AMIP II Diagnostic Subproject No. 23:

Contents:
Background
Objectives
Methodology
Data Requirements
References
Further information

Major modes of climate variability such as the Southern Oscillation, the North Atlantic Oscillation and the North Pacific Oscillation may be viewed as perturbations in the mean locations and intensities of a few atmospheric centers of action (COA).  It follows that our understanding of interannual variability can be improved by diagnosing the simulated COAs in terms of model physical parameterizations.  This proposed subproject is a continuation of AMIP I subproject 23 with the aim to investigate important aspects of the morphology, the genesis and the variability of the COAs in the Pacific,  Atlantic and Indian oceans in AMIP II model simulations, to compare the model simulated COAs with observations and reanalysis data, and to attempt to relate inter-model differences in the COA features in terms of the differences in model physical parameterizations.

A comprehensive study of the atmospheric centers of action was carried out by Gilbert Walker based on the calculation of correlation coefficients between pressure, temperature and precipitation, and reported in a series of papers with the title ?World Weather? published between 1924 and 1937.  Rossby (1939) discussed fluctuation of pressure in a COA in terms of changes in the intensity of zonal circulation.  At present we lack satisfactory dynamical explanations for the genesis and observed structures of the COAs.  Santer (1988) developed objective indices for the locations and intensities of the COA's and compared their values for the Azores High and the Icelandic Low in the OSU AGCM, the OSU CGCM and the GISS GCM. Using the procedure of Santer (1988) we have developed indices for the intensities and latitude and longitude locations for the Azores High, the Hawaiian High, the Icelandic Low and the Aleutian Low in observational data and several AMIP I simulations.  The seasonal variations of the four COA's in the MPI simulation were compared with observations by Hameed et al. (1995).  The relationship between surface pressures in the Azores High and the Hawaiian High and local diabatic cooling rates was compared between NCEP reanalysis fields and the MPI AMIP I simulation by Shi and Hameed (1996).  The pressure and the locations of the Icelandic Low in winter and the Azores High in winter and summer are significantly correlated with pre-season values of the indices of the four COA's and to SST anomalies in the North Atlantic (Hameed et al., 1996).  Our current work aims to use these for developing prediction schemes for the indices of the Icelandic Low and the Azores High.  Diagnostic studies on a relationship between the Asian monsoon and the intensity of the Hawaiian High in summer have been carried out by Shi and Hameed (1997).

Our experience with AMIP I suggests that monthly mean data may be used for diagnostic studies of the summer subtropical high pressure systems. However archived daily data is needed to analyze winter circulation that affects the subtropical highs and the low pressure systems at higher latitudes.  The expanded data base of AMIP II offers unique advantages in such analyses.  Transient momentum and heat fluxes in a number of GCMs, with different orographic effects, gravity wave drag, land-sea temperature differences and boundary layer parameterizations can be compared to obtain a better understanding of the genesis of the four COAs in winter and summer.
 

Methodology

We expect to carry out this investigation with  AMIP II simulations, comparing  models with distinct parameterizations of topography and diabatic heating rates.  Detailed diagnostics of the Centers of Action in the North and South Pacific and Atlantic Oceans and the Indian Ocean will be carried out in the following tentative sequence:

i.   Describe the seasonal variation in each of the COA parameters, compare
with observations, interpret in terms of GCM simulated thermal, topographic
and transient forcings, and NCEP reanalysis fields.  Temporal variation
will be characterized by Fourier analysis.
ii.  Describe the interannual variations in the COA parameters, compare
with observations, interpret in terms of GCM simulated thermal, topographic
and transient forcings and NCEP reanalysis fields.
iii. Relate the interannual variations in the COA parameters to the
simulated NAO, the NPO and the SO and their teleconnections.

Data available in the AMIP II standard output including monthly averaged values of SLP, Geopotential Heights, SSTs, Precipitation, Diabatic Heating Rates, Vertical Velocities, Temperature at standard pressure levels; Daily averaged wind and air temperature, OLR, 500 mb vertical velocity, Geopotential Height, Radiation Fluxes, Potential Vorticity.

AGREEMENT TO PROTOCOL

We agree to cooperate with PCMDI and the AMIP panel, to provide a set of our principal diagnostic results for archival storage at PCMDI, and to prepare a report for the WGNE/WCRP series.

Hameed, S., Pittalwala I. I. and Shi, W., 1996:  Predictability of the Icelandic Low and the Azores High, Proceedings of the twenty first annual climate diagnostics and prediction workshop, Huntsville, Alabama, 304-308.

Hameed, S., Shi, W., Boyle, J. and Santer, B., 1995:  Investigation  of the centers of action in the North Atlantic and North Pacific in the ECHAM AMIP simulation, Proceedings of the First International AMIP Scientific Conference, WCRP-92, WMO/TD-No. 732, 221-226.

Rossby, C. G., 1939:  Relation between variations in the intensity of the  zonal circulation of the atmosphere and the displacements of the semi-permanent centers of action.  J. Mar. Res. 2, 38-55.

Santer, B. D., 1988:  Regional validation of general circulation models. Ph.D. Thesis, University of East Anglia, Norwich, England.

Shi, W. and Hameed, S., 1996:  Effects of local diabatic heating on the  subtropical high pressure systems, Proceedings of the twenty first annual climate diagnostics and prediction workshop, Huntsville, Alabama, 308-311.

Shi, W. and Hameed, S., 1997:  Monsoon circulations and the interannual variability of the Hawaiian High in summer (ms in preparation).

LLNL Disclaimers

UCRL-MI-127350