The central American midsummer drought
(MSD, veranillo or canicula)
Brian Mapes
University of Miami
midsummer drought
30-year climatology
Miami
Key West
1999
MSD
(
?
What causes the MSD?
• Local, fussy:
– The ingredients for convection get worse
(moisture, instability, lifting mechanisms).
• Global, vague:
– Solar declination angle ultimately drives all
aspects of the seasonal cycle.
– Continent-ocean contrasts involved
• Somewhere in between lies a useful truth...A
What causes the MSD?
• Regional:
– North Atlantic Subtropical High (NASH or
Bermuda High) & tradewinds are enhanced and
extended west in mid-summer.
Midsummer nose on N. Atlantic
Subtropical High (NASH)
Veracruz Cayman
late Mayearly June
Valparaiso
wet
dry
July
wet
upwind
slopes
wetter in
midsummer
Valparaiso, FL
midsummer drought
Miami
Key West
Cayman
MSD broader in
time than
Veracruz
CMAP
Individual years - still discernable
(in big regional Central American box average)
intraseasonal can
mask it a bit
9 year
mean
TAIWAN 25N
MSD
happens in
other places
too
May
CUBA 25N
May
SE BRAZIL 25S
Nov.
MADAGASCAR
25S
CMAP rainfall
Nov.
Where are those boxes? W. subtropical oceans
High
igh
High
High
What causes the MSD?
• Regional:
– NASH (Bermuda High) & tradewinds are
enhanced and extended west in mid-summer.
• Locally, less rain is due to:
–
–
–
–
–
frictional divergence or other subsidence mech.?
flow direction brings drier airmass?
wind leads to lower SST (albeit via enhanced LHF)...?
capping inversion strength/height?
different meso/synoptic disturbance characteristics?
• But what causes the NASH/trades to vary?
A broader view of the summertime
oceanic subtropical highs
• Meteorology 101:
– Hot air rises, cool air sinks.
• Hydrostatic sinking is all about PGF to push
horizontal branch of circulation (divergence)
• e.g. Hadley cell: eq. trough, subtropical H belt
• In summer, subtrop. oceans cooler than land
=>Highs over the ocean basins
Annual mean SLP
July SLP (same color scale)
cooling
cooling
January SLP (same color scale)
• Meteorology 301: stratified
– Heated air rises, cooled air tends to sink.
• heating rate = rate of flow across isentropes =
vertical velocity (in  coordinates)
– E. subtropical oceans: cooling (even in winter)
• little latent heat release (rain), much rad. cooling
(stratocu decks under dry upper troposphere)
“The Usual Explanation”
• 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
2001
Model experiments, forced by realistic heating (above), in
realistic JJA zonal mean flow, with real topography
Rodwell and Hoskins 2001
Asian Heating Q_Asia
Y887 (Q_Asia, mts)
Pure heating effect:
Y887 (Q_Asia, mts)Y887 (mts only)
Rossby
??
Kelvin
Try NAm and
NPac
Heating:
Y887 forced by
mountains + NAm Q
Y887 forced by mtns +
NAm Q + NPac Q
Rossby
??
Kelvin
Red: global Q
Small et al. 2007 concur: imposed
heating anomalies in the region can
explain most of the flow anomalies
• some westward
influence from
E. Atl
• MSD negative
heating drives
NASH/trade
enhancement
• positive
feedback
positive feedback
• Easterlies / High drive (negative) rain
anomalies
• (negative) latent heating anomalies drive
easteries/ High
• An amplified, twitchy system - but what
drives it, to make this climatological
feature?
One hypothesis:
One hypothesis:
MSD
wet
midsummer tradewind surge
Drivers: 2. Mean flow changes in midsummer?
Chen, Hoerling & Dole 2001
Heating
Eddy Z1000
w/o shear
Eddy Z1000
July u(y,p)
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, Transients?
Drivers: 3. Influences via the midlatitude westerlies?
2006
Hudson Bay
contributes too
west coast
thermal gradient
Japan’s
midsummer
drought
mechanism: the
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.
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?
Clues from climate models
• Do free-running, full-physics climate
models have the MSD?
– Short answer: yes
• What drives it there?
– Global, vague:
• solar declination, like all aspects of seasons
• continent-ocean contrasts
• Experiment!
5
Observations
CMAP rain
1
SLP
July Aug
IPRC/
ECHAM
model:
quite good
8 IPRC GCM rain
2
GCM SLP
Experiment: global monsoon system
on steroids
• Increase continent-ocean contrasts
– perpetual June (and every other month):
• fix SST and solar decl. for each day of year
• run model for many days, so atm+land come
into equilibrium
– Warmest NH now in June, not July
– Continents hotter in summer (dried out?)
– What happens to monsoons (and MSD)?
Northern
continents get
hotter in
summer
Dec
Jan
Our MSD
is a major
part of the
global
rainfall
response
Annual mean rainfall change
Americas
drier
Indian
Ocean
wetter
Dec
in MSD
Jan
early onset
Conclusions (so far)
• The Central American MSD is a significant aspect
of the seasonal cycle, both regionally and globally.
• Useful for understanding phys. mechanisms
– how exactly does the NASH affect rainfall/convection
• U. for u. summer teleconnections
– from Asia, zonal [u] easterlies, waves on westerlies...
• A “natural mode” of the regional system - relevant
to other types of S-I climate variability?
Future work
• Starting fall 07 in earnest (thanks NSF)
• Atlantic summer climate reading group
– me, 2-3 students on project, other experts
(please!)
• where to start?
software demo
u200 HF anomaly plot, July-Aug
HF anomalies
(total - annual - semiannual):
Yes, the Bonin
High and 2 other
stationary waves
are evident, but
also a zonally
elongated u
anomaly spanning
all Asia-WPAC
Japan
-4
-2
HF
0 m/s 2
time
slice
Jul Aug
10d
100d
HF wavelet analysis
Interannual consistency of the
Asian HF jet anomaly ~1 Aug
Low-level heating dominant. Influences from
monsoons and planetary waves, but mainly local
feedbacks. Eastern-basin oriented.
EASM
ISM
ENASM?
EASM
NAM
WNPM
(2nd active)
May 1
Jun 1
Jul 1
May 1
Jun 1
Jul 1