal and that might be too artificial in this case. Note that the
"heat content in the thermocline" is a factor of both the actual
temperatures in the thermocline and the depth of the thermocline.
Looking at the leading patterns, the leading SST pattern (explaining 36%
of the variance) looks much like the quintessential ENSO pattern. It was
found to be very highly correlated to the leading heat content pattern,
especially in the time series. This leading heat content pattern
contained its largest anomalies in the western Pacific where,
ironically, the SSTs change very little. Obviously this is due to the
large swings in the depth of the thermocline. The pattern in heat
content that explains the second most variance is in apparent quadrature
(in the time series) to the first pattern with about a nine month lag.
This pattern shows a negative anomaly along the equator and a positive
anomaly to the north. We speculated on how a zonal wind stress gradient
could force westerly moving Rossby waves which would move more slowly
away from the equator and give the thermocline more time to form large
variances We were at somewhat of a loss to explain exactly why this
second pattern would be so asymmetric about the equator. The location of
the ITCZ was mentioned, the importance of ZONAL winds and the fact that
the climatology in heat content is more symmetric than the ENSO pattern.

7. With regard to convection and precipitation anomalies, we saw that
the ITCZ moves south toward the equator and that convection moves west
to east during an ENSO event. Interestingly, this was not observed until
recently because the satellite OLR observations had been tuned to focus
on the very cold high clouds in the western Pacific and that the clouds
which form the ITCZ in the east are not so high/cold.

8. ENSO (using the CT index) has strong relationships to teleconnection
patterns in the extratropics. The northern hemisphere teleconnection
pattern is somewhat akin to the Pacific North American pattern (PNA),
especially on decadal time scales. The southern hemisphere pattern is
not surprisingly strongest in the Pacific and is called the PSA pattern.
We observed that in the summertime, the teleconnection patterns and
their relationship to the tropics is much less pronounced due to the
weaker summertime jets. This is not the case in the southern hemisphere
where the jets remain strong year round. Because of these teleconnection
patterns we can find the storm track changes that are associated with
ENSO events. Over the North American continent this indicates much
greater storminess across the southern US states and much less in the
north central states.

9. Finally, we looked at temperature, precipitation, and streamflow
anomalies around the globe during ENSO events. Some notable anomalies
include much higher temperature in the far northern reaches of the North
American continent, eastern Australia and South Africa. Precipitation
anomalies include much wetter weather in Peru, Ecuador and the southeast
US, and much drier conditions across the Amazon Basin and the Pacific
Northwest (though is this due to the mountain stations?). Streamflow was
correlated to precipitation.




Craig Brown