海洋科学技術センター試験研究報告　第41号　JAMSTECR, 41（March 2000）

57

＊１ Institute for Global Change Research, Frontier Research System for Global Change

＊２ Graduate School of Environmental Earth Science, Hokkaido University

＊３ 地球フロンティア研究システム

＊４ 北海道大学大学院地球環境科学研究科

Inter-Hemisphere Decadal Variations

in SST, surface wind and heat flux over the Atlantic basin

Youichi TANIMOTO＊１ Shang-Ping XIE＊２

Decadal climate variations are examined using a new observed dataset of marine meteorological variables in the Pan-

Atlantic basin. Leading empirical orthogonal functions (EOFs: spatial patterns) of sea surface temperature (SST) anomalies

on the decadal band (8-16 years) conducted in independent two subdomains north and south of equator feature one center

of action at 15oN and another center with opposite polarity at the same latitudes in the Southern Tropics. The same EOF

analysis for sea level pressure (SLP) anomalies also indicates two centers of action with opposing polarities around 30

degree latitudes, straddling the equator. The accompanying four principle components (PCs: time series) that contain

three cycles of decadal variations are correlated well with one another, indicative of the existence of the tropical dipole

mode in the ocean-atmosphere system.

Composite anomaly maps of wind velocity and heat fluxes, based on the PCs of leading modes of SST and SLP

anomalies, indicate that the latent heat flux induced by the cross-equatorial wind plays an important role in forcing the

dipole mode of the decadal SST variability. Anomalies on either side of equator show comparable amplitudes in the SST

field, but have quite different amplitudes in wind velocity and flux fields. The role of low-level clouds in forcing the SST

anomalies is discussed.

Key Words : Pan-Atlantic, decadal variability, ocean-atmosphere interaction

大西洋の海面水温，海面気圧海上風，海面熱フラックス，

下層雲におけるInter-hemispheric 10年スケール変動

谷本　陽一＊３　謝　　尚平＊４

熱帯大気海洋系には強い結合変動があり，太平洋ではエルニーニョとしてしられている。大西洋においても結合変動は

みられるものの，その空間構造は太平洋のものと大きく異なり赤道反対称の双極子構造をとる。この双極子構造は熱帯大

西洋に卓越する唯一のモードではないため，観測的見地からの存在には未だ決定的な証拠が出されていない。本研究で

は，これまでとは異なった観点から卓越モードとしての双極子構造の存在を示す。

キーワード：環大西洋，10年スケール変動，大気海洋相互作用

JAMSTECR, 41 (2000)58

1. Introduction

Decadal sea surface temperature (SST) variations in

the tropical Atlantic are organized into dipole patterns

with centers of action around 10-15 degree latitudes

(Servain 1991, Nobre and Shukla 1996, Chang et al. 1997,

among others). At the same time, sea level pressure (SLP)

anomaly field also displays a dipole structure, whose cen-

ters of action locate slightly more poleward at 30 degree

latitudes and sign opposite to local SST anomalies

(Tanimoto and Xie 1999). This air-sea coupled tropical

dipole structure could be associated with the mid-latitudes

decadal variations via the atmospheric teleconnections

(Watanabe and Kimoto 1999). Rajagopalan et al. (1998)

presented that high coherences and in-phase (out-of phase)

relation were found between North Atlantic Oscillation

(NAO, Hurrell 1995) index and SST time series in north-

ern (southern) tropics. The extratropical North Atlantic

also displays pronounced decadal SST variations, which

are associated with a decadal change of mid-latitudes

westerlies (Deser and Blackmon 1993, Kushnir 1994,

Delworth, 1993, Halliwell and Mayer 1996). Similar situ-

ations occur in the SST and SLP fields in the Southern

Hemisphere (Venegas et al., 1997).

The dipole mode, however, is not the only mode of

SST variability in the tropical Atlantic. The interference

of different modes is considered to cause an apparent in-

ter-hemispheric decorrelation in both SST and SLP field

(Fig. 1). Previous empirical decomposition analyses and

their variations performed in domains that include the

whole tropics produce contradictory results as to whether

the northern and southern tropical SST anomalies vary

independently or can be decomposed into a pair of mono-

pole and dipole modes. The leading rotated EOFs of tropi-

cal SST anomalies in recent four decades (Houghton and

Tourre 1992) have one major center of action each con-

fined to one hemisphere but show no substantial signal in

the other hemisphere. Cross spectral analyses by Mehta

(1998) and Enfield et al. (1999) with longer data records

present little coherence of SST anomalies between north-

ern and southern tropics on any frequency domain. In

contrast, a singular value decomposition (SVD) analysis

of SST, wind stress and heat flux anomalies over 40 years

by Chang et al. (1997) showed that an SST dipole struc-

ture was maintained by equatorial anti-symmetric heat

flux anomalies, in association with cross-equatorial at-

mospheric flows. The same conclusion is reached from a

joint empirical orthogonal function (EOF) analysis of SST

and zonal wind anomalies (Nobre and Shukla 1996).

Most previous modal decompositions are applied to

both northern and southern parts of the tropical Atlantic

at the same time, and then tropics-extratropics linkages

are examined. Here we will further reexamine this inter-

hemispheric relation in the SST variability from a differ-

ent perspective. First, we will perform EOF analysis sepa-

Fig. 1 Scatter plot of zonal mean (a) SST (ºC) and (b) SLP (hPa) anomalies in the Northern tropics ( horizontal axis)

and Southern tropics (vertical axis). Zonal mean anomalies are calculated form unfiltered boreal winter in 8-

20º latitudes for SST and 20-40º latitudes for SLP.

JAMSTECR, 41 (2000) 59

rately for the North and South Atlantic, thus avoiding the

criticism that the EOF analysis over the whole tropics

might force artificial interhemispheric correlation. Sec-

ond, we will include the extratropics into our analysis

domains, recognizing the effects of atmospheric

teleconnections linking the tropics and mid-latitudes,

which in turn produce the Pan-Atlantic Decadal Oscilla-

tion (PADO; Xie and Tanimoto 1998). We will repeat

the same analysis on the SLP variability that presumably

most directly interacts with SST anomalies to see if these

interacting fields lead to a coherent interhemispheric re-

lation.

Observational and model studies indicate that surface

heat flux is crucial in forcing the dipole mode of SST

variability in the tropical Atlantic. Wagner (1996) exam-

ined contributions to SST dipole mode from each surface

heat flux component. His analysis method inevitably

emphasizes variability on interannual time scales where

significant positive contribution is seen only from wind-

induced latent heat variations. This latent heat flux con-

tribution has been independently confirmed in ocean gen-

eral circulation model (GCM) simulation (Carton et al.

1996), but it is still unclear if other heat flux components

might also contribute on longer decadal time scales. Di-

pole-associated wind variability is weak in amplitude and

not well organized in the South Atlantic and may not be

able to account for all the local SST anomalies that have

comparable amplitudes with those in the northern tropics

(Chang et al. 1997, Nobre and Shukla 1996, Tanimoto

and Xie 1999). In these analyses, wind anomaly ampli-

tudes north of equator are generally two or three times

larger than those south of equator even though SST

anomaly amplitudes are comparable on either side of

equator. Thus it is interesting to examine why this asym-

metric amplitudes of the atmospheric field could be as-

sociated with the symmetric amplitudes of the SST field.

We will show that low-level cloud variability is one of

the missing forcings for SST anomalies in the southern

tropics.

Now two types of gridded SST datasets are available

for the statistical analyses: One intends a complete spa-

tial coverage via an optimal method. This benefits nu-

merical experiments with a complete boundary condition,

just like GISST (Folland and Parker 1995) and GOSTA

(Bottomley et al. 1990). The other gridding method fills

a grid point only when there are enough number of ob-

servations in a grid for monthly seasonal averaging. Pre-

vious empirical studies (Mehta 1998, Enfield et al. 1999,

Xie et al. 1999) have employed the former dataset to cap-

ture the large-scale structure of Atlantic SST anomalies.

Over data sparse region like the South Atlantic, however,

heavy temporal and spatial interpolations can be a source

of signal distortion. Here in order to ensure the fidelity

of the gridded data, we require all grid points in the analy-

sis to have uninterrupted seasonal means (see the next

section for details). In the present study, we construct

such a new observational dataset of marine meteorologi-

cal variables including monthly SST, SLP, cloud amount

and ocean heat fluxes, calculated only from observations

by ship of opportunities. We did not use satellite-based

observations in calculation to avoid possible system bias

due to instrument changes. No spatial interpolation is

allowed. Later we will compare the results from this cut-

to-the-bone dataset with more heavily interpolated

datasets like the GISST and GOSTA.

The rest of paper is organized as follows; Section 2

describes the datasets and the analysis procedures. Sec-

tion 3 shows leading modes that explain the meridional

gradients of the inter-tropical SST and SLP anomaly

fields. Section 4 presents the Pan-Atlantic patterns for

SST, SLP, wind velocity and ocean heat fluxes. Section

5 gives a discussion of a lower cloud effect on the asym-

metric amplitudes of SLP and wind velocity, associated

with the tropical SST dipole. Concluding remarks are

given in Section 6.

2. Data set and analysis procedure

A monthly 4-degree latitude-longitude dataset of ma-

rine meteorological variables is constructed from qual-

ity-controlled ship and buoy observations compiled in

Long Marine Reports in fixed length records (LMRF) of

comprehensive ocean-atmosphere dataset (COADS; Woo-

druff et al. 1987) for the North and South Atlantic (70ºN-

50ºS) from 1950 through 1995. The domain contains the

Norwegian, North and Caribbean Seas, southern part of

Labrador Sea, Gulf of Mexico and Mediterranean. In the

present study, we examine SST, SLP, vector and scalar

wind speed, sensible and latent heat flux fields. A higher

resolution (2 degree x 2 degree) dataset of cloudiness is

also constructed from ship reports in LMRF of COADS.

The cloud amount is visually measuring a cloud cover-

age of the whole sky. Turbulent heat fluxes are calcu-

JAMSTECR, 41 (2000)60

lated using Kondo’s (1975) aerodynamic bulk method for

each of ship-buoy measurement. In regions like the South

Atlantic where there are few ship observations, the

COADS suffers sampling errors, and SLP and wind ve-

locity may not even satisfy basic dynamic constraints like

the geostrophic balance. We also complement the

COADS with the NCEP (National Centers for Environ-

mental Prediction, monthly 2 degree latitude-longitude

resolution) reanalysis dataset over 1958-1995, which pro-

vides dynamically-consistent SLP and wind velocity data

even with insufficient observed input in the Southern

Hemisphere to an assimilation model. In contrast, grid

point values of COADS are calculated from independent

observations of SLP and wind velocity. For each of these

variables we calculate a monthly climatological mean

annual cycle based on the entire period of record

(COADS: 46 years, NCEP: 38 years), and the monthly

anomalies are defined as departures from the climatologi-

cal means.

Seasonal averages are used in the following sections.

Averages are calculated only for boreal winters (Decem-

ber through March) only on those grid points with more

than three monthly-mean values. This study is based on

the modal decomposition (EOF and SVD analyses) of

SST and SLP anomalies at those grid points that contain

no missing boreal winter averages for the entire period of

1950/51 through 1994/95. Large variance is found on

interannual and decadal time scales (Servain 1991, Huang

and Shukla 1997, Mehta 1998) with a clear spectral gap

between the two time scales (Mehta and Delworth 1995,

Mehta 1998). Based on the above spectral structure of

tropical Atlantic SST anomalies, Tanimoto and Xie (1999)

applied a time-scale separation method to 51-year SST

time series and found negative (positive) correlation on

decadal (interannual) time scale in SST anomalies across

the equator. A band-pass filter is accepted to extract

decadal variations when substantial variances are found

in a frequency band. Explained variance of the leading

EOFs depends on the number of the grid points for which

a modal decomposition is performed. The grid points

employed in the modal decomposition are 312 (103) for

the North (South) Atlantic SST field, and 314 (113) for

the North (South) Atlantic SLP field. Composite anoma-

lies of SST, SLP, lower cloud amount and heat fluxes are

calculated based on the time series of the leading modes

to examine regional signals of the PADO.

3. Inter-hemispheric mode of decadal climate vari-

ability

Large variance appears in a decadal frequency domain

of 0.05-0.12 cycle per year in the tropical Atlantic SST

anomalies (Mehta 1998, Rajagopalan et al. 1998). In-

deed, Tanimoto and Xie (1999) showed that the cross-

equatorial gradient index of annual mean SST anomalies

varied on decadal time scales, often having an anti-sym-

metric dipole-like spatial pattern. Regressed SST, SLP

and wind vector anomalies onto the cross-equatorial gra-

dient index revealed a PADO pattern with the dipole in

the tropics. In this section, we present collaborative evi-

dence for the PADO based on different analysis methods.

Before we perform an EOF analysis, we divide the

Atlantic basin into two independent parts north and south

of the equator to eliminate an artificial-correlation prob-

lem in modal decomposition of tropical SST anomalies

(Houghton and Tourre 1992). Then, SST anomalies are

averaged for boreal winter and filtered through the decadal

band (8-16 year). The upper two curves in Figure 2 show

normalized principle components (PCs) of the leading

mode of SST anomalies in the North and South Atlantic,

respectively. Figure 3 depicts the SST regressions onto

the PCs instead of EOF eigenvectors. Grid points used

in the EOF analysis are shaded in the background. These

leading EOFs explain 37.0% and 54.9% of band-passed

variance of the used grid point values in the Northern and

Southern hemisphere, respectively. The second modes

explain only 19.0% and 13.2% of the variance in the

Fig. 2 Upper curves: the normalized principal components (PCs)

of SST anomalies in the North (solid) and South (dashed

line) Atlantic. Lower curves: PCs of SLP anomalies. The

vertical axis of SST anomalies is reversed.

JAMSTECR, 41 (2000) 61

Northern and Southern Hemisphere, respectively, ensur-

ing a fair separation of leading EOFs. The leading SST

patterns feature two tropical centers of action with op-

posing polarities across the equator with maximum re-

gression of 0.3º C in the Northern Tropics. SST anoma-

lies display a PADO pattern, with centers of action lining

up meridionally in the North Atlantic. Significant nega-

tive regressions, with a magnitude of -0.4ºC, extend from

south of Newfoundland through Gulf of Mexico and Car-

ibbean Sea, while small positive regressions of about

0.1ºC appear between South Greenland and south-west-

ern Europe. Negative regressions cover the whole the

Southern Hemisphere domain except in the southeastern

corner where poor data coverage generates an apparently

spurious center of action. The spatial structure of SST

anomalies will be discussed in Section 4.

These coherent patterns in fact fluctuate almost in phase

on the decadal time scales (Fig. 2, simultaneous correla-

tion coefficient is 0.63). Note again that these leading

EOFs are derived from independent fields so that there is

no a priori reason for them to be correlated. Decadal

variability is pronounced after mid 1960s while the agree-

ment of two PCs may be insignificant in the first 15 years

and the last 5 years. Simultaneous correlation maps onto

two PCs (not shown) present a similar pattern to regres-

sion maps. All centers of action have statistically-sig-

nificant correlations above 0.8 in the Pan-Atlantic domain.

The same analysis performed with the GISST dataset

gives a similar PADO pattern (Xie et al. 1999).

We perform the same analysis to the boreal winter SLP

anomalies in the two subdomains (Figure 4). The lead-

ing EOFs of SLP fields explain 36.4% and 50.2% of band-

passed variance of used grid point values in the Northern

and Southern Hemisphere, respectively. A subtropical

SLP center of action appears in 20-40ºN band in the cen-

tral North Atlantic, which is 10-15 degrees poleward of

the tropical SST center and has the opposite polarity to

the tropical SST anomalies. Another extratropical center

west of Europe also has significant regressions. The South

Atlantic EOF does not have much spatial structure, with

the SLP varying more or less uniformly over the whole

subdomain. The accompanying PCs of the SLP field (the

lower two curves of Figure 2), correlate to one another.

More strikingly, the SST and SLP pairs of PCs are well

correlated among themselves, despite the fact that they

are all derived from independent samples (Table 1). Two

minima in early 1970s and 1980s and three maxima in

late 1960s, late 1970s and early 1990s are shown up in all

Fig. 3 The first SST EOFs for the North and the South Atlantic,

which explain 37.0% and 54.9% of decadal band-passed (8-

16 years) boreal winter SST anomalies, respectively. Re-

gressed SST anomalies onto the PCs for the North (South)

Atlantic are shown in the upper (lower) panel. Negative

values are dashed. Contour interval is 0.1ºC.

Fig. 4 Same as Figure 3, but for the SLP anomalies. They explain

36.4% and 50.2% of decadal band-passed (8-16 years) bo-

real winter SLP anomalies for the North and the South At-

lantic, respectively. Contour interval is 0.2hPa

JAMSTECR, 41 (2000)62

four time series (Figure 2). These results indicate that an

air-sea coupled PADO dominates the recent three decades.

Scatter plots of zonal mean values in the 20-40ºN and

20-40ºS bands, calculated from bandpass filtered SLP

anomalies (not shown), confirm this out-of-phase rela-

tionship, with a tilted elliptical track much like the scat-

ters of decadal SST anomalies (see Fig. 5 in Tanimoto

and Xie 1999).

We also perform an SVD analysis to examine coupled

modes of SST and SLP fields in the combined Atlantic.

The leading SVD mode explains 60.1% of total squared

covariance. The heterogeneous regression maps of SST

and SLP fields (not shown) are similar to the pieced up

regression fields. All decomposed modes (EOFs and

SVDs) feature distinct NAO and associated SST patterns

(Deser and Blackmon 1993, Kushnir 1994) in the North-

ern Hemisphere, but show spatially uniform patterns in

the Southern Hemisphere.

The agreement between four time series of PCs is fairly

pronounced during three cycles of the decadal variability

from 1966 through 1990. Before and after this period,

however, the correlation among the PCs does not hold so

well. Although this could be due to the end effect of the

band-pass filter, similar EOF analyses of SST and SLP

fields for a shorter period of 1966-1995 reproduce the

three cycles of a decadal oscillation, raising the more

explained variance by about 15% (not shown). This re-

sult seems to suggest the nonlinear relationship between

this decadal variability and lower frequency fluctuations.

But a nonlinear diagnosis is out of scope in the present

study.

4. Decadal climate variability in Pan-Atlantic

To examine features common to three distinct cycles

of the distinct PADO, we made composite maps of me-

teorological variables based on the PCs of leading SST

and SLP modes. Compositing helps us to see the PADO

signature outside areas of EOF analyses. Six years each

are chosen to represent the positive phase (1968-70, 79-

81) and the negative phase (1972-74, 84-86) of the PADO.

Table 1 Correlation between leading PCs of SST and SLP fields in the two

subdomains north and south of equator

SLP N. Atl.

SLP S. Atl.

SST N. Atl.

SLP N.Atl.

1.00

-

-

SLP S.Atl.

0.63

1.00

-

SST N.Atl.

0.56

0.50

1.00

SST N.Atl.

0.69

0.78

0.63

Fig. 5 Difference maps of unfiltered boreal winter SST and SLP anomalies associated with the PADO, defined as the difference between

six positive phase years (1968, 69, 70, 79, 80, 81) and six negative phase years (1972, 73, 74, 84, 85,86). Contour interval is 0.2ｼC

and 0.5hPa for SST and SLP fields, respectively. Positive (negative) values are represented by the solid (dashed) contours.

JAMSTECR, 41 (2000) 63

form between 20 degree latitudes on either side of equa-

tor, while SST amplitudes increase to the east, exceeding

0.8ºC in the Northeastern Tropics. The composite SST

anomalies south of the equator exceed 0.6º C, but the spa-

tial pattern is less coherent. In the eastern boundary re-

gions, seasonal variations are largely due to the develop-

ment of the Guinea dome -the shallow domelike ther-

mocline feature in subsurface ocean- and the Angola dome

-a counterpart of Guinea dome in the Southern Hemi-

sphere-, respectively (Yamagata and Iizuka, 1995). These

domes develop due not only to an one-dimensional sur-

face heat flux, but also to active divergence of horizontal

heat transport in subsurface. Further investigations into

such subsurface variations in response to anomalous wind

stresses associated with cross-equatorial SST gradient are

desired.

In the difference map of wind vectors (Fig. 6), anoma-

lous southwesterlies in geostrophic balance with the SLP

difference reduce the climatological northeasterly trade.

This leads to a reduction in scalar wind speed by up to

1.5ms-1 (light shade in Fig. 6), suppressing the latent heat

flux release (negative anomalies) in the same region (Fig.

7a). The sensible heat flux (Fig. 7b) also depends on the

wind speed, but there is no substantial difference between

the two phases in the tropics. In the Southern Tropics,

the SLP di fference fie ld suggests anomalous

southeasterlies, which enhance the heat release (positive

anomalies) from the ocean surface. This is consistent

with the negative SST anomalies south of the equator.

Fig. 6 Same as Figure 4, but for wind velocity anomalies (vec-

tors). The wind velocity scale (5.0ms-1) are indicated on the

bottom of panel. Dark (light) shades are the regions in which

scalar wind speed anomalies are more than 0.5ms-1 (less than

-0.5ms-1).

Fig. 7 Same as Figure 4, but for (a) latent and (b) sensible heat flux anomalies. Contour interval is 10Wm-2.

The positive phase corresponds to a northward SST or a

southward SLP gradient in the tropics, and vice versa.

We will show difference maps of climate anomalies be-

tween the two phases.

Figures 5 shows the difference map of unfiltered SST

and SLP anomalies between the two phases. The polar-

ity of tropical SST and SLP anomalies is zonally uni-

JAMSTECR, 41 (2000)64

Wind velocity differences between the two phases dis-

play southeasterlies in the Southern Tropics. But the

amplitudes in the Southern Tropics are about one-third

of those in the Northern Tropics. Note that anomalous

southerlies on the equatorial grid points are stronger than

those further north in 8-12ºN. These strong southerly

anomalies are associated with a northward shift of the

ITCZ, a manifestation of interhemispheric interactions.

The heat flux anomalies show incoherent structures in

the Southern Tropics, becoming even worse south of 40ºS.

Few observations result in large sampling errors, espe-

cially in those higher order fields such as wind veloci-

ties, heat and momentum fluxes.

The atmospheric composites (right panel in Fig. 5 and

Fig. 6) in the Southern Hemisphere indicate a noisy anti-

cyclonic SLP pattern, small speeds and disorganized di-

rections of wind velocities. Composite maps of SLP and

wind vectors calculated from the NCEP reanalysis

datasets (Figure 8) are quite similar to those from COADS

in an overall sense. But the anticyclone centered on 30ºS

is better defined and associated wind pattern is well or-

ganized in the Southern tropics. Inter-hemispheric flows

within 10 degree latitudes have comparable amplitudes

both sides of the equator. Although the SLP and wind

velocity difference fields now have coherent structure,

their magnitudes poleward of 20ºS are still smaller than

those in the Northern subtropics.

In the extratropical North Atlantic, an atmospheric

NAO-like SLP and SST patterns (Figs. 5 and 8) are domi-

nant (Deser and Blackmon 1993, Kushnir 1994). Sub-

tropical SLP anomaly pattern in the North Atlantic shows

an intensification of the climatological Azores high, and

is sandwiched by negative SST anomalies off east coast

of United States and positive ones around Newfoundland

and south of Greenland. This association indicates an

ocean surface response to the atmospheric forcing by the

intensified westerlies to the south of the SLP center and

by weakened ones to its north. Positive SLP anomalies

in the Norwegian Strait are cooperative in weakening the

westerlies. The latent flux is one of the major compo-

nent in forming SST anomalies over most of the

extratropics, but the sensible heat flux makes comparable

contribution in the southern Labrador Sea. Such a change

in heat inputs into the extratropical atmosphere may in

turn maintain the polarity of SLP anomalies (Peng et al.

1997, Nakamura and Yamagata 1999, Rodwell et al.

1999), but a controversial issue needs further investiga-

tion.

Positive SST anomalies in 30-40ºS, 10-40ºW have a

somewhat coherent structure, but the SLP and wind ve-

locity anomalies do not. Because large sampling errors

are likely involved, we will not further discuss anomalies

in the extratropical South Atlantic.

5. Discussion

The previous studies of decadal variability have re-

vealed characteristic in the SST and SLP anomaly pat-

terns in the North Atlantic (Deser and Blackmon 1993,

Kushnir 1994, Peng and Fyfe 1996, Robertson 1996,

Bojariu 1997, Zorita et al. 1992, Watanabe et al. 1999)

and the South Atlantic (Venegas et al. 1997). These

anomaly patterns seem components of a single PADO

mode in the air-sea coupled field, as is demonstrated in

the Sections 3 and 4. Investigations of tropics-extratropics

connections (Xie and Tanimoto 1998, Tanimoto and Xie

1999) indicated that change in cross-equatorial SST gra-

dient, showing a distinct decadal variations, was strongly

Fig. 8 Same as Figure 5, but for SLP and wind velocity anomalies

from the NCEP reanalysis dataset. The reference wind ve-

locity (5.0ms-1) are indicated on the bottom of panel.

Contour interval is 0.5hPa for the SLP field. Positive (nega-

tive) values are represented by the solid (dashed) lines.

JAMSTECR, 41 (2000) 65

linked with extratropical decadal variability.

It remains controversial whether the meridional SST

gradient variability in the tropics represents an intrinsic

coupled mode (Chang et al. 1997, Xie and Tanimoto 1998,

Tanimoto and Xie 1999) or arises from fortuitous super-

position of two independent modes of SST variations

confined on either side of the equator (Houghton and

Tourre 1992, Mehta 1998). The results of our EOF analy-

ses conducted separately in two subdomains north and

south of the equator in Section 3 support the existence of

an air-sea coupled inter-hemispheric interaction, as is in-

dicated by the mutual temporal correlation among the four

PCs of SST and SLP fields and by a tropical dipole con-

figuration in their EOF spatial patterns.

Recent theoretical analysis with a generalized coupled

model (Xie et al. 1999) that includes both Bjerknes and

wind-evaporation-SST feedbacks provides some physi-

cal basis for our empirical modal decompositions. In an

ocean of Atlantic zonal size, arbitrary initial disturbances

in the model disperse into two sets of modes: equatorially

symmetric and anti-symmetric, respectively. Further-

more, the dispersion relations of these coupled models

also are such that their frequencies are well separated:

the anti-symmetric mode prefers decadal and longer time

scales, while the symmetric mode exhibits higher

interannual frequencies.

Simple/intermedium coupled models such as Xie et al.’s

(1999) produce a dipole mode of similar amplitudes of

the equator. While the observed SST pattern indicates

such a rough symmetry in amplitude, the SLP and wind

speed anomalies in the Southern Hemisphere are only one-

third of those in the Northern Hemisphere. While the

SLP and wind anomalies can be understood as the

baroclinic response to the SST dipole, the anomalous

cyclonic circulation centered on 35oN is largely a surface

signature of a deep barotropic response. It appears at

500hPa with its center shifted slightly westward (Fig. 9).

It will be interesting to include this barotropic response

in the atmospheric component of simple/intermedium

coupled models and see how coupled modes, particularly

the dipole, will change. We note that this hemispheric

difference in atmospheric response can be important for

understanding upper ocean variability that is largely wind-

driven. We can expect larger subsurface decadal vari-

ability in the North than the South Atlantic as ocean GCM

simulations seem to suggest (Huang and Shukla 1997).

Here we consider two factors which might cause asym-

metric amplitudes of the dipole-associated atmospheric

anomalies in the Atlantic. First, there are few observa-

tions outside of merchant ship routes in the Southern

Hemisphere. Tropical SST anomalies usually have a per-

sistence of more than one season. The persistence of at-

mospheric anomalies, by contrast, is much shorter than

that of SST anomalies. Few observation gives rise to

larger sampling errors in the atmospheric fields than in

the ocean, and in turn might mask climate signals. Simi-

larities between the results from NCEP and COADS

datasets, however, suggest that sampling errors are not a

critical problem in the tropics.

Second, the cloud shielding of solar radiation may con-

tribute to maintaining the tropical dipole mode of SST

anomalies. In boreal spring when the SST field in the

equatorial Atlantic is nearly uniform in both the zonal

and meridional directions, the ITCZ is sensitive to the

changes in an interhemispheric SST gradient associated

with the dipole (Nobre and Shukla 1996). This meridi-

onal shift of the ITCZ causes the decadal variability in

northeastern Brazil rainfall (Servain 1991, Mehta 1998,

among others). Composites based on a 2 degree latitude-

longitude higher-resolution dataset from the COADS cap-

ture this shift in ITCZ's latitude. Associated with an

anomalous northward SST gradient is an increase (de-

crease) in cloudiness at 5ºN (5ºS; Fig. 10). These near-

Fig. 9 Same as Figure 5, but for 1000hPa (thin contours) and

500hPa (thick contours) anomalies from the NCEP reanaly-

sis dataset.

JAMSTECR, 41 (2000)66

equatorial changes in cloudiness indeed seem associated

with convective activity as indicated by the convergence

(divergence) of anomalous winds at the surface (dotted

line in Fig. 10b). These near-equatorial changes in high

clouds act as a negative feedback to diminish the cross-

equatorial SST gradient.

Additional cloudiness anomalies are found in off-equa-

torial tropics poleward of 10 degree latitudes, which have

not been noted previously to our knowledge. These off-

equatorial cloudiness anomalies are negatively correlated

with local SST anomalies, but apparently not associated

with significant changes in surface wind convergence.

Thus they most likely correspond to changes in low-level

stratiform clouds. These low-level cloud anomalies are

spatially organized (Fig. 10). The cloud anomaly pattern

in the southern tropics is particularly robust, seen in all

the seasons. In annual mean, the southern cloudiness

anomalies are twice as large as the northern ones (not

shown). Two mechanisms are possible for causing

changes in low-level clouds. First is a top-down mecha-

nism: the shifts in the ITCZ change the subsidence in

off-equatorial tropics, affecting the height of and stratifi-

cation at the top of the planetary boundary layer (PBL).

The other is bottom-up: negative (positive) SST anoma-

lies increase (decrease) the stratification across the top of

the PBL provided temperatures in the free atmosphere

do not change. Enhanced (weakened) capping of the PBL

leads to an increase (decrease) in stratiform cloud cover

trapped near the top of the PBL. Increased (reduced) cloud

cover will in turn cause a further cooling (warming) in

SST through insolation change, completing a positive

feedback loop between local SST and stratus clouds.

In a coupled ocean-atmosphere model, the temporal

spectrum of the dipole mode response to a white-noise

forcing is sensitive to thermal damping rate in the SST

equation (Xie and Tanimoto 1998, Xie 1999). The SST

dependence of surface evaporation provides a major

mechanism for thermal damping, which can be linear-

ized as a Newtonian cooling term with a damping rate of

(1 year) -1 (Xie 1999). The rate of SST change due to

cloud shielding is -0.62(1-A)S0C'/(cpρH), where Reed's

(1977) formula for shortwave radiation has been used,

S0=300 Wm-2 is the solar radiation in clear sky, A=0.96

the albebo of sea surface, C’ the perturbation cloud

amount, cp and ρ are the specific heat at constant pres-

sure and density of sea water, and H=50m is the depth of

the mixed layer. Assuming that low-level clouds in off-

equatorial tropics vary with local SSTs, we have C'=-αT’

with α =0.1 K-1 from the right panels of Fig. 10. This

leads to an SST-stratus feedback coefficient, b = 0.62α

Fig.10 (a) Boreal spring (Feb.-Apr.) composite of cloud cover anomalies based on a higher resolu-

tion (2x2) COADS (left panel; heavy shading < -3.0% & light shading > 3.0%). (b) Zonal

mean anomalies of SST (upper right) and cloud cover (%; solid) along with surface wind

divergence (10-6s-1 in dotted line; lower right panel).

JAMSTECR, 41 (2000) 67

(1-A)S0/(cpρH)=(3.5 years) -1. Thus the local SST-stratus

feedback can reduce the Newtonian cooling rate for SST

by as much as 30%.

6. Concluding Remarks

We have examined dominant modes of the decadal

variability in the Pan-Atlantic basin, using the new ob-

servational datasets of marine meteorological variables

calculated from LMRF of COADS. An EOF analysis

was performed for decadal SST anomalies in the North

Atlantic and in the South Atlantic separately, avoiding

the artificial correlation between the northern and south-

ern tropics. The leading EOFs featured a dipole struc-

ture in the tropics across the equator and were associated

with substantial extratropical signals. The SLP EOF fea-

tured a similar dipole albeit with subtropical centers of

action on either side of the equator. Time series of these

leading modes correlated well with one another and pre-

sented three cycles of distinct decadal oscillations during

1966-90, indicative of the existence of an inter-hemi-

spheric dipole mode that involved ocean-atmosphere in-

teraction.

Composite maps, based on the positive and negative

phases of the tropical dipole mode, clearly showed that

the latent heat flux induced by anomalous wind vector

anomalies played a major role in coupling the atmosphere

and ocean. The composite SST anomalies had compa-

rable amplitudes on either side of the equator. However,

the magnitude of the SLP and wind velocity anomalies to

north of equator were three times larger than those to the

south. Furthermore an NCEP reanalysis dataset showed

a deep barotropic atmospheric pattern in the Northern

Hemisphere associated with the NAO pattern, but not in

the Southern Hemisphere at all. Whereas this asymmet-

ric amplitude structure might be a consequence of large

sampling errors due to insufficient observations in the

Southern Hemisphere, low-level clouds might play a role

in keeping SST anomalies having comparable amplitudes

across the equator. The response of cloud fields to the

SST dipole differed near and off the equator. Within 10

degrees latitude, it involved north-southward shift in the

ITCZ and changes in deep convective clouds, acting to

dampen the cross-equatorial SST gradient. Outside this

equatorial zone, low-level clouds responded to and posi-

tively fed back onto the local SST, reducing the thermal

damping rate by 30%.

Acknowledgment

The authors are grateful to Prof. T. Yamagata, Dr.

H. Nakamura, Dr. Iwasaka and Prof. Matsuno for stimu-

lating discussion. We also thank the NOAA/NCEP for

providing the reanalysis data and the NCAR for provid-

ing the COADS/LMRF dataset. This work was partly

supported by Frontier Research System for Global change.

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