JOURNAL

OF GEOPHYSICALRESEARCH,

VOL. 99, NO. Dll,

PAGES 23,071-23,078, NOVEMBER

20, 1994

Daily surface wind variations over the equatorial Pacific Ocean
Clara

Deser

CIRES, University of Colorado, Boulder

Abstract. Daily surface wind variations over the equatorial eastern Pacific Ocean
during summer 1992 are documented, using hourly observations from the tropicalatmosphere ocean moored buoy array. Diurnal and semidiurnal variations are apparent
in the zonal wind component, with peak-to-peak amplitudes of a few tenths of a meter
per second. The phase of the semidiurnal cycle in zonal wind is approximately uniform
across the equatorial eastern Pacific Ocean, with westerly wind maxima at -0300 and
1500 LT. The diurnal cycle dominates the daily march of meridional wind. The rangeof the diurnal meridional wind variations is -0.6-0.8 ms-• at most locations' morethan twice as large as the daily zonal wind changes. The low-level flow is southward
across the equator at night (relative to the daily mean), regardless of whether the mean
winds are southerly or northerly. The diurnal meridional wind variations along theequator may be related to the diurnal cycle of deep convection in the Intertropical
Convergence Zone (ITCZ) to the north. In particular, surface wind divergence along
the equator (which is dominated by the meridional component) exhibits a pronounced
diurnal cycle, with the strongestdivergence in the early morning when deep convectionin the ITCZ is at a maximum. The average daily range of the equatorial surface winddivergenceis 1.5 x 10-6 s-1 or --30%of thedailymeanThe semidiurnal
zonalwind9

ø

variations are dynamically consistent with the well-known semidiurnal cycle in surfacepressure, which is thought to be a manifestation of the atmospheric thermal tide.
1.

Introduction

Little is known about the magnitudeand spatial pattern ofdaily surface wind variations over the open ocean in thetropics. (In this study, "daily" refers to all variations within
a 24-hour period, and "diurnal" and "semidiurnal" refer tothe first and secondharmonics of the daily cycle, respectively.) Field programs such as the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) andthe Barbados Oceanographic and Meteorological Experiment (BOMEX) gathered useful but limited information on
daily wind changes in the tropical Atlantic [Nitta and Esbensen, 1974; Gray and Jacobson, 1977; Pedder, 1978;
McBride and Gray, 1980; Albright et al., 1981]. Thesestudies emphasized the diurnal variation in the verticalprofile of horizontal wind divergence and related the inferred
upward motion to daily variations in deep convective activity. Diurnal wind variations at island stations in the western
tropical Pacific were examined by Wallace and Hartranft[1969] and Hastenrath [1972]. These studies were concernedwith the vertical structure of the diurnal wind changesin the
troposphere and lower stratosphereand its relation to atmospheric tidal theory. A similar theme was pursued by Williams and Avery [1992], who documented the vertical structure of diurnal

zonal wind variations

at Christmas

Island inthe central equatorial Pacific. The only study to examine
daily wind variability over the equatorial eastern Pacific isthat by Halpern [1988]. He documentedthe diurnal surface
wind amplitudes at four mooring sites along the equator eastof 140øW.

The tropical atmosphere ocean (TAO) moored buoy array
Copyright 1994 by the American Geophysical Union.
Paper number 94JD02155.
0148-0227/94/94JD-02155505.00

in the equatorial Pacific has been collecting hourly surface
wind observations since -1990 [Hayes et al., 1991]. (Windrecords at a few locations go back to -1980 [Halpern,
1988].) The TAO array provides an unprecedented opportunity to study daily wind variability over a vast expanse of theequatorial ocean. The purposeof this study is to describethe
large-scale spatial pattern of daily surface wind variations
over the equatorial Pacific Ocean during summer 1992. This
period contained the most complete data coverage at thetime this investigation was undertaken. In addition to describing the diurnal and semidiurnal cycles of surface zonal
and meridional wind, we investigate the daily variations of
the large-scale surface wind divergence field over the equatorial eastern Pacific and relate those variations to the daily
cycle of deep convection in the ITCZ.

2.

Data

and Methods

Conceived by S. P. Hayes, the TAO array was designedto
monitor upper ocean thermal conditions and surface atmospheric properties for improved prediction of the E1 Nino/
Southern Oscillation cycle [Hayes et al., 1991; McPhaden,
1993]. Beginningwith two buoys in late 1983, the array in its
fully implemented form in late 1994 will consist of 70
moorings distributed within 8ø of the equator, spaced approximately every 15ø of longitude across the Pacific basin[see McPhaden, 1993]. Winds are measured at -3.8 m above
the sea surface using cup and vane or propeller and vaneassemblies (see McPhaden and Hayes [1990] and Hayes etal. [ 1991] for more details). The expected instrumental errors

aretypically-0.1 ms- •, withmaximum
calibration
residuals
lessthan0.2 ms-• for anyparticularinstrument,
basedonpredeployment and postdeployment calibration tests[McPhaden and Hayes, 1990; Freitag et al., 1991]. Most of

23,071

23,072

DESER: SURFACE WIND CYCLES OVER EQUATORIAL

Table la. Locations and Wind Sampling Characteristicsof the 14 TAO Moorings Used in This Study
Buoy Location

Resolution

Averaging

No. of Days

5øN, 170øW
0øN, 170øW
5øN, 155øW
2øN, 155øW
0øN, 155øW
2øS, 155øW
5øS, 155øW
0øN, 140øW
2øS, 140øW
2øS, 125øW
5øS, 125øW
2øN, 110øW

1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
1 hour
1 hourcentered
centered
centered
centered
centered
centered
centered
centered
centered
centered
centered
centered

92
92
72
92
74
75
77
92
75
92
92
92

0øN, 110øW2øS, 110øW

2 hours
1 hourbeginning
centered

91
92

TAO, tropical atmosphere ocean. In column 3, "centered" meansthat the hourly or 2 hourly wind average is centered on the hour, and"beginning" means the hourly average is assignedto the beginningof the hour. Column 4 lists the number of days on which winds were
measured during the 92-day period June 1-August 31, 1992.

PACIFIC OCEAN

10N

0
10S

180

170W

160W

150W

140W

130W

120W

110W

100W

Figure 1. Mean surface wind field over the equatorial
eastern Pacific during June-August 1992 from the tropicalatmosphere ocean (TAO) array. Only those sites with atleast 60 days of hourly observations are shown. The longest
vector is 8 ms

prevail across the equatorial eastern Pacific, with magni-

tudesbetween5 and8 ms-t . Thewindsexhibita southerly
component at 110øW and a northerly component west of140øW.The mean winds over the equatorial eastern Pacificin summer1992werewithin1-2 ms-t of climatology[Cli-mate Analysis Center, 1992].
Figure 2 shows the average daily march (in local time) of
the zonal wind component during summer 1992 at all locations where data were reported on at least 60 days. The daily
the buoys provide hourly wind observations,but sampling mean zonal winds have been removed and the hourly values
within the hour is variable. At some buoys, winds are smoothed with a three-point binomial filter. Positive (negameasured continuously, at others the "hourly" wind con- tive) values indicate westerly (easterly) winds relative to thesists of a 6-min average centered on the hour, or of spot daily mean. The 24-hour period has been repeated forsamples every 10 min (P. Freitag, personal communication, clarity. The standard errors of the hourly wind averages are1993). Depending on the buoy, the hourly values are as- in the range0.1-0.2 ms-• for the set of buoywindsused
signedto either the beginning, the middle, or the end of the here. Hence attention will be paid only to those daily windhour. The hourly wind data are recorded on cassettetape or changes
that havepeak-to-peakamplitudes-0.3 ms-• or
solid state memory devices on board the buoys; the data greater. In Figure 2, diurnal and/or semidiurnal zonal windtapes are retrieved every 6 months-1 year and processedat variations are apparent at almost all of the sites, with
the TAO project office at the Pacific Marine Environmental peak-to-peak amplitudes of a few tenths of a meter per
Laboratory in Seattle, Washington.
second.The semidiurnalfluctuationsare particularly evident
This study uses data from the 92-day period June 1-Aualong 155ø and 170øW where lower harmonics of the dailygust 31, 1992. Fourteen moorings reported hourly wind dataon at least 60 days during this period; their locations and.6
samplingcharacteristicsare listed in Table la. The long-term(9 years) wind records at 0ø, 110øWand 0ø, 140øWare used to
5N
examine the seasonaldependenceof the daily wind variations. The samplingcharacteristicsfor these buoys are given
in Table lb. All of the hourly or 2 hourly wind data used inthis study were interpolatedto the top of the hour.
3.

024
48
•

2N

Results

Daily Variations of Surface Wind

The mean surfacewind field during summer1992from the
TAO array is shown in Figure 1. Only those sites with atleast 60 days of observations are included. Easterly winds

Table lb. Wind Sampling Characteristicsfor the Buoys
at 0ø, 110øWand 0ø, 140øWDuring 1983-1983
Buoy Location

Time Period0ø, 110øW

Nov. 1983 to April 1989
May 1989 to Oct. 1992
April 1984 to April 1987
May 1987 to Oct. 1987
Nov. 1987 to Nov. 1988
Dec. 1988 to March 1990
April 1990 to April 1993

0ø, 140øW

Resolution Averaging
1 hour2 hours
1 hour2 hours
1 hour2 hours
1 hourbeginning
beginning
beginning
beginning
beginning
beginning
centered

170 W

Figure 2.

155 W

140 W

125 W

110 W

Average daily march of the zonal wind compo-

nentin metersper second(ms-•) duringJune-August
1992
from the TAO array. All boxes are drawn to the same scale,with local time on the abscissa(0-48 hrs) and wind speed on

the ordinate(-0.6-0.6 ms-t), as shownin the top left
corner. The daily mean zonal winds have been removed
from each site, and the hourly values smoothed with athree-pointbinomial filter. The 24-hour period is repeatedfor
clarity. Positive (negative) values indicate westerly (easterly) winds relative to the daily mean.

DESER: SURFACE WIND CYCLES OVER EQUATORIAL

cycle are suppressed. The phase and amplitude of thesemidiurnal cycle is similar at all locations.
In contrast to the zonal wind the daily march of meridional
wind exhibits a strong diurnal cycle with little semidiurnal
variability (Figure 3). (In Figure 3, positive (negative) values
indicate southerly (northerly) winds relative to the daily
mean.) At most locations the range of daily meridional wind

PACIFIC OCEAN

23,073

0

12

24

36

48

0

12

24

36

48

variationsis --•1ms-1, almosttwiceas largeas the daily
range of the zonal wind variations. The low-level flow issouthward across the equator at night (relative to the dailymean), regardless of whether the mean winds are southerly
or northerly (see Figure 1). There appears to be a regularsouthward progressionof the nighttime northerly wind maximum, particularly at 110ø and 125øW(rate --•1 hour/deg oflatitude).
To give an idea of the regularity of the daily wind cycles,
Figure 4 shows the hourly meridional wind deviations fromthe daily means for each day during summer 1992 at 2øN,
155øW. Each column in Figure 4 shows the deviations fromthe daily means for a particular hour, with each dot representing a different day. The 24-hour period has been repeated for clarity. The daily cycle of meridional wind isvisually apparent, but there is also a high proportion of
scatter within each hour-column.

Hour (local time)

It should be noted that theamplitude of the mean daily meridional wind cycle at 2øN,
155øW is among the largest of the set of buoy winds
considered here (see Figure 3). The large proportion of
scatter in Figure 4 underscores the need for lengthy, highquality wind records with which to document daily cycles.
The wind records shown in Figures 2 and 3 were decomposed into diurnal and semidiurnal harmonics according to

Figure4. Scatterdiagramof hourlymeridional
winddevi-

ations from the daily means (ms-') for each day during
summer 1992 at 2øN, 155øW. Each column shows the deviations from the daily means for a particular hour, with each
dot representinga different day. The 24-hour period has been
repeated for clarity.

Xk(t ) = Ak cos (7rk/12t) + B• sin (7rk/12t)
where

X is the deviationof the zonal

or meridional

windcomponent from the daily mean, the index k = 1 (k = 2)
refers to the diurnal (semidiurnal) harmonic, and t is time inhours. A• and B• were determinedby the method of leastsquares [cf. Draper and Smith, 1966].
The amplitudes and phases of the diurnal and semidiurnal
harmonics of zonal and meridional winds at each buoy arelisted in Table 2. Figure 5a (Figure 5b) shows the amplitude

and phase of the diurnal (semidiurnal) harmonic of the zonal
wind component in vectorial format. The length of the arrowdenotes the amplitude of the harmonic and the direction
indicates the phase of the westerly wind maximum in local
time. An arrow pointing due west indicates a local noon
westerly maximum, with time increasing in the counterclockwise direction. The amplitudes of the diurnal andsemidiurnal harmonics of zonal wind are comparable, withthemajorityof valuesaround0.15ms-• (seealsoTable2).
The phase of the semidiurnalharmonic of zonal wind (Figure5b) is approximately uniform across the basin, with westerly
maxima at --•0300and 1500 LT (see also Table 2). The phase

5N
0

24

48

of the diurnal

harmonic

is less uniform

than

that

of thesemidiurnal harmonic, with the westerly wind maximum
occurring in the evening at 110øW and in the afternoon

2N

elsewhere.

Figure 6 shows the harmonic analysis results for themeridional wind component. An arrow pointing due west
indicates a local noon southerly wind maximum, with timeincreasingin the counterclockwise direction. The dominanceof the diurnal harmonic (Figure 6a) over the semidiurnal
harmonic (Figure 6b) is readily apparent. The amplitude ofthediurnalmeridional
windharmonic
rangesfrom0.09ms-•
to 0.56 ms-• with valuesgenerallyaround0.3-0.4 ms-•
(Table 2): more than twice as large as the mean amplitude ofthe diurnal harmonic of zonal wind (see Table 2). Southward
170 W
155W
140W
125W
110W
phase propagation of the diurnal meridional wind harmonic
Figure 3. As in Figure 2 but for the meridional wind is evident at 110ø, 125ø, and 155øW south of IøN.
The amplitudes of the diurnal harmonics obtained in thiscomponent(ms-I). Positive (negative)values indicate
southerly (northerly) winds relative to the daily mean.
study may be compared with those from previous studies.

23,074

Table 2.

DESER: SURFACE WIND CYCLES OVER EQUATORIAL

Amplitude (meters per second) and Phase (LT)of the Diurnaland Semidiurnal

Harmonics

PACIFIC OCEAN

ION

of the Zonaland Meridional Wind Components at Each Buoy During
June-August 1992
Diurnal

DiurnalSemidiurnal

Semidiurnal

Zonal
Wind

Amplitude,
ms-iPhase,
LT

Amplitude,
ms-I

Phase,LT

5øN, 170øW
0øN, 170øW
5øN, 155øW
2øN, 155øW
0øN, 155øW
2øS, 155øW
5øS, 155øW
0øN, 140øW
2øS, 140øW
2øS, 125øW
5øS, 125øW
2øN, 110øW
0øN, 110øW2øS, 110øW

0.14
0.15
0.15
0.10
0.12
0.02
0.03
0.04
0.05
0.14
0.21
0.11
0.19
0.18

1436
1612
1412
1306
1624
1442
0642
1754
0900
1618
1518
0030
2218
1906

0.15
0.17
0.17
0.15
0.09
0.17
0.17
0.03
0.11
0.02
0.10
0.10
0.07
0.03

1606
1542
1400
1548
1430
1512
1512
1406
1530
1412
1518
1542
1530
1312

Diurnal

DiurnalSemidiurnal

Semidiurnal

MeridionalWind

Amplitude,
ms-I

Phase,
LT

Amplitude,
ms-I

Phase,LT

5øN, 170øW
0øN, 170øW
5øN, 155øW
2øN, 155øW
0øN, 155øW
2øS, 155øW
5øS, 155øW
0øN, 140øW
2øS, 140øW
2øS, 125øW
5øS, 125øW
2øN, 110øW
0øN, 110øW2øS, 110øW

0.25
0.12
0.56
0.46
0.17
0.13
0.29
0.22
0.09
0.37
0.28
0.37
0.30
0.31

0730
1124
1318
0932
0642
1230
1706
1000
2124
1236
1554
0930
1036
1312

0.03
0.02
0.29
0.06
0.07
0.05
0.07
0.05
0.02
0.05
0.10
0.14
0.11
0.10

1300
0900
1436
1236
1142
1242
1024
1354
1554
1254
1536
1412
1530
1648

10S

180

170W

160W

150W

140W

130W

120W

110W

100W

180

170W

160W

150W

140W

130W

120W

110W

100W

10N

Nitta and Esbensen [1974] obtained diurnal harmonic ampli-lOS

Figure 5. (a) Vector representation of the amplitude andphase of the diurnal harmonic of the zonal wind component
during June-August 1992. The length of the arrow denotes

the amplitudeof the harmonic(ms-1) and the direction
indicates the phase in local time. An arrow pointing due west
(south, east, north) indicates a westerly wind maximum at
local noon (1800, 2400, 0600). The longest arrow is 0.25

ms-1.(b)Asin Figure5abutforthesemidiurnal
harmonicofthe zonal wind component. The arrow scale is the same forboth panels.

wave is thought to be a manifestation of the thermal tide, due
principally to the absorptionof solar radiation by ozone in
the stratosphereand water vapor in the troposphere [Chapman and Lindzen, 1970]. At the equator during June-JulyAugust the semidiurnal surface pressure cycle may be expressed as [Haurwitz and Cowley, 1973]
P (mbar) = 1.05 sin (2t + 159ø)
where t is local time in degrees. This pressure wave, showngraphically in Figure 7, migrates westward with the Sun,
giving rise to zonal pressure gradients. The zonal wind (u)acceleration due to the semidiurnal pressure wave at theequator can be determined using a linear inviscid balance:

tudesof 0.5ms-1for bothwindcomponents
using9 daysof
hourly wind data from BOMEX in the western Atlantic
trades. Pedder [1978] reported diurnal harmonic amplitudesof 0.23ms-1 for bothwindcomponents
in hisanalysisof 21 10m
days of 6 hourly wind data collected during GATE. Halpern[1988]founddiurnalharmonicamplitudes--•0.18ms- • forthe zonalwindcomponent
and--•0.34ms-1 for the meridi-

0

onal wind component along the equator in the eastern Pacificusing1 year of hourly measurements.
Williamsand Avery 10s[1992]obtainedan amplitudeof 0.15 ms-1 for the diurnal
zonal wind harmonic at Christmas Island (2øN, 157øW) from
4 years of hourly profiler measurements. Hence the results
reported here are compatible with those from previous
studies.

180

170W

160W

150W

140W

130W

120W

110W

100W

170W

160W

150W

140W

130W

120W

110W

100W

10N

0

Semidiurnal Cycle of Zonal Wind
10S

The uniformity of phase of the semidiurnal cycle of zonal
wind is striking (Figure 5b). It is well known that surfacepressure in the tropics undergoes a strong semidiurnal variation [cf. Haurwitz and Cowley, 1973]. The local phase andamplitude of the semidiurnal pressure wave are approximately uniform within 20ø of the equator, with pressure
maximima at 0942 and 2142 LT and amplitude --•1 mbar
[Haurwitz and Cowley, 1973;Hamilton, 1981]. This pressure

180

Figure 6. (a) As in Figure 5a but for the diurnal harmonicof the meridional wind component. An arrow pointing due
west (south, east, north) indicates a southerly wind maximum at local noon (1800, 2400, 0600). The longest arrow is

0.6 ms-1 (b) As in Figure 6a but for the semidiurnal.

harmonic of the meridional wind component. The arrow
scale is the same for both panels.

DESER: SURFACE WIND CYCLES OVER EQUATORIALPACIFIC OCEAN

23,075

1.0

o.o

13bw

-1
.o • • •
0

6

12

18

24

Hour (local)

Figure 7. Semidiurnal cycle of sea level pressure (millibars) at the equator in local time [from Haurwitz andCowley, 1973].

12ow

110W

100W

Figure 9. Distribution of outgoing longwave radiation(OLR) (watts per square meter) during June-August 1992.
The contour interval is 10 W m -2 and values <240 W m -2
are shaded. The rectangle indicates the region for whichsurface wind divergence is calculated.

tropical ocean exhibits a strong diurnal cycle (amplitude---1-2mmd-]), with theheaviestprecipitation
occurringin&u/&t - -l/p(&P/&x)(1)

According to (1) the zonal wind will be a maximum (minimum) at 0400 and 1600 (1000 and 2200) LT when the pressure
gradient vanishes. The predicted zonal wind is compared totheobservedsemidiurnal

variationof zonal

windin theequatorial Pacific in Figure 8. To obtain a robust estimate of
the observed semidiurnal cycle of zonal wind, the semidiurnal zonal wind harmonics at each buoy were averaged foreach latitude band (5øN, 2øN, 0ø, 2øS, 5øS) and then averaged
together. Figure 8 shows there is good agreement between
the observed and the predicted semidiurnal zonal winds atthe equator. However, the amplitude of the predicted wind islarger than observed, and the phase is lagged by 1 hour.
The observed lack of a semidiurnal cycle in the meridional
wind component (see Figure 3) is consistentwith the lack of
meridional variation in the semidiurnalpressurewave in theequatorial zone.
Daily Cycle of Surface Wind Divergence and Its Relation
to Convectionin the ITCZ

Studies by Gray and Jacobsen [1977] and Hendon andWoodbury [1993] have shown that rainfall over the open

i

i

i

0.1

0.0
-0.1

-0.2

0

6

12

18

diurnalvariationthat is consistentwiththat in the convec-

tion. Specifically, the circulation should be strongestduring
early morning when the convection is most active andweakest during early evening.
Figure 9 shows the distribution of outgoing longwave
radiation (OLR), a proxy for deep convection, over thetropical eastern Pacific during summer 1992. Convection in
the ITCZ was active during summer 1992, as evidenced bythelowOLR values(<240 W m-2) along---8øN.Winddata

0.2

E

the early morning (0600-0900 LT). (Hendon and Woodbury[1993] inferred the diurnal variation of rainfall from an index
of deep convective activity based on outgoing longwave
radiation.) The Intertropical Convergence Zone (ITCZ)
along ---8øN in the eastern Pacific exemplifies this behavior[Hendon and Woodbury, 1993]. Mechanisms responsible forthe diurnal rainfall variations have been proposed by Grayand Jacobsen [1977] and Randall et al. [1991]. Gray andJacobsen [ 1977] emphasize the role of day-night variations intropospheric longwave cooling in clear versus cloudy regions, while Randall et al. [1991] advocate the role ofday-night variations in cloud top heating and cooling. In
simplest terms, deep convection in the ITCZ is associatedwith upward motion and low-level convergence locally andwith large-scale compensating subsidence and low-level divergence to the north and south (i.e., a local Hadley circulation). This meridional circulation cell should exhibit a24

Hour (local)

Figure 8. Observed (open circles) and predicted (solid
circles) semidiurnal variation of the zonal wind component(ms-] ) at theequatorin localtime.Theobserved
windis anaverage for 5øN-5øS. The predicted wind is determined fromthe zonally averaged semidiurnalcycle of sea level pressureusing a linear inviscid balance.

from buoys within the region (2øN-2øS, 155øW-l10øW; seerectangle in Figure 9) were used to compute the mean daily
variation of surface wind divergence during summer 1992.
Note that the area used for the divergence calculation lies tothe south of the ITCZ, in the region of compensating
subsidence. Surface wind divergence south of the ITCZ
exhibits a diurnal cycle, with divergence in the early morningand convergence in the early evening relative to the dailymean (Figure 10a). The phase of the diurnal cycle in surface
wind divergence is thus consistent with that of deep convection in the ITCZ to the north. The mean daily range of theequatorialsurfacewinddivergenceis ---1.5x 10-6 s-] or---30%of the dailymeandivergence(5.4 x 10-6 s-l). The
divergence is determined mainly by the meridional windcomponent (not shown).
The daily marches of meridional wind at 2øN and 2øS (thenorthern and southern boundaries of the region used to
calculate the divergence) are shown in Figure 10b. The

23,076

DESER: SURFACE WIND CYCLES OVER EQUATORIAL
10

I

-lOo

I

I

I

PACIFIC OCEAN

the strongest convection at 0800 LT, and no shading for the
weakest convection at 0200 LT. At 0200 LT (Figure 11, top)
the wind vectors are mostly northerly relative to the daily
mean. By 0800 LT (Figure 11, top middle), when convectionin the ITCZ is near its peak, the winds north of 2øS havebecome southerly (except at 5øN, 155øW). Equatorial divergence is clearly visible at this time. At 1400 LT (Figure 11,
bottom middle), almost all of the wind vectors are southerly.
By 2000 LT (Figure 11, bottom), when convection in the
ITCZ is at a minimum, the winds north of 2øS have become
northerly, resulting in convergence along the equator.

I

SeasonalDependenceof the Diurnal Wind Variations at 0%110øW and 0ø, 140øW
6

12

18

24

30

36

Figure 12a shows the mean monthly amplitudes of thediurnal harmonic of meridional wind at 0ø, 110øW and 0ø,140øW, based on the period 1984-1992. Both sites show asimilar seasonal cycle, with the smallest diurnal amplitude.5
2N(0.05ms-l ) in spring(February-March-April)andthelargestdiurnalamplitude(0.35ms-i ) in summerandautumn.
The seasonalcycle of the diurnal meridional wind amplitude
is compared to the seasonal cycle of OLR in the ITCZ

180

170W

160W

150W

140W

130W

120W

110W

100W

.............
•'•;•:•:.
•..-.•.,:.::::.::..•:•:e•..................
•,:•:.5.'•
'*

1

-.5

0

6

12

18

24

3O

36
2

Hour (localtime)
Figure 10. (a) Daily march of surfacewind divergence 10N(10-7 S-l) duringJune-August
1992fortheregion(2øN-2øS,
155øW-l10øW). Times are local, with the first 12 hours
repeated for clarity. The daily mean divergence has been

removed.(b) Dailymarchof surfacemeridionalwind(ms-l)01

at 2øN and 2øS during June-August 1992. The winds havebeen averaged over the longitude band 155ø-110øWand thedaily means removed. Times are local, with the first 12 hours
repeated for clarity.

lot4

winds have been averaged over the longitude band 155ø-01

1i0øw. The diurnalcycleof meridionalwindat 2øNleads
that at 2øSby --•4 hours, and the amplitude is approximately
twice as large.
The diurnal amplitude of the surface wind divergence iswithin the range of previous studies: Nitta and Esbensen

--..

x,

10s

14
10S

10N

[1974]obtained
a diurnalamplitudeof 2 x 10-6 s-1- Pedder[ 1978] and Wallace and Hartranft [ 1969] reported amplitudesof 0.4 x 10-6 s-1 and0.5 x 10-6 s-1 respectively'
andGrayandJacobsen[1977]obtained
anamplitudeof 4 x 10-6
s-• for dayswithdeepconvection.
Thephaseof thesurface
winddivergenceis alsosimilarto thatobtainedby Nitta andEsbensen [ 1974], Gray and Jacobsen [1977], and Albright etal. [1981].
Figure 11 shows the mean wind vectors during summer
1992 at 0200, 0800, 1400 and 2000 LT. The wind vectors are

deviations from the daily mean and represent 3-hour aver-agescenteredon the timesgivenabove.The 240 W m-2
OLR

contouris also shown.

The diurnal

variationin con-vection is indicated schematically, with heavy shading for

10S

180

170W

160W

150W

140W

130W

120W

1 I 0W

100W

Figure 11. Mean wind vectors during June-August 1992at
0200, 0800, 1400, and 2000 LT.

The wind vectors are

deviations from the daily mean and represent 3-hour averagescentered on the times given above. The longestarrow is

0.8 ms-•, andthearrowscaleis thesamefor all panels.The
240 W m -20LR contour is also shown. The diurnal variation in convection is indicated schematically, with dark
shading for the strongest convection at 0800 LT and noshading for the weakest convection at 2000 LT

DESER: SURFACE WIND CYCLES OVER EQUATORIAL(Figure 12b). OLR was averaged over the area (9ø-13øN,
150ø-100øW]and the period 1984-1992. The meridional windrecord shown in Figure 12b is the average of the diurnal
harmonics at 0ø, 110øW and 0ø, 140øW. The seasonal cycles
of both parameters are remarkably similar, with the largest
diurnal wind amplitude occurring when convection is strongest and vice versa. While this comparisondoes not directly
test the hypothesisthat diurnal variations in deep convection
are associated with diurnal variations in surface wind, it issuggestive, particularly in light of evidence presented byGray and Jacobsen [1977], that the amplitude of the diurnal
cycle in rainfall is proportional to the daily mean rainfall. A
similar seasonaldependenceof the daily surface wind variations at 0ø, 150øEwas reported by Morrissey [1990].
The mean monthly amplitudes of the diurnal and semidiurnal harmonics of zonal and meridional wind at 0ø, 110øW
and 0ø, 140øW for 1984-1992 are listed in Table 3. None ofthe other harmonics

exhibits as dramaticas the diurnal harmonicof meridional

harmonic

windof meridionala seasonal variation

wind. The semidiurnal

is several

times smaller

modest(-0.05-0.15 ms-1) in all months.Note that theamplitude of the diurnal zonal wind harmonic at 0ø, 110øW
i

i

i

I

i

i

i

i

I

i

i

I

i

a

m

23,077

Table 3. Mean Monthly Amplitudes (meters per second)of the Diurnal and Semidiurnal Harmonics of Zonal (U)and Meridional Wind (V) at 0ø, 110øW and 0ø, 140øW
for 1984-1992

U (0ø, 11oøw)
Diurnal,ms-1

v (0ø, 11oow)

Semidiurnal,
ms

Diurnal,

-1

ms

-1

Semidiurnal,
ms-1

January
February

0.08
0.07

0.07
0.10

0.25
0.05

0.16
0.07

March

0.16

0.08

0.05

0.02

April
May

0.23
0.14

0.09
0.02

0.06
0.24

0.02
0.08

June

0.08

0.03

0.32

0.10

July
AugustSeptember

0.12
0.13
0.10

0.07
0.05
0.03

0.36
0.37
0.42

0.07
0.09
0.08

OctoberNovember
December

0.07
0.12
0.11

0.05
0.06
0.07

0.36
0.16
0.21

0.12
0.11
0.13

than

the diurnal harmonic in all months (except spring, when theamplitudesare negligible). The amplitudesof the diurnal andsemidiurnal harmonics of zonal wind are comparable andi

PACIFIC OCEAN

U (0ø, 140øW)

January
February

V (0ø, 140øW)

Diurnal,Semidiurnal,

Diurnal,Semidiurnal,

ms-1

ms-1

ms-1

ms-•

0.10
0.06

0.07
0.07

0.27
0.16

0.04
0.02

March

0.10

0.06

0.06

0.08

April
May
June
July
AugustSeptember

0.11
0.06
0.02
0.08
0.11
0.09

0.08
0.10
0.10
0.07
0.09
0.09

0.08
0.20
0.31
0.33
0.37
0.31

0.02
0.03
0.03
0.02
0.03
0.05

OctoberNovember
December

0.04
0.01
0.04

0.09
0.11
0.05

0.26
0.27
0.20

0.06
0.12
0.06

duringsummer1992(0.19 ms-1) was considerablylargerthanthelong-termmean(0.11ms-1 Table3)
,

b

4.

Summary

Daily variations of surface wind and wind divergence overthe equatorial eastern Pacific during summer 1992 have been
documented. The zonal wind component exhibits diurnaland semidiurnal variations of a few tenths of a meter per
second.The phase of the semidiurnal cycle is approximately
uniform over the domain, with westerly wind maxima at--•0300 and 1500 LT. The diurnal cycle dominates the dailymarchof themeridional
windcomponent.
The rangeof thediurnal meridional wind variations is •0.6-0.8 ms-1 at most
locations: more than twice as large as the daily zonal wind
J FMAMJJ ASONDJ
FMAchanges. The low-level flow is generally southward acrossFigure 12. (a) Average seasonalcycle of the amplitude of the equator at night (relative to the daily mean), regardlessof
thediurnalharmonicof meridionalwind(ms- 1)at 0ø,110øW whether the mean wind is southerly or northerly. There is(solid circles) and 0ø, 140øW (open circles), based on the someevidencei•ora southward
progressionof thediurnal
period 1984-1992. The first four months have been repeated cycle of meridional wind.
for clarity. (b) Average seasonalcycle of the amplitude of the
The diurnal meridional wind variations along the equator
diurnal harmonic of meridional wind at 0ø, 110øW and 0ø,
may be related to the diurnal cycle of deep convection in the140øWaveraged together (solid circles) and OLR in the ITCZ
(9ø-13øN, 150ø-100øW)(open circles). Both records are stan- ITCZ to the north. In particular, surface wind divergencealong the equator (which is dominated by the meridional
dardized
and the OLR data have been inverted
so thatpositive (negative) values correspond to stronger (weaker) component) exhibits a pronounced diurnal cycle, with thestrongest divergence in the early morning when deep conthan average deep convection. Both the wind and the OLR
vection in the ITCZ is at a maximum. The average daily
records are based on the period 1984-1992.

23,078

DESER: SURFACE WIND CYCLES OVER EQUATORIAL PACIFIC OCEAN

rangeof theequatorialsurface
winddivergenceis 1.5x 10-6s- • or -•30%of thedailymean.
The semidiurnal cycle in zonal wind is dynamically consistent with the observed zonally uniform semidiurnal cyclein surfacepressureat the equator. The semidiurnalpressure
wave is thought to be a manifestation of the atmospheric
thermal tide [Chapman and Lindzen, 1970].
The TAO array is continuingto provide an unprecedented
data set for the study of daily surface wind variations overthe equatorial Pacific. The results presented here are basedon a relatively short (92 days) period of record and sparse
network of observations.A more comprehensiveassessmentof the daily wind variations is planned when the completed
TAO array has been operational for several years.

Halpern,D., Mooredsurfacewindobservati9ns
at four sitesalong
the Pacific equator between 140ø and 95øW, J. Clim., 1, 12511260, 1988.

Hamilton, K., Latent heat release as a possible forcing mechanism
for atmospherictides, Mon. Weather Rev., 109, 3-17, 1981.
Hastenrath, S. L., Daily wind, pressure and temperature variation
up to 30 km over the tropical western Pacific, Q. J. R. Meteorol.
$oc., 98, 49-59, 1972.
Haurwitz, B., and A.D.
Cowley, The diurnal and semidiurnal
barometric pressure oscillations: Global distribution and annual
variation, Pure Appl. Geophys., 102, 193-222, 1973.
Hayes, S. P., L. J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi,
TOGA-TAO: A moored array for real-time measurementsin thetropical Pacific Ocean, Bull. Am. Meteorol. Soc., 72, 339-347,
1991.

Hendon, H. H., and K. Woodberry, The diurnal cycle of tropicalconvection, J. Geophys. Res., 98, 16,623-16,637, 1993.
McBride, J. L., and W. M. Gray, Mass divergence in tropical
weather systems, I, Diurnal variation, Q. J. R. Meteorol. Soc.,

106, 501-516, 1980.
Acknowledgments.i thank Michael J. McPhaden,directorof the
TOGA-TAO project, for his continuinginterest in and support of McPhaden, M. J., TOGA-TAO and the 1991-93 E1 Nino-Southern
Oscillation event, Oceanography, 6, 36-44, 1993.
this study. Linda Stratton, Linda Mangum, and Paul Freitag provided the wind data and crucial information on the hourly averaging McPhaden, M. J., and S. P. Hayes, Variability in the easternequatorialPacificduring 1986-1988, J. Geophys.Res., 95, 13,195procedures. I also thank William Gray for informative discussions
13,208, 1990.
on radiative forcing of the diurnal cycle and John M. Wallace andMorrissey, M. L., An evaluation of ship data in the equatorial
Harry Hendon for their valuable comments on an earlier draft of thewestern Pacific, J. Clim., 3, 99-112, 1990.
manuscript.I gratefully acknowledgethe hospitalityof T. VonderNitta, T., and S. Esbensen, Diurnal variations in the western
Haar and CIRA, Colorado State University, where much of thisAtlantic Trades during BOMEX, J. Meteorol. Soc. Jpn., 52,
work was completed. This study was supported by a grant fromEPOCS.

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C. Deser, CIRES, University of Colorado, Campus Box 449,
Boulder, CO 80309.

(Received March 15, 1994' revised August 15, 1994;
accepted August 17, 1994.)