摘要:

Seasonal variability in the isothermal and isopycnal surface mixed layers of the North Pacific Ocean is examined using the Naval Research Laboratory Ocean Mixed Layer Depth (NMLD) Climatology. A comparison with observations from 11 ocean weather stations in the northeast Pacific Ocean is performed that validates the NMLD climatology in this region. The general features of the isothermal layer depth (ILD) and mixed layer depth (MLD) obtained from these mixed layers are explained with wind stress, surface net heat flux, and freshwater flux climatologies, given guidance from a mixed layer model. Departures from a surface‐forced interpretation of turbulent mixing are found near the Kuroshio, where horizontal heat transport is important. The much deeper ILD in the northeast Pacific in winter and spring relative to the MLD reveals a 50 m “barrier layer” between the bottom of the MLD and the top of the thermocline. A detailed analysis shows this barrier layer extends over most of the North Pacific subpolar gyre. It forms when the seasonal thermocline is deepened in winter by surface cooling, such that salinity stratification due to evaporation minus precipitation less than zero (E‐P<0) becomes important in the formation of the MLD. A shallower halocline forms over the subpolar gyre than in other regions of the North Pacific because of precipitation dominating over evaporation in the annual mean. A mechanism for maintaining the shallow halocline is provided by upward vertical motion driven by positive wind stress curl in the presence of diapycnal mixing. Numerical models show this as part of a shallow meridional overturni

ISSN:0148-0227    

DOI:10.1029/2000JC900071

年代:2000

数据来源: WILEY

摘要:

A new method is introduced for determining ocean isothermal layer depth (ILD) from temperature profiles and ocean mixed layer depth (MLD) from density profiles that can be applied in all regions of the world's oceans. This method can accommodate not only in situ data but also climatological data sets that typically have much lower vertical resolution. The sensitivity of the ILD and MLD to the temperature difference criteria used in the surface layer depth definition is discussed by using temperature and density data, respectively: (1) from 11 ocean weather stations in the northeast Pacific and (2) from theWorld Ocean Atlas 1994. Using these two data sets, a detailed statistical error analysis is presented for the ILD and MLD estimation by season. MLD variations with location due to temperature and salinity are properly accounted for in the defining density (Δσt) criterion. Overall, the optimal estimate of turbulent mixing penetration is obtained using a MLD definition of ΔT=0.8°0, although in the northeast Pacific region the optimal MLD criterion is found to vary seasonally. The method is shown to produce layer depths that are accurate to within 20 m or better in 85% or more of the cases. The MLD definition presented in this investigation accurately represents the depth to which turbulent mixing has penetrated and would be a useful aid for validation of one‐dimensional bulk mixed layer models and ocean general circulation models with an embedded mixed

ISSN:0148-0227    

DOI:10.1029/2000JC900072

年代:2000

数据来源: WILEY

摘要:

A 50‐year coupled atmosphere‐ocean model integration is used to study sea surface temperature (SST) and mixed layer depth (h), and the processes which influence them. The model consists of an atmospheric general circulation model coupled to an ocean mixed layer model in ice‐free regions. The midlatitude SST variability is simulated fairly well, although the maximum variance is underestimated and located farther south than observed. The model is clearly deficient in the vicinity of the Gulf Stream and in the eastern tropical Pacific where advective processes are important. The model generally reproduces the observed structure of the meanhin both March and September but underestimates it in the North Atlantic during winter. The net surface heat flux strongly regulates both the mean (¯) and the anomalous (') SSTs throughout the year. The entrainment heat flux, which is proportional to the product of the entrainment rate (We) and the temperature jump at the base of the mixed layer (ΔT), influences SSTs in summer and fall, especially north of ∼35°N (45°N) in the Pacific (Atlantic).We'ΔT¯is more important for the development of SST' in fall compared toWe¯ΔT', which is larger in summer. The entrainment rate is dominated by wind‐induced mixing in summer and surface buoyancy forcing in winter; the density jump at the base of the mixed layer is of secondary importance. In addition, anomalies inhhave a significant impact on the heat balance of the mixed layer during spring and summer. Deep winter mixed layers and the storage of thermal anomalies beneath the shallow mixed layer in summer leads to large winter‐to‐winter persistence of SST anomalies in the far North Atlantic, in accord with observations and stoc

ISSN:0148-0227    

DOI:10.1029/2000JC900074

年代:2000

数据来源: WILEY

摘要:

Seasonal changes and vertical mixing processes in the upper layers of the North Atlantic Ocean are simulated with a basin‐scale sigma coordinate ocean model that uses the Mellor‐Yamada turbulence closure scheme. The cause of insufficient surface mixing and a too shallow summertime thermocline, common problems of ocean models of this type, is investigated in detail by performing a series of sensitivity experiments with different surface forcing conditions and different turbulence parameterizations. A recent improvement in the parameterization of the dissipation term in the Mellor‐Yamada turbulence scheme, which has shown a significant improvement in one‐dimensional calculations, had a positive but relatively small influence on the three‐dimensional calculations. The results quantify the improvement in the model upper ocean thermal structure as surface forcing becomes more realistic from one experiment to another, for example, when monthly mean winds are replaced by 6 hour variable winds. The inclusion of shortwave radiation penetration is especially important to prevent overly shallow model mixed layers during the summer and seems to affect not only the surface layer but also the thermal structure of the upper 200 m of the ocean. The difficulty of evaluating turbulent mixing processes in three‐dimensional models due to errors in surface fluxes, spatial changes, and three‐dimensional effects, as shown here, points to the important role still left for one‐dimensional turbulence models in improving parameterizations used in three‐dimensional

ISSN:0148-0227    

DOI:10.1029/2000JC900088

年代:2000

数据来源: WILEY

摘要:

The main goal of this exploratory study is to determine how the temperature‐salinity relationship changes with horizontal length scale and depth in the ocean. Temperature and salinity were measured on a range of scales from 4 m to 1000 km, towing a SeaSoar along isobars and isopycnals in the subtropical gyre of the North Pacific, during the winter of 1997. The wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location. Measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed; that is, horizontal temperature and salinity gradients tend to cancel each other in their effect on density. Thermohaline variability at small scales is intermittent and clusters around large‐scale thermohaline anomalies. Below the base of the mixed layer, horizontal gradients of temperature are only partially opposed by salinity, and the density ratio is close to 2. In the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals. Density‐compensated variability, ubiquitous in the mixed layer, is reduced along deeper isopycnals. Compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density grad

ISSN:0148-0227    

DOI:10.1029/2000JC900057

年代:2000

数据来源: WILEY

摘要:

The distribution and formation of mesothermal structure (temperature inversions) in the North Pacific subarctic region are investigated through analysis of climatological hydrographic data. It is suggested that the heat and salt that maintain the mesothermal water and thus the halocline in the density range of 26.7–27.2σθare transported as a crossgyre flow from the transition domain just east of Japan, where the waters are influenced by the subtropical gyre water mass, to the eastern subarctic region. Along the transport route the isopycnal potential temperature and thus salinity are well conserved. In the western subarctic gyre, the Bering Sea, and the northern Gulf of Alaska, the temperature reaches its minimum at the surface in winter and the areal coverage agrees well with the distribution of the mesothermal structure. In the southeastern part of the zonally distributed mesothermal structure in the area of 170°E–150°W and 45°–50°N, where the winter sea surface temperature is higher than that in the deeper layer, dichothermal water is formed by subsurface intrusion of the low‐temperature and low‐salinity water that outcropped in the previous winter over the warm and saline water transported from the tr

ISSN:0148-0227    

DOI:10.1029/2000JC900020

年代:2000

数据来源: WILEY

摘要:

The dependence of the normalized radar cross section of the sea on wind variability within the resolution cell is examined by considering probability distributions of cross sections and wind vectors. If a threshold wind speed exists below which backscatter is negligible for steady winds, variability of the wind over the resolution cell is shown to cause significant backscatter at mean wind speeds below the threshold. In fact, if the variability is sufficiently high, cross sections become essentially constant at very low wind speeds. The viability of this model is tested by comparing its predictions based on the NASA scatterometer 2 (NSCAT2) model function with probability distributions obtained from NSCAT cross sections that are collocated with buoy measurements. Both the overall probability distribution of cross sections and the probability of negative cross sections obtained from the NSCAT data are shown to be in good agreement with the predictions. A means of improving the accuracy of low wind speed scatterometer measurements is suggested when wind variability is not too high.

ISSN:0148-0227    

DOI:10.1029/2000JC900043

年代:2000

数据来源: WILEY

摘要:

Oceanic heat storage and heat storage rate are correlated to the sea surface height anomaly signals from the TOPEX/Poseidon altimeter data from 1992 to 1998. The height anomaly data are decomposed through two‐dimensional finite impulse response filtering. The signal components are the basin‐scale (seasonal), westward propagating (Rossby waves), eastward propagating (Kelvin waves), and mesoscale eddies and a small‐scale residual. The sum of these filtered components does not depart significantly from the original signal. Correlations and rms differences are calculated to compare heat storage estimates on the basis of altimeter and in situ data at four sites: Tropical Atmosphere‐Ocean Array, Hawaii Ocean Time‐series Program, California Cooperative Oceanic Fisheries Investigations, and hydrostation “S.” The heat storage anomaly differences range from 52 to 64×107J m−2, and the correlations range from 0.67 to 0.88. The heat storage rate rms differences range from 97 to 228 W m−2, and the correlations range from 0.66 to 0.82. The fraction of the variance associated with Rossby waves varies from 13% in the equatorial Pacific to 62% in the tropical North Pacific. Conversely, in the same regions the nonpropagating signal g

ISSN:0148-0227    

DOI:10.1029/1999JC000048

年代:2000

数据来源: WILEY

摘要:

The Indian Ocean underwent substantial changes in 1997–1998. The observations show not only the appearance of a dipole mode in the tropical region but also a persistent basin‐scale warming. We present in this study an analysis of the basin‐scale sea surface temperature (SST) variations during 1997–1998 using satellite observations, in situ temperature measurements, and National Centers for Environmental Prediction reanalyses. We find that the Indian SST anomaly peaks occurred at two periods, i.e., November–December–January coinciding with the Niño3 peak and the following April–May–June, and were phase locked to the Indian Ocean seasonal cycle. The changes of SST in the equatorial ocean were related to a coupled interaction between the atmosphere and the ocean. Oceanic upwelling in the east and downwelling in the west played a major role in giving rise to the SST anomalies associated with the dipole mode structure. The upwelling off the coast of Sumatra elevated the regional thermocline by more than 80 m in December 1997. On the other hand, the changes of SST in the southern Indian Ocean were largely induced by the changes of local latent flux. During boreal fall‐winter of 1997 the southeasterly trades were displaced and abnormally prolonged in their northernmost equatorial position. This shifted the center of the trades toward the equator, weakened the winds in the central Southern Ocean, reduced the latent heat flux in the region, and subsequently induced a surface warming. The total change of the SST anomalies integrated over the two periods, July–December 1997 and January–May 1998, were explained well by the same period latent flux integral in both intensity and pattern. The cross‐basin upper ocean temperature sections show that the extratropical warming was rather uniformly distributed in the upper 60 m, further supporting the role of mixed‐laye

ISSN:0148-0227    

DOI:10.1029/2000JC900068

年代:2000

数据来源: WILEY

摘要:

The semienclosed western Mediterranean Sea has proven to be a useful location to evaluate surface heat flux estimates. In the past the directly measured average oceanic heat transport from the Atlantic into the Mediterranean Sea through the Strait of Gibraltar of ∼5.2±1.3 W m−2has been compared to estimates of the average heat flux across the surface of the Mediterranean Sea. On long timescales both should closely balance each other. By using a monthly temperature climatology of the western Mediterranean Sea we offer the possibility to extend the comparison to the seasonal timescale. This gives additional information with which different surface heat flux data sets can be evaluated. The seasonal heat content changes of the western Mediterranean and the advective exchange of heat through the Straits of Gibraltar and Sicily are estimated on the basis of a new extensive hydrographic data set and of published values for the volume transports. To demonstrate the method, a limited number of surface heat flux data sets are compared with the oceanographically calculated counterpart. The comparison reveals that some heat fluxes do not only agree well for the long‐term averages but also for the seasonal timescale, whereas others show larger deviations. The remaining rms discrepancies of ±10.2 W m−2for the best heat flux data set are smaller than the uncertainty of the oceanographic estimate and of a reasonable magnitude compared to the uncertainty of the long‐term average

ISSN:0148-0227    

DOI:10.1029/2000JC900039

年代:2000

数据来源: WILEY