# Presentation: 919334

## Interannual Variability of Surface Longwave Radiation over the African Continent as Derived from AVHRR

- * Cooperative Institute for Meteorological Satellite Studies (CIMSS) / Department of Atmospheric & Oceanic Sciences.
-   Currently of Earth System Science Interdisciplinary Center / Department of Meteorology, University of Maryland-College Park.

**Introduction**

-      The variability of outgoing longwave radiation (OLR) over the African continent is of notable importance on both regional and global scales, and its distribution and magnitude is a key factor of the radiation budget of the Earth and the climate system.
-      Regionally, Sahel Africa is particularly sensitive to the impacts of climate change because of such factors as poverty, frequent drought, and over-dependence of precipitation-sensitive agriculture.
-      Also, for the first time, the relationship between the meridional OLR gradient and the frequency of Atlantic tropical cyclones is statistically analyzed.

**Results**

- 1974                  2003

- JJA OLR Max.

- JJA OLR Min.

- Max. – Min.

**Methods**

-      There exists a permanent OLR gradient over the African continent: a maximum to the north and a minimum over central Africa.  The intensity of max. and min. centers of OLR over 30 years (1974-2003) were composited as JJA and DJF means.  30-year timeseries for max., min., and resulting contrast were then constructed for analysis and comparison to tropical Atlantic activity.
-      The OLR data, collected by the Advanced Very High Resolution Radiometer (AVHRR) on TIROS-N and other NOAA- series satellites, has 1.1 km (nadir) spatial resolution.
-      Atlantic hurricane/tropical cyclone data were filtered to remove any non-Cape Verde-type storms, as it is presumed storms originating elsewhere are not related to easterly waves propagating off the west coast of Africa.

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- been known for severe land degradation through over-grazing and exploitation, which increases surface albedo. It can be shown from space that changes in population affect the vegetative cover which directly influences the OLR from the surface. Note the collocation of population maxima *(below right)* with the 1999 OLR minima.

-      The interannual variability of the OLR gradient is closely related to that of Atlantic tropical cyclones.  Both the trends *(top left)* and the detrended, normalized variability *(bottom left)* of OLR gradient and tropical cyclone frequency were analyzed.  Between 1986 and 1998, the correlation is 0.84.
-      The meridional gradient of temperature is a first-order term in vertical zonal sheer.  By OLR emission’s relation to temperature, it is proposed that an intensification of the OLR contrast signifies a strengthening of the African Easterly Jet (AEJ).

**Summary**

-      This work resulted in a number of findings regarding the interannual variability of OLR that is of great importance to the climate system:

**Acknowledgements**

-      Thanks to Drs. Steve Ackerman, Jim Kossin, Jon Martin, Mr. Tim Schmit, and three anonymous reviewers for their helpful words.  OLR data courtesy of NOAA Climate Diagnostics Center, Boulder, CO.  Tropical storm data courtesy of NOAA Tropical Prediction Center in Miami, FL.  Population data courtesy of Socioeconomic Data and Applications Center.  Thanks to Dr. Tony Busalacchi for poster materials.

**Implications**

-      The above relationships may be useful in understanding and verifying land-atmosphere coupling in general circulation models.  The potential usefulness of using the OLR gradient to study the variability of Atlantic tropical cyclones is also demonstrated.  Most importantly, it should remind us of the power that humans have in changing the surface of the Earth and the careful regard with which we must proceed.

-      To illustrate the spatial variability of climatological OLR features, a persistent OLR minimum over Cameroon in 1998 shifted into Nigeria by the following year *(below left).*  The original minimum split into two distinct minima: one near Zaria (north) and one near Port Harcourt (south), Nigeria.  JJA mean OLR over Port Harcourt decreased from 211 to 196 Wm-2 between 1998 and 1999.  Sahel Africa, especially near the southern coast of Nigeria, has long

*K.B. Karnauskas, University of Wisconsin- Madison****

- kris@essic.umd.edu

-  OLR contrast over Africa is increasing by 0.46-0.67 Wm-2 per year.

-  Individual centers of OLR minima and maxima can shift hundreds of kilometers over periods of years to decades.  There is evidence to suggest that anthropogenic activity may contribute to these changes.

-  A compelling correlation exists between the OLR contrast over Africa and the incidence of Cape Verde-type Atlantic tropical cyclones.