<P> There are several difficulties with satellite - based absolute SST measurements . First, in infrared remote sensing methodology the radiation emanates from the top "skin" of the ocean, approximately the top 0.01 mm or less, which may not represent the bulk temperature of the upper meter of ocean due primarily to effects of solar surface heating during the daytime, reflected radiation, as well as sensible heat loss and surface evaporation . All these factors make it somewhat difficult to compare satellite data to measurements from buoys or shipboard methods, complicating ground truth efforts . Secondly, the satellite cannot look through clouds, creating a cool bias in satellite - derived SSTs within cloudy areas . However, passive microwave techniques can accurately measure SST and penetrate cloud cover . Within atmospheric sounder channels on weather satellites, which peak just above the ocean's surface, knowledge of the sea surface temperature is important to their calibration . </P> <P> The SST has a diurnal range, just like the Earth's atmosphere above, though to a lesser degree due to its greater specific heat . On calm days, the temperature can vary by 6 ° C (10 ° F). The temperature of the ocean at depth lags the Earth's atmosphere temperature by 15 days per 10 metres (33 ft), which means for locations like the Aral Sea, temperatures near its bottom reach a maximum in December and a minimum in May and June . Near the coastline, offshore winds move the warm waters near the surface offshore, and replace them with cooler water from below in the process known as Ekman transport . This pattern increases nutrients for marine life in the region . Offshore river deltas, freshwater flows over the top of the denser seawater, which allows it to heat faster due to limited vertical mixing . Remotely sensed SST can be used to detect the surface temperature signature due to tropical cyclones . In general, an SST cooling is observed after the passing of a hurricane primarily as the result of mixed layer deepening and surface heat losses . In the wake of several day long Saharan dust outbreaks across the adjacent northern Atlantic Ocean, sea surface temperatures are reduced 0.2 C to 0.4 C (0.3 to 0.7 F). Other sources of short - term SST fluctuation include extratropical cyclones, rapid influxes of glacial fresh water and concentrated phytoplankton blooms due to seasonal cycles or agricultural run - off . </P> <P> The Atlantic Multidecadal Oscillation (AMO) is important for how external forcings are linked with North Atlantic SSTs . </P> <P> El Niño is defined by prolonged differences in Pacific Ocean surface temperatures when compared with the average value . The accepted definition is a warming or cooling of at least 0.5 ° C (0.9 ° F) averaged over the east - central tropical Pacific Ocean . Typically, this anomaly happens at irregular intervals of 2--7 years and lasts nine months to two years . The average period length is 5 years . When this warming or cooling occurs for only seven to nine months, it is classified as El Niño / La Niña "conditions"; when it occurs for more than that period, it is classified as El Niño / La Niña "episodes". </P>

At which time during this period is the rate of evaporation expected to be the highest