Abstract

Thirty-six XBT temperature profiles have been used in a parametric model introduced by Hendry to model the Gulf Stream's thermal structure at 65°W between 200 and 1200 dbar, with an rms residual error of 0.56°C. Velocity has been computed geostrophically relative to 1200 dbar, and has been included in calculating potential vorticity analytically from the model. The resulting potential vorticity section for 65°W has been compared with the analogous result from Hendry's parametric model at 59°W, as well as observed potential vorticity sections from 68° to 55°W. There is a significant feature in the potential vorticity structure at 65°W not found at 59°W-namely, a relative minimum in potential vorticity along isopycnals, centered at the Gulf Stream's axis and 350 dbar. The modeled potential vorticity sections are consistent with the observation including the downstream disappearance of this feature. The dynamical implications of these results are briefly discussed.

Abstract

A simplistic interpretation of eddy heat fluxes from a two-year current meter mooring deployment in the Kuroshio Extension leads to the conclusion that the eddy field is denying at 152°E, contradicting observations from the surface to 300 m that indicate the region to be one of steady or growing eddy energy. Thus, a simplified version of the method used by Hall to construct the velocity field of the current from the moored data has been used to examine the baroclinic and barotropic energy conversions in the cyclonic and anticyclonic portions of the current, for both geographic and ‘stream’ coordinates. Although the error bars are large, in stream coordinates significant conversions of mean to eddy potential energy occur on the anticyclonic side of the current at both 350 and 625 dbar, with smaller average conversions of eddy to mean energy over the cold portion. Barotropic conversions in this coordinate system are small, but qualitatively the calculated Reynolds stresses agree with previous observations showing that ∂(u′v′)/∂y < 0 across the current, so that on average they converge mean momentum. For geographical coordinates, integrated energy balances still suggest overall decay of eddy energy, though not as strong as that found in the “simplistic” interpretation. Reynolds stresses are much stronger than for stream coordinates, and are still convergent, resulting in relatively large apparent conversions of eddy to mean kinetic energy in this coordinate system. Comparison with a similar energetic analysis by Rossby in the Gulf Stream at 73°W shows that: 1) the effects of going from geographical to stream coordinates are similar for the two currents, and 2) at locations that are geographically comparable for the two currents, very different energetic regimes prevail. Dynamical differences are also reflected in the vertical velocity structure. It is hypothesized that external factors, such as the nature of the underlying deep flow, may influence the western boundary current systems in the two oceans in an important way.

Abstract

A curent meter mooring, instrumented from the bottom into the thermocline, was deployed in the Gulf stream at 68°W for a year. Data from the uppermost instrument indicate the Gulf Stream moved back and forth across the mooring site, so that the horizontal as well as vertical structure of the Stream may be deduced. The two key points to the success of the analysis are: 1) the well-defined relationship between temperature and cross-stream distance in the thermocline, enabling the use of the former as a horizontal coordinate; and 2) a daily-changing definition of Gulf Stream flow direction based on the shear between the thermocline and 2000 m depth. Time-series of daily-rotated velocities may be used to calculate empirical orthogonal functions for the along- and cross-stream vertical structures, which are decoupled and are respectively baroclinic and barotropic. Using the inferred horizontal coordinate one can estimate masss, momentum and kinetic energy fluxes agree well with historical data. Bryden's method has been used to calculate vertical velocities from the temperature equation; the resulting time-series of w are visually coherent throughout the water column and their vertical amplitude structure looks like that of a first baroclinic mode. The rms vertical velocities are large [O(.05 cm s⁻¹)], and these as well as other estimates have been used to explore the validity of the quasi-geostrophic approximation at the mooring site. The Rossby number for the thermocline flow is about 0.3, and for the deep flow is ≤ 0.1.

Abstract

A current meter mooring maintained for over three years at 28°N, 152°W, in the eastern North Pacific has yielded velocity and temperature data throughout the water column, with particularly good thermocline resolution The flow is characterized by weak primarily westward mean velocities, with a superimposed eddy field having rms velocities ranging from 10 cm s⁻¹ in the upper thermocline to 3 cm s⁻¹ at 1000 m depth. The eddy energy is divided into two main bands: the low frequency eddies have spatial scales of 250–300 km and periods of 100–200 days, propagate southwestward, and have slightly more zonal than meridional energy. The high frequency eddies also propagate southwestward, have spatial scales of 150–175 km and periods of 40–80 days, and are strongly meridionally oriented. Vertical EOF structure calculated in the frequency domain suggests that the low frequency eddies are more wavelike (linear) in nature than are the high frequency. The entire band appears to derive energy baroclinically from a secularly varying background flow; as a function of time, the eddy heat flux tends to be down the very low frequency varying temperature gradient. Some interesting points of comparison are found with eddies in a three-layer nonlinear model of the eastern North Pacific recently described by Lee.

Abstract

Two CTD sections across the Gulf Stream at 68° and 55°W were acquired in late March of 1988 within 11 days of one another as part of an effort to look at downstream changes in the current. Using complementary current meter measurements, sections of total barotropic and geostrophic baroclinic velocity are constructed and used to calculate transport in potential density classes. Potential vorticity sections are presented for both locations, including the effects of planetary, stretching, and relative vorticity. The data are also used to examine the core properties of recently formed 18°Water at the two sections. It is found that: 1) water parcels in the exposed surface layers experience downstream density and potential vorticity changes consistent with surface forcing; 2) thermocline Gulf Stream transport is conserved downstream and below the exposed layers is conserved within individual density classes; 3) subthermocline Gulf Stream transport increases modestly at levels above the sill depth of the New England Seamounts but quadruples at levels below that; 4) the calculated potential vorticity structure is consistent with the transport distribution and historical observations and displays several distinct layers; and 5) transport and potential vorticity distributions together suggest that five active layers and steep bottom topography are required to fully describe downstream evolution of the Gulf Stream as an open-ocean eastward jet.

Abstract

A moored array at the equator in the western basin of the Atlantic provides a 604-day time series of abyssal currents and temperatures spanning the full breadth of the Antarctic Bottom Water (AABW) flowing from the Brazil Basin to the Guiana Basin. Mean AABW transport is estimated to be 2.0 Sv (Sv ≡ 10⁶ m³ s⁻¹), comprising organized westward flow of 2.24 Sv and return flow of 0.24 Sv. The low-frequency variability is dominated by a quasi-annual transport cycle of amplitude 0.9 Sv and a 120-day period of amplitude 0.6 Sv. Maximum transports occur in September–October, while minimum transports occur in February–March. Allowing for this quasi-annual cycle and extrapolating the 604-day record to a full two years adds about 7% to the estimated mean AABW transport. The array also provides limited sampling in the overlying lower North Atlantic Deep Water (LNADW), where a southern boundary intensified flow of LNADW gives the strongest recorded mean speed through the array, 9.9 cm s⁻¹ into the Brazil Basin. The LNADW records also have a quasi-annual cycle with strong LNADW flow episodes occurring in April–May. Time series of temperature indicate that the LNADW/AABW transition layer rises and falls in synchrony with the quasi-annual AABW transport cycle (uplifted transition layer during strong AABW transport periods). An observed overall warming trend appears to be accompanied by a decline in AABW transport.

Abstract

One of the major objectives of the Deep Basin Experiment, a component of the World Ocean Circulation Experiment, was to quantify the intensity and spatial distribution of deep vertical mixing within the Brazil Basin. In this study, basin-averaged estimates of deep vertical mixing rates are calculated using two independent methodologies and datasets: 1) vertical fluxes are derived from large-scale temperature and density budgets using direct measurements of deep flow through passages connecting the Brazil Basin to surrounding basins and a comprehensive hydrographic dataset within the basin interior and 2) vertical mixing rates are estimated from finescale bathymetry and hydrographic data using a functional relationship between turbulent dissipation and bathymetric roughness, deduced from localized measurements of ocean microstructure obtained during the Deep Basin Experiment. The space–time mean estimates of vertical mixing diffusivities across representative surfaces within the Antarctic Bottom Water layer fell in the range κ ∼ 1–5(× 10⁻⁴ m² s⁻¹) and were indistinguishable from each other within the estimation uncertainties. The mixing rates inferred from potential temperature budgets update, and are consistent with, earlier estimates that were based on less data. Mixing rates inferred from budgets bounded by neutral surfaces are not significantly different from the former. This implies that lateral eddy fluxes along isopycnals are not important in the potential temperature budgets, at least within the large estimation uncertainties. Unresolved processes, such as cabbeling and low frequency variability, which complicate inference of mixing from large-scale budgets, have been considered. The agreement between diffusivity estimates based on a modeled relationship between bathymetric roughness and turbulent dissipation, with those inferred from large-scale budgets, provides independent confirmation that the mixing rates have been accurately quantified.

Abstract

—J. BLUNDEN, T. BOYER, AND E. BARTOW-GILLIES

Earth’s global climate system is vast, complex, and intricately interrelated. Many areas are influenced by global-scale phenomena, including the “triple dip” La Niña conditions that prevailed in the eastern Pacific Ocean nearly continuously from mid-2020 through all of 2022; by regional phenomena such as the positive winter and summer North Atlantic Oscillation that impacted weather in parts the Northern Hemisphere and the negative Indian Ocean dipole that impacted weather in parts of the Southern Hemisphere; and by more localized systems such as high-pressure heat domes that caused extreme heat in different areas of the world. Underlying all these natural short-term variabilities are long-term climate trends due to continuous increases since the beginning of the Industrial Revolution in the atmospheric concentrations of Earth’s major greenhouse gases.

In 2022, the annual global average carbon dioxide concentration in the atmosphere rose to 417.1±0.1 ppm, which is 50% greater than the pre-industrial level. Global mean tropospheric methane abundance was 165% higher than its pre-industrial level, and nitrous oxide was 24% higher. All three gases set new record-high atmospheric concentration levels in 2022.

Sea-surface temperature patterns in the tropical Pacific characteristic of La Niña and attendant atmospheric patterns tend to mitigate atmospheric heat gain at the global scale, but the annual global surface temperature across land and oceans was still among the six highest in records dating as far back as the mid-1800s. It was the warmest La Niña year on record. Many areas observed record or near-record heat. Europe as a whole observed its second-warmest year on record, with sixteen individual countries observing record warmth at the national scale. Records were shattered across the continent during the summer months as heatwaves plagued the region. On 18 July, 104 stations in France broke their all-time records. One day later, England recorded a temperature of 40°C for the first time ever. China experienced its second-warmest year and warmest summer on record. In the Southern Hemisphere, the average temperature across New Zealand reached a record high for the second year in a row. While Australia’s annual temperature was slightly below the 1991–2020 average, Onslow Airport in Western Australia reached 50.7°C on 13 January, equaling Australia's highest temperature on record.

While fewer in number and locations than record-high temperatures, record cold was also observed during the year. Southern Africa had its coldest August on record, with minimum temperatures as much as 5°C below normal over Angola, western Zambia, and northern Namibia. Cold outbreaks in the first half of December led to many record-low daily minimum temperature records in eastern Australia.

The effects of rising temperatures and extreme heat were apparent across the Northern Hemisphere, where snow-cover extent by June 2022 was the third smallest in the 56-year record, and the seasonal duration of lake ice cover was the fourth shortest since 1980. More frequent and intense heatwaves contributed to the second-greatest average mass balance loss for Alpine glaciers around the world since the start of the record in 1970. Glaciers in the Swiss Alps lost a record 6% of their volume. In South America, the combination of drought and heat left many central Andean glaciers snow free by mid-summer in early 2022; glacial ice has a much lower albedo than snow, leading to accelerated heating of the glacier. Across the global cryosphere, permafrost temperatures continued to reach record highs at many high-latitude and mountain locations.

In the high northern latitudes, the annual surface-air temperature across the Arctic was the fifth highest in the 123-year record. The seasonal Arctic minimum sea-ice extent, typically reached in September, was the 11th-smallest in the 43-year record; however, the amount of multiyear ice—ice that survives at least one summer melt season—remaining in the Arctic continued to decline. Since 2012, the Arctic has been nearly devoid of ice more than four years old.

In Antarctica, an unusually large amount of snow and ice fell over the continent in 2022 due to several landfalling atmospheric rivers, which contributed to the highest annual surface mass balance, 15% to 16% above the 1991–2020 normal, since the start of two reanalyses records dating to 1980. It was the second-warmest year on record for all five of the long-term staffed weather stations on the Antarctic Peninsula. In East Antarctica, a heatwave event led to a new all-time record-high temperature of −9.4°C—44°C above the March average—on 18 March at Dome C. This was followed by the collapse of the critically unstable Conger Ice Shelf. More than 100 daily low sea-ice extent and sea-ice area records were set in 2022, including two new all-time annual record lows in net sea-ice extent and area in February.

Across the world’s oceans, global mean sea level was record high for the 11th consecutive year, reaching 101.2 mm above the 1993 average when satellite altimetry measurements began, an increase of 3.3±0.7 over 2021. Globally-averaged ocean heat content was also record high in 2022, while the global sea-surface temperature was the sixth highest on record, equal with 2018. Approximately 58% of the ocean surface experienced at least one marine heatwave in 2022. In the Bay of Plenty, New Zealand’s longest continuous marine heatwave was recorded.

A total of 85 named tropical storms were observed during the Northern and Southern Hemisphere storm seasons, close to the 1991–2020 average of 87. There were three Category 5 tropical cyclones across the globe—two in the western North Pacific and one in the North Atlantic. This was the fewest Category 5 storms globally since 2017. Globally, the accumulated cyclone energy was the lowest since reliable records began in 1981. Regardless, some storms caused massive damage. In the North Atlantic, Hurricane Fiona became the most intense and most destructive tropical or post-tropical cyclone in Atlantic Canada’s history, while major Hurricane Ian killed more than 100 people and became the third costliest disaster in the United States, causing damage estimated at $113 billion U.S. dollars. In the South Indian Ocean, Tropical Cyclone Batsirai dropped 2044 mm of rain at Commerson Crater in Réunion. The storm also impacted Madagascar, where 121 fatalities were reported.

As is typical, some areas around the world were notably dry in 2022 and some were notably wet. In August, record high areas of land across the globe (6.2%) were experiencing extreme drought. Overall, 29% of land experienced moderate or worse categories of drought during the year. The largest drought footprint in the contiguous United States since 2012 (63%) was observed in late October. The record-breaking megadrought of central Chile continued in its 13th consecutive year, and 80-year record-low river levels in northern Argentina and Paraguay disrupted fluvial transport. In China, the Yangtze River reached record-low values. Much of equatorial eastern Africa had five consecutive below-normal rainy seasons by the end of 2022, with some areas receiving record-low precipitation totals for the year. This ongoing 2.5-year drought is the most extensive and persistent drought event in decades, and led to crop failure, millions of livestock deaths, water scarcity, and inflated prices for staple food items.

In South Asia, Pakistan received around three times its normal volume of monsoon precipitation in August, with some regions receiving up to eight times their expected monthly totals. Resulting floods affected over 30 million people, caused over 1700 fatalities, led to major crop and property losses, and was recorded as one of the world’s costliest natural disasters of all time. Near Rio de Janeiro, Brazil, Petrópolis received 530 mm in 24 hours on 15 February, about 2.5 times the monthly February average, leading to the worst disaster in the city since 1931 with over 230 fatalities.

On 14–15 January, the Hunga Tonga-Hunga Ha'apai submarine volcano in the South Pacific erupted multiple times. The injection of water into the atmosphere was unprecedented in both magnitude—far exceeding any previous values in the 17-year satellite record—and altitude as it penetrated into the mesosphere. The amount of water injected into the stratosphere is estimated to be 146±5 Terragrams, or ∼10% of the total amount in the stratosphere. It may take several years for the water plume to dissipate, and it is currently unknown whether this eruption will have any long-term climate effect.