VOLUME 21 JOURNAL OF CLIMATE 15FEBRUARY 2008 A Climatology of Diurnal and Semidiurnal Surface Wind Variations over the Tropical Pacific Ocean Based on the Tropical Atmosphere Ocean Moored Buoy Array REI UEYAMA Department of Atmospheric Sciences, University of Washington, Seattle, Washington CLARA DESER National Center for Atmospheric Research, Boulder, Colorado (Manuscript received 5 September 2006, in final form 31 May 2007) ABSTRACT Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N–8°S, 165°E– 95°W) during 1993–2004 are used to document the climatological seasonal and annual mean patterns of diurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, the amplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is at least 3 times as large as the semidiurnal harmonic for the meridional wind component, both averaged across the buoy array. Except for the eastern equatorial Pacific, the semidiurnal zonal wind harmonic exhibits uniform amplitude (ϳ0.14 m sϪ1) and phase [maximum westerly wind anomalies ϳ0325/1525 local time (LT)] across the basin in all seasons. This pattern is well explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over the cold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer stability. Diurnal meridional wind variations tend to be out of phase north and south of the equator (maximum southerly wind anomalies ϳ0700 LT at 5°N and ϳ1900 LT at 5°S), while a noon southerly wind anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season (June–November). The diurnal meridional wind variations result in enhanced divergence along the equator and convergence along the southern border of the intertropical convergence zone ϳ0700 LT (opposite conditions ϳ1900 LT); the amplitude of the divergence diurnal cycle is ϳ5 ϫ 10Ϫ7 sϪ1. The diurnal meridional wind variations are largely consistent with the diurnal pressure gradient force. 1. Introduction and Slingo 2001; Yang and Smith 2006). These daily cycles are associated with large variations in the solar The tropical atmosphere varies on a multitude of forcing and represent fundamental modes of variability time scales and influences weather and climate on re- in the global climate system. However, in part because gional and global scales. The intraseasonal Madden– of the scarcity of observational data over the oceans, Julian oscillation and the interannual El Niño–Southern much work still remains in characterizing and interpret- Oscillation (ENSO) are two well-documented modes of ing daily variations of tropical atmospheric properties. variability whose effects extend beyond the tropical Pa- Deser and Smith (1998, hereafter DS) analyzed 4 yr cific Ocean basin. The climate of the tropical Pacific (1993–96) of hourly near-surface wind data from the also undergoes daily variations in, for example, baro- Tropical Atmosphere Ocean (TAO) moored buoy ar- metric pressure, cloudiness, rainfall, and near-surface ray and showed that the mean daily march of the zonal wind (e.g., Haurwitz and Cowley 1973; Hamilton 1980; (meridional) wind component is primarily semidiurnal Hendon and Woodberry 1993; Gutzler and Hartten (diurnal). They further noted that the phase, amplitude, 1995; Deser and Smith 1998; Dai and Wang 1999; Yang and spatial pattern of the semidiurnal zonal wind varia- tions are consistent with the classical theory of atmo- spheric thermal tides. Briefly, diurnal and semidiurnal Corresponding author address: Rei Ueyama, Department of At- mospheric Sciences, University of Washington, Box 351640, Se- atmospheric thermal tides are generated by the absorp- attle, WA 98195. tion of solar radiation by ozone in the stratosphere and E-mail: [email protected] water vapor in the troposphere. The tides propagate DOI:10.1175/2007JCLI1666.1 © 2008 American Meteorological Society 593 Unauthenticated | Downloaded 10/05/21 08:55 PM UTC JCLI1666 594 JOURNAL OF CLIMATE VOLUME 21 TABLE 1. Percent data coverage at each TAO buoy during 1993–2004 for wind, SST, and air temperature data, and during 2000–04 for pressure data. The parentheses denote less than 50% wind data coverage. A dash indicates lack of pressure measurements. For example, for the buoy at 8°N, 95°W, 43% of the wind records, 79% of the SST records, and 72% of the air temperature records during 1993–2004 have data; and 60% of the pressure records during 2000–04 have data. 165°E 180°W 170°W 155°W 140°W 125°W 110°W95°W 9°N ————76/100/97/———— 8°N 64/93/80/— 59/80/66/— 95/99/98/— 65/90/84/—— 63/92/86/— 85/92/88/— (43)/79/72/60 5°N 77/96/100/— 74/88/92/— 79/87/93/— 85/100/99/— 85/95/95/— (40)/96/79/— 61/87/87/— 60/82/83/48 2°N 78/97/90/— 74/95/92/— 66/81/91/— 75/85/89/— 66/91/90/— 84/91/75/— 78/95/88/74 (38)/67/61/37 0° (44)/80/67/— 79/94/92/— 90/99/95/— 93/99/92/— 91/96/92/— 76/90/89/— 65/91/79/68 64/79/78/53 2°S 63/83/90/— 93/98/98/— 72/96/84/— 71/86/83/— 81/98/98/— 92/99/89/— 68/94/91/25 56/86/83/48 5°S 53/95/90/— 66/91/87/— 78/92/92/— 90/99/91/— 100/100/100/— 95/95/95/— 76/89/88/— 68/76/76/37 8°S 66/87/88/— 83/83/88/— 88/91/95/— 86/100/97/—— 77/94/84/— 78/97/91/— 76/79/81/55 downward to the earth’s surface, affecting sea level tropical Pacific using 12 yr (1993–2004) of hourly wind pressure and thereby surface winds. Tidal signals ob- data from the TAO moored buoy array. Extending ear- served at the surface are mainly semidiurnal (Chapman lier results of DS, we document the climatological an- and Lindzen 1970) because the energy of the diurnal nual and semiannual mean patterns of diurnal and forcing is trapped near the level of excitation in the semidiurnal surface wind variations. lower stratosphere and upper troposphere (Haurwitz 1964; Lindzen 1967). In particular, the semidiurnal 2. Data and methods component of the sea level pressure tidal amplitude a. Data over the tropics is 2–3 times larger than the diurnal component. While the semidiurnal zonal wind varia- There are ϳ55 moored TAO buoys in the tropical tions documented by DS are consistent with atmo- Pacific Ocean (9°N–8°S, 165°E–95°W) located approxi- spheric thermal tidal theory, the observed diurnal me- mately every 2°–3° of latitude and 15° of longitude. The ridional wind variations over the tropical Pacific have spacing of the buoys was chosen to be commensurate yet to be explained, although we note that tidal theory with the narrow meridional scale of oceanic processes may also be relevant. at the equator, such as equatorially trapped waves and The diurnal cycle in tropical convection is clearly im- upwelling motions that determine sea surface tempera- portant for understanding the diurnal variability in the ture (SST) anomalies during ENSO. This arrangement surface wind field. Many studies have documented the of narrow meridional spacing of the buoys is not opti- diurnal cycles in deep convection and rainfall over the mal for the study of large-scale atmospheric circulation. tropical oceans from a variety of data sources including Nevertheless, the TAO array is a valuable tool for satellite products from the Tropical Rainfall Measuring monitoring daily variations of surface winds across the Mission (Bowman et al. 2005; Yang and Smith 2006) tropical Pacific because of its unprecedented spatial and International Satellite Cloud Climatology Project and temporal coverage. This study examines 12 yr (1 (Hendon and Woodberry 1993), in situ ship reports January 1993–31 December 2004) of hourly wind data from the Comprehensive Ocean Atmosphere Data Set from the 51 buoys that had at least 50% of data cover- (COADS; Janowiak et al. 1994), and rain gauges from age (Table 1). For the selected buoys, the average num- island stations (Gray and Jacobson 1977) and the TAO ber of days per year containing data is 278, ranging buoys (Serra and McPhaden 2004). The various from a minimum of 193 at 5°S, 165°E to a maximum of datasets are all broadly consistent in indicating an early 365 at 5°S, 140°W. These numbers account for the time morning [ϳ0300–0600 local time (LT)] rainfall maxi- span of data collection at each buoy as well as the fre- mum over the tropical oceans. Despite documentation quent gaps in the record due to instrument failure. of a clear diurnal signal in oceanic deep convective rain- The TAO array samples winds at 3.8 m above sea fall, neither its mechanism nor its connection to the level. Wind speed measurements are made with a pro- diurnal cycle of surface wind and wind divergence are peller and have a resolution of 0.2 m sϪ1 and an accu- well understood (Yang and Smith 2006). racy of Ϯ0.3 m sϪ1. A vane or a fluxgate compass is Given the importance of the surface wind field to the used to measure wind direction at 1.4° resolution with large-scale circulations of the atmosphere and ocean, an accuracy of 5°–7.8°. Wind data were recorded for a this study seeks to expand our knowledge of daily (di- 6-min interval at the beginning of each hour until 1996, urnal and semidiurnal) surface wind variability over the after which they are recorded every 10 min.