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Observations of Marine Atmospheric Boundary Layer Transitions across theSummer Kuroshio Extension*
YOUICHI TANIMOTO, 1,# SHANG-PING XIE, @,& KOHEI KAI, 1 HIDEKI OKAJIMA, &,#HIROKI TOKINAGA,**,@ TOSHIYUKI MURAYAMA, 11 MASAMI NONAKA, #AND HISASHI NAKAMURA ##,#
1 Graduate School of Environmental Science, Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan# Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan@ International Pacific Research Center, School of Ocean and Earth Science and Technology,
University of Hawaii at Manoa, Honolulu, Hawaii& Department of Meteorology, School of Ocean and Earth Science and Technology,
University of Hawaii at Manoa, Honolulu, Hawaii** Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Scienceand Technology, Yokosuka, Japan
11 Faculty of Marine Technology, Tokyo University of Marine Science and Technology, Tokyo, Japan## Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan(Manuscript received 10 January 2008, in final form 9 September 2008)
ABSTRACTThe baiu and Kuroshio Extension (KE) fronts, both zonally oriented and nearly collocated east of Japan, arethe dominant summertime features of the atmosphere and ocean, respectively, over the midlatitude northwest
Pacific. An atmospheric sounding campaign was conducted on board the R/V Roger Revelle during the 2005
summer. Transects of soundings across the KE front are analyzed to study its effects on the atmosphere, alongwith continuous surface meteorological and ceilometer cloud-base observations. While the KE front remained
nearly stationary during the cruise, the baiu front displayed large meridional displacements that changed winddirection across the KE front. The presence of sharp sea surface temperature (SST) gradients anchored by the
KE enhanced the thermal and moisture advection, causing substantial changes in the marine atmosphericboundary layer (MABL) structure. When the baiu front was displaced north of the KE front, southwesterly
winds advected warm, humid air from the subtropics over the cold water, producing a surface inversion
favorable to fog formation. When the baiu front was to the south, on the other hand, northerly winds across theKE front destabilized the MABL, leading to the formation of a solid low-cloud deck beneath a strong capping
inversion. The wind changes with the meridional displacement of the baiu front thus caused large variations innear-surface atmospheric stability and surface turbulent heat fluxes, with potential feedback on deep convection and fog/low-cloud formation around the front.

1. Introduction
Over the northwest Pacific east of the Japanese mainisland of Honshu, sea surface temperature (SST) exhibitsrich structures associated with the Kuroshio Extension* International Pacific Research Center Publication Number
553 and School of Ocean and Earth Science and Technology
Publication Number 7566.

Corresponding author address: Youichi Tanimoto, Faculty ofEnvironmental Earth Science, Hokkaido University, N10 W5,
Kita-ku, Sapporo 060-0810, Japan.
E-mail: tanimoto@ees.hokudai.ac.jp
DOI: 10.1175/2008JCLI2420.1
Ó 2009 American Meteorological Society(KE) meanders and pinched-off mesoscale eddies between the northern edge of the subtropical gyre and theKuroshio–Oyashio interfrontal zone (see Fig. 1 in Yasuda
et al. 1996). The narrow KE jet displays larger decadal variability (Nonaka et al. 2006; Taguchi et al. 2007) in response
to basin-scale changes in surface wind curls (Schneider andMiller 2001; Qiu 2003). Strong ocean advection and the
deep winter mixed layer allow subsurface variability toaffect SST (Xie et al. 2000; Tomita et al. 2002) and surface
heat flux (Tanimoto et al. 2003). Strong SST gradients eastof Japan maintain baroclinicity in the marine atmospheric boundary layers (MABL), which has been suggested as being important for atmospheric storm tracks(Inatsu et al. 2003; Nakamura et al. 2004). While most

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studies of the Kuroshio–atmosphere interaction have so
far been based on models or model-assimilated analyses,
in situ vertical soundings of the atmosphere have beenlacking, hampering the progress in characterizing andunderstanding the KE’s influence on the atmosphere.
Tokinaga et al. (2006) conducted global positioning
system (GPS) sonde soundings over the KE region on
board research vessels during the 2003/04 winter. They
report strong modulations of the atmospheric mixed
layer by synoptic weather disturbances. The atmospheric
mixed layer was found to develop as high as 1500 m with
reduced vertical wind shear in response to the northerly
cold advection across the KE front, in association withthe passage of cold fronts. The southerly warm advection,by contrast, was found to suppress the mixed layer development with strong vertical wind shear. These results
support the idea that the mixed layer’s adjustment tochanges in near-surface atmospheric stability causes a
positive correlation between SST and wind speed aroundthe KE front (Nonaka and Xie 2003) and other extratropical SST fronts as observed by satellite microwave
measurements [see recent reviews by Xie (2004), Chelton
et al. (2004), and Small et al. (2008)].
During the 2005 summer, we carried out a similar
atmospheric sounding campaign on board the research
vessel (R/V) Roger Revelle over the KE region, complemented by continuous laser ceilometer and surface
meteorological measurements. Drastically different fromwinter, the summer climate over the KE region ischaracterized by a convective rainband called the baiu/
mei-yu front and by an extensive low-cloud deck to thenorth that occupies the entire midlatitude/subpolar North
Pacific (Klein and Hartmann 1993; Norris 1998b). This
low-cloud deck displays large temporal variability thatis important for maintaining low-frequency SST anomalies during summer (Zhang et al. 1998; Norris 2000;
Mochizuki and Awaji 2008).
Figure 1 depicts distributions of SST, surface winds,
and rainfall observed during our summer survey. In ourstudy area (black box in Fig. 1), the KE anchors a zonal
SST front just east of Tokyo (358–368N), with SST decreasing from 238 to 208C just over ;100 km across thefront. The baiu/mei-yu rainband extends from eastern
China through our study area into the North Pacific witha general northeastward tilt. Southwesterly surface winds
converge from the south onto the rainband where thesubtropical warm/moist and subpolar cool/dry air masses
meet. In our study area, a precipitation band and theassociated strong surface southwesterlies tend to be
confined to the warmer flank of the quasi-steady KE
front, suggestive of an SST influences.
The present study reports on the results from the cruise
survey during the 2005 summer over the KE region,

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where strong SST fronts are featured along the KE aswell as to the north and south. To our knowledge, this is
the first successful atmospheric sounding campaign under
the summer baiu front across the KE front. As pointed
out by Ninomiya (1984), the baiu/mei-yu front overthe East China Sea is characterized by a pronounced
meridional gradient of humidity rather than of temperature,
while the front east of Japan accompanies a strong meridional gradient not only of humidity but also of temperature. Our Kuroshio Extension System Study (KESS)
cruise thus offers a unique opportunity to study the nature of the interaction between the baiu and KE fronts.
As a first step, we focus on the effects of the KE front on
the MABL structure including low clouds. On monthly
mean, the atmospheric baiu front was roughly collocatedwith the KE front in our study region, but the former
displayed strong variability on the synoptic time scale.
We show that the MABL experienced major changes as
the atmospheric baiu front moved north and south of thequasi-stationary KE front in the ocean. While we will use
soundings taken for a short period, mechanisms discussed
in the present study may operate for the climatology anddecadal variability near the KE front.
In the rest of the paper, section 2 describes the field
campaign and the data. Section 3 describes the synoptic
ocean and atmospheric conditions during the campaign
and presents the observations results in vertical sections
across the KE front. Section 4 discusses the implications
for the baiu. Section 5 offers a summary and discussion.

2. Observations and data
As part of the KESS (Donohue et al. 2008), a joint
survey of the ocean–atmosphere system (KESS-05) was
conducted from 17 June through 16 July 2005 over the
KE and Kuroshio recirculation (the black rectangle inFig. 1) on board the R/V Roger Revelle of the Scripps
Institution of Oceanography. The present study focuseson two meridional transects of the atmospheric soundings across the KE front completed within slightly morethan 3 days to illustrate the MABL modulation by theoceanic front. Figures 2a and 2b show the sounding stations (green X marks) along the transects overlaid on the
concurrent SST field (colors). These two transects arechosen for detailed analysis because they are geographically close but represent contrasting atmospheric conditions characterized by the southwesterly and northerly
winds, respectively, at the surface. The first transect took
1.5 days to complete (Fig. 2a; hereafter line A) with 11
soundings, starting from the cold flank of the meandering
KE front at station A1 (36.568N, 146.888E) at 0510 UTC
11 July 2005 onto the warm flank at station A11 (34.378N,
145.998E) at 1846 UTC 12 July 2005 (hereafter period A).

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FIG. 1. AMSR-E SST (contours at 18C intervals), rainfall (color shading, mm day21), and
QuikSCAT surface winds (vectors, m s21) averaged during the KESS-05 cruise (17 Jun–16 Jul
2005). The inset rectangle indicates the study area of the cruise.

Consisting of six GPS soundings, the second transect(Fig. 2b; line/period B) west of line A started a half-day
later from the warm flank of the KE front at station B1(34.838N, 144.108E) at 0741 UTC 13 July 2005 onto thecold flank at station B6 (36.708N, 144.838E) at 0733 UTC
14 July 2005.
On board the ship, we used GPS sondes of the latest
type (Vaisala RS92-SGP) to measure air temperature,
relative humidity (RH), pressure, and wind velocity
every 2 s from the sea surface to about 20 km in thelower stratosphere. The sonde data were transmitted toan onboard receiver (Vaisala Digicora III of the Shigaraki Middle and Upper Atmosphere Observatory,
Kyoto University) and then linearly interpolated tovertical intervals of 10 m. We use the data from thesurface to 4.0-km height to study the MABL.
Cloud base was continuously monitored at 1-min intervals with a ceilometer (Vaisala CT25K of Tokyo University of Marine Science and Technology) mounted on
top of the bridge. In each measurement, the ceilometer
emitted a train of pulsed laser beams vertically andrecorded the averaged backscattered signal for 12 s. Then,the ceilometer automatically reported up to three cloudbase heights detected by a build-in algorithm as well as
the backscatter profile (Kahn et al. 2004; Vaisala 1999).
Surface marine meteorological observations of SST,
surface air temperature (SAT), surface wind velocity,
RH, rainfall, and solar radiation are available at 1-min
intervals. We use 60-min running averages except for

RH. Since RH records contain some spikelike noises,
we use the original records at 1-min intervals. As ameasure of the static stability near the surface, the parameter S 5 SST 2 SAT is calculated. Positive (negative)
values of S indicate the unstable (stable) near-surface
atmosphere. Steep SST changes of 4.28 and 3.18C are recorded across lines A and B, respectively.
To capture the synoptic conditions of the ocean andatmosphere during the KESS-05 cruise, we employ the
Advanced Microwave Scanning Radiometer for the Earth
Observing System (AMSR-E) SST and rainfall products
from remote sensing systems, available in 3-day runningmeans on a 0.258 grid, as well as the Quick Scatterometer(QuikSCAT) surface wind vector product, available atweekly intervals on a 0.258 grid (Liu et al. 2000).
We also use the mesoscale operational weather analysis (M-ANAL) for the Far East (208–508N, 1208–1508E)
provided by the Japan Meteorological Agency (JMA),
including wind velocity, temperature, and RH in thelower troposphere (975, 950, 925, 900, 800, and 700 hPa),available at 6-h intervals on a 0.208 latitude 3 0.258 longitude grid. The sounding data from the KESS-05 cruise
were not assimilated into the M-ANAL system.

3. MABL modifications across the KE SST front
a. Synoptic conditions of the ocean and atmosphere
During the 3-day period for lines A and B, SST(colors in Fig. 2a) displays a steep front along 368N with

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FIG. 2. (a) AMSR-E SST in 3-day running means centered on 11 Jul 2005 (color shading, 8C), equivalent potential
temperatures at 850 hPa (contours at 1-K intervals), and surface winds (vectors, m s21) from M-ANAL averaged forthe first day of period A (from 0600 UTC 11 Jul to 0000 UTC 12 Jul 2005). (b) As in (a) but for period B (13 Jul 2005
for SST and from 0000 to 1800 UTC 13 Jul for M-ANAL). Cross symbols in (a) and (b) indicate the sounding stations
along lines A and B, respectively. (c) Time–height section of equivalent potential temperature (color contours, K)
and wind velocities (vectors, m s21) at 358N, 1458E based on the M-ANAL.

a cold meander and a cold ocean eddy centered at 368N,
1468E. The eddy was about 18 3 18 latitude–longitude indiameter. Line A was nearly meridional (Fig. 2a) andcut across the cold eddy located just to the north of the
KE front, while line B was located west of the eddy but
cut across the SST front of KE (Fig. 2b). As expectedfrom the large heat capacity of the ocean, SST was
nearly the same during periods A and B.

During the 3-day period, the baiu front stretched over
several thousand kilometers from the lower Yangtze
River in eastern China through Japan into the western
North Pacific. A close look into the JMA surface weather
maps indicates that the baiu front was located northof the KE front during period A (Fig. 3a) and thendisplaced to its south during period B (Fig. 3b). As
displayed in the satellite infrared (IR) images, line A

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FIG. 3. JMA surface weather maps at (a) 0000 UTC 11 Jul and (b) 0000 UTC 13 Jul 2005, showing the analyzed baiu
front. The ship tracks, lines A and B, are superimposed with heavy black lines in (a) and (b), respectively. (c),(d) The
corresponding infrared images from the Geostationary Operational Environmental Satellite-9 (GOES-9). The ship
tracks are superimposed with heavy green lines. The satellite images are provided by Kochi University (information
online at http://weather.is.kochi-u.ac.jp/) and JMA.

(Fig. 3c) was overlaid by high-level clouds (i.e., lower
cloud-top temperatures) extending southwestward fromthe surface low at 448N, 1528E along the baiu front,
while line B (Fig. 3d) was under low-level clouds (higher
cloud-top temperatures) prevailing to the north of thebaiu front.
In association with the southward shift of the baiu
front from periods A to B, the lower atmosphere on the
KE SST front at 358N, 1458E experienced rapid changes(Fig. 2c). Specifically, the M-ANAL shows a large decrease in equivalent potential temperature (ue ) by 8 K,
a rapid deceleration of the southwesterly wind by 15
m s21, and a sign reversal of the meridional wind component from 1200 to 1800 UTC 12 July 2005, indicatingthat the baiu front passed this location sometime in thisperiod. Therefore, the sounding location of A9 (34.958N
at 0424 UTC 12 July) was still a little south of the baiu

front, while the last two soundings of A10 (34.708N at
1421 UTC 12 July) and A11 (34.378N at 1846 UTC 12
July) along line A were affected by disturbances in thebaiu front as described later. For period B, the baiu
front was always located south of line B. Though thebaiu front was nearby, surface rainfall was not reported
on board the ship in either transect. Consistently, noneof the seven AMSR-E snapshots for periods A and B
indicates any precipitation in the sounding area (notshown).
During period A prior to these rapid changes, surface
winds were southwesterly and convergent across a meridional maximum of ue at 850 hPa (ue850 ) that was associated with moist air on the southern flank of the baiu
front (Fig. 2a). This is rather similar to the monthly
mean conditions in Fig. 1a. As a result of warm/moist
advections by the southwesterlies, ue850 was high, ranging

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from 337 to 342 K over line A (contours in Fig. 2a). On
the first day of period B (0000–1800 UTC 13 July 2005),
surface winds turned northerly over the KE SST front,with high ue850 (.340 K) displaced to the south (Fig. 2b).
On line B, ue850 decreased by 10 K to 327–333K.
Meridional displacements of the baiu front relative tothe stationary KE front, via the associated changes insurface advection of temperature and humidity, brought
about large changes in the MABL, as described in therest of this section.

b. Line A under surface southwesterlies
Figure 4 displays observations along line A, which cut
through the cold ocean eddy (35.58–36.18N), then thesteep SST front of KE (35.28–35.38N), and finally arrived in a warm pool south of 35.28N. The SST front of
KE was quite strong, with SST changing by 48C in lessthan 20 km (;35.258N). The corresponding meridional
gradient of the SAT was much more relaxed, varying
only by 48C over ;90 km (from 36.08 to 35.28N) as opposed to an SST change of 78C on line A. The SAT
profile is consistent with the warm advection by thesoutherlies, with the near-surface atmosphere nearly inequilibrium with underlying SST on the upwind side of
the SST front and in rather slow adjustment to the rapid
decrease in SST on its downwind side. As a result, thenear-surface stratification was nearly neutral (20.58 .
S . 21.78C) on the warmer flank of the KE front, while
it was strongly stable (S , 248C) on the colder flank of
the front and over the cold eddy (blue curve in Fig. 4c).
(Small ripples on the surface variable records are due tostops made for ocean hydrographic observations, eachlasting for a few hours.)
Figure 5 shows a sounding in the near-neutral regime(station A9 at ;34.958N) on the warmer flank of the KE
front (gray curves). It features a surface mixed layer anda major inversion at 1100 m. In between there were twominor inversions at 300 and 550 m. Figure 4a shows thata surface mixed layer with nearly uniform virtual potential temperature developed over the near-neutral
regime on the warmer flank of the KE front (A8–A9)
and stratification was still weak above the mixed layer
top (at ;300 m) up to the bottom of the main inversion(at 1100 m or above). The greater turbulent mixing in
the MABL over the near-neutral regime is indicated bya decrease in surface RH (Fig. 4c) and a slight increasein surface wind speed (Fig. 4d) in the section between
A8 and A9 relative to the other sections north of A7 andsouth of A10. In the area near 34.88–35.68N, the ceilometer measurements identified a double cloud base ataround 400 and 1100 m, corresponding to the top of themixed layer and the main inversion base, respectively

1365(Fig. 4a). In fact, the histogram of cloud-base heights fora 6-h period (a total of 360 ceilometer samples) just southof the KE front (35.208–34.928N; gray bars in Fig. 6)
clearly shows double peaks in layers between 300 and
500 m and between 1000 and 1200 m. The histogram in
Fig. 6 also indicates that there were some occasions
when the ceilometer detected no cloud base. During thisdaytime period (0900–1500 local time 12 July 2005), theonboard pyranometer recorded 591 W m22 of solar
radiation on average with a standard deviation of 190
W m22, and the radiation fluctuated between over 1000
W m22 and below 300 W m22. It was partly sunny at A8
according to our field notes. These results suggest theformation of scattered stratocumulus and/or shallow
cumulus in the MABL on the warmer flank of the KE
front in the presence of warm/moist southerly winds.
These cloud types are climatologically prevalent in thisregion during summer (Norris 1998b).
As mentioned in section 3a, the southward-moving
baiu front at the surface was probably close to the ship
at sounding locations A10 and A11. The surface southerlies observed on the ship mean that the baiu front at thesurface was still to the north. The movement of the baiu
front may cause variations in vertical structure among
the soundings south of the KE front. The increase insurface wind speed, decrease in surface relative humidity,
and increase in cloud-base height for A7–A9 all seemconsistent with greater turbulent mixing in the MABL. A
strong inversion develops at 1100 m, above which the freetroposphere is dry. The soundings A10 and A11, bycontrast, observe higher relative humidity up to higher
levels at 1800 m as well as at the bottom of the MABL
probably due to meso-a features associated with the baiu
front nearby.
To the north of station A8, a thin surface inversion
formed as the southwesterly winds carried warm andmoist air across the KE front over the cold ocean surface (Fig. 4a). In this stable regime, both surface sensible and latent heat fluxes were downward because the
SAT and surface specific humidity were higher than the
SST and surface saturated specific humidity, respectively (Figs. 4b and 4e). Figure 5 displays a typicalsounding at station A4 (black curves). The surface inversion was about 150 m thick. The atmosphere wassaturated from the surface to 800 m, indicating theformation of a thick fog layer. Dewpoint temperaturesin the lower MABL were nearly uniform meridionally
between A4 (black dashed line in Fig. 5) and farther
upwind (e.g., at A9; gray dashed line in Fig. 5), while
the air temperature at A4 was typically cooler by
28–38C. The formation of strong surface stratificationaccompanying overcast fog under southerlies has beenreported in the composited soundings at the ocean

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FIG. 4. (a) Latitude–height section of virtual potential temperature (black contours at 1-K
intervals) and RH (color shades, %) observed by GPS sondes along line A. White dots denotethe cloud base determined from the ceilometer. Shipboard marine meteorological observations
along line A for (b) SST (blue curve, 8C) and SAT (red, 8C), (c) SST 2 SAT (S; blue curve, 8C),
100% 2 surface RH (black dots, %), (d) surface zonal (black, m s21) and meridional (gray,m s21) wind velocities, and (e) saturated specific humidity at the sea surface (blue, g kg21) andsurface specific humidity (red, g kg21). Except for RH, the 60-min running mean is shown. For
RH, the original 1-min interval is employed.

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FIG. 6. Histogram of ceilometer-detected cloud-base heights online A . Black (gray) bars are for the strongly stable (near neutral)
surface regime. A total of 360 one-minute samples are used foreach regime.

FIG. 5. Vertical profiles of air temperature (solid curves, 8C) anddewpoint (dashed, 8C) at stations A4 (black) and A9 (gray).

weather station C in the subpolar North Atlantic(Norris 1998a).
Figure 5 thus suggests that fog formation was facilitated by the cooling of warm and moist air through heatexchanges with the cool ocean, as it crossed the oceanic
front. In fact, both on the cold flank of the KE front andover the cold ocean eddy, fog formation was recognizedby visual observations and confirmed by ceilometermeasurements of cloud-base height (Fig. 4a). A histogram of the cloud-base height thus measured for 6-h
periods over the strongly stable regime from 36.358 to
35.888N (black bars in Fig. 6) shows the predominanceof a near-surface mode with the average cloud base at
35 m. Norris (1998b) has pointed out that the formationof a surface inversion and associated fog are due to warm,
moist airflow across an SST front toward the cooler side,
as confirmed by our transect observations. Whereas theclimatological frequency of fog occurrence is rather low(;5%) south of 408N in the western North Pacific(Norris 1998b), cloud-type composites by Norris andIacobellis (2005) show that the warm, moist southerlies
across the SST front over the North Pacific favor fog
formation.
The onsets of the surface inversion layer and surface
humidity saturation (RH 5 100) were almost coincidental, occurring at A8 (35.28N) and at 35.258N between
A8 and A7, respectively (Figs. 4a and 4c), whereas thegrounding of the ceilometer-measured cloud base occurred farther downwind to the north of A6 (35.78N).
This delay must be due either to the presence of a fog
layer that was too thin to be detected by the ceilometeror to strong surface wind speed south of A6. Wind speed
dropped rapidly from more than 10 m s21 at A8 to about
5 m s21 at A5 (Fig. 4d), as a resut of the suppressedvertical mixing of wind momentum within the MABL
due to the rapid decrease in SST and the resultant stabilization of the near-surface atmosphere (Nonaka andXie 2003; Tokinaga et al. 2006). The suppressed turbulent mixing within the MABL favors fog formation.
Similar formations (breakups) of sea fog were observedon the colder (warmer) flank of the KE front on 4–5 July
2005 during the same cruise, though the transition was
not as clear as in Fig. 4.

c. Line B under surface northerlies
Along line B, SST (blue curve in Fig. 7b) displayed asteep front at 35.48–36.08N between nearly uniform
warm and cold pools. As evident in Fig. 7d, northeasterly winds of 3–5 m s21 prevailed during the first half of1368

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FIG. 7. As in Fig. 4 but for line B. Black dots in (c) represent 90% 2 RH (%).

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FIG. 8. As in Fig. 5 but for stations B3 (gray) and B4 (black).

the 24-h transect, while southeasterlies of 3–5 m s21
dominated during its second half. The northeasterlies
brought cold air from the colder flank of the KE front,
and the southeasterlies for the second half of period B
still advected cooler air from the region of the cold
ocean eddy located southeast of the transect (Fig. 2b).
The northeasterly cool advection was anomalous compared to the climatological southwesterlies during thebaiu season in this region (Fig. 1). Correspondingly, theSAT profile was characteristic of cold advection (Fig.
7b), nearly in equilibrium with SST on the colder flankand adjusting slowly to the rapid SST increase on thefront and its warmer flank. The SAT gradient was thus
more relaxed than the SST gradient across the transitof line B (red curve in Fig. 7b). As a result, the nearsurface stratification was close to being neutral orslightly unstable (20.18 , S , 0.58C) on the colder flankof the SST front, while it was strongly unstable (1.58 ,1369

S , 2.58C) on its warmer flank. Correspondingly, surface
sensible heat flux was upward to the south of the SST
front, while it was diminished to its north. Nevertheless,Fig. 7e suggests that surface latent heat flux was upwardon either side of the SST front under the unsaturated
cool air, though enhanced over the warmer water. These
upward heat fluxes along line B under the cool advection were in sharp contrast to the downward fluxes alongline A with strong warm advection.
Line B was located north of the baiu front, where theMABL was topped by low clouds with their base at 400–
800 m under a strong inversion at the 1000-m level (Fig.
7a). Above the inversion, RH was typically around 50%,
while it exceeded 90% in the MABL above the mixed
layer top at 600–700 m. In this moist layer, the ceilometer detected a cloud bases that was elevated south of
the SST front relative to its north (800 versus 600 m).
Immediately above the inversion, the virtual potential
temperature was almost uniform meridionally (Fig. 7a).
Thus, the variations in the MABL temperature (from298 to 295 K) observed along line B must reflect theeffects of the SST front. Moving from the north acrossthe SST front, we observed the inversion base rose by
100 m and the RH right underneath increased by 10%,likely due to the enhanced surface heat fluxes and vertical mixing on the warmer flank of the front. This rise in
the inversion base due to the cool advection on the warm
flank of the SST front is consistent with a mixed layer
model proposed by Schubert et al. (1979). In their experiment of cool advection toward warm SST, a gradual
SST increase accompanies a deepening of the MABL
associated with enhanced heat supply from the ocean.
Figure 8 compares vertical profiles of air temperatureand dewpoint between stations B3 and B4. At station
B3 (south of the SST front in the unstable surface regime), a layer between 700 and 900 m was saturated
with moisture (gray curves in Fig. 8), indicating cloudformation below the main inversion. At station B4(north of the SST front in the near-neutral surface regime; black curves in Fig. 8), the lapse rate was about
68C km21 in a layer between 600 and 750 m below themain inversion. It was close to the moist-adiabatic lapse
rate, suggestive of cloud formation in that nearly saturated layer (Fig. 7) as detected by the ceilometer (Fig.
7a). Owing to the reduced vertical mixing over low SST,
the MABL at station B4 was more strongly stratified
than at B3. On the poleward flank of the SST front,
higher RH in the cooler surface mixed layer probably
helped lower the cloud base as observed.
The lower cloud base north of the KE front relative toits south is evident also in the time–height section of theceilometer backscatter intensity measured at 1-min intervals (Fig. 9a). As the ship was traveling northward

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FIG. 9. (a) Time–height section of ceilometer backscatter intensity (colors, 1022 km21 sr21)
and primary cloud base (black dots). The second cloud base is plotted with gray dots whenreported. (b) Time series of shipboard SST (blue, 8C) and SAT (red, 8C) from 1300 UTC 13 Jul
to 0300 UTC 14 Jul 2005, when the ship was sailing from the warmer to the colder flank of the
KE SST front. (c) As in Fig. 6 but for line B.

across the intense SST front during the 60-min period of
1930–2030 UTC 13 July 2005 (blue curve in Fig. 9b), theheight of the upper boundary of strong backscatter (redshading . 1.0 3 1022 km21 sr21) showed a 200-m decrease from 650–800 to 450–600 m. Especially during
1600–1930 UTC in the unstable surface regime on thewarmer flank of the SST front, the layer of high backscatter was very steady in altitude, indicating full overcast with a flat cloud base. In the near-neutral surface
regime on the colder flank of the front, by contrast, thehigh backscatter layer displayed much larger verticaldisplacements. This feature is indicative of the presenceof thin clouds without a solid cloud base on the colder
flank of the SST front, where a cloud base was sometimes identified below 400 m. When this happened, asecond cloud base was detected at 600 m, close to thelevel at which single cloud bases are identified. Sporadic, weak backscatters of 0.05–0.1 3 1022 km21 sr21
(blue shading in Fig. 9a) spread in a wider verticalrange, sometimes reaching all the way to the surface,indicating drizzles. Indeed, no surface rainfall was observed on board the ship.

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Figure 9c compares histograms of the ceilometercloud-base height between the unstable and near-neutral
surface regimes. As in Fig. 6, a total of 360 (6 h) samples
are used for each of the regimes. In a manner consistent
with Fig. 9a, the cloud-base height tends to be lower in
the near-neutral regime than in the unstable regime, at
583 and 749 m on average, respectively, and its standard
deviation is larger in the near-neutral regime (114 m)
than in the unstable regime (66 m). Occasionally in thenear-neutral surface regime, no cloud base was identified (Fig. 9c). These ceilometer statistics confirm theprevalence of broken, scattered clouds in the nearneutral surface regime on the colder flank of the SST
front and that of a solid low-cloud deck in the unstablesurface regime on its warmer flank.

1371

The sharp contrasts in temperature, moisture, andcloud distributions between the two soundings are mainly
caused by the synoptic meridional displacement of thebaiu front. As the subtropical or subpolar air mass isbeing advected across the SST front, strong adjustments
take place in the MABL as reflected in the surface turbulent heat flux and vertical structures of temperature,moisture, and cloud. Meridional thermal advection isimportant in the MBAL where large temperature differences (48–68C) are observed between periods A and B.
It is interesting to note that because of strong warm advection, the MABL is much warmer at A6 than B3, despite a much lower SST at the former station (178 versus
23.58C). The importance of the meridional advection is
also manifested as a more pronounced difference in
MABL dewpoint temperature between A6 and B3.

4. MABL structure in and outside the baiu front
Along the subtropical flank of the baiu front, there isa rainband that often accompanies a deep moist layer, asdetected by several soundings on line A. Thus, the baiu
front separates a moist, subtropical air mass from adrier, subpolar air mass to the north. Figure 10a compares a moist, subtropical profile based on a sounding at
A6 and a drier, subpolar profile based on a sounding at
B3. These two soundings conducted roughly at the same
latitude (35.68N) are chosen to avoid latitudinal biases.
The underlying SST was 178C at A6 (at the center of thecold eddy) and 23.58C at B3.
The sounding A6 indicates that two layers, 0–1700
and 4500–6100 m, were almost saturated, in which temperature roughly followed the moist-adiabatic profile
punctuated by six inversions at 4500-, 2600-, 1700-, 1200-,
600-, and 100-m levels. All these inversions except the
one near the surface were weak and likely to be causedby the differential advection of temperature and moisture. In contrast, the near-surface inversion formed as
the warm tropical air blew across the SST front onto thecold eddy, as discussed earlier. High moisture contents
in sounding A6 were maintained by the southwesterly
advection of moist, subtropical air (Fig. 10b) that became saturated probably due to a weak synoptic ascentassociated with the baiu front. In fact, satellite IR imagery indicates relatively high clouds at around A6 (notshown). In contrast, the subpolar sounding at B3 indicates a much drier condition below 3500 m, where moisture was trapped mostly in the MABL with a thin cloudlayer between 600 and 900 m capped by a strong inversion. A significant increase in dewpoint at 3500 m on asounding at B3 indicates that the subtropical air mass still
remained in the midtroposphere. Indeed, the meridional
winds were northerly in a layer of 4800–6500 m but still
southerly aloft.

5. Summary and discussion
We have analyzed atmospheric in situ observations
on the KESS-05 summer cruise to study the effects of
the KE front on the MABL. The data include GPS
sonde soundings, and continuous observations of surface meteorological variables and cloud-base height witha laser ceilometer. These in situ data are complemented
with a suite of satellite observations and an operational
mesoscale atmospheric analysis. The baiu and KE fronts
are the dominant summertime features of the atmosphere and ocean in the region, respectively. The KE
front remained quite stationary during our study period, while the baiu front moved substantially northand south on the synoptic weather time scale, accompanying sign reversals of the meridional component of
surface winds. The presence of the strong SST front
anchored by the KE enhances the effects of thermal
and moisture advection by meridional winds, causing
substantial changes in the MABL structure. The effects of the KE front on the MABL are illustrated withtwo cross-frontal transects.
Line A sampled the baiu front, with a few soundings
capturing a deep moist layer with temperatures nearlyat
the moist-adiabatic lapse rate. Robust southwesterly
winds prevailed at the surface on line A. As warm, humid subtropical air moved across the KE front onto cold
water to the north, a surface inversion and a thick fog
layer formed. The MABL stratification was highly stable, with downward surface sensible heat flux on thecolder flank of the SST front. On the warmer flank, bycontrast, the MABL stratification was nearly neutral,and stratocumulus (or shallow cumulus) clouds formed
with elevated cloud-base heights of either 300–600 m(the top of the surface mixed layer) or ;1000 m (bottomof the main inversion).

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FIG. 10. Vertical profiles of (a) air temperature (solid curves, 8C) and dewpoint (dashed, 8C) and(b) meridional wind (solid curves, m s21) at stations A6 (black) and B3 (gray).

Measurements along line B were carried out after thebaiu front had moved away to the south, sampling the
subpolar air mass with a strong inversion capping amoist MABL. Moderate northeasterlies prevailingnorth of the baiu front advected the cold subpolar air
across the KE front to the south. In this circumstance,
the MABL stratification was highly unstable, activating
surface turbulence to enhance the heat and moisturesupply from the warm ocean. The associated intensification of vertical mixing led to the deepening of the
MABL and moistening of its upper portion where asolid cloud deck formed below the inversion. A sharptransition was observed across the KE front. Over thecooler ocean north of the front, the cloud deck was
replaced with broken clouds scattering with lower cloudbase. Under the almost neutral or stable stratification in
the MABL, suppressed vertical mixing of moisture wasunfavorable for maintaining a solid cloud deck near the
MABL top. Similar modulations of low clouds can befound over a meandering SST front over the easternequatorial Pacific (Deser et al. 1993; Hashizume et al.
2001).

Changes in thermal advection associated with themeridional migration of the baiu front can exert different effects on the near-surface and free atmospheres.
Under the southerly warm advection, the near-surface
stratification is strongly stable, but the high moisture
content advected from the south forms a deep moist
layer in the free atmosphere. The northerly advection ofcool, dry air, on the other hand, destabilizes the MABL
stratification while suppressing deep convection. The
presence of the quasi-stationary SST front along the KE
can substantially strengthen the aforementioned advective effects. On line A, for example, enhanced advection of warm, moist air from the south, once acrossthe SST front, increased the surface stability over thecool ocean surface. A surface inversion and sea fog
formed, shutting off surface evaporation that would
otherwise moisten the MABL and increase convective
instability.
The surface latent heat flux around the KE front
observed in our survey varied from zero under the moist
southerlies (line A) to 120 W m22 under the driernortherlies (line B). This and other possible effects of

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TANIMOTO ET AL.

the KE front on the baiu front and associated cloudformation and precipitation need further investigation.
Unlike the observed cross sections, the vertical gradientof the M-ANAL virtual potential temperature is foundto be much more uniform within the lower troposphere,
failing to represent the inversions near the surface andat the top of the mixed layer along lines A and B, respectively. The further improvement of numerical models,continuous monitoring of atmospheric and oceanic conditions, and the subsequent improvement in the qualityof (mesoscale) analysis data are highly desired. Thereby,
we could deepen our understanding of the mechanismsof decadal SST anomalies in the oceanic frontal zone
along the KE and Oyashio Extension observed in conjunction with variability in low-level cloud cover (Norris
2000) and in heat release from the Kuroshio with possible impacts on the baiu-frontal precipitation (Tomita
et al. 2007).
Acknowledgments. We thank Prof. Randy Watts, thechief scientist of the KESS-05 cruise; the captain andcrew of the R/V Roger Revelle for support for atmospheric soundings; and G. Takagaki, M. Roxy, and A.
Goto for their participation in the cruise survey. Drs. M.
Yamamoto and J. Furumoto of the Shigaraki Middle
and Upper Atmosphere Observatory, Kyoto University, kindly lent us the sounding system and provided
practical advices on the usage. Advice from Dr. K.
Ishikawa, Ocean Research Institute, University of Tokyo,
and Prof. Y. Fujiyoshi, Institute of Low Temperature
Science, Hokkaido University, was helpful in executing
our observations. This work was supported in part bya grant-in-aid for scientific research defrayed by the
Ministry of Education, Culture, Sports, Science andTechnology of Japan (17340137, 15340153, and 18204044),
the Sumitomo Foundation (043426), the Japan Agency
for Marine–Earth Science and Technology, the U.S.
National Science Foundation, and the National Aeronautic and Space Administration. The first author conducted this work at IPRC, University of Hawaii, ona sabbatical leave from Hokkaido University. Graphic
outputs were made by use of GrADS.
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