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A Satellite Study of the Relationship between Sea Surface Temperature andColumn Water Vapor over Tropical and Subtropical Oceans
KAYA KANEMARU
Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan

HIROHIKO MASUNAGA
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan(Manuscript received 24 May 2012, in final form 14 December 2012)
ABSTRACTThe known characteristics of the relationship between sea surface temperature (SST) and column water
vapor (CWV) are reevaluated with recent satellite observations over tropical and subtropical oceans. Satellite
data acquired by the Aqua Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E),
Atmospheric Infrared Sounder (AIRS)/Advanced Microwave Sounder Unit (AMSU) suite, the TropicalRainfall Measuring Mission (TRMM) precipitation radar (PR), and the Quick Scatterometer (QuikSCAT)
SeaWinds are analyzed together for 7 years from October 2002 to September 2009. CWV is decomposed into
surface humidity, presumably coupled closely to SST, and the water vapor scale height as an index of vertical
moisture gradient between the boundary layer and the free troposphere. Surface relative humidity is climatologically homogeneous across tropical and subtropical oceans, while the dependence of CWV on SST
varies from one region to another. SST mainly accounts for the variation of CWV with the water vapor scaleheight, which is virtually invariant over subtropical oceans. On the other hand, over tropical oceans, thevariability of CWV is explained not only by SST but also by a systematic change of the water vapor scaleheight. The regional contrast between tropical and subtropical oceans is discussed in the context of the regional moisture budget including vertical moisture transport through convection.

1. Introduction
Atmospheric water vapor has an important role inmaintaining the hydrological cycle on the earth’s climate
system. Water vapor integrated from the surface to the
top of the atmosphere, often called column water vapor(CWV), is known to be related closely with sea surface
temperature (SST) over the global ocean (e.g., Prabhakara
et al. 1979; Raval and Ramanathan 1989; Stephens 1990;
Gaffen et al. 1992; Jackson and Stephens 1995; Wentz
and Schabel 2000; Trenberth et al. 2005). Prabhakara
et al. (1979) and Stephens (1990) pointed out that the relationship between SST and CWV resembles the Clausius–Clapeyron (C–C) relation. Two major assumptions,however, are required to physically ascribe the SST–CWV

Corresponding author address: Kaya Kanemaru, Graduate Schoolof Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8601, Japan.
E-mail: kanemaru@satellite.hyarc.nagoya-u.ac.jp
DOI: 10.1175/JCLI-D-12-00307.1
Ó 2013 American Meteorological Society

relation to the C–C relation. First, surface relative humidity should be regionally homogeneous so that surface
moisture depends exclusively on SST. Second, water vapor at all heights should vary coherently, that is, vertical
moisture gradient should be invariable.
The first assumption of the constant surface relative
humidity is indeed supported by past observations (Peixoto
and Oort 1996; Dai 2006). The second assumption ofinvariant vertical moisture gradient may be valid regionally (Reitan 1963; Bolsenga 1965), whereas it is lesslikely the case on a global scale because vertical moisture gradient changes with latitudes and seasons (Smith
1966; Liu and Niiler 1984). The characteristics of the
SST–CWV relation indeed differ regionally between
tropical and subtropical oceans. Stephens (1990) and
Gaffen et al. (1992) found that the SST–CWV relation is
not entirely consistent with the C–C relation over tropicaloceans. Jackson and Stephens (1995) showed that observed CWV is poorly predicted by the C–C relation with
SST over tropical oceans, although the C–C relation is a

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reasonable approximation over subtropical oceans. Thesestudies suggest that the assumption of invariant vertical
moisture gradient may not hold over tropical oceans.
The regional variability of vertical moisture gradient
may be considered to be related to convective activities.
Chaboureau et al. (1998) found that the mid- and uppertropospheric moisture systematically relates with highcloud occurrence by satellite data. Bretherton et al. (2004)
and Holloway and Neelin (2009) showed from observational data that the precipitation intensity relates withthe free tropospheric (FT) moisture rather than theboundary layer (BL) moisture.
To comprehensively understand the roles of SST andtropospheric moisture in convective development, thesystematic regional variation of vertical moisture gradient is investigated in this paper. Recent technologicalprogress in satellite remote sensing has made it possible
to observe surface relative humidity and vertical moisture gradient thoroughly across the globe. The purposesof this study are to 1) understand the regional differencein the SST–CWV relation focusing on the potential rolesof surface relative humidity and vertical moisture gradient, and 2) discuss the relevance between the regional
characteristics of the SST–CWV relation and dynamical
environments. Data analyzed are introduced in section
2. Analysis procedures are described in section 3. The
results are presented in section 4 and discussed in section
5. Current findings are concluded in section 6.

2. DataThis study uses satellite data from the Aqua Advanced
Microwave Scanning Radiometer for Earth Observing
System (AMSR-E), the Aqua Atmospheric InfraredSounder (AIRS)/Advanced Microwave Sounder Unit
(AMSU) suite, the Quick Scatterometer (QuikSCAT)
SeaWinds, and the Tropical Rainfall Measuring Mission(TRMM) precipitation radar (PR). The AMSR-E andQuickSCAT products (versions 7 and 4, respectively)
are provided by Remote Sensing Systems (RSS). RSS
provides daily gridded data with ascending and descending orbits separately stored at 0.258 3 0.258 spatial
resolution. The AMSR-E product contains SST and CWVover ocean. The root-mean-square (RMS) error of
AMSR-E SST and CWV is 0.588C and 0.57 kg m22, respectively (Wentz and Meissner 2000). The QuikSCAT
product contains wind vector and convergence at ocean
surface. The RMS difference of QuikSCAT wind speed
and direction is 1 m s21 and 238, respectively (Ebuchi et al.
2002). Since the accuracy of the wind vector is affected byrain, wind data in raining scenes are excluded using rainflags. This study uses the Level-2 AIRX product version 5
utilizing AIRS and AMSU instruments (hereafter AIRS

collectively) provided by the National Aeronautics andSpace Administration (NASA). The AIRS productyields temperature and humidity profiles at about 1-km
vertical resolution and 45-km spatial resolution at nadir,where the contamination of clouds is algorithmically
cleared (Susskind et al. 2003, 2011). The accuracy of
AIRS temperature and humidity estimates is 1 K km21
and 20% (2 km)21, respectively (Tobin et al. 2006), andretains acceptable quality up to 70% of cloud fraction(Fetzer et al. 2006). In this study, AIRS temperature andhumidity profiles are analyzed only where flagged as
best. The TRMM PR data are provided by the Japan
Aerospace Exploration Agency (JAXA) Earth Observation Research Center (EORC). The PR productcontains rain profiles at the 250-m vertical resolution
and about 5-km horizontal resolution after August 2001
when the TRMM orbit was boosted. Surface rain andvertical rain profiles, averaged to the 1-km vertical resolution, are adopted from the PR 2A25 product version
6 (Iguchi et al. 2000).
All the satellite datasets are projected onto a common
18 3 18 gridded daily data with ascending and descending orbits combined. These data are analyzed for 7 years
from October 2002 to September 2009 over all oceans
within the latitudinal band between 258N and 258S. Data
are sampled only where both AMSR-E and AIRS observations are available for the analysis in section 4, andwhere all AMSR-E, AIRS, QuikSCAT, and PR measurements are available for the discussion in section 5.
AMSR-E and QuikSCAT data are averaged from theoriginal 0.258 3 0.258 grid to a 18 3 18 grid. AIRS and PR
data are averaged from individual footprints in level-2
data to a 18 3 18 grid. AMSR-E, AIRS, and QuikSCAT
estimates are interpolated from the neighboring eightgrid boxes averaged to fill in pixels contaminated by rain
and clouds. The accuracy of interpolated data and thesampling errors caused by missing data are evaluated inappendix B.

3. Analysis procedure
The analysis procedure is introduced in this section.
The water vapor scale height Hy is introduced to describe vertical moisture gradient, where
Hy [

W
W
5
.
r y ,s r a qy ,s(1)Here, W, ry,s, ra, and qy,s are CWV, surface water vapor
density, the density of dry air (51.2 kg m23), and surface water vapor mixing ratio, respectively. Here, Hy isderived from CWV observed by AMSR-E, and qy,s isestimated by AIRS and AMSR-E. The method to estimate

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Ts for T0 to derive the SST–CWV relation, (1) and (2)give
RH0 3 Hy 5

Ry WTs.
es (Ts )(4)

Equation (4) shows that RH0 3 Hy is a function of SST
and CWV. The theoretical relation between SST and ry,sis thus written as
RH0 5

Ry ry,s Ts
es (Ts ).

(5)

Equations (4) and (5) are compared with observational
results in the next section. In these equations, T0 has
been substituted by Ts for practical convenience, whichslightly affects the RH0 variability to evaluate the relationship between SST and moisture.

4. Results
FIG. 1. Vertical profiles of water vapor mixing ratio normalized
by its surface value sorted by the water vapor scale height. The
normalization is applied to instantaneous measurements before
averaged into each scale height bin.

qy,s is described in appendix A. If water vapor density
exponentially decreases with height, Hy provides thee-folding scale. Figure 1 shows the scaled vertical structure of moisture sorted as a function of Hy, where water
vapor mixing ratio qy is normalized by its surface valueqy,s. Figure 1 illustrates that Hy well represents the contrast between the BL and FT water vapor mixing ratios.
In this work, Hy is used as a measure quantifying vertical
moisture gradient.
A simple analytic equation is derived next as guidance
to examine the observed SST–CWV relation. The equation of state for vapor is expressed asry,s 5 ra qy,s 5

e0
RH0 3 es (T0 )
5
,
Ry T0
Ry T0

(2)

where e0, Ry, T0, RH0, and es(T0) are surface vapor pressure,the gas constant for water vapor (5461.5 J K21 kg21),
surface air temperature, surface relative humidity, andsaturation vapor pressure for temperature T0, respectively; RH0 is obtained from qy,s and AIRS surface airtemperature. Saturated vapor pressure may be written
as (Bolton 1980)

17:67t,(3)es (T0 ) 5 6:11 exp
t 1 243:5
where t is Celsius temperature (t 5 T0 2 273.15). We
rewrite SST and CWV with Ts and W, and substitutingThis section reveals the regional characteristics of
the SST–CWV relation over tropical and subtropicaloceans. Figure 2a shows the joint probability density
function of SST and CWV (solid) with the histogram
ridge indicated by thick line over the latitudinal band
between 258N and 258S. Theoretical lines of RH0 3 Hy(dashed) predicted from (4) are also shown. The histogram pattern represents the daily average and overall
captures the climatological SST–CWV relation seen in
the monthly average (Gaffen et al. 1992; Zhang and Qiu
2008). The histogram ridge follows a theoretical RH0 3
Hy line of 1000 m for SSTs lower than 248C and begins
to depart from 1000 to 1500 m for SSTs from 248 to 278C.
The joint histogram of SST and ry,s (Fig. 2b) has a ridgein the close vicinity of a RH0 of 70%–80%. It is impliedthat RH0 does not change much over a broad range of
SST, as also found by past observations (Peixoto andOort 1996; Dai 2006). Figure 2 implies that Hy, undera virtually constant RH0, is also almost invariant for low
SSTs. When SST reaches 278C, the histogram ridge (Fig.
2a) rapidly rises nearly vertically from 1500 to 2000 m
while RH0 remains near 70%–80% (Fig. 2b). Once theridge reaches near a RH0 3 Hy of 2000 m, the histogram
ridge ceases to rise. This is confirmed by Fig. 3. Figure
3a offers evidence that the histogram peak extends overa narrow band where observed RH0 is 80% as seen in
Fig. 2b. Observed RH0 is somewhat higher than inferredfrom the theoretical line in Fig. 2b, which is because wehave replaced T0 with Ts in (5). Figure 3b shows that Hystays around 1500 m for relatively low SSTs, while it
drastically increases from 1500 to 2500 m for higher
SSTs. This result suggests that two distinct regimes

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FIG. 2. (a) The joint probability density function (solid lines) of SST and CWV and RH0 3 Hy predicted from (4)(m; dashed lines). Bin sizes are 0.758C and 1.5 kg m22. Contour interval is 0.1%. The thick line shows the maximum
occurrence for each SST. (b) As in (a), but for SST and ry,s and RH0 predicted from (5) (%). Bin sizes are 0.758C and0.75 kg m23. Contour interval is 0.2%.

exist with respect to how Hy changes with SST and
CWV. In the first regime, CWV is controlled by SST
through the C–C relation. On the other hand, CWV isattributed not only to a regional gradient of SST but also
to a systematic change in Hy in the second regime. These
regimes are separated by SST with a boundary near

278C. Standard deviations of RH0 and Hy in the SST–CWV plane are shown in Fig. 4. The climatologicalmeans of RH0 and Hy are well representative of their
daily values since standard deviations are both small inthe vicinity of the histogram and on its lower right side.
The standard deviation is, however, relatively large

FIG. 3. (a) Surface relative humidity (%) as a function of SST and CWV. (b) As in (a), but for the water vapor scaleheight (m). Contours and dashed lines are taken from Fig. 2a.

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FIG. 4. As in Fig. 3, but for standard deviation.

where SST is low and CWV is large, which is discussed
later.
The findings above are quantitatively assessed next.
CWV exhibits contrasting sensitivities to SST dependingon SSTs (Fig. 5a) as expected from the results above.
The CWV sensitivity to SST is decomposed into different contributions as follows. Differentiating (4) with respect to SST, one finds›lnW ›lnRH0 ›lnHy ›lnes ›lnTs
5
1
1
2
›Ts
›Ts
›Ts
›Ts
›Ts
5

›lnRH0 ›lnHy Ly 2 Ry Ts
1
1
.
›Ts
›Ts
Ry Ts2(6)

Here, Ly is the specific latent heat of vapor (52.5 3
106 J K21 kg21). The first and second terms on the

FIG. 5. (a) As in Fig. 2a, but for CWV binned by SST (thick line). Standard deviation of 61 around mean CWV isshaded. Bin size is 0.758C. (b) Observed estimates of each term in (6): ›lnW/›Ts (orange), ›lnRH0/›Ts (green), ›lnHy /›Ts
(blue), the C–C relation (red), the sum of decomposed terms (black), and the residual term (violet). Shaded histogram shows SST occurrence (%).

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FIG. 6. Occurrence (%) of the threshold Hy over 2200 m.

right-hand side in the bottom line of (6) are the sensitivity of RH0 and Hy to SST, respectively. The third term
represents the C–C relation. One can readily find that
RyTs is an order of magnitude smaller than Ly, so thatthe last term is virtually determined by the C–C relation.
Figure 5b shows the contribution of each term in (6).
The sum of the decomposed terms in (6) almost agrees
with the actual sensitivity of CWV to SST, which justifies
the utility of (6). The contribution of the third term isalmost constant at about 6% 8C21 as predicted by the
C–C relation. The C–C relation dominates the other two
terms when SSTs are below 278C; that is, the behavior of
CWV is dominated primarily by the C–C relation with
SST. On the other hand, the contribution of Hy isgreatest from 278 to 298C, implying that the increase of

Hy with SST is critical for the dependence of CWV on
SST. The contribution of RH0 to SST remains minimal
as expected. The residual sensitivity to SST, defined as
the difference between the CWV sensitivity and the sumof decomposed terms, arises from the difference between Ts and T0 that has been neglected in (6).
Figure 6 shows the frequency of occurrence when Hy
exceeds 2200 m on the global map. The threshold of
2200 m separates the two SST regimes as seen from Fig.
3b as discussed in the next section. Regions where Hy ishigher than this threshold include the Indian Ocean, thetropical western Pacific, the intertropical convergencezone (ITCZ), and the South Pacific convergence zone
(SPCZ). On the other hand, regions where Hy is generally below the threshold distribute over subtropical

FIG. 7. As in Fig. 2a, but for the joint probability density function (PDF) separated for 7 years as indicated at the top of each panel. Color
illustrates the anomaly against the 7-yr mean (Fig. 2).

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FIG. 8. As in Fig. 2a, but overlaid with (a) precipitation P (mm day21), (b) evaporation E (mm day21), (c) P 2 E(mm day21), and (d) surface convergence (1026 s21).

oceans surrounding tropical oceans. The two SST regimes
are thus hereafter called subtropics and deep tropic regimes: Hy hardly changes in the subtropic regime and Hy
increases with SST in the deep tropics regime.
The temporal variability of CWV is closely relatedwith SST on a monthly time scale (Wentz and Schabel
2000; Trenberth et al. 2005). Trenberth et al. (2005) andSudradjat et al. (2005) pointed out that the temporalvariability of CWV relates with the El Ni~
no–Southern

Oscillation (ENSO) variability. Figure 7 shows yearly
histograms for the consecutive 12 months from October
to September. Histograms are overall similar, but somesystematic deviation is visible relative to the mean
histogram for all 7 years combined. El Ni~
no events areobserved in 2002/03 and 2004/05 and a La Ni~
na event isobserved in 2007/08, and both El Ni~
no and La Ni~
naevents occurred during 2006/07 according to the oceanic Ni~no index. The joint histograms between El Ni~no15 JUNE 2013

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4211

FIG. 9. As in Fig. 8, but for standard deviation.

and La Ni~
na years are in contrast with each other.
The occurrence in El Ni~no (La Ni~na) years enhances(reduces) at the top end of the histogram ridge andincreases (decreases) along its lower tail. These ENSOrelated changes are as expected given that the tropics
as a whole has a warmer SST and a more humid atmosphere during El Ni~
no. This variability, however, is
not so large that it essentially alters the climatologicalhistogram.

5. Discussion
In this section, contrasting behaviors of Hy between
the subtropics and deep tropics regimes seen in theprevious section are discussed. Figure 8 illustrates theparameters critical for the moisture budget in the SST–CWV plane. Figure 8a shows that precipitation P depends
strongly on CWV rather than SST. The importance ofwater vapor for convective development was emphasized

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FIG. 10. The difference in P (mm day21) conditioned or unconditioned (unconditioned 2 conditioned) on AIRS and QuikSCAT
data losses in heavily raining areas.

by Inoue (1990) and Neelin et al. (2009) while the strengthof convection is also known to relate with SST (Gadgil
et al. 1984; Graham and Barnett 1987) presumably throughthe tight correlation between SST and CWV. The sharp
dependence of P on CWV is consistent with the known
exponential relationship between CWV and P (Zeng
1999; Bretherton et al. 2004; Peters and Neelin 2006).
The influence of SST, however, appears to be indirect
because P remains relatively weak even for the highest
SSTs as far as CWV is low. Evaporation E (Fig. 8b) isrelatively homogeneous across the entire domain whenplotted in logarithmic scale, in sharp contrast to P, whichchanges drastically in the same color scale (Fig. 8a). The
difference between the two, P 2 E, is thus virtually amirror image of P, decreasing from positive to negativeacross their boundary around a CWV of 50 kg m22 (Fig.
8c). An excess and deficit of P 2 E is nearly balancedwith moisture convergence and divergence (Peixoto andOort 1992), as seen in Fig. 8d although only the surface
divergence is plotted instead of moisture divergence.
Figure 8d shows that the surface convergence crosses
zero at about a CWV of 45 kg m22, similarly to P 2 E(Fig. 8c). Figure 9 shows the standard deviation for Fig.
8. The standard deviation of P (Fig. 9a) varies sharply
depending on both SST and CWV while the variation ofE (Fig. 9b) is rather homogeneous. Consequently, thestandard deviation of P 2 E (Fig. 9c) mainly reflects that

VOLUME 26

FIG. 11. As in Fig. 1, but for vertical precipitation structure (mm day21 ). The solid line shows surface precipitation(mm day21) binned by the water vapor scale height.

of P, which is roughly mirrored by the standard deviation of surface convergence (Fig. 9d). The standard
deviation stays small along the histogram ridge, so therelation between CWV and SST may be considered to
be overall stable around the climatology. The large variability of P is observed where P itself is high, and might
be partly linked with a significant reduction of sample size
because of rain contamination in AMSR-E, AIRS, and

FIG. 12. Surface convergence (solid) (1026 s21) and P 2 E(dashed) (mm day21) sorted by the water vapor scale height.
Shaded histogram shows Hy occurrence (%).

15 JUNE 2013

KANEMARU AND MASUNAGATABLE 1. Frequency-of-occurrence chart of Hy and P 2 E withgiven thresholds (Hy 5 2200 m and P 2 E 5 0 mm day21). The
classification is performed with the local climatology of Hy andP 2 E. Numbers show the count of 18 3 18 grid boxes over theocean in the analysis area.
P 2 E (mm day21)

Hy (m)

.2200
,2200
Total

.0

,0

Total

2218
226
2444

666
10 027
10 6932884
10 25313 137

QuikSCAT observations. Figure 10 shows the difference in P conditioned or unconditioned on AIRS and
QuikSCAT data losses in heavily raining areas. Figure 10
virtually illustrates the sampling bias owing to precipitation in water vapor and surface wind estimates. A
systematic bias is found where P is very high, but the significantly biased domain does not extend down to thehistogram ridge and its close vicinity. Rainfall biases doexist but would not alter the main conclusions of thispaper as far as global-scale climatology is concerned.
AMSR-E SST (used for the abscissa) may be also affected by rain, but this effect is minor since Fig. 10 remains almost unchanged when the National Oceanicand Atmospheric Administration (NOAA) Optimum
Interpolation SST (OISST; Reynolds et al. 2002) is used
for plotting the same diagram instead of AMSR-E SST(not shown).
Figure 11 shows the vertical precipitation structure
separately for different Hy. An increase of Hy accompanies a rapid deepening of convection and an increase

4213

of P. The systematic change in Hy related with P corresponds well to FT moisture variation in Fig. 1, which isconsistent with past observational studies (Bretherton
et al. 2004; Holloway and Neelin 2009). Figure 12 shows
the moisture budget parameters as a function of Hy.
Surface convergence and P 2 E intersect the zero when
Hy is 2100 and 2400 m, respectively. The threshold of
Hy in the moisture budget can be therefore considered
to be about 2200 m, which separates the subtropics regime and the deep tropics regime as already seen in Fig. 6.
To ensure this possible link between Hy and climate regimes, Table 1 shows a frequency occurrence chart of Hy
and the sign of P 2 E computed from the global climatology map. Areas with Hy . 2200 m generally havemore precipitation than evaporation and vice versa,
with some exceptions with relatively less frequenciesof occurrence. This result overall supports the regime
classification based on the Hy threshold of 2200 m. It isnear the boundary between the deep tropics and subtropics (Fig. 6) that the classification fails.
Based on the present results and discussions above,
the regional difference in the SST–CWV relation along
the histogram ridge between subtropical and tropicaloceans can be schematically illustrated in Fig. 13. In thesubtropics regime, surface divergence removes the BL
moisture supplied by surface evaporation while vertical
moisture transport from the BL to the FT is suppressedin the absence of vigorous convection. The inefficientvertical moisture transport holds moisture close to thesurface, resulting in a consistently low value of Hy. On
the other hand, in the deep tropics regime where deep
convection dominates, convection effectively ventilates

FIG. 13. The schematics of the moisture budget characteristics in the subtropics and deep
tropics regimes. Solid arrows show the moisture flux. Dashed lines with arrows show surface
divergence and convergence.

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FIG. 14. (a) Frequency of occurrence (%) for SSTs from 188 to 248C and CWVs from 40 to 55 kg m22. (b)Precipitation (mm day21) conditioned on the same SST and CWV ranges.

the BL against surface moisture convergence, which leads
to an efficient FT moistening. As a result, Hy increases asconvection intensifies unlike in the subtropics regime.
Both the deep tropics and subtropics regimes result in thebalance of surface moisture although individual processes
are quite different from each other.
Stephens (1990) found that the CWV variability iscontrolled by SST in a way resembling the C–C relation
over global oceans, while the relationship is somewhatloosened over tropical oceans (Gaffen et al. 1992; Jacksonand Stephens 1995). In light of the present study, theconclusion of Stephens (1990) applies to the subtropics
but does not to the deep tropics, where Hy changes
systematically with SST and CWV no longer behaves as
expected from the C–C relation. On the other hand,
Holloway and Neelin (2009) showed that the CWV variability observed in situ sounding in the western Pacific is
explained mainly by the FT moisture rather than coupledclosely to SST. This is consistent with our findings forthe deep tropics regime. It was also pointed out by otherwork (e.g., Bretherton et al. 2004) that precipitation
nonlinearly increases with the FT moisture. This steep
increase of precipitation signifies the characteristics
typical of the deep tropics, although not always specifically intended so in past work. As such, the present
conclusions provide a comprehensive picture coveringboth the deep tropics and subtropics that have been individually examined by difference studies.

In some domains in the SST–CWV diagram, P 2 E andsurface convergence have opposite signs, implying thatthe balance as shown in Fig. 13 does not hold. To identify
the origins of such an imbalance, the domain of SSTs
from 188 to 248C and CWVs from 40 to 55 kg m22 is
projected back onto a global map (Fig. 14). Figure 14ashows that the selected SSTs and CWVs are found overthe cold tongue in the south of the ITCZ where P isgenerally low. A substantial amount of precipitation that
falls in these SST and CWV ranges is attributed to extratropical storms (Fig. 14b) although observed occurrence is extremely infrequent. The apparent budget
imbalance in this case thus likely arises because statistically rare high-precipitation events in midlatitudes
contaminate the cold tongue climatology when sorted by
SST and CWV. In addition, cold tongue regions may be
exposed to occasional intrusions of ITCZ disturbances,which could make it difficult to characterize these regions in terms of the simple two-regime classifications asshown in Fig. 13.

6. Summary
In the current study, the regional difference in therelationship between SST and CWV is evaluated overtropical and subtropical oceans with a focus on the potential roles of vertical moisture gradient. Satellite dataacquired by the Aqua AMSR-E and AIRS/AMSU suite,15 JUNE 2013

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the QuikSCAT SeaWinds, and the TRMM PR are
analyzed together for 7 years from October 2002 to
September 2009 over the latitudinal band between
258N and 258S.
This study focuses on the water vapor scale height Hy
as an index of vertical moisture gradient between theboundary layer and the free troposphere. It is found that
the SST–CWV relation changes its nature depending on
the range of SST. The SST–CWV relation resembles the
C–C relation when SST is lower than 278C. For higher
SSTs, the SST–CWV relation does not follow the C–C
relation because the sensitivity of CWV to Hy is larger
than the SST contribution, although surface relative
humidity remains constant. When mapped globally, Hy
hardly changes over relatively cold ocean in the subtropics and increases with SST in the deep tropics where
SST is high. The subtropics and deep tropics regimes
clearly separate two climate regimes with distinct behaviors of Hy as noted above. The interannual variabilityof the SST–CWV relation related with the El Ni~no–Southern Oscillation variability is found that the histograms of SST and CWV is slightly changed from year toyear while the overall pattern is almost unchanged.
The different behaviors of Hy between the regimes are
discussed in terms of the regional moisture budget including vertical moisture transport through convection.
In the subtropics regime, surface divergence removes
the boundary layer moisture supplied by surface evaporation while moisture transport from the boundary
layer to the free troposphere is suppressed in the absence of vigorous convection because of the subsidence
branch of the Hadley and Walker circulation. The inefficient vertical moisture transport holds moisture close
to the surface, which results in a consistently low Hy. On
the other hand, in the deep tropics regime where deep
convection dominates, convection effectively ventilates
the boundary layer moisture against surface moisture
convergence, which leads to an efficient free tropospheremoistening. As a result, Hy increases as convection intensifies unlike in the subtropics regime. This two-regime
picture does break down in some regions including theeast Pacific cold tongue. The temporal variation of moisture between the boundary layer and the free troposphere
will be explored in future work.
Acknowledgments. The authors thank the anonymous reviewers for their valuable comments. The Aqua
AMSR-E and the QuikSCAT SeaWinds datasets areproduced by the Remote Sensing Systems (www.remss.
com). The AIRX Level-2 product was acquired as partof the activities of NASA’s Science Mission Directorate
and is archived and distributed by the Goddard Earth
Sciences (GES) Data and Information Services Center(DISC). The TRMM PR 2A25 dataset are provided by
the Japan Aerospace Exploration Agency (JAXA)
Earth Observation Research Center (EORC). The
Tropical Atmosphere/Ocean (TAO) Project Office of
the National Oceanic and Atmospheric Administration(NOAA)/Pacific Marine Environmental Laboratory(PMEL) provided ocean buoy data. NOAA OISST V2
data are provided by the NOAA/OAR/ESRL PSD,
Boulder, Colorado (from their web site at http://www.
esrl.noaa.gov/psd/). Oceanic Ni~no index data are available online (at http://www.cpc.ncep.noaa.gov/products/
analysis_monitoring/ensostuff/ensoyears.shtml).

APPENDIX A
Estimation of Surface Moisture and Evaporation
In this study, surface water vapor mixing ratio qy,s isestimated by a simple BL model including a surface
layer (SL) (Fig. A1) to obtain the water vapor scale heightand evaporation. The BL model is based on cloud-topped
BL within the SL characterized by the logarithmic law.
The BL model is originally formulated by Masunaga(2012) for estimating water vapor mixing ratio below themixed layer (ML) or qy,ML. When the SL is neutrally
stable, the vertical gradient of moisture is expressed asa logarithmic profile, that is,qy,s 5 qy,ML 1ln(z/zSL )[q*(T ) 2 qy,ML ], (A1)ln(zq /zSL ) y sq*(Tys ), z, zSL, and zq are saturation vapor mixing ratio
for sea surface temperature Ts, the height, the SL height,
and the roughness length for moisture, respectively. The
theoretical bulk transfer coefficients for momentum CD
and for evaporation CE are (Garratt 1992)
2k
CD (z) 5ln(z/z0 )CE (z) 5

and

(A2)

k
k
,
ln(z/z0 ) ln(z/zq )(A3)

where k and z0 are von Karm
an constant (50.40) andthe surface roughness length corresponding to theheight of zero wind speed, respectively. From (A2) and(A3), the relationship between CD and CE becomesCE (z) 5

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃkCD (z) .
ln(z/zq )(A4)Large et al. (1994) gives the empirical relationship at
10-m height between CD and CE as

4216

JOURNAL OF CLIMATE

FIG. A1. (left) The schematics of the BL and SL model to estimate surface water vapor mixing ratio qy,s. The BL model estimating mixed layer water vapor mixing ratio qy,ML is taken from by
Masunaga (2012). (right) The estimation of qy,ML in the SL model islogarithmically extrapolated down to qy,s and saturated water vapor mixing ratio q*(Tys ) for sea surface temperature Ts.

CE (10) 5

34:6 qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
CD (10) .
103

(A5)

where zq is obtained as 9.534 3 1025 m and fixed in thecurrent SL model. In this study, zSL is assumed to be
50 m. Figure A2 shows the result of comparison between satellite estimated qy,s and the in situ qy,s. The insitu qy,s is obtained by the Tropical Atmosphere Ocean(TAO) arrayed buoys over tropical oceans. Satellite
overpasses and buoy locations are matched up within 18
resolution. The observed height of qy,s in the TAO buoys
is 3-m height, so that qy,s is calculated in (A1) as 3 m.
Figure A2a is the joint probability density function between in situ qy,s and AIRS lowest qy, which corresponds
to layer mean between 1000 and 925 hPa. While AIRS
lowest qy is clearly underestimated, the estimation of qy,sfrom the BL model by (A1) reasonably agrees with thein situ measurements in the current method (Fig. A2b).
Finally, evaporation E is calculated from the bulk
method,E 5 ra CE ju10 j[qy*(Ts ) 2 qy,s ],

(A6)

(A7)

where ra and ju10j are the density of dry air (51.2 kg m23)
and the wind speed at 10 m, respectively. Here, CE isadopted from the empirical formulas derived by Largeet al. (1994),

1 2:701 0:142 1 0:0764ju10 j ,CD 5 3
10 ju10 j

Therefore, zq is calculated from (A4) and (A5),


103
k ,
zq 5 10 exp 2
34:6

VOLUME 26(A8)

and (A5). In this study, ju10j is obtained by QuikSCAT.

FIG. A2. (a) The joint probability density function between qy,s observed by TAO buoy and AIRS layer-mean qy
between 1000 and 925 hPa. Bin sizes are both 0.25 g kg21. Contour interval is 0.1%. (b) As in (a), but for qy,sestimated by AMSR-E and AIRS.

15 JUNE 2013

TABLE B1. Error analysis for each observed parameter. Bias,
RMSE, and correlation coefficient r are calculated against in situ
data. ‘‘Intp’’ indicates whether or not interpolation is applied.
Precipitation P is computed with TRMM PR each for sample.

All samples
Ts (8C)
T0 (8C)qy,s (g kg21)
ju10j (m s21)
u10 (m s21)y10 (m s21)
E (mm day21)

4217

KANEMARU AND MASUNAGA

Intp

No.

Bias

RMSE

r

P
(mm day21)

—
no
yes
no
yes
no
yes
no
yes
no
yes
no
yes
no
yes

53 71651 13653 60347 20652 59146 43152 56852 36653 70653 36653 70653 36653 70645 68852 560

—
20.03
20.03
21.43
21.14
20.21
20.20
0.16
0.18
0.00
20.02
0.20
0.20
0.20
0.24

—
0.51
0.51
1.68
1.73
1.16
1.16
1.22
1.24
1.59
1.62
1.59
1.59
1.26
1.32

—
0.97
0.97
0.77
0.76
0.82
0.82
0.86
0.86
0.91
0.90
0.91
0.90
0.75
0.73

3.65
2.93
3.59
2.47
3.32
2.34
3.32
3.20
3.64
3.20
3.64
3.20
3.64
2.34
3.32

APPENDIX B
Error Estimation of Interpolated Data
The accuracy of satellite estimations by AMSR-E,
AIRS, and QuikSCAT is affected by rain and/or clouds.
In this study, data gaps caused by rain and clouds are
spatially interpolated from surrounding areas to reduce
sampling bias that could lead to an underestimation of
precipitation. The interpolation is done by averaging
together the neighboring eight grid boxes. The impact of
the interpolation is evaluated in comparison with in situ
data obtained by the TAO arrayed buoys. Methodology
is as described in appendix A, but data are sampled only
when AMSR-E, AIRS, QuikSCAT, and PR observations and in situ data are not subject to this limitation.
QuikSCAT measures wind speed at 10-m height, while
the TAO anemometers observe wind speed at 4-m height.
The wind speed measured by the TAO buoys is converted to the equivalent neutral wind speed at the 10-m
height by the method of Liu and Tang (1996). Table B1
shows bias, root-mean-square error (RMSE), correlation coefficient r, and PR precipitation P computed foreach sample composed of satellite estimations and insitu observations. Parameters assessed are sea surface
temperature Ts, surface air temperature T0, surface water
vapor mixing ratio qy,s, scalar wind speed at 10-m heightju10j, zonal wind u10, meridional wind y 10, and evaporation E. Estimates with and without interpolation arecompared for each parameter. Estimated qy,s and E rely
on AIRS and 10%–15% of samples are lost without
interpolation, while the loss of data is a few percent for

AMSR-E Ts and QuikSCAT wind speed. Interpolated
data tend to be somewhat lower in accuracy than noninterpolated data but do not introduce a significant
sampling bias in precipitation. Precipitation can besubstantially underestimated because of the samplingbias when the interpolations are not applied. The accuracy of estimated qy,s is 1.2 g kg21, which is same or
better than the typical accuracy of satellite product in
the range from 1.2 to 2.0 g kg21 (Iwasaki and Kubota
2012). Bourras (2006) showed that the accuracy of E
from existing satellite products against tested in situ dataover tropical oceans is from 24 to 41 W m22, which corresponds to a range from 0.8 to 1.4 mm day21. RMSE ofthe current estimation of E is 1.3 mm day21.
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