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Air‐Sea Interactions and Water Mass Transformation During a Katabatic Storm in the Irminger Sea (2022)

Gutjahr, O. ; Jungclaus, J. H. ; Brüggemann, N. ; [et al.]

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Publication Date: 2022-10-04

Description: We use a global 5‐km resolution model to analyze the air‐sea interactions during a katabatic storm in the Irminger Sea originating from the Ammassalik valleys. Katabatic storms have not yet been resolved in global climate models, raising the question of whether and how they modify water masses in the Irminger Sea. Our results show that dense water forms along the boundary current and on the shelf during the katabatic storm due to the heat loss caused by the high wind speeds and the strong temperature contrast. The dense water contributes to the lightest upper North Atlantic Deep Water as upper Irminger Sea Intermediate Water and thus to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). The katabatic storm triggers a polar low, which in turn amplifies the near‐surface wind speed due to the superimposed pressure gradient, in addition to acceleration from a breaking mountain wave. Overall, katabatic storms account for up to 25% of the total heat loss (20 January 2020 to 30 September 2021) over the Irminger shelf of the Ammassalik area. Resolving katabatic storms in global models is therefore important for the formation of dense water in the western boundary current of the Irminger Sea, which is relevant to the AMOC, and for the large‐scale atmospheric circulation by triggering polar lows.

Description: Plain Language Summary: Katabatic storms are outbursts of cold air associated with strong winds from coastal valleys of Greenland, in particular from the Ammassalik valleys in southeast Greenland. These storms are not resolved in global climate models because of their small spatial extent. However, they are important for the formation of dense water on the Irminger Sea shelf, because they induce a substantial heat loss from the coastal water. In this study, we resolve katabatic storms for the first time in a global climate model and analyze the water transformation caused by a single storm before quantifying the importance of katabatic storms for the entire simulation period. We find that a water mass is formed during the katabatic storm that is dense enough to contribute to the cooling and sinking of the global conveyor belt in the subpolar North Atlantic. Overall, katabatic storms account for up to 25% of the heat loss over the Irminger shelf of the Ammassalik area.

Description: Key Points: For the first time, the direct effect of a katabatic storm on the Irminger Sea has been simulated in a global climate model. The katabatic storm induces strong heat loss and dense water formation over the Irminger shelf (Sermilik Trough) and in the boundary current. Dense water forming in the western boundary current during katabatic storms contributes to the lightest upper North Atlantic Deep Water.

Description: Collaborative Research Centre TRR181 funded by DFG

Description: Max Planck Society for Advancement of Science

Description: NextGEMS

Description: European Union’s Horizon 2020

Description: https://hdl.handle.net/21.11116/0000-0008-ECF1-E

Description: https://cera-www.dkrz.de/WDCC/ui/Compact.jsp?acronym=DKRZ_LTA_033_ds00010

Description: https://mpimet.mpg.de/en/science/modeling-with-icon/code-availability

Keywords: ddc:551.5

Language: English

Type: doc-type:article

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Corrigendum to "Observed and simulated estimates of the meridional overturning circulation at 26.5 degrees N in the Atlantic" published in Ocean Sci., 5, 575–589, 2009 (2010)

Baehr, J. ; Cunningham, S. ; Haak, H. ; [et al.]

Copernicus Publications (EGU)

In: Ocean Science, 6 (1). p. 1.

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Publication Date: 2013-01-31

Type: Article , PeerReviewed

Format: text

DOI: 10.5194/os-6-1-2010

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Seasonal variability of the Atlantic meridional overturning circulation at 26.5°N (2010)

Kanzow, Torsten ; Cunningham, S. A. ; Johns, W. E. ; [et al.]

AMS (American Meteorological Society)

In: Journal of Climate, 23 (21). pp. 5678-5698.

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Publication Date: 2020-08-04

Description: The Atlantic meridional overturning circulation (AMOC) makes the strongest oceanic contribution to the meridional redistribution of heat. Here, an observation-based, forty-eight-month-long time series of the vertical structure and strength of the AMOC at 26.5°N is presented. From April 2004 to April 2008 the AMOC had a mean strength of 18.7 ±2.1 Sv with fluctuations of 4.8 Sv rms. The best guess of the peak-to-peak amplitude of the AMOC seasonal cycle is 6.7 Sv, with a maximum strength in autumn and a minimum in spring. While seasonality in the AMOC was commonly thought to be dominated by the northward Ekman transport, this study reveals that fluctuations of the geostrophic mid-ocean and Gulf Stream transports of 2.2 Sv and 1.7 Sv rms, respectively, are substantially larger than those of the Ekman component (1.2 Sv rms). A simple model based on linear dynamics suggests that the seasonal cycle is dominated by wind stress curl forcing at the eastern boundary of the Atlantic. Seasonal geostrophic AMOC anomalies might represent an important and previously underestimated component of meridional transport and storage of heat in the subtropical North Atlantic. There is evidence that the seasonal cycle observed here is representative of much longer intervals. Previously, hydrographic snapshot estimates between 1957 and 2004 had suggested a long-term decline of the AMOC by 8 Sv. This study suggests that aliasing of seasonal AMOC anomalies might have accounted for a large part of the inferred slowdown.

Type: Article , PeerReviewed

Format: text

DOI: 10.1175/2010JCLI3389.1

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The contribution of eastern-boundary density variations to the Atlantic meridional overturning circulation at 26.5° N (2010)

Chidichimo, M. P. ; Kanzow, Torsten ; Cunningham, S. A. ; [et al.]

Copernicus Publications (EGU)

In: Ocean Science, 6 (2). pp. 475-490.

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Publication Date: 2013-01-31

Description: We study the contribution of eastern-boundary density variations to sub-seasonal and seasonal anomalies of the strength and vertical structure of the Atlantic Meridional Overturning Circulation (AMOC) at 26.5° N, by means of the RAPID/MOCHA mooring array between April 2004 and October 2007. The major density anomalies are found in the upper 500 m, and they are often coherent down to 1400 m. The densities have 13-day fluctuations that are apparent down to 3500 m. The two strategies for measuring eastern-boundary density – a tall offshore mooring (EB1) and an array of moorings on the continental slope (EBH) – show little correspondence in terms of amplitude, vertical structure, and frequency distribution of the resulting basin-wide integrated transport fluctuations, implying that there are significant transport contributions between EB1 and EBH. Contrary to the original planning, measurements from EB1 cannot serve as backup or replacement for EBH: density needs to be measured directly at the continental slope to compute the full-basin density gradient. Fluctuations in density at EBH generate transport variability of 2 Sv rms in the AMOC, while the overall AMOC variability is 4.8 Sv rms. There is a pronounced deep-reaching seasonal cycle in density at the eastern boundary, which is apparent between 100 m and 1400 m, with maximum positive anomalies in spring and maximum negative anomalies in autumn. These changes drive anomalous southward upper mid-ocean flow in spring, implying maximum reduction of the AMOC, and vice-versa in autumn. The amplitude of the seasonal cycle of the AMOC arising from the eastern-boundary densities is 5.2 Sv peak-to-peak, dominating the 6.7 Sv peak-to-peak seasonal cycle of the total AMOC. Our analysis suggests that the seasonal cycle in density may be forced by the strong near-coastal seasonal cycle in wind stress curl.

Type: Article , PeerReviewed

Format: text

DOI: 10.5194/os-6-475-2010

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Decadal Climate Prediction: Opportunities and Challenges (2010)

Hurrell, J. ; Delworth, T. ; Danabasoglu, G ; [et al.]

In: In: Proceedings of the "OceanObs'09: Sustained Ocean Observations and Information for Society". , ed. by Hall, J., Harrison, D. E. and Stammer, D. ESA Publication, WPP-306 . UNSPECIFIED.

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Publication Date: 2012-02-23

Type: Book chapter , NonPeerReviewed

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The present and future system for measuring the Atlantic meridional overturning circulation and heat transport (2010)

Cunningham, S. ; Baringer, M. ; Johns, B. ; [et al.]

OceanObs'09

In: In: Proceedings of the "OceanObs'09: Sustained Ocean Observations and Information for Society" Conference. , ed. by Hall, J., Harrison, D. E. and Stammer, D. ESA Publication, WPP-306 . OceanObs'09, Venice, Italy.

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Publication Date: 2012-07-06

Description: of the global combined atmosphere-ocean heat flux and so is important for the mean climate of the Atlantic sector of the Northern Hemisphere. This meridional heat flux is accomplished by both the Atlantic Meridional Overturning Circulation (AMOC) and by basin-wide horizontal gyre circulations. In the North Atlantic subtropical latitudes the AMOC dominates the meridional heat flux, while in subpolar latitudes and in the subtropical South Atlantic the gyre circulations are also important. Climate models suggest the AMOC will slow over the coming decades as the earth warms, causing widespread cooling in the Northern hemisphere and additional sea-level rise. Monitoring systems for selected components of the AMOC have been in place in some areas for decades, nevertheless the present observational network provides only a partial view of the AMOC, and does not unambiguously resolve the full variability of the circulation. Additional observations, building on existing measurements, are required to more completely quantify the Atlantic meridional heat transport. A basin-wide monitoring array along 26.5°N has been continuously measuring the strength and vertical structure of the AMOC and meridional heat transport since March 31, 2004. The array has demonstrated its ability to observe the AMOC variability at that latitude and also a variety of surprising variability that will require substantially longer time series to understand fully. Here we propose monitoring the Atlantic meridional heat transport throughout the Atlantic at selected critical latitudes that have already been identified as regions of interest for the study of deep water formation and the strength of the subpolar gyre, transport variability of the Deep Western Boundary Current (DWBC) as well as the upper limb of the AMOC, and inter-ocean and intrabasin exchanges with the ultimate goal of determining regional and global controls for the AMOC in the North and South Atlantic Oceans. These new arrays will continuously measure the full depth, basin-wide or choke-point circulation and heat transport at a number of latitudes, to establish the dynamics and variability at each latitude and then their meridional connectivity. Modeling studies indicate that adaptations of the 26.5°N type of array may provide successful AMOC monitoring at other latitudes. However, further analysis and the development of new technologies will be needed to optimize cost effective systems for providing long term monitoring and data recovery at climate time scales. These arrays will provide benchmark observations of the AMOC that are fundamental for assimilation, initialization, and the verification of coupled hindcast/forecast climate models.

Type: Book chapter , NonPeerReviewed

Format: text

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Dynamics of Decadal Climate Variability and Implications for its Prediction (2010)

Hurrell, J. ; Delworth, T. ; Danabasoglu, G ; [et al.]

.

In: In: Proceedings of the "OceanObs'09: Sustained Ocean Observations and Information for Society". , ed. by Hall, J., Harrison, D. E. and Stammer, D. ESA Publication, WPP-306 . ., ., o.S..

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Publication Date: 2012-07-06

Description: The temperature record of the last 150 years is characterized by a long-term warming trend, with strong multidecadal variability superimposed. The multidecadal variability is also seen in other (societal important) parameters such as Sahel rainfall or Atlantic hurricane activity. The existence of the multidecadal variability makes climate change detection a challenge, since Global Warming evolves on a similar timescale. The ongoing discussion about a potential anthropogenic signal in the Atlantic hurricane activity is an example. A lot of work was devoted during the last years to understand the dynamics of the multidecadal variability, and external as well as internal mechanisms were proposed. This White Paper focuses on the internal mechanisms relevant to the Atlantic Multidecadal Oscillation/Variability (AMO/V) and the Pacific Decadal Oscillation/Variability (PDO/V). Specific attention is given to the role of the Meridional Overturning Circulation (MOC) in the Atlantic. The implications for decadal predictability and prediction are discussed.

Type: Book chapter , NonPeerReviewed

Format: text

DOI: 10.5270/OceanObs09.cwp.53

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Continuous, Array-Based Estimates of Atlantic Ocean Heat Transport at 26.5°N (2011)

Johns, W. E. ; Baringer, M. O. ; Beal, L. M. ; [et al.]

AMS (American Meteorological Society)

In: Journal of Climate, 24 (10). pp. 2429-2449.

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Publication Date: 2017-08-24

Description: Continuous estimates of the oceanic meridional heat transport in the Atlantic are derived from the Rapid Climate Change–Meridional Overturning Circulation (MOC) and Heatflux Array (RAPID–MOCHA) observing system deployed along 26.5°N, for the period from April 2004 to October 2007. The basinwide meridional heat transport (MHT) is derived by combining temperature transports (relative to a common reference) from 1) the Gulf Stream in the Straits of Florida; 2) the western boundary region offshore of Abaco, Bahamas; 3) the Ekman layer [derived from Quick Scatterometer (QuikSCAT) wind stresses]; and 4) the interior ocean monitored by “endpoint” dynamic height moorings. The interior eddy heat transport arising from spatial covariance of the velocity and temperature fields is estimated independently from repeat hydrographic and expendable bathythermograph (XBT) sections and can also be approximated by the array. The results for the 3.5 yr of data thus far available show a mean MHT of 1.33 ± 0.40 PW for 10-day-averaged estimates, on which time scale a basinwide mass balance can be reasonably assumed. The associated MOC strength and variability is 18.5 ± 4.9 Sv (1 Sv ≡ 106 m3 s−1). The continuous heat transport estimates range from a minimum of 0.2 to a maximum of 2.5 PW, with approximately half of the variance caused by Ekman transport changes and half caused by changes in the geostrophic circulation. The data suggest a seasonal cycle of the MHT with a maximum in summer (July–September) and minimum in late winter (March–April), with an annual range of 0.6 PW. A breakdown of the MHT into “overturning” and “gyre” components shows that the overturning component carries 88% of the total heat transport. The overall uncertainty of the annual mean MHT for the 3.5-yr record is 0.14 PW or about 10% of the mean value.

Type: Article , PeerReviewed

Format: text

DOI: 10.1175/2010JCLI3997.1

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