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

Abstract

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 1Њ-2ЊC. 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 140Њ to 110ЊW along 2ЊN during 21-28 September 1999. A strong temperature inversion is observed throughout the cruise along 2ЊN, 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.

Observation of the marine atmospheric boundary layer in the Gulf of Guinea during the 2006 boreal spring

“…This is confirmed by the very good spatial correlation obtained between the SST and the MABL height (and to a lesser extent the mixed layer height) when the latter was averaged over a 10-day period (Figure 6). These results are thus in agreement with previous studies (Sweet et al, 1981;Wayland and Raman, 1989;Kwon et al, 1998;Hashizume et al, 2002), which discussed in particular how the adjustment of the mixed layer and the MABL to a SST front differs: the thin mixed layer responds to the local changes in the SST, whereas the thicker MABL follows the SST variations with a spatial and temporal lag, due to the advection by the mean atmospheric flow.…”

“…This agrees with the clear spatial and temporal coherence found in the previous section between the surface wind and SST patterns. Several previous studies of the SST front in the Pacific or North Atlantic also emphasized the same characteristics (Businger and Shaw, 1984;Friehe et al, 1991;Kwon et al, 1998;Hashizume et al, 2002;Small et al, 2008). To obtain a wide view of the differences between both sides of the SST front, ECMWF operational analyses are used to compute the mixed layer and the MABL heights.…”

“…The cloud base was often used as a cut-off for the MABL (Kwon et al, 1998). Wayland and Raman (1989) and Hashizume et al (2002) used virtual potential temperature profiles, searching the maximal gradient. Mathieu et al (2004) computed the MABL height by using the Richardson number, but this method requires the fixing of a critical Richardson number, on which the MABL height depends strongly.…”

“…As shown in the previous section, EGEE-3 soundings led to defining the MABL top as the altitude where the gradient of the potential temperature between 700 and 2500 m is maximal. The definition of the upper limit prevents the top rises up to several kilometres in the deep convection areas; the lower limit stops at the mixed layer top, which can also be characterized by a strong potential temperature gradient (Hashizume et al, 2002). Thus the mixed layer top can be defined as being collocated with the lowest level where the potential temperature increases by more than 0.3 K with a 100 m increase in height.…”

Ocean‐atmosphere in situ observations at the Brazil‐Malvinas Confluence region

“…Figure 3 suggests that during 2 November 2004 the OA exchanges were closely correlated with the SST field: over cold waters we observe weak winds on a colder atmosphere (stable MABL) and vice-versa. This process has been documented at the synoptic scale for other regions of the World Ocean [Rouault et al, 2000;Hashizume et al, 2002] but not yet for the BMC region. Supposing that the atmosphere at the BMC region is not being affected by the passage of a frontal system or any other factor disrupting its mesoscale pattern (as it was the case in 2 November 2004), one would expect the OA interactions to occur as they do across other ocean fronts.…”