# Presentation: 789013

## P1.7

-       ***ABSTRACT***
*      **A study of the effects of slow vacillations of the Southern Hemisphere zonal index, or Antartic Oscillation (AAO) on South American climate is presented. Nearly zonally symmetric variations of the zonal wind are the dominant mode of low frequency variability in the Southern Hemisphere. The zonal mean zonal wind slowly oscillates between periods when the tropospheric midlatitude westerly jet is centered near 55 ̊S (high index), and periods when the tropospheric midlatitude westerly jet is centered near 40 ̊S (low index). *
* *
*      **Daily 1980-2000 NCEP reanalysis as well as daily Outgoing Longwave Radiation (OLR) from NOAA-CIRES are used to study the effect of zonal index variability on the tropospheric circulation of  the Southern Hemisphere . *
*     **The South American Low-Level Jet (SALLJ) appears to be stronger during Low Index periods than during High Index periods. The Utah Global Dry Primitive Equations model is being used to try to understand the observed zonal index-induced SALLJ variability.*

***Rosana Nieto Ferreira, NASA/GSFC, Mailcode 913.0, Greenbelt, MD 20720, USA, ferreira@janus.gsfc.nasa.gov***

**     *****Zonal Index Structure and Variability***
*     **An Empirical Orthogonal Function (EOF) analysis of the NCEP reanalysis Southern Hemisphere daily zonal mean zonal wind (U) with the annual cycle removed and filtered with a 10-360 day bandpass filter was performed. In agreement with previous studies (e.g. Hartmann and Lo, 1998), we find that:*
*The first EOF obtained in this analysis (Figure 1a) is the Zonal Index mode. This mode is characterized by nearly equivalent barotropic centers of variability of opposite sign near 40**o**S and 60oS. *
*The zonal index mode accounts for over 30% of the variability of the zonal mean U in the Southern Hemisphere.*
* **The power spectrum of the first principal component (Figure 1b) is essentially undistinguishable from a red noise spectrum. *
* **According to Figure 1c there is a tendency for less days of Low (High) index mode during December-February (February-March).*

***Baroclinic Wave Structure***
***High Index:**** *
*                   **Midlatitude Jet is displaced poleward -> **anticyclonic shear** prevails in the midlatitudes and subtropics -> baroclinic wave life cycles of the **thinning trough** type predominate -> stronger equatorward propagation of energy. Note how waves tilt equatorward in every correlation map.*
***Low Index:**** *
*                   **Midlatitude Jet is displaced equatorward -> **cyclonic shear** prevails in the midlatitudes and subtropics -> baroclinic wave life cycles of the **broadening trough** type predominate.*

*Figure 3: Simultaneous correlation maps of 10-day high pass filtered 200 mb relative vorticity at 45**o**, 50**o**, and 55**o**S for the high and low index periods. *

*Figure 4: Low minus High Index 10-day high pass filtered a) EKE (m**2**/s**2**) and b) momentum fluxes (m**2**/s**2**). *

* ****EKE and Momentum Fluxes***
* **The Low minus High index high frequency Eddy Kinetic Energy (EKE; Figure 4a) shows an **enhancement of high frequency eddy activity near** **40**oS** (60oS) during** Low** (High) **Index periods**. This variability has stronger amplitude in the Pacific and Indian Oceans.*
* **High frequency eddy momentum **flux convergence in the 40**oS belt** is in the sense of **strengthening westerly winds near 40oS during Low Index periods** (Figure 4b), thereby **contributing to the maintenance of the Low Index state**. The same is true at 60oS during High Index periods. These tendencies have stronger amplitude in the regions where the jet index anomalies are stronger, i.e. in the Pacific and Indian Oceans.*

***W***

*The SALLJ is stronger during Low Index than during High Index periods (Figure 5b).*

***SUMMARY***
*The Zonal Index mode is characterized by nearly equivalent barotropic centers of variability of opposite sign near 40oS and 60oS. *
* **The Zonal Index mode accounts for over 30% of the variability of the zonal mean U in the Southern Hemisphere.*
* **Low index periods** are characterized by an **equatorward displacement of the midlatitude jet at all longitudes**, *
* **The zonal index oscillation has strongest amplitude in the Pacific and Indian Oceans. *
* **High frequency eddy momentum fluxes contribute to maintain the zonal index oscillation in either of its extremes. *
* **The subtropical jet over South America is stronger during Low Index periods. *
* **The **SALLJ** appears to be **stronger during Low Index periods**. *
* **Preliminary results with a Global, Dry PE model suggest that the zonal index-induced midlatitude jet variability is not responsible for the zonal index-induced variability of the SALLJ.*
* **Additional experiments are being conducted to study the effect of the zonal index-induced Subtropical Jet variability on the variability of the SALLJ.*
*The results presented in this study are in good agreement with Hartmann and Lo (1998) *

***References***
*Hartmann, D. L., and F. Lo, 1998: Wave-driven zonal flow Vacillation in the Southern Hemisphere. J. Atmos. Sci., 55, 1303-1315.*

***MJ***