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Patrick J. Hogan
and
Harley E. Hurlburt

Abstract

A regional primitive equation ocean model is used to investigate the impact of grid resolution, baroclinic instability, bottom topography, and isopycnal outcropping on the dynamics of the wind and throughflow-forced surface circulation in the Japan/East Sea. The results demonstrate that at least 1/32° (3.5 km) horizontal grid resolution is necessary to generate sufficient baroclinic instability to produce eddy-driven cyclonic deep mean flows. These abyssal currents follow the f/h contours of the bottom topography and allow the bottom topography to strongly influence mean pathways of the upper-ocean currents in the Japan/East Sea. This upper ocean–topographical coupling via baroclinic instability (actually a mixed baroclinic–barotropic instability) requires that mesoscale variability be very well resolved to obtain sufficient coupling. For example, 1/32° resolution is required to obtain a realistic separation latitude of the East Korean Warm Current (EKWC) from the Korean coast when Hellerman–Rosenstein monthly climatological wind stress forcing is used. Separation of the EKWC is more realistic at 1/8° resolution when the model is forced with climatological winds formed from the ECMWF 10-m reanalysis due to strong positive wind stress curl north of the separation latitude, but at 1/8° the level of baroclinic instability is insufficient to initiate upper ocean–topographical coupling. Hence, this major topographical effect is largely missed at coarser resolution and leads to erroneous conclusions about the role of bottom topography and unexplained errors in the pathways of current systems. Results from a 1/64° simulation are similar to those at 1/32°, particularly where the EKWC separates from the Korean coast, suggesting statistical simulation convergence for mesoscale variability has been nearly achieved at 1/32° resolution. Isopycnal outcropping and associated vertical mixing provide an alternate mechanism to topographical control in developing and maintaining a boundary current along the west coast of Japan, but are less important than baroclinic instability in driving deep mean flows.

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Prasad G. Thoppil
and
Patrick J. Hogan

Abstract

The circulation and mesoscale eddies in the Persian Gulf are investigated using results from a high-resolution (∼1 km) Hybrid Coordinate Ocean Model (HYCOM). The circulation in the Persian Gulf is composed of two spatial scales: basin scale and mesoscale. The progression of a cyclonic circulation cell dominates the basin-scale circulation in the eastern half of the gulf (52°–55°E) during March–July. This is primarily the consequence of density-driven outflow–inflow through the Strait of Hormuz and strong stratification. A northwestward-flowing Iranian Coastal Current (ICC; 30–40 cm s−1) between the Strait of Hormuz and north of Qatar (∼52°E) forms the northern flank of the cell. Between July and August the ICC becomes unstable because of the baroclinic instability mechanism by releasing the potential energy stored in the cross-shelf density gradient. As a result, the meanders in the ICC evolve into a series of mesoscale eddies, which is denoted as the Iranian coastal eddies (ICE). The ICE have a diameter of about 115–130 km and extend vertically over most of the water column. Three cyclonic eddies produced by the model during August–September 2005 compared quite well with the Moderate Resolution Imaging Spectroradiometer (MODIS) SST and chlorophyll-a observations. The remnants of ICE are seen until November, after which they dissipate as the winter cooling causes the thermocline to collapse.

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Prasad G. Thoppil
and
Patrick J. Hogan

Abstract

Observations in the Strait of Hormuz (26.26°N, 56.08°E) during 1997–98 showed substantial velocity fluctuations, accompanied by episodic changes in the salinity outflow events with amplitude varying between 1 and 2 psu on time scales of several days to a few weeks. These events are characterized by a rapid increase in salinity followed by an abrupt decline. The mechanisms behind these strong pulses of salinity events are investigated with a high-resolution (∼1 km) Hybrid Coordinate Ocean Model (HYCOM) with particular reference to the year 2005. In accordance with the observations, the simulated salinity events are characterized by strong coherence between the enhanced flows in zonal and meridional directions. It is inferred that most of the simulated and observed outflow variability is associated with the continuous formation of strong mesoscale cyclonic eddies, whose origin can be traced upstream to around 26°N, 55.5°E. These cyclonic eddies have a diameter of about 63 km and have a remnant of Persian Gulf water (PGW) in their cores, which is eroded by lateral mixing as the eddies propagate downstream at a translation speed of 4.1 cm s−1. The primary process that acts to generate mesoscale cyclones results from the barotropic instability of the exchange circulation through the Strait of Hormuz induced by fluctuations in the wind stress forcing. The lack of salinity events and cyclogenesis in a model experiment with no wind stress forcing further confirms the essential ingredients required for the development of strong cyclones and the associated outflow variability.

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Robert Wood
,
Matthew Wyant
,
Christopher S. Bretherton
,
Jasmine Rémillard
,
Pavlos Kollias
,
Jennifer Fletcher
,
Jayson Stemmler
,
Simone de Szoeke
,
Sandra Yuter
,
Matthew Miller
,
David Mechem
,
George Tselioudis
,
J. Christine Chiu
,
Julian A. L. Mann
,
Ewan J. O’Connor
,
Robin J. Hogan
,
Xiquan Dong
,
Mark Miller
,
Virendra Ghate
,
Anne Jefferson
,
Qilong Min
,
Patrick Minnis
,
Rabindra Palikonda
,
Bruce Albrecht
,
Ed Luke
,
Cecile Hannay
, and
Yanluan Lin

Abstract

The Clouds, Aerosol, and Precipitation in the Marine Boundary Layer (CAP-MBL) deployment at Graciosa Island in the Azores generated a 21-month (April 2009–December 2010) comprehensive dataset documenting clouds, aerosols, and precipitation using the Atmospheric Radiation Measurement Program (ARM) Mobile Facility (AMF). The scientific aim of the deployment is to gain improved understanding of the interactions of clouds, aerosols, and precipitation in the marine boundary layer.

Graciosa Island straddles the boundary between the subtropics and midlatitudes in the northeast Atlantic Ocean and consequently experiences a great diversity of meteorological and cloudiness conditions. Low clouds are the dominant cloud type, with stratocumulus and cumulus occurring regularly. Approximately half of all clouds contained precipitation detectable as radar echoes below the cloud base. Radar and satellite observations show that clouds with tops from 1 to 11 km contribute more or less equally to surface-measured precipitation at Graciosa. A wide range of aerosol conditions was sampled during the deployment consistent with the diversity of sources as indicated by back-trajectory analysis. Preliminary findings suggest important two-way interactions between aerosols and clouds at Graciosa, with aerosols affecting light precipitation and cloud radiative properties while being controlled in part by precipitation scavenging.

The data from Graciosa are being compared with short-range forecasts made with a variety of models. A pilot analysis with two climate and two weather forecast models shows that they reproduce the observed time-varying vertical structure of lower-tropospheric cloud fairly well but the cloud-nucleating aerosol concentrations less well. The Graciosa site has been chosen to be a permanent fixed ARM site that became operational in October 2013.

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Tim Boyer
,
Huai-Min Zhang
,
Kevin O’Brien
,
James Reagan
,
Stephen Diggs
,
Eric Freeman
,
Hernan Garcia
,
Emma Heslop
,
Patrick Hogan
,
Boyin Huang
,
Li-Qing Jiang
,
Alex Kozyr
,
Chunying Liu
,
Ricardo Locarnini
,
Alexey V. Mishonov
,
Christopher Paver
,
Zhankun Wang
,
Melissa Zweng
,
Simone Alin
,
Leticia Barbero
,
John A. Barth
,
Mathieu Belbeoch
,
Just Cebrian
,
Kenneth J. Connell
,
Rebecca Cowley
,
Dmitry Dukhovskoy
,
Nancy R. Galbraith
,
Gustavo Goni
,
Fred Katz
,
Martin Kramp
,
Arun Kumar
,
David M. Legler
,
Rick Lumpkin
,
Clive R. McMahon
,
Denis Pierrot
,
Albert J. Plueddemann
,
Emily A. Smith
,
Adrienne Sutton
,
Victor Turpin
,
Long Jiang
,
V. Suneel
,
Rik Wanninkhof
,
Robert A. Weller
, and
Annie P. S. Wong

Abstract

The years since 2000 have been a golden age in in situ ocean observing with the proliferation and organization of autonomous platforms such as surface drogued buoys and subsurface Argo profiling floats augmenting ship-based observations. Global time series of mean sea surface temperature and ocean heat content are routinely calculated based on data from these platforms, enhancing our understanding of the ocean’s role in Earth’s climate system. Individual measurements of meteorological, sea surface, and subsurface variables directly improve our understanding of the Earth system, weather forecasting, and climate projections. They also provide the data necessary for validating and calibrating satellite observations. Maintaining this ocean observing system has been a technological, logistical, and funding challenge. The global COVID-19 pandemic, which took hold in 2020, added strain to the maintenance of the observing system. A survey of the contributing components of the observing system illustrates the impacts of the pandemic from January 2020 through December 2021. The pandemic did not reduce the short-term geographic coverage (days to months) capabilities mainly due to the continuation of autonomous platform observations. In contrast, the pandemic caused critical loss to longer-term (years to decades) observations, greatly impairing the monitoring of such crucial variables as ocean carbon and the state of the deep ocean. So, while the observing system has held under the stress of the pandemic, work must be done to restore the interrupted replenishment of the autonomous components and plan for more resilient methods to support components of the system that rely on cruise-based measurements.

Open access