FY 2006

Cloud, radiation, and surface forcing in the equatorial eastern Pacific

Hare, J., C. Fairall, T. Uttal, D. Hazen, M.F. Cronin, N. Bond, and D. Veron

NOAA Tech Memo OAR-PSD 307, NOAA ESRL, Boulder, CO, 64 pp (2005)

Cloud and surface flux processes play an important role in defining sea surface temperature in the tropical eastern Pacific Ocean. The need to increase our knowledge of air-sea interaction in this region has led to the refinement and deployment of the enhanced Tropical Atmosphere-Ocean Eastern Pacific Investigation of Climate (TAO-EPIC) buoys in that region and the development of the Pan-American Climate Studies (PACS) program. In conjunction with these efforts, we developed and implemented a regular and repeated ship-based cloud and flux measurement program onboard the NOAA Ships Ronald H. Brown and Ka’imimoana. These intensive measurement campaigns were made during NOAA’s fall (Ronald H. Brown; 1999- 2002) and spring (Ka’imimoana; 2000-2002) maintenance cruises along the 95°W and 110°W TAO buoy lines. With this dataset, we have developed a limited climatology of the PACS area, and we continue to repeat the deployments during the fall cruises to the TAO lines. The analysis presented shows seasonal (spring vs. fall) contrasts in the latitude averaged variables.

At a given latitude, the year-to-year and seasonal variabilities of many of the meteorological and oceanic means are relatively small. However, we find notable seasonal variability in the northern branch of the Inter-Tropical Convergence Zone (ITCZ), the north-south sea surface temperature gradient, and heat fluxes north of the equator. In the fall, the strengthening of the north-south SST contrast enhances convective activity (more and deeper clouds, precipitation, southerly inflow) in the area around 6°N, 95°W; diurnal variations of low cloud fraction were weak. In contrast, the spring clouds varied significantly over the diurnal cycle with substantially lower cloud fraction during the day south of 5°N. This may be a result of greater subsidence or surface heating during the day. Relatively low average cloud base heights around the equator are due to decoupling of the marine boundary layer (MBL) as southerly air flows over the region of the cold tongue. In the fall, the cold tongue surface water is significantly cooler, corresponding with very low and evenly distributed cloud base observations.

Estimates of cloud forcing of surface radiation in the visible and infrared (IR) are presented. In the IR, cloud forcing strongly correlates with cloud fraction and IR cloud forcing shows significant seasonal variability. In the solar band, less variability in seasonal cloud forcing was seen. From the observations, it is determined that clouds in the eastern equatorial Pacific tend to cool the surface by about 40 W m in both seasons.

The spring net heat flux is nearly symmetrical about the equator with a maximum (175 W m) at the equator decreasing to about 100 W m at 10°N and 8°S. The equatorial maximum is associated with lower turbulent fluxes and modestly lower cloudiness at the equator. In the fall, the maximum net heat flux (180 W m) is at 2°S and the minimum (essentially zero) is at 6°N. Much of the fall net heat flux asymmetry is caused by cloud radiative forcing.