Moisture and latent heat flux variabilities in the tropical Pacific derived from satellite data

Abstract

A method of determining ocean‐atmosphere moisture and latent heat fluxes using spaceborne sensors is presented and evaluated. The satellite data used are the geophysical parameters: sea surface temperature, precipitable water, and surface wind speed, derived from observations of the microwave radiometer on the Nimbus 7 spacecraft from 1980 to 1983 in the tropical Pacific. The period included one of the most intense El Niño‐Southern Oscillation (ENSO) episodes of this century. The surface humidity is first derived from the precipitable water using a global empirical relation and is found to be in close agreement with in situ measurements in the ascending and the descending branches of the Hadley and Walker circulations during both ENSO and non‐ENSO periods. The latent heat flux is then computed with a bulk parameterization model. At monthly and 2° latitude by 2° longitude resolution, the random error of the computed latent heat flux is estimated to approximately 26 W/m2 with the flux variability in the order of 100 W/m2. In areas covered by surface convergence, the moisture flux is found to be low, with the temporal variability due mostly to the variability of wind speed. Elsewhere, the variability of sea‐air humidity difference plays a more important role. During the early phase of the 1982–1983 ENSO, a surface convergence center moves east leading the anomalous equatorial westerlies. At this center, the low wind and high humidity caused negative (low) latent heat flux anomalies despite anomalously high sea surface temperatures. Latent heat flux is found to play an important role in the seasonal cooling of the upper ocean except in areas covered by major surface convergence zones where the variability of insolation is expected to be large and in areas of ocean upwelling where ocean dynamics perturb the upper ocean heat balance. The latent heat flux from the general circulation model of the European Center of Medium Range Weather Forecast is found to be inadequate in capturing the temporal and spatial variations computed from the satellite data.

Estimating the Bowen ratio over the open and ice‐covered ocean

“…Current methods for obtaining surface fluxes from satellites are always based on a bulk flux algorithm, such as represented by (4) [e.g., Liu, 1988;Thadathil et al, 1995;Konda et al, 1996]. Consequently, from satellites, we need surface temperature and surface-level estimates of wind speed, air temperature, and humidity.…”

“…[70] The necessary near-surface gradients  s À  r and Q s À Q r are typically found by first estimating the total precipitable water from the scanning multichannel microwave radiometer [Liu, 1988] or the Special Sensor Microwave/Imager [Thadathil et al, 1995;Bentamy et al, 2003]. The next step, often, is to obtain surface-level humidity, Q r , from a relation between precipitable water and Q r such as those described by Liu [1986] or Schulz et al [1993] [e.g., Liu, 1988Konda et al, 1996;Bentamy et al, 2003].…”

Development of Near Sea Surface Air Specific Humidity Retrieval by Satellite Remote Sensing

“…Because of the inclusion of the 22 GHz water vapor absorption line onboard the SSM/I, W can be retrieved with great accuracy from this platform. Then, by using this empirical Q a -W relation in conjunction with satellite-derived surface wind speeds and sea surface temperature (SST), determination of latent heat flux over remote areas in the open oceans became feasible [9]. Unfortunately, errors in Q a originating from the Q a -W relation can result in large errors in latent heat flux.…”

On the Annual Cycle of Latent Heat Fluxes over the Equatorial Pacific Using TAO Buoy Observations

“…In the past, there have been many efforts to estimate sensible and latent heat fluxes over the tropical Pacific (e.g., Weare et al, 1981;Esbensen and Reynolds, 1981;Esbensen and Kushnir, 1981;Reed, 1985;Oberhuber, 1988;Liu, 1988;Chou et al, 1995;da Silva et al, 1995). With the exception of Liu (1988) and Chou et al (1995), long-term ship records and marine weather reports are generally used in research communities to obtain surface heat fluxes for climate studies.…”

“…With the exception of Liu (1988) and Chou et al (1995), long-term ship records and marine weather reports are generally used in research communities to obtain surface heat fluxes for climate studies. These studies provided useful information regarding spatial and temporal variability of fluxes.…”