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Indo-Pacific Sea Surface Temperature Influences on Failed Consecutive Rainy Seasons over Eastern Africa**. Andy Hoell 1 and Chris Funk 1,2.

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Indo-Pacific Sea Surface Temperature Influences on Failed Consecutive Rainy Seasons over Eastern Africa**

Andy Hoell1and Chris Funk1,2

**Hoell, A. and C. Funk, in press: Indo-Pacific sea surface temperature influences on failed consecutive rainy seasons over eastern Africa, Climate Dynamics, doi: 10.1007/s00382-013-1991-6

1University of California Santa Barbara; 2U.S. Geological Survey

Contact: hoell@geog.ucsb.edu

1. Introduction

Below average precipitation during consecutive long and short rains seasons over eastern Africa (Fig. 1a) can have devastating long-term impacts on water availability and agriculture. We examine the forcing of drought during consecutive long and short rains seasons over eastern Africa by Indo-Pacific sea surface temperatures (SSTs). The forcing of eastern Africa precipitation and circulation by SSTs is tested using ten ensemble simulations of a global weather forecast model forced by 1950-2010 observed SSTs.

4. Seasonal SST and Precipitation

SST areas likely responsible for forcing precipitation over eastern Africa were identified through correlation of SST-forced precipitation and SST (Fig. 6). During MAM, eastern Africa rainfall is significantly related with SST over the Indo-Pacific Warm Pool and with Pacific SST that weakly resembles PDV. During OND, eastern Africa rainfall is significantly related with El Nino-Southern Oscillation (ENSO) and the Indian Ocean dipole.

5. MAM SST-Forced Precipitation

The trend in west Pacific SST (Fig. 9e,f) force significant changes to the regional circulation that reduces precipitation over the entire northwest Indian Ocean Rim. The Indian Ocean dipole also forces similar circulation and precipitation changes (Fig. 9c,d), but over a smaller spatial extent. However, ENSO (Figs. 9a,b) and interannual variability in west Pacific SSTs (Figs. 9g,h) do not force significant changes to the low-level circulations and precipitation over eastern Africa.

3. Decadal to Multi-Decadal Variability

During MAM, SST forced significant precipitation departures during 1950-1978 and 1999-2010 over eastern Africa, the Arabian Peninsula and extreme southern portions of central-southwest Asia. This highlights the long-term trend in eastern Africa precipitation and a possible decadal variation in precipitation. Meanwhile, during OND over eastern Africa, the significant SST-forced precipitation departures were confined to the early period of 1950-1978.

Fig. 6: Correlation of SST-forced precipitation departure over eastern Africa and observed SST for a) MAM and b) OND. The white contour denotes values significant to p<0.05.

Fig. 4: SST-forced percent precipitation departure for (top row) 1950-1978, (middle row) 1979-1998 and (bottom row) 1999-2010 during (left column) MAM and (right column) OND. The white contour denotes values significant to p<0.05 using a resampling approach.

Fig. 9: (left column) Regression of SST-forced 850 hPa wind magnitude (shaded) and normalized wind vectors to standardized indices of SST variation during MAM. (right column) Regression of SST-forced precipitation to standardized indices of SST variation during MAM. The white contour denotes values significant to p<0.05 using a student t test.

Fig. 1: a) The eastern Africa domain, taken to be the land area bounded by the purple box (10°S-10°N, 35°E-50°E). b) Average monthly observed precipitation for 1981-2010 spatially averaged over the eastern Africa domain, highlighting the long rains of March-May (MAM) and the short rains of October-December (OND).

The temporal variability of ENSO, the IOD and the west Pacific SSTs are shown in Fig. 7.

2. Consecutive SST-Forced Droughts

The frequency of SST-forced drought spanning consecutive MAM and OND seasons increased substantially during the late 1990s, lasing into the 2000s (Fig. 2b). The most persistent drought conditions occurred between OND 1998 and MAM 2002, during which there were eight consecutive dry MAM and OND seasons. For MAM, precipitation during 1950-2010 over eastern Africa decreased considerably (Fig. 3a). Three precipitation periods are distinguishable during MAM within the long-term decreasing trend: 1950-1983, 1984-1998 and 1999-2010. For OND, precipitation varied primarily on interannual time scales within a slight long-term decreasing trend.

6. OND SST-Forced Precipitation

Seasonal-to-interannual variability associated with ENSO, the IOD and the interannual component of west Pacific SSTs significantly impact the lower tropospheric circulation and precipitation over the Northwest Indian Ocean Rim (Fig. 10). However, while the trend in west Pacific SSTs appears to influence precipitation over the Northwest Indian Ocean Rim, these influences are not statistically significant.

Decadal to multi-decadal SST changes resembling Pacific Decadal Variability (PDV) and a long-term warming of the global oceans (Fig. 5) occur concomitantly with eastern Africa precipitation changes (Fig. 4). Cool PDV SST signatures are distinguishable during 1950-1978 and 1999-2010 despite strong tropical warming between the two periods. A warm PDV signature was present during 1979-1998.

Fig. 7: a) ENSO time series as measured by the b) NINO3.4 index. b) IOD time series as measured by the d) Dipole Mode Index. c) Western Pacific SST variability as measured by area-averaged SSTs over the boxed region in f).

The long-term trend and interannual variability of west Pacific SSTs are separated in Fig. 8. West Pacific SSTs steadily increased throughout the entire period, but sharply increased after 1999. The interannual component of west Pacific SSTs and ENSO differ in terms of magnitude.

Fig. 10: (left column) Regression of SST-forced 850 hPa wind magnitude (shaded) and normalized wind vectors to standardized indices of SST variation during OND. (right column) Regression of SST-forced precipitation to standardized indices of SST variation during OND. The white contour denotes values significant to p<0.05 using a student t test.

Fig. 5: Observed SST anomaly (°C) for a) 1950-1978, b) 1979-1998 and c) 1999-2010. All plots are significant to p<0.05 using a resampling approach.

Fig. 8: a) Observed western Pacific SST variability as measured by the area-averaged SSTs over the boxed region in Fig. 7f. b) Observed western Pacific SST trend, as captured by a five-year running mean of the observed western Pacific SST time series. c) Observed de-trended western Pacific SST, which is calculated as the difference between the western Pacific SST time series (a) and the western Pacific SST trend (b).

Fig. 2: a) SST-forced MAM and OND percent precipitation departure spatially averaged over eastern Africa for 1950-2010. b) The maximum number of consecutive dry MAM and OND seasons that occurred within a 5-yr window centered around the labeled year.

Fig. 3: Observed SST-forced percent precipitation departure during a) MAM and b) OND spatially-averaged over eastern Africa for 1950-2010.