Nandini Ramesh IPCC AR5 WG1 - Climate Change 2013: The Physical Science Basis Seminar in Atmospheric Science21 st February, 2014

1. Introduction The ocean exchanges heat, freshwater, and C with the atmosphere. Large heat capacity Currently storing 30% of our C emissions Large heat capacity + inertia = the ocean “integrates” short term variability. This makes it a good indicator of long-term change. Less long-term coverage of ocean variables than for the atmosphere.

2. Ocean Temperature & Heat Content Our observation system has changed over time and space Data from 1970 onwards is fit for this analysis – therefore changes since 1971 are documented here. Biases in XBTs & MBTs have been corrected for since AR4. These produced spurious decadal variability. Differences in background climatology & how poorly- sampled regions are dealt with produce disagreement among estimates. Upper ocean heat content (UOHC) changes on many timescales, deep ocean only on long timescales.

Upper Ocean Temperature (<700 m) Most warming in the North Atlantic 25-65N: consistent with poleward displacement of global temperature field Southern Ocean – may be an artefact. Other reanalyses don’t have such strong warming; warming was stronger in the upper 1000 m of the SO between 1930 & % increase in density stratification Arctic (not shown) has also undergone warming.

Upper Ocean Heat Content Dips following volcanic eruptions of ‘63, ‘82, ‘91. d/dt at < decadal scales is poorly constrained TW of heating depending on dataset decrease in rate of change: uncertain because this was the time of transition from XBT to Argo.

Deep Ocean Temperature & Heat Content

Very little data below 700 m! We know that the global ocean (at m) warmed from 2005 to m depth range contributes an estimated 30% of the total trend from 0 to 2000 m. (previous figure) Below 2000 m: only from transects. Estimates of heat content change are only feasible for the North Atlantic m: no significant trend (likely) Below 3000 m: Significant warming trend (likely). Most pronounced at ~4500 m: influence of AABW

AABW influence can be felt on multidecadal timescales because of waves, not spreading. AABW warming may be due to recovery from cool 1970s Weddell polynya. But measurements from further north suggest that warming started in ‘91. Net cooling trend in N Atlantic: highly variable on decadal timescales. Deep Ocean Temperature & Heat Content

3. Changes in Salinity & Freshwater Content Influenced by hydrological cycle, ocean dynamics and ice formation/melting We don’t have good measurements of E or P over the ocean, but we do have salinity measurements. Reflects “rich-gets-richer” trend – very likely

Changes in Sea Surface Salinity

Upper Ocean Salinity Changes Salinity increases in the gyres. Freshening of AAIW and SAMW Atlantic: saltier. Pacific: fresher. Fresher surface waters  more stable stratification  alters ocean circulation. Western Tropical Pacific & North Pacific in particular. Saltier Mediterranean has contributed to saltier Atlantic. Freshening in the Arctic (medium confidence)

4. Changes in Ocean Surface Fluxes Exchanges of heat, water, & momentum (via wind stress) with the atmosphere. The accuracy of our measurements is insufficient to make inferences on a global scale. Positive flux = into the ocean. A useful constraint: changes in ocean heat content. Net heat flux must be consistent with this.

Small positive trend in ocean precipitation, but low confidence.

Wind stress: Southern Ocean: increase since the 1980s (medium confidence) Tropical Pacific: increase since the 1990s (medium confidence) but this may be the IPO. Net decrease since North Atlantic: Winter wind stress curl related to the NAO. Poleward shift of the zero wind stress curl line: low confidence.

Surface waves Changes in Surface Wave Height of 8-10 cm/decade for North Pacific, 14 cm/decade for North Atlantic over reported in AR4. More recent work: up to 20 cm/decade in the eastern/central North Pacific. No trend in extreme waves. Satellite record is still too short to infer trends.

5. Changes in Water-mass Properties North Pacific Intermediate Water: freshened. Antarctic Intermediate Water: dipole pattern. Cooler & fresher on lighter isopycnals, warmer and more saline on denser isopycnals. Variability may also be linked to ENSO & SAM. Upper NADW: freshening from ; reversal after that. Heat entered during low NAO phase of the 60s. Lower NADW: Net cooling – possibly due to entrainment of surrounding waters. Sparse data. AABW: Warmed since the 80s or 90s. Export rate has decreased (more likely than not). Indian & Pacific sectors have seen freshening; Atlantic has variability on multidecadal timescales. Smaller volume in the Atlantic is related to changes in mixing and not formation rates.

6. Changes in Ocean Circulation Velocity data is predominantly from Argo drifters. Measurements of SSH are from satellites – 1992 onwards. Pacific: Intensification of the N subpolar gyre, S subtropical gyre, subtropical cells. Southward expansion of the N subtropical gyre because of the migration of the North Equatorial Current from 13 o to 11 o N. Southward shift of ACC. Likely to be decadal variability. AMOC: likely to slow down. No net trend; weakened in but recovered soon after.

Exchange between basins Indonesian Throughflow: Low confidence in a trend. Dominated by ENSO variability. ACC: poleward shift. Increases in wind stress have been compensated by eddies, and not changes in transport. North Atlantic/Nordic: No evidence of a trend.

7. Sea Level Change Changes because of exchanges with land: melting of ice. Changes because of thermal expansion. Spatial variations: tides, winds. Measured using tide gauges. Corrections need to be made for vertical land movement: isostatic adjustment, groundwater mining, tectonic activity, hydrocarbon extraction. Altimetry/tide gauges measure effects of both thermal expansion and addition of mass. GRACE has allowed us to estimate mass changes.

Significant progress made since AR4 on quantifying uncertainties in global mean sea level associated with VLM. Change in estimate = one standard error. Interannual fluctuations due to ENSO. The rate of thermosteric rise in sea level: 50% higher than estimated in AR4 (for 0-700m).

8. Ocean Biogeochemical Changes Carbon Ocean’s inorganic C reservoir is 50x that of the atmosphere. Even small perturbations to the ocean reservoir can have huge consequences for the atmosphere. CO2 uptake is measured from pCO2 difference across air-sea interface. Measurements are few: too much uncertainty to estimate a global trend. Estimates of global uptake rate: 1.9 [1.2 to 2.5] PgC/yr and 2 [1 to 3] PgC/yr

Increase in pCO2 in both atmosphere & ocean at all locations. Faster increase in pCO2 in the ocean means oceanic uptake. Uptake rates are strongly affected by ENSO and NAO.

Estimated using Transit Time Distribution method – based on CFC measurements. Doesn’t include changes in biological productivity. Unlikely that there has been a decrease in net uptake rate of global sinks.

Highest storage rates in deep water formation regions.

Ocean Acidification Surface waters are mildly basic: pH High confidence that pH has decreased by 0.1 over the industrial era. Most acidification in the North Atlantic, least in subtropical South Pacific. Uptake of anthropogenic CO2 is the dominant cause of changes in seawater chemistry.

Oxygen Oxygen concentrations are decreasing. 15% attributable to warming (reduction in solubility of O2) and rest attributable to reduced downward mixing because of increased stratification. North Pacific, North Atlantic, and Indian Ocean show decreases; Southern Ocean – studies disagree on the trends. Coastal regions are becoming hypoxic at a faster rate.

Nutrients N-based fertilizers in runoff  high nutrient concentration in coastal waters  eutrophication  more CO2 removal. Up to 3% of oceanic new production is because of this nitrogen source. Oligotrophic provinces in the four basins grew by 0.8 – 4.3 % per year in (from chlorophyll) No published studies on long-term changes.

Synthesis Substantial progress since AR4 Virtually certain that the upper 700 m have warmed. Very likely that m depth range contributed 30% of the total warming. Global mean sea level has risen by 19 cm since The rate of increase was higher during Rise in mean sea level explains most extreme sea level increases. Regional trends in salinity have very likely increased the salinity contrast. Virtually certain that the ocean is storing CO2; very likely that this has contributed to acidification. Consistency between patterns of change across different parameters enhances confidence. Circulation change: southward shift of the ACC.