Masanori Konda

Japanese Ocean Flux Data Sets with Use of Remote Sensing Observations (J-OFURO)

We have constructed ocean surface data sets using mainly satellite data and called them Japanese Ocean Flux data sets with Use of Remote sensing Observations (J-OFURO). The data sets include shortwave radiation, longwave radiation, latent heat flux, sensible heat flux, and momentum flux etc. This article introduces J-OFURO and compares it with other global flux data sets such as European Centre for Medium Range Weather Forecasting (ECMWF) and National Center for Environmental Prediction (NCEP) reanalysis data and da Silva et al. (1994). The usual ECMWF data are used for comparison of zonal wind. The comparison is carried out for a meridional profile along the dateline for January and July 1993. Although the overall spatial variation is common for all the products, there is a large difference between them in places. J-OFURO shortwave radiation in July shows larger meridional contrast than other data sets. On the other hand, J-OFURO underestimates longwave radiation flux at low- and mid-latitudes in the Southern Hemisphere. J-OFURO latent heat flux in January overestimates at 10°N–20°N and underestimates at 25°N–40°N. Finally, J-OFURO shows a larger oceanic net heat loss at 10°N–20°N and a smaller loss north of 20°N in January. The data of da Silva et al. in July show small net heat loss around 20°S and large gain around 20°N, while the NCEP reanalysis (NRA) data show the opposite. The da Silva et al. zonal wind speed overestimates at low-latitudes in January, while ECMWF wind data seem to underestimate the easterlies.

A new method to determine near‐sea surface air temperature by using satellite data

We present a new algorithm with which to determine near-sea surface air temperature from satellite observations. A relationship between air temperature, sea surface temperature (SST), wind speed, and humidity is obtained from the aerodynamical equation and the bulk formula. We solve air temperature from this relationship by giving other variables with the observations and without explicit assumption of boundary layer parameters. Our new method is validated using observed monthly mean data at the Japan Meteorological Agency (JMA) and Tropical Ocean-Global Atmosphere (TOGA)-Tropical Atmosphere Ocean (TAO) buoys. Air temperature and sensible heat flux can be determined with accuracies of 0.0° ± 1.2°C and 0.1 ± 8.7 W/m2, respectively, when SST, wind speed, and humidity are given by in situ observations. In order to retrieve near-sea surface air temperature and sensible heat flux from satellite data, SST is obtained from multi channel sea surface temperature (MCSST) of advanced very high resolution radiometer (AVHRR), wind speed is obtained from special sensor microwave/imager (SSM/I), and humidity is obtained from SSM/I together with the empirical equation proposed by Liu [1986]. An error of monthly mean satellite-derived air temperature is −0.3° ± 3.1°C, and that of sensible heat flux is 10.0 ± 37.6 W/m2. Errors of both satellite-derived values are enlarged, possibly because the satellite observations have seasonably systematic error. Time evolution of the interannual variation of air temperature estimated by satellite agrees well with that of in situ measurements. The estimation error of the interannual variation of air temperature is 0.0° ± 1.5°C, and that of sensible heat flux is 0.3 ± 19.0 W/m2. At an interannual timescale the satellite-derived sensible heat flux catches the time evolution of the in situ observation. Both the air temperature and the sensible heat flux obtained by using our method have fewer errors than estimations made by assuming relative humidity as the climatological mean value.

Surface Heat Flux Variations across the Kuroshio Extension as Observed by Surface Flux Buoys

Abstract Wintertime sea surface heat flux variability across the Kuroshio Extension (KE) front is analyzed using data from the Kuroshio Extension Observatory (KEO) buoy in the Kuroshio recirculation gyre south of the KE front and from the Japan Agency for Marine–Earth Science and Technology KEO (JKEO) buoy in the north of the front. The coincident data used are from periods during two winters (2007 and 2008), when both buoys had a complete suite of meteorological data. In these two winter periods, the focus of this research is on three types of typical weather patterns referred to here as the northerly wind condition, the monsoon wind condition, and the normal condition. During the northerly wind condition, latent and sensible heat fluxes were large and often varied simultaneously at both sites, whereas during the monsoon wind condition the latent heat flux at the KEO site was significantly larger than that at the JKEO site. The difference between these heat flux patterns is attributed to the different ai...

Measurement of the sea surface emissivity

The sea surface emissivity in the infrared region is determined on the basis of data analyses. Net radiation, surface irradiance and other oceanographical and meteorological variables are measured throughout most of the year at the oceanographical observatory tower in Tanabe Bay, Japan. We have found that 0.984±0.004 is a reliable emissivity value from the night time data. Surface emission radiates not from the subsurface water but from the sea surface. The thermal skin layer on the sea surface, however, is disturbed and disappears under high wind speed over 5 m/s through the analyses of the radiation observation using the emissivity value of 0.984. Under low wind speed, the sea surface can be cooler or warmer than the subsurface due to overlying thermal conditions and the skin layer can be neutral as the transient process between them. By using an emissivity value of 0.984, the temperature difference between the sea surface temperature and the temperature determined from surface irradiance that has been reported in the satellite data analyses is found to be reduced by half.

Interannual variability of the sea-surface temperature in the Indian Ocean in response to the air–sea turbulent heat exchange

Abstract We investigate the interannual variability of the sea-surface temperature (SST) regulated by the thermal and dynamical forcings at the sea surface associated with seasonal changes of the monsoon circulation in the Indian Ocean. Analysis is done with satellite observations from July 1987 to December 1998. Interannual variations of the air–sea thermal exchange are examined along a path that defines the main monsoon circulation. Taking into account the advection by meridional atmospheric circulation, we show that the anomalous moisture contents are transferred meridionally from south of the equator to the north during the southwest monsoon. It is also shown that the interannual signals from north and south of the equator can be reduced in the equatorial region as a result of convergence of the wind during the northeast monsoon. Due to the synchronous changes of wind and SST anomalies, which have opposite effects on the turbulent heat flux, the air–sea thermal coupling is minimized in the western tropical Indian Ocean on an interannual time scale. In the equatorial region, the resultant air–sea thermal exchange at low latitude almost depends on the atmospheric condition in the mid latitude instead of the thermal condition at the sea surface. In addition, it is thought that the interannual variation of the SST at the equatorial zone is mainly controlled by the wind-induced heat transport in the western and the central areas. In contrast, the SST anomalies in the eastern half of the basin are usually controlled by the surface thermal forcing except for the marked ENSO years. It is suggested that the east–west contrast of heating anomalies due to the wind driven heat transport may affect the oceanic heat balance in the eastern tropical Indian Ocean. The SST anomaly in the southern mid latitude is thermally controlled by the air–sea heat exchange. Our results suggest that the SST anomalies in the mid and low latitudes are linked and modulated through the meridional transport of atmospheric anomalies, although they are determined by the distinct processes.

Evaluation of Vector Winds Observed by NSCAT in the Seas around Japan

In order to validate wind vectors derived from the NASA Scatterometer (NSCAT), two NSCAT wind products of different spatial resolutions are compared with observations by buoys and research vessels in the seas around Japan. In general, the NSCAT winds agree well with the wind data from the buoys and vessels. It is shown that the root-mean-square (rms) difference between NSCAT-derived wind speeds and the buoy observations is 1.7 ms−1, which satisfies the mission requirement of accuracy, 2 ms−1. However, the rms difference of wind directions is slightly larger than the mission requirement, 20°. This result does not agree with those of previous studies on validation of the NSCAT-derived wind vectors using buoy observations, and is considered to be due to differences in the buoy observation systems. It is also shown that there are no significant systematic trends of the NSCAT wind speed and direction depending on the wind speed and incidence angle. Comparison with ship winds shows that the NSCAT wind speeds are lower than those observed by the research vessels by about 0.7 ms−1 and this bias is twice as large for data observed by moving ships than by stationary ships. This result suggests that the ship winds may be influenced by errors caused by ships motion, such as pitching and rolling.

Analysis of the global relationship of biennial variation of sea surface temperature and air-sea heat flux using satellite data

We investigated the phase difference and the cross correlation coefficient between the band-pass filtered biennial variations of sea surface temperature (SST) and air-sea heat flux estimated by the monthly mean 2°×2° satellite data of Advanced Very High Resolution Radiometer (AVHRR) and Special Sensor Microwave/Imager (SSM/I) from July 1987 to June 1991. Judging from the phase difference, it can be determined whether the biennial variation of SST is controlled by local thermal air-sea interaction or oceanic processes of horizontal transport. When the local air-sea heat flux controls the biennial variation of SST, the phase of SST advances π/2 (∼6 months) against that of the air-sea heat flux. In contrast, when the biennial variation of SST is controlled by the oceanic process, the phase difference between the SST and the air-sea heat flux becomes 0 or π(∼12 months). In this case, two types of the phase differences are determined, depending on which variability of SST and air-sea heat flux is larger. The close thermal air-sea interaction is noticeable in the tropics and in the western boundary current region. The phase difference of π/2 appears mainly in the north Pacific, the southeast Indian Ocean, and the western tropical Pacific; zero in the eastern tropical Pacific and the northeast and equatorial Atlantic; and that of π in the central equatorial Pacific and north of the intertropical convergence zone (ITCZ) of the Atlantic. Phase differences of 0, π, or π/2 are possible in the western boundary current regions. This fact indicates that each current plays a different role to the biennial variation of SST. It is inferred that SST anomalies in the tropics are mutually correlated, and the process in which marked SST anomalies in the tropics are transferred to the remote area was probed. In the equatorial Pacific, the SST anomaly is transferred by the long planetary wave. On the other hand, it is found from the phase relationship and the horizontal correlation of SST that the SST anomaly in the central and western equatorial Pacific is connected through atmospheric mediation. It is suggested that the biennial variation of SST in the eastern Indian Ocean is affected by heat transport due to the Indonesian throughflow from the western tropical Pacific. It is found that the mentioned pattern of the interannual variation of SST in the tropical Atlantic as a dipole is not tenable.