Abstract
The quasigeostrophic and the generalized omega equations are the most widely used methods to reconstruct vertical velocity
w
from in situ data. As observational networks with much higher ...spatial and temporal resolutions are being designed, the question arises of identifying the approximations and scales at which an accurate estimation of
w
through the omega equation can be achieved and what critical scales and observables are needed. In this paper we test different adiabatic omega reconstructions of
w
over several regions representative of main oceanic regimes of the global ocean in a fully eddy-resolving numerical simulation with a 1/60° horizontal resolution. We find that the best reconstructions are observed in conditions characterized by energetic turbulence and/or weak stratification where near-surface frontal processes are felt deep into the ocean interior. The quasigeostrophic omega equation gives satisfactory results for scales larger than ~10 km horizontally while the improvements using a generalized formulation are substantial only in conditions where frontal turbulent processes are important (providing improvements with satisfactory reconstruction skill down to ~5 km in scale). The main sources of uncertainties that could be identified are related to processes responsible for ocean thermal wind imbalance (TWI), which is particularly difficult to account for (especially in observation-based studies) and to the deep flow that is generally improperly accounted for in omega reconstructions through the bottom boundary condition. Nevertheless, the reconstruction of mesoscale vertical velocities may be sufficient to estimate vertical fluxes of oceanic properties in many cases of practical interest.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) is a multinational program initiated in 1997 in the tropical Atlantic to improve our understanding and ability to predict ...ocean‐atmosphere variability. PIRATA consists of a network of moored buoys providing meteorological and oceanographic data transmitted in real time to address fundamental scientific questions as well as societal needs. The network is maintained through dedicated yearly cruises, which allow for extensive complementary shipboard measurements and provide platforms for deployment of other components of the Tropical Atlantic Observing System. This paper describes network enhancements, scientific accomplishments and successes obtained from the last 10 years of observations, and additional results enabled by cooperation with other national and international programs. Capacity building activities and the role of PIRATA in a future Tropical Atlantic Observing System that is presently being optimized are also described.
Plain Language Summary
Long data records are essential for improving our understanding of the weather and climate, their variability and predictability, and how the climate may change in the future in response to anthropogenic greenhouse gas emissions. Climate variability in the tropical Atlantic Ocean has strong impacts on the coastal climate in particular and, consequently, the economies of the surrounding regions. Since 1997, the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) program has maintained a network of moored buoys in the tropical Atlantic in order to provide instantaneous high‐quality data to research scientists and weather forecasters around the world. This paper describes PIRATA successes in terms of scientific discoveries and observing technology enhancements. Perspectives are also provided on PIRATA's role in the future Tropical Atlantic Observing System, currently under design, that will consist of a variety of coordinated measurements from satellites, ships, buoys, and other ocean technologies.
Key Points
We describe the sustained tropical Atlantic observing system PIRATA maintained through a multinational cooperative program in ocean science
Major scientific results and accomplishments obtained from the last 10 years are presented
The potential role of PIRATA in a future Tropical Atlantic Observing System is described
A diagnostic analysis of the climatological annual mean and seasonal cycle of
the Angola–Benguela Frontal Zone (ABFZ) is performed by applying an ocean
frontogenetic function (OFGF) to the
ocean ...mixing layer (OML). The OFGF reveals that the meridional confluence and
vertical tilting terms are the most dominant contributors to the
frontogenesis of the ABFZ. The ABFZ shows a well-pronounced semiannual cycle
with two maximum (minimum) peaks in April–May and November–December
(February–March and July–August). The development of the two maxima of
frontogenesis is due to two different physical processes: enhanced tilting
from March to April and meridional confluence from September to October. The
strong meridional confluence in September to October is closely related to
the seasonal southward intrusion of tropical warm water to the ABFZ that
seems to be associated with the development of the Angola Dome northwest of
the ABFZ. The strong tilting effect from March to April is attributed to the
meridional gradient of vertical velocities, whose effect is amplified in this
period due to increasing stratification and shallow OML depth. The proposed
OFGF can be viewed as a tool to diagnose the performance of coupled general
circulation models (CGCMs) that generally fail at realistically simulating
the position of the ABFZ, which leading to huge warm biases in the
southeastern Atlantic.
A single-column version of the CNRM-CM6-1 global climate model has been developed to ease development and validation of the boundary layer physics and air-sea coupling in a simplified environment. ...This framework is then used to assess the ability of the coupled model to represent the sea surface temperature (SST) diurnal cycle. To this aim, the atmospheric-ocean single-column model (AOSCM), called CNRM-CM6-1D, is implemented in a case study derived from the CINDY2011/DYNAMO campaign over the Indian Ocean, where large diurnal SST variabilities have been well documented.
The HYdrological cycle in the Mediterranean Experiment (HyMeX) Special Observing Period 2 (SOP2, January 27–March 15, 2013) was dedicated to the study of dense water formation in the Gulf of Lion in ...the northwestern Mediterranean. This paper outlines the deep convection of winter 2012–2013 and the meteorological conditions that produced it. Alternating phases of mixing and restratification are related to periods of high and low heat losses, respectively. High-resolution, realistic, three-dimensional models are essential for assessing the intricacy of buoyancy fluxes, horizontal advection, and convective processes. At the submesoscale, vertical velocities resulting from symmetric instabilities of the density front bounding the convection zone are crucial for ventilating the deep ocean. Finally, concomitant atmospheric and oceanic data extracted from the comprehensive SOP2 data set highlight the rapid, coupled evolution of oceanic and atmospheric boundary layer characteristics during a strong wind event.
A simplified model of the atmospheric boundary layer (ABL)
of intermediate complexity between a bulk parameterization and a three-dimensional
atmospheric model is developed and integrated to the ...Nucleus for European Modelling of the Ocean (NEMO) general circulation model.
An objective in the derivation of such a simplified model, called ABL1d, is
to reach an apt representation in ocean-only numerical simulations of some of the
key processes associated with air–sea interactions at the characteristic scales of
the oceanic mesoscale. In this paper we describe the formulation of the
ABL1d model and the strategy to constrain this model with large-scale
atmospheric data available from reanalysis or real-time forecasts. A particular
emphasis is on the appropriate choice and calibration of a turbulent closure scheme
for the atmospheric boundary layer. This is a key ingredient to properly represent
the air–sea interaction processes of interest. We also provide a detailed description
of the NEMO-ABL1d coupling infrastructure and its computational efficiency.
The resulting simplified model is then tested for several boundary-layer regimes
relevant to either ocean–atmosphere or sea-ice–atmosphere coupling. The coupled
system is also tested with a realistic 0.25∘ resolution global configuration.
The numerical results are evaluated using standard metrics
from the literature to quantify the wind–sea-surface-temperature
(a.k.a. thermal feedback effect),
wind–current (a.k.a. current feedback effect), and ABL–sea-ice couplings.
With respect to these metrics, our results show very good agreement with observations
and fully coupled ocean–atmosphere models for a computational overhead of about
9 % in terms of elapsed time compared to standard uncoupled simulations.
This moderate overhead, largely due to I/O operations, leaves room for further
improvement to relax the assumption of horizontal homogeneity behind ABL1d
and thus to further improve the realism of the coupling while keeping the flexibility
of ocean-only modeling.
This study presents the principles of the new coupling interface based on the SURFEX multi-surface model and the OASIS3-MCT coupler. As SURFEX can be plugged into several atmospheric models, it can ...be used in a wide range of applications, from global and regional coupled climate systems to high-resolution numerical weather prediction systems or very fine-scale models dedicated to process studies. The objective of this development is to build and share a common structure for the atmosphere–surface coupling of all these applications, involving on the one hand atmospheric models and on the other hand ocean, ice, hydrology, and wave models. The numerical and physical principles of SURFEX interface between the different component models are described, and the different coupled systems in which the SURFEX OASIS3-MCT-based coupling interface is already implemented are presented.
Estimating the vertical velocity (
w
) in the oceanic upper-layers is a key issue for understanding the cold tongue development in the Eastern Equatorial Atlantic. In this methodological paper, we ...develop an expanded and general formulation of the vertical velocity equation based on the primitive equation (PE) system, in order to gain new insight into the physical processes responsible for the Equatorial and Angola upwellings. This approach is more accurate for describing the real ocean than simpler considerations based on just the wind-driven patterns of surface layer divergence. The
w
-sources/forcings are derived from the PE
w
-equation and diagnosed from a realistic ocean simulation of the Equatorial Atlantic. Sources of
w
are numerous and express the high complexity of terms related to the turbulent momentum flux, to the circulation and to the mass fields, some of them depending explicitly on
w
and others not. The equatorial upwelling is found to be mainly induced by the (i) the zonal turbulent momentum flux, (ii) the curl of turbulent momentum flux and (iii) the imbalance between the circulation and the pressure fields. The Angola upwelling in the eastern part of the basin is controlled by strong curl of turbulent momentum flux. A strong cross-regulation is evidenced between the
w
-forcings independent of
w
and dependent on
w
, which suggests an equatorial balanced-dynamics. The
w
-forcing depending on
w
represents the negative feedback of the ocean to the
w
-forcing independent of
w
: in the equatorial band, this adjustment is led by non-linear processes and by vortex stretching outside.
Abstract
A comparison of June 2005 and June 2006 sea surface temperatures in the eastern equatorial Atlantic exhibits large variability in the properties of the equatorial cold tongue, with far ...colder temperatures in 2005 than in 2006. This difference is found to result mainly from a time shift in the development of the cold tongue between the two years. Easterlies were observed to be stronger in the western tropical Atlantic in April–May 2005 than in April–May 2006, and these winds favorably preconditioned oceanic subsurface conditions in the eastern Atlantic. However, it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid-May 2005 over a broad region extending from 20°W to the African coast and from 6°S to the equator. This particular event underscores the ability of local intraseasonal wind stress variability in the Gulf of Guinea to initiate the cold tongue season and thus to dramatically impact the SST in the eastern equatorial Atlantic. Such intraseasonal wind intensifications are of potential importance for year-to-year variability in the onset of the African monsoon.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This study investigates the nonlinear processes by which the ocean diurnal variations can affect the intraseasonal sea surface temperature (SST) variability in the Atlantic Ocean. The Centre National ...de Recherches Météorologiques one-dimensional ocean model (CNRMOM1D) is forced with the 40-yr ECMWF Re-Analysis (ERA-40) surface fluxes with a 1-h frequency in solar heat flux in a first simulation and with a daily forcing frequency in a second simulation. This model has a vertical resolution of 1 m near the surface. The comparison between both experiments shows that the daily mean surface temperature is modified by about 0.3°–0.5°C if the ocean diurnal variations are represented, and this correction can persist for 15–40 days in the midlatitudes and more than 60 days in the tropics. The so-called rectification mechanism, by which the ocean diurnal warming enhances the intraseasonal SST variability by 20%–40%, is found to be robust in the tropics. In contrast, in the midlatitudes, diurnal variations in wind stress and nonsolar heat flux are shown to affect the daily mean SST. For example, an intense wind stress or nonsolar heat flux toward the atmosphere during the first half of the day followed by weak fluxes during the second half result in a shallow mixed layer. The following day, the preconditioning results in heat being trapped near the surface and the daily mean surface temperature being higher than if these diurnal variations in surface forcings were not resolved.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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