A case study of wind and sea surface properties at 8°N 95°W

Abstract

In this study, buoy data from the Tropical Atmosphere Ocean (TAO) array location at 8°N 95°W are analyzed to determine the connection between the atmosphere and ocean. Wind, sea surface temperature (SST), and sea surface salinity (SSS) data from 1994 through 2008 are examined for overall and yearly trends. The TAO buoy at 8°N 95°W is a unique location with regard to meteorological data for a number of reasons. It is located at the far northeast corner of the TAO array, which spans the tropical Pacific Ocean from latitudes 8°N to 8°S and longitudes 95°W to 165Έ. The buoy is located approximately 415 nautical miles southwest of the coast of Guatemala. In addition to the buoy's proximity to land relative to other TAO buoys, the Inter-Tropical Convergence Zone (ITCZ) moves across the buoy during the year. The TAO buoys measure meteorological and oceanographie properties at 10-minute intervals for a total of 144 measurements per day from each instrument. Daily averages are then calculated for each data type and transmitted via the Advanced Research and Global Observations Satellite (ARGOS). The near-real-time data are quality controlled through automated scripts and manual checks. Once the buoy is recovered, the high-resolution 10-minute data are downloaded from the buoy and subsurface instruments and returned to shore. The high-resolution data, known as delayed-mode data, are also quality controlled through automated scripts and manual checks. The delayed-mode data then replace the near-real-time data as the official data record for a station. The data used in this study are available from the Web site of the National Oceanic and Atmospheric Administration's (NOAA) National Data Buoy Center (NDBC). The data set is daily average data calculated from the high-resolution delayed-mode data. Observed wind directions from the TAO buoy at 8°N 95W° show a yearly cycle of northeast winds from January to March and southwest winds from May to October, with April, November, and December acting as transition months. The pattern in the wind direction shows the position of the ITCZ in relation to the buoy. The transition months indicate approximately when the ITCZ is passing over the buoy. These months are characterized by large changes in the day-to-day wind direction. A yearly cycle similar to the wind direction is seen in the SST and SSS data as well. Monthly averages were calculated after the discovery of the yearly cycle. The data show a lag in the response between the ocean and the atmosphere when plotted side-by-side. Cross correlations between winds and sea surface data with varying lags are calculated to determine the response time of the ocean compared to the atmosphere. Correlation values show a three-month lag between wind direction and both SST and SSS. Correlations between wind speeds and both SST and SSS show a five-month lag between the yearly wind speed and sea surface patterns. The calculated lag between the winds and the sea surface properties show multiple water types moving past the buoy. Wind directions from the northeast bring warmer, more saline water to the buoy location while southwest winds bring cooler, less saline water to the area around the buoy. Wind speeds appear to have less of an effect than wind direction on SST and SSS, as evidenced by correlation values near zero. Sea surface temperatures respond more quickly to wind changes than SSS. Salinity shows a broad peak early in the year followed by a gradual decrease over several months. Meanwhile, SST has a dramatic rise from March to May followed by a decrease during the following four months, two months shorter than the salinity decrease. Wind patterns show a similar response to those seen across the Isthmus of Panama. During the dry season in Panama (December, January, February, and March), northeast winds are observed over Panama and at 8°N 95°W. The opposite happens during the rainy season in Panama, which is broken into the two time periods of April through June and August through November. Winds are from the southwest across Panama and at 8°N 95°W. The observed winds illustrate the extent to which Central American weather patterns extend into offshore marine weather environments. The area around 8°N 95°W can be traced as the source region of the moisture for the Panama rainy season.