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Abstract
Stommel argued that the seasonal cycle leads to a bias in the coupling between the surface mixed layer and the main thermocline of the ocean. He suggested that a “demon” operated that effectively only allowed fluid at the end of winter to pass from the mixed layer into the main thermocline. In this study, Stommel's hypothesis is examined using diagnostics from a time-dependent coupled mixed layer-primitive equation model of the North Atlantic (CME). The influence of the seasonal cycle on the properties of the main thermocline is investigated using two methods. In the first, the rate and timing of subduction into the main thermocline is diagnosed using kinematic methods from the 1° resolution CME fields. In the second, tracer diagnostics of the CME and idealized experiments using a “date” tracer identifying the timing of subduction are performed. Over the subtropical gyre, both approaches generally support Stommel's hypothesis that fluid is only transferred from the mixed layer into the main thermocline over a short period, ∼1 month, in late winter/early spring. Tracer date experiments are also conducted using the eddy-resolving ⅓° CME fields. Eddy stirring is found to enhance the rate at which the tracer spreads into unventilated regions, but does not alter the seasonal bias of the Stommel demon mechanism.
Abstract
Stommel argued that the seasonal cycle leads to a bias in the coupling between the surface mixed layer and the main thermocline of the ocean. He suggested that a “demon” operated that effectively only allowed fluid at the end of winter to pass from the mixed layer into the main thermocline. In this study, Stommel's hypothesis is examined using diagnostics from a time-dependent coupled mixed layer-primitive equation model of the North Atlantic (CME). The influence of the seasonal cycle on the properties of the main thermocline is investigated using two methods. In the first, the rate and timing of subduction into the main thermocline is diagnosed using kinematic methods from the 1° resolution CME fields. In the second, tracer diagnostics of the CME and idealized experiments using a “date” tracer identifying the timing of subduction are performed. Over the subtropical gyre, both approaches generally support Stommel's hypothesis that fluid is only transferred from the mixed layer into the main thermocline over a short period, ∼1 month, in late winter/early spring. Tracer date experiments are also conducted using the eddy-resolving ⅓° CME fields. Eddy stirring is found to enhance the rate at which the tracer spreads into unventilated regions, but does not alter the seasonal bias of the Stommel demon mechanism.
Abstract
This paper examines trends in the southern annular mode (SAM) and the strength, position, and width of the Southern Hemisphere surface westerly wind jet in observations, reanalyses, and models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). First the period over 1951–2011 is considered, and it is shown that there are differences in the SAM and jet trends between the CMIP5 models, the Hadley Centre gridded SLP (HadSLP2r) dataset, and the Twentieth Century Reanalysis. The relationships between these trends demonstrate that the SAM index cannot be used to directly infer changes in any one kinematic property of the jet. The spatial structure of the observed trends in SLP and zonal winds is shown to be largest, but also most uncertain, in the southeastern Pacific. To constrain this uncertainty six reanalyses are included and compared with station-based observations of SLP. The CMIP5 mean SLP trends generally agree well with the direct observations, despite some climatological biases, while some reanalyses exhibit spuriously large SLP trends. Similarly, over the more reliable satellite era the spatial pattern of CMIP5 SLP trends is in excellent agreement with HadSLP2r, whereas several reanalyses are not. Then surface winds are compared with a satellite-based product, and it is shown that the CMIP5 mean trend is similar to observations in the core region of the westerlies, but that several reanalyses overestimate recent trends. The authors caution that studies examining the impact of wind changes on the Southern Ocean could be biased by these spuriously large trends in reanalysis products.
Abstract
This paper examines trends in the southern annular mode (SAM) and the strength, position, and width of the Southern Hemisphere surface westerly wind jet in observations, reanalyses, and models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). First the period over 1951–2011 is considered, and it is shown that there are differences in the SAM and jet trends between the CMIP5 models, the Hadley Centre gridded SLP (HadSLP2r) dataset, and the Twentieth Century Reanalysis. The relationships between these trends demonstrate that the SAM index cannot be used to directly infer changes in any one kinematic property of the jet. The spatial structure of the observed trends in SLP and zonal winds is shown to be largest, but also most uncertain, in the southeastern Pacific. To constrain this uncertainty six reanalyses are included and compared with station-based observations of SLP. The CMIP5 mean SLP trends generally agree well with the direct observations, despite some climatological biases, while some reanalyses exhibit spuriously large SLP trends. Similarly, over the more reliable satellite era the spatial pattern of CMIP5 SLP trends is in excellent agreement with HadSLP2r, whereas several reanalyses are not. Then surface winds are compared with a satellite-based product, and it is shown that the CMIP5 mean trend is similar to observations in the core region of the westerlies, but that several reanalyses overestimate recent trends. The authors caution that studies examining the impact of wind changes on the Southern Ocean could be biased by these spuriously large trends in reanalysis products.
Abstract
A low-cost tide gauge was developed and field tested to demonstrate a technology that would enable more cost-effective and greater sampling of spatially variable water levels and ocean surface waves. The gauge was designed to be adaptable to expendable and, possibly, air-deployed use for applications such as support of naval operations. The gauge incorporates a single printed circuit board that includes a very low power 3.3-Vdc microprocessor and 1 Mbyte of nonvolatile flash memory. A low-cost solid-state pressure sensor provides pressure data that are corrected automatically as a function of measured pressure and temperature and are processed within the gauge to provide low-frequency water levels and nondirectional surface wave information. Gauge-operating lifetimes range from more than four months to more than two years, depending on the data collection mode (tide or tide–wave) and the data collection interval (half-hourly or hourly). Gauge measurements are compared to measurements from a Sea-Bird Electronics, Inc., wave and tide gauge that uses a high quality quartz sensor.
Abstract
A low-cost tide gauge was developed and field tested to demonstrate a technology that would enable more cost-effective and greater sampling of spatially variable water levels and ocean surface waves. The gauge was designed to be adaptable to expendable and, possibly, air-deployed use for applications such as support of naval operations. The gauge incorporates a single printed circuit board that includes a very low power 3.3-Vdc microprocessor and 1 Mbyte of nonvolatile flash memory. A low-cost solid-state pressure sensor provides pressure data that are corrected automatically as a function of measured pressure and temperature and are processed within the gauge to provide low-frequency water levels and nondirectional surface wave information. Gauge-operating lifetimes range from more than four months to more than two years, depending on the data collection mode (tide or tide–wave) and the data collection interval (half-hourly or hourly). Gauge measurements are compared to measurements from a Sea-Bird Electronics, Inc., wave and tide gauge that uses a high quality quartz sensor.
Abstract
A two-season Observing System Experiment (OSE) was used to quantify the impacts of assimilating the Advanced Scatterometer (ASCAT) surface winds product distributed by the European Organization for the Exploitation of Meteorological Satellites (EUMESAT) and the National Environmental Satellite, Data, and Information Service (NESDIS). The ASCAT wind retrievals were provided by the Royal Netherlands Meteorological Office (KNMI) and the 50-km resolution ASCAT products were assimilated. The impact of assimilating the ASCAT surface wind product in the National Centers for Environmental Prediction (NCEP) Global Data Assimilation/Global Forecast System (GDAS/GFS) was assessed by comparing the forecast results through 168 h for the months of August 2008 and January 2009. The NCEP GDAS/GFS was used, at a resolution of T382–64 layers, as the assimilation system and forecast model for these experiments.
A control simulation utilizing all the data types assimilated in the operational GDAS was compared to an experimental simulation that added the ASCAT surface winds. Quality control procedures required to assimilate the ASCAT surface winds are discussed. Anomaly correlations (ACs) of geopotential height forecasts as well as geographic distribution of AC of geopotential height forecasts at 1000 and 500 hPa were evaluated for the control and experiment during both seasons. The geographical distribution of forecast impact (FI) on the wind and temperature fields near the surface is also presented.
The results of this study show that assimilation of the surface wind retrievals from the ASCAT sensor improve the NCEP GFS wind and temperature forecasts. A positive FI, which suggests the error growth of the experiment is slower than the control, has been realized in the NCEP GDAS/GFS wind and temperature forecasts through 24 h. The ASCAT experiment AC scores show modest forecast improvements from days 4 through 7.
Abstract
A two-season Observing System Experiment (OSE) was used to quantify the impacts of assimilating the Advanced Scatterometer (ASCAT) surface winds product distributed by the European Organization for the Exploitation of Meteorological Satellites (EUMESAT) and the National Environmental Satellite, Data, and Information Service (NESDIS). The ASCAT wind retrievals were provided by the Royal Netherlands Meteorological Office (KNMI) and the 50-km resolution ASCAT products were assimilated. The impact of assimilating the ASCAT surface wind product in the National Centers for Environmental Prediction (NCEP) Global Data Assimilation/Global Forecast System (GDAS/GFS) was assessed by comparing the forecast results through 168 h for the months of August 2008 and January 2009. The NCEP GDAS/GFS was used, at a resolution of T382–64 layers, as the assimilation system and forecast model for these experiments.
A control simulation utilizing all the data types assimilated in the operational GDAS was compared to an experimental simulation that added the ASCAT surface winds. Quality control procedures required to assimilate the ASCAT surface winds are discussed. Anomaly correlations (ACs) of geopotential height forecasts as well as geographic distribution of AC of geopotential height forecasts at 1000 and 500 hPa were evaluated for the control and experiment during both seasons. The geographical distribution of forecast impact (FI) on the wind and temperature fields near the surface is also presented.
The results of this study show that assimilation of the surface wind retrievals from the ASCAT sensor improve the NCEP GFS wind and temperature forecasts. A positive FI, which suggests the error growth of the experiment is slower than the control, has been realized in the NCEP GDAS/GFS wind and temperature forecasts through 24 h. The ASCAT experiment AC scores show modest forecast improvements from days 4 through 7.
Abstract
A two-season observing system experiment (OSE) was used to quantify the impacts of assimilating the WindSat surface winds product developed by the Naval Research Laboratory (NRL). The impacts of assimilating these surface winds were assessed by comparing the forecast results through 168 h for the months of October 2006 and March 2007. The National Centers for Environmental Prediction’s (NCEP) Global Data Assimilation/Global Forecast System (GDAS/GFS) was used, at a resolution of T382-64 layers, as the assimilation system and forecast model for these experiments.
A control simulation utilizing all the data types assimilated in the operational GDAS was compared to an experimental simulation that added the WindSat surface winds. Quality control procedures required to assimilate the surface winds are discussed. Anomaly correlations (ACs) of geopotential heights at 1000 and 500 hPa were evaluated for the control and experiment during both seasons. The geographical distribution of the forecast impacts (FIs) on the wind field and temperature fields at 10-m height and 500 hPa is also discussed.
The results of this study show that assimilating the surface wind retrievals from the WindSat satellite improve the NCEP GFS wind and temperature forecasts. A positive FI, which suggests that the error growth of the experiment is slower than the control, has been realized in the NCEP GDAS/GFS wind and temperature forecasts through 24 h. The WindSat experiment AC scores are similar to the control simulation AC scores until the day 6 forecasts, when the improvements in the WindSat experiment become greater for both seasons and in most of the cases.
Abstract
A two-season observing system experiment (OSE) was used to quantify the impacts of assimilating the WindSat surface winds product developed by the Naval Research Laboratory (NRL). The impacts of assimilating these surface winds were assessed by comparing the forecast results through 168 h for the months of October 2006 and March 2007. The National Centers for Environmental Prediction’s (NCEP) Global Data Assimilation/Global Forecast System (GDAS/GFS) was used, at a resolution of T382-64 layers, as the assimilation system and forecast model for these experiments.
A control simulation utilizing all the data types assimilated in the operational GDAS was compared to an experimental simulation that added the WindSat surface winds. Quality control procedures required to assimilate the surface winds are discussed. Anomaly correlations (ACs) of geopotential heights at 1000 and 500 hPa were evaluated for the control and experiment during both seasons. The geographical distribution of the forecast impacts (FIs) on the wind field and temperature fields at 10-m height and 500 hPa is also discussed.
The results of this study show that assimilating the surface wind retrievals from the WindSat satellite improve the NCEP GFS wind and temperature forecasts. A positive FI, which suggests that the error growth of the experiment is slower than the control, has been realized in the NCEP GDAS/GFS wind and temperature forecasts through 24 h. The WindSat experiment AC scores are similar to the control simulation AC scores until the day 6 forecasts, when the improvements in the WindSat experiment become greater for both seasons and in most of the cases.
Abstract
The large regional summer warming on the east coast of the northern Antarctic Peninsula (AP), which has taken place since the mid-1960s, has previously been proposed to be caused by a trend in the Southern Hemisphere Annular Mode (SAM). The authors utilize a high-resolution regional atmospheric model climatology (14-km grid spacing) to study the mechanisms that determine the response of the near-surface temperature to an increase in the SAM (ΔT/ΔSAM). Month-to-month variations in near-surface temperature and surface pressure are well represented by the model. It is found that north of ∼68°S, ΔT/ΔSAM is much larger on the eastern (lee) side than on the western (windward) side of the barrier. This is because of the enhanced westerly flow of relatively warm air over the barrier, which warms (and dries) further as it descends down the lee slope. The downward motion on the eastern side of the barrier causes a decrease in surface-mass balance and cloud cover. South of ∼68°S, vertical deflection across the barrier is greatly reduced and the contrast in ΔT/ΔSAM between the east and west sides of the barrier vanishes. In the northeastern part of the AP, the modeled ΔT/ΔSAM distribution is similar to the distribution derived from satellite infrared radiometer data. The region of strongest modeled temperature sensitivity to the SAM is where ice shelf collapse has recently taken place and does not extend farther south over the Larsen-C Ice Shelf.
Abstract
The large regional summer warming on the east coast of the northern Antarctic Peninsula (AP), which has taken place since the mid-1960s, has previously been proposed to be caused by a trend in the Southern Hemisphere Annular Mode (SAM). The authors utilize a high-resolution regional atmospheric model climatology (14-km grid spacing) to study the mechanisms that determine the response of the near-surface temperature to an increase in the SAM (ΔT/ΔSAM). Month-to-month variations in near-surface temperature and surface pressure are well represented by the model. It is found that north of ∼68°S, ΔT/ΔSAM is much larger on the eastern (lee) side than on the western (windward) side of the barrier. This is because of the enhanced westerly flow of relatively warm air over the barrier, which warms (and dries) further as it descends down the lee slope. The downward motion on the eastern side of the barrier causes a decrease in surface-mass balance and cloud cover. South of ∼68°S, vertical deflection across the barrier is greatly reduced and the contrast in ΔT/ΔSAM between the east and west sides of the barrier vanishes. In the northeastern part of the AP, the modeled ΔT/ΔSAM distribution is similar to the distribution derived from satellite infrared radiometer data. The region of strongest modeled temperature sensitivity to the SAM is where ice shelf collapse has recently taken place and does not extend farther south over the Larsen-C Ice Shelf.
Abstract
Since the mid-1960s, rapid regional summer warming has occurred on the east coast of the northern Antarctic Peninsula, with near-surface temperatures increasing by more than 2°C. This warming has contributed significantly to the collapse of the northern sections of the Larsen Ice Shelf. Coincident with this warming, the summer Southern Hemisphere Annular Mode (SAM) has exhibited a marked trend, suggested by modeling studies to be predominantly a response to anthropogenic forcing, resulting in increased westerlies across the northern peninsula.
Observations and reanalysis data are utilized to demonstrate that the changing SAM has played a key role in driving this local summer warming. It is proposed that the stronger summer westerly winds reduce the blocking effect of the Antarctic Peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern Antarctic Peninsula. When this occurs, a combination of a climatological temperature gradient across the barrier and the formation of a föhn wind on the lee side typically results in a summer near-surface temperature sensitivity to the SAM that is 3 times greater on the eastern side of the peninsula than on the west. SAM variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula (∼62°S), both at the surface and throughout the troposphere. This is in contrast to a station farther south (∼65°S) where the SAM exerts little influence.
Abstract
Since the mid-1960s, rapid regional summer warming has occurred on the east coast of the northern Antarctic Peninsula, with near-surface temperatures increasing by more than 2°C. This warming has contributed significantly to the collapse of the northern sections of the Larsen Ice Shelf. Coincident with this warming, the summer Southern Hemisphere Annular Mode (SAM) has exhibited a marked trend, suggested by modeling studies to be predominantly a response to anthropogenic forcing, resulting in increased westerlies across the northern peninsula.
Observations and reanalysis data are utilized to demonstrate that the changing SAM has played a key role in driving this local summer warming. It is proposed that the stronger summer westerly winds reduce the blocking effect of the Antarctic Peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern Antarctic Peninsula. When this occurs, a combination of a climatological temperature gradient across the barrier and the formation of a föhn wind on the lee side typically results in a summer near-surface temperature sensitivity to the SAM that is 3 times greater on the eastern side of the peninsula than on the west. SAM variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula (∼62°S), both at the surface and throughout the troposphere. This is in contrast to a station farther south (∼65°S) where the SAM exerts little influence.
Abstract
Commencing in 1956, observations made at Halley Research Station in Antarctica provide one of the longest continuous series of near-surface temperature observations from the Antarctic continent. Since few other records of comparable length are available, the Halley record has been used extensively in studies of long-term Antarctic climate variability and change. The record does not, however, come from a single location but is a composite of observations from a sequence of seven stations, all situated on the Brunt Ice Shelf, that range from around 10 to 50 km in distance from the coast. Until now, it has generally been assumed that temperature data from all of these stations could be combined into a single composite record with no adjustment. Here, we examine this assumption of homogeneity. Application of a statistical changepoint algorithm to the composite record detects a sudden cooling associated with the move from Halley IV to Halley V station in 1992. We show that this temperature step is consistent with local temperature gradients measured by a network of automatic weather stations and with those simulated by a high-resolution atmospheric model. These temperature gradients are strongest in the coastal region and result from the onshore advection of maritime air. The detected inhomogeneity could account for the weak cooling trend seen in the uncorrected composite record. In future, studies that make use of the Halley record will need to account for its inhomogeneity.
Abstract
Commencing in 1956, observations made at Halley Research Station in Antarctica provide one of the longest continuous series of near-surface temperature observations from the Antarctic continent. Since few other records of comparable length are available, the Halley record has been used extensively in studies of long-term Antarctic climate variability and change. The record does not, however, come from a single location but is a composite of observations from a sequence of seven stations, all situated on the Brunt Ice Shelf, that range from around 10 to 50 km in distance from the coast. Until now, it has generally been assumed that temperature data from all of these stations could be combined into a single composite record with no adjustment. Here, we examine this assumption of homogeneity. Application of a statistical changepoint algorithm to the composite record detects a sudden cooling associated with the move from Halley IV to Halley V station in 1992. We show that this temperature step is consistent with local temperature gradients measured by a network of automatic weather stations and with those simulated by a high-resolution atmospheric model. These temperature gradients are strongest in the coastal region and result from the onshore advection of maritime air. The detected inhomogeneity could account for the weak cooling trend seen in the uncorrected composite record. In future, studies that make use of the Halley record will need to account for its inhomogeneity.
Abstract
Summer near-surface temperatures over the northeast coast of the Antarctic Peninsula have increased by more than 2°C over the past 40 years, a temperature increase 3 times greater than that on the northwest coast. Recent analysis has shown a strong correlation between this striking warming trend and significant change in the summer Southern Hemisphere annular mode (SAM), which has resulted in greatly increased summer westerlies across the northern peninsula. It has been proposed that the strengthening westerlies have resulted in increased vertical deflection of relatively warm maritime air over the northern peninsula, contributing significantly to the observed warming and the recent collapse of northern sections of the Larsen Ice Shelf. In this study, laboratory and numerical modeling of airflow incident to the peninsula are employed to further understand this mechanism. It is shown that the effect of the strengthening westerlies has led to a distinct transition from a “blocked” regime to a “flow-over” regime, that is, confirmation of the proposed warming mechanism. The blocked regime is dominated by flow stagnation upstream (i.e., little vertical deflection) and consequent lateral deflection of flow along the western side of the peninsula. The flow-over regime is dominated by vertical deflection of mid/upper-level air over the peninsula, with strong downslope winds following closely to the leeward slope transporting this air (which warms adiabatically as it descends) to the near-surface of the northeast peninsula. The strong rotation typical of high latitudes considerably increases the flow over the peninsula, particularly strengthening it over the southern side (verified by aircraft measurements), suggesting that the warming trend is not solely confined to the northeast. Globally, flow regime transitions such as this may be responsible for other local climate variations.
Abstract
Summer near-surface temperatures over the northeast coast of the Antarctic Peninsula have increased by more than 2°C over the past 40 years, a temperature increase 3 times greater than that on the northwest coast. Recent analysis has shown a strong correlation between this striking warming trend and significant change in the summer Southern Hemisphere annular mode (SAM), which has resulted in greatly increased summer westerlies across the northern peninsula. It has been proposed that the strengthening westerlies have resulted in increased vertical deflection of relatively warm maritime air over the northern peninsula, contributing significantly to the observed warming and the recent collapse of northern sections of the Larsen Ice Shelf. In this study, laboratory and numerical modeling of airflow incident to the peninsula are employed to further understand this mechanism. It is shown that the effect of the strengthening westerlies has led to a distinct transition from a “blocked” regime to a “flow-over” regime, that is, confirmation of the proposed warming mechanism. The blocked regime is dominated by flow stagnation upstream (i.e., little vertical deflection) and consequent lateral deflection of flow along the western side of the peninsula. The flow-over regime is dominated by vertical deflection of mid/upper-level air over the peninsula, with strong downslope winds following closely to the leeward slope transporting this air (which warms adiabatically as it descends) to the near-surface of the northeast peninsula. The strong rotation typical of high latitudes considerably increases the flow over the peninsula, particularly strengthening it over the southern side (verified by aircraft measurements), suggesting that the warming trend is not solely confined to the northeast. Globally, flow regime transitions such as this may be responsible for other local climate variations.
No Abstract available.
No Abstract available.
