Journal of the Meteorological Society of Japan, Vol. 88, No. 2, pp. 183--202, 2010.

183

DOI:10.2151/jmsj.2010-205

Subtropical Paciﬁc SST Variability Related to the Local Hadley Circulation
during the Premature Stage of ENSO

Yoshimitsu CHIKAMOTO
Center for Climate System Research, University of Tokyo, Kashiwa, Japan

Youichi TANIMOTO
Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan

Hitoshi MUKOUGAWA
Disaster Prevention Research Institute, Kyoto University, Kyoto, Japanand
Masahide KIMOTO
Center for Climate System Research, University of Tokyo, Kashiwa, Japan(Manuscript received 9 April 2009, in ﬁnal form 4 January 2010)

Abstract
Formation processes of negative (positive) sea surface temperature anomalies (SSTAs) in the subtropicalNorth and South Paciﬁc associated with the ENSO warm (cold) events are examined using reanalysis and in-situ
observational datasets. During the premature stage of the ENSO warm events, negative SSTAs appear over thesubtropical North Paciﬁc in the February–March period and over the subtropical South Paciﬁc after April, andvice versa in the ENSO cold events. One month prior to the formation of these subtropical negative SSTAs, thenegative air humidity anomaly and anomalous downward motion appear at the same location in either the
Northern or Southern hemisphere. Associated with these atmospheric anomalies, the strengthened descending
branch of local Hadley circulation is observed during the January–February period in the Northern hemisphereand after March in the Southern hemisphere, which coincides with the seasonal transition of the climatologicallocal Hadley circulation from the Northern to Southern hemisphere. Our linear decomposition analysis of surface
heat ﬂux anomalies indicates that the negative air humidity anomaly, as well as anomalies in wind speed, contributes to the formation of the subtropical negative SSTAs through the enhanced latent heat ﬂux induced by theanomalous air–sea humidity di¤erence. These results suggest that the anomalous downward motion associatedwith the changes in local Hadley circulation can induce the subtropical negative SSTAs through the surface humidity variability. A possible mechanism for the subtropical air–sea interaction associated with the local Hadleycirculation is discussed.

Corresponding author: Yoshimitsu Chikamoto, Centerfor Climate System Research, University of Tokyo 5-15, Kashiwanoha, Kashiwa, Chiba, 277-8568, Japan.
E-mail: chika44@ccsr.u-tokyo.ac.jp
6 2010, Meteorological Society of Japan

1. Introduction
The well-known feature of the subtropical Paciﬁcsea surface temperature anomaly (SSTA) during
the El Niño and the Southern Oscillation (ENSO)

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warm events is a horse shoe like structure of cold
SST encompassing the tongue-like structure ofwarm SST in the eastern equatorial Paciﬁc andvice versa in the cold events (e.g., Rasmusson andCarpenter 1982; Trenberth et al. 1998; Wallace
et al. 1998; Larkin and Harrison 2002; Alexander
et al. 2004). This subtropical SST variability shows
the seasonal evolution associated with the development of equatorial SSTAs during the ENSO event.
Figure 1 shows the composite di¤erence maps ofseasonal SSTAs between the ENSO warm andcold events (the deﬁnitions of the ENSO warm andcold events are described in Section 2). During theJanuary-February-March (JFM) period in the premature stage of the ENSO events, negative SSTAs
are observed west of 150 W in the subtropicalVol. 88, No. 2

North Paciﬁc from 15 N to 30 N, while such significant anomalies are not apparent in the subtropicalSouth Paciﬁc (Fig. 1a). In the subtropical North
Paciﬁc, the negative SSTAs become prominent at
the center of 165 W during the April-May-June(AMJ) period (Fig. 1b), and move further northof 35 N in the following July-August-September(JAS) period (Fig. 1c). In the subtropical South Paciﬁc, on the other hand, negative SSTAs begin to
emerge over the region east of Australia, extending
from the equator to 40 S and tilting southeastward
during the April-May-June (AMJ) period, threemonths later than in the subtropical North Paciﬁcand persist until the mature stage of the ENSO
event with their area extending eastwards (Fig. 1d).
In association with the ENSO events, the SSTAs

Fig. 1. Composite di¤erence maps of SSTAs in (a) January-February-March, (b) April-May-June, (c) JulyAugust-September, and (d) October-November-December periods between the premature stage of the ﬁvewarm and three cold events of ENSO. The contour interval is 0.4 K. Zero contours are omitted. Anomalies
exceeding the thresholds for signiﬁcance at the 90% and 95% levels are indicated by light and dark shaded
regions, respectively.

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Y. CHIKAMOTO et al.

in the equatorial Paciﬁc a¤ect atmospheric meridional circulations via changes in precipitation andupward motion over the central and western tropical Paciﬁc. Oort and Yienge (1996) indicate that theHadley circulation, calculated from the zonal mean
streamfunction by using the zonal mean meridional wind component, is strengthened in bothhemispheres during the ENSO warm events. Wang
(2002), by analyzing the regional meridional circulation, shows that the strengthened local Hadleycirculation is observed in the western North Paciﬁcduring the November–January period, one year before the mature stage of the ENSO warm events.
The strengthened Hadley circulation induced by
the ENSO forcing may a¤ect humidity variationsin the lower atmosphere and thereby produce thesubtropical SST variability through a change in sea
surface evaporation, because the descending branchof local Hadley circulation induces a dry conditionin the lower atmosphere by vertical moisture advection and surface moisture ﬂux divergence (Sun and
Lindzen 1993; Sun and Oort 1995; Sohn et al. 2001;
Kawamura et al. 2002; Yoneyama 2003). Chikamoto and Tanimoto (2005) showed that a latent
heat ﬂux anomaly induced by an air–surface humidity anomaly is the major factor to form a local
SST anomaly in the Caribbean region after the mature stage of the ENSO events. These studies suggest that the air–surface humidity variability under
the descending blanch of the Hadley circulation
over the subtropical Paciﬁc is important to induce
the local SST variability during the ENSO events.
Weng (2009) also implied the existence of atmospheric forced subtropical SST variability on thebasis of a statistical mode obtained from the timemode extended singular value decomposition analysis between the upper ocean heat content and SST
anomalies.
Recent studies focus on the roles of the subtropical SST variability in the phase transition and ampliﬁcation of the ENSO events. Wang and Zhang(2002) pointed out that o¤-equatorial, cool SSTA
maintains anomalous Philippine Sea anti-cyclone
through a positive thermodynamic feedback (Wanget al. 2000). The resultant surface easterlies over theequatorial region can excite a cold Kelvin wave,
which is indispensable for the phase transition of
the ENSO events (Kug and Kang 2006; Ohba andUeda 2007). Another role of the subtropical SST
variability is the link between extratropical andtropical oceans (Liu et al. 1994; McCreary and Lu
1994; Gu and Philander 1997). Shin and Liu (2000)

185

showed that thermocline anomalies in the midlatitudes can e‰ciently penetrate into the equator. On
the basis of the results of these studies, Sun et al.
(2004) demonstrated that enhanced ocean coolingin the subtropical Paciﬁc results in a regime withstronger ENSO activity. Thus, the subtropical
SSTAs may feed back to the ENSO cycle and/or
modulate the amplitude of ENSO.
In the present study, we examine how the subtropical negative (positive) SSTAs associated withthe variations in the local Hadley circulation are
produced during the premature stage of the ENSO
warm (cold) event. Since the changes in the local
Hadley circulation will accompany surface air humidity variability, we show the formation processof air humidity anomaly associated with the seasonal cycles of the local Hadley circulation. This
air humidity variability is key to understanding theair–sea interaction over subtropical regions.
The rest of this paper is organized as follows.
Section 2 describes the datasets and deﬁnes the
ENSO warm and cold events from the Niño3 index. In Section 3, we examine the interannual variability of air–surface humidity related to the descending branch of local Hadley circulation. In
Section 4, we analyze the formation processes ofthe subtropical SSTAs induced by the air humidity
variability. On the basis of the results from ouranalyses, we discuss a possible mechanism to induce the air humidity variation and suggest an atmospheric positive feedback in Section 5. The results are summarized in Section 6.
2. Datasets
To capture the ENSO-related variability in thetropical Paciﬁc, monthly horizontal wind, verticalp-velocity, and speciﬁc humidity ﬁelds at eight pressure levels are obtained from the National Centers
for Environmental Prediction-Department of Energy Atmospheric Model Intercomparison ProjectII Reanalysis (NCEP2; Kanamitsu et al. 2002)
data on a 2:5  2:5 grid. Monthly latent and sensible heat ﬂuxes from the ocean surface (upward
positive), surface longwave and shortwave radiation ﬂuxes, zonal and meridional wind components
at 10-m height, and speciﬁc humidity at 2-m height
are also taken from the NCEP2 approximately on a
1:875  1:875 Gaussian grid. Monthly SST data
are obtained from the National Oceanic and Atmospheric Administration optimum interpolation SST
version 2 (OISST; Reynolds et al. 2002) on a1  1 grid. To calculate monthly latent heat ﬂux

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components, we used surface ﬂux variables of surface wind speed at 10-m height, saturation speciﬁc
humidity at the ocean surface, and air–sea humidity di¤erence in speciﬁc humidity between the ocean
surface and at a 2-m height on the Gaussian grid.
The surface wind speed at a 10-m height is derived
from 4 times daily of zonal and meridional windcomponents at 10-m height in the NCEP2 data.
From the linearly interpolated SST and surfacepressure, the monthly saturation speciﬁc humidity
at the ocean surface and the air–sea di¤erences in
speciﬁc humidity between the ocean surface and at
2-m height are calculated on the Gaussian grid.
The surface latent heat ﬂuxes of the reanalysis
data are less reliable than other elements in thefree atmosphere due to insu‰cient treatments ofthe planetary boundary layer (Trenberth and Guillemot 1995; Trenberth and Guillemot 1998). Therefore, we employ the 2  2 monthly dataset of surface latent heat ﬂuxes based on individual marine
meteorological reports archived in the Comprehensive Ocean-Atmosphere Data Set (COADS; Woodru¤ et al. 1987). This dataset contains some missing
values in the equatorial and South Paciﬁc regions
since satellite observations and spatial interpolation
were not used. Details of the calculation procedure
are described in Tanimoto and Xie (2002) and Tanimoto et al. (2003). We calculate the monthly climatological means based on a 21- or 22-year period
from December 1981 to December 2002 for NCEP2
and OISST datasets and for a 46-year period fromJanuary 1950 to December 1995 for COADS. The
monthly anomalies are deﬁned as departures fromthe climatological mean values.
To represent the magnitude of the ENSO warmand cold events, we extract eight warm (cold) yearsof 1957/58, 65/66, 72/73, 82/83, 87/88, 91/92, 94/
95, and 97/98 (1955/56, 67/68, 70/71, 73/74, 75/76,
84/85, 88/89, and 99/00) from the SSTAs in theNiño3 region, as described in Chikamoto and Tanimoto (2006), which is deﬁned as the averaged
monthly SSTAs from November through January
in the following year (exceeding one standard deviation). Hence, we take the warm (cold) events of 7(7) years in COADS and of 5 (3) years in NCEP2
and OISST. We analyze composite di¤erence maps
between the ENSO warm and cold events, hereafter
simply referred to as composite in the ENSO warm
event. The anomalies with the opposite polarity areobserved in the ENSO cold event. In the COADS
datasets, composites calculated from more thanfour events are employed.

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3. Subtropical air humidity variability during thepremature stage of the ENSO events
As we stated in the introduction, the subtropical
SSTAs during the premature stage of the ENSO
event have a di¤erent time evolution between the
North and South Paciﬁc. In Fig. 2, we have a closer
look at the monthly evolution of the SSTAs fromJanuary to April during the premature stage of the
ENSO event. In January, negative SSTAs lower
than 0:5 C appear in the subtropical North Paciﬁc near the date line. These negative SSTAs become prominent in February (Fig. 2b) and are zonally elongated along 25 N in the following March.
In contrast, a coherent structure of negative SSTA
is not so apparent in the subtropical South Paciﬁcduring this period (Figs. 2b,c). In April, the prominent negative SSTAs exceeding 1:0 C begin to
emerge in the subtropical South Paciﬁc (Fig. 2d)and maintain their anomalies until the mature stageof the ENSO event (Figs. 1b–d).
To examine the temporal evolution of surface air
humidity anomalies related to the SSTA formation
over the subtropical Paciﬁc, we drew monthly composite maps of speciﬁc humidity anomaly at a 2-m
height (qa0 ) and surface wind anomalies from January to April (Fig. 3). In the subtropical North Paciﬁc, signiﬁcant negative qa0 lower than 1:0 g kg1
associated with northerly wind anomalies is observed near the date line in January (Fig. 3a) andextends in the westward direction along 25 N in
February (Fig. 3b). In the following March andApril, the local maximum of negative qa0 in the subtropical North Paciﬁc gradually decreases its amplitude (Figs. 3c,d). Instead, signiﬁcant negative qa0
and southerly wind anomalies emerge in the subtropical South Paciﬁc during this period.
Figure 4 shows the temporal evolution of the speciﬁc humidity anomaly at a 2-m height (qa0 ) and thesaturation speciﬁc humidity anomaly at the sea surface (q0o ) averaged over the subtropical North Paciﬁc (160 E–170 W, 20 N–40 N) and the subtropical South Paciﬁc (160 E–170 W, 20 S–40 S). Notethat q0
o is a function of the local SSTA. As shownin Fig. 4, negative qa0 , much lower than q0o , isformed over the subtropical North Paciﬁc in January and February before the mature stage of the
ENSO events. This anomalous air-sea di¤erence in
speciﬁc humidity indicates that the negative air
humidity anomalies contribute to form negative
SSTAs in the subtropical North Paciﬁc, as will beshown in the next section. After March, however,April 2010

Y. CHIKAMOTO et al.

187

Fig. 2. Composite di¤erence maps of SSTAs in (a) January, (b) February, (c) March, and (d) April between the ﬁve warm and three cold events of ENSO. The contour interval is 0.5 K. Zero contours areomitted. Anomalies exceeding the thresholds for signiﬁcance at the 90% and 95% levels are indicated bylight and dark shaded regions, respectively.

temporal evolutions of qa0 and q0o tend to show similar behavior. Particularly from August to October,a decrease in qa0 follows a decrease in q0
o . Alexander
et al. (2004) indicated that a reduced short wave radiation associated with an increase in low clouds isa major factor to form the negative SSTAs in thesubtropical North Paciﬁc during this period. Overthe subtropical South Paciﬁc, signiﬁcant negative
qa0 is formed in March and persists until November(Fig. 4b). Throughout this period, qa0 is lower thanq0o except for August, indicating that the anoma-

lous air–sea di¤erence in speciﬁc humidity contributes to enhance evaporation from the ocean surface. During the same period, signiﬁcant negative
SSTAs less than 0:4 C actually persist over thesubtropical South Paciﬁc during the premature
stage of the ENSO warm event (Figs. 4b, 1). These
temporal evolutions of qa0 and q0o suggest that thesurface air humidity anomaly is important for theformation of negative SSTAs in the subtropical Paciﬁc of both hemispheres.
In general, surface air humidity variation is

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Fig. 3. Composite di¤erence maps of air humidity anomalies at a 2-m height and surface wind anomalies in(a) January, (b) February, (c) March, and (d) April between the ﬁve warm and three cold events of ENSO.
The contour interval is 0.5 g kg1 and zero contours are omitted. The reference vector at the upper rightcorner indicates 5 m s1 . Air humidity anomalies exceeding the thresholds for signiﬁcance at the 90% and
95% levels are indicated by light and dark shaded regions, respectively. Wind vectors with either the zonal
or meridional component exceeding the 90% signiﬁcance level are plotted.

mainly controlled by local SSTAs over the tropics.
Over the equatorial regions, for example, a positive
SSTA leads to surface moist air associated with thedevelopment of ENSO (Figs. 2, 3). Over the subtropical regions, on the other hand, the negative
SSTA follows the surface dry air, suggesting that
surface air humidity variation associated with achange in atmospheric circulation induces the local
SST variability. In fact, the equatorward surface

wind anomalies are observed over the subtropicalnegative air humidity anomalies at the same time.
Therefore, we turn our attention to the relationshipbetween surface air humidity anomaly and achange in the local Hadley circulation.
Figures 5a,b show the latitude–time diagrams ofcomposite di¤erence for surface humidity and vertical motion at 500 hPa zonally averaged over the160 E–170 W. The climatological cycle for theseApril 2010

Y. CHIKAMOTO et al.

Fig. 4. Composite di¤erence anomalies of
air humidity at a 2-m height (qa0 ; solidline), saturation speciﬁc humidity at thesea surface (q0o ; dashed line), and SSTAs(shaded) averaged over (a) the subtropicalNorth Paciﬁc (20 N–40 N, 160 E–
170 W) and (b) the subtropical South Paciﬁc (20 S–40 S, 160 E–170 W) between
the ﬁve warm and three cold events of
ENSO. The units of speciﬁc humidity and
SSTA are g kg1 and K. Anomalies of the
air-sea humidity di¤erence exceeding thethresholds for signiﬁcance at the 90% levels
are indicated by circles.

variables is also shown in Figs. 5c,d. During the
ENSO warm event, anomalous upward motion in
the equator and downward motion in the subtropi-

189

cal North Paciﬁc are observed in January and February (Fig. 5b), thereby strengthening the climatological local Hadley circulation in the Northernhemisphere (Fig. 5d). Associated with the strengthened climatological descending branch of local
Hadley circulation, negative surface air humidity
anomalies appear over the subtropical North Paciﬁc during the dry season (Figs. 5a,c). In the following March and April, the anomalous downward
motion begins to take place in the subtropicalSouth Paciﬁc associated with the seasonal cycle ofthe climatological descending branch, enforcing
the local Hadley circulation in the Southern hemisphere. This anomalous downward motion overthe subtropical South Paciﬁc persists from March
to September with the strengthened climatologicallocal Hadley circulation in the Southern hemisphere, while the climatological local Hadley circulation in the Northern hemisphere disappears during this period. Associated with this transition of
intensity in the climatological downward motion
from the northern to the southern subtropics, anegative speciﬁc humidity anomaly (i.e., surface
dry air) is prominent in the subtropical North Paciﬁc during the January–February period and thenappears in the subtropical South Paciﬁc fromMarch to September (Fig. 5a). Consistent withthese transitions, the subtropical negative SSTAs
are also formed in February in the subtropicalNorth Paciﬁc and in April in the subtropical South
Paciﬁc, respectively (Figs. 2, 4).
Figure 6 shows the latitude–vertical cross sectionof anomalous meridional circulation and speciﬁc
humidity anomaly zonally averaged in 160 E–
170 W during the January–April period. As stated,
air humidity anomalies are closely related to theanomalous meridional circulation: positive (negative) speciﬁc humidity anomaly with anomalous
upward (downward) motion near the equator (subtropics). On the basis of these vertical cross sections, we also capture the seasonal transition of theanomalous meridional circulation. In the January–
February period, the negative speciﬁc humidity
anomaly and the anomalous downward motion are
dominant in the Northern hemisphere around 25 N(Figs. 6a,b). These anomalies in the subtropicalNorth Paciﬁc accompany a positive humidity
anomaly and an anomalous upward motion near
the equator. In the following March, however, theamplitudes of these anomalies in the subtropicalNorth Paciﬁc gradually decrease (Fig. 6c). Instead,the anomalous downward motion and the negative

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Fig. 5. Time evolution of air humidity at a 2-m height (left panels) and vertical p-velocity at 500 hPa (rightpanels) in (a,b) the composite di¤erence between the ﬁve warm and three cold events of ENSO and (c,d)
climatology. The deviations from annual means of climatological air humidity are shaded in (c). The units
in the left and right panels are g kg1 and Pa s1 , respectively. Anomalies exceeding the thresholds for
signiﬁcance at the 90% and 95% levels are indicated by light and dark shaded regions, respectively.

speciﬁc humidity anomaly become prominent in theSouthern hemisphere extending from 20 S to 40 S.
This dry air in the lower atmosphere associatedwith the anomalous downward motion over thesubtropical South Paciﬁc ampliﬁes and shifts equatorward in the following April (Fig. 6d).
During the premature stage of the ENSO events,
equatorial positive SSTAs can induce the anomalous meridional circulation near the Date Line.
This anomalous meridional circulation strengthens

the climatological local Hadley circulation andthen causes the strengthened descending branch of
local Hadley circulation. Associated with the seasonal cycle of climatological local Hadley circulation, the strengthened descending branch inducedby the anomalous meridional circulation also shows
the seasonal transition from the Northern to Southern hemisphere during the boreal spring. Sincechanges in the descending branch of the local
Hadley circulation a¤ect the surface air humidity

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Y. CHIKAMOTO et al.

Fig. 6. Composite di¤erence anomalies of meridional wind (m s1 ), vertical p-velocity (upward positive;
102 Pa s1 ), and speciﬁc humidity (shaded) zonally averaged from 160 E to 170 W in (a) January,(b) February, (c) March, and (d) April between the ﬁve warm and three cold events of ENSO. The reference vector above the color bar is 3 m s1 or 3  102 Pa s1 . The contour interval is 0.3 g kg1 and zero
contours are omitted.

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Journal of the Meteorological Society of Japanvariability, the observed negative air humidity
anomalies in the subtropical Paciﬁc also show theseasonal transition from the Northern to Southernhemisphere. As we stated in the introduction, thesurface air humidity variability can induce the local
SST variability through changes in surface evaporation. In the next section, we will examine the formation processes of the subtropical negative SSTAs
induced by the surface air humidity anomalies.
4. Formation processes of the subtropical Paciﬁc
SSTAs
To examine the formation process of negative
SSTA in the subtropical North Paciﬁc, we draw
composite maps of SSTA tendency and net heatﬂux anomaly in January (Fig. 7). The SSTA tendency in a given month is deﬁned as the di¤erence
between the SSTA in the subsequent month andthat in the previous month. A positive heat ﬂux
anomaly indicates a heat loss from the ocean surface. Since the surface latent and sensible heat
ﬂuxes in the reanalysis data are less reliable thanother elements in the free atmosphere, we will also
examine the COADS dataset. In both datasets, decreases in SSTA of 0.3 C for two months are observed in the subtropical North Paciﬁc with a zonally elongated pattern along 25 N (Figs. 7a,b).
Over this region, positive net heat ﬂux anomaliesof 20 W m2 are also observed in both datasets(Figs. 7c,d), although those anomalies are somewhat stronger in NCEP2. Given that SSTA tendencies of 0:3 C for 2-month are only due to surface
heat ﬂux anomalies of 20 W m2 , we obtain an
equivalent mixed layer depth of 85-m, comparable
to the observed isothermal layer in the subtropicalNorth Paciﬁc in January (Kara et al. 2003). This estimation suggests that the January surface heat ﬂux
anomaly mainly contributes to the formation of thenegative SSTA in this region.
Over the tropics, a primarily important component of net heat ﬂux anomaly is a latent heatﬂux anomaly, particularly induced by a change inwind speed (e.g., Cayan 1992; Alexander and Scott
1997; Xie et al. 1999). Indeed, the January composite map of heat ﬂux anomaly component indicates that positive latent heat ﬂux anomalies of
20 W m2 are observed over the subtropical North
Paciﬁc with the zonally elongated pattern along25 N while short wave radiation anomaly is quite
small over most of the North Paciﬁc during thisperiod (Fig. 7e–h). However, as stated below, theanomalous scalar wind speed is insu‰cient in ex-

Vol. 88, No. 2

plaining the latent heat ﬂux anomalies over the subtropical North Paciﬁc.
The latent heat ﬂux anomaly can be decomposed
into three linearized components as in Enﬁeld andMayer (1997) and Tanimoto et al. (2003):
Flh0 ¼ rce Le fW 0 Dq þ W Dq 0
þ ðW 0 Dq 0  W 0 Dq 0 Þg;

ð1Þ

where Flh is the latent heat ﬂux, W is the scalarwind speed, Dq is the air-sea di¤erence in speciﬁc
humidity, r is the atmospheric density, ce is thebulk coe‰cient, Le is the latent heat of vaporization for water, and the overbar and prime indicate
the climatological mean and anomaly, respectively.
The bulk coe‰cients are assumed to be constant in
NCEP2 and to be dependent on scalar wind speed
and static stability in COADS. However, when welinearize the surface heat ﬂux anomalies, we ignore
the dependence of the coe‰cients on those variables. On the right-hand side of (1), the ﬁrst two
terms represent contributions of the wind speed
anomaly (W 0 component) and the anomalous airsea di¤erence in speciﬁc humidity (Dq 0 component)
to the total amount of latent heat ﬂux anomalies,
respectively. The sum of these ﬁrst two terms(Figs. 8e,f ) is comparable to the total latent heatﬂux anomaly (Figs. 7e,f ) and explains most of the
net heat ﬂux anomalies (Figs. 7c,d).
Figure 8 shows the January composite maps ofthe linearized components of latent heat ﬂux
anomalies based on COADS and NCEP2-OISST
datasets. In both datasets, positive anomalies ofW 0 component are observed with the zonally elongated pattern along 25 N (Figs. 8a,b). Positive
anomalies of Dq 0 component are also observed
over the subtropical North Paciﬁc particularly near
the Date Line (Figs. 8c,d). As a result, the latent
heat ﬂux anomalies with the zonally elongated pattern along 25 N, approximately represented by sumof these two components, enhance the heat loss
from the subtropical North Paciﬁc Ocean and thencontribute to the formation of negative SSTAs.
These results indicate that the Dq 0 and W 0 components, is important for generating a substantial latent heat ﬂux anomaly in January, and hence, negative SSTAs in the following February over thesubtropical North Paciﬁc.
To examine the formation of the negative SSTAs
in the subtropical South Paciﬁc from February to
April, we drew composite maps of SSTA tendency
and net heat ﬂux anomaly in March (Fig. 9). In

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Y. CHIKAMOTO et al.

193

Fig. 7. Composite di¤erence maps of (a,b) SSTA tendency, (c,d) surface net heat ﬂux anomaly, (e,f ) latent
heat ﬂux anomaly, and (g,h) short wave radiation anomaly in January between the ENSO warm and cold
events. The left (right) panels are based on NCEP2 and OISST (COADS) in ﬁve (seven) warm and three(seven) cold events of ENSO. The SSTA tendency in a given month is deﬁned as the di¤erence between the
SSTA in the subsequent month and that in the previous one. Positive anomalies in surface heat ﬂux indicate
heat loss from the ocean. The contour intervals of SSTA tendency (heat ﬂux anomalies) are 0.3 K(20 W m2 ) and zero contours are omitted. Heat ﬂux anomalies exceeding the thresholds for signiﬁcanceat the 90% and 95% levels are indicated by light and dark shaded regions, respectively.

both datasets, decreases in SSTA of 0.6 C for twomonths are observed in the South Paciﬁc extending
from the equator to 40 S and tilting southeastward(Figs. 9a,b). Over this region, positive net heat ﬂux

anomalies of 40 W m2 are also observed in bothdatasets (Figs. 9c,d). The observed isothermal layerover this region is at about a 50-m, depth almost
comparable to the equivalent mixed layer depth

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Vol. 88, No. 2

Fig. 8. Composite di¤erence maps of contributions of (a,b) wind speed anomaly and (c,d) anomalous air-sea
di¤erence in speciﬁc humidity to latent heat ﬂux anomaly and (e,f ) sum of these two contributions inJanuary. The left (right) panels are based on NCEP2 and OISST (COADS) in ﬁve (seven) warm and three(seven) cold events of ENSO. The contour interval is 10 W m2 , which is half for Fig. 7. Zero contours areomitted.

obtained from those anomalies. Over the tropicalPaciﬁc, a signiﬁcant westerly wind anomaly appears along 10 S over the western tropical Paciﬁc
in March (Fig. 3c). These westerly wind anomalies
can enhance the surface wind speed, and therefore
contribute to the positive latent heat ﬂux anomaly.
To the east of Australia, on the other hand, the surface wind anomaly is weak in March (Fig. 3c),while signiﬁcant negative SSTAs are observed in

the following April (Fig. 2d). This result suggests
the relative importance of Dq 0 for generating negative SSTAs in the subtropical South Paciﬁc, similar
to the subtropical North Paciﬁc.
Figure 10 shows March composite maps of thelinearized components of latent heat ﬂux anomalies. The net heat ﬂux anomaly that is mainly contributed by the latent heat ﬂux anomaly is well represented by the sum of the W 0 and Dq 0 components

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Y. CHIKAMOTO et al.

Fig. 9.

195

Figure legends are the same as those in Fig. 7, but for March.

(Figs. 9, 10e,f ). In NCEP2 and OISST, the positive
latent heat ﬂux anomaly over the South Paciﬁc ismainly contributed by the W 0 component northof 20 S, but by the Dq 0 component south of 20 S(Figs. 10a,c). As a result the positive latent heatﬂux anomalies have a meridionally large area extending equator to 40 S. In COADS, the positive
anomalies of the Dq 0 component south of 20 S arealso observed, while there are many missing values

north of 20 S (Figs. 10b,d). These results indicatethat the Dq 0 component is important for generatinga substantial latent heat ﬂux anomaly in March andhence negative SSTAs in the following April overthe subtropical South Paciﬁc.
Over the tropics, SSTAs are normally higher
than surface air temperature anomalies, implyingthat surface anomalies in humidity and temperatureare caused by SSTAs. Over the equatorial Paciﬁc,Journal of the Meteorological Society of Japan

196

Fig. 10.

Vol. 88, No. 2

Figure legends are the same as these in Fig. 8, but for March.

for example, SSTAs and Dq 0 component show thesame polarity (Figs. 2c, 10c), indicating that a positive SSTA induces positive air humidity due to enhanced surface evaporation. Over the subtropics,
however, SSTAs and Dq 0 component show opposite
polarities in the subtropical North Paciﬁc in January (Figs. 2a, 8c) and in the subtropical South Paciﬁc in March (Figs. 2c, 10c). These results indicatethat a negative air humidity anomaly can produce
the positive anomaly of Dq 0 component, and therefore induce negative SSTAs in the subtropics.

5. Discussion
To understand the subtropical air-sea interaction, we will discuss the role of the anomalous
downward motion related to the strengthened local
Hadley circulation. The anomalous downward motion can induce a dry condition in the lower atmosphere due to moisture ﬂux advection, which affects the formation of subtropical negative SSTAs
through the contribution from anomalous air-sea
humidity di¤erence to latent heat ﬂux anomaly,April 2010

Y. CHIKAMOTO et al.

as stated in the previous section. In fact, qa0 is signiﬁcantly anti-correlated with vertical p-velocity
at 500 hPa in January and February over the subtropical North Paciﬁc (correlation coe‰cients r are0:65 and 0:59, respectively, and exceed the statistical signiﬁcance at the 99% level) and in Marchand April over the subtropical South Paciﬁc (r ¼
0:56 and 0:76). The anomalous downward motion associated with the strengthened local Hadleycirculation coincides with the local maxima of
equatorward surface wind anomalies in January
over the subtropical North Paciﬁc and in Marchover the subtropical South Paciﬁc, which locates ina slightly poleward region of the low-level return
ﬂow of the local Hadley circulation (Figs. 3, 6).
This relationship between the anomalous downward motion and the equatorward surface wind
anomaly satisﬁes the Sverdrup balance. Therefore,
these vertical and meridional wind anomalies inthe subtropics can e¤ectively induce a dry conditionin the lower atmosphere due to vertical and horizontal moisture ﬂux advection, and cause enhanced
evaporation from the ocean surface in these regions.
The enhanced evaporation induced by the anomalous downward motion also a¤ects the moisturebalance. The moisture budget equation shows abalance among a vertically integrated moisture ﬂux
divergence, evaporation from the ocean surface,and precipitation as follow:
ð Ps‘  ðqVÞ 0 ¼ E 0  P 0 :ð2Þ
0Under the descending branch of the local Hadleycirculation, however, the precipitation variation issmall when compared to the other terms. Therefore, the enhanced evaporation mainly balances
the vertically integrated moisture ﬂux anomaly divergence over the subtropical region associatedwith the anomalous downward motion. Table 1
shows each term of the moisture budget equation averaged over regions (20 N–40 N, 160 E–
170 W) of the subtropical North Paciﬁc in Januaryand (40 S–20 S, 160 E–170 W) of the subtropicalSouth Paciﬁc in March. Note that the left-handside of Eq. (2) is estimated by the vertical integration of the moisture ﬂux anomaly divergence fromthe surface to 700-hPa. As we have suggested, themagnitude of the evaporation anomaly is almost
comparable to that of moisture ﬂux anomaly divergence in the lower atmosphere over these regions,while the precipitation anomaly is much smaller

197

than those terms. Since the moisture ﬂux anomaly
divergence in the lower atmosphere is closely related to the anomalous downward motion, theanomalous downward motion in the subtropics
can a¤ect the enhanced evaporation from the ocean
surface through the anomalous wind divergence inthe lower atmosphere.
On the basis of these balances, the role of air-sea
interaction in the subtropical Paciﬁc is explained asfollows. During the premature stage of the ENSO
warm event, the growth of equatorial SSTAs will
drive an anomalous downward motion in the subtropics associated with the strengthened local Hadley circulation. This anomalous downward motion
accompanies an equatorward wind anomaly in thelower atmosphere at the same location to satisfy
the Sverdrup balance. These anomalous downward
motion and equatorward wind anomaly induce a
dry condition in the lower atmosphere due to vertical and horizontal moisture advections, which inturn cause enhanced evaporation from the ocean
surface. During the dry season in the subtropicalPaciﬁc, the enhanced evaporation balances the vertically integrated moisture ﬂux divergence mainly
contributed from the wind anomaly divergence inthe lower atmosphere, implying a low-level return
ﬂow of the local Hadley circulation would be
strengthened in the o¤-equatorial regions. In otherwords, the subtropical moisture associated with theenhanced evaporation from the ocean surface istransported to the equator by the low-level equatorward wind anomaly in the o¤-equatorial regions.
Actually, the meridional component of the moisture ﬂux anomaly in the lower atmosphere showsthe equatorward moisture transports around 20
after the enhanced evaporation in the subtropics(Fig. 11). If this equatorward moisture transport
can contribute to a strengthened initial local Hadley circulation due to the increasing moisture supply in the equatorial lower atmosphere, the processdescribed above indicates the presence of an atmospheric positive feedback loop. However, it is still
unclear whether this positive feedback plays a signiﬁcant role in the activity of Hadley circulation
and/or has a ENSO phase dependency. To reveal
the dynamical processes and causality involved in
the subtropical air-sea interaction, further data diagnosis and model experimentation is required.
This atmospheric positive feedback would be
suppressed by the subtropical negative SSTAs associated with an atmosphere-ocean interaction. Oncethe atmospheric positive feedback becomes active,

198

Journal of the Meteorological Society of Japan

Vol. 88, No. 2

Fig. 11. Composite di¤erence anomalies of the meridional component of vertically integrated moisture ﬂux
from the surface to 700-hPa (zonally averaged from 160 E to 170 W) in (a) February and (b) April
between the ﬁve warm and three cold events of ENSO. Anomalies exceeding the thresholds for signiﬁcanceat the 90% levels are indicated by circles. Anomalies are normalized by atmospheric thickness, and theirunit is g kg1 m s1 . Northward (southward) anomalous moisture transport is light (dark) shaded.

the enhanced evaporation accompanied by thisfeedback induces negative SSTAs in the subsequentmonth. Since the negative SSTAs reduce the ampli-

tude of enhanced evaporation through the reduction of air-sea humidity di¤erence, the atmosphericpositive feedback would not be strengthened dueApril 2010

Y. CHIKAMOTO et al.

to this SSTA damping e¤ect. According to thisprocess, SSTAs act to adjust to the surface air humidity anomaly associated with the atmosphericpositive feedback. This idea is similar to the regulatory mechanism suggested by Chiang and Lintner(2005), who examined the gross oceanic and land
surface temperature warming in the entire tropics
after the mature stage of the ENSO warm event. If
the strengthened descending branch of the local
Hadley circulation retains the subtropical enhanced
evaporation, the subtropical negative SSTAs may
be maintained during the premature stage of the
ENSO warm event. During the subtropical wet season, however, the atmospheric feedback becomesinactive because of the large precipitation amountthat induces a new moisture balance. This could bea reason why negative SSTAs in the subtropical regions show seasonal variations associated with thelocal Hadley circulation.
6. Summary
We have examined the formation processes ofnegative (positive) SSTAs in the subtropical Northand South Paciﬁc during the premature stage of
the ENSO warm (cold) events from reanalysis andin-situ observational datasets. Our composite difference maps between the ENSO warm and cold
events indicate that signiﬁcant negative SSTAs
are observed over the subtropical North Paciﬁcnear the Date Line in February–March period andover the subtropical South Paciﬁc after April. One
month before the formation of these subtropicalnegative SSTAs, negative air humidity anomalies
appear at the same location of these subtropicalnegative SSTAs in both hemispheres. Our analysis
shows that the subtropical negative air humidity
anomalies accompany the anomalous downward
motion that corresponds to the strengthened descending branch of the local Hadley circulation.
These negative air humidity anomalies and anomalous downward motion show a seasonal transition
from the Northern hemisphere in the January–
February period to the Southern hemisphere in the
March–April period. This transition in anomalous
surface dry air and downward motion from theNorthern to Southern hemisphere has the sametiming to climatological seasonal cycle for the descending branch of the local Hadley circulation.
These results suggest that the negative air humidity anomalies associated with the strengthened descending branch of the local Hadley circulation
can induce negative SSTAs in the subtropics

199

through an enhanced latent heat ﬂux at the sea surface. Our linear decomposition analysis of surface
heat ﬂux anomalies indicate that the enhanced latent heat ﬂux contributed by anomalous air-sea humidity di¤erence, as much as by anomalous scaler
wind speed, is the major factor to form the subtropical negative SSTAs in both hemispheres.
Over the tropical ocean such as the climatologically warm SST region an anomalous vertical motion is mainly controlled by local SSTAs. In theequatorial Paciﬁc, for example, SSTAs and anomalous air-sea humidity di¤erence show the same polarity (Figs. 2c, 10c), implying that the equatorial
SST variability a¤ects the intensity in the equatorial upward motion because of atmospheric dynamic and thermodynamic responses. Over the subtropics, however, anomalous downward motion
plays an important role in the formation of subtropical SSTAs through surface air humidity
anomalies. According to the principle of Sverdrup
balance, an anomalous downward motion in the
middle troposphere accompanies the equatorwardsurface wind anomaly in the subtropics. These
anomalous downward motion and equatorwardsurface wind anomaly contribute to induce the dry
condition in the lower atmosphere through verticaland horizontal moisture ﬂux advections and thencause the negative SSTAs through the enhanced
evaporation at the sea surface. As expected fromthese processes in the lower atmosphere, SSTAs
and anomalous air-sea humidity di¤erence show
an opposite polarity in the subtropical North Paciﬁc in January (Figs. 2a, 8c) and in the subtropicalSouth Paciﬁc in March (Figs. 2c, 10c). On the basisof the moisture budget equation, the anomalous
moisture ﬂux divergence related to the anomalous
downward motion mostly balances the enhanced
Table 1. Each term of the moisture budget equation
averaged in the subtropical North Paciﬁc (NP; 20 N–
40 N, 160 E–170 W) and South Paciﬁc (SP; 20 S–
40 S, 160 E–170 W). The moisture ﬂux anomaly divergence is estimated by the vertical integration ofmoisture ﬂux anomaly divergence from surface to
700-hPa. Anomalies of vertically integrated moistureﬂux divergence, evaporation, and precipitation represent ‘  ðqVÞ 0 , E 0 , and P 0 in units of mm day1 , respectively.

NP
SP

Month

‘  ðqVÞ 0

E0

P0

Jan
Mar

1.28
1.22

0.91
1.09

0.35
0.13

Journal of the Meteorological Society of Japan

200

Vol. 88, No. 2

Hadley circulation

Moisturetransport

Dry air

Atmosphere

Enhanced
evaporation

Ocean
Warm SST

Cold SST

Eq.

30o

Poleward

Fig. 12. Schematic diagram of the proposed atmosphere-ocean interaction near the Date Line during thepremature stage of the ENSO warm event. The white open arrows indicate the strengthened vertical andmeridional winds associated with the local Hadley circulation.

evaporation at the sea surface because the precipitation anomaly is small under the climatologicaldescending branch of the local Hadley circulation(Table 1). This relationship between anomalous
downward motion and SSTA will be the uniquecharacteristic of subtropical air-sea interaction
when compared to the tropics. On the basis of theresults of our analysis, we discussed a possible feedback loop in the subtropical atmosphere, which issummarized in the schematic diagram in Fig. 12.
This atmospheric positive feedback could be a reason why the negative SSTAs in the subtropicalSouth Paciﬁc persist for the half year during thepremature stage of the ENSO events.
As we have seen in Section 4, the short wave radiation anomaly plays a minor role in the formation of subtropical SSTA in the NCEP2 and
COADS datasets (Figs. 7, 9). However, Alexander
et al. (2004) claimed that the short wave radiation

obtained from the reanalysis dataset has large errors. In addition, the number of surface observations in the South Paciﬁc is not very large in the
COADS dataset. Accurate surface observations ofsurface heat ﬂux components are highly desired toreveal the mechanism underlying subtropical airsea interaction.
Acknowledgments
The authors are grateful to S.-P. Xie, and M.
Watanabe for their stimulating discussions. The
manuscript beneﬁtted from the constructive comments by anonymous reviewers. The NCEP2 andNOAA OI SST datasets were obtained from thewebsite of the NOAA/OAR/ESRL PSD, Boulder,
Colorado, USA, (http://www.cdc.noaa.gov/). This
work was supported by Kakushin project of theJapanese ministry of education, culture, sports,science, and technology.

April 2010

Y. CHIKAMOTO et al.

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