Commentary on the Paper of Karoly

David E. Parker

U.K. Meteorological Office

I was particularly interested in Dr. Karoly's video, which raises several questions. I think it would be possible to verify and improve his land-based analyses, because we have large numbers of marine data that we at the Met Office, and Henry Diaz and others at NCAR, are hoping to put together in a blend of our marine data bank and the COADS data base. The COADS data and ours include pressures and winds, so that they can be checked against one another. They should be a good method of filling in this area of the world, which is not very well covered with observations.

Some of you may also have read Neil Ward's work (J. Climate, 1992) in which he corrects marine winds. The measured winds appear to have become stronger relative to the geostrophic winds calculated from the pressures. So far, this work has gone back only to 1949, although similar work could be carried out for earlier years. It is very labor-intensive, but one of my recommendations is that the Southern Hemisphere surface data base be improved. An improved surface data base, especially in the Southern Hemisphere, might help us better understand the mechanisms of climate. For example, Section C of the 1992 IPCC Science Supplement shows what happens to estimates of trends in oceanic latent heat fluxes if we apply appropriate corrections to the winds.

Unfortunately, many of the upper-air data from Southern Hemisphere rawinsonde stations are missing, e.g., in Brazil, Antarctica, and southern tropical Africa. I would therefore recommend that everything be done to maintain the network, especially in developing countries and on remote islands. The other problem with the upper-air data is that changes of instrument type and changes of operating practice must be compensated for. To do this, we need adequate station histories. These, as well as the data, are being collated in the Comprehensive Aerological Reference Data Set (CARDS) project under Tom Karl and others at Asheville.

Ultimately, of course, the adjusted data should be put together in a fixed-model-based reanalysis, to yield self-consistent analyses. This is being done by the European Centre for Medium-range Weather Forecasting, and data back to 1958 are being analyzed at NOAA/NMC.

And last, the South Pacific Convergence Zone is an intriguing phenomenon, and vital to our understanding of the Southern Hemisphere climate. So, extra effort should be made to measure and understand it.

Discussion

GHIL: You called your first two EOFs high latitude and low latitude, but what matters is where the nodal line is. Perhaps you have pools of air trapped behind the high-latitude and subtropical jets that happen to have different temperature anomalies.

KAROLY: The interaction between the storm tracks and the main jets is a complicated process that can lead to long-lived low-frequency anomalies. The general symmetry of the anomalies is perhaps more characteristic of the Southern Hemisphere because of the topography. But I agree that it could be described in terms of symmetric air-mass contrast between warmer and colder regions.

GROOTES: Ice-core records for different parts of Antarctica might be able to give you an idea of whether this kind of circulation persisted.

SHUKLA: In the video you showed, almost all the major fluctuations around Australia were coming from the south. Was that a data problem?

KAROLY: It looks that way until the 1950s, but then till the 1970s or 1980s they seem to come more from the north and after that there is no obvious signal. I imagine it's a data problem, but the only way to know is to look at the individual station data. I'm hoping to persuade Phil Jones to do some analysis of this.

TRENBERTH: The dominant mode in the Southern Hemisphere is a double jet structure, with the jets alternating in strength. This mode has been simulated by the Canadian Climate Center and by the Japanese, and in fact turns out to be the dominant global mode. It is entirely derived from natural variability, and seems to have no connection to any external forcing.

KAROLY: It's not clear that internal fluctuations in the atmosphere alone can provide enough longevity to extend it into the decadal range. There might be a link with the Antarctic circumpolar current or the distribution of ocean temperature in the Southern Hemisphere that could provide a longer duration. The Max Planck Hamburg model produces a 50-year episode of this mode, as I remember. It is internal to these sorts of models.



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