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

QQ

H

QQ

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: mapes@miami.edu

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