Direct Observations of Atmospheric Boundary Layer Response to SST Variations Associated with Tropical Instability Waves over the Eastern Equatorial Pacific

Tropical instability waves (TIWs), with a typical wavelength of 1000 km and period of 30 days, cause the equatorial front to meander and result in SST variations on the order of 18‐28C. Vertical soundings of temperature, humidity, and wind velocity were obtained on board a Japanese research vessel, which sailed through three fully developed SST waves from 1408 to 1108W along 28N during 21‐28 September 1999. A strong temperature inversion is observed throughout the cruise along 28N, capping the planetary boundary layer (PBL) that is 1‐ 1.5 km deep. Temperature response to TIW-induced SST changes penetrates the whole depth of the PBL. In response to an SST increase, air temperature rises in the lowest kilometer and shows a strong cooling at the mean inversion height. As a result, this temperature dipole is associated with little TIW signal in the observed sea level pressure (SLP). The cruise mean vertical profiles show a speed maximum at 400‐500 m for both zonal and meridional velocities. SST-based composite profiles of zonal wind velocity show weakened (intensified) vertical shear within the PBL that is consistent with enhanced (reduced) vertical mixing, causing surface wind to accelerate (decelerate) over warm (cold) SSTs. Taken together, the temperature and wind soundings indicate the dominance of the vertical mixing over the SLP-driving mechanism. Based on the authors’ measurements, a physical interpretation of the widely used PBL model proposed by Lindzen and Nigam is presented.

[1]  R. Reynolds,et al.  The NCEP/NCAR 40-Year Reanalysis Project , 1996, Renewable Energy.

[2]  Michael H. Freilich,et al.  Observations of coupling between surface wind stress and sea surface temperature in the Eastern Tropical Pacific , 2001 .

[3]  K. Gage,et al.  ENSO's impact on the annual cycle: The view from Galápagos , 2000 .

[4]  D. Chelton,et al.  Air-sea heat exchange along the northern sea surface temperature front in the eastern tropical Pacific , 2002 .

[5]  Steven P. Anderson.  On the Atmospheric Boundary Layer over the Equatorial Front , 2001 .

[6]  Xiaosu Xie,et al.  Atmospheric manifestation of tropical instability wave observed by QuikSCAT and tropical rain measuring mission , 2000 .

[7]  A. E. Gill.  Some simple solutions for heat‐induced tropical circulation , 1980 .

[8]  C. Bretherton,et al.  Cloudiness and Marine Boundary Layer Dynamics in the ASTEX Lagrangian Experiments. Part I: Synoptic Setting and Vertical Structure , 1995 .

[9]  R. Wu,et al.  On Equatorial Pacific Surface Wind Changes around 1977: NCEP–NCAR Reanalysis versus COADS Observations* , 2003 .

[10]  P. Webster,et al.  The influence of cross‐equatorial pressure gradients on the location of near‐equatorial convection , 1999 .

[11]  J. David Neelin,et al.  ENSO theory , 1998 .

[12]  B. Stevens,et al.  Entrainment, Rayleigh Friction, and Boundary Layer Winds over the Tropical Pacific , 2002 .

[13]  S. Oltmans,et al.  Ozonesonde Observations in the Equatorial Eastern Pacific —the Shoyo-Maru Survey— , 2002 .

[14]  John M. Wallace,et al.  The Influence of Sea-Surface Temperature on Surface Wind in the Eastern Equatorial Pacific: Seasonal and Interannual Variability , 1989 .

[15]  C. Deser.  Diagnosis of the Surface Momentum Balance over the Tropical Pacific Ocean , 1993 .

[16]  J. Schultz.  Carrier-mediated photodiffusion membranes. , 1977, Science.

[17]  G. McFarquhar,et al.  Marine boundary layers associated with ocean upwelling over the eastern equatorial Pacific Ocean , 1999 .

[18]  Frank J. Wentz,et al.  Satellite microwave SST observations of transequatorial tropical instability waves , 2000 .

[19]  S. Zebiak,et al.  Surface Wind over Tropical Oceans: Diagnosis of the Momentum Balance, and Modeling the Linear Friction Coefficient , 2000 .

[20]  W. Liu,et al.  Local and remote atmospheric response to tropical instability waves: A global view from space , 2001 .

[21]  B. Albrecht,et al.  Low-Level Inversions over the Tropical Pacific—Thermodynamic Structure of the Boundary Layer and the Above-Inversion Moisture Structure , 1989 .

[22]  M. Cane,et al.  Relative Roles of Elevated Heating and Surface Temperature Gradients in Driving Anomalous Surface Winds over Tropical Oceans , 2001 .

[23]  Richard H. Johnson,et al.  Diurnal Variability of the Marine Boundary Layer during ASTEX , 2001 .

[24]  Smith,et al.  Satellite measurements of sea surface temperature through clouds , 2000, Science.

[25]  R. Lindzen,et al.  On the role of sea surface temperature gradients in forcing low-level winds and convergence in the tropics , 1987 .

[26]  F. Wentz.  A well‐calibrated ocean algorithm for special sensor microwave / imager , 1997 .

[27]  Bin Wang,et al.  A Simple Tropical Atmosphere Model of Relevance to Short-Term Climate Variations , 1993 .

[28]  C. Deser,et al.  The Influence of Sea Surface Temperature Gradients on Stratiform Cloudiness along the Equatorial Front in the Pacific Ocean , 1993 .

[29]  N. Bond.  Observations of Planetary Boundary-Layer Structure in the Eastern Equatorial Pacific , 1992 .

[30]  W. Timothy Liu,et al.  NASA scatterometer provides global ocean‐surface wind fields with more structures than numerical weather prediction , 1998 .

[31]  S. Hastenrath.  Climate dynamics of the tropics , 1985 .

[32]  B. Albrecht,et al.  Spatial Variability of Atmospheric Boundary Layer Structure over the Eastern Equatorial Pacific , 2000 .

[33]  T. Matsuno,et al.  How and why climate variability differs between the tropical Atlantic and Pacific , 1999 .

[34]  C. Deser,et al.  Diurnal and Semidiurnal Variations of the Surface Wind Field over the Tropical Pacific Ocean , 1998 .

[35]  N. Lau,et al.  Observed Structure and Propagation Characteristics of Tropical Summertime Synoptic Scale Disturbances , 1990 .

[36]  Masaki Ishiwatari,et al.  Coupled ocean‐atmospheric waves on the equatorial front , 1998 .

[37]  R. Sutton,et al.  The influence of subseasonal wind variability on tropical instability waves in the Pacific , 2001 .

[38]  S. Philander.  Instabilities of zonal equatorial currents, 2 , 1976 .

[39]  Chih-Pei Chang.  Westward Propagating Cloud Patterns in the Tropical Pacific as seen from Time-Composite Satellite Photographs , 1970 .

[40]  I. Held,et al.  Modeling Tropical Convergence Based on the Moist Static Energy Budget , 1987 .

[41]  J. McCreary,et al.  Meridional Asymmetry and Energetics of Tropical Instability Waves , 1995 .

[42]  J. Wallace,et al.  The Influence of Sea-Surface Temperature on Surface Wind in the Eastern Equatorial Pacific: Weekly to Monthly Variability , 1989 .

[43]  R. Legeckis.  Long Waves in the Eastern Equatorial Pacific Ocean: A View from a Geostationary Satellite , 1977, Science.

[44]  Lucio López Cavazos,et al.  Case‐studies of developing east Pacific easterly waves , 1998 .