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
Miami
*1 Aug
Key West
1999
a.k.a. veranillo, canicula
MSD
*1 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).
• useful explanation lies in betweenA
EPAC/WATL (IAS-centric)
summer climate system
H500
H
qV
NASH Z field peaks in Jul-Aug
Romero et al. 2008
Z_500
Z_600
Z_700
Z_850
Midsummer nose on N. Atlantic
Subtropical High (NASH)
H
rain - CMAP
wet
dry
H
wet
wet
late Mayearly June
July
Valparaiso, FL
midsummer drought
Miami
Key West
Cayman
MSD broader in
time than
Veracruz
CMAP
a “corner” on the NASH...
850 mb Z and winds
at peak of MSD
(NCEP clim.)
“bubble”
in mean
anticyclone
dry
date of clim. HF SLP bump
“Bubble”
high moves
around mean
NASH
...or “jet extension”
850 mb Z and winds
at peak of MSD
(NCEP clim.)
scatterometer winds
(Romero et al 2007)
...or should we avoid jet and bubble talk and just use
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!
H
NAM
wet
wet
dry
wet
H
late Mayearly June
July
Can we predict late summer NAM rain
from NASH SLP rise in early summer?
date of clim. HF SLP bump
“Bubble”
high moves
around mean
NASH
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).
• useful explanation lies in betweenA
Different perspectives on the MSD
• A local curiosity?
• A regional phenomenon?
• An aspect of the global monsoon system?
reproduced by many GCMs
e.g. this 11-model superensemble:
MSD
Gets stronger in a (good) GCM
when continents are made hotter
in summer
MSD
dominant WH precip response
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?
“The usual explanation”:
anticyclone driven by east basin
cooling
cooling,
vortex tube
H
shrinking v ~S
S
cool ocean
(upwelling, adv.)
Rossby waves on beta plane
“push” by east basin cooling
H
S
H
S
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
Q
H
Q
2001
Model experiments, forced by realistic heating (above), in
realistic JJA zonal mean flow, with real topography
Dynamical model of response to heating
Rodwell and Hoskins 2001
Asian Heating
Its pure
effect:
Rossby
??
??
Kelvin
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
??
Local heatings (push + pull)
Kelvin
Red: global Q can explain most of the NASH
Similar results: response to specific
MSD heating anomalies
Small et al. 2007
MSD NASH enhancement
partly “pushed” from east
use
observed
heating
anomalies
here
The Central American Midsummer Drought: Regional Aspects and Large-Scale Forcing
Richard Justin O. Small, Simon P. de Szoeke, and Shang-Ping Xie
Journal of Climate
Volume 20, Issue 19 (October 2007) pp. 4853–4873
response to MSD heating anomalies
Small et al. 2007
push + pull can
explain most of
MSD flow anomaly
dry
positive feedback: MSD (reduced) heating
--> anticyclone & easterlies
MSD
Need to calibrate that second step more carefully!
(moist processes, maybe surface couplings too)
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
60N
Westerlies retreat to >30N in midsummer
Qu i ck Ti me ™a nd a
T IF F (LZ W )d ec om pres so r
ne ed ed to s ee th i s pi c tu re.
<0
Eq
J F M A M J J A S O N D
time slice
easterlies protrude
to 30N suddenly
in mid summer
WHY, in terms of [u] budget?
Not f[v]: ~barotropic; [v](t) wrong
[u’v’]: Tilted TUTTs, Tibetan High,
summer TUTTs pump u momentum
v >0,
u >0
v <0,
u <~0
TUTTs: driven by Asian monsoon
heating largely...
Global/ Asian
ultimate cause
for MSD?
A Model of the Asian Summer Monsoon. Part I: The Global Scale
Brian J. Hoskins and Mark J. Rodwell
Journal of the Atmospheric Sciences
Volume 52, Issue 9 (May 1995) pp. 1329–1340
Beyond the Usual Explanation III
• Stationary waves coming through the
midlatitude westerlies
– more barotropic
Stationary wave activity flux
2006
Hudson Bay
contributes too
west coast
thermal gradient
Apparent exp.
for Japan’s
midsummer
drought
(“Bonin High”)
“Formation mechanism
of the Bonin High in
August”
(Enomoto, Hoskins, & Matsuda 2003)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Beyond the usual explanation
NAM
H500
H
[u] sweep
pull
Qconv
push
Q?
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 all
journaled
• assigned - students or others
on web
– some topic, w/ discussion leader
for steady
• a paper, presentation, etc.
progress
– discussion, dangle questions for next time
• Email if interested: mapes@miami.edu
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:
Beyond the usual explanation
H
pull
Qconv >0
[u] sweep
push
Qrad <0
Local heating: a + feedback
• MSD neg. latent heating anomalies can explain
much of the associated flow change
» Small et al. 2007 - talk this session
er)?
• a positive feedback (Amplifi
• dry => easterlies & anticyclone => dry
But logically the MSD can’t explain its own Origin...
e.g. seasonal timing?
Dynamical hypotheses for MSD Origin
w/ (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 easterlies
–
but what is the origin of those?
–
a global monsoon issue
1. Pulled by (subseasonal climatological)
Q’ to west?
subseasonal-timescale rain anomalies in
CMAP climatology, at MSD time
MSDITCZ+
WNPM+
ITCZ+
Jan
Jun
Dec
WNPM+
longitude
MSD-
1b. Pulled by Q’ in NAM?
Big OLR signal, but little CMAP rain’
climatological subseasonal OLR anomalies
at MSD time
NAM
WNPM
ITCZ+
MSD
2. Pushed by Q’ in Atlantic/ Africa?
subseasonal anomalies of heating Q’
WNPM
like CMAP,
(dynamically diagnosed via vort. budget)
not OLR:
(AM) NAM weak
WNPM+
ITCZ+
MSD-
Jan
Jun
Dec
ITCZ
MSD-
longitude
not
much