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OCEAN HEAT BUDGET (PERIMBANGAN BAHANG)

Penting:mengerti iklim dunia variabilitas jangka pendek dan panjang

Peran Lautan:

50% cahaya mthr diserap dan disimpan sementara pd perm bumi

(daratan & lautan)

Sbgn bahang yg tersimpan di laut: dilepas ke atm (evaporasi + radiasi

infra merah) Selebihnya disebarkan oleh arus laut ke lintang tinggi

Total dari perimbangan fluks bahang kedalam atau keluar volume air:

Heat Budget (perimbangan bahang)

Umumnya fluks lewat permukaan.

Fluks lewat lap. dalamjauh lebih kecil

Secara global: fluks bahang balance (berimbang) kalau tdk

lautmkn hangatatau dingin purba@14

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Komponen utama (major term) heat budget pd perm:

Insolation (QSW):fluks sinar mthr ke laut

Radiasi bersih Inframerah (QLW): fluks bersih (net) radiasi

infra mrh dari lautan

Sensible Heat Flux (QS): fluks bahang dr perm akibat konduksi

Latent Heat Flux (QL): fluks bahang yg dibawa uap air

Adveksi (QV): bahang dibawa arus

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Konservasi Bahang mengharuskan:

QT= QSW+ QLW+ QS+ QL+ QV

QT adalah resultan bahang yg diperoleh (gain) atau hilang (loss)

Unit fluks bahang: watts/m2 .

Perkalian fluks x luas permukaan x waktu = energi dalam joule

Perubahan temperatur (T) airberhubungan dgn perubahan energi

(E):

E = Cpm T,

dimana: Cp = sp. heat pd tek tetap (~ 4.0 x 103 J.kg-1.oC-1)

m = massa yg dipanaskan atau didinginkan

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Pentingnya laut dalam Neraca Bahang Bumi

Perbandingan sederhana: bahang yg tersimpan di laut dan daratan

dalam 1 siklus tahunan: pd summer(bahang disimpan) dan winter

(bahang dilepas).Ternyata laut menyimpan dan melepas bahang yang lebih banyak

dari daratan.

Hal ini terjadi karena Cp laut> Cp daratan dan batuan (5:1) dan airlaut masih dapat kontak dengan atm sampai ked. 100 m sedangkan

daratan dan batuan hanya ked 1m, jadi lebih banyak massa air laut

yang dpt kontak dgn atm dibanding daratan dan batuan.

Konsekuensinya: kisaran perubahan suhu udara musiman diatas

daratan makin tinggi bila makin jauh dari laut, dan dapat melebihi

40 oC ditengah benua dan mencapai 60oC di Siberia.

Sedangkan di laut dan sepanjang pantai < 10 oC

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Faktor2 Yang Mempengaruhi Komponen Neraca Bahang

I. Insolation (QSW) (fluks sinar matahari ke laut):

1. Tinggi matahari di atas horizon, tgt: lintang, musim dan waktu pd

siang hr (tdk ada mlm hr)2. Lamanya waktu siang hr, tgt: lintang dan musim

3. Penampang melintang perm yg menyerap snr mthr, tgt: ketinggian

mthr di atas horizon

4. Attenuasi, yg tgt:

* pjg lint di atm, yg bervariasi sbg csc , = sdt mthr di atas horz* Awan : absorp dan scatter

* mol gas yg absorp pd kanal tertentu. H2O, O3dan CO2: penting

* aerosol: absorp dan scatter

* debu: scatter5. Pantulan dari perm, tgt sdt elevasi mthr dan kekasaran muka laut

Yg plg dominan: sdt inklinasi dan keawanan.

Penyerapan oleh uap air dan ozon: lemah

Insolasi tahunan rata-rata berada pada kisaran: +30 s/d + 260 W/m25(37)purba@14

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II Fluks Inframerah (QLW) (fluks bersih (net) radiasi lautan) :

Perm laut mengemisi bahang spt black bodydgn suhu air ~ 290 oK.

Puncak emisi ~ 10 m. Emisi REM pada ini diserap awan dan uap airpd bgn ttt dari spektrum dan pd bgn lain di teruskan jendela atm

Transmissi: 813 m, tgt uap airAbsorpsi: 3.54.0 m, tgt konsentrasi CO2 Fig 5.4

CO2 jendela tertutup dan makin banyak radiasi bumiterperangkap di atm.

Atm spt rumah kaca green house effect(bebas sr mthr msk,sbgn emisi bm terperangkap) ~ suhu perm bumi 33 oC lebih hangat

bila dibandingkan dgn kondisi tidak ada atm.

Gas green house effect: CO2, Uap air, methane dan O3

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Uap air

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Net fluks infra red tgt:

1. Kecerahan jendela atm., yg tgt: ketebalan awan, ketinggian

awan dan kandungan uap air di atm2. Suhu air: Radisi ~ T4 , T dalam oK

3. Penutupan es dan salju mengisolasi muka laut

Variasi uap air dan awan lebih penting dalam menentukan

radiasi balik dibanding variasi suhu permukaan.

Dari kutub ke ekuator terdapat kenaikan radiasi kembali ~ 42

%, tetapi pada jarak yg sama, uap air dapat mempengaruhi

emisi radiasi infra mrh bersih ~ 200 %,

Fluks infra merah tahunan rata-rata berkisar: - 60 W/m2

sampai - 30 W/m2

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III. Fluks Latent Heat (QL): fluks bahang akibat

Evaporasi

Tgt terutama: kec. angin dan kelembaban relatif.Bila angin kencang dan udara kering evaporasi Juga laut terbuka lebih besar dari laut yg tertutup es di kutub

Fluks evaporasi tahunan rata-rata berkisar: - 130 W/m2sampai - 10

W/m2

IV. Sensible Heat Flux (QS): akibat konduksi

Tgt terutama: kec angin dan perbedaan temperatur mk laut dan

udara

Fluks Sensible Heat tahunan rata-rata berkisar: - 42 W/m2sampai

- 2 W/m2

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Penghitungan fluks secara langsung:

Hanya ada satu cara yg akurat menentukan sensible dan latent heat

dan momemtum: pengukuran langsung besaran turbulent pd lapisanbatas (boundary layer) atm yg diukur dgn gust probe(alat yg

berespons sangat cepat) pada pesawat yg terbang rendah atau

platform

Mahal dan hanya ada bbrp pengukuran dan untuk kalibrasi metode

lainnya.

Gust probemengukur: kec angin vertikal dan horizontal,

kelembaban dan suhu udara pada lapisan batas (boundary layer) atm

dgn perm bumi ( kisaran 30 m diatas mk laut, karena fluks tidaktergantung ketinggian pada lapisan ini)

Pengukuran harus super cepat (gust probe), bbrp pengukuran per

detik

Fluks dihitung dari informasi tsb 10(37)purba@14

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Cara lain: Radiometer (alat mengukur radiasi dr mk bumi)

Radiometers padaships, offshore platforms, and bahkansmall islands

dapat mengukur langsung fluks radiasi.

Kanal yang lebar yg sensitif thd radiasi: 0.3m s/d 50m: dpt

mengukur incoming solar and infrared radiation (accuracy: 3%)

Alat lain: radiometer khusus dapat mengukur: Radiasi mthr yang

datang, radiasi infra merah kebawah dan ke atas.

Akan tetapi biasanya, radiasi ifra merah keatas ditentukan

berdasarkan pengukuran suhu laut permukaan

Cara ini lebih akurat dari pada mengukur radiasi

Penghitungan Fluks secara tidak langsung: Bulk Formula

Kedua cara secara langsung: mahal utk jangka pjg

Utk pengukuran praktis: diamati korelasi antar fluks dgn variabel yg

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Fluks dari sensible heat dan latent heat serta momentum.

Formula Bulk :

Suhu Udara Tadiukur dengan termometer pada kapal

TsSuhu permukaan laut diukur dengan termometer dari kapal atau

dari ruang angkasa dengan infrared radiometersseperti AVHRR.

Specific humidity udara pada ketinggian 10 m diatas paras laut (qa)

dihitung dari pengukuran relative humidity dari kapal.

Specific humidity pada paras laut (qs) dihitung dari Tsdgnasumsi

udara pada muka laut sudah jenuh (saturated) dgn uap air.U10:kec angin pd 10 m diatas perm laut

Koef: CD, CSdan CLdihitung mengkorelasikan pengukuran

langsung fluks yang dilakukan dengan probesdengan variabel yang

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The problem now becomes:

How to calculate the fluxes across the sea surface required for

studies of ocean dynamics?

The fluxes include: 1) stress; 2) solar heating; 3) evaporation; 4)

net infrared radiation; 5) rain; 5) sensible heat; and 6) others such

as CO2 and particles (which produce marine aerosols).

Furthermore, the fluxes must be accurate.

We need an accuracy of approximately 15 W/m2.

This is equivalent to the flux of heat which would warm or cool a

column of water 100 m deep by roughly 1oC in one year.

Table 5.2 lists typical accuracies of fluxes measured globally

from space.

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Hasil-hasil pengukuran: Global Data Sets for Fluxes

Ship and satellite data: produce global maps of fluxes.

Observations from ship measurements made over the past 150 years

yield maps of the long-term mean values of the fluxes, especially inthe northern hemisphere.

Ship data, however, are sparse in time and space, and they are

increasingly supplemented by data from space.

Space data give the variability of some of the fluxes.

Output from numerical weather models is also used.

Ada beberapa sumber data:1. Comprehensive Ocean-Atmosphere Data Set

Data collected from observers on ships are the richest source ofmarine information. These marine reports have now been edited and

published as the Comprehensive Ocean-Atmosphere Data Setcoads

(Woodruff et al. 1987) available through the National Oceanic and

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2. Data Satelit.

Raw data are available from the many satellite projects, but

the data must be further processed to be useful.

Data are available from the instruments on operational meteorologicalsatellites including:

1) NOAAseries of polar-orbiting, meteorological satellites;

2) Defense Meteorological Satellite Program DMSPpolar-orbiting

satellites, which carry the Special Sensor Microwave/ Imager

(SSM/I);

3) Geostationary meteorological satellitesoperated by NOAA

(GOES), Japan(GMS) and the European Space Agency

(METEOSATS). Data are also available from :

(i) Nimbus-7, Earth Radiation Budget Instruments;(ii) Earth Radiation Budget Satellite, Earth Rad. Budget Experiment;

(iii) The European Space Agencys ERS1 & 2;

(iv) The Japanese ADvanced Earth Observing System (ADEOS);

(v) Quicksat, and (vi) Topex/Poseidon. 18(37)purba@14

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3. International Satellite Cloud Climatology Project.

The International Satellite Cloud Climatology Project is to collect

observations of clouds made by dozens of Meteorological

satellites from 1985 to 1995, to calibrate the the satellite data, to

calculate cloud cover using carefully verified techniques, and to

calculate surface insolation.

4. Global Precipitation Climatology Project.

Uses three sources of data to calculate rain rate:(i) Infrared observations of the height of cumulus clouds from GOES

satellites.

(ii) Measurements by rain gauges on islands and land.

(iii) Radio-frequency emissions from from water droplets in theatmosphere observed by the SSMI.

Accuracy is about 1 mm/day. Data on a 2.5 lat by 2.5 long grid

from July 1987 to December 1995 from the Global Land Ocean

Precipitation Analysis at the NASA Goddard Space Flight Center.19(37)purba@14

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5. Reanalyzed Data From Numerical Weather Models

Surface heat fluxes have been calculated from weather data using

numerical weather models by various reanalysis projects.

The fluxes are consistent with atmospheric dynamics, they are

global, and they are available for many years on a uniform grid.

For example, the NCAR/NCEP reanalyzed data are available on a

CD-ROM including daily averages of wind stress, sensible and latent

heat fluxes, net long and short wave fluxes, near-surface

temperature, and precipitation.

6. Data From Numerical Weather Models

Some projects require flux data a few hours after observations are

collected.The surface analysis from numerical weather models is a good

source for this type

of data.

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DISTRIBUSI GEOGRAFIS KOMPONEN NERACA BAHANG

Bbrp grup: data kapal, satelit dan model numerik dr sirkulasi atm utk

menghitung rata-rata dari komponen Neraca Bahang.

Fig 5.6: Menggambarkan pentingnya secara global nilai-nilai dari

berbagai komponen

Pd puncak atm. insolation balances radiasi infra merah

Pd permukaan latent heat flux and net infrared radiation tend to

balance insolation, and sensible heat flux is small.

Hanya 20% of insolation reaching Earth is absorbed directly by

the atmosphere while 49% is absorbed by the ocean and land.

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What then warms the atm and drives the atm circulation.

Sunlight warms the tropical oceans which must evaporate water to

keep from warming up.

It also radiates heat to the atm, but the net radiation term is smaller

than the evaporative term.

Trade winds carry the heat in the form of water vapor to the tropical

convergence zone where it falls as rain.

Rain releases the latent heat evaporated from the sea, and it heats the

air in cumulus rain clouds by as much as 125 W/m2 averaged over a

six-year period

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The zonal average of the oceanic heat-budget terms (figure 5.7):

(1)insolation is greatest in the tropics,

(2)evaporation balances insolation, and

(3) sensible heat flux is small.

Zonal average is an average along lines of constant latitude.

The terms in Figure 5.7 dont sum to zero.

The areal-weighted integral of the curve for total heat flux is not zero.

Because the net heat flux into the oceans averaged over several yearsmust be less than a few watts per square meter, the non-zero value

must be due to errors in the various terms in the heat budget.

Errors in the heat budget terms can be reduced by using additional

information.

Heat and fresh water are transported by the oceans and atmosphere, and

the known values for the transports can be used to constrain the

calculations of net heat fluxes (figure 5.7).

The constrained fluxes show that the ocean gains heat in the tropics and

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Figure 5.7 Upper: Zonal averages of heat transfer to the ocean by insolation QSW, and loss

by infrared radiation QLW, sensible heat flux QS, and latent heat flux QL. Lower:Net heat flux

through the sea surface calculated from the data above (solid line) and net heat flux constrained

to give heat and fresh-water transports by the ocean that match independent calculations of

these transports. The area under the lower curves ought to be zero, but it is 16 W/m2 for

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Maps of the regional distribution of fluxes give clues to the

processes producing the fluxes.

Net downward short-wave radiation (insolation) at the sea surface(Figure 5.8 a) shows that the large flux into the tropical region is

modulated by the distribution of clouds, and that heating is

everywhere positive.

The net infrared heat flux (Figure 5.8 b) is largest in regions with

the least clouds, such as the central gyres and the eastern central

Pacific.

The net infrared flux is everywhere negative.

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Figure 5.8 a. Annual-mean insolation QSWthrough the sea surface during 1989

calculated by the Satellite Data Analysis Center at the NASA Langley Research

Center using data from the International Satellite Cloud Climatology Project.

Units are W/m2, contour interval is 10 W/m2.

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Figure 5.8 b. Annual-mean infrared radiation QLWthrough the sea surface

during 1989 calculated by the Satellite Data Analysis Center at the NASA

Langley Research Center using data from the International Satellite Cloud

Climatology Project. Units are W/m2, contour interval is 10 W/m2.

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Fig. 5.9: Latent heat fluxes. Dominated by evaporation in the trade wind regions

and the offshore flow of cold air masses behind cold fronts in winter offshore of

Japan and North America.

Figure 5.9 Annual-mean latent heat flux from the sea surface QLin W/m2during 1989

calculated from data compiled by the Data Assimilation Office of NASAS Goddard

Space Flight Center using reanalyzed output from the ECMWF numerical weather

prediction model. Contour interval is 10 W/m2. 29(37)purba@14

Fi 5 10 S ibl h t fl d i t d b ld i bl i ff ti t

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Fig. 5.10 a. Sensible heat fluxes are dominated by cold air blowing off continents.

Figure 5.10 a. Annual-mean upward sensible heat flux QSthrough the sea surface in

W/m2calculated by DaSilva, Young, and Levitus (1995) using the COADS data set from

1945 to 1989. Contour interval is 2 W/m2.30(37)purba@14

Fi 5 10 b Th t h ti i i l t i t i l i d t

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Fig. 5.10 b. The net heating gain is largest in equatorial regions and net

heat loss is largest downwind on Asia and North America (westerlies).

Figure 5.10b. Annual-mean constrained, net, downward heat flux through the sea surface

in W/m2calculated by DaSilva, Young, and Levitus (1995) using the COADS data set

from 1945 to 1989. Contour interval is 20 W/m2(bottom). 31(37)purba@14

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MERIDIONAL HEAT TRANSPORT

Overall, earth gains heat at the top of the tropical atmosphere, and it

loses heat at the top of the polar atmosphere.

The atmospheric and oceanic circulation together must transport heatfrom low to high latitudes to balance the gains and losses.

This north-south transport is called the meridional transport.

How much heat is carried by the ocean and how much by the

atmosphere?

The sum of the meridional heat transport by the ocean andatmosphere together is calculated accurately from the divergence of

the zonal average of the heat budget measured at the top of the

atmosphere by satellites.

To make the calculation, we assume steady state transports over

many years so that any long term net heat gain or loss through the top

of the atmosphere must be balanced by a meridional transport and not

by heat storage in the ocean or atmosphere.32(37)

y

Q

x

QQ

TyTxT

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Jadi ada 2 heat transport:

1) Heat Budget at the top of the Atmosphere

* Insolation is calculated from the solar constant and observations of

reflected sunlight made by meteorological satellites and by specialsatellites such as the Earth Radiation Budget Experiment Satellite.

* Back radiation is measured by infrared radiometers on the satellites.

The difference between insolation and net infrared radiation is the net

heat flux across the top of the atmosphere.

2) Oceanic Heat Transport

Oceanic heat transport are calculated three ways:

Surface Flux Method uses measurements of wind, insolation, air, and

sea temperature, cloudiness, and bulk formulas to estimate the heat

flux through the sea surface.Then the fluxes are integrated to obtain the zonal average of the heat

flux (figure 5.7).

Finally, the meridional derivative of the net flux gives the flux

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* Direct Methoduses measured values of current velocity and

temperature from top to bottom along a zonal section spanning an

ocean basin.

The values are used to calculate the flux from the product of

northward velocity and heat content derived from the temperature

measurement.

* Residual Methoduses atmospheric observations or the output ofnumerical models of the atmospheric circulation to calculate the heat

transport in the atmosphere.

This is the direct method applied to the atmosphere.

The atmospheric transport is subtracted from the total meridional

transport calculated from the top-of-the-atmosphere heat flux to

obtain the oceanic contribution as a residual

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Important Concepts

1. Sunlight is absorbed primarily in the tropical ocean.

The amount of sunlight is modulated by season, latitude, time of

day, and cloud cover.2. Most of the heat absorbed by the oceans in the tropics is released

as water vapor which heats the atmosphere when water is

condenses as rain.

Most of the rain falls in the tropical convergence zones, lesser

amounts fall in mid-latitudes near the polar front.3. Heat released by rain and absorbed infrared radiation from the

ocean are the primary drivers for the atmospheric circulation.

4. The net heat flux from the oceans is largest in mid-latitudes and

offshore of Japan and New England.5. Heat fluxes can be measured directly using fast response

instruments on low-flying aircraft, but this is not useful for

estimating heat budgets of the ocean.

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6. Heat fluxes through large regions of the sea surface can be

estimated from bulk formula.

Seasonal, regional, and global maps of fluxes are available

based on ship and satellite observations.

7. The most widely used data sets for studying heat fluxes are the

Comprehensive Ocean-Atmosphere Data Set and the reanalysis

of meteorological data by numerical weather prediction models.

8. The oceans transport about one-half of the heat needed to warm

higher latitudes, the atmosphere transports the other half.

9. Solar output is not constant, and the observed small variations inoutput of heat and light from the sun seem to produce the

changes in global temperature observed over the past 400 years.

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