Dear MetSoc Wellington members:

Please see below for information on two seminars, both to be given by Dr
Alex Pezza of Melbourne University. Apologies for the rather late notice
on these...


Regards,
Jim Renwick
MetSoc VP (Wellington)

--------

NIWA Seminar
10:30am Wednesday 30 August 2006
Main Conference Room, Allen Building

CATARINA: THE FIRST SOUTH ATLANTIC HURRICANE AND LARGE SCALE LINKS

Alexandre Bernardes Pezza
The University of Melbourne, Victoria, Australia


It has long been believed that hurricanes could not form over the South
Atlantic Ocean due to a combination of relatively cold waters and strong
windshear. This concept has been consistently presented in text books
but is now under revision after Catarina hit southern Brazil in March
2004. Pezza and Simmonds (2005, GRL) proposed a large scale mechanism
to explain how this extremely rare event was formed, and such ideas have
recently received further support in the literature (McTaggart-Cowan et
al, MWR in press). In this seminar we will show what Catarina really was
and why it was so unusual, with a perspective from local eyes (Pezza was
in Brazil when the hurricane occurred). A possible hemispheric link with
the trend towards a more positive phase of the Southern Annular Mode
(SAM) will be discussed in terms of climate change and future south
Atlantic storms.


--------

VUW research School of Earth Sciences Seminar
10am Friday 1st September
in Cotton-304 (tea room), VUW

SOUTHERN HEMISPHERE CYCLONES AND ANTICYCLONES: RECENT TRENDS AND LINKS
WITH DECADAL VARIABILITY IN THE PACIFIC OCEAN

Alexandre Bernardes Pezza
School of Earth Sciences, The University of Melbourne, Australia
e-mail: apezza@...

ABSTRACT

In this seminar we will talk about a link between the extratropical
Southern Hemisphere cyclone and anticyclone behaviour and the decadal
climate variability in the Pacific Ocean (PO). We discuss a pattern of
coherent large scale anomalies and trends in cyclone and anticyclone
behaviour in light of the climate variability in the PO over the ERA40
reanalysis period (1957 – 2002). Two representative PO indices are the
Pacific Decadal (Interdecadal) Oscillation (PDO and IPO) and here the
PDO is chosen due to it being less associated with the Southern
Oscillation Index (SOI).
Composites of indicators of density and intensity of
cyclones/anticyclones given by an automatic tracking scheme were
calculated for the years when the PDOI was more than one standard
deviation above and below its mean. Although the ERA40 is not free from
noise and assimilation changes, the results show a large scale feature
which seems robust and agrees with earlier studies using different datasets.
The sea level pressure shows a strong annular structure related to the
PDO which is not seen for the SOI, with lower pressure around Antarctica
during the positive phase and vice-versa. More intense (and fewer)
cyclones and anticyclones were observed during the positive PDO. This is
less consistent for the SOI, particularly during the summer when an
opposite PDO/SOI pattern arises at high latitudes. The trends project a
pattern coincident with the positive PDO phase and seem to be linked
with the main climate shift in the late seventies. Trends observed over
the Tasman Sea are consistent with declining winter rainfall over
south-eastern Australia.
Most patterns are statistically significant and seem robust, but random
changes in ENSO may participate to a certain degree as a modulator.
Although global warming and related changes in the Southern Annular Mode
may also help to explain the observed behaviour, the large scale
response here presented gives a new insight and would be of considerable
interest for further modelling studies.