1730

JOURNAL OF CLIMATEVOLUME 11

Diurnal and Semidiurnal Variations of the Surface Wind Field over the
Tropical Pacific Ocean
CLARA DESER* AND CATHERINE A. SMITH
CIRES, University of Colorado, Boulder, Colorado(Manuscript received 25 March 1997, in final form 17 August 1997)
ABSTRACTThe climatological large-scale patterns of diurnal and semidiurnal near-surface wind variations over the tropical
Pacific Ocean are documented using 3 yr of hourly measurements from the Tropical Atmosphere–Ocean mooredbuoy array. Semidiurnal variations account for 68% of the mean daily variance of the zonal wind component,
while diurnal variations account for 82% of the mean daily variance of the meridional wind component. The
spatially uniform amplitude (0.15 m s21 ) and phase (0300 LT) of the semidiurnal zonal wind variations areshown to be consistent with atmospheric thermal tidal theory.
The diurnal meridional wind variations on either side of the equator are approximately out of phase. Thispattern results in a diurnal variation of wind divergence along the equator, with maximum divergence in theearly morning (;0800 LT). The average amplitude of the diurnal cycle in zonal mean divergence is 0.45 31026 s21 , which corresponds to a day–night change of 45% relative to the daily mean. The relative day–nightchanges in near-surface equatorial wind divergence are larger in the western Pacific (78%) than in the eastern
Pacific (31%) due mainly to differences in the daily mean divergence. The diurnal amplitude of equatorial winddivergence changes seasonally and interannually in proportion to the strength of the mean divergence.
It is suggested that diurnal heating of the sea surface may contribute to the zonally symmetric diurnal cycleof equatorial wind divergence.

1. Introduction
Tropical meteorologists have long noted the existenceof daily1 variations in a variety of weather elements,
including cloudiness, rainfall, and barometric pressure
over the open ocean. Semidiurnal fluctuations have been
identified most prominently in sea level pressure records(Haurwitz 1956; Haurwitz and Cowley 1973; Hamilton
1980), but also in tropospheric zonal winds (Nitta andEsbensen 1974; Pedder 1978; Morrissey 1990; Williams
et al. 1992), light rain, and associated stratiform cloudiness (Rickenbach and Rutledge 1997, manuscript submitted to J. Atmos. Sci., hereafter RR97). The classicaltheory of the atmospheric thermal tide accounts wellfor the zonally uniform phase and amplitude of the semidiurnal pressure changes within the Tropics (Lindzen
1968; Chapman and Lindzen 1970).

* Current affiliation: National Center for Atmospheric Research,
Boulder, Colorado.
1
In this study, the term ‘‘daily’’ refers to all variations within a
24-h period, and the adjectives ‘‘diurnal’’ and ‘‘semidiurnal’’ referto the first and second harmonics of the daily cycle, respectively.

Corresponding author address: Dr. Clara Deser, Climate and Global Dynamics Division, NCAR, P.O. Box 3000, Boulder, CO 803073000.
E-mail: cdeser@ucar.edu

q 1998 American Meteorological Society

Diurnal variations over the tropical oceans are strongly apparent in heavy rainfall and deep convective cloudiness, with an early morning maximum in most regions(cf. Ruprecht and Gray 1976; Gray and Jacobson 1977).
The mass circulations associated with the diurnallyvarying deep convective systems also exhibit day–nightchanges, with strongest ascent within the cloud clusterand descent in adjacent clear regions in the early morning (McBride and Gray 1980). Recently, the large-scale
nature of the diurnal rainfall cycle over the tropicaloceans has been revealed by satellites (cf. Hendon andWoodbury 1993; Janowiak et al. 1994). These studies
show that the regions of time-mean deep convection(e.g., the intertropical and South Pacific convergencezones) exhibit the most pronounced diurnal variability.
Several complementary mechanisms have been proposed to explain the diurnal cycle in deep convective
rainfall over the tropical oceans including day–nightvariations in tropospheric radiative cooling rates (Grayand Jacobsen 1977; Foltz and Gray 1979), cloud-radiation interactions (Randall et al. 1991; Xu and Randall
1995), and daytime heating of the sea surface (Hendon
and Woodberry 1993; Chen and Houze 1997). However,
the relative importance of these processes has not yet
been resolved.
With the exception of the satellite-based estimates ofrainfall, the large-scale spatial patterns of daily atmospheric circulation variations over the tropical oceans

1731

227/94–132/96
634/635
69/93–135/96
1121/1161
1/93–253/96
875/1347
1/93–217/96
0/1311
1/93–217/96
1306/1311
1/93–354/95
807/1083
1/93–188/96
1203/1282
88S

326/93–334/95
737/738

114/94–291/96
457/907
1/93–208/96
810/1302
1/93–74/95
624/803
1/93–249/96
1335/1343
1/93–88/96
1025/1182
1/93–212/96
836/1306
110/94–122/95
378/377
58S

84/93–125/95
608/771

1/93–130/96
1219/1224
1/93–284/96
1094/1378
1/93–239/96
1234/1333
1/93–251/95
902/980
307/93–219/96
827/1007
1/93–346/95
604/1075
1/93–191/96
1112/1285
28S

85/93–180/96
1133/1190

1/93–293/96
743/1387
129/93–222/95
641/823
1/93–174/96
1259/1268
1/93–252/95
972/981
1/93–360/95
1081/1089
1/93–209/96
1289/1303
1/93–104/94
458/46808

86/93–336/95
977/980

1/93–134/96
412/1228
73/93–236/96
836/1258
124/93–45/96
1013/1016
117/93–80/95
469/693
1/93–97/95
823/826
1/93–258/95
877/987
1/93–26/96
790/1120
28N

87/93–179/96
1083/1187

73/93–280/96
885/1302
59/93–40/96
925/1076
290/93–243/96
926/1048
1/93–183/95
907/912
1/93–206/96
1292/1300
1/93–194/96
1282/1288
58N

88/93–15/96
917/1022

958W
1108W

75/93–279/96
961/1299
126/93–260/96
1225/1229

1258W
1408W

1/93–82/95
707/811304/93–205/95
466/631

1558W
1708W

1/93–343/95
978/1072
334/93–340/95
574/736

1808
1658E

The TAO array was designed to monitor upper-ocean
thermal conditions and surface atmospheric properties
for improved prediction of the El Niño–Southern Oscillation cycle (Hayes et al. 1991; McPhaden 1993).
Beginning with two buoys in late 1983, the array was
fully implemented in December 1994 with 70 moorings
distributed within 88 of the equator and spaced approximately every 158 of longitude across the Pacific basin(see McPhaden 1993). Winds are sampled at 2 Hz for
6 min every hour centered on the hour at ;3.8 m above
the sea surface using cup-and-vane or propeller-andvane assemblies [see McPhaden and Hayes (1990) andHayes et al. (1991) for more details]. The expected instrumental errors are typically ;0.1 m s21 , with maximum calibration residuals less than 0.2 m s21 for any
particular instrument, based on pre- and postdeploymentcalibration tests (McPhaden and Hayes 1990; Freitag etal. 1991). The hourly wind data are recorded on cassettetape or solid-state memory devices on board the buoys;
the data archives are retrieved every 6 months to 1 yr
and processed at the TAO project office at the Pacific
Marine Environmental Laboratory in Seattle, Washington.
This study uses data from the period 1 January 1993–
31 December 1996. The record lengths are highly variable from buoy to buoy, with frequent gaps within eachrecord (Table 1). The average record length is 895 days
(approximately 2.5 yr), with a maximum of 1335 days
and a minimum of 412 days. Although it is desirable
to use a common period of record for all the buoys, we
felt it was more important to include as many sites as
possible since our emphasis is upon the large-scale spatial patterns of daily variability. To minimize any potential aliasing of the seasonal cycle and interannualchanges, the average daily march was computed for each

1/93–313/95
1038/1042

2. Data and methods

88N

are not known. An unprecedented observing network
now exists for the documentation of daily variations ofnear-surface winds across the tropical Pacific: the Tropical Atmosphere–Ocean (TAO) moored buoy array. In
this study, approximately 3 yr of hourly measurementsfrom nearly 60 buoys between 1658E and 958W are used
to document the large-scale climatological daily variations in the low-level atmospheric circulation. We address the following issues. What are the spatial patternsof the climatological daily wind variations across thetropical Pacific? Are there associated large-scale dailychanges in horizontal wind divergence? What is themagnitude and phase of the climatological daily windand divergence variations? Are there seasonal and interannual modulations of the daily wind patterns? What
are the relative contributions of the diurnal and semidiurnal harmonics to the mean daily wind cycles? Apilot study by Deser (1994) used a limited subset of theTAO data to examine similar issues in the eastern equatorial Pacific.

267/94–267/96
675/730

DESER AND SMITH
TABLE 1. Record length information for the TAO buoy wind data used in this study. For each buoy, the top entry lists the start and end dates (day/year), and the bottom entry lists thetotal number of days with data followed by the total number of days in the record (separated by a ‘‘/’’).

JULY 1998

1732

JOURNAL OF CLIMATEVOLUME 11

FIG. 1. (top) Mean near-surface wind vectors during 1993–96 from the TAO array superimposed upon the mean distribution of deep
convection as inferred from OLR values less than 240 W m22 (shading). The wind vector scale is indicated at the lower right (7 m s 21 ).
(bottom); divergence of the mean wind vectors above. Contour interval is 2 3 1026 s21 , with negative values dashed and the zero contour
thickened.

2-month segment, from January–February 1993 to November–December 1996. Then, the mean daily marches
from the three years were averaged for each bimonth.
Finally, the resulting daily marches from the six ‘‘longterm mean’’ bimonths were averaged together to form
the mean daily march during 1993–96.
3. Resultsa. Mean daily march
Figure 1 shows the mean surface wind vectors during
1993–96 (top) and their divergence (bottom). Easterlies
are present across the basin, with anticyclonic flow
around the South Pacific subtropical high and northeast
trade winds along 88N. The low-level flow is strongly
divergent along the equator (maximum values ;6 31026 s21 ) and convergent north of ;58N [along the intertropical convergence zone (ITCZ)] and in the far southwestern Pacific (the South Pacific convergence zone;
SPCZ). The locations of the wind convergence zones
are qualitatively consistent with the mean distributionof deep convection as inferred from outgoing longwave

radiation (OLR) values less than 240 W m22 (Fig. 1,
top).
The average daily march (in local time) of the zonal
wind component at each buoy is shown in Fig. 2a. The
mean standard error of an hourly wind average is 0.09
m s21 . Semidiurnal fluctuations are evident at most locations, with peak-to-peak amplitudes of a few tenthsof a m s21 . The phase of the semidiurnal zonal windvariations is nearly uniform across the basin, with maxima around 0300–0400 and 1500–1600 local time (LT).
Diurnal variations are evident at the three easternmost
equatorial sites.
The average daily march of the meridional wind component (Fig. 2b) exhibits strong diurnal fluctuations withlittle semidiurnal variability. At many locations, therange of daily meridional wind variations exceeds 0.7
m s21 , more than twice the daily range of the zonal windvariations. A general tendency for morning maxima
north of the equator and evening maxima south of theequator can be noted. There does not appear to be any
simple relationship between the phase of the diurnal
cycle of the meridional wind and the direction of themean meridional wind component. For example, along

JULY 1998

DESER AND SMITH

1733

and cross spectra. Figure 3a shows the power spectrum
for the meridional wind component at 28N, 1258W (top)and 88S, 1108W (middle), and the squared coherence
between them (bottom). Only submonthly timescales areshown. Analogous results for the zonal wind component
are shown in Fig. 3b. The spectra are based on 1016
days of data (4 May 1993–13 February 1996), with five
2-day periods of missing data filled by linear interpolation.
The meridional wind spectra (Fig. 3a) exhibit significant peaks at the diurnal and synoptic (4–5 day) frequencies. However, the squared coherence is significant
only at the diurnal frequency. The synoptic peak may
be associated with mixed Rossby–gravity waves, a ubiquitous feature of the near-equatorial meridional windfield (Liebmann and Hendon 1990).
The zonal wind spectra (Fig. 3b) exhibit a generalenhancement of power at timescales longer than abouta week. A modest spectral peak is present at the semidiurnal frequency (and at the diurnal frequency at 88S,
1108W). The squared coherence between the two zonal
wind records is significant at the 99% level at the semidiurnal frequency and periods longer than about one
week.
Power spectra for numerous other buoy wind records(not shown) confirm that significant diurnal and/or semidiurnal spectral peaks are present when the amplitudeof the mean daily cycle (see Fig. 2) exceeds the mean
standard error (approximately 0.1 m s21 ).
c. Harmonic analysis
FIG. 2. (a) Average daily march of the zonal wind component during
1993–96 from the TAO array. All boxes are drawn to the same scale,with local time on the abscissa (0–48 h) and wind speed on theordinate (20.6 to 0.6 m s21 ), as shown in the lower-left box. The
daily mean zonal winds have been removed from each site, and thehourly values smoothed with a three-point binomial filter. The 24-h
period is repeated for clarity. Positive (negative) values indicate westerly (easterly) winds relative to the daily mean. (b) As in (a) but forthe meridional wind component. Positive (negative) values indicate
southerly (northerly) winds relative to the daily mean.

88S, the maximum southerly winds occur near 1800 LT,
although the mean meridional winds change from southerly in the east to northerly in the west (recall Fig. 1).
A scatterplot of the phase of the diurnal cycle of meridional wind versus the mean meridional wind velocity(not shown) confirms the lack of a simple relationship:
the linear correlation coefficient between these twoquantities is not significantly different from zero.
b. Sample power spectra
Do the wind records exhibit distinct spectral peaks at
the diurnal and/or semidiurnal frequencies, or are thedaily fluctuations part of a continuum of high-frequency
variability? We selected two buoys with long continousrecords to illustrate the nature of the wind power spectra

The wind records shown in Figs. 2a and 2b were
decomposed into diurnal and semidiurnal harmonics according to
X k (t) 5 A k cos(pk/12t) 1 B k sin(pk/12t),
where X is the deviation of the zonal or meridional windcomponent from the daily mean, the index k 5 1 (k 52) refers to the diurnal (semidiurnal) harmonic, and t is
time in hours. Here A k and B k were determined by themethod of least squares (cf. Draper and Smith 1966).
The amplitudes and phases of the diurnal and semidiurnal wind harmonics at each buoy are listed in Table
2. Figure 4a (b) shows the amplitude and phase of thediurnal (semidiurnal) harmonic of the zonal wind component in vectorial format. The length of the arrow denotes the amplitude of the harmonic and the direction
indicates the phase of the westerly wind maximum inlocal time. An arrow pointing due west indicates a local
noon westerly maximum, with time increasing in the
counterclockwise direction. Note that for the semidiurnal harmonic, an arrow oriented in the east–west direction indicates zonal wind maxima at midnight andnoon local time.
The diurnal harmonic of the zonal wind component(Fig. 4, top) is generally weaker than the semidiurnal
harmonic except at the three easternmost equatorial sites

1734

JOURNAL OF CLIMATEVOLUME 11

FIG. 3. (a) Power spectrum for the meridional wind component at 28N, 1258W (top) and 88S, 1108W (middle) based on 1016 days. The
spectra are shown in variance conserving form: power spectral density multiplied by frequency along the ordinate, and log (period) in days
along the abscissa. Only periods less than 1 month are shown. The thin dashed lines represent the spectra for the appropriate red noisebackground continuum and its 99% a priori confidence limit. The bottom panel shows the squared coherence between the two wind records.
The thin dashed line is the 99% a priori significance level. (b) As in (a) but for the zonal wind component.

JULY 1998

1735

DESER AND SMITH(see also Table 2), and shows no coherent spatial pattern.
The average amplitude of the diurnal harmonic is 0.08
m s21 compared to 0.15 m s21 for the semidiurnal harmonic. The semidiurnal (diurnal) harmonic accounts for
68% (25%) of the total variance of the long-term dailymarch of the zonal wind component over the basin.
The semidiurnal harmonic of the zonal wind component is remarkably uniform in both phase and amplitude across the basin (Fig. 4, bottom; Table 2). The
average phase (time of maximum westerly wind) 6 one
standard deviation is 0310 and 1510 LT 6 30 min, andthe average amplitude is 0.15 m s21 6 0.04 m s21 . The
amplitude is somewhat larger in the west (0.18 m s21
for 1608E–1708W) than in the east (0.12 m s21 for 1408–
1108W).
Figure 5 shows the results of harmonic analysis forthe meridional wind component. An arrow pointing due
west indicates a southerly wind maximum at local noon,with time increasing in the counterclockwise direction.
The dominance of the diurnal harmonic over the semidiurnal harmonic is readily apparent, consistent with
Fig. 2. The average amplitude of the diurnal (semidiurnal) harmonic is 0.19 m s21 6 0.09 m s21 (0.07 m
s21 6 0.03 m s21 ). The diurnal (semidiurnal) harmonic
accounts for 82% (12%) of the total variance of thelong-term daily march of the meridional wind component over the basin.
The diurnal meridional wind harmonic displays a distinctive spatial pattern, which in broadest terms may bedescribed as a morning (0600–0900 LT) southerly windmaximum north of the equator, and an early evening(1800–2000 LT) southerly wind maximum south of theequator. Some differences may be noted between thewestern and eastern halves of the domain, particularily
in the equatorial zone where the phase transition occurs.
West of ;1508W, the diurnal cycle is weak along thelatitude of the phase transition (28S); east of ;1508W,
the diurnal cycle is large in the equatorial zone, andcharacterized by a southerly wind maxima near local
noon, making the phase transition more gradual than in
the western Pacific.
The semidiurnal harmonic of the meridional windcomponent (Fig. 5b) exhibits a general pattern of crossequatorial asymmetry (as evidenced by the horizontal
bars north of the equator and vertical bars south of theequator), as well as a quadrature relationship to the zonal
wind semidiurnal cycle (Fig. 4b). As discussed below,
these phase relationships are consistent with the notionthat the semidiurnal meridional winds result from theaction of the Coriolis force.
The large-scale spatial patterns of the diurnal andsemidiurnal wind harmonics are similar for the two solsticial seasons (not shown) and closely resemble thosefor the annual mean shown above.
The mean daily march of wind speed (not shown) is
nearly identical to that of the zonal wind component,
despite the generally lower amplitude of the mean daily
cycle of the zonal versus meridional winds. The simi-

larity of the mean daily marches of wind speed and zonal
wind is due to the fact that the contributions of the zonal
and meridional wind daily cycles to the wind speed daily
cycle are weighted by their daily mean magnitudes (ascan be shown formally with a Taylor series expansion),with the mean zonal winds being generally stronger thanthe mean meridional winds.
d. The semidiurnal wind cycle
It is well known that sea level pressure in the Tropics
undergoes a strong semidiurnal variation, with amplitude ;1 mb and maximima ;0945 and 2145 LT (cf.
Haurwitz and Cowley 1973; Hamilton 1980). This semidiurnal pressure cycle can be explained on the basis of
atmospheric tidal theory (Chapman and Lindzen 1970).
Briefly, atmospheric thermal tides are caused by heating
due to the absorption of solar radiation by ozone in thestratosphere and to a lesser extent by water vapor andclouds in the troposphere. The ozone and water vapor
heating excites vertically propagating modes that carry
the signal from the stratosphere and upper troposphereto the ground. The surface tidal pressure signal is predominantly semidiurnal because 80% of the diurnal
driving goes into modes whose energy is trapped near
the level of forcing (Lindzen 1967).
Haurwitz and Cowley (1973) document the geographical pattern and seasonal variation of the semidiurnalpressure wave based on station data (note that a handfulof island stations form the basis for their description
over the tropical oceans). They show that the phase andamplitude of the semidiurnal pressure wave are approximately uniform within 208 of the equator, withslightly larger amplitude over the continents relative tothe oceans. [The slight reduction in amplitude of theobserved semidiurnal zonal wind fluctuations between
1408W and 1108W (recall Fig. 4b) is qualitatively consistent with the semidiurnal pressure field shown in
Haurwitz and Cowley (1973), and may be related to theremoteness of this region from continental influences.]
The annual mean semidiurnal surface pressure cycle
may be expressed as (Haurwitz and Cowley 1973)
P 5 1.05 · cos 3 (u) · sin(2t 1 1598),(1)

where u is latitude, t is local time in degrees, and theamplitude (1.05 mb) is characteristic of that over thetropical Pacific Ocean (see Haurwitz 1956). The 1598
phase lag corresponds to pressure maxima at 0942 and2142 LT. The pressure wave, shown graphically in Fig.
6, migrates westward with the sun, giving rise to zonalpressure gradients. The zonal wind (u) acceleration due
to the semidiurnal pressure wave near the equator may
be determined using a linear inviscid balance:

du/dt 5 21/r(dP/dx),(2)where the symbols have their conventional meanings.
Note that the zonal wind is out of phase with the pressure
field and in quadrature with the pressure gradient field.

1736

JOURNAL OF CLIMATEVOLUME 11

TABLE 2a. Amplitude, phase, and percent variance explained by the diurnal and semidiurnal harmonics of the mean daily march of the zonal
wind component. For each buoy, the top entry lists the amplitude (m s21) and phase (hour and tenths in LT; separated by a ‘‘/’’) of the diurnal
harmonic, the middle entry the amplitude (m s21) and phase (hour in LT; separated by a ‘‘/’’) of the semidiurnal harmonic, and the bottom entry
the percent of the total mean daily variance explained (%) by the diurnal and semidiurnal harmonics (separated by a ‘‘/’’). For example, at 88N,
1658E, the amplitude and phase of the diurnal (semidiurnal) harmonic are 0.11 m s21 (0.20 m s21) and 1218 LT (1506 LT), respectively; andthe variance explained by the first (second) harmonic is 21% (74%).
1658E

1808

1708W

1558W

1408W

1258W

1108W

88N

0.11/12.3
0.20/15.1
21/74

0.09/14.7
0.14/15.5
25/64

0.08/18.5
0.16/15.2
18/75

0.17/17.2
0.19/14.8
39/51

0.05/21.5
0.14/15.5
11/77

0.07/19.0
0.17/15.4
13/85

0.07/20.6
0.17/15.4
13/84

58N

0.12/16.8
0.17/15.2
33/65

0.12/16.4
0.11/14.9
48/42

0.10/18.0
0.18/15.3
22/75

0.08/18.5
0.15/15.4
23/73

0.03/17.9
0.12/15.4
7/83

0.02/12.2
0.13/14.8
1/92

0.09/25.1
0.18/15.5
18/76

0.05/9.0
0.15/16.4
8/84

28N

0.18/16.6
0.19/15.3
45/51

0.10/16.9
0.16/15.1
29/69

0.03/20.4
0.19/15.5
3/93

0.05/21.8
0.14/15.5
11/84

0.09/16.9
0.14/15.2
27/63

0.07/18.0
0.12/15.0
22/75

0.06/18.7
0.16/15.5
11/84

0.07/10.6
0.18/15.5
14/81

08

0.16/16.0
0.22/15.3
32/60

0.08/16.8
0.19/15.4
15/79

0.07/20.7
0.17/15.2
12/85

0.06/21.5
0.15/15.5
13/81

0.06/14.3
0.06/15.049/41

0.11/24.6
0.06/13.4
79/19

0.19/24.2
0.04/13.1
89/5

0.20/25.2
0.15/14.8
62/36

28S

0.11/17.2
0.19/15.8
24/69

0.05/21.7
0.19/15.1
6/92

0.09/21.8
0.16/15.1
22/74

0.04/19.3
0.13/15.0
8/88

0.06/15.2
0.10/15.1
23/71

0.03/23.8
0.08/14.6
16/77

0.03/19.9
0.08/14.9
9/78

0.12/25.1
0.13/15.4
44/53

58S

0.01/18.5
0.14/15.2
0/54

0.03/19.2
0.21/14.6
1/92

0.08/13.5
0.20/15.2
12/83

0.03/15.5
0.16/15.3
2/93

0.10/14.7
0.12/15.4
39/57

0.05/10.8
0.13/15.2
13/79

0.10/13.9
0.10/15.6
46/49

0.03/10.3
0.19/15.3
2/89

88S

0.13/13.0
0.17/14.3
35/62

0.07/12.8
0.18/15.1
11/80

0.13/12.1
0.25/15.1
20/73

0.09/16.1
0.14/15.6
31/66

0.14/13.3
0.11/14.8
58/40

0.11/11.4
0.10/15.3
51/47

0.13/14.8
0.18/15.4
35/62

The zonal wind computed from (1) and (2) is comparedto the observed semidiurnal zonal wind in Fig. 7. (Theamplitude and phase of the observed semidiurnal zonal
wind was obtained by averaging the semidiurnal harmonics at all the buoys excluding the three equatorialsites between 1408W and 1108W.) There is good agreement between the observed and modeled semidiurnal
zonal wind variations both in amplitude and phase. The
observed winds are slightly weaker (0.16 m s21 vs 0.18
m s21 ) and earlier (1520 LT vs 1540 LT) compared tothe model winds. It remains to be seen whether surface
frictional effects (not included in the simple model) can
account for these small differences.
The three equatorial buoys between 1408W and
1108W exhibit highly suppressed semidiurnal fluctuations (Figs. 2a and 4b). We speculate that the stable
thermodynamic conditions in the planetary boundary
layer in this region (cf., Wallace et al. 1989; Deser andWallace 1990; Bond 1992) may inhibit the propagationof the semidiurnal tidal signal down to the sea surface.
Are the meridional wind semidiurnal variations consistent with the semidiurnal pressure field? To first order,
the inviscid meridional momentum balance isdy /dt 5 21/r(dP/dy) 2 fu,(3)

where f is the Coriolis parameter (note that the Coriolis
term is of similar magnitude to the pressure gradientterm in the meridional momentum balance). Since themeridional pressure gradient and Coriolis force are inphase with each other and in quadrature with the zonal

958W

pressure gradient term (recall Fig. 6), the meridional
wind acceleration will be in quadrature with the zonal
wind acceleration. Further, the semidiurnal meridional
wind variations should be asymmetric about the equator,
and identically zero at the equator. These characteristics
are generally consistent with the observations shown in
Fig. 5b.
Table 3 lists the amplitude and phase of the observed
and modeled semidiurnal meridional wind perturbations
at 88N and 88S. The observed winds are out of phase,as predicted from (3), with nearly identical magnitudes(0.065 m s21 ). Note that the phases (0520 and 1130 LT)
are approximately 1 h earlier than those expected froma quadrature relationship with the zonal wind perturbation (Table 3). The modeled semidiurnal meridional
wind variations are nearly identical to the observationsin amplitude and show the expected phases (0645 LT
and 1245 LT).
SEMIDIURNAL VARIATION OF SURFACE WIND
DIVERGENCE
The semidiurnal cycle of surface wind divergence
may be reconstructed from the semidiurnal harmonics
of the zonal and meridional wind components. The resulting amplitude of the semidiurnal divergence averaged across the basin is 0.9 3 1027 s21 , with maximum
convergence at 0620 and 1820 LT (not shown).
Several studies have reported a semidiurnal variationin rainfall over the tropical western Pacific using mea-

JULY 1998

1737

DESER AND SMITH
TABLE 2b. As in Table 2a but for the meridional wind component.
1658E

1808

1708W

1558W

1408W

1258W

1108W

88N

0.09/1.8
0.07/12.8
52/28

0.12/9.4
0.01/17.8
69/1

0.11/14.9
0.08/13.1
61/29

0.23/8.6
0.17/9.1
58/30

0.16/14.5
0.11/23.7
62/29

0.07/12.8
0.13/11.6
18/61

0.07/8.6
0.10/12.4
27/59

958W

58N

0.20/7.8
0.04/12.6
92/4

0.27/8.3
0.02/22.8
93/0

0.29/9.1
0.08/13.7
89/8

0.31/9.6
0.04/14.1
92/2

0.25/9.7
0.04/13.4
95/2

0.15/7.3
0.01/21.3
88/0

0.14/11.2
0.12/13.5
53/41

0.09/29.0
0.07/13.2
49/29

28N

0.20/8.7
0.06/13.7
82/9

0.23/6.7
0.06/14.1
91/6

0.31/6.2
0.05/15.5
96/2

0.33/8.8
0.05/15.8
96/2

0.44/9.5
0.06/15.0
98/2

0.23/9.8
0.02/16.1
98/0

0.23/10.0
0.05/14.4
93/5

0.21/11.4
0.01/22.9
93/008

0.09/11.2
0.05/16.045/17

0.16/5.7
0.09/14.6
73/20

0.15/7.7
0.04/15.4
93/5

0.20/8.4
0.03/15.9
96/2

0.25/10.2
0.04/16.0
96/3

0.22/11.7
0.04/16.2
96/3

0.17/13.0
0.14/15.3
59/39

0.26/12.1
0.14/15.7
76/22

28S

0.06/18.6
0.05/15.1
42/33

0.04/9.4
0.03/15.9
37/20

0.14/15.7
0.07/15.8
72/18

0.05/11.7
0.04/17.1
48/32

0.14/12.8
0.05/17.7
84/12

0.20/14.8
0.06/17.1
92/7

0.18/15.5
0.09/16.4
76/21

0.16/12.5
0.11/15.9
66/33

58S

0.14/21.5
0.08/16.2
62/20

0.15/16.5
0.09/17.8
61/22

0.25/19.5
0.07/7.7
89/8

0.17/19.9
0.06/18.5
86/9

0.13/20.4
0.07/18.7
75/21

0.09/17.4
0.12/16.8
32/60

0.21/18.1
0.10/18.1
81/17

0.24/14.4
0.11/17.0
81/16

88S

0.10/21.2
0.07/16.1
55/26

0.25/17.7
0.12/14.9
76/17

0.28/19.9
0.10/17.7
86/10

0.28/21.9
0.08/19.1
90/7

0.13/19.5
0.05/18.081/14

0.24/18.7
0.07/16.7
88/8

0.24/17.7
0.10/17.5
83/15

FIG. 4. (top) Vector representation of the amplitude and phase of the diurnal harmonic of the zonal wind component during 1993–96. The
length of the arrow denotes the amplitude of the harmonic (m s 21 ) 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, 0000, 0600 LT). The wind vector scale is indicatedat the lower right (0.5 m s21 ). (bottom) As in the top panel but for the semidiurnal harmonic of the zonal wind component. The wind vector
scale is the same for both panels.

1738

JOURNAL OF CLIMATEVOLUME 11

FIG. 5. (top) As in Fig. 4 but for the diurnal harmonic of the meridional wind component. An arrow pointing due west (south, east, north)indicates a southerly wind maximum at local noon (0600, 0000, 0600 LT). The wind vector scale is indicated at the lower right (0.5 m s 21 ).
(bottom) As in the top panel but for the semidiurnal harmonic of the meridional wind component. The wind vector scale is the same forboth panels.

surements from small atolls (Hamilton 1981; Gray andJacobson 1977) and aboard ships (RR97) where heatisland effects are minimized. These studies indicate asemidiurnal rainfall rate amplitude of ;0.3–1 mm day21
and a phase (time of maximum) near 0300–0400 LT
and 1500–1600 LT.
The observed semidiurnal surface wind convergencemaxima evidently follow the semidiurnal rainfall maxima in the tropical western Pacific by about 2 h. In
addition, the nearly constant phase and amplitude of thesemidiurnal zonal wind variations through the depth of

FIG. 6. The sun-synchronous semidiurnal cycle of sea level pressure(mb) at 0000 UTC according to Haurwitz and Cowley (1973). Positive(negative) pressure perturbations relative to the daily mean are shown
as solid (dashed) contours. Contour interval is 0.2 mb.

the free troposphere (Williams et al. 1992) suggests that
there is no appreciable vertical velocity associated withthe semidiurnal pressure oscillation (only a change innet tropospheric mass locally; see also Lindzen 1967).
Thus, it is difficult to implicate the semidiurnal thermal
tide as the cause of the semidiurnal rainfall variation.
e. The diurnal wind cycle
Figure 8 shows the mean diurnal wind vector deviations relative to the daily means at 0700, 1300, 1900,
and 0100 LT. The wind vectors were reconstructed fromthe phase and amplitude of the diurnal harmonics ofboth wind components and smoothed in the zonal direction with a three-point binomial filter. The corresponding diurnal wind divergence fields are shown in
Fig. 9. The divergences were computed from the reconstructed wind vectors in UTC, and then converted
back to LT for display. At 0700 LT, the wind vectors
show a general pattern of southerly flow relative to thedaily mean at and north of the equator, and northerly
flow relative to the daily mean south of 28S. This pattern

JULY 1998

1739

DESER AND SMITH

FIG. 7. Observed (solid curve) and modeled (dashed curve) semidiurnal variation of the zonal wind component (m s21 ) over the tropical Pacific in local time. The predicted wind is determined from thezonally averaged semidiurnal cycle of sea level pressure using a linear
inviscid balance.

results in convergence north of ;38N and divergenceto the south relative to the daily mean (Fig. 9). Maximum divergence anomalies are 8 3 1027 s21 near 08,
1708W. Six hours later (1300 LT), the meridional winddeviations have weakened substantially except over theeastern equatorial Pacific where strong southerly flow
is evident. The wind pattern during this phase of thediurnal variation has little effect on divergence values
in the western Pacific, while moderately enhancing thedivergence (convergence) immediately to the south(north) of the equator in the eastern Pacific (Fig. 9). The
wind patterns at 1900 and 0100 LT, shown for completeness, are reversed from those 12 h earlier.
1) ZONALLY SYMMETRIC COMPONENTIn view of the large zonally symmetric signal associated with the diurnal cycles of meridional wind anddivergence, we show the mean diurnal cycle of the zo-

nally averaged surface wind divergence across the Pacific as a function of latitude in Fig. 10. The diurnal
cycle is approximately out of phase north and south of
38N, with maximum convergence (divergence) at 58N(08–58S) in the early morning (0600–0900 LT). The
largest amplitude (6 3 1027 s21 ) occurs at the equator.
The diurnal cycle at 58S is delayed relative to that at
the equator by several hours, with maximum divergenceoccuring at 0900–1100 LT.
Figure 11 shows the latitudinal profiles of zonallyaveraged wind divergence near the times of maximum
deviation from the daily mean (0800 and 2000 LT). Notethat the long-term mean wind divergence is included inthese profiles. The divergent area near the equatorstrengthens and expands southward in the early morning
relative to the early evening. The total equatorial divergence, as estimated from the area under the positive
portions of the two curves normalized by the (daily)
mean latitudinal extent with actual divergence (6.1 3
10 5 m) decreases from 2.45 3 1026 s21 at 0800 LT to
1.55 3 1026 s21 at 2000 LT, a morning to evening ratioof 1.6 or a 45% percent change relative to the daily
mean. Presumably, this extra mass divergence in theearly morning is compensated by convergence to thenorth and south of the domain (there is a hint of thecompensation at 58N along the southern edge of the
ITCZ; Fig. 11).
2) WEST VERSUS EAST PACIFIC
In addition to a zonally symmetric signal, the diurnal
cycles of meridional wind and divergence exhibit east–west differences. In particular, the west Pacific (1658E–
1708W) shows more asymmetry about the equator, while
the east Pacific (1408–958W) exhibits a distinctive diurnal cycle in the equatorial zone (recall Fig. 5a). These
regions may also be distinguished on the basis of theirclimatological conditions, as illustrated by the wind divergence and OLR distributions in Fig. 1. The west
Pacific is characterized by off-equatorial convergenceand deep convection in both hemispheres, while the east
Pacific has a single convergence zone north of the equator.
The latitudinal profiles of total wind divergence near
the extremes of the diurnal cycle (0700 LT and 1900
LT) in the west Pacific are shown in Fig. 12a. Comparedto the zonal averages (Fig. 11), the west Pacific exhibits
more convergence north and south of the equator, andthe equatorial divergence is more limited in strengthand latitudinal extent. The total equatorial divergence

TABLE 3. Amplitude (m s21) and phase (hour in LT) of the observed and modeled semidiurnal zonal and meridional wind fluctuations at
88N and 88S.
U (88N and S avg)
Observed
Modeled21

0.16 m s ; 0320 LT
0.18 m s21; 0340 LT

V (88N)V (88S)

21

0.067 m s21; 1130 LT
0.063 m s21; 1240 LT

0.064 m s ; 0520 LT
0.063 m s21; 0640 LT

1740

JOURNAL OF CLIMATEVOLUME 11

FIG. 8. Mean wind vector deviations from the daily means at 0700, 1300, 1900, and 0100 LT based on the period 1993–96. The windvectors were smoothed in the zonal direction with a three-point binomial filter and represent 3-h averages centered on the times given above.
The wind vector scale is shown at the lower right (0.5 m s21 ).

JULY 1998

DESER AND SMITH

1741

FIG. 9. As is Fig. 8 but for wind divergence. Contour interval is 2
3 1027 s21 , with negative values dashed and positive values shaded.

FIG. 11. The mean latitudinal profiles of zonally averaged surface
wind divergence (1026 s21 ) across the Pacific at the extremes of thediurnal cycle: 0800 LT (solid curve) and 2000 LT (dashed curve).

is 2.14 3 1026 s21 at 0600 LT and 0.93 3 1026 s21 at
1800 LT, a morning to evening ratio of 2.3 or a 78%
percent change relative to the daily mean. The relative
day–night differences are substantially larger than thosefor the zonal average, although the mean divergence is
less.
Figure 12b shows analogous results for the east Pacific, where the morning and evening maxima occur near
0900 and 2100 LT, respectively, 2 h later than in thewest Pacific. The equatorial divergence is stronger thanin the west Pacific and zonal mean, and there is noconvergence south of the equator. The total equatorial
divergence is 2.74 3 1026 s21 at 0900 LT and 2.01 31026 s21 at 2100 LT, a morning to evening ratio of 1.4
or a 31% percent change relative to the daily mean. The
relative day–night differences are smaller than those forthe zonal average, although the mean divergence isgreater.
3) SEASONAL AND INTERANNUAL VARIATIONS OF
THE DIURNAL CYCLE OF EQUATORIAL WIND
DIVERGENCE

FIG. 10. Mean diurnal cycle (LT) of the zonally averaged surface
wind divergence (1027 s21 ) across the Pacific as a function of latitude
based on the period 1993–96. The daily means at each latitude havebeen removed. Positive (negative) values indicate divergence (convergence) relative to the daily mean.

Figure 13 shows the amplitude and phase of the diurnal harmonic of equatorial wind divergence in thewestern Pacific (1658E–1708W) at 2-month intervals
computed from the difference between the meridional

1742

JOURNAL OF CLIMATEVOLUME 11

FIG. 12. The mean latitudinal profiles of surface wind divergence (10 26 s21 ) at the extremes of the diurnal cycle: (a) western Pacific at
0700 LT (solid curve) and 1900 LT (dashed curve) and (b) eastern Pacific at 0900 LT (solid curve) and 2100 LT (dashed curve).

winds at 28–58N and 58–88S. Note that it was not possible to reliably define the total wind divergence at eachgrid point for each 2-month segment due to large
amounts of missing data. There is large secular variability in the diurnal amplitude of equatorial wind divergence in the western Pacific, with approximately
twice as much diurnal variation during the last half of1995 (8.1 3 1027 s21 ) than during the previous year
and a half (3.9 3 1027 s21 ). The phase remains nearly
constant during the entire period (0600–0800 LT).
Figure 14 shows the amplitude and phase of the diurnal cycle of equatorial wind divergence in the eastern
Pacific (1408–958W) at 2-month intervals, formed fromthe difference between the meridional winds at 08 and
88S (there is little seasonal or interannual variation of
the diurnal meridional winds north of the equator). A
pronounced seasonal cycle is evident, with minimum
diurnal amplitude (2–3 3 1027 s21 ) in March–April andmaximum (;6 3 1027 s21 ) in July–August. The phaseof the diurnal cycle is fairly uniform throughout the year
(0800–0100 LT).
The seasonal and interannual variations of the diurnal
cycle of equatorial surface wind divergence are associated with changes in the mean divergence. In the western Pacific, there was enhanced equatorial wind divergence during the last half of 1995 compared to the previous 18 months (Fig. 15). The enhanced divergence
was accompanied by increased OLR and reduced SST

along the equator associated with the termination ofwarm episode conditions (not shown). In the eastern
Pacific, the mean wind divergence between the equatorand 88S is considerably stronger in July–August thanMarch–April as the South Pacific high strengthens andexpands equatorward (Fig. 16). Thus, for both the interannual variations in the western Pacific and seasonalchanges in the eastern Pacific, the diurnal amplitude ofwind divergence varies in proportion to the strength ofthe mean divergence.
4. Discussion
a. Vertical and latitudinal extent of the diurnallyvarying circulation
We have shown that there is a strong diurnal variationin the low-level meridional wind field and associatedequatorial mass divergence across the Pacific. Is there
any evidence that the diurnal cycle in low-level equatorial divergence is linked to the day–night variation inheavy rainfall in the off-equatorial convergence zones?
A simple moisture budget and preliminary analysis of
the large-scale vertical motion field supports the notionof a diurnally varying deep tropospheric circulation that
extends to at least 158 lat.
The extra low-level mass diverged from the equatorin the early morning (see Figs. 8–11) is compensatedJULY 1998

DESER AND SMITH

FIG. 13. Amplitude (top) and phase (bottom) of the diurnal harmonic of equatorial wind divergence in the western Pacific (1658E–
1708W) at 2-month intervals computed from the difference between
the meridional winds at 28–58N and 58–88S.

presumably by enhanced convergence to the north andsouth, although the buoy array does not extend far
enough to verify this (there is a hint of the compensating
mass convergence along 58N at the southern edge of the
ITCZ). Such a compensation would be consistent withthe known early morning (0600–0900 LT) maximum indeep convection and heavy rainfall in the off-equatorialconvergence zones (Hendon and Woodberry 1993; Grayand Jacobson 1977). One can make a simple estimateof the amplitude of the implied off-equatorial diurnal
rainfall variation from the equatorial divergence diurnal
amplitude (0.45 3 1026 s21 ). Assuming that the divergence extends over a layer 200 mb deep, the averagewater vapor content of the converging air is 16 g kg21 ,and all of the converged moisture falls out as rain, theresulting diurnal rainfall amplitude is 1.4 mm day21 , avalue that compares favorably with direct estimates byGray and Jacobson (1977) and others and indirect estimates by Hendon and Woodberry (1993) of 1–2 mmday21 .
The above arguments rest on the assumption that the1743

FIG. 14. As in Fig. 13 but for the eastern Pacific (1408–958W),
formed from the difference between the meridional winds at 08 and88S.

meridional overturning circulation in the tropical troposphere undergoes a diurnal modulation that links the
low-level divergence along the equator with the precipitation maxima off the equator. To test this notion, apreliminary analysis of the diurnal cycle of large-scalevertical motion over the tropical Pacific was carried out
using reanalysis data from the National Centers for Environmental Prediction (NCEP; Kalnay et al. 1996). The
reanalyses are computed four times daily (at 0000, 0600,
1200, and 1800 UTC) on a 2.58 lat 3 2.58 long grid.
For the longitude band 1808–1358W in the central Pacific, the 0600 and 1800 UTC profiles correspond to
0600–0900 LT and 1800–2100 LT, respectively, near
the extremes of the diurnal cycle of near-surface equatorial wind divergence.
Figure 17 shows the latitudinal profiles of morningand evening pressure vertical velocity (v) averaged overthe depth of the troposphere (925–200 mb) for 1995.
Subsidence in the equatorial zone and rising motion in
the SPCZ (7.58–17.58S) are more than twice as strong
in the morning than in the evening. There is also enhanced morning subsidence in the northern subtropics.

1744

JOURNAL OF CLIMATE

FIG. 15. Latitudinal profiles of mean surface wind divergence (10 26s21 ) in the western Pacific (1658E–1708W) for July–December 1995
(solid curve) and January 1994–June 1995 (dashed curve) based ondata from the Comprehensive Ocean–Atmosphere Data Set.

The day–night changes in v at the equator and SPCZ
extend throughout the free troposphere (not shown). The
magnitude of the morning-to-evening change in equatorial subsidence (v ; 0.02 Pa s21 ) corresponds to achange in low-level divergence of ;1 3 1026 s21 for aboundary layer 200 mb deep: comparable to the buoy
wind divergence estimate of 0.9 3 1026 s21 . Thus, thevertical motion field supports the notion that the diurnal
cycle in near-surface wind divergence is linked to a deep
meridional tropospheric circulation.
b. Comments on proposed radiative mechanisms forthe diurnal cycle of deep convection
The role of radiative processes in driving day–nightdifferences in oceanic deep convection has been emphasized by Gray and Jacobson (1977), McBride andGray (1980), Foltz and Gray (1979), Randall et al.
(1991), and Xu and Randall (1995). The work of Grayand colleagues emphasize the role of day–night variations in tropospheric longwave cooling in clear versus
cloudy regions, while Randall and colleagues advocate
the role of day–night variations in cloud-top shortwave
absorption and longwave cooling.
Briefly, Gray and colleagues argue that day–nightchanges in net longwave cooling are largely compensated by subsidence warming. The radiatively driven

VOLUME 11

FIG. 16. Long-term mean surface wind divergence (10 26 s21 ) in
the eastern Pacific for July–August (top) and March–April (bottom)
based on data from the Comprehensive Ocean–Atmosphere Data Set.
Contour interval is 2 3 1026 s21 .

day–night variation in subsidence rate will be larger for
clear regions than cloudy areas since clouds reduce thenet upward longwave flux. Horizontal contrasts in theradiatively driven subsidence rate will lead to a diurnal
cycle in low-level mass divergence, with maximum convergence into the cloudy regions and divergence out of
the clear regions toward dawn. While this mechanism
depends only on the presence of adjacent cloud andcloud-free regions in the mean, Gray and colleagues
note that the resulting diurnal cycle of convergence into
the cloudy areas should result in increased deep convection and rainfall in the early morning.
In contrast, Randall and colleagues suggest that theclouds themselves force a diurnal cycle in rainfall byabsorbing solar radiation during the day (thereby stabilizing lapse rates and inhibiting deep convection) andcooling by infrared emission during the night (thereby
destabilizing lapse rates and promoting deep convection). It should be noted that this mechanism will also
impact adjacent clear regions through conservation ofmass.
Both radiative mechanisms should result in a diurnal
circulation that is symmetric with respect to the distribution of time-mean deep convection. Figure 18 shows
the mean buoy wind vector deviations from the dailymeans at 0700 and 1900 LT (taken from Fig. 8) super-JULY 1998

DESER AND SMITH

FIG. 17. Latitudinal profiles of pressure vertical velocity ( v; Pas21 ) averaged over the depth of the troposphere (925–200 mb) at
0600–0900 LT (solid curve) and 1800–2100 LT (dashed curve) in
the central Pacific (1808–1358W) based on the NCEP reanalyses for1995. Positive values indicate downward motion.

1745

waters induce lower temperatures in the atmosphericplanetary boundary layer that are reflected hydrostatically as higher surface pressures (Lindzen and Nigam
1987).]The mean diurnal amplitude of SST, constructed fromthe TAO buoys, exhibits a pronounced zonally symmetric component with maximum values along theequator (Fig. 19, bottom). The diurnal amplitude is twoto three times stronger at the equator (0.168C–0.278C)
than at 88N or 88S. The phase of the diurnal cycle isapproximately uniform across the domain, with a meanof 1700 LT 6 10 min (not shown).
Driven by the diurnal cycle of incoming solar radiation, the diurnal variation of SST is also influenced bycloud amount and wind strength (Bond and McPhaden
1995; Webster et al. 1996). The reduction of the climatological mean cloudiness and wind speed along theequator is likely responsible for much of the spatial
pattern of the SST diurnal amplitude shown in Fig. 19(see Deser and Smith 1996).
The impact of the equatorial cold tongue on surface
wind divergence in the mean, and the spatial pattern of
the SST diurnal cycle, leads us to speculate that diurnal
heating of the sea surface might contribute to reducing
the zonally symmetric component of equatorial winddivergence during the day and enhancing it at night, as
observed. Modeling experiments are needed to quantitatively test this hypothesis.
5. Conclusions

imposed upon the time-mean distribution of deep convection (as given by OLR , 240 W m22 ). Over thewestern tropical Pacific, the equatorial symmetry evident in the diurnal winds is also present in the mean
rainfall pattern. That is, the flow of low-level air awayfrom the clear equator into the regions of time-mean
deep convection is strongest in the early morning: qualitatively consistent with both radiative mechanisms discussed above. However, in the eastern tropical Pacific,
the equatorial symmetry in the winds is absent in therainfall distribution. Hence, unless OLR is a poor indicator of deep clouds and high moisture contents in theeastern tropical South Pacific, it is difficult to see how
the proposed radiative mechanisms can account for theequatorial symmetry in the diurnal wind cycles in theeastern tropical Pacific.
c. A possible role for SST in the diurnal cycle of
low-level winds
The climatological distribution of sea surface temperature (SST) exhibits a narrow tongue of cold upwelled surface waters along the equator (Fig. 19, top).
This ‘‘equatorial cold tongue’’ has a pronounced effecton the mean low-level atmospheric circulation, causing
the winds to diverge above it (recall Fig. 1). [The cold

The climatological large-scale patterns of diurnal andsemidiurnal near-surface wind fluctuations over thetropical Pacific have been documented using several
years of hourly measurements from the TAO mooredbuoy array. The main results may be summarized asfollows.
1) Semidiurnal variations account for 68% of the mean
daily variance of the zonal wind component, while
diurnal variations account for 82% of the mean daily
variance of the meridional wind component.
2) The amplitude and phase of the semidiurnal zonal
wind variations are nearly uniform across the basin,
averaging 0.15 6 0.04 m s21 and 0310 6 0030 LT,
respectively.
3) The observed semidiurnal near-surface wind variations are dynamically consistent with the zonally averaged semidiurnal pressure field, which is forced by
the atmospheric thermal tide in the stratosphere andupper troposphere.
4) The mean diurnal cycle of the meridional wind field
exhibits a pronounced zonally symmetric component
with opposite phase on either side of the equator.
This pattern results in a diurnal variation of winddivergence in the equatorial zone with maximum divergence in the early morning (;0800 LT). The average amplitude of the diurnal cycle in zonal mean

1746

JOURNAL OF CLIMATEVOLUME 11

FIG. 18. Mean buoy wind vector deviations from the daily means at 0700 LT (top) and 1900 LT (bottom) superimposed upon the time-mean
distribution of deep convection (as given by OLR , 240 W m22 ; shaded). The wind vector scale is shown at the lower right (0.5 m s 21 ).

divergence is 0.45 3 1026 s21 , which corresponds
to a day–night change of 45% relative to the daily
mean. The relative day–night changes in near-surface
wind divergence are larger in the west (78%) thanin the east (31%), due mainly to differences in thedaily mean divergence.
5) In addition to the zonally symmetric component,
there are east–west differences in the structure of thediurnal meridional winds. In particular, the diurnal
cycle at the equator is large in the eastern Pacificand negligible in the western Pacific. These differences may be related to the absence of a Southern
Hemisphere convergence zone in the eastern tropical
Pacific in the mean.
6) The diurnal cycle of equatorial wind divergencechanges seasonally in the eastern Pacific and interannually in the western Pacific in proportion to thestrength of the mean divergence.
7) We suggest that diurnal heating of the sea surface
may play a role in forcing the zonally symmetriccomponent of the diurnal cycle of equatorial winddivergence.

Future work
The wind data used in this study afford only a narrow
equatorial view of the diurnal low-level circulation
changes. A more complete description of the diurnal
cycle throughout the global Tropics and subtropics isneeded, including the vertical structure of the circulation
changes. Wind profiles from the tropical island networkof Doppler radars (Gage et al. 1991) may be particularily
helpful in providing vertical structure information (seeGutzler and Hartten 1995; Williams and Avery 1996).
Remotely sensed parameters such as tropospheric water
vapor may provide insight into the large-scale nature of
the diurnal circulation changes. In addition to observational studies, modeling work is needed to investigate
the relative importance of radiative versus surface heating mechanisms in the diurnal cycle of the tropical maritime circulation.
As noted by Randall et al. (1991), the diurnal cycle may
have implications for the time-mean surface energy budget and the partitioning of precipitation between landand sea. As such, the importance of the diurnal cycle

JULY 1998

DESER AND SMITH

1747

FIG. 19. (top) Mean SST distribution (8C) from Reynolds and Smith (1994). (bottom) Amplitude (8C) of the mean diurnal cycle of SST fromthe TAO buoys during 1993–96. Contour interval is 0.028C, with values .0.168C shaded. The buoys measure SST at a depth of 0.5 m.

may reach beyond the daily timescale to the mean stateof the tropical atmosphere.
Acknowledgments. We wish to thank Mr. Michael S.
Timlin for his invaluable assistance with the data processing and graphics. Discussions with Professor William Gray and Drs. John Knaff, John Scheaffer, andJonathan Friedman are gratefully acknowledged. We arealso indebted to Dr. Michael McPhaden and his staff at
PMEL for providing the wind data used in this study.
The graphics were produced with the GrADs package
developed at the University of Maryland under the direction of Brian Doty. This study was supported by agrant from NOAA’s Office of Global Programs. The
comments of the anonymous reviewers helped us to
improve the presentation of the results.
This paper is dedicated to the memory of Dr. Stanley
P. Hayes, whose vision helped to establish the TAO
array.
REFERENCES
Bond, N. A., 1992: Observations of planetary boundary layer structure in the eastern equatorial Pacific. J. Climate, 5, 699–706.
, and M. J. McPhaden, 1995: An indirect estimate of the diurnal

cycle in upper ocean turbulent heat fluxes at the equator, 1408W.
J. Geophys. Res., 100, 18 369–18 378.
Chapman, S., and R. S. Lindzen, 1970: Atmospheric Tides. D. Reidel,
200 pp.
Chen, S. S., and R. A. Houze, 1997: Diurnal variation and lifecycleof deep convective systems over the tropical Pacific warm pool.
Quart. J. Roy. Meteor. Soc., 123, 357–388.
Deser, C., 1994: Daily surface wind variations over the equatorial
Pacific Ocean. J. Geophys. Res., 99, 23 071–23 078.
, and J. M. Wallace, 1990: Large-scale atmospheric circulation
features of warm and cold episodes in the tropical Pacific. J.
Climate, 3, 1254–1281.
, and C. A. Smith, 1996: Daily cycles of SSTs and surface winds
over the tropical Pacific from the TAO moored buoy array. Proc.
20th NOAA Climate Diagnostics Workshop, Seattle, WA,
NOAA, 256–259.
Draper, N. R., and H. Smith, 1966: Applied Regression Analysis. John
Wiley and Sons, 407 pp.
Foltz, G. S., and W. M. Gray, 1979: Diurnal variation in the troposphere’s energy balance. J. Atmos. Sci., 36, 1450–1466.
Freitag, H. P., M. J. McPhaden, C. H. Coho, and A. J. Shepherd,
1991: Equatorial wind, current and temperature data: 1088W to
1408W; April 1983 to October 1987. NOAA Data Rep. ERL
PMEL-35, 116 pp.
Gage, K. S., G. G. Balsley, W. L. Ecklund, D. A. Carter, and J. R.
McAfee, 1991: Wind profiler related research in the tropical
Pacific. J. Geophys. Res., 96, 3209–3220.
Gray, W. M., and R. W. Jacobson Jr., 1977: Diurnal variation of deep
cumulus convection. Mon. Wea. Rev., 105, 1171–1188.
Gutzler, D. S., and L. M. Hartten, 1995: Daily variability of lower

1748

JOURNAL OF CLIMATE

tropospheric winds over the tropical western Pacific. J. Geophys.
Res., 100, 22 999–23 008.
Hamilton, K., 1980: The geographical distribution of the solar semidiurnal surface pressure oscillation. J. Geophys. Res., 85, 1945–1949.
, 1981: Latent heat release as a possible forcing mechanism foratmospheric tides. Mon. Wea. Rev., 109, 3–17.
Haurwitz, B., 1956: The geographical distribution of the solar semidiurnal pressure oscillation. N.Y. University Meteor. Paper, 2(5),36 pp.
, and A. D. Cowley, 1973: The diurnal and semidiurnal barometric pressure oscillations: Global distribution and annual variation. Pure Appl. Geophys., 102, 193–222.
Hayes, S. P., L. J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi,
1991: TOGA–TAO: A moored array for real-time measurementsin the tropical Pacific Ocean. Bull. Amer Meteor. Soc., 72, 339–
347.
Hendon, H. H., and K. Woodberry, 1993: The diurnal cycle of tropicalconvection. J. Geophys. Res., 98, 16 623–16 637.
Janowiak, J. E., P. A. Arkin, and M. Morrissey, 1994: An examinationof the diurnal cycle in oceanic tropical rainfall using satellite
and in situ data. Mon. Wea. Rev., 122, 2296–2311.
Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437–471.
Liebmann, B., and H. H. Hendon, 1990: Synoptic-scale disturbancesnear the equator. J. Atmos. Sci., 47, 1463–1479.
Lindzen, R. S., 1967: Thermally driven diurnal tide in the atmosphere.
Quart. J. Roy. Meteor. Soc., 93, 18–42.
, 1968: The application of classical atmospheric tidal theory.
Proc. Roy. Soc. London, A303, 299–316.
, and S. Nigam, 1987: On the role of sea surface temperature
gradients in forcing low-level winds and convergence in the
Tropics. J. Atmos. Sci., 44, 2440–2458.
McBride, J. L., and W. M. Gray, 1980: Mass divergence in tropical
weather systems. Paper I: Diurnal variation. Quart. J. Roy. Meteor. Soc., 106, 501–516.

VOLUME 11

McPhaden, M. J., 1993: TOGA–TAO and the 1991–93 El Niño–Southern Oscillation event. Oceanography, 6, 36–44.
, and S. P. Hayes, 1990: Variability in the eastern equatorial
Pacific during 1986–88. J. Geophys. Res., 95, 13 195–13 208.
Morrissey, M. L., 1990: An evaluation of ship data in the equatorial
western Pacific. J. Climate, 3, 99–112.
Nitta, T., and S. Esbensen, 1974: Diurnal variations in the western
Atlantic trades during BOMEX. J. Meteor. Soc. Japan, 52, 254–
257.
Pedder, M. A., 1978: Diurnal and semidiurnal variations in the A/B
Scale-averaged wind fields during phase III of GATE. Mon. Wea.
Rev., 106, 782–788.
Randall, D. A., Harshvardhan, and D. A. Dazlich, 1991: Diurnal
variability of the hydrologic cycle in a general circulation model.
J. Atmos. Sci., 48, 40–62.
Reynolds, R. W., and T. M. Smith, 1994: Improved global sea surface
temperature analyses using optimum interpolation. J. Climate,7, 929–948.
Ruprecht, E., and W. M. Gray, 1976: Analysis of satellite-observed
cloud clusters, Paper I and II. Tellus, 28, 391–425.
Wallace, J. M., T. P. Mitchell, and C. Deser, 1989: The influence ofsea surface temperature variability upon surface wind in theeastern equatorial Pacific: Seasonal and interannual variability.
J. Climate, 2, 1492–1499.
Webster, P. J., C. A. Clayson, and J. A. Curry, 1996: Clouds, radiation,and the diurnal cycle of sea surface temperature in the tropicalwestern Pacific. J. Climate, 9, 1712–1730.
Williams, C. R., and S. K. Avery, 1996: Diurnal winds observed in
the tropical troposphere using 50 MHz wind profilers. J. Geophys. Res., 101, 15 051–15 060.
,
, J. R. McAfee, and K. S. Gage, 1992: Comparison ofobserved diurnal and semidiurnal tropospheric winds at Christmas Island with tidal theory. Geophys. Res. Lett., 19, 1471–1474.
Xu, K., and D. A. Randall, 1995: Impact of interactive radiativetransfer on the macroscopic behavior of cumulus ensembles. Part
II: Mechanisms for cloud-radiative interactions. J. Atmos. Sci.,
52, 800–817.