Results Frontier Newsletter No.15 Jul.2001

Interhemispheric Oscillations in the Atmosphere

Dr.Zhaoyong Guan(Climate Variations Research Program) , Dr.Toshio Yamagata(Program Director,Climate Variations Research Program)

The low-frequency variations in the global climate system have received much attention in recent decades. Several strong climate signals are well-known in the atmosphere; those are the Madden-Julian Oscillation, the Southern Oscillation, the North Atlantic Oscillation, the Pacific/North America teleconnection pattern / the Arctic Oscillation, and some low-frequency principal modes related to the polar vortex in the Southern Hemisphere on several different time scales.

Another very important manifestation of the low-frequency variability we note in the atmosphere is the Interhemispheric Oscillation (IHO). The IHO represents the air mass redistribution between the two hemispheres because the total mass of the global atmosphere must be conserved on any time scale. We have analyzed the surface air pressure (SAP) field obtained from NCEP/NCAR 40-year monthly mean reanalysis to clarify the nature of IHO.

What the IHO displays is that the global oscillations of the anomalous SAP in the Northern and Southern Hemispheres are basically out of phase (Fig. 1). A strong annual cycle of the IHO derived from the anomalous SAP is seasonally phase-locked. The IHO signal is also detected at periods of approximately 150 months, 33 months, 5 months and 3 months.



Figure 1: (a) Time evolution of the area mean surface air pressure in units of hPa for the Northern Hemisphere (solid red), Southern Hemisphere (dashed blue) and the whole globe (green color filled). We have smoothed the time series using an 11-month running mean. The time dependence of the global mean surface air pressure is considered as the data error.
(b) The time series of IHO index in units of hPa (thin line) and the same after smoothed with an 11-month running mean (thick line).

The EOF analysis for zonal mean detrended anomalous SAP reveals further interesting features. The first EOF, explaining 36.4% of total variance, shows the general characteristics of the oscillation located in the Southern Hemisphere, whereas the second EOF, explaining 21.1% of total variance, shows the oscillation in the Northern Hemisphere. The correlation between the principal components of EOF1 (EOF2) and SAP anomalies shows a pattern similar to well-known Antarctic Oscillation (Arctic Oscillation).

The air mass associated with EOF1 (EOF2) is almost conserved in the Southern (Northern) Hemisphere. Thus, the major contributor to IHO varaibility is found in the EOF3 of the SAP anomaly. The EOF3, explaining 14.5% of total variance, reflects the fluctuation in the mid- and high- latitudes in both hemispheres. The spatial pattern of the IHO shows some lows (highs) in the mid- and high- latitudes in the Northern Hemisphere and one high (low) in the mid- and high- latitudes in the Southern Hemisphere at the same time (Figure 2). Since the total air mass should be conserved in principle, the SAP rise in one hemisphere should be accompanied with the drop in the other hemisphere.

The multidecadal trend of IHO is also seen in the anomalous SAP field, which seems to be related to the global warming signal. The dynamical mechanism of IHO as well as its real nature will be investigated further using observational data and a high-resolution numerical model in the near future.

Figure 2. The spatial pattern of the IHO. The contour interval is 0.25hPa.

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