VOLUME 21

JOURNAL OF CLIMATE

15 FEBRUARY 2008

A Climatology of Diurnal and Semidiurnal Surface Wind Variations over the Tropical
Pacific Ocean Based on the Tropical Atmosphere Ocean Moored Buoy Array
REI UEYAMA
Department of Atmospheric Sciences, University of Washington, Seattle, Washington

CLARA DESER
National Center for Atmospheric Research, Boulder, Colorado(Manuscript received 5 September 2006, in final form 31 May 2007)
ABSTRACT
Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N–8°S, 165°E–
95°W) during 1993–2004 are used to document the climatological seasonal and annual mean patterns ofdiurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, theamplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal
wind component, while the diurnal harmonic is at least 3 times as large as the semidiurnal harmonic for themeridional wind component, both averaged across the buoy array. Except for the eastern equatorial Pacific,
the semidiurnal zonal wind harmonic exhibits uniform amplitude (⬃0.14 m s⫺1) and phase [maximum
westerly wind anomalies ⬃0325/1525 local time (LT)] across the basin in all seasons. This pattern is well
explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over thecold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer
stability. Diurnal meridional wind variations tend to be out of phase north and south of the equator(maximum southerly wind anomalies ⬃0700 LT at 5°N and ⬃1900 LT at 5°S), while a noon southerly wind
anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season(June–November). The diurnal meridional wind variations result in enhanced divergence along the equatorand convergence along the southern border of the intertropical convergence zone ⬃0700 LT (opposite
conditions ⬃1900 LT); the amplitude of the divergence diurnal cycle is ⬃5 ⫻ 10⫺7 s⫺1. The diurnal
meridional wind variations are largely consistent with the diurnal pressure gradient force.

1. Introduction
The tropical atmosphere varies on a multitude oftime scales and influences weather and climate on regional and global scales. The intraseasonal Madden–
Julian oscillation and the interannual El Niño–Southern
Oscillation (ENSO) are two well-documented modes ofvariability whose effects extend beyond the tropical Pacific Ocean basin. The climate of the tropical Pacificalso undergoes daily variations in, for example, barometric pressure, cloudiness, rainfall, and near-surface
wind (e.g., Haurwitz and Cowley 1973; Hamilton 1980;
Hendon and Woodberry 1993; Gutzler and Hartten
1995; Deser and Smith 1998; Dai and Wang 1999; Yang

Corresponding author address: Rei Ueyama, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195.
E-mail: rei@atmos.washington.edu

and Slingo 2001; Yang and Smith 2006). These daily
cycles are associated with large variations in the solar
forcing and represent fundamental modes of variability
in the global climate system. However, in part becauseof the scarcity of observational data over the oceans,much work still remains in characterizing and interpreting daily variations of tropical atmospheric properties.
Deser and Smith (1998, hereafter DS) analyzed 4 yr
(1993–96) of hourly near-surface wind data from theTropical Atmosphere Ocean (TAO) moored buoy array and showed that the mean daily march of the zonal(meridional) wind component is primarily semidiurnal(diurnal). They further noted that the phase, amplitude,
and spatial pattern of the semidiurnal zonal wind variations are consistent with the classical theory of atmospheric thermal tides. Briefly, diurnal and semidiurnal
atmospheric thermal tides are generated by the absorption of solar radiation by ozone in the stratosphere andwater vapor in the troposphere. The tides propagate

DOI:10.1175/2007JCLI1666.1© 2008 American Meteorological Society

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TABLE 1. Percent data coverage at each TAO buoy during 1993–2004 for wind, SST, and air temperature data, and during 2000–04
for pressure data. The parentheses denote less than 50% wind data coverage. A dash indicates lack of pressure measurements. For
example, for the buoy at 8°N, 95°W, 43% of the wind records, 79% of the SST records, and 72% of the air temperature records during
1993–2004 have data; and 60% of the pressure records during 2000–04 have data.

9°N
8°N
5°N
2°N
0°
2°S
5°S
8°S

165°E

180°W

170°W

155°W

140°W

125°W

110°W

95°W

—
64/93/80/—
77/96/100/—
78/97/90/—(44)/80/67/—
63/83/90/—
53/95/90/—
66/87/88/——
59/80/66/—
74/88/92/—
74/95/92/—
79/94/92/—
93/98/98/—
66/91/87/—
83/83/88/——
95/99/98/—
79/87/93/—
66/81/91/—
90/99/95/—
72/96/84/—
78/92/92/—
88/91/95/——
65/90/84/—
85/100/99/—
75/85/89/—
93/99/92/—
71/86/83/—
90/99/91/—
86/100/97/—

76/100/97/——
85/95/95/—
66/91/90/—
91/96/92/—
81/98/98/—
100/100/100/———
63/92/86/—(40)/96/79/—
84/91/75/—
76/90/89/—
92/99/89/—
95/95/95/—
77/94/84/——
85/92/88/—
61/87/87/—
78/95/88/74
65/91/79/68
68/94/91/25
76/89/88/—
78/97/91/——(43)/79/72/60
60/82/83/48(38)/67/61/37
64/79/78/53
56/86/83/48
68/76/76/37
76/79/81/55

downward to the earth’s surface, affecting sea levelpressure and thereby surface winds. Tidal signals observed at the surface are mainly semidiurnal (Chapman
and Lindzen 1970) because the energy of the diurnal
forcing is trapped near the level of excitation in thelower stratosphere and upper troposphere (Haurwitz
1964; Lindzen 1967). In particular, the semidiurnal
component of the sea level pressure tidal amplitude
over the tropics is 2–3 times larger than the diurnal
component. While the semidiurnal zonal wind variations documented by DS are consistent with atmospheric thermal tidal theory, the observed diurnal meridional wind variations over the tropical Pacific have
yet to be explained, although we note that tidal theory
may also be relevant.
The diurnal cycle in tropical convection is clearly important for understanding the diurnal variability in thesurface wind field. Many studies have documented thediurnal cycles in deep convection and rainfall over thetropical oceans from a variety of data sources including
satellite products from the Tropical Rainfall Measuring
Mission (Bowman et al. 2005; Yang and Smith 2006)
and International Satellite Cloud Climatology Project(Hendon and Woodberry 1993), in situ ship reports
from the Comprehensive Ocean Atmosphere Data Set(COADS; Janowiak et al. 1994), and rain gauges fromisland stations (Gray and Jacobson 1977) and the TAO
buoys (Serra and McPhaden 2004). The various
datasets are all broadly consistent in indicating an earlymorning [⬃0300–0600 local time (LT)] rainfall maximum over the tropical oceans. Despite documentationof a clear diurnal signal in oceanic deep convective rainfall, neither its mechanism nor its connection to thediurnal cycle of surface wind and wind divergence are
well understood (Yang and Smith 2006).
Given the importance of the surface wind field to thelarge-scale circulations of the atmosphere and ocean,
this study seeks to expand our knowledge of daily (diurnal and semidiurnal) surface wind variability over thetropical Pacific using 12 yr (1993–2004) of hourly winddata from the TAO moored buoy array. Extending earlier results of DS, we document the climatological annual and semiannual mean patterns of diurnal andsemidiurnal surface wind variations.

2. Data and methods
a. Data
There are ⬃55 moored TAO buoys in the tropical
Pacific Ocean (9°N–8°S, 165°E–95°W) located approximately every 2°–3° of latitude and 15° of longitude. The
spacing of the buoys was chosen to be commensuratewith the narrow meridional scale of oceanic processes
at the equator, such as equatorially trapped waves andupwelling motions that determine sea surface temperature (SST) anomalies during ENSO. This arrangementof narrow meridional spacing of the buoys is not optimal for the study of large-scale atmospheric circulation.
Nevertheless, the TAO array is a valuable tool formonitoring daily variations of surface winds across thetropical Pacific because of its unprecedented spatial
and temporal coverage. This study examines 12 yr (1January 1993–31 December 2004) of hourly wind datafrom the 51 buoys that had at least 50% of data coverage (Table 1). For the selected buoys, the average number of days per year containing data is 278, ranging
from a minimum of 193 at 5°S, 165°E to a maximum of
365 at 5°S, 140°W. These numbers account for the timespan of data collection at each buoy as well as the frequent gaps in the record due to instrument failure.
The TAO array samples winds at 3.8 m above sea
level. Wind speed measurements are made with a propeller and have a resolution of 0.2 m s⫺1 and an accuracy of ⫾0.3 m s⫺1. A vane or a fluxgate compass isused to measure wind direction at 1.4° resolution withan accuracy of 5°–7.8°. Wind data were recorded for a
6-min interval at the beginning of each hour until 1996,
after which they are recorded every 10 min.

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FIG. 1. Mean surface wind vectors during 1993–2004 from the TAO array superimposed on mean outgoing longwave radiation during
1997–2001 (shading and contours at 20 W m⫺2 intervals): (a) annual mean, (b) December–May, and (c) June–November. Scale at
top-left corner of (a).

In addition to the wind data, hourly measurements ofbarometric pressure, SST, and air temperature from theTAO buoys are also examined for further analysis.
Pressure data became available after 1 January 2000
when pressure sensors were added to the 10 easternmost TAO buoys along 110° and 95°W (Table 1) as partof the East Pacific Investigation of Climate (EPIC)
study. Owing to this data limitation, all available pressure data between 1 January 2000 and 31 December
2004 are analyzed regardless of the low temporal coverage at some buoys (Table 1). Data coverage for SST
and air temperature is high (Table 1).
Barometric pressure is measured using a pressuretransducer at 3 m above sea level with 0.1-hPa resolution and an accuracy of ⫾0.01%. SST is measured at 1
m below the sea surface using a thermistor with 0.001°C
resolution and an accuracy of ⫾0.02°–0.03°C. Air temperature is measured at 3 m above sea level using a
resistance temperature recorder with 0.01°C resolutionand ⫾0.2°C accuracy. Further details of the TAO arraydatasets can be found on the TAO Project Office Web
site of the National Oceanic and Atmospheric Admin-

istration (NOAA)/Pacific Marine Environmental
Laboratory (available online at http://www.pmel.noaa.
gov/tao/index.shtml).

b. Methods
Seasonal (December–February, March–May, June–
August, and September–November) mean daily windmarches (i.e., mean variation of wind from hour to hourover the course of a day) were computed for each buoy
and year by averaging all available measurements ofthe two wind components at every hour within thegiven 3-month period. The seasonally averaged daily
wind marches were then averaged over all 12 yr foreach buoy. Annual and semiannual (December–May
and June–November) mean marches were constructedby averaging the seasonal mean marches. Finally, thedaily means of the annual and semiannual mean hourly
wind marches were removed.
All annual and semiannual mean hourly windmarches constructed in this manner were decomposed
into diurnal and semidiurnal harmonics following

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FIG. 2. Annual mean amplitudes and phases of the (a) diurnal and (b) semidiurnal harmonics of the zonal wind. The length of each
vector represents amplitude of the corresponding harmonic (scale at top-left corner of both panels) and the direction represents the LTof maximum westerly wind anomaly (clock at middle right). For example, an arrow pointing due north in (a) implies maximum westerly
wind anomaly at 0600 LT, and a vertical bar in (b) implies maximum westerly wind anomalies at 0600 and 1800 LT.

Xk共t兲 ⫽ Ak cos共2␲k Ⲑ24t兲 ⫹ Bk sin共2␲k Ⲑ24t兲,

共1兲

where X denotes the zonal or meridional wind anomalies at the tth hour of the day (t in local time), with kreferring to the diurnal harmonic (k ⫽ 1) or semidiurnal harmonic (k ⫽ 2). Here A and B are values determined by the method of least squares (Draper andSmith 1966). To portray the spatial patterns of the diurnal and semidiurnal wind variations, the amplitudes

and phases of the harmonics at each buoy are plottedon a map in vector format.

3. Resultsa. Mean wind field
As background to the analysis of diurnal and semidiurnal variability, we first present the annual and semi-

FIG. 3. Same as in Fig. 2, but for the meridional wind. The direction of the vectors represents the LT of maximum southerly wind
anomaly (clock at middle right).

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annual (December–May and June–November) mean
wind fields from the TAO buoys during 1993–2004 (Fig.
1). December–May and June–November correspond to
the warm and cold seasons, respectively, in the tropical
Pacific (Wallace et al. 1989; Hartten and Datulayta
2004, hereafter HD). Surface winds are predominantly
easterly across the basin throughout the year, converging into the region of deep convection (as inferred fromoutgoing longwave radiation values less than 240 W m⫺2)
over the western Pacific. Surface convergence associated with the meridional component of the wind is evident within the areas of deep convection along the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). The semiannual
variation of the surface wind field is qualitatively consistent with that of deep convection, with increased meridional flow into the enhanced convective region along
the ITCZ from June through November (Fig. 1c) andalong the SPCZ from December through May (Fig. 1b).

b. Daily wind harmonics: Annual mean
Figures 2 and 3 show the annual mean amplitudes
and phases of the diurnal and semidiurnal harmonics ofthe zonal and meridional winds, respectively. The
length of the vector represents the amplitude of theharmonic at each buoy, and its direction denotes thephase in LT. Arrowheads on the bars of the diurnal
harmonics of the zonal (meridional) wind point to thetime of maximum westerly (southerly) wind anomaly(the term “anomaly” is used with respect to the dailymean wind). Due north indicates a maximum at 0600
LT, with time increasing in the counterclockwise direction (e.g., due west is 1200 LT, due south is 1800 LT,
due east is 0000 LT). Arrowheads are not present on
the bars of the semidiurnal harmonics because maximum winds occur twice a day, 12 h apart. For example,
a vertical bar in the semidiurnal zonal (meridional)
wind plot indicates maximum westerly (southerly) wind
anomalies at 0600 and 1800 LT.
The phases and amplitudes of the semidiurnal harmonics of the zonal wind are strikingly uniform; maximum westerly wind anomalies are observed consistently ⬃0325/1525 LT with amplitudes ⬃0.14 m s⫺1(Fig. 2b). The semidiurnal signal is weak along 125° and
110°W at and south of the equator. Diurnal variability
has generally smaller amplitudes and less coherent
phases than semidiurnal variability (Fig. 2a).
The daily meridional wind cycle is dominated by diurnal variability whose amplitude is approximately 3
times the amplitude of the semidiurnal variability at
most buoys (Fig. 3). Diurnal variability of the meridional wind in the western Pacific (165°E–155°W) exhibits an approximately 12-h phase difference between

597

FIG. 4. Annual mean amplitudes (m s⫺1) of diurnal (black) andsemidiurnal (white) harmonics of zonal and meridional winds averaged over the TAO buoy domain.

buoys along 2°–5°N and those along 5°–8°S; maximum
southerly wind anomalies are observed between 0600
and 0900 LT in the Northern Hemisphere, and between
1800 and 2100 LT in the Southern Hemisphere. In theeastern Pacific (125°–95°W), the phase propagates
more gradually from north to south, with maximum
southerly wind anomalies ⬃1000 LT at 5°N progressing
to ⬃2000 LT at 8°S. Semidiurnal variations of the meridional wind, despite their small amplitudes, exhibit
coherent phases with maximum southerly wind anomalies ⬃0000/1200 LT (⬃0600/1800 LT) in the Northern(Southern) Hemisphere (Fig. 3b). This pattern of crossequatorial asymmetry of the semidiurnal meridional
wind anomalies is in quadrature to the semidiurnal
cycle of the zonal wind (Fig. 2b) and is likely a result of
the Coriolis force, as discussed in DS.
Figure 4 quantifies the relative amplitudes of the diurnal and semidiurnal harmonics of each wind component averaged across the buoy array. The mean amplitude of the semidiurnal harmonics of the zonal wind istwice as large as that of the diurnal harmonics (0.14
versus 0.07 m s⫺1), while the mean diurnal amplitude of
the meridional wind is more than 3 times as large as its
mean semidiurnal amplitude (0.19 versus 0.06 m s⫺1).

c. Daily wind harmonics: Semiannual means
Figures 5 and 6 show semiannual (December–May
and June–November) mean amplitudes and phases ofthe diurnal and semidiurnal harmonics of the zonal andmeridional winds, respectively. In addition to the general patterns of their annual mean counterparts, thesemiannual harmonics exhibit some noteworthy differences. For example, the suppression of the semidiurnal
zonal wind harmonic along 125° and 110°W at, andsouth of, the equator, is even more apparent in June–

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FIG. 5. December–May and June–November amplitudes and phases of the (a), (b) diurnal and (c), (d) semidiurnal harmonics of thezonal wind. The length of each vector represents amplitude of the corresponding harmonic [scales at top-left corners of (a) and (c)] andthe direction represents the LT of maximum westerly wind anomaly (clock at middle right).

November than in the annual mean and December–
May (Figs. 2b and 5c,d). The diurnal meridional windharmonics are also seasonally dependent, with larger
amplitudes in June–November than in December–May(Figs. 6a,b). The enhancement is particularly evident
over the eastern basin.
Averaged over the buoy array, the daily zonal andmeridional wind cycles are dominated by semidiurnaland diurnal variability, respectively, for both semiannual means (Fig. 7). The basin-averaged amplitude of
the semidiurnal zonal wind harmonics does not varysignificantly from season to season, while that of thediurnal meridional wind harmonics is clearly larger dur-ing June–November compared with December–May.
The semidiurnal variability of the zonal wind component and the diurnal variability of the meridional windcomponent are analyzed further, as they dominate theclimatological daily mean wind cycle over the tropical
Pacific Ocean.

4. Discussion
a. Semidiurnal variability
In the absence of friction and nonlinear effects, themomentum balance for the zonal wind component may
be expressed as

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FIG. 6. Same as in Fig. 5, but for the meridional wind. The direction of the vectors represents the LT of maximum southerly wind
anomaly (clock at middle right).

␦u
1 ␦p
⫽⫺
⫹ f␷,
␦t
␳ ␦x

共2兲

where the zonal wind acceleration (␦u/␦t) is associatedwith the pressure gradient in the east–west direction(␦p/␦x), the density of air (␳ ⫽ 1.2 kg m⫺3), and the
Coriolis force ( f␷, where f is the Coriolis parameter and␷ is the meridional wind). We use Eq. (2) to assess
whether the observed semidiurnal zonal wind variations over the tropical Pacific are consistent with thosederived from the local pressure gradient force associated with atmospheric thermal tides.
Classical atmospheric thermal tidal theory (e.g.,
Chapman and Lindzen 1970) predicts a zonally uniform

FIG. 7. Same as in Fig. 4, but for semiannual mean amplitudes:
December–May (DM) and June–November (JN).

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semidiurnal pressure wave in the tropics. Haurwitz andCowley (1973, hereafter HC) show that the amplitude
and phase of the semidiurnal pressure variability are
relatively constant within 20° of the equator, with values ⬃1 hPa and ⬃0945/2145 LT, respectively. Their
formula for the annual mean semidiurnal surface pressure variation based on zonally averaged land station
records isp ⫽ 1.161 cos3共␪兲 sin共2t ⫹ 159⬚兲,

共3兲

where ␪ is latitude and t is local time in degrees (i.e.,
one day is divided into 360 equal parts). Since HC approximated the global distribution of the semidiurnalpressure wave based only on land station data, the accuracy of their results over the ocean is questionable.
Nevertheless, ship measurements of sea level pressure
over the tropical Pacific (9°N–9°S, 162.5°E–92.5°W)
from the COADS as analyzed by Dai and Wang (1999,
hereafter DW) indicate semidiurnal pressure variationsof similar spatially uniform amplitude and phase (⬃1hPa and ⬃0950/2150 LT, respectively).
We use the pressure data analyzed by DW, available
as seasonal (3 month) and annual mean amplitudes andphases of the diurnal and semidiurnal harmonics on a
2° latitude ⫻ 5° longitude grid, and we use the semidiurnal surface pressure formula from HC [Eq. (3)] to
compute the semidiurnal variation of the zonal windcomponent from our Eq. (2). The pressure harmonics
analyzed by DW were first smoothed along the latitude
and longitude directions using a 4-point boxcar average. The smoothed pressure harmonics were then used
to reconstruct the daily pressure marches at each grid
point and the zonal pressure gradients centered at theTAO buoy locations were computed (e.g., the zonalpressure gradient for the buoys at 165°E was approximated by the pressure difference between DW pressure
data at 167.5° and 162.5°E). Two neighboring pressure
gradients in the longitude direction were averaged forbuoys situated at the midpoints of the DW pressure
grid (e.g., pressure gradients computed at 1°N and 1°S
were averaged to obtain the pressure gradient for thebuoys along the equator). All available meridional
wind data, including those of buoys with ⬍50% data
coverage (Table 1), were used to compute the Coriolis
term in Eq. (2).
The annual mean semidiurnal variability of the observed zonal wind averaged over all buoys in the tropical Pacific agrees well with those derived from the twopressure data sources, although the observed amplitudeis smaller than the pressure-derived amplitudes (0.14
versus 0.20 m s⫺1; Fig. 8a). Differences in the amplitudes between the observed and pressure-derived zonal

FIG. 8. (a) Comparison of semidiurnal zonal wind averaged over
the tropical Pacific (⬃9°N–9°S, 165°E–95°W): observed from
TAO array (solid) and derived from sea level pressure measurements from DW COADS data (dash–dot) and from HC land
station records (dot). (b) Same as (a), but averaged over the eastern equatorial Pacific (⬃1°N–5°S, 125°–110°W).

winds may be due to neglected effects of surface friction and nonlinear terms in the momentum balance aswell as measurement errors. Comparisons of the seasonal averages of the observed and pressure-derived
winds are similar to the annual mean results (notshown). We note that the lack of seasonal variation in
the semidiurnal zonal wind and pressure harmonics is
consistent with atmospheric thermal tidal theory(Chapman and Lindzen 1970; HC).
Next we examine whether the reduction in amplitudeof the semidiurnal zonal wind harmonics along 125° and
110°W at, and south of, the equator is reflected in thepressure field. The semidiurnal zonal wind amplitude
derived from the pressure data of DW in the region(1°N–5°S, 127.5°–107.5°W) is the same as the basinwide
average (0.20 m s⫺1) and is thus considerably larger
than observed (0.07 m s⫺1; Fig. 8b). Note that regionalpressure information is not available from HC. The

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FIG. 9. Amplitudes and phases of semidiurnal harmonics of sea level pressure: (a) annual mean, (b) December–May, and (c)June–November. The length of each vector represents amplitude of the corresponding harmonic [scale at bottom-left corner of (a)] andthe direction represents the LT of pressure anomaly maximum (clock at top left).

spatial coverage of the TAO buoy pressure data (2°N–
2°S along 110°W and 8°N–8°S along 95°W) is too limited to allow a meaningful assessment of the semidiurnal zonal pressure gradient force. However, the observed semidiurnal pressure harmonic field (Fig. 9)shows no evidence of a reduction in amplitude at thelocations where the semidiurnal zonal wind harmonic issuppressed, except for the buoy at 2°S, 110°W in December–May (however, this estimate is based on only
16% data coverage during 2000–04; Table 1). Indeed,
the semidiurnal pressure harmonics from the TAO
buoys are highly uniform in both amplitude and phase(⬃1.4 hPa and ⬃1010/2210 LT, respectively) for annual
and semiannual means (Fig. 9).
Since it appears that the zonal pressure gradient
force cannot account for the reduction in semidiurnal
zonal wind amplitude over the cold surface waters in
the eastern Pacific, as an alternative mechanism we explore a possible role of enhanced boundary layer stability that may attenuate the surface wind amplitude by
inhibiting the vertical mixing of momentum. Boundary
layer stability has been shown to affect the strength ofnear-surface winds in the eastern equatorial Pacific ona variety of time scales from annual to subseasonal(Wallace et al. 1989; Chelton et al. 2001; Xie 2004).
Clearly, a full investigation of the role of boundary
layer stability and associated boundary layer depth requires observations of the vertical structure of the semidiurnal zonal wind cycle, information not currently
available over the tropical Pacific Ocean. However, as afirst step, we analyzed the spatial structure of atmospheric boundary layer stability as estimated from thedifference between long-term daily mean SST and airtemperature from the TAO buoys.
SSTs exceed near-surface air temperatures over theentire domain in the annual and semiannual means,reflecting that the tropical marine boundary layer isgenerally unstable (Fig. 10). However, the magnitudeof the SST–air temperature difference is greatly diminished over the cold surface waters of the eastern equatorial upwelling zone and southeast tropical Pacific, approaching near-zero values in the cold season (June–
November; Fig. 10c). This reduction of boundary layer
instability (or relative enhancement of boundary layer
stability) is consistent with the prevalence of stratiform
low clouds at, and south of, the equator in the eastern
Pacific (Deser et al. 1993; Norris 1998). The annual andsemiannual distributions of mean boundary layer stability (Fig. 10) compare well with those of semidiurnal
zonal wind amplitude (Figs. 2b and 5c,d): their patterncorrelation coefficients are in the range of 0.7–0.8. In
particular, the region of enhanced stability over theeastern Pacific is associated with a reduction in semidiurnal amplitude. This correspondence is confirmed by ascatterplot between seasonal (3 month) mean boundary
layer stability and semidiurnal zonal wind amplitude,
which reveals a strong positive relationship for SST–airtemperature differences ⬍0.6°C (e.g., buoys in the eastern Pacific; Fig. 11). Whether the similarity between thespatial patterns of mean boundary layer stability andsemidiurnal zonal wind harmonic amplitude is causal orincidental remains to be seen.
One source of information on the vertical structureof the daily wind cycle is the lower-tropospheric windprofiler at Isla San Cristóbal in the Galápagos Islands(0.90°S, 89.61°W), which provides half-hourly winddata between 300 and 2600 m. Using these data, HD
found that the daily cycle of winds below ⬃500 m isdecoupled from that aloft during periods of low(⬍23°C) SST. However, contrary to our findings,
higher-amplitude semidiurnal zonal wind signals wereobserved near the surface during periods of increased
boundary layer stability, and these were speculated tobe due to water vapor tidal forcing in the lowest 500 m(HD). Further work is needed to reconcile the different
dependencies of the semidiurnal zonal wind amplitudes

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FIG. 10. Long-term daily mean boundary layer stability (daily mean SST minus air temperature) during 1993–2004: (a) annual mean,(b) December–May, and (c) June–November. Scale at top-left corner of (a). Small vectors (i.e., small temperature differences) indicate
enhanced stability. Directions of the bars are arbitrary.

on boundary layer stability in the Galápagos profiler
and TAO buoy data.

b. Diurnal variability

FIG. 11. Scatterplot of the seasonal (3 month) mean boundary
layer stability (daily mean SST minus air temperature in °C) vs theseasonal mean semidiurnal amplitude (m s⫺1) of zonal wind at all
51 buoys. Dots in the bottom-left quadrant represent data fromeastern equatorial buoys.

The spatial pattern of the diurnal meridional windharmonics is more complex than that of the semidiurnal
zonal wind harmonics. Diurnal meridional wind variations tend to be out of phase north and south of theequator (southerly wind anomaly maxima ⬃0700 LT at
5°N and ⬃1900 LT at 5°S), while on the equator in theeastern Pacific, southerly wind anomaly maxima occur
around noon, particularly during the cold season(June–November; Fig. 6b). We note that the noonphase and seasonal dependence of the amplitude of thediurnal meridional wind variations in the eastern equatorial Pacific are consistent with the Galápagos windprofiler results of HD.
Following the same method used to analyze the semidiurnal zonal wind variability, we compare the observed diurnal meridional wind variability with that derived from the diurnal pressure harmonics of DW

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FIG. 12. Annual mean amplitudes and phases of the diurnal harmonics of the meridional wind derived from DW COADS pressure
data based on Eq. (4). The length of each vector represents amplitude (scale at top-left corner) and the direction represents the LT of
maximum southerly wind anomaly (clock at top left).

(smoothed as described in section 4a) based on the momentum balance for the meridional wind component:

␦␷
1 ␦p
⫽⫺
⫺ fu.
␦t
␳ ␦y

共4兲

The annual mean spatial patterns of the observed andpressure-derived diurnal meridional wind harmonics
are qualitatively similar, with opposite phases in the
Northern and Southern Hemispheres of the western
Pacific (⬃0600–0900 and 1800–2100 LT, respectively)
and approximately noon phases at the equator in theeastern Pacific (Figs. 3a and 12). The annual mean amplitudes tend to be 15% larger for the pressure-derived
harmonics than the observed harmonics. Semiannual
mean patterns of the pressure-derived diurnal meridional wind harmonics are also largely consistent withthose of the TAO winds (not shown). The good agreement between the observed and pressure-derived windharmonics indicate that the pressure gradient force
plays an important, if not dominant, role in determining
the diurnal cycle of the meridional winds over the tropical Pacific, while other factors such as boundary layer
stability may be of secondary importance.
Hourly wind vector snapshots representing departures from the daily mean winds, reconstructed fromthe amplitudes and phases of the diurnal zonal and meridional wind harmonics, are shown at 3-h intervals
from 0100 to 1300 LT for the June–November season in
Fig. 13 (results based on annual and December–May
means exhibit similar patterns with generally smaller
amplitudes; not shown). These snapshots confirm theremarkable asymmetry of the diurnal meridional windcomponent about the equator, especially in the western
Pacific. This pattern results in enhanced equatorial divergence ⬃0700 LT (convergence ⬃1900 LT). There isalso an indication of a diurnal cycle in surface winddivergence along the southern border of the ITCZ thatis out of phase with the one on the equator, although
the spatial coverage of the TAO array is not sufficient
to fully delineate this feature.
The diurnal cycles of wind divergence relative to thedaily mean along the near-ITCZ (8°–5°N) and nearequatorial (0°–2°S; similar results are obtained for 2°N–
2°S) bands for December–May and June–November
are shown in Fig. 14. Solid lines denote zonal averages
across the tropical Pacific (165°E–95°W) and dashedlines denote averages for the western basin (165°E–
155°W). All available wind data within the domains,
including those of buoys with ⬍50% data coverage(Table 1), were used to compute the divergence. The
diurnal cycles in the near-ITCZ and near-equatorial
bands are approximately out of phase in both seasonsand longitudinal domains, with maximum equatorial divergence (ITCZ convergence) at 0700–0900 LT, consistent with Fig. 13. In addition, they are slightly larger inamplitude and earlier in phase over the western basinthan the full tropical Pacific, also consistent with thewind vectors in Fig. 13. In the western Pacific, the diurnal amplitude of the near-equatorial divergence is
7.1 ⫻ 10⫺7 s⫺1 in December–May, similar to the value
for June–November (8.0 ⫻ 10⫺7 s⫺1), while that of thenear-ITCZ divergence is 6.2 ⫻ 10⫺7 s⫺1 in December–
May, larger than the value for June–November (3.6 ⫻10⫺7 s⫺1). We speculate that the smaller diurnal amplitude of the near-ITCZ divergence in June–November
may be due to the more northerly position of the mean
ITCZ in that season such that the TAO buoy domain
does not fully capture the associated low-level convergence (recall Fig. 1).
The out-of-phase relationship between the diurnal
cycles of surface wind divergence for the near-ITCZ
and near-equatorial regions suggests that the two may
be linked through a diurnally oscillating meridional
overturning circulation, at least in the western Pacificwhere the diurnal wind harmonics exhibit a relatively

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FIG. 13. June–November wind vector departures from the daily means reconstructed from the diurnal harmonics at (a) 0100, (b)0400, (c) 0700, (d) 1000, and (e) 1300 LT. Scale is shown at top-left corner of (a).

simple pattern (see also DS). Although it remains to be
seen how this diurnal surface wind divergence patternrelates to diurnal precipitation changes over the tropical Pacific, the early morning maximum in rainfall reported in numerous studies (Gray and Jacobson 1977;
Hendon and Woodberry 1993; Janowiak et al. 1994;

Serra and McPhaden 2004; Bowman et al. 2005; Yang
and Smith 2006) appears to be broadly consistent withthe morning maximum in surface wind convergence
into the western Pacific ITCZ. Furthermore, a simple
estimate of the magnitude of the diurnal rainfall change
expected from the amplitude of the diurnal surface

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605

FIG. 14. (left) December–May and (right) June–November diurnal cycle (LT) of surface wind divergence (s⫺1)
averaged over all longitudes (165°E–95°W; solid) and western longitudes (165°E–155°W; dashed) for (a), (c)near-ITCZ (8°–5°N) and (b), (d) near-equatorial (0°–2°S) regions. All available wind data (including buoys with⬍50% data coverage, Table 1) were analyzed. The daily means at each buoy have been removed. Positive(negative) values indicate divergence (convergence) relative to the daily mean.

wind divergence compares favorably with the observed
amplitude of diurnal rainfall variations (DS). However,a joint analysis of diurnal precipitation and wind variability is needed to link the two processes more definitively.
As we have shown, the pattern of diurnal meridional
wind variations from the TAO buoys is generally consistent with the diurnal pressure gradient force. The
zonally symmetric portion of the pressure gradient
force that results in out-of-phase meridional wind variations north and south of the equator appears to be consistent with the structure of the diurnal atmospheric
thermal tide (Braswell and Lindzen 1998; DW). The
diurnal component of sea level pressure variability overthe eastern Pacific cold tongue that drives the noon
equatorial southerly wind maximum may result fromthe diurnal cycle of SST and associated atmosphericboundary layer specific humidity. We note that thismechanism would account for the confinement of thenoon southerly wind maximum below ⬃500 m during
periods of low SST at the Galápagos Islands as documented by HD. Further work is needed to explore
these mechanisms more fully.

5. Conclusions
The diurnal and semidiurnal variations of the zonal
and meridional near-surface wind components over thetropical Pacific presented in this study generally agreewith the findings of DS, which were based on a smaller
sample of years (1993–96 versus 1993–2004). In particular, we confirm that the zonal wind variability is mainly
semidiurnal, with near-uniform amplitude (⬃0.14
m s⫺1) and phase (maximum westerly wind anomalies⬃0325/1525 LT) across the basin (except the easternequatorial Pacific) in all seasons. This semidiurnal
zonal wind pattern is well explained by classical atmospheric thermal tidal theory. We also confirm that themeridional wind variability is mainly diurnal, and tends
to be out of phase north and south of the equator(maximum southerly wind anomalies ⬃0700 LT at 5°N
and ⬃1900 LT at 5°S) particularly over the western
Pacific, while on the equator in the eastern Pacific,
there is a noon southerly wind anomaly maximum. In
the eastern Pacific (125°–95°W), the diurnal meridional
wind cycle exhibits a southward phase progression from
5°N to 8°S.

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We have shown that the amplitudes of the diurnal
meridional wind variations are larger during the coldseason (June–November) than the warm season (December–May), with similar spatial patterns in the two
seasons. The diurnal meridional wind variations over
the western Pacific result in enhanced (relative to thedaily mean) divergence along the equator and convergence along the southern border of the ITCZ at ⬃0700
LT, with opposite conditions at ⬃1900 LT. The amplitude of the diurnal cycle in surface wind divergence is
4–8 ⫻ 10⫺7 s⫺1 depending on region and season, similar
to that found by DS.
The spatial pattern of the diurnal meridional windvariations agrees well with that derived from the diurnal pressure harmonics of DW, suggesting the dominant role of the pressure gradient force in determining
the diurnal cycle of the meridional winds over the tropical Pacific. Further work is needed to understand theprocesses contributing to the diurnal pressure gradient
force, including the role of atmospheric thermal tides,
diurnal SST variations, and diurnal variations in deep
convection. In this regard, improved observations of
the vertical structure of diurnal wind variations, as wellas an overarching investigation that examines the diurnal wind variability in conjunction with that of deep
convection and precipitation over the tropical oceans,would be beneficial.
One result that is more prominent in our study compared with DS is the suppression the semidiurnal zonal
wind amplitude at and south of the equator in the eastern Pacific. This feature is most striking during the coldseason (June–November), which is why it was only
hinted at in the annual mean analyses of DS. Based on
the available sea level pressure data, we found no evidence for a concomitant reduction of the semidiurnal
zonal pressure gradient force, leading us to propose analternative mechanism, namely a reduction of vertical
momentum mixing due to enhanced boundary layer
stability over the cold surface waters. Although wehave no direct evidence for this mechanism, we notethat the spatial patterns of boundary layer stability (estimated from the long-term daily mean SST–air temperature difference) and semidiurnal zonal wind amplitude are similar, as are their seasonal dependencies.
Additional evidence is required to support or refute
this hypothesis. In particular, observations of the vertical structure of semidiurnal wind variations are needed
to improve our understanding of the role of boundary
layer processes in downward-propagating tidal signals.
Acknowledgments. This work began under the auspices of the Significant Opportunities in AtmosphericResearch and Science (SOARS) program of the Uni-

VOLUME 21

versity Corporation for Atmospheric Research, withfunding from the National Science Foundation, theU.S. Department of Energy, the National Oceanic andAtmospheric Administration, and the NASA Goddard
Space Flight Center. It was completed in the Department of Atmospheric Sciences at University of Washington under the guidance of Dr. John M. Wallace, whois acknowledged for his invaluable comments. We
thank Dr. Aiguo Dai for providing and sharing his expertise in the COADS pressure data, Mr. Adam Phillips for his generous assistance with data processing,and two anonymous reviewers for many constructive
suggestions. We also thank NOAA/Pacific Marine Environmental Laboratory for providing the TAO data
and the TAO–EPIC Project for providing the TAO
pressure data. Financial support was provided in partby the SOARS program and the National Science
Foundation Graduate Research Fellowship.
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