The potential for sea ice-albedo feedback to give rise to nonlinear climate change in the Arctic Ocean – defined as a nonlinear relationship between polar and global temperature change or, equivalently, a time-varying polar amplification – is explored in IPCC AR4 climate models. Five models supplying SRES A1B ensembles for the 21 st century are examined and very linear relationships are found between polar and global temperatures (indicating linear Arctic Ocean climate change), and between polar temperature and albedo (the potential source of nonlinearity). Two of the climate models have Arctic Ocean simulations that become annually sea ice-free under the stronger CO 2 increase to quadrupling forcing. Both of these runs show increases in polar amplification at polar temperatures above-5 o C and one exhibits heat budget changes that are consistent with the small ice cap instability of simple energy balance models. Both models show linear warming up to a polar temperature of-5 o C, well above the disappearance of their September ice covers at about-9 o C. Below-5 o C, surface albedo decreases smoothly as reductions move, progressively, to earlier parts of the sunlit period. Atmospheric heat transport exerts a strong cooling effect during the transition to annually ice-free conditions. Specialized experiments with atmosphere and coupled models show that the main damping mechanism for sea ice region surface temperature is reduced upward heat flux through the adjacent ice-free oceans resulting in reduced atmospheric heat transport into the region.

Geophysical fluid dynamics.

The Max-Planck-Institute global ocean/sea ice model with orthogonal curvilinear coordinates

ユーザーガイドのこのバージョンは,1991年以降生じた変更を認めている.コード自体は若干の最近の変更 を取り入れている.Fortran名:tmean,smeanは,関数と残りの関数からくるこれらの変数の取扱いを識別す るために,tclim,sclimに変更された.より明確にそれらの機能を示すために,名称:trnu,trnvは,drx2d, dry2dに,名称:advuu,advvvは,adx2d,ady2dに変更された.風によって駆動される閉じた水域の代わり に,pom97.fは現在,領域の中央に島または海山を含む水路を通過する流れの問題を解く.このように,サブ ルーチン bcondは,アクティブな開境界条件を含む.しかしながら,これらの実例となる境界条件は,多くの 可能性の一部であり,従って,地域的なモデルのための開境界条件は,モデルのユーザーにとって難しい選択 となる.この 1998年の改訂は,第 16節の開境界条件に関するより完全な議論を含む.

/pdf/users-guide-for-a-three-dimensional-primitive-equation-4gcaryry2p.pdf

USERS GUIDE for A THREE-DIMENSIONAL, PRIMITIVE EQUATION, NUMERICAL OCEAN MODEL

Open boundary conditions for long-term integration of regional oceanic models

https://coaps.fsu.edu/pub/eric/papers_html/Chassignet_et_al_05b.pdf

The HYCOM (HYbrid Coordinate Ocean Model) data assimilative system

Much has been written of the error in computing the horizontal pressure gradient associated with sigma coordinates in ocean or atmospheric numerical models. There also exists the concept of “hydrostatic inconsistency” whereby, for a given horizontal resolution, increasing the vertical resolution may not be numerically convergent. In this paper, it is shown that the differencing scheme cited here, though conventional, is not hydrostatically inconsistent; the sigma coordinate, pressure gradient error decreases with the square of the vertical and horizontal grid size. Furthermore, it is shown that the pressure gradient error is advectively eliminated after a long time integration. At the other extreme, it is shown that diagnostic calculations of the North Atlantic Ocean using rather coarse resolution, and where the temperature and salinity and the pressure gradient error are held constant, do not exhibit significant differences when compared to a calculation where horizontal pressure gradients are c...

/pdf/the-pressure-gradient-conundrum-of-sigma-coordinate-ocean-47ct1gbphr.pdf

The Pressure Gradient Conundrum of Sigma Coordinate Ocean Models

[1] Recent studies indicate that the rates of sea level rise (SLR) along the U.S. mid-Atlantic coast have accelerated in recent decades, possibly due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and its upper branch, the Gulf Stream (GS). We analyzed the GS elevation gradient obtained from altimeter data, the Florida Current transport obtained from cable measurements, the North Atlantic Oscillation (NAO) index, and coastal sea level obtained from 10 tide gauge stations in the Chesapeake Bay and the mid-Atlantic coast. An Empirical Mode Decomposition/Hilbert-Huang Transformation (EMD/HHT) method was used to separate long-term trends from oscillating modes. The coastal sea level variations were found to be strongly influenced by variations in the GS on timescales ranging from a few months to decades. It appears that the GS has shifted from a 6–8 year oscillation cycle to a continuous weakening trend since about 2004 and that this trend may be responsible for recent acceleration in local SLR. The correlation between long-term changes in the coastal sea level and changes in the GS strength was extremely high (R = −0.85 with more than 99.99% confidence that the correlation is not zero). The impact of the GS on SLR rates over the past decade seems to be larger in the southern portion of the mid-Atlantic Bight near Cape Hatteras and is reduced northward along the coast. The study suggests that regional coastal sea level rise projections due to climate change must take into account the impact of spatial changes in ocean dynamics.

/pdf/gulf-stream-s-induced-sea-level-rise-and-variability-along-1ddub53b6w.pdf

Gulf Stream's induced sea level rise and variability along the U.S. mid‐Atlantic coast

Progress in numerical models of the Loop Current, rings, and related circulation during the past three decades is critically reviewed with emphasis on physical phenomena and processes

/pdf/loop-current-rings-and-related-circulation-in-the-gulf-of-4hktsufejv.pdf

Loop Current, Rings and Related Circulation in the Gulf of Mexico: A Review of Numerical Models and Future Challenges

Recent studies identified the U.S. East Coast north of Cape Hatteras as a “hotspot” for accelerated sea-level rise (SLR), and the analysis presented here shows that the area is also a “hotspot for accelerated flooding.” The duration of minor tidal flooding [defined as 0.3 m above MHHW (mean higher high water)] has accelerated in recent years for most coastal locations from the Gulf of Maine to Florida. The average increase in annual minor flooding duration was ∼20 h from the period before 1970 to 1971–1990, and ∼50 h from 1971–1990 to 1991–2013; spatial variations in acceleration of flooding resemble the spatial variations of acceleration in sea level. The increase in minor flooding can be predicted from SLR and tidal range, but the frequency of extreme storm surge flooding events (0.9 m above MHHW) is less predictable, and affected by the North Atlantic Oscillations (NAO). The number of extreme storm surge events since 1960 oscillates with a period of ∼15 year and interannual variations in the number of storms are anticorrelated with the NAO index. With higher seas, there are also more flooding events that are unrelated to storm surges. For example, it is demonstrated that week-long flooding events in Norfolk, VA, are often related to periods of decrease in the Florida Current transport. The results indicate that previously reported connections between decadal variations in the Gulf Stream (GS) and coastal sea level may also apply to short-term variations, so flood predictions may be improved if the GS influence is considered.

/pdf/accelerated-flooding-along-the-u-s-east-coast-on-the-impact-u85tqbf1h0.pdf

Accelerated flooding along the U.S. East Coast: On the impact of sea‐level rise, tides, storms, the Gulf Stream, and the North Atlantic Oscillations

In a recent paper by Mellor et al., it was found that, in two-dimensional ( x, z) applications with finite horizontal viscosity and zero diffusivity, the velocity error, associated with the evaluation of horizontal density or pressure gradients on a sigma coordinate grid, prognostically disappeared, leaving behind a small and physically insignificant distortion in the density field. The initial error is numerically consistent in that it decreases as the square of the grid increment size. In this paper, we label this error as a sigma error of the first kind. In three-dimensional applications, the authors have encountered an error that did not disappear and that has not been understood by us or, apparently, others. This is a vorticity error that is labeled a sigma error of the second kind and is a subject of this paper. Although it does not prognostically disappear, it seems to be tolerably small. To evaluate these numerical errors, the authors have adopted the seamount problem initiated by Beckman and Haidvogel. It represents a stringent test case, as evidenced by their paper, wherein the model is initialized with horizontal isopycnals, zero velocity, and no forcing; then, any velocities that develop must be considered errors. Two appendices are important adjuncts to the paper, the first providing theoretical confirmation and understanding of the numerical results, and the second delving into additional errors related to horizontal or isosigma diffusion. It is, however, shown that satisfactory numerical solutions are obtained with zero diffusivity.

/pdf/sigma-coordinate-pressure-gradient-errors-and-the-seamount-2t7476vqj3.pdf

Tal Ezer

Papers

The Pressure Gradient Conundrum of Sigma Coordinate Ocean Models

Gulf Stream's induced sea level rise and variability along the U.S. mid‐Atlantic coast

Loop Current, Rings and Related Circulation in the Gulf of Mexico: A Review of Numerical Models and Future Challenges

Accelerated flooding along the U.S. East Coast: On the impact of sea‐level rise, tides, storms, the Gulf Stream, and the North Atlantic Oscillations

Sigma Coordinate Pressure Gradient Errors and the Seamount Problem