SOLAR EFFECTS ON RAINFALL AT LAKE VICTORIA, EAST AFRICA. J. Curt Stager, Natural Resources, Paul Smith's College

Transcription

1 SOLAR EFFECTS ON RAINFALL AT LAKE VICTORIA, EAST AFRICA. J. Curt Stager, Natural Resources, Paul Smith's College

2 LAKE VICTORIA World's second largest lake (ca. 69,000 sq.km) Relatively shallow (ca. 80 m) Outflow into the Nile. Most inputs from rainfall, most losses by evaporation: like a huge rain gauge.

3 "LONG RAINS" (MAM) & "SHORT RAINS" (OND).

4 SUNSPOTS AND THE LEVELS OF LAKE VICTORIA From 1896 to 1927 AD, a strong (0.87) correlation existed between SSN and the levels of Lake Victoria. Victoria became a classic example of sun-rainfall effects.

5 After 1927, SSN-lake correlations decayed or reversed. This is now most often described as a classic case of false correlations. A common explanation is that the record examined was originally too short...

6 AN UPDATED RECORD 1. Since 1967, TSI-lake CC's = TSI-lake CC's were significant (0.62) for most of the century (same for SSN). 2. Every sunspot peak coincided with a lake level rise; but the pulse timing fits > relative magnitude.

7 NOT JUST LIMITED TO LAKE VICTORIA Other lakes here have similar decade-scale pulses yr lacustrine pulses in Holocene varves (Kenya) and in the Nile flood minimum record (lake outflow).

8 "Accepted" rain-triggers for this region: ENSO = heavier OND "short" rains...but not always (0.35). Indian Ocean Dipole = some with ENSO, better fit (0.58).

9 DOES INSTABILITY NEGATE AN ASSOCIATION? Linkages between tropical rainfall, ENSO, and IOD are unstable, shutting on & off; as does the "solar association."

10 POSSIBLE MECHANISMS 1. If higher TSI increases tropical SST's: a. Western Indian Ocean warming is associated with enhanced East African rainfall (Black et al., Nicholson et al., Mistry & Conway, Linthicum et al.). b. Tropical Indian Ocean SST's track solar cycles (W. White et al., JGR 1997) 2. Warm-wet, cool-dry pattern dominated late Quaternary.

11 WARM = WET, and COOL = DRY A dense paleosol underlies early Holocene sediments, even in mid-lake. This core from Damba Channel, Uganda, contains a shell deposit at the paleosol contact. (Stager, 1984)

12 DIATOMS

13 Climate cycles present in the GISP2 ice core related to NH insolation and solar variability. Calendar years before 2000 A.D a b 21,000 yr 11,400 yr (ppb) GISP2 sodium yr GISP2 bpc's 1460 yr 950 yr c stronger insolation? When added together, they produce irregular pulses that track the levels of Lake Victoria. They also occur in monsoon records from the Arabian Sea. Victoria CA2 higher relative lake level lower d YD OD OLD dry H-1 Calendar years before 2000 A.D. (Stager et al., 2002) dry? GISP2 CT series weaker insolation?

14 If higher TSI increases rainfall, then RVF outbreaks might be predictable years in advance, simply by tracking the sunspot cycle... TOO GOOD TO BE TRUE?

15 TAKING A CLOSER LOOK: How does the 20th century sun-lake association relate to sun-rainfall relationships?

16 Rainfall pulses are abrupt, shut on/off with ITCZ passage. Lake levels change gradually, smoothing and lagging rainfall by ca. 1 year. Rainfall-lake relations are imperfect (0.4 overall): a. Rain pulses not reflected in lake levels b. Lake rise after 1960 >> rain increase c. Long rains have > consistent lake signal than short rains

17 Early century: highest SSN-lake CC's (0.8). 0 lag: RAIN v. LEVEL = 0.16; RAIN vs. SSN = yr lag: RAIN v. LEVEL = 0.50; and RAIN vs. SSN = 0.34

18 A SEASONAL SIGNAL: Linked to ITCZ system? = SIG.DIFF. LONG RAINS slightly wetter in rising phases of solar cycle. SHORT RAINS slightly wetter in closing phases of solar cycle.

19 No significant correlation between annual rainfall and sunspots/tsi, with 0 or +1 yr lags. With 3-yr smoothing, the highest CC is (0.25).

20 CC's rise to levels resembling those with ENSO when annual rainfall precedes TSI by -1 year = 0.36 (NINO4 = 0.32) = 0.21 (NINO4 = 0.27) = 0.25 (NINO4 = 0.20) African rains cause sunspots? Fortuitous? OR...Rainfall increases most during early phases of solar cycles?

21 Rising phase of each sunspot peak coincided with a large rainfall pulse. Closing phases also had rain pulses in mid-century; relative sizes gradually reversed, cancelling CC's then. Lake pulses fit solar peaks better than the rain series does.

22 SSN series shifted back 1-2 years: SSN peaks overlay rain pulses, as with lake series. Alternating 5-6 yr rain pulses weaken the CC's with SSN. Is the "solar signal" just hidden by ENSO, or coincidental, or does the solar cycle entrain the ENSO rhythm?

23 Lake level (5,8,9,12,16 yrs) Smoothed rainfall (5,6.5,8,9,12,16 yrs) Monthly SSN (5,8,10-11 yrs) NINO4 index (5.5,9-10,13 yrs)

24 LINGERING QUESTIONS: 1. Why are sun-rainfall and ENSO/IOD-rainfall associations so unstable and non-linear? 2. Does the solar cycle interact with ENSO/IOD in setting the pace of tropical rainfall pulses? 3. Why a 12-yr pulse in rains and lake levels? (Solar = yrs, ENSO/IOD = 13 yrs) and 4. Why the recent rise in Victoria levels?

25 TSI, SST's, and rains increased in late 20th century. Smoothed rain vs. SSN = 0.92, with 8 year lag.

26 WHAT CAN WE CONCLUDE? 1. Victoria lake-sun linkages are visually appealing, statistically significant, and plausible. 2. They are also unstable (as are links between lake levels and ENSO/IOD, as well as levels and rainfall series). 3. Most major ENSO/IOD-driven rain pulses occur on either side of SSN maxima, not on the peaks. 5. The 12-year periodicity in rain/lake is close to but not identical to SSN or ENSO/IOD periodicities.

27 CONFUSING ENOUGH YET? Pilkington Bay core P2K-1; shallow-water diatoms (SWD), amoeba remains, and inferred conductivity.

28 WARM SS PEAKS NOW: WET WARM E. HOLOCENE: WET COOL GLACIAL: DRY COOL LIA: WET!? Can there be more than one mode of sun-rainfall interactions here?

29 HYPOTHESIS Could a weaker sun reduce the range over which the ITCZ migrates? This might increase the duration of rainy seasons over East Africa (COOL = WET).

30 SOLAR VARIABILITY vs. ORBITAL CYCLES If this sun-rainfall linkage is real, then the LIA differed fundamentally from "true" ice ages, as well as from today. "Real" ice ages are related to long-term cooling of alternating hemispheres (by orbital cycles). Wouldn't solar weakening during the LIA encourage symmetrical cooling of both hemispheres?

31 BUT if minima caused wetting in LIA, why would rainfall increase during solar maxima NOW? Teleconnections between East Africa and higher latitudes switched phasing at the close of the LIA. If these sun-rain links are real, then they are localized and UNSTABLE!

32 ACKNOWLEDGEMENTS National Science Foundation R. Anyah, D. Conway, J. Lean, P. Mayewski, L.D. Meeker, R. Sutcliffe Paul Smith's College Climate Change Institute, U. Maine, Orono