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C. A. Luecke
,
H. W. Wijesekera
,
E. Jarosz
,
D. W. Wang
,
T. G. Jensen
,
S. U. P. Jinadasa
,
H. J. S. Fernando
, and
W. J. Teague

Abstract

The formation of a sharp oceanic front located south-southeast of Sri Lanka during the southwest monsoon is examined through in situ and remote observations and high-resolution model output. Remote sensing and model output reveal that the front extends approximately 200 km eastward from the southeast coast of Sri Lanka toward the southern Bay of Bengal (BoB). This annually occurring front is associated with the boundary between the southwest monsoon current with high-salinity water to the south, and a weak flow field comprised of relatively fresh BoB water to the north. The front contains a line of high chlorophyll extending from the coastal upwelling zone, often for several hundred kilometers. Elevated turbulent diffusivities ∼10−2 m2 s−1 along with large diapycnal fluxes of heat and salt were found within the front. The formation of the front and vertical transports are linked to local wind stress curl. Large vertical velocities (∼50 m day−1) indicate the importance of ageostrophic, submesoscale processes. To examine these processes, the Ertel potential vorticity (PV) was computed using the observations and numerical model output. The model output shows a ribbon of negative PV along the front between the coastal upwelling zone and two eddies (Sri Lanka Dome and an anticyclonic eddy) typically found in the southern BoB. PV estimates support the view that the flow is susceptible to submesoscale instabilities, which in turn generate high vertical velocities within the front. Frontal upwelling and heightened mixing show that the seasonal front is regionally important to linking the fresh surface water of the BoB with the Arabian Sea.

Significance Statement

Within the ocean, motions span extraordinarily wide ranges of sizes and time scales. In this study we focus on a narrow, intensified feature called a front. This front occurs in the southern Bay of Bengal during the summer monsoon and forms a boundary between fresher water to the north and saltier water to the south. Features such as this are difficult to study, however, by combining observations made from ships and satellites with output from numerical models of the ocean, we are able to better understand the front. This is important because fronts like the one studied here play a role in determining the pathways of heat within the ocean, which, in turn, may feedback into the atmosphere and weather patterns.

Open access
C. A. Luecke
,
H. W. Wijesekera
,
E. Jarosz
,
D. W. Wang
,
J. C. Wesson
,
S. U. P. Jinadasa
,
H. J. S. Fernando
, and
W. J. Teague

Abstract

Long-term measurements of turbulent kinetic energy dissipation rate (ε), and turbulent temperature variance dissipation rate (χ T ) in the thermocline, along with currents, temperature, and salinity were made at two subsurface moorings in the southern Bay of Bengal (BoB). This is a part of a major international program, conducted between July 2018 and June 2019, for investigating the role of the BoB on the monsoon intraseasonal oscillations. One mooring was located on the typical path of the Southwest Monsoon Current (SMC), and the other was in a region where the Sri Lanka dome is typically found during the summer monsoon. Microstructure and finescale estimates of vertical diffusivity revealed the long-term subthermocline mixing patterns in the southern BoB. Enhanced turbulence and large eddy diffusivities were observed within the SMC during the passage of a subsurface-intensified anticyclonic eddy. During this time, background shear and strain appeared to influence high-frequency motions such as near-inertial waves and internal tides, leading to increased mixing. Near the Sri Lanka dome, enhanced dissipation occurred at the margins of the cyclonic feature. Turbulent mixing was enhanced with the passage of Rossby waves and eddies. During these events, values of χ T exceeding 10−4 °C2 s−1 were recorded concurrently with ε values exceeding 10−5 W kg−1. Inferred diffusivity peaked well above background values of 10−6 m2 s−1, leading to an annually averaged diffusivity near 10−4 m2 s−1. Turbulence appeared low throughout much of the deployment period. Most of the mixing occurred in spurts during isolated events.

Free access
H. W. Wijesekera
,
W. J. Teague
,
D. W. Wang
,
E. Jarosz
,
T. G. Jensen
,
S. U. P. Jinadasa
,
H. J. S. Fernando
, and
Z. R. Hallock

Abstract

High-resolution currents and hydrographic fields were measured at six deep-water moorings in the southern Bay of Bengal (BoB) by the Naval Research Laboratory as part of an international effort focused on the dynamics of the Indian Ocean. Currents, temperature, and salinity were sampled over the upper 500 m for 20 months between December 2013 and August 2015. One of the major goals is to understand the space–time scales of the currents and physical processes that contribute to the exchange of water between the BoB and the Arabian Sea. The observations captured Southwest and Northeast Monsoon Currents, seasonally varying large eddies including a cyclonic eddy, the Sri Lanka dome (SLD), and an anticyclonic eddy southeast of the SLD. The observations further showed intraseasonal oscillations with periods of 30–70 days, near-inertial currents, and tides. Monthly averaged velocities commonly exceeded 50 cm s−1 near the surface, and extreme velocities exceeded 150 cm s−1 during the southwest monsoon. Tides were small and dominated by the M2 component with velocities of about 3 cm s−1. The average transport into the BoB over the measurement period was 2 Sv (1 Sv ≡ 106 m3 s−1) but likely exceeded 15 Sv during summer of 2014. This study suggests the water exchange away from coastal boundaries, in the interior of the BoB, may be largely influenced by the location and strength of the two eddies that modify the path of the Southwest Monsoon Current. In addition, there is a pathway below 200 m for transport of water into the BoB throughout the year.

Full access
S. Saha
,
S. Nadiga
,
C. Thiaw
,
J. Wang
,
W. Wang
,
Q. Zhang
,
H. M. Van den Dool
,
H.-L. Pan
,
S. Moorthi
,
D. Behringer
,
D. Stokes
,
M. Peña
,
S. Lord
,
G. White
,
W. Ebisuzaki
,
P. Peng
, and
P. Xie

Abstract

The Climate Forecast System (CFS), the fully coupled ocean–land–atmosphere dynamical seasonal prediction system, which became operational at NCEP in August 2004, is described and evaluated in this paper. The CFS provides important advances in operational seasonal prediction on a number of fronts. For the first time in the history of U.S. operational seasonal prediction, a dynamical modeling system has demonstrated a level of skill in forecasting U.S. surface temperature and precipitation that is comparable to the skill of the statistical methods used by the NCEP Climate Prediction Center (CPC). This represents a significant improvement over the previous dynamical modeling system used at NCEP. Furthermore, the skill provided by the CFS spatially and temporally complements the skill provided by the statistical tools. The availability of a dynamical modeling tool with demonstrated skill should result in overall improvement in the operational seasonal forecasts produced by CPC.

The atmospheric component of the CFS is a lower-resolution version of the Global Forecast System (GFS) that was the operational global weather prediction model at NCEP during 2003. The ocean component is the GFDL Modular Ocean Model version 3 (MOM3). There are several important improvements inherent in the new CFS relative to the previous dynamical forecast system. These include (i) the atmosphere–ocean coupling spans almost all of the globe (as opposed to the tropical Pacific only); (ii) the CFS is a fully coupled modeling system with no flux correction (as opposed to the previous uncoupled “tier-2” system, which employed multiple bias and flux corrections); and (iii) a set of fully coupled retrospective forecasts covering a 24-yr period (1981–2004), with 15 forecasts per calendar month out to nine months into the future, have been produced with the CFS.

These 24 years of fully coupled retrospective forecasts are of paramount importance to the proper calibration (bias correction) of subsequent operational seasonal forecasts. They provide a meaningful a priori estimate of model skill that is critical in determining the utility of the real-time dynamical forecast in the operational framework. The retrospective dataset also provides a wealth of information for researchers to study interactive atmosphere–land–ocean processes.

Full access
W. Feng
,
M. P. Chipperfield
,
H. K. Roscoe
,
J. J. Remedios
,
A. M. Waterfall
,
G. P. Stiller
,
N. Glatthor
,
M. Höpfner
, and
D.-Y. Wang

Abstract

An offline 3D chemical transport model (CTM) has been used to study the evolution of the Antarctic ozone hole during the sudden warming event of 2002 and to compare it with similar simulations for 2000. The CTM has a detailed stratospheric chemistry scheme and was forced by ECMWF and Met Office analyses. Both sets of meteorological analyses permit the CTM to produce a good simulation of the evolution of the 2002 vortex and its breakup, based on O3 comparisons with Total Ozone Mapping Spectrometer (TOMS) column data, sonde data, and first results from the Environmental Satellite–Michelson Interferometer for Passive Atmospheric Sounding (ENVISAT–MIPAS) instrument. The ozone chemical loss rates in the polar lower stratosphere in September 2002 were generally less than in 2000, because of the smaller average active chlorine, although around the time of the warming, the largest vortex chemical loss rates were similar to those in 2000 (i.e., −2.6 DU day−1 between 12 and 26 km). However, the disturbed vortex of 2002 caused a somewhat larger influence of polar processing on Southern Hemisphere (SH) midlatitudes in September. Overall, the calculations show that the average SH chemical O3 loss (poleward of 30°S) by September was ∼20 DU less in 2002 compared with 2000. A significant contribution to the much larger observed polar O3 column in September 2002 was due to the enhanced descent at the vortex edge and increased horizontal transport, associated with the distorted vortex.

Full access
P. Bechtold
,
S. K. Krueger
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W. S. Lewellen
,
E. van Meijgaard
,
C.-H. Moeng
,
D. A. Randall
,
A. van Ulden
, and
S. Wang

Several one-dimensional (ID) cloud/turbulence ensemble modeling results of an idealized nighttime marine stratocumulus case are compared to large eddy simulation (LES). This type of model intercomparison was one of the objects of the first Global Energy and Water Cycle Experiment Cloud System Study boundary layer modeling workshop held at the National Center for Atmospheric Research on 16–18 August 1994.

Presented are results obtained with different 1D models, ranging from bulk models (including only one or two vertical layers) to various types (first order to third order) of multilayer turbulence closure models. The ID results fall within the scatter of the LES results. It is shown that ID models can reasonably represent the main features (cloud water content, cloud fraction, and some turbulence statistics) of a well-mixed stratocumulus-topped boundary layer.

Also addressed is the question of what model complexity is necessary and can be afforded for a reasonable representation of stratocumulus clouds in mesoscale or global-scale operational models. Bulk models seem to be more appropriate for climate studies, whereas a multilayer turbulence scheme is best suited in mesoscale models having at least 100- to 200-m vertical resolution inside the boundary layer.

Full access
Tammy M. Weckwerth
,
John Hanesiak
,
James W. Wilson
,
Stanley B. Trier
,
Samuel K. Degelia
,
William A. Gallus Jr.
,
Rita D. Roberts
, and
Xuguang Wang

Abstract

Nocturnal convection initiation (NCI) is more difficult to anticipate and forecast than daytime convection initiation (CI). A major component of the Plains Elevated Convection at Night (PECAN) field campaign in the U.S. Great Plains was to intensively sample NCI and its near environment. In this article, we summarize NCI types observed during PECAN: 1 June–16 July 2015. These NCI types, classified using PECAN radar composites, are associated with 1) frontal overrunning, 2) the low-level jet (LLJ), 3) a preexisting mesoscale convective system (MCS), 4) a bore or density current, and 5) a nocturnal atmosphere lacking a clearly observed forcing mechanism (pristine). An example and description of each of these different types of PECAN NCI events are presented. The University of Oklahoma real-time 4-km Weather Research and Forecasting (WRF) Model ensemble forecast runs illustrate that the above categories having larger-scale organization (e.g., NCI associated with frontal overrunning and NCI near a preexisting MCS) were better forecasted than pristine. Based on current knowledge and data from PECAN, conceptual models summarizing key environmental features are presented and physical processes underlying the development of each of these different types of NCI events are discussed.

Full access
Will Hobbs
,
Paul Spence
,
Amelie Meyer
,
Serena Schroeter
,
Alexander D. Fraser
,
Philip Reid
,
Tian R. Tian
,
Zhaohui Wang
,
Guillaume Liniger
,
Edward W. Doddridge
, and
Philip W. Boyd

Abstract

In recent years, the Southern Ocean has experienced extremely low sea ice cover in multiple summers. These low events were preceded by a multidecadal positive trend that culminated in record high ice coverage in 2014. This abrupt transition has led some authors to suggest that Antarctic sea ice has undergone a regime shift. In this study we analyze the satellite sea ice record and atmospheric reanalyses to assess the evidence for such a shift. We find that the standard deviation of the summer sea ice record has doubled from 0.31 million km2 in 1979–2006 to 0.76 million km2 for 2007–22. This increased variance is accompanied by a longer season-to-season sea ice memory. The atmosphere is the primary driver of Antarctic sea ice variability, but using a linear predictive model we show that sea ice changes cannot be explained by the atmosphere alone. Identifying whether a regime shift has occurred is difficult without a complete understanding of the physical mechanism of change. However, the statistical changes that we demonstrate (i.e., increased variance and autocorrelation, and a changed response to atmospheric forcing), as well as the increased spatial coherence noted by previous research, are indicators based on dynamical systems theory of an abrupt critical transition. Thus, our analysis is further evidence in support of a changed Antarctic sea ice system.

Significance Statement

In recent years, there have been several summers with extremely low Antarctic sea ice cover, including consecutive record lows in February 2022 and February 2023. Since then, the 2023 winter has seen a remarkably low sea ice growth with an anomaly far below expected climatology. This has led researchers to question whether there has been a regime shift, and we assess the observational evidence for such a shift. In the last decade or so, the variability of summer sea ice has almost doubled, accompanied by a much longer sea ice memory from season to season. These statistical changes, as well an increased spatial coherence noted by other researchers, are consistent with theoretical indicators of a critical transition, or regime shift.

Open access
E. Kalnay
,
M. Kanamitsu
,
R. Kistler
,
W. Collins
,
D. Deaven
,
L. Gandin
,
M. Iredell
,
S. Saha
,
G. White
,
J. Woollen
,
Y. Zhu
,
M. Chelliah
,
W. Ebisuzaki
,
W. Higgins
,
J. Janowiak
,
K. C. Mo
,
C. Ropelewski
,
J. Wang
,
A. Leetmaa
,
R. Reynolds
,
Roy Jenne
, and
Dennis Joseph

The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system.

The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a “quick look” CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CDROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, “C” variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems.

The 40 years of reanalysis (1957–96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data.

NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.

Full access
G. E. Bodeker
,
S. Bojinski
,
D. Cimini
,
R. J. Dirksen
,
M. Haeffelin
,
J. W. Hannigan
,
D. F. Hurst
,
T. Leblanc
,
F. Madonna
,
M. Maturilli
,
A. C. Mikalsen
,
R. Philipona
,
T. Reale
,
D. J. Seidel
,
D. G. H. Tan
,
P. W. Thorne
,
H. Vömel
, and
J. Wang

Abstract

The three main objectives of the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) are to provide long-term high-quality climate records of vertical profiles of selected essential climate variables (ECVs), to constrain and calibrate data from more spatially comprehensive global networks, and to provide measurements for process studies that permit an in-depth understanding of the properties of the atmospheric column. In the five years since the first GRUAN implementation and coordination meeting and the printing of an article (Seidel et al.) in this publication, GRUAN has matured to become a functioning network that provides reference-quality observations to a community of users.

This article describes the achievements within GRUAN over the past five years toward making reference-quality observations of upper-air ECVs. Milestones in the evolution of GRUAN are emphasized, including development of rigorous criteria for site certification and assessment, the formal certification of the first GRUAN sites, salient aspects of the GRUAN manual and guide to operations, public availability of GRUAN’s first data product, outcomes of a network expansion workshop, and key results of scientific studies designed to provide a sound scientific foundation for GRUAN operations.

Two defining attributes of GRUAN are 1) that every measurement is accompanied by a traceable estimate of the measurement uncertainty and 2) that data quality and continuity are maximized because network changes are minimized and managed. This article summarizes how these imperatives are being achieved for existing and planned data products and provides an outlook for the future, including expected new data streams, network expansion, and critical needs for the ongoing success of GRUAN.

Full access