The central American mid-summer drought

(MSD, veranillo or canicula)

Brian Mapes

University of Miami

midsummer drought

30-year climatology

Key WestKey West

Miami

MSD

1999

(

?

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 usefuluseful truth...A

What causes the MSD?

• Regional: – North Atlantic Subtropical High (NASH or

Bermuda High) & tradewinds are enhanced and extended west in mid-summer.

late May-early June

July

Midsummer nose on N. Atlantic Subtropical High (NASH)

dry

Carib easterly jet

VeracruzVeracruz

ValparaisoValparaiso

wet

wet

CaymanCayman

upwind upwind slopesslopeswetter wetter inin midsummermidsummer

midsummer drought

Key WestKey West

Miami

Valparaiso, FL

CaymanCaymanMSD broader in

time than VeracruzVeracruz

CMAP

Individual years - still discernable

(in big regional Central American box average)

intraseasonal canmask it a bit

9 year mean

CMAP rainfall

TAIWAN 25N

CUBA 25N

SE BRAZIL 25S

MADAGASCAR 25S

May

Nov.

Nov.

May

MSD happens in other places too

Where are those boxes? W. subtropical oceans

High HighHighHigh

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=>Highs over the ocean basins

Annual mean SLP

July SLP (same color scale)

January SLP (same color scale)

coolingcooling coolingcooling

• 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

887 (Q_Asia, mts)

Pure heating effect: 887 (Q_Asia, mts)-887 (mts only)

RossbyRossby

KelvinKelvin

??

Try NAm and NPac Heating:

887 forced by mountains + NAm Q

887 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:

MSD

wet midsummer tradewind surge

One hypothesis:

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

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

Drivers: 3. Influences via the midlatitude westerlies?

west coast thermal gradient

Hudson Baycontributes too

2006

Japan’s midsummer

drought mechanism: the

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.

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!

Observations

IPRC/ ECHAM model:

quite good

SLP

IPRC GCM rain

July Aug

CMAP rain

GCM SLP

1

5

2

8

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 changeAnnual mean rainfall change

IndianIndianOceanOceanwetterwetter

early onsetearly onset

AmericasAmericasdrierdrier

in MSDin MSD

Dec

Jan

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

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

time slice

0 m/s 2-4 -2

u200 HF anomaly plot, July-Aug

HF wavelet analysis

100d

10d

total HF

Japan

Jul Aug

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.

ISM

WNPM

EASM

May 1 Jun 1 Jul 1

(2nd active)

EASMNAM

ENASM?

May 1 Jun 1 Jul 1