The mid-summer “drought” (MSD) in the tropical Americas and, announcing... Atlantic Summer...
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Transcript of The mid-summer “drought” (MSD) in the tropical Americas and, announcing... Atlantic Summer...
The mid-summer “drought” (MSD) in the tropical Americas
and, announcing...
Atlantic Summer Klimate (ASK) study group - alternate Fridays in
Map & Chart Room
Brian Mapes, MPO
midsummer drought
30-year climatology
Key WestKey West
Miami
**1 Aug1 Aug
MSD
1999
a.k.a.a.k.a. veranillo, canicula veranillo, canicula
**1 Aug1 Aug
OK, a 10-20% notch in mean rain. Who cares?
• Has some societal impacts» to be honest... not my main motivation
• A distinctive specimen of summer (convective) season hydro-climate variability– slippery, like so much CliVar
• modest amplitude, low frequency– requires statistics to discern
» a sporting challenge
– real, mysterious, tractable (since GCMs have it)!
• use to calibrate understanding of mechanisms
26y mean Observations
(CMAP)
Mean climatology of 10 GCMs
What causes the MSD?• Global, vague:
– Insolation seasonality ultimately drives all aspects of the seasonal cycle.
– Continent-ocean contrasts involved
• Local, begs questions: – The ingredients for convection get worse
(moisture, instability, lifting mechanisms).
• usefuluseful explanation lies in betweenA
EPAC/WATL (IAS-centric) summer climate system
qV
HH500500
HH
NASH Z field peaks in Jul-Aug
Z_850
Z_500
Z_700
Z_600
Romero et al. 2008
rain - CMAP
late May-early June
July
Midsummer nose on N. Atlantic Subtropical High (NASH)
dry
Carib easterly jet
wet
wet
H
H
wet
midsummer drought
Key WestKey West
Miami
Valparaiso, FL
CaymanCaymanMSD broader in
time than VeracruzVeracruz
CMAP
dry
a “corner” on the NASH...
850 mb Z and winds at peak of MSD(NCEP clim.)
“bubble”in mean
anticyclone
“Bubble” high moves
around mean NASH
date of clim. HF SLP bump
...or “jet extension”
850 mb Z and winds at peak of MSD(NCEP clim.)
Carib easterly jet
scatterometer winds (Romero et al 2007)
...or should we avoid jet and bubble talk and just use ...or should we avoid jet and bubble talk and just use neutral “field anomalies” language? neutral “field anomalies” language?
high frequency anomaly fields(midsummer - broad summer)
SLP
rain - TRMM 3B43
rain - CMAP
OK, an “explanation” (story) about the mean annual cycle.
But mean is a fiction of averaging!
late May-early June
July
dryCarib easterly jet
wet
wet
H
Hwet
NAM
“Bubble” high moves
around mean NASH
date of clim. HF SLP bump
Can we predict late summer NAM rain from NASH SLP rise in early summer?
Can we predict late summer NAM rain from NASH SLP rise in early summer?
What causes the MSD?• Global, vague:
– Insolation seasonality ultimately drives all aspects of the seasonal cycle.
– Continent-ocean contrasts involved
• Local, begs questions: – The ingredients for convection get worse
(moisture, instability, lifting mechanisms).
• usefuluseful explanation lies in betweenA
Different perspectives on the MSD
• A local curiosity?
• A regional phenomenon?
• An aspect of the global monsoon system?
MSD
dominant WH precip response
Gets stronger in a (good) GCMwhen continents are made hotterin summer
reproduced by many GCMs e.g. this 11-model superensemble:
MSD
What causes the MSD?
• Suppose it’s regional low-level flow – NASH enhancement– Process-level mechanisms of rainfall reduction
still need elaboration...• (...but for now, accept High Dry connection)
• Then what causes the NASH enhancement?
cool oceancool ocean(upwelling, adv.)(upwelling, adv.)
“The usual explanation”: anticyclone driven by east basin
cooling
Rossby waves on beta plane
cooling, cooling, vortex tube vortex tube shrinkingshrinking
SS
v ~SH
“push” by east basin cooling
H SSH SS
Beyond the Usual Explanation I
• Hoskins (1996): • “the usual explanation for the subtropical
anticyclones (radiative cooling)” in summer is “inadequate”
• “monsoon latent heat release over the neighboring continent” is essential
“pull” from the west
H
H
2001
Model experiments, forced by realistic heating (above), in realistic JJA zonal mean flow, with real topography
Dynamical model of response to heatingRodwell and Hoskins 2001
Asian Heating
Its pure effect:
RossbyRossby
KelvinKelvin
??
??
2-3 m/s Caribbean easterlies “pulled” from west
(by Kelvin wave from Asia)
Americas Heating Q:
(from Rodwell & Hoskins
2001 again)
forced by NAm Q
as above, +NPac Q Rossby
Kelvin
??
Red: global QLocal heatings (push + pull)
can explain most of the NASH
Similar results: response to specific MSD heating anomalies
Small et al. 2007
useuseobserved observed heating heating
anomaliesanomaliesherehere
MSD NASH enhancement partly “pushed” from east
The Central American Midsummer Drought: Regional Aspects and Large-Scale ForcingRichard Justin O. Small, Simon P. de Szoeke, and Shang-Ping XieJournal of Climate Volume 20, Issue 19 (October 2007) pp. 4853–4873
positive feedback: MSD (reduced) heating
--> anticyclone & easterlies MSD
Need to calibrate that second step more carefully! (moist processes, maybe surface couplings too)
response to MSD heating anomaliesSmall et al. 2007
push + pull can explain most of
MSD flow anomaly dry
Beyond the Usual Explanation II
• Zonal mean flow– basic state for heating-induced circulations
MSD ultimate driver: [u] changes in midsummer?
Heating
Eddy Z1000
w/o shear
Eddy Z1000
July u(y,p)
Chen, Hoerling & Dole 2001
u300, zonal mean Jul-Aug time slice
Westerlies retreat to >30N in midsummerQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.
<0
time slice
easterlies protrudeto 30N suddenlyin mid summer
WHY, in terms of [u] budget? Not f[v]: ~barotropic; [v](t) wrong
[u’v’]: Tilted TUTTs, Tibetan High, Transients?
Eq
60N
J F M A M J J A S O N D
summer TUTTs pump u momentum
v >0,u >0
v <0,u <~0
TUTTs: driven by Asian monsoonheating largely...
A Model of the Asian Summer Monsoon. Part I: The Global ScaleBrian J. Hoskins and Mark J. RodwellJournal of the Atmospheric Sciences Volume 52, Issue 9 (May 1995) pp. 1329–1340
Global/ Asian ultimate cause
for MSD?
Beyond the Usual Explanation III
• Stationary waves coming through the midlatitude westerlies– more barotropic
Stationary wave activity flux
west coast thermal gradient
Hudson Baycontributes too
2006
Apparent exp. for Japan’s midsummer
drought (“Bonin High”)
“Formation mechanism of the Bonin High in
August”(Enomoto, Hoskins, & Matsuda 2003)
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
HH500500
HHNAMNAM
[u] sweep[u] sweeppushpushQ?Q?
pullpullQconvQconv
RossbyRossby
waveswaves
Beyond the usual explanationBeyond the usual explanation
qV
Summary• Our midsummer dryness is an interesting, unexplained,
tractable specimen of cli. var. – simulated in many GCMs
• dynamics must not be tooo subtle or fine-scale!
• Occurs with low-level anticyclone/E’ly anom.• largely self-forced
– MSD amplified by pos. feedbacks, but what is source?
– moist process “amplifier” needs study, MSD could calibrate it
• Three dyn. hypotheses offered – cooling push, heating pull, [u] shear, st’y wave flux
• Other ideas out there– feedbacks to ocean, land
Atlantic Summer Klimate (ASK) study group
• Will misspell anything for acronym• Not overawed by “climate” grandeur• Va. Key expertise sufficient to get to the
bottom of all this science, and to bring students along from scratch - don’t be shy!
• Asking, refining and following questions is the job
other seasonal processes?
• Winter before (via SST? NAO/ENSO?)
• African and North American monsoons– have onsets/transitions about same time
• push, pull; transports of q, dust
• Asian monsoon – via its zonal mean impacts, & ultralong waves– Study Asian or Brazilian (SACZ) analogues...
• Hurricane season
TC track density midsummer vs. late
MSD relationships to the weather
SYNOPTIC EXPERIENCE/ TEXTURE NEEDED
courtesy Jason Dunion
linkages/ analogies to other scales?
• Timescale: – TAV, AMO, AMM, MOC, AGW, Paleo
• Reframings of spatial scale/region? – IAS– EPac– NAtl– Pan-Atlantic
ASK group• Fridays, Map & Chart Room, 12-1:30
– STARTING THIS WEEK
• Agenda: – chat, snacks, swap stories/code/data/ideas
– followup on last time’s dangling questions• assigned - students or others
– some topic, w/ discussion leader • a paper, presentation, etc.
– discussion, dangle questions for next time
• Email if interested: [email protected]
all journaled
on web for steadyprogress
regional klymut
• unglamorous poor cousin to Researching Hurricanes or Global Climate Change – not sexy
• like Powerful Storms with Huge Impacts
– not elegant • like the Grand Average over Everything
– but so many environmental issues lie in this middle ground...
regional klymut tools
• statistics– the only power in regional averaging comes from statistical “laws” - no grippy integral conservatons govern 5% of globe
• regional klymut modeling– parameterizations for both convection and interactions with larger scales. ugh.
Tools available
• Data– global: reanalysis, GCM archives
• deduction from interannual, intermodel comparisons
– regional: long records at some sites– local: soundings archives, Explorer, radars,...
• vertical structure
– Satellite: all of the above!
positive feedback• An amplified, twitchy system - what drives it, to make
this climatological feature?
1. Local SST cycle hypothesis?– not well supported by obs
2. Global easterlies in midsummer?– but where do they come from?
3. Midlatitude wave pathways?
4. Asian monsoon? – via Rodwell-Hoskins’ wraparound Kelvin wave?
One hypothesis:
H
[u] sweep[u] sweep
pushpushQrad <0Qrad <0
pullpullQconv >0Qconv >0
st’yst’ywaves
waves
Beyond the usual explanation
Local heating: a + feedback
• MSD neg. latent heating anomalies can explain much of the associated flow change
» Small et al. 2007 - talk this session
• a positive feedback (Amplifier)?• dry => easterlies & anticyclone => dry
But logically the MSD can’t explain its own Origin... e.g. seasonal timing?
Dynamical hypotheses for MSD Originw/ (imperfect) evidence: similar climatological timing
1. anticyclone/easterlies “pulled” by Q’ to west– EPAC ITCZ?
– WNP monsoon onset?
– NAM?
2. ... “pushed” by Q’ to east– something about Africa? (monsoon onset, dust?)
– Marine processes over Atlantic?
3. ... “sheared in” by zonal mean easterlies1. but what is the origin of those?
1. a global monsoon issue
1. Pulled by (subseasonal climatological) Q’ to west?
MSD-MSD-WNPM+WNPM+
ITCZ+ITCZ+
subseasonal-timescale rain anomalies in CMAP climatology, at MSD time
MSD-MSD-
Jan
Jun
Dec
longitude
WNPM+WNPM+
ITCZ+ITCZ+
1b. Pulled by Q’ in NAM?Big OLR signal, but little CMAP rain’
MSDMSD
NAMNAM
climatological subseasonal OLR anomalies at MSD time
WNPMWNPM
ITCZ+ITCZ+
2. Pushed by Q’ in Atlantic/ Africa?
MSD-MSD-(AM)(AM)
subseasonal anomalies of heating Q’ (dynamically diagnosed via vort. budget)
MSD-MSD-
Jan
Jun
Dec
longitude
ITCZ+ITCZ+WNPM+WNPM+
WNPMWNPM
ITCZITCZ
like CMAP, not OLR: NAM weak
not much