Boreal Sum

mer Intraseasonal Variability and

Air-Sea Interaction in the Tropical N

ortheast PacificE

ric D. M

aloney and Steven K

. Esbensen

College of O

ceanic and Atm

ospheric Sciences

Oregon S

tate University

Correspondence: m

aloney@coas.oregonstate.edu

Project W

eb Site: http://oregonstate.edu/~m

aloneye/

1. Introduction and Background

The Madden-Julian oscillation (M

JO) produces variations in low

-level winds over the

east Pacific w

arm pool that force variability in sum

mertim

e precipitation, and an associated m

odulation of tropical cyclones. Recent studies have show

n that these sum

mertim

e MJO

-related variations may be predictable up to 2-3 w

eeks in advance (e.g. W

aliseret al. 1999). As show

n below, one such event occurred during the

EP

IC2001 experim

ent, expressed in terms of 200 hPa

velocity potential.

A m

ore generalized lag-correlation analysis of NC

EP

reanalysis 30-90 day bandpassfiltered zonal w

ind (7.5oN

-12.5oN

averaged) shows significant eastw

ard propagation of M

JO w

ind anomalies into the tropical eastern north Pacific during sum

mertim

e. S

pectral analysis indicates that a strong intraseasonal peak (~50 days) in the east P

acific warm

pool occurs then (see below). The intraseasonal spectral peak in this

region in precipitation and winds is as dom

inant as anywhere in the Tropics.

Wind speed over the w

arm pool is

enhanced during periods of MJO

westerly

anomalies and enhanced precipitation, w

ith suppression of w

ind speed during MJO

easterly periods (see above).

Wind speed anom

alies associated with the

MJO

appear to be caused both by intraseasonal vector w

ind anomalies added

to the climatologicalsouthw

esterly flow,

and by variations in eddy activity, including easterly w

aves and tropical cyclones.

For example, the suppression of w

ind speed during M

JO easterly periods appears to be

about equally due to easterly anomalies added

to the climatologicalsouthw

esterly flow and

suppression of eddy variance (see left).

3. Analysis of Intraseasonal SST

Intraseasonal SS

T variance during summ

ertime m

aximizes over the

east Pacific w

arm pool (see right), w

ith another maxim

um in a band

just to the north of the equator. The intraseasonal band (30-100 days) explains about 30%

of the total SS

T variance (including interannual) over the east P

acific warm

pool during summ

ertime.

A spectral analysis using eight years of TM

I SST data indicates a significant 50 day peak over the east Pacific w

arm pool during

boreal summ

er.Only the clim

atologicalseasonal cycle was rem

oved before the spectra w

ere computed. A

s will be show

n below, the M

JO

explains a large fraction of the intraseasonal SS

T variance in this region.

The equatorial variance maxim

um is likely associated w

ith tropical instability w

aves, having a dominant period closer to 30 days than the

40-50 day periodicity associated with the M

JO.

6. Conclusions

Satellite and buoy data show that the M

JO is associated w

ith strong precipitation and w

ind variations over the east Pacific warm

pool during June-October that are

supported by wind-induced latent heat flux variability. These latent heat flux

anomalies are generated both by 1) vector m

ean winds adding constructively or

destructively to the climatologicalw

ind field, and by 2) MJO

-induced variations in easterly w

ave and tropical cyclone activity.

A significant 50-day intraseasonal spectral peak in SST occurs in the east Pacific

warm

pool during summ

ertime, gnerated

in large extent by the MJO

. SSTs vary by up to 1

oC over the east Pacific w

arm pool during an M

JO lifecycle. SST anom

alies precede M

JO precipitation anom

alies by 5-10 days.

A full reference list can be found in M

aloney and Esbensen (2006), in press in Monthly

Weather R

eview.

We w

ould like to thank the NO

AA

Clim

ate Prediction P

rogram for the A

mericas w

ithin the Clim

ate Program

Office

(Grant# N

A05O

AR

31006) for support of this research. The statements, findings, conclusions, and

recomm

endations do not necessarily reflect the views of N

OA

A, orthe D

epartment of C

omm

erce.

Sept 7

Sept 27

Oct 2

Oct 7

Oct 12

Sept 22

Sept 17

Sept 12

Figure 1.

MJO

Convectively S

uppressed

MJO

Convectively E

nhanced

TRM

M P

recipand Q

uikSC

AT W

ind Anom

aliesQ

uikSC

AT V

ector Wind and W

ind Speed A

nomalies

MJO

Convectively S

uppressed

MJO

Convectively E

nhanced

Intraseasonal Precipitation vs. TA

O B

uoy Latent Heat Flux

r=0.59slope=5.1

Intraseasonal anomalies

(10-100 days). TRM

M P

recipA

nomalies

r=0.44slope=2.8

r=0.37slope=1.8

r=-0.34slope=-2.0

~50 day

~30 days (TIWs)

~50 day

Analysis of June-O

ctober Intraseasonal SS

T

Intraseasonal U850 G

lobal EO

FsU

sed to Construct M

JO Index

EO

F2 (24%)

EO

F1 (35%)Longitude

2. Analysis of the June-O

ctober MJO

We conduct an analysis of the M

JO in the east P

acific warm

pool during June-October of

1998-2005 using satellite and buoy data. Enhanced TAO

array measurem

ents associated for E

PIC

2001 were available from

2000-2004.

Surface MJO

westerly (easterly) anom

alies are associated with an

enhancement

(suppression) of convection over the warm

pool, and a suppression (enhancement)

of convection to the east of 110oW

(see below left).W

ind jets appear to be active during periods of M

JO easterly anom

alies (not shown).

Convectively S

uppressed Phase

Com

posite Wind S

peed Anom

alies

Total

Vector W

ind

Eddies

(Regression)

(Regression)

(Com

posite)

The MJO

-related variations in wind speed contribute to

variations in latent heat flux during MJO

events. In fact, latent heat flux anom

alies are primarily w

ind-driven over this region.

An analysis from

TAO

buoys and TRM

M indicates a

significant coupling between latent heat flux and

precipitation over the east Pacific warm

pool (see left), suggesting that w

ind-evaporation feedback may help

support MJO

convection over the east Pacific warm

pool during sum

mertim

e.

Recent m

odeling work also suggests that w

ind-evaporation feedback supports M

JO convection in this region (M

aloney and E

sbensen 2005), and is consistent with the m

ore generalized view

that wind-evaporation feedback supports

MJO

convection across the Tropics in regions of mean low

-level w

esterly flow (e.g. M

aloney and Sobel2004).

SSTs in the 30-90 day band are coherent across the east Pacific warm

pool during sum

mertim

e.Using a reference S

ST tim

e series was at 9

oN, 92

oW,

coherence squared is significant and exceeds 0.4 across much of the w

arm pool.

While phase vectors broadly indicate an in-phase relationship for intraseasonal

SS

Ts within the w

arm pool, S

STs north of 14

oN tend to lag those to the south by

about 1/8 of a cycle (~5 days). Interestingly, no coherence occurs between w

arm

pool and equatorial SS

T in the intraseasonal band, contradictingthe results of

Maloney and K

iehl (2002) who used R

eynolds SS

T data.

Coherence squared betw

een SST and precipitation is also significant (0.3-0.4) in the intraseasonal band across the east Pacific w

arm pool, w

ith precipitation lagging S

ST by a 1/4 phase (~10 days) w

hen they are colocatedspatially, although at increasing lags tow

ard the north.

95%

Intraseasonal SST variations in the east Pacific will be related

to the MJO

using the tropical equatorial M

JO index of M

aloney and Kiehl (2002).The

leading EO

Fsof the 30-90 day equatorial averaged 850 hP

azonal w

ind represent a quadrature

pair that defines the eastward propagating M

JO (see

right). Although peak variance for these E

OFs

occurs in the Eastern

Hem

isphere, substantial zonal wind variance also is captured in the east P

acific. The principal com

ponents of the leading EO

Fsare linearly com

bined to form an

MJO

index, as described in Maloney and K

iehl (2002).

A lag correlation betw

een the MJO

index (multiplied by -1) and intraseasonal SST

indicates that the MJO

index explains about 40% of the intraseasonal SST variance in

the east Pacific warm

pool during June-October (see below

). Thus, the MJO

is an im

portant factor in controlling east Pacific w

arm pool S

ST during sum

mertim

e.SS

T leads the M

JO index, the im

plications of which w

ill become apparent below

.

Significant MJO

events are defined as maxim

a of the index exceeding 1σ, and a com

posite MJO

event is created (as a function of lag in days) byaveraging these

significant events. Suppressed (enhanced) convection typically precedes positive (negative) SST anom

alies by 5-10 days over the warm

pool (see below). Peak to

peak variations in SST can be as high as 1oC

over an MJO

lifecycle. An asym

metry in

the response is apparent, with w

arm S

ST anom

alies that precede enhanced convection being stronger and m

ore widespread than the corresponding cold anom

alies after convection. This asym

metry is sim

ilar if events are defined using minim

a of the MJO

index.Precipitation anom

alies west of 110

oW are not associated w

ith strong SS

T variations.M

aloney and Kiehl (2002) used an atm

ospheric GC

M coupled to a slab ocean

to show that intraseasonal S

ST anom

alies of even lesser magnitude than show

n below

are likely important for producing realistic M

JO convective variability .

95% significant regions shaded