• Introduction
• Data
• Analysis and discussion
• Conclusion
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3

The climate system, encompassing complex interactions between the different subsystems such as the oceans, the land surface, and the ice coverage of land and oceans, incorporates many feedbacks. These account for the systems behavior. The El Nino/ Southern Oscillation (ENSO) is an example of a natural irregular climate fluctuation that involves extensive interaction between atmosphere and ocean. El Nino influences climate in distant regions via teleconnections. This has been evidenced, for instance, by consistent changes in precipitation in specific areas of the globe. Droughts in South-east Asia, parts of Australia, Parts of Africa, and heavy rainfall and flooding in arid areas of South America and East Africa have been observed during El Nino years. ENSO events have a significant economic impact on developing countries in the tropics, East Africa not an exception, primarily through effects on food production. In this regard, therefore, the present work presents an analysis of the temporal characteristics of the ENSO-Total ozone relationship over Kenya in East Africa.

Dobson spectrophotometer and Total Ozone Mapping System (TOMS) total ozone data, together with ozone sounding data are used in the study. Statistically significant relationships between NINO3 and total ozone in the first half of the year are observed. The long rains season over Kenya (March to May) suggest a negative connection with NINO3. The period before the start of the October-to-November (short) rains, on the other hand, depicts a positive relationship.

Connection between Convective activity and total ozone’s temporal pattern is also investigated. METEOSAT Cold Cloud Duration (CCD) data show appreciable relationship with total ozone over the region. Distinct patterns for the two rainy seasons are evident. The relationships suggest possible significant association between convective activity and possible tropospheric ozone production and/or stratospheric ozone redistribution. The results seem to show signs of applicability of total ozone over East Africa as an indicator of climate variability and/or change.

Introduction

The traditional view in climate has been that the stratosphere can only play a limited role in climate change. However, there has been increasing evidence in recent years that the stratopshere is a sensitive component of the climate system, which can affect the troposphere through coupling mechanisms.

There are three principal mechanisms by which the stratosphere can affect tropospheric climate. The first is through radiative transfer, either by changes in the amount of solar radiation that reaches the surface (e.g. after a volcanic eruption), or by changes in the amount of downwelling longwave radiation emitted by the stratopshere (e.g. because of stratospheric ozone depletion). The impact depends very sensitively on the vertical, latitudinal and seasonal structure of the changes in radiatively active substances, particularly in the vicinity of the tropopause. (Forster et al. 1997; Hansen et al., 1997).

The fact that the distribution of the radiatively active substances is controlled by the Brewer-Dobson circulation together with quasi-horizontal in mixing into the lower stratosphere emphasizes that climate models need to represent these processes with sufficient fidelity in order to capture this sensitivity.

The second and third mechanisms by which the stratosphere can affect tropospheric climate take account of the basic dynamical fact that tropospherically forced waves propagate up, while zonal mean anomalies propagate down. Thus, the second mechanism is that the stratosphere can affect the "upper boundary condition" of troposphere by affecting the propagation characteristics of tropospheric waves. The possibility goes back to the classic work of Harney and Darzin, but there has been remarkebly little investigation on this issue in recent years. The possibility of wave reflection at the tropopause has obvious implications for regional climate purtabations.

The third mechanism is then the downward propagation of zonal-mean anomalies, whose mechanism is ‘downward control’. Such downaward influence has been seen in model studies (Kodera et. al., 1996).

In view of the above, therefore, the present work presents an analysis of the temporal characteristics of the convection and ENSO versus Total ozone relationship over Kenya in East Africa.

Data

TOMS total ozone data were collected from the NASA TOMS website. The period 1991 to 1994 and 1996 to 1999 were considered.

Dobson total ozone data were obtained from the Nairobi station of the Dobson total ozone observatory. The period 1984 to 1999 was considered. Meteorological data were collected from the Kenya Meteorological Department. They included Rainfall and Lightning events.

Analysis and discussion

To investigate the relationship between El Nino and total ozone over Nairobi NINO3 data were correlated with total ozone over Nairobi. El Nino has been widely documented as significantly associated with rainfall activities over East Africa. In deed the various phases of El Nino are now been used operationally for the climate outlook over the East African region.

Kenya is characterized by two rainy seasons, namely October to December (commonly known as ‘short’) rains and the March to May (called ‘Long’) rains. Convective activity is most prominent during the long rains and much of the rain then is attributable to thunderstorm activities.

Figure 1: Temporal plot of normalized NINO3 values

An attempt to establish a relationship between stratospheric dynamics and climate variability is made by correlating total ozone with NINO3. Table 2 below shows results of the analysis.

Jan	-0.57034
Feb	-0.23762
Mar	-0.41252
Apr	-0.28679
May	-0.18552
Jun	-0.09095
Jul	-0.29339
Aug	0.17345
Sep	0.406873
Oct	0.020952
Nov	-0.27222
Dec	-0.35314

Table 2: correlation between total ozone and nino3

Statistically significant relationships between NINO3 and total ozone in the first half of the year are observed. The long rains season over Kenya (March to May) suggest a negative connection with NINO3. The period before the start of the October-to-November (short) rains, on the other hand, depicts a positive relationship.

Figure 3: Temporal patterns of normalized ozone values

Figures 1 and 3 below illustrates the temporal patterns the normalized values of both total ozone and NINO3 during the long rains season in Kenya. Table 4 below shows the anomalous months/years with respect to both variables.

Year	Month	Ozone	NINO3
1985		÷ (positive)
1987		÷ (negative)	÷ (Positive)
1988			÷ (negative)
1992		÷ (negative)	÷ (Positive)
1997		÷(negative)	÷(Positive)

Table 4: anomalous years with respect to NINO3 and total ozone.

An notable inverse relationship of the anomalous years during the long rains is a good indication of a potential use of total ozone data for prediction of extreme weather events.

Connection between Convective activity and total ozone’s temporal pattern is also investigated. METEOSAT Cold Cloud Duration (CCD) data show appreciable relationship with total ozone over the region. This is demonstrated in Table 5 and Figure 6 below:

Distinct patterns for the two rainy seasons are evident. The relationships suggest possible significant association between convective activity and possible tropospheric ozone production and/or stratospheric ozone redistribution.

lag	correlation
0	-0.097
1	0.121808
2	0.257247
3	0.318821
4	0.325792
5	0.093563

Table 5: Lag correlation coefficient of total ozone versus thunder events

Figure 6: Temporal variablity of total ozone versus thunderstorm events in Nairobi, Kenya.

Conclusion

A notable inverse relationship between ozone and NINO3 anomalous years during the long rains in Kenya is evident. This is an indication of a potential use of total ozone data for prediction of extreme weather events.

Cold Cloud Duration (CCD) data show appreciable relationship with total ozone over the region. Distinct patterns for the two rainy seasons are evident. The relationships suggest possible significant association between convective activity and possible tropospheric ozone production and/or stratospheric ozone redistribution. The results seem to show signs of applicability of total ozone over East Africa as an indicator of climate variability and/or change.

References

Forster , P. M. de F., R. S. Freckleton and K. P. Shine: On aspects of the concept of radiative forcing, Clim. Dyn., 13, 547-560, 1997

Hansen , J. M. Sato and R. Ruedy: Radiative forcing and climate response, J. Geophysical Res., 102, 6831-6864, 1997

Kodera , K. M. Chiba, H. Koide A. Kitoh, and Y. Nikaidou: Interannual variability of the winter startosphere and troposphere in the Northern Hemisphere, J. Meteor. Soc. Japan, 74 365-382, 1996