Comment on egusphere-2022-354

Abstract. We investigate the spatial and temporal variability of sea surface height (SSH) in the North Atlantic basin using satellite altimeter data from October 1992–January 2004. Our primary aim is to provide a detailed description of such variability, including that associated with propagating signals. We also investigate possible correlations between SSH variability and atmospheric pressure changes as represented by climate indices. We first investigate interannual SSH variations by deriving the complex empirical orthogonal functions (CEOFs) of altimeter data lowpass-filtered at 18 months. We determine the spatial structure of the leading four modes (both in amplitude and phase) and also the associated principal component (PC) time series. Using wavelet analysis we derive the time-varying spectral density of the PCs, revealing when particular modes were strongest between 1992–2004. The spatial pattern of the leading CEOF, comprising 30% of the total variability, displays a 5-year periodicity in phase; signal propagation is particularly marked in the Labrador Sea. The second mode, with a dominant 3-year signal, has strong variability in the eastern basin. Secondly, we focus on the Azores subtropical frontal zone. The leading mode (35%) is strong in the south and east of this region with strong variations at 3- and 5-year periods. The second mode (21%) has a near-zonal band of low variance between 22°–27° N, sandwiched between two regions of high variance. Thirdly, we lowpass filter the altimeter data at a cutoff of 30 days, instead of 18 months, in order to retain signals associated with propagating baroclinic Rossby waves and/or eddies. The leading mode is the annual steric signal, around 46% of the SSH variability. The third and fourth CEOFs, 11% of the remaining variability, are associated with westward propagation which is particularly dominant in a "waveband" between 32°–36° N. For all three cases considered above, no significant cross-correlation is found between the North Atlantic Oscillation index and the amplitude of the leading four PCs of interannual SSH variability. The only exception is an anti-correlation found over the North Atlantic basin between the NAO and the 4th PC. In the subtropical front, the East Atlantic Pattern index is anti-correlated with the leading PC for SSH variations lowpass filtered at 30 days. Further investigation of forcing mechanisms is suggested using hindcasts from ocean general circulation models.

This study investigates the sea surface height (SSH) and current responses to synoptic winter wind in the Bohai, Yellow, and East China Seas (BYES) using a numerical model. The basin‐scale responses are primarily interpreted as two leading coastal trapped waves (CTWs) identified through the complex empirical orthogonal function analysis of SSH. One CTW is a Kelvin wave (KW) explaining 76% of the total SSH variance; the other is a wind‐forced lowest shelf wave (SW1) explaining 17% of the total SSH variance. The KW dominates the current in the Bohai and Yellow Seas (BYS), and the SW1 dominates the current in the western East China Sea (ECS). The KW is a free relaxation response to large wind‐driven SSH variations and propagates cyclonically in the BYES around one SSH node. The SW1 propagates cyclonically in the semienclosed BYS around two nodes and southward along the western ECS with two nodes on the shelf. The wavelength and e‐folding scale of amplitude vary spatially, producing regionally distinct features and mechanisms. In the semienclosed BYS, the associated geostrophic current is along the cophase line of SSH, and its magnitude is proportional to both wave amplitude and wavenumber. In the western ECS, the associated geostrophic current is perpendicular to the cophase line of SSH, and its magnitude is proportional to the wave amplitude and inversely proportional to the e‐folding scale of amplitude. This study reveals not only the basin‐scale SSH and current variations in the BYES but also new features of CTWs in a semienclosed embayment.

This study considers the relation of the annular mode to the kinematics of a fluctuating jet in zonal-mean zonal wind and to the zonal index, using an idealized model of fluctuations in the eddy-driven jet. When the sphericity of the domain is accounted for, observed and numerically simulated annular modes for the Southern Hemisphere summertime are found to be in excellent agreement. In particular, the annular mode and zonal index mode are shown to be related but distinct. Although the annular mode is strongly (but not identically) related to fluctuations in jet position, fluctuations in jet strength and width are shown to also be important for its simulation. When the sphericity of the domain is neglected, analytic expressions for the leading empirical orthogonal function (EOF) modes of zonal-mean geopotential for the cases of individual fluctuations in jet strength, position, and width can be obtained. None of these EOF modes have the characteristics of the annular mode. In the presence of simultaneous fluctuations in jet strength and position, the leading zonal-mean geopotential EOF mode (strongly resembling the annular mode) is shown to mix the zonal index mode of zonal-mean zonal wind with other EOF modes, demonstrating why the annular mode and zonal index mode are related but distinct. The greater sensitivity to domain size of EOF modes of geopotential relative to the EOF modes of zonal-mean zonal wind is also discussed. This study focuses on the Southern Hemisphere summertime, which is characterized by a single, eddy-driven jet; the generality of the results presented suggest that the conclusions should be qualitatively unchanged in the presence of both subtropical and eddy-driven jets.

<p>Interannual variations in the upper ocean heat and freshwater contents in the subpolar North Atlantic has important climatic effect. It affects the intensity of deep convection, which, in turn, forms the link between upper and deep ocean circulation of the global ocean Conveyor Belt.</p><p>The upper ocean heat content is primarily affected by two main process: by the ocean-atmosphere heat exchange and by oceanic heat advection. The intensity of both fluxes in the subpolar gyre is linked to the character of atmospheric circulation, largely determined by the phase of the North Atlantic Oscillation (NAO).</p><p>To study the interannual variability of the oceanic heat advection (in the upper 500<sup>th</sup> meters layer) we compare the results from four different data-sets: ARMOR-3D (1993-2018), SODA3.4.2 and SODA3.12.2 (1980-2017), and ORAS5 (1958-2017). The ocean-atmosphere heat exchange is accessed as the sum of the latent and the sensible heat fluxes, obtained from OAFlux data-set (1958-2016).</p><p>The oceanic heat advection to the Labrador and to the Irminger seas has high negative correlation (-0.79) with that into the Nordic Seas. During the years with high winter NAO Index (NAOI) the oceanic heat advection into the Subpolar Gyre decreases, while to the Nordic Seas – increases. These variations go in parallel with the intensification of the Norwegian, the West Spitsbergen and the slope East Greenland currents and weakening of the West Greenland and the Irminger Currents. During the years with high NAOI, the ocean heat release (both sensible and latent) over the Labrador and Irminger seas increases, but over the Norwegian Sea it decreases.</p><p>In summary, the results show that, during the positive NAO phase, the observed decrease of the heat content in the upper Labrador and Irminger seas is linked to both, a higher oceanic het release and a lower intensity of advection of warm water from the south. In the Norwegian Sea, the opposite sign of variations of the fluxes above leads to a simultaneous warming of the upper ocean.</p><p>The investigation is supported by the Russian Scientific Foundation (RSF), number of project 17-17-01151.</p><p> </p><p> </p>

The sea surface height (SSH) variations observed by the TOPEX/POSEIDON satellite in the Yellow and East China Seas are examined over the 3 years from 1993 through 1995 assuming a linear response to wind stress across the region. The transfer function relating wind stress to SSH is modeled by a series of increasingly complex functions that gradually allow wind stress over successively broader regions and time periods to influence the SSH solution at a particular point. The SSH response to instantaneous wind stress at the same point in space implies an Ekman transport, but the response explains only a small fraction of the SSH variability. Relating SSH to wind stress averaged over two subregions and including both instantaneous and 12‐hour lagged wind stress indicates that a significant portion of SSH variability responds to winds that are remote in both space and time. An optimal estimation of the transfer function by minimizing the error variance leads to an extended empirical orthogonal function (EEOF) analysis of the wind stress field. The SSH response to each wind mode is determined. The analysis shows the principle variations in the wind stress to consist of northerly wind bursts during winter, which produce large SSH drops in the Bohai Bay and northern Yellow Sea region. The principal wind mode also is significantly related to variations along the shelf break near the Kuroshio Current. The third and fourth wind stress modes indicate the passage of typhoons.

Abstract. The Nordic Seas constitute the main ocean conveyor of heat between the North Atlantic Ocean and the Arctic Ocean. Although the decadal variability in the subpolar North Atlantic has been given significant attention lately, especially regarding the cooling trend since the mid-2000s, less is known about the potential connection downstream in the northern basins. Using sea surface heights from satellite altimetry over the past 25 years (1993–2017), we find significant variability on multiyear to decadal timescales in the Nordic Seas. In particular, the regional trends in sea surface height show signs of a weakening since the mid-2000s, as compared to the rapid increase in the preceding decade since the early 1990s. This change is most prominent in the Atlantic origin waters in the eastern Nordic Seas and is closely linked, as estimated from hydrography, to heat content. Furthermore, we formulate a simple heat budget for the eastern Nordic Seas to discuss the relative importance of local and remote sources of variability; advection of temperature anomalies in the Atlantic inflow is found to be the main mechanism. A conceptual model of ocean heat convergence, with only upstream temperature measurements at the inflow to the Nordic Seas as input, is able to reproduce key aspects of the decadal variability in the heat content of the Nordic Seas. Based on these results, we argue that there is a strong connection with the upstream subpolar North Atlantic. However, although the shift in trends in the mid-2000s is coincident in the Nordic Seas and the subpolar North Atlantic, the eastern Nordic Seas have not seen a reversal of trends but instead maintain elevated sea surface heights and heat content in the recent decade considered here.

A primitive equation ocean model has been applied to the study of mesoscale ocean dynamics and prediction using sea surface height information derived from a satellite‐borne altimeter. Results from a model of the Gulf of Mexico were directly compared with the altimeter data from GEOS 3 and Seasat reported by Marsh et al. (1984) and in situ hydrographic data analyzed by Maul and Herman (1985). In the eastern Gulf the amplitude and position of relative maxima of sea surface height variability were found to be similar for model, altimeter, and in situ data. In the central Gulf the model and the altimeter‐derived sea surface height variability maps were similar but differed considerably from the in situ results. The mean sea surface in the model and from the in situ data also agreed well in the eastern Gulf, but there was little agreement in the central Gulf. While there is no geoid adequate for determining an independent altimetric estimate of the mean dynamic height of the sea surface in the Gulf by differencing the geoid with the altimeter‐derived mean sea surface, the model dynamic mean sea surface can be subtracted from the mean surface of Marsh et al. (1984) to obtain a new estimate of the Gulf geoid, which may be the best geoid available in that region for several years. A benchmark experiment (“truth”) was integrated to statistical equilibrium and compared with results from four experiments in which the model was initialized with fields modified from archived benchmark data. The experiments differed only in the initialization fields. Each experiment was integrated for 100 days with inflow transport remaining constant throughout the integration. The idealized experiments were initialized geostrophically (1) with the exact sea surface and pycnocline height fields, (2) with only the exact sea surface heights and a pycnocline assumed to compensate it such that the deep pressure perturbations vanished, (3) just as in case 2 but with a geoid error component added on small spatial scales, and (4) just as in case 3 except the geoid error model included an additional contribution in strong geoid gradient regions. The sequence of four numerical experiments showed that (1) geostrophic initialization with exact sea surface and pycnocline height information provided accurate forecasts to 100 days, (2) even when only the sea surface height information was provided to the numerical model for geostrophic initialization and deep pressure perturbations assumed to vanish at the initial time, the forecasts were superior to persistence or climatology over the 100‐day forecast period, and (3) errors in the geoid on spatial scales comparable to the grid resolution of the model did not seriously degrade the forecast, even in dynamically active regions with large gradients in the geoid height where instability processes were observed to occur.

The variability of winter North Atlantic blocking and cold extremes connected to North Atlantic Sea surface temperature modes and ENSO: The mediated role of North Atlantic jet

A 'regime shift' is characterized by an abrupt transition from one quasi-steady climatic state to another, and its transition period is much shorter than the lengths of the individual epochs of each climatic state. In the present study, we investigate when regime shifts occurred and what was the difference in climatic states before and after the shifts, using the wintertime sea surface temperature (SST) field in the Northern Hemisphere. The relationship between changes in the SST field, and those in the atmospheric circulation, is also investigated. In order to detect organized patterns of the SST variations, we apply an empirical orthogonal function (EOF) analysis. As the results, the first mode is identical to El Nino/Southern Oscillation (ENSO) and so-called Pacific Decadal Oscillation (PDO), and corresponds to the Pacific/North American (PNA) pattern. The second mode, which relates to the Arctic Oscillation (AO), has a zonally elongated signal in both the North Atlantic, and North Pacific. EOF analyses to each oceanic basin are made separately, and the robustness of these modes is confirmed. In the present study, we define the regime shifts as the 'significant' and 'systematic' changes between the two quasi-steady states, continuing more than 5 years. Then, in order to identify the years when regime shifts occurred in the SST field, we carefully inspect the time series of original gridded SST data and those of the EOF modes. As a result, six regime shifts are detected in the study period from the 1910s to the 1990s: 1925/26, 1945/46, 1957/58, 1970/71, 1976/77 and 1988/89. It is ascertained that the shifts at almost all grids are completed within one year. All regime shifts having similar SST and atmospheric circulation pattern, including the changes in an intensity of the Aleutian Low (AL), and the corresponding SST changes in the central North Pacific. All regime shifts can be well described by the combination of the first and the second EOF modes. The duration between each regime shift is about 10 years, which are identical to the PDO. The simultaneous shifts in the first, and the second EOF modes, imply that the change in the AL activity associated with the PNA pattern, might have some connection with that of the AO.

Altimeter data from May 1995 to February 2002, together with wind, heat, and fresh water fluxes and hydrographic data, are used to study the annual variability in sea surface height (SSH) in the Nordic Seas. The main contributions to the SSH are quantified with error estimates. The amplitude of the annual variation of the SSH ranges from 4 to 8 cm with largest variability in the deeper basins (Lofoten, Norwegian, and Greenland Basins) and along the Norwegian coast. The seasonal steric height can explain 1.5-4 cm of the SSH variation and arises from changes in the air-sea heat flux. However, the associated seasonal changes in the surface currents that arrive from the steric height and the heat flux are weak (a few mm/s). This is because their contributions to the SSH are nearly constant over large areas. From altimetry and hydrography, we calculated seasonal changes in bottom pressure and currents. In the Greenland and Norwegian Seas, the seasonal anomalous large-scale circulation is cyclonic during spring and anticyclonic during autumn. It is most apparent in the Norwegian Sea with a seasonal variation in speed of 1-2 cm/s. This variation can be explained by changes in the averaged wind-stress curl over the Norwegian Sea. The annual cycle of the Norwegian Atlantic slope current is also revealed. It is strongest in winter and weakest in summer with an annual amplitude in speed of 2-4 cm/s.

The present study investigated the joint impacts of the spring Arctic Oscillation (AO) and the Scandinavia (SCAND) teleconnection pattern on spring surface air temperature (SAT) variations over the midhigh latitude Eurasia. It is found that the connection of the spring AO (SCAND) to the first two empirical orthogonal function (EOF) modes of the spring Eurasian SAT variations depends strongly on the phase of the spring SCAND (AO) index. The EOF1 and EOF2 modes display same sign variation and tripole pattern, respectively. In the in‐phase AO‐SCAND years, the mid‐high latitude Eurasia is covered by same sign SAT anomalies, and the spring AO and SCAND indices both have a strong (weak) connection with the EOF1 (EOF2) mode. In the out‐of‐phase AO‐SCAND years, the SAT anomalies over the mid‐high latitude Eurasia are featured by a tripole pattern, and the connection of the spring AO and SCAND indices with the EOF1 (EOF2) is insignificant (significant). Differences in the distribution of SAT anomalies between the in‐phase and out‐of‐phase AO‐SCAND years are attributed to the constructive and destructive superposition of atmospheric circulation anomalies related to the spring AO and SCAND indices. The observed joint impacts of the spring AO and SCAND on spring Eurasian SAT variations are captured by a historical simulation of the flexible global ocean‐atmosphere‐land system model, spectral version 2. Analysis indicates that the prediction skill of the spring Eurasian SAT variations related to the first two EOF modes is enhanced when taking both the spring AO and SCAND signals into account.

Temporal and spatial variability of Chl- a and SST on the South Atlantic Bight: Revisiting with cloud-free reconstructions of MODIS satellite imagery

North Atlantic atmospheric and ocean inter-annual variability over the past fifty years – Dominant patterns and decadal shifts

Physical forcing and biological response are highly variable over a wide range of scales in the South China Sea. The present paper analyzed interannual variability of the surface chlorophyll-a concentration of the South China Sea using NASA standard SeaWiFS monthly products from 1997 to 2007. Time series of monthly data were first smoothed using a 12-month running mean filter. An empirical orthogonal function (EOF) analysis was performed to evaluate the interannual variability. The first EOF mode is characterized by a higher surface chlorophyll-a concentration in the deep basin of the South China Sea with a maximum value southwest of Luzon Strait. The corresponding time coefficient function is highly correlated with the multivariate ENSO index (MEI). The correlation coefficient is −0.61 when the time coefficient function lags the MEI by 9 months. The second EOF mode is characterized by a northwest lower chlorophyll-a concentration. The corresponding time coefficient function correlates with the MEI at a correlation coefficient equal to 0.88, with a lag of 1 month. The third EOF mode shows the interannual variability of the chlorophyll-a concentration has some relationship with Indian Ocean dipole mode as well. The link between the climate and ocean biological states in the South China Sea is due to changes in upper-ocean temperature and wind field, which influence the availability of nutrients for phytoplankton growth.

Using pentad data of the Northern Hemisphere extended winter (November–March) from 1979 to 2012 derived from the daily rainfall of the National Meteorological Information Center of China, subseasonal variability of precipitation in China is analyzed. The two dominant modes of subseasonal variability are identified with an empirical orthogonal function (EOF) analysis. The first EOF mode (EOF1) is characterized by a monopole in South China, whereas the second EOF mode (EOF2) has a meridional dipole structure with opposite precipitation anomalies over the Yangtze River basin and the coastal area of South China. These two modes tend to have a phase shift to each other in both space and time, indicating that part of their variability represents a southward-propagating pattern. The subseasonal variability is decomposed into two components: one related to the Madden–Julian oscillation (MJO) and the other independent of MJO. It is found that the MJO contributes to about 10% of the precipitation variability in South China. EOF1 is associated with MJO phase 3, corresponding to enhanced equatorial convection in the Indian Ocean and depressed convection in the western Pacific, while EOF2 is related to MJO phase 5 when the enhanced tropical convection moves to the Maritime Continent region. Subseasonal precipitation variability in China that is independent of the MJO is especially affected by processes including tropical convection variability and the “cold surge” phenomenon or the development of a Siberian high and cold-air outbreak in East Asia associated with a wave train from the North Atlantic.