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- Author or Editor: Stefan Hastenrath x
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Abstract
Updated estimates of meridional heat transport in the Atlantic Ocean water body derived from the surface energy badget agree with the evaluation of hydrographic sections, annual mean northward transports being about 8 and 11 × 1014 W at 30°8 and 25°N, respectively. For the World Ocean as a whole, a southward transport of 6 × 1014 W is obtained at 60°S. Concerning the satellite-derived not radiation at the top of the atmosphere, the five data sets published in the literature are adjusted to form a zero annual mean for the globe as a whole; the required adjustments are ±10 W m−2 for the various sets. Even so, the required poleward heat transport in the atmosphere-ocean system obtained from the five data sets differs conspicuously, with a range of 20 × 1014 W at 30°N. This range may reflect observational errors and real interannual variability. The comfortable numerical agreement notwithstanding, plausible error tolerances far exceed the differences between the heat transport estimates derived by various independent methods.
Abstract
Updated estimates of meridional heat transport in the Atlantic Ocean water body derived from the surface energy badget agree with the evaluation of hydrographic sections, annual mean northward transports being about 8 and 11 × 1014 W at 30°8 and 25°N, respectively. For the World Ocean as a whole, a southward transport of 6 × 1014 W is obtained at 60°S. Concerning the satellite-derived not radiation at the top of the atmosphere, the five data sets published in the literature are adjusted to form a zero annual mean for the globe as a whole; the required adjustments are ±10 W m−2 for the various sets. Even so, the required poleward heat transport in the atmosphere-ocean system obtained from the five data sets differs conspicuously, with a range of 20 × 1014 W at 30°N. This range may reflect observational errors and real interannual variability. The comfortable numerical agreement notwithstanding, plausible error tolerances far exceed the differences between the heat transport estimates derived by various independent methods.
Abstract
Heat budget estimates for the global tropics are derived from recent calculations of the oceanic heat budget and satellite measurements of net radiation at the top of the atmosphere. Annual mean heat export from the zone 30°N–30°S amounts to ∼101 × 1014 W (=100 units). Of this total 39 and 61 units are performed within the oceanic water body and the atmospheric column over sea and land, respectively. In the zone 0–10°N, to which the planetary cloud band (ITCZ) is essentially limited throughout the year, atmospheric heat export reaches only 13 units, as compared to an oceanic export of 18 units from the zone 0–10°S. In particular, oceanic export in the belt 0–5°S alone contributes 11 units which is 90% of the net radiative heat gain at the top of the atmosphere in this latitude zone. Accordingly, the atmospheric heat export from the realm of the ITCZ related to hot tower mechanisms seems to play a more, modest relative role in the global heat budget than heretofore believed. By comparison, oceanic export from the cold water zones immediately to the south of the Atlantic and Pacific equator emerges as an important factor in global energetics.
Oceanic meridional heat transport in the Pacific is directed from the tropics into either hemisphere; in the Atlantic it is northward from high southern latitudes all the way to the arctic; and it is directed south-ward in the Indian Ocean. Oceanic heat gain in the Pacific offsets deficits in the higher southern latitudes of the Atlantic and Indian Ocean sectors, as well as in the Atlantic as a whole. Meridional heat transport for all oceans combined is largest around 30°N and 25°S, where it accounts for 53 and 35% of the total poleward transport. Atmospheric transport is largest and effects the bulk of the total transport in midlatitudes.
Appreciably different estimates of net radiation at the top of the atmosphere, and of oceanic and atmospheric heat export must be regarded as compatible within the broad error limits indicated at present for all three terms.
Abstract
Heat budget estimates for the global tropics are derived from recent calculations of the oceanic heat budget and satellite measurements of net radiation at the top of the atmosphere. Annual mean heat export from the zone 30°N–30°S amounts to ∼101 × 1014 W (=100 units). Of this total 39 and 61 units are performed within the oceanic water body and the atmospheric column over sea and land, respectively. In the zone 0–10°N, to which the planetary cloud band (ITCZ) is essentially limited throughout the year, atmospheric heat export reaches only 13 units, as compared to an oceanic export of 18 units from the zone 0–10°S. In particular, oceanic export in the belt 0–5°S alone contributes 11 units which is 90% of the net radiative heat gain at the top of the atmosphere in this latitude zone. Accordingly, the atmospheric heat export from the realm of the ITCZ related to hot tower mechanisms seems to play a more, modest relative role in the global heat budget than heretofore believed. By comparison, oceanic export from the cold water zones immediately to the south of the Atlantic and Pacific equator emerges as an important factor in global energetics.
Oceanic meridional heat transport in the Pacific is directed from the tropics into either hemisphere; in the Atlantic it is northward from high southern latitudes all the way to the arctic; and it is directed south-ward in the Indian Ocean. Oceanic heat gain in the Pacific offsets deficits in the higher southern latitudes of the Atlantic and Indian Ocean sectors, as well as in the Atlantic as a whole. Meridional heat transport for all oceans combined is largest around 30°N and 25°S, where it accounts for 53 and 35% of the total poleward transport. Atmospheric transport is largest and effects the bulk of the total transport in midlatitudes.
Appreciably different estimates of net radiation at the top of the atmosphere, and of oceanic and atmospheric heat export must be regarded as compatible within the broad error limits indicated at present for all three terms.
Abstract
The subsurface thermal structure in the tropical Atlantic Ocean (30°N–20°S, East of 80°W) is studied on the basis of an extensive data bank of subsurface soundings. Calendar monthly maps are presented showing mixed layer depth, base of thermocline, thermocline thickness, and vertical temperature gradient across the thermocline. These maps are complemented by vertical cross sections depicting mixed layer depth, base of thermocline, and selected isotherms: a zonal profile along the equator (50°W–10°E), a meridional transect across the Eastern Atlantic (4°N–18°S), and a meridional section across the Central Atlantic (30°N–18°S).
The basinwide subsurface thermal structure is dominated by the annual cycle of the surface wind field with extrema around April and August. The mixed layer is relatively shallow between 20°N and 10°S, with overall grater depth in the western as compared to the eastern portion of the basin. Two systems of annual cycle variation of mixed layer depth stand out. (i) Along the equator, the mixed layer depth increases from around April to about August, with largest variations to the west, in direct response to the annual variance of the zonal wind component in the equatorial zone. (ii) In the North Equatorial Atlantic, a northward migration of a band of shallowest mixed layer is apparent from April to August, broadly concordant with the seasonal migration of the confluence zone between the northeast trades and the cross-equatorial airstreams from the Southern Hemisphere, Concomitant with this northward displacement, a belt of maximum mixed layer depth builds up immediately to the north of the equator. The evolution of this trough–ridge structure in mixed layer depth is related to the seasonal reversal of the North Equatorial Countercurrent. Various recent numerical model experiments are in qualitative agreement with the present empirical documentation of the annual cycle of the basinwide pattern of mixed layer depth.
The thermocline is likewise thinnest and most intense and most intense in the low latitudes, especially in the eastern portion of the basin, but its spatial pattern and seasonal variations differ from those of mixed layer. The near-equatorial thermocline structure is characterized by a wide vertical separation of isothermal surfaces at the Equator and extremum zones of thinnest and most intense thermocline at 4°S year round and at 4°N especially in the latter part of the boreal winter semester. It is conjectured that easterly surface winds produce, at the equator, upwelling above and downwelling below the thermocline and the opposite pattern of vertical motion at some distance from the equator, thus leading to the observed thick and weak thermocline at the equator and the isotherm packing around 4°N and S. The marked asymmetry of the surface wind field and the associated wind stress curl pattern within the cross-equatorial airstreams at the height of the boreal summer may be factors for the absence of this extremum zone of thermocline characteristics at 4°N at this time of the year. The comprehensive documentation of subsurface thermal structure presented here is relevant in recent and ongoing empirical and modeling studies of the tropical Atlantic Ocean.
Abstract
The subsurface thermal structure in the tropical Atlantic Ocean (30°N–20°S, East of 80°W) is studied on the basis of an extensive data bank of subsurface soundings. Calendar monthly maps are presented showing mixed layer depth, base of thermocline, thermocline thickness, and vertical temperature gradient across the thermocline. These maps are complemented by vertical cross sections depicting mixed layer depth, base of thermocline, and selected isotherms: a zonal profile along the equator (50°W–10°E), a meridional transect across the Eastern Atlantic (4°N–18°S), and a meridional section across the Central Atlantic (30°N–18°S).
The basinwide subsurface thermal structure is dominated by the annual cycle of the surface wind field with extrema around April and August. The mixed layer is relatively shallow between 20°N and 10°S, with overall grater depth in the western as compared to the eastern portion of the basin. Two systems of annual cycle variation of mixed layer depth stand out. (i) Along the equator, the mixed layer depth increases from around April to about August, with largest variations to the west, in direct response to the annual variance of the zonal wind component in the equatorial zone. (ii) In the North Equatorial Atlantic, a northward migration of a band of shallowest mixed layer is apparent from April to August, broadly concordant with the seasonal migration of the confluence zone between the northeast trades and the cross-equatorial airstreams from the Southern Hemisphere, Concomitant with this northward displacement, a belt of maximum mixed layer depth builds up immediately to the north of the equator. The evolution of this trough–ridge structure in mixed layer depth is related to the seasonal reversal of the North Equatorial Countercurrent. Various recent numerical model experiments are in qualitative agreement with the present empirical documentation of the annual cycle of the basinwide pattern of mixed layer depth.
The thermocline is likewise thinnest and most intense and most intense in the low latitudes, especially in the eastern portion of the basin, but its spatial pattern and seasonal variations differ from those of mixed layer. The near-equatorial thermocline structure is characterized by a wide vertical separation of isothermal surfaces at the Equator and extremum zones of thinnest and most intense thermocline at 4°S year round and at 4°N especially in the latter part of the boreal winter semester. It is conjectured that easterly surface winds produce, at the equator, upwelling above and downwelling below the thermocline and the opposite pattern of vertical motion at some distance from the equator, thus leading to the observed thick and weak thermocline at the equator and the isotherm packing around 4°N and S. The marked asymmetry of the surface wind field and the associated wind stress curl pattern within the cross-equatorial airstreams at the height of the boreal summer may be factors for the absence of this extremum zone of thermocline characteristics at 4°N at this time of the year. The comprehensive documentation of subsurface thermal structure presented here is relevant in recent and ongoing empirical and modeling studies of the tropical Atlantic Ocean.
Abstract
The annual cycle and spatial patterns of subsurface heat storage Qt and divergence of heat transport Qv in the tropical Atlantic Ocean (30°N–20°S, east of 80°W) are studied on the basis of subsurface temperature soundings complied until 1978 and evaluations of the net heat gain through the ocean surface (Qt +Qv ) from long-term ship observations (1911–70). The net oceanic heat gain (Qt +Qv ) follows in large part, but not exclusively, the annual cycle of insolation with largest gain in the respective summer, and loss in the winter half-year. The Qt has an annual range and spatial gradients considerably larger than those of (Qt +Qv ). Poleward of about 15°N, Qt exhibits an annual cycle similar to (Qt +Qv ) and insolation. By contrast, temporal and spatial variations of Qt are more complicated in the equatorial Atlantic (about 10°N–10°S). For the average over this latitude band, heat depletion (negative Qt ) is found around March-May and largest storage around August–September. The divergence of oceanic heat transport Qv is obtained as the difference between (Qt +Qv ) and Qt , and exhibits patterns broadly complementary to those of Qt .
Calendar monthly maps indicate two major systems of annual cycle changes of Qt . (i) A west–east seasaw variation is apparent in the equatorial belt (about 10°S–10°N), with heat depletion prevailing in the western equatorial Atlantic around March and April, and heat storage continuing in the Gulf of Guinea from January to around May. This heat budget pattern is associated with a deepening of isothermal surfaces to the west from about May to July, with concomitant shallowing in the Gulf of Guinea. (ii) The zone 0–15°N is dominated by a northwestward shift of a band of negative Qt from around March to August, and an inverse displacement thereafter. Both systems (i) and (ii) of seasonal changes in Qt broadly parallel the annual cycle of the surface wind field over the tropical Atlantic, characterized by extrema around April and August.
Abstract
The annual cycle and spatial patterns of subsurface heat storage Qt and divergence of heat transport Qv in the tropical Atlantic Ocean (30°N–20°S, east of 80°W) are studied on the basis of subsurface temperature soundings complied until 1978 and evaluations of the net heat gain through the ocean surface (Qt +Qv ) from long-term ship observations (1911–70). The net oceanic heat gain (Qt +Qv ) follows in large part, but not exclusively, the annual cycle of insolation with largest gain in the respective summer, and loss in the winter half-year. The Qt has an annual range and spatial gradients considerably larger than those of (Qt +Qv ). Poleward of about 15°N, Qt exhibits an annual cycle similar to (Qt +Qv ) and insolation. By contrast, temporal and spatial variations of Qt are more complicated in the equatorial Atlantic (about 10°N–10°S). For the average over this latitude band, heat depletion (negative Qt ) is found around March-May and largest storage around August–September. The divergence of oceanic heat transport Qv is obtained as the difference between (Qt +Qv ) and Qt , and exhibits patterns broadly complementary to those of Qt .
Calendar monthly maps indicate two major systems of annual cycle changes of Qt . (i) A west–east seasaw variation is apparent in the equatorial belt (about 10°S–10°N), with heat depletion prevailing in the western equatorial Atlantic around March and April, and heat storage continuing in the Gulf of Guinea from January to around May. This heat budget pattern is associated with a deepening of isothermal surfaces to the west from about May to July, with concomitant shallowing in the Gulf of Guinea. (ii) The zone 0–15°N is dominated by a northwestward shift of a band of negative Qt from around March to August, and an inverse displacement thereafter. Both systems (i) and (ii) of seasonal changes in Qt broadly parallel the annual cycle of the surface wind field over the tropical Atlantic, characterized by extrema around April and August.
Abstract
Interannual variations in the large-scale atmospheric and oceanic fields over the tropical Atlantic are studied in relation to rainfall anomalies on the Angola coast. Departure patterns are constructed by stratification with respect to extremely dry and wet years and by correlation with rainfall in Angola, which is concentrated in March–April.
The analysis suggests a causality chain of atmospheric-oceanic anomalies. Variations of westward wind stress on the western equatorial Atlantic constitute an early link in this chain. The annual cycle is characterized by a relaxation of the wind stress from September–November to February–March. Anomalous seasonal relaxation of easterly wind stress over the western equatorial Atlantic remotely forces the sea surface temperature (SST) departures in the eastern South Atlantic, a large relaxation being followed by positive SST departures. Sea surface temperature modulates the rainfall over downstream Angola by controlling the atmospheric moisture and stability. Within each link of this causality chain, a substantial portion of the variance stems from processes other than the direct line wind stress → SST → rainfall.
Abstract
Interannual variations in the large-scale atmospheric and oceanic fields over the tropical Atlantic are studied in relation to rainfall anomalies on the Angola coast. Departure patterns are constructed by stratification with respect to extremely dry and wet years and by correlation with rainfall in Angola, which is concentrated in March–April.
The analysis suggests a causality chain of atmospheric-oceanic anomalies. Variations of westward wind stress on the western equatorial Atlantic constitute an early link in this chain. The annual cycle is characterized by a relaxation of the wind stress from September–November to February–March. Anomalous seasonal relaxation of easterly wind stress over the western equatorial Atlantic remotely forces the sea surface temperature (SST) departures in the eastern South Atlantic, a large relaxation being followed by positive SST departures. Sea surface temperature modulates the rainfall over downstream Angola by controlling the atmospheric moisture and stability. Within each link of this causality chain, a substantial portion of the variance stems from processes other than the direct line wind stress → SST → rainfall.
Abstract
The heat budget of the atmosphere-ocean-land system in the Indian Ocean region (30°N–30°S, 30–120°E) is studied on the basis of ocean surface heat flux calculations from long-term ship observations and satellite-derived estimates of net radiation at the top of the atmosphere.
The hydrosphere to the north of the equator exports heat at rates of 5 × 1014 W for the year as a whole, and more than 8 × 1014 W during the northern summer (May–October) half-year, respectively. In contrast, the heat budget of the Southern Hemisphere water is dominated by the seasonal storage/depletion of heat transferred through the ocean surface. Oceanic heat export/import is small for this region during both the November–April and May–October half-years, and near zero for the year as a whole. The mean annual net meridional oceanic heat transport is directed southward throughout the study area, reaching a maximum of 8 × 1014 W at 10–15°S. From heat balance considerations, the annual average upwelling north of the equator is calculated to be ∼6 × 10−7 m s−1. Most of the compensatory down-welling must occur outside the tropical Indian Ocean.
Residually determined heat export by the atmosphere north of the equator averages 18 and 4 × 10−14 W during the northern summer and winter half-years, respectively. South of the equator the atmosphere exports heat at a mean annual rate of 19 × 10−14 W, with little seasonal variation. During northern summer, the atmospheric energy export from the southern tropical Indian Ocean is largely in the form of latent heat and is directed northward across the equator. The southern tropical Indian Ocean is the major source of the atmospheric water vapor carried across the coastline of southern Asia during the northern summer southwest monsoon. The larger water vapor flux divergence south of the equator at this time is fed by strong evaporation. This is supported by a combination of the seasonal depletion of the local oceanic heat content and oceanic heat import from north of the equator, in addition to the surface net radiation.
South of about 10°S, the atmosphere must dispose of both the net radiative heat input at the top of the system and the heat imported within the oceanic water body. In contrast, to the north the atmosphere and hydrosphere make similar contributions to the lateral energy export.
Abstract
The heat budget of the atmosphere-ocean-land system in the Indian Ocean region (30°N–30°S, 30–120°E) is studied on the basis of ocean surface heat flux calculations from long-term ship observations and satellite-derived estimates of net radiation at the top of the atmosphere.
The hydrosphere to the north of the equator exports heat at rates of 5 × 1014 W for the year as a whole, and more than 8 × 1014 W during the northern summer (May–October) half-year, respectively. In contrast, the heat budget of the Southern Hemisphere water is dominated by the seasonal storage/depletion of heat transferred through the ocean surface. Oceanic heat export/import is small for this region during both the November–April and May–October half-years, and near zero for the year as a whole. The mean annual net meridional oceanic heat transport is directed southward throughout the study area, reaching a maximum of 8 × 1014 W at 10–15°S. From heat balance considerations, the annual average upwelling north of the equator is calculated to be ∼6 × 10−7 m s−1. Most of the compensatory down-welling must occur outside the tropical Indian Ocean.
Residually determined heat export by the atmosphere north of the equator averages 18 and 4 × 10−14 W during the northern summer and winter half-years, respectively. South of the equator the atmosphere exports heat at a mean annual rate of 19 × 10−14 W, with little seasonal variation. During northern summer, the atmospheric energy export from the southern tropical Indian Ocean is largely in the form of latent heat and is directed northward across the equator. The southern tropical Indian Ocean is the major source of the atmospheric water vapor carried across the coastline of southern Asia during the northern summer southwest monsoon. The larger water vapor flux divergence south of the equator at this time is fed by strong evaporation. This is supported by a combination of the seasonal depletion of the local oceanic heat content and oceanic heat import from north of the equator, in addition to the surface net radiation.
South of about 10°S, the atmosphere must dispose of both the net radiative heat input at the top of the system and the heat imported within the oceanic water body. In contrast, to the north the atmosphere and hydrosphere make similar contributions to the lateral energy export.