NUMERICAL SIMULATION OF BOUNDARY LAYER STRUCTURE AND CROSS-EQUATORIAL FLOW IN THE EASTERN PACIFIC

Justin Small

International Pacific Research Center

School of Ocean and Earth Science and Technology

University of Hawaii

2525 Correa Rd

Honolulu, HI 96815

e-mail: small@hawaii.edu

Tel: 808-956-4471

Abstract

Recent satellite observations from scatterometer and microwave imager have mapped the low level flow over the equatorial eastern Pacific and its relationship with sea surface temperature (SST). A marked deceleration towards the cold tongue and acceleration across its northern SST front is regularly seen. A high-resolution numerical model is used to examine this change in flow. The model used here has a 1.5 level turbulence closure scheme to help explain the atmospheric Planetary Boundary Layer (PBL) response. The principal result is that a sharp pressure gradient drives the northward acceleration. Because of thermal advection by the mean flow, the pressure gradient is located downstream of the SST gradient and consequently divergence occurs over the SST front, as observed by satellite. Comparisons are made between the model results and observations from the recent Eastern Pacific Investigation of Climate Processes (EPIC) campaign. Similar PBL structures are seen in model and EPIC observations along 95 W, suggesting the model has reasonably captured the physics. In particular, a shallow PBL is seen over the cold tongue as a result of the higher static stability there, compared with unstable regimes to the north and south which have deeper PBLs. Cloud is seen in the inversion layer which slopes upwards away from its lowest level over the cold tongue and towards higher latitudes. There is a maximum of low level cloudiness over the SST front.